How do I set environment variables from Java

Question

How do I set environment variables from Java   I see that I can do this for subprocesses using ProcessBuilder  I have several subprocesses to start  though  so I d rather modify the current process s environment and let the subprocesses inherit it   There s a System getenv String  for getting a single environment variable  I can also get a Map of the complete set of environment variables with System getenv    But  calling put   on that Map throws an UnsupportedOperationException -- apparently they mean for the environment to be read only  And  there s no System setenv     So  is there any way to set environment variables in the currently running process   If so  how   If not  what s the rationale    Is it because this is Java and therefore I shouldn t be doing evil nonportable obsolete things like touching my environment    And if not  any good suggestions for managing the environment variable changes that I m going to need to be feeding to several subprocesses

User · Accepted Answer

Is it because this is Java and therefore I shouldn t be doing evil nonportable obsolete things like touching my environment     I think you ve hit the nail on the head   A possible way to ease the burden would be to factor out a method  void setUpEnvironment ProcessBuilder builder        Map lt String  String gt  env   builder environment           blah blah     and pass any ProcessBuilders through it before starting them   Also  you probably already know this  but you can start more than one process with the same ProcessBuilder  So if your subprocesses are the same  you don t need to do this setup over and over

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Linux MacOS only Setting single environment variables  based on answer by Edward Campbell   public static void setEnv String key  String value        try           Map lt String  String gt  env   System getenv            Class lt   gt  cl   env getClass            Field field   cl getDeclaredField  quot m quot            field setAccessible true           Map lt String  String gt  writableEnv    Map lt String  String gt   field get env           writableEnv put key  value         catch  Exception e            throw new IllegalStateException  quot Failed to set environment variable quot   e            Usage  First  put the method in any class you want  e g  SystemUtil  Then call it statically  SystemUtil setEnv  quot SHELL quot    quot  bin bash quot     If you call System getenv  quot SHELL quot   after this  you ll get  quot  bin bash quot  back

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For use in scenarios where you need to set specific environment values for unit tests  you might find the following hack useful  It will change the environment variables throughout the JVM  so make sure you reset any changes after your test   but will not alter your system environment   I found that a combination of the two dirty hacks by Edward Campbell and anonymous works best  as one of the does not work under linux  one does not work under windows 7  So to get a multiplatform evil hack I combined them   protected static void setEnv Map lt String  String gt  newenv  throws Exception     try       Class lt   gt  processEnvironmentClass   Class forName  java lang ProcessEnvironment        Field theEnvironmentField   processEnvironmentClass getDeclaredField  theEnvironment        theEnvironmentField setAccessible true       Map lt String  String gt  env    Map lt String  String gt   theEnvironmentField get null       env putAll newenv       Field theCaseInsensitiveEnvironmentField   processEnvironmentClass getDeclaredField  theCaseInsensitiveEnvironment        theCaseInsensitiveEnvironmentField setAccessible true       Map lt String  String gt  cienv    Map lt String  String gt       theCaseInsensitiveEnvironmentField get null       cienv putAll newenv       catch  NoSuchFieldException e        Class   classes   Collections class getDeclaredClasses        Map lt String  String gt  env   System getenv        for Class cl   classes          if  java util Collections UnmodifiableMap  equals cl getName               Field field   cl getDeclaredField  m            field setAccessible true           Object obj   field get env           Map lt String  String gt  map    Map lt String  String gt   obj          map clear            map putAll newenv                         This Works like a charm  Full credits to the two authors of these hacks

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public static void set Map lt String  String gt  newenv  throws Exception       Class   classes   Collections class getDeclaredClasses        Map lt String  String gt  env   System getenv        for Class cl   classes            if  java util Collections UnmodifiableMap  equals cl getName                   Field field   cl getDeclaredField  m                field setAccessible true               Object obj   field get env               Map lt String  String gt  map    Map lt String  String gt   obj              map clear                map putAll newenv                       Or to add update a single var and removing the loop as per thejoshwolfe s suggestion    SuppressWarnings    unchecked       public static void updateEnv String name  String val  throws ReflectiveOperationException       Map lt String  String gt  env   System getenv        Field field   env getClass   getDeclaredField  m        field setAccessible true         Map lt String  String gt   field get env   put name  val

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A version in Kotlin  in this algorithm I created a decorator that allows you to set and get variables from the environment   import java util Collections import kotlin reflect KProperty   class EnvironmentDelegate       operator fun getValue thisRef  Any   property  KProperty lt   gt    String           return System getenv property name      -              operator fun setValue thisRef  Any   property  KProperty lt   gt   value  String            val key   property name           val classes  Array lt Class lt   gt  gt    Collections  class java declaredClasses         val env   System getenv             val cl   classes first    java util Collections  UnmodifiableMap     it name             val field   cl getDeclaredField  m           field isAccessible   true         val obj   field env          val map   obj as MutableMap lt String  String gt          map putAll mapOf key to value             class KnownProperties       var JAVA HOME  String by EnvironmentDelegate       var sample  String by EnvironmentDelegate       fun main         val knowProps   KnownProperties       knowProps sample    2        println  Java Home    knowProps JAVA HOME        println  Sample    knowProps sample

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You can pass parameters into your initial java process with -D   java -cp  lt classpath gt  -Dkey1 value -Dkey2 value

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public static void set Map lt String  String gt  newenv  throws Exception       Class   classes   Collections class getDeclaredClasses        Map lt String  String gt  env   System getenv        for Class cl   classes            if  java util Collections UnmodifiableMap  equals cl getName                   Field field   cl getDeclaredField  m                field setAccessible true               Object obj   field get env               Map lt String  String gt  map    Map lt String  String gt   obj              map clear                map putAll newenv                       Or to add update a single var and removing the loop as per thejoshwolfe s suggestion    SuppressWarnings    unchecked       public static void updateEnv String name  String val  throws ReflectiveOperationException       Map lt String  String gt  env   System getenv        Field field   env getClass   getDeclaredField  m        field setAccessible true         Map lt String  String gt   field get env   put name  val

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This is the Kotlin evil version of the  pushy s evil answer      Suppress  UNCHECKED CAST    Throws Exception  class  fun setEnv newenv  Map lt String  String gt         try           val processEnvironmentClass   Class forName  java lang ProcessEnvironment           val theEnvironmentField   processEnvironmentClass getDeclaredField  theEnvironment           theEnvironmentField isAccessible   true         val env   theEnvironmentField get null  as MutableMap lt String  String gt          env putAll newenv          val theCaseInsensitiveEnvironmentField   processEnvironmentClass getDeclaredField  theCaseInsensitiveEnvironment           theCaseInsensitiveEnvironmentField isAccessible   true         val cienv   theCaseInsensitiveEnvironmentField get null  as MutableMap lt String  String gt          cienv putAll newenv        catch  e  NoSuchFieldException            val classes   Collections  class java getDeclaredClasses           val env   System getenv           for  cl in classes                if   java util Collections  UnmodifiableMap     cl getName                      val field   cl getDeclaredField  m                   field setAccessible true                  val obj   field get env                  val map   obj as MutableMap lt String  String gt                  map clear                   map putAll newenv                                  It is working in macOS Mojave at least

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public static void set Map lt String  String gt  newenv  throws Exception       Class   classes   Collections class getDeclaredClasses        Map lt String  String gt  env   System getenv        for Class cl   classes            if  java util Collections UnmodifiableMap  equals cl getName                   Field field   cl getDeclaredField  m                field setAccessible true               Object obj   field get env               Map lt String  String gt  map    Map lt String  String gt   obj              map clear                map putAll newenv                       Or to add update a single var and removing the loop as per thejoshwolfe s suggestion    SuppressWarnings    unchecked       public static void updateEnv String name  String val  throws ReflectiveOperationException       Map lt String  String gt  env   System getenv        Field field   env getClass   getDeclaredField  m        field setAccessible true         Map lt String  String gt   field get env   put name  val

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A version in Kotlin  in this algorithm I created a decorator that allows you to set and get variables from the environment   import java util Collections import kotlin reflect KProperty   class EnvironmentDelegate       operator fun getValue thisRef  Any   property  KProperty lt   gt    String           return System getenv property name      -              operator fun setValue thisRef  Any   property  KProperty lt   gt   value  String            val key   property name           val classes  Array lt Class lt   gt  gt    Collections  class java declaredClasses         val env   System getenv             val cl   classes first    java util Collections  UnmodifiableMap     it name             val field   cl getDeclaredField  m           field isAccessible   true         val obj   field env          val map   obj as MutableMap lt String  String gt          map putAll mapOf key to value             class KnownProperties       var JAVA HOME  String by EnvironmentDelegate       var sample  String by EnvironmentDelegate       fun main         val knowProps   KnownProperties       knowProps sample    2        println  Java Home    knowProps JAVA HOME        println  Sample    knowProps sample

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For use in scenarios where you need to set specific environment values for unit tests  you might find the following hack useful  It will change the environment variables throughout the JVM  so make sure you reset any changes after your test   but will not alter your system environment   I found that a combination of the two dirty hacks by Edward Campbell and anonymous works best  as one of the does not work under linux  one does not work under windows 7  So to get a multiplatform evil hack I combined them   protected static void setEnv Map lt String  String gt  newenv  throws Exception     try       Class lt   gt  processEnvironmentClass   Class forName  java lang ProcessEnvironment        Field theEnvironmentField   processEnvironmentClass getDeclaredField  theEnvironment        theEnvironmentField setAccessible true       Map lt String  String gt  env    Map lt String  String gt   theEnvironmentField get null       env putAll newenv       Field theCaseInsensitiveEnvironmentField   processEnvironmentClass getDeclaredField  theCaseInsensitiveEnvironment        theCaseInsensitiveEnvironmentField setAccessible true       Map lt String  String gt  cienv    Map lt String  String gt       theCaseInsensitiveEnvironmentField get null       cienv putAll newenv       catch  NoSuchFieldException e        Class   classes   Collections class getDeclaredClasses        Map lt String  String gt  env   System getenv        for Class cl   classes          if  java util Collections UnmodifiableMap  equals cl getName               Field field   cl getDeclaredField  m            field setAccessible true           Object obj   field get env           Map lt String  String gt  map    Map lt String  String gt   obj          map clear            map putAll newenv                         This Works like a charm  Full credits to the two authors of these hacks

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Kotlin implementation I recently made based on Edward s answer   fun setEnv newEnv  Map lt String  String gt         val unmodifiableMapClass   Collections unmodifiableMap lt Any  Any gt  mapOf    javaClass     with unmodifiableMapClass getDeclaredField  m              isAccessible   true          Suppress  UNCHECKED CAST           get System getenv    as MutableMap lt String  String gt        apply           clear           putAll newEnv

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You can pass parameters into your initial java process with -D   java -cp  lt classpath gt  -Dkey1 value -Dkey2 value

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Poking around online  it looks like it might be possible to do this with JNI   You d then have to make a call to putenv   from C  and you d  presumably  have to do it in a way that worked on both Windows and UNIX   If all that can be done  it surely wouldn t be too hard for Java itself to support this instead of putting me in a straight jacket   A Perl-speaking friend elsewhere suggests that this is because environment variables are process global and Java is striving for good isolation for good design

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this is a dirty hack - but should be ok for a unittest  private void setNewEnvironmentHack Map lt String  String gt  newenv  throws Exception     Class lt   gt  processEnvironmentClass   Class forName  java lang ProcessEnvironment      Field theEnvironmentField   processEnvironmentClass getDeclaredField  theEnvironment      theEnvironmentField setAccessible true     Map lt String  String gt  env    Map lt String  String gt   theEnvironmentField get null     env clear      env putAll newenv     Field theCaseInsensitiveEnvironmentField   processEnvironmentClass getDeclaredField  theCaseInsensitiveEnvironment      theCaseInsensitiveEnvironmentField setAccessible true     Map lt String  String gt  cienv    Map lt String  String gt   theCaseInsensitiveEnvironmentField get null     cienv clear      cienv putAll newenv

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Tim Ryan s answer worked for me    but I wanted it for Groovy  Spock context for example   and simplissimo   import java lang reflect Field  def getModifiableEnvironmentMap         def unmodifiableEnv   System getenv       Class cl   unmodifiableEnv getClass       Field field   cl getDeclaredField  m       field accessible   true     field get unmodifiableEnv     def clearEnvironmentVars  def keys         def savedVals           keys each  key - gt          String val   modifiableEnvironmentMap remove key             thinking about it  I m not sure why we need this test for null            but haven t yet done any experiments         if  val    null                 savedVals put  key  val                       savedVals    def setEnvironmentVars Map varMap        modifiableEnvironmentMap putAll varMap        pretend existing Env Var doesn t exist def PATHVal1   System env PATH println  PATH val1   PATHVal1   String   keys     PATH    key2    key3   def savedVars   clearEnvironmentVars keys  def PATHVal2   System env PATH println  PATH val2   PATHVal2       return to reality setEnvironmentVars savedVars  def PATHVal3   System env PATH println  PATH val3   PATHVal3   println  System env   System env       pretend a non-existent Env Var exists setEnvironmentVars     key4     key4Val     println  key4 val   System env key4

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Tried pushy s answer above and it worked for the most part  However  in certain circumstances  I would see this exception   java lang String cannot be cast to java lang ProcessEnvironment Variable   This turns out to happen when the method was called more than once  owing to the implementation of certain inner classes of ProcessEnvironment  If the setEnv     method is called more than once  when the keys are retrieved from the theEnvironment map  they are now strings  having been put in as strings by the first invocation of setEnv        and cannot be cast to the map s generic type  Variable  which is a private inner class of ProcessEnvironment   A fixed version  in Scala   is below  Hopefully it isn t too difficult to carry over into Java   def setEnv newenv  java util Map String  String    Unit       try       val processEnvironmentClass   JavaClass forName  java lang ProcessEnvironment       val theEnvironmentField   processEnvironmentClass getDeclaredField  theEnvironment       theEnvironmentField setAccessible true       val variableClass   JavaClass forName  java lang ProcessEnvironment Variable       val convertToVariable   variableClass getMethod  valueOf   classOf java lang String       convertToVariable setAccessible true       val valueClass   JavaClass forName  java lang ProcessEnvironment Value       val convertToValue   valueClass getMethod  valueOf   classOf java lang String       convertToValue setAccessible true       val sampleVariable   convertToVariable invoke null          val sampleValue   convertToValue invoke null          val env   theEnvironmentField get null  asInstanceOf java util Map sampleVariable type  sampleValue type       newenv foreach   case  k  v    gt            val variable   convertToVariable invoke null  k  asInstanceOf sampleVariable type          val value   convertToValue invoke null  v  asInstanceOf sampleValue type          env put variable  value                     val theCaseInsensitiveEnvironmentField   processEnvironmentClass getDeclaredField  theCaseInsensitiveEnvironment       theCaseInsensitiveEnvironmentField setAccessible true      val cienv   theCaseInsensitiveEnvironmentField get null  asInstanceOf java util Map String  String       cienv putAll newenv         catch       case e   NoSuchFieldException   gt          try           val classes   classOf java util Collections  getDeclaredClasses         val env   System getenv           classes foreach  cl   gt              if  java util Collections UnmodifiableMap     cl getName                val field   cl getDeclaredField  m               field setAccessible true              val map   field get env  asInstanceOf java util Map String  String                  map clear      Not sure why this was in the code  It means we need to set all required environment variables              map putAll newenv                                 catch           case e2  Exception   gt  e2 printStackTrace                     case e1  Exception   gt  e1 printStackTrace

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It turns out that the solution from  pushy  anonymous  Edward Campbell does not work on Android because Android is not really Java   Specifically  Android does not have java lang ProcessEnvironment at all   But it turns out to be easier in Android  you just need to do a JNI call to POSIX setenv     In C JNI    JNIEXPORT jint JNICALL Java com example posixtest Posix setenv    JNIEnv  env  jclass clazz  jstring key  jstring value  jboolean overwrite        char  k    char      env - gt GetStringUTFChars env  key  NULL       char  v    char      env - gt GetStringUTFChars env  value  NULL       int err   setenv k  v  overwrite         env - gt ReleaseStringUTFChars env  key  k         env - gt ReleaseStringUTFChars env  value  v       return err      And in Java   public class Posix        public static native int setenv String key  String value  boolean overwrite        private void runTest             Posix setenv  LD LIBRARY PATH    foo   true

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Like most people who have found this thread  I was writing some unit tests and needed to modify the environment variables to set the correct conditions for the test to run   However  I found the most upvoted answers had some issues and or were very cryptic or overly complicated   Hopefully this will help others to sort out the solution more quickly   First off  I finally found  Hubert Grzeskowiak s solution to be the simplest and it worked for me   I wish I would have come to that one first   It s based on  Edward Campbell s answer  but without the complicating for loop search   However  I started with  pushy s solution  which got the most upvotes  It is a combo of  anonymous and  Edward Campbell s    pushy claims both approaches are needed to cover both Linux and Windows environments   I m running under OS X and find that both work  once an issue with  anonymous approach is fixed    As others have noted  this solution works most of the time  but not all   I think the source of most of the confusion comes from  anonymous s solution operating on the  theEnvironment  field   Looking at the definition of the ProcessEnvironment structure   theEnvironment  is not a Map lt  String  String   but rather it is a Map lt  Variable  Value     Clearing the map works fine  but the putAll operation rebuilds the map a Map lt  String  String    which potentially causes problems when subsequent operations operate on the data structure using the normal API that expects Map lt  Variable  Value     Also  accessing removing individual elements is a problem   The solution is to access  theEnvironment  indirectly through  theUnmodifiableEnvironment    But since this is a type UnmodifiableMap the access must be done through the private variable  m  of the UnmodifiableMap type   See getModifiableEnvironmentMap2 in code below   In my case I needed to remove some of the environment variables for my test  the others should be unchanged    Then I wanted to restore the environment variables to their prior state after the test   The routines below make that straight forward to do   I tested both versions of getModifiableEnvironmentMap on OS X  and both work equivalently   Though based on comments in this thread  one may be a better choice than the other depending on the environment   Note  I did not include access to the  theCaseInsensitiveEnvironmentField  since that seems to be Windows specific and I had no way to test it  but adding it should be straight forward   private Map lt String  String gt  getModifiableEnvironmentMap         try           Map lt String String gt  unmodifiableEnv   System getenv            Class lt   gt  cl   unmodifiableEnv getClass            Field field   cl getDeclaredField  m            field setAccessible true           Map lt String String gt  modifiableEnv    Map lt String String gt   field get unmodifiableEnv           return modifiableEnv        catch Exception e            throw new RuntimeException  Unable to access writable environment variable map              private Map lt String  String gt  getModifiableEnvironmentMap2         try           Class lt   gt  processEnvironmentClass   Class forName  java lang ProcessEnvironment            Field theUnmodifiableEnvironmentField   processEnvironmentClass getDeclaredField  theUnmodifiableEnvironment            theUnmodifiableEnvironmentField setAccessible true           Map lt String String gt  theUnmodifiableEnvironment    Map lt String String gt  theUnmodifiableEnvironmentField get null            Class lt   gt  theUnmodifiableEnvironmentClass   theUnmodifiableEnvironment getClass            Field theModifiableEnvField   theUnmodifiableEnvironmentClass getDeclaredField  m            theModifiableEnvField setAccessible true           Map lt String String gt  modifiableEnv    Map lt String String gt   theModifiableEnvField get theUnmodifiableEnvironment           return modifiableEnv        catch Exception e            throw new RuntimeException  Unable to access writable environment variable map              private Map lt String  String gt  clearEnvironmentVars String   keys         Map lt String String gt  modifiableEnv   getModifiableEnvironmentMap         HashMap lt String  String gt  savedVals   new HashMap lt String  String gt          for String k   keys            String val   modifiableEnv remove k           if  val    null    savedVals put k  val               return savedVals     private void setEnvironmentVars Map lt String  String gt  varMap        getModifiableEnvironmentMap   putAll varMap          Test public void myTest         String   keys      key1    key2    key3         Map lt String  String gt  savedVars   clearEnvironmentVars keys           do test      setEnvironmentVars savedVars

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This is a combination of  paul-blair  s answer converted to Java which includes some cleanups pointed out by paul blair and some mistakes that seem to have been inside  pushy  s code which is made up of  Edward Campbell and anonymous   I cannot emphasize how much this code should ONLY be used in testing and is extremely hacky  But for cases where you need the environment setup in tests it is exactly what I needed   This also includes some minor touches of mine that allow the code to work on both Windows running on   java version  1 8 0 92  Java TM  SE Runtime Environment  build 1 8 0 92-b14  Java HotSpot TM  64-Bit Server VM  build 25 92-b14  mixed mode    as well as Centos running on  openjdk version  1 8 0 91  OpenJDK Runtime Environment  build 1 8 0 91-b14  OpenJDK 64-Bit Server VM  build 25 91-b14  mixed mode    The implementation          Sets an environment variable FOR THE CURRENT RUN OF THE JVM    Does not actually modify the system s environment variables      but rather only the copy of the variables that java has taken      and hence should only be used for testing purposes      param key The Name of the variable to set     param value The value of the variable to set      SuppressWarnings  unchecked   public static  lt K V gt  void setenv final String key  final String value        try               we obtain the actual environment         final Class lt   gt  processEnvironmentClass   Class forName  java lang ProcessEnvironment            final Field theEnvironmentField   processEnvironmentClass getDeclaredField  theEnvironment            final boolean environmentAccessibility   theEnvironmentField isAccessible            theEnvironmentField setAccessible true            final Map lt K V gt  env    Map lt K  V gt   theEnvironmentField get null            if  SystemUtils IS OS WINDOWS                   This is all that is needed on windows running java jdk 1 8 0 92             if  value    null                    env remove key                 else                   env put  K  key   V  value                           else                  This is triggered to work on openjdk 1 8 0 91                The ProcessEnvironment Variable is the key of the map             final Class lt K gt  variableClass    Class lt K gt   Class forName  java lang ProcessEnvironment Variable                final Method convertToVariable   variableClass getMethod  valueOf   String class               final boolean conversionVariableAccessibility   convertToVariable isAccessible                convertToVariable setAccessible true                   The ProcessEnvironment Value is the value fo the map             final Class lt V gt  valueClass    Class lt V gt   Class forName  java lang ProcessEnvironment Value                final Method convertToValue   valueClass getMethod  valueOf   String class               final boolean conversionValueAccessibility   convertToValue isAccessible                convertToValue setAccessible true                if  value    null                    env remove convertToVariable invoke null  key                  else                      we place the new value inside the map after conversion so as to                    avoid class cast exceptions when rerunning this code                 env put  K  convertToVariable invoke null  key    V  convertToValue invoke null  value                        reset accessibility to what they were                 convertToValue setAccessible conversionValueAccessibility                   convertToVariable setAccessible conversionVariableAccessibility                                      reset environment accessibility         theEnvironmentField setAccessible environmentAccessibility               we apply the same to the case insensitive environment         final Field theCaseInsensitiveEnvironmentField   processEnvironmentClass getDeclaredField  theCaseInsensitiveEnvironment            final boolean insensitiveAccessibility   theCaseInsensitiveEnvironmentField isAccessible            theCaseInsensitiveEnvironmentField setAccessible true              Not entirely sure if this needs to be casted to ProcessEnvironment Variable and  Value as well         final Map lt String  String gt  cienv    Map lt String  String gt   theCaseInsensitiveEnvironmentField get null           if  value    null                   remove if null             cienv remove key             else               cienv put key  value                     theCaseInsensitiveEnvironmentField setAccessible insensitiveAccessibility         catch  final ClassNotFoundException   NoSuchMethodException   IllegalAccessException   InvocationTargetException e            throw new IllegalStateException  Failed setting environment variable  lt   key   gt  to  lt   value   gt    e         catch  final NoSuchFieldException e               we could not find theEnvironment         final Map lt String  String gt  env   System getenv            Stream of Collections class getDeclaredClasses                       obtain the declared classes of type  UnmodifiableMap                  filter c1 - gt   java util Collections UnmodifiableMap  equals c1 getName                      map c1 - gt                        try                           return c1 getDeclaredField  m                          catch  final NoSuchFieldException e1                            throw new IllegalStateException  Failed setting environment variable  lt   key   gt  to  lt   value   gt  when locating in-class memory map of environment   e1                                                             forEach field - gt                        try                           final boolean fieldAccessibility   field isAccessible                            field setAccessible true                              we obtain the environment                         final Map lt String  String gt  map    Map lt String  String gt   field get env                           if  value    null                                   remove if null                             map remove key                             else                               map put key  value                                                        reset accessibility                         field setAccessible fieldAccessibility                         catch  final ConcurrentModificationException e1                               This may happen if we keep backups of the environment before calling this method                            as the map that we kept as a backup may be picked up inside this block                             So we simply skip this attempt and continue adjusting the other maps                            To avoid this one should always keep individual keys value backups not the entire map                         LOGGER info  Attempted to modify source map    field getDeclaringClass       field getName    e1                         catch  final IllegalAccessException e1                            throw new IllegalStateException  Failed setting environment variable  lt   key   gt  to  lt   value   gt   Unable to access field    e1                                                       LOGGER info  Set environment variable  lt   key   gt  to  lt   value   gt   Sanity Check    System getenv key

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Linux MacOS only Setting single environment variables  based on answer by Edward Campbell   public static void setEnv String key  String value        try           Map lt String  String gt  env   System getenv            Class lt   gt  cl   env getClass            Field field   cl getDeclaredField  quot m quot            field setAccessible true           Map lt String  String gt  writableEnv    Map lt String  String gt   field get env           writableEnv put key  value         catch  Exception e            throw new IllegalStateException  quot Failed to set environment variable quot   e            Usage  First  put the method in any class you want  e g  SystemUtil  Then call it statically  SystemUtil setEnv  quot SHELL quot    quot  bin bash quot     If you call System getenv  quot SHELL quot   after this  you ll get  quot  bin bash quot  back

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You can pass parameters into your initial java process with -D   java -cp  lt classpath gt  -Dkey1 value -Dkey2 value

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jython variant based on  pushy s answer  works on windows   def set env newenv       from java lang import Class     process environment   Class forName  java lang ProcessEnvironment       environment field    process environment getDeclaredField  theEnvironment       environment field setAccessible True      env   environment field get None      env putAll newenv      invariant environment field   process environment getDeclaredField  theCaseInsensitiveEnvironment        invariant environment field setAccessible True      invevn   invariant environment field get None      invevn putAll newenv    Usage   old environ   dict os environ  old environ  EPM ORACLE HOME     r E  Oracle Middleware EPMSystem11R1  set env old environ

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If you work with SpringBoot you can add specifying the environmental variable in the following property   was app config properties toSystemProperties

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jython variant based on  pushy s answer  works on windows   def set env newenv       from java lang import Class     process environment   Class forName  java lang ProcessEnvironment       environment field    process environment getDeclaredField  theEnvironment       environment field setAccessible True      env   environment field get None      env putAll newenv      invariant environment field   process environment getDeclaredField  theCaseInsensitiveEnvironment        invariant environment field setAccessible True      invevn   invariant environment field get None      invevn putAll newenv    Usage   old environ   dict os environ  old environ  EPM ORACLE HOME     r E  Oracle Middleware EPMSystem11R1  set env old environ

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It turns out that the solution from  pushy  anonymous  Edward Campbell does not work on Android because Android is not really Java   Specifically  Android does not have java lang ProcessEnvironment at all   But it turns out to be easier in Android  you just need to do a JNI call to POSIX setenv     In C JNI    JNIEXPORT jint JNICALL Java com example posixtest Posix setenv    JNIEnv  env  jclass clazz  jstring key  jstring value  jboolean overwrite        char  k    char      env - gt GetStringUTFChars env  key  NULL       char  v    char      env - gt GetStringUTFChars env  value  NULL       int err   setenv k  v  overwrite         env - gt ReleaseStringUTFChars env  key  k         env - gt ReleaseStringUTFChars env  value  v       return err      And in Java   public class Posix        public static native int setenv String key  String value  boolean overwrite        private void runTest             Posix setenv  LD LIBRARY PATH    foo   true

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This is a combination of  paul-blair  s answer converted to Java which includes some cleanups pointed out by paul blair and some mistakes that seem to have been inside  pushy  s code which is made up of  Edward Campbell and anonymous   I cannot emphasize how much this code should ONLY be used in testing and is extremely hacky  But for cases where you need the environment setup in tests it is exactly what I needed   This also includes some minor touches of mine that allow the code to work on both Windows running on   java version  1 8 0 92  Java TM  SE Runtime Environment  build 1 8 0 92-b14  Java HotSpot TM  64-Bit Server VM  build 25 92-b14  mixed mode    as well as Centos running on  openjdk version  1 8 0 91  OpenJDK Runtime Environment  build 1 8 0 91-b14  OpenJDK 64-Bit Server VM  build 25 91-b14  mixed mode    The implementation          Sets an environment variable FOR THE CURRENT RUN OF THE JVM    Does not actually modify the system s environment variables      but rather only the copy of the variables that java has taken      and hence should only be used for testing purposes      param key The Name of the variable to set     param value The value of the variable to set      SuppressWarnings  unchecked   public static  lt K V gt  void setenv final String key  final String value        try               we obtain the actual environment         final Class lt   gt  processEnvironmentClass   Class forName  java lang ProcessEnvironment            final Field theEnvironmentField   processEnvironmentClass getDeclaredField  theEnvironment            final boolean environmentAccessibility   theEnvironmentField isAccessible            theEnvironmentField setAccessible true            final Map lt K V gt  env    Map lt K  V gt   theEnvironmentField get null            if  SystemUtils IS OS WINDOWS                   This is all that is needed on windows running java jdk 1 8 0 92             if  value    null                    env remove key                 else                   env put  K  key   V  value                           else                  This is triggered to work on openjdk 1 8 0 91                The ProcessEnvironment Variable is the key of the map             final Class lt K gt  variableClass    Class lt K gt   Class forName  java lang ProcessEnvironment Variable                final Method convertToVariable   variableClass getMethod  valueOf   String class               final boolean conversionVariableAccessibility   convertToVariable isAccessible                convertToVariable setAccessible true                   The ProcessEnvironment Value is the value fo the map             final Class lt V gt  valueClass    Class lt V gt   Class forName  java lang ProcessEnvironment Value                final Method convertToValue   valueClass getMethod  valueOf   String class               final boolean conversionValueAccessibility   convertToValue isAccessible                convertToValue setAccessible true                if  value    null                    env remove convertToVariable invoke null  key                  else                      we place the new value inside the map after conversion so as to                    avoid class cast exceptions when rerunning this code                 env put  K  convertToVariable invoke null  key    V  convertToValue invoke null  value                        reset accessibility to what they were                 convertToValue setAccessible conversionValueAccessibility                   convertToVariable setAccessible conversionVariableAccessibility                                      reset environment accessibility         theEnvironmentField setAccessible environmentAccessibility               we apply the same to the case insensitive environment         final Field theCaseInsensitiveEnvironmentField   processEnvironmentClass getDeclaredField  theCaseInsensitiveEnvironment            final boolean insensitiveAccessibility   theCaseInsensitiveEnvironmentField isAccessible            theCaseInsensitiveEnvironmentField setAccessible true              Not entirely sure if this needs to be casted to ProcessEnvironment Variable and  Value as well         final Map lt String  String gt  cienv    Map lt String  String gt   theCaseInsensitiveEnvironmentField get null           if  value    null                   remove if null             cienv remove key             else               cienv put key  value                     theCaseInsensitiveEnvironmentField setAccessible insensitiveAccessibility         catch  final ClassNotFoundException   NoSuchMethodException   IllegalAccessException   InvocationTargetException e            throw new IllegalStateException  Failed setting environment variable  lt   key   gt  to  lt   value   gt    e         catch  final NoSuchFieldException e               we could not find theEnvironment         final Map lt String  String gt  env   System getenv            Stream of Collections class getDeclaredClasses                       obtain the declared classes of type  UnmodifiableMap                  filter c1 - gt   java util Collections UnmodifiableMap  equals c1 getName                      map c1 - gt                        try                           return c1 getDeclaredField  m                          catch  final NoSuchFieldException e1                            throw new IllegalStateException  Failed setting environment variable  lt   key   gt  to  lt   value   gt  when locating in-class memory map of environment   e1                                                             forEach field - gt                        try                           final boolean fieldAccessibility   field isAccessible                            field setAccessible true                              we obtain the environment                         final Map lt String  String gt  map    Map lt String  String gt   field get env                           if  value    null                                   remove if null                             map remove key                             else                               map put key  value                                                        reset accessibility                         field setAccessible fieldAccessibility                         catch  final ConcurrentModificationException e1                               This may happen if we keep backups of the environment before calling this method                            as the map that we kept as a backup may be picked up inside this block                             So we simply skip this attempt and continue adjusting the other maps                            To avoid this one should always keep individual keys value backups not the entire map                         LOGGER info  Attempted to modify source map    field getDeclaringClass       field getName    e1                         catch  final IllegalAccessException e1                            throw new IllegalStateException  Failed setting environment variable  lt   key   gt  to  lt   value   gt   Unable to access field    e1                                                       LOGGER info  Set environment variable  lt   key   gt  to  lt   value   gt   Sanity Check    System getenv key

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public static void set Map lt String  String gt  newenv  throws Exception       Class   classes   Collections class getDeclaredClasses        Map lt String  String gt  env   System getenv        for Class cl   classes            if  java util Collections UnmodifiableMap  equals cl getName                   Field field   cl getDeclaredField  m                field setAccessible true               Object obj   field get env               Map lt String  String gt  map    Map lt String  String gt   obj              map clear                map putAll newenv                       Or to add update a single var and removing the loop as per thejoshwolfe s suggestion    SuppressWarnings    unchecked       public static void updateEnv String name  String val  throws ReflectiveOperationException       Map lt String  String gt  env   System getenv        Field field   env getClass   getDeclaredField  m        field setAccessible true         Map lt String  String gt   field get env   put name  val

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on Android the interface is exposed via Libcore os as a kind of hidden API   Libcore os setenv  VAR    value   bOverwrite   Libcore os getenv  VAR       The Libcore class as well as the interface OS is public  Just the class declaration is missing and need to be shown to the linker  No need to add the classes to the application  but it also does not hurt if it is included   package libcore io   public final class Libcore       private Libcore            public static Os os     package libcore io   public interface Os       public String getenv String name       public void setenv String name  String value  boolean overwrite  throws ErrnoException

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Poking around online  it looks like it might be possible to do this with JNI   You d then have to make a call to putenv   from C  and you d  presumably  have to do it in a way that worked on both Windows and UNIX   If all that can be done  it surely wouldn t be too hard for Java itself to support this instead of putting me in a straight jacket   A Perl-speaking friend elsewhere suggests that this is because environment variables are process global and Java is striving for good isolation for good design

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Poking around online  it looks like it might be possible to do this with JNI   You d then have to make a call to putenv   from C  and you d  presumably  have to do it in a way that worked on both Windows and UNIX   If all that can be done  it surely wouldn t be too hard for Java itself to support this instead of putting me in a straight jacket   A Perl-speaking friend elsewhere suggests that this is because environment variables are process global and Java is striving for good isolation for good design

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Tried pushy s answer above and it worked for the most part  However  in certain circumstances  I would see this exception   java lang String cannot be cast to java lang ProcessEnvironment Variable   This turns out to happen when the method was called more than once  owing to the implementation of certain inner classes of ProcessEnvironment  If the setEnv     method is called more than once  when the keys are retrieved from the theEnvironment map  they are now strings  having been put in as strings by the first invocation of setEnv        and cannot be cast to the map s generic type  Variable  which is a private inner class of ProcessEnvironment   A fixed version  in Scala   is below  Hopefully it isn t too difficult to carry over into Java   def setEnv newenv  java util Map String  String    Unit       try       val processEnvironmentClass   JavaClass forName  java lang ProcessEnvironment       val theEnvironmentField   processEnvironmentClass getDeclaredField  theEnvironment       theEnvironmentField setAccessible true       val variableClass   JavaClass forName  java lang ProcessEnvironment Variable       val convertToVariable   variableClass getMethod  valueOf   classOf java lang String       convertToVariable setAccessible true       val valueClass   JavaClass forName  java lang ProcessEnvironment Value       val convertToValue   valueClass getMethod  valueOf   classOf java lang String       convertToValue setAccessible true       val sampleVariable   convertToVariable invoke null          val sampleValue   convertToValue invoke null          val env   theEnvironmentField get null  asInstanceOf java util Map sampleVariable type  sampleValue type       newenv foreach   case  k  v    gt            val variable   convertToVariable invoke null  k  asInstanceOf sampleVariable type          val value   convertToValue invoke null  v  asInstanceOf sampleValue type          env put variable  value                     val theCaseInsensitiveEnvironmentField   processEnvironmentClass getDeclaredField  theCaseInsensitiveEnvironment       theCaseInsensitiveEnvironmentField setAccessible true      val cienv   theCaseInsensitiveEnvironmentField get null  asInstanceOf java util Map String  String       cienv putAll newenv         catch       case e   NoSuchFieldException   gt          try           val classes   classOf java util Collections  getDeclaredClasses         val env   System getenv           classes foreach  cl   gt              if  java util Collections UnmodifiableMap     cl getName                val field   cl getDeclaredField  m               field setAccessible true              val map   field get env  asInstanceOf java util Map String  String                  map clear      Not sure why this was in the code  It means we need to set all required environment variables              map putAll newenv                                 catch           case e2  Exception   gt  e2 printStackTrace                     case e1  Exception   gt  e1 printStackTrace

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Poking around online  it looks like it might be possible to do this with JNI   You d then have to make a call to putenv   from C  and you d  presumably  have to do it in a way that worked on both Windows and UNIX   If all that can be done  it surely wouldn t be too hard for Java itself to support this instead of putting me in a straight jacket   A Perl-speaking friend elsewhere suggests that this is because environment variables are process global and Java is striving for good isolation for good design

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This is the Kotlin evil version of the  pushy s evil answer      Suppress  UNCHECKED CAST    Throws Exception  class  fun setEnv newenv  Map lt String  String gt         try           val processEnvironmentClass   Class forName  java lang ProcessEnvironment           val theEnvironmentField   processEnvironmentClass getDeclaredField  theEnvironment           theEnvironmentField isAccessible   true         val env   theEnvironmentField get null  as MutableMap lt String  String gt          env putAll newenv          val theCaseInsensitiveEnvironmentField   processEnvironmentClass getDeclaredField  theCaseInsensitiveEnvironment           theCaseInsensitiveEnvironmentField isAccessible   true         val cienv   theCaseInsensitiveEnvironmentField get null  as MutableMap lt String  String gt          cienv putAll newenv        catch  e  NoSuchFieldException            val classes   Collections  class java getDeclaredClasses           val env   System getenv           for  cl in classes                if   java util Collections  UnmodifiableMap     cl getName                      val field   cl getDeclaredField  m                   field setAccessible true                  val obj   field get env                  val map   obj as MutableMap lt String  String gt                  map clear                   map putAll newenv                                  It is working in macOS Mojave at least

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If you work with SpringBoot you can add specifying the environmental variable in the following property   was app config properties toSystemProperties

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this is a dirty hack - but should be ok for a unittest  private void setNewEnvironmentHack Map lt String  String gt  newenv  throws Exception     Class lt   gt  processEnvironmentClass   Class forName  java lang ProcessEnvironment      Field theEnvironmentField   processEnvironmentClass getDeclaredField  theEnvironment      theEnvironmentField setAccessible true     Map lt String  String gt  env    Map lt String  String gt   theEnvironmentField get null     env clear      env putAll newenv     Field theCaseInsensitiveEnvironmentField   processEnvironmentClass getDeclaredField  theCaseInsensitiveEnvironment      theCaseInsensitiveEnvironmentField setAccessible true     Map lt String  String gt  cienv    Map lt String  String gt   theCaseInsensitiveEnvironmentField get null     cienv clear      cienv putAll newenv

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Like most people who have found this thread  I was writing some unit tests and needed to modify the environment variables to set the correct conditions for the test to run   However  I found the most upvoted answers had some issues and or were very cryptic or overly complicated   Hopefully this will help others to sort out the solution more quickly   First off  I finally found  Hubert Grzeskowiak s solution to be the simplest and it worked for me   I wish I would have come to that one first   It s based on  Edward Campbell s answer  but without the complicating for loop search   However  I started with  pushy s solution  which got the most upvotes  It is a combo of  anonymous and  Edward Campbell s    pushy claims both approaches are needed to cover both Linux and Windows environments   I m running under OS X and find that both work  once an issue with  anonymous approach is fixed    As others have noted  this solution works most of the time  but not all   I think the source of most of the confusion comes from  anonymous s solution operating on the  theEnvironment  field   Looking at the definition of the ProcessEnvironment structure   theEnvironment  is not a Map lt  String  String   but rather it is a Map lt  Variable  Value     Clearing the map works fine  but the putAll operation rebuilds the map a Map lt  String  String    which potentially causes problems when subsequent operations operate on the data structure using the normal API that expects Map lt  Variable  Value     Also  accessing removing individual elements is a problem   The solution is to access  theEnvironment  indirectly through  theUnmodifiableEnvironment    But since this is a type UnmodifiableMap the access must be done through the private variable  m  of the UnmodifiableMap type   See getModifiableEnvironmentMap2 in code below   In my case I needed to remove some of the environment variables for my test  the others should be unchanged    Then I wanted to restore the environment variables to their prior state after the test   The routines below make that straight forward to do   I tested both versions of getModifiableEnvironmentMap on OS X  and both work equivalently   Though based on comments in this thread  one may be a better choice than the other depending on the environment   Note  I did not include access to the  theCaseInsensitiveEnvironmentField  since that seems to be Windows specific and I had no way to test it  but adding it should be straight forward   private Map lt String  String gt  getModifiableEnvironmentMap         try           Map lt String String gt  unmodifiableEnv   System getenv            Class lt   gt  cl   unmodifiableEnv getClass            Field field   cl getDeclaredField  m            field setAccessible true           Map lt String String gt  modifiableEnv    Map lt String String gt   field get unmodifiableEnv           return modifiableEnv        catch Exception e            throw new RuntimeException  Unable to access writable environment variable map              private Map lt String  String gt  getModifiableEnvironmentMap2         try           Class lt   gt  processEnvironmentClass   Class forName  java lang ProcessEnvironment            Field theUnmodifiableEnvironmentField   processEnvironmentClass getDeclaredField  theUnmodifiableEnvironment            theUnmodifiableEnvironmentField setAccessible true           Map lt String String gt  theUnmodifiableEnvironment    Map lt String String gt  theUnmodifiableEnvironmentField get null            Class lt   gt  theUnmodifiableEnvironmentClass   theUnmodifiableEnvironment getClass            Field theModifiableEnvField   theUnmodifiableEnvironmentClass getDeclaredField  m            theModifiableEnvField setAccessible true           Map lt String String gt  modifiableEnv    Map lt String String gt   theModifiableEnvField get theUnmodifiableEnvironment           return modifiableEnv        catch Exception e            throw new RuntimeException  Unable to access writable environment variable map              private Map lt String  String gt  clearEnvironmentVars String   keys         Map lt String String gt  modifiableEnv   getModifiableEnvironmentMap         HashMap lt String  String gt  savedVals   new HashMap lt String  String gt          for String k   keys            String val   modifiableEnv remove k           if  val    null    savedVals put k  val               return savedVals     private void setEnvironmentVars Map lt String  String gt  varMap        getModifiableEnvironmentMap   putAll varMap          Test public void myTest         String   keys      key1    key2    key3         Map lt String  String gt  savedVars   clearEnvironmentVars keys           do test      setEnvironmentVars savedVars

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Tim Ryan s answer worked for me    but I wanted it for Groovy  Spock context for example   and simplissimo   import java lang reflect Field  def getModifiableEnvironmentMap         def unmodifiableEnv   System getenv       Class cl   unmodifiableEnv getClass       Field field   cl getDeclaredField  m       field accessible   true     field get unmodifiableEnv     def clearEnvironmentVars  def keys         def savedVals           keys each  key - gt          String val   modifiableEnvironmentMap remove key             thinking about it  I m not sure why we need this test for null            but haven t yet done any experiments         if  val    null                 savedVals put  key  val                       savedVals    def setEnvironmentVars Map varMap        modifiableEnvironmentMap putAll varMap        pretend existing Env Var doesn t exist def PATHVal1   System env PATH println  PATH val1   PATHVal1   String   keys     PATH    key2    key3   def savedVars   clearEnvironmentVars keys  def PATHVal2   System env PATH println  PATH val2   PATHVal2       return to reality setEnvironmentVars savedVars  def PATHVal3   System env PATH println  PATH val3   PATHVal3   println  System env   System env       pretend a non-existent Env Var exists setEnvironmentVars     key4     key4Val     println  key4 val   System env key4

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I stumbled upon this thread as I had a similar requirement where I needed to set  or update  an Environment Variable  permanently   So I looked in to - How to set an environment variable permanently from Command Prompt and it was very simple  setx JAVA LOC C  Java JDK  Then I just implemented the same in my code Here s what I used  suppose - JAVA LOC is the env  variable name          String cmdCommand    quot setx JAVA LOC  quot     quot C  Java JDK quot               ProcessBuilder processBuilder   new ProcessBuilder                processBuilder command  quot cmd exe quot    quot  c quot   cmdCommand               processBuilder start     ProcessBuilder fires-up a cmd exe and passes the command that you want  The environment variable is retained even if you kill JVM reboot the system as it has no relation with the JVM Program s lifecycle

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I stumbled upon this thread as I had a similar requirement where I needed to set  or update  an Environment Variable  permanently   So I looked in to - How to set an environment variable permanently from Command Prompt and it was very simple  setx JAVA LOC C  Java JDK  Then I just implemented the same in my code Here s what I used  suppose - JAVA LOC is the env  variable name          String cmdCommand    quot setx JAVA LOC  quot     quot C  Java JDK quot               ProcessBuilder processBuilder   new ProcessBuilder                processBuilder command  quot cmd exe quot    quot  c quot   cmdCommand               processBuilder start     ProcessBuilder fires-up a cmd exe and passes the command that you want  The environment variable is retained even if you kill JVM reboot the system as it has no relation with the JVM Program s lifecycle

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Kotlin implementation I recently made based on Edward s answer   fun setEnv newEnv  Map lt String  String gt         val unmodifiableMapClass   Collections unmodifiableMap lt Any  Any gt  mapOf    javaClass     with unmodifiableMapClass getDeclaredField  m              isAccessible   true          Suppress  UNCHECKED CAST           get System getenv    as MutableMap lt String  String gt        apply           clear           putAll newEnv

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on Android the interface is exposed via Libcore os as a kind of hidden API   Libcore os setenv  VAR    value   bOverwrite   Libcore os getenv  VAR       The Libcore class as well as the interface OS is public  Just the class declaration is missing and need to be shown to the linker  No need to add the classes to the application  but it also does not hurt if it is included   package libcore io   public final class Libcore       private Libcore            public static Os os     package libcore io   public interface Os       public String getenv String name       public void setenv String name  String value  boolean overwrite  throws ErrnoException

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You can pass parameters into your initial java process with -D   java -cp  lt classpath gt  -Dkey1 value -Dkey2 value

[java] How do I set environment variables from Java?

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