How to read the value of a private field from a different class in Java

Question

I have a poorly designed class in a 3rd-party JAR and I need to access one of its private fields  For example  why should I need to choose private field is it necessary   class IWasDesignedPoorly       private Hashtable stuffIWant     IWasDesignedPoorly obj          How can I use reflection to get the value of stuffIWant

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You can use jOOR for that  class Foo       private final String value    quot ABC quot     class Bar       private final Foo foo   new Foo        public String value             return org joor Reflect              on this foo               field  quot value quot                get            class BarTest        Test     void accessPrivateField             Assertions assertEquals new Bar   value     quot ABC quot

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If using Spring  In a testing context  ReflectionTestUtils provides some handy tools that can help out here with minimal effort  It s described as being  quot for use in unit and integration testing scenarios quot   In a non-testing context  there is also a similar class named ReflectionUtils but this is described as  quot Only intended for internal use quot  - see this answer for a good interpretation of what this means  To address the example in the original post  Hashtable iWantThis    Hashtable ReflectionTestUtils getField obj   quot stuffIWant quot

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You need to do the following   private static Field getField Class lt   gt  cls  String fieldName        for  Class lt   gt  c   cls  c    null  c   c getSuperclass              try               final Field field   c getDeclaredField fieldName               field setAccessible true               return field            catch  final NoSuchFieldException e                   Try parent           catch  Exception e                throw new IllegalArgumentException                       Cannot access field     cls getName           fieldName  e                       throw new IllegalArgumentException               Cannot find field     cls getName           fieldName

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In order to access private fields  you need to get them from the class s declared fields and then make them accessible   Field f   obj getClass   getDeclaredField  stuffIWant      NoSuchFieldException f setAccessible true   Hashtable iWantThis    Hashtable  f get obj     IllegalAccessException   EDIT  as has been commented by aperkins  both accessing the field  setting it as accessible and retrieving the value can throw Exceptions  although the only checked exceptions you need to be mindful of are commented above   The NoSuchFieldException would be thrown if you asked for a field by a name which did not correspond to a declared field    obj getClass   getDeclaredField  misspelled      will throw NoSuchFieldException   The IllegalAccessException would be thrown if the field was not accessible  for example  if it is private and has not been made accessible via missing out the f setAccessible true  line   The RuntimeExceptions which may be thrown are either SecurityExceptions  if the JVM s SecurityManager will not allow you to change a field s accessibility   or IllegalArgumentExceptions  if you try and access the field on an object not of the field s class s type   f get  BOB      will throw IllegalArgumentException  as String is of the wrong type

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Reflection isn t the only way to resolve your issue  which is to access the private functionality behaviour of a class component   An alternative solution is to extract the class from the  jar  decompile it using  say  Jode or Jad  change the field  or add an accessor   and recompile it against the original  jar  Then put the new  class ahead of the  jar in the classpath  or reinsert it in the  jar   the jar utility allows you to extract and reinsert to an existing  jar   As noted below  this resolves the wider issue of accessing changing private state rather than simply accessing changing a field   This requires the  jar not to be signed  of course

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It is quite easy with the tool XrayInterface  Just define the missing getters setters  e g   interface BetterDesigned     Hashtable getStuffIWant      is mapped by convention to stuffIWant     and xray your poor designed project   IWasDesignedPoorly obj   new IWasDesignedPoorly    BetterDesigned better        System out println better getStuffIWant       Internally this relies on reflection

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Using the Reflection in Java you can access all the private public fields and methods of one class to another  But as per the Oracle documentation  in the section drawbacks they recommended that      Since reflection allows code to perform operations that would be illegal in non-reflective code  such as accessing private fields and methods  the use of reflection can result in unexpected side-effects  which may render code dysfunctional and may destroy portability  Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform   here is following code snapts to demonstrate basic concepts of Reflection   Reflection1 java  public class Reflection1       private int i   10       public void methoda                  System out println  method1              public void methodb                  System out println  method2              public void methodc                  System out println  method3               Reflection2 java  import java lang reflect Field  import java lang reflect InvocationTargetException  import java lang reflect Method    public class Reflection2       public static void main String ar    throws IllegalAccessException  IllegalArgumentException  InvocationTargetException               Method   mthd   Reflection1 class getMethods       for axis the methods           Field   fld   Reflection1 class getDeclaredFields        for axis the fields               Loop for get all the methods in class         for Method mthd1 mthd                         System out println  method    mthd1 getName                 System out println  parametes    mthd1 getReturnType                           Loop for get all the Field in class         for Field fld1 fld                        fld1 setAccessible true               System out println  field    fld1 getName                 System out println  type    fld1 getType                 System out println  value    fld1 getInt new Reflaction1                           Hope it will help

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Just an additional note about reflection  I have observed in some special cases  when several classes with the same name exist in different packages  that reflection as used in the top answer may fail to pick the correct class from the object  So if you know what is the package class of the object  then it s better to access its private field values as follows   org deeplearning4j nn layers BaseOutputLayer ll    org deeplearning4j nn layers BaseOutputLayer  model getLayer 0   Field f   Class forName  org deeplearning4j nn layers BaseOutputLayer   getDeclaredField  solver    f setAccessible true   Solver s    Solver  f get ll      This is the example class that was not working for me

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You can use Manifold s  JailBreak for direct  type-safe Java reflection    JailBreak Foo foo   new Foo    foo stuffIWant    123   public class Foo       private String stuffIWant       JailBreak unlocks the foo local variable in the compiler for direct access to all the members in Foo s hierarchy   Similarly you can use the jailbreak   extension method for one-off use   foo jailbreak   stuffIWant    123     Through the jailbreak   method you can access any member in Foo s hierarchy   In both cases the compiler resolves the field access for you type-safely  as if a public field  while Manifold generates efficient reflection code for you under the hood   Discover more about Manifold

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As oxbow lakes mentions  you can use reflection to get around the access restrictions  assuming your SecurityManager will let you    That said  if this class is so badly designed that it makes you resort to such hackery  maybe you should look for an alternative  Sure this little hack might be saving you a few hours now  but how much will it cost you down the road

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Try to go around the problem for the case  the calass of which you want to set get data is one of your own classes   Just create a public setter Field f  Object value  and public Object getter Field f  for that  You can even do some securoty check on your own inside theses member functions  E g  for the setter   class myClassName       private String aString       public set Field field  Object value                A  do some checkings here  for security              B  set the value         field set this  value             Of course  now you have to find out the java lang reflect Field for sString prior to setting of field value   I do use this technique in a generic ResultSet-to-and-from-model-mapper

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Try FieldUtils from apache commons-lang3   FieldUtils readField object  fieldName  true

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Use the Soot Java Optimization framework to directly modify the bytecode  http   www sable mcgill ca soot   Soot is completely written in Java and works with new Java versions

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One other option that hasn t been mentioned yet  use Groovy   Groovy allows you to access private instance variables as a side effect of the design of the language   Whether or not you have a getter for the field  you can just use   def obj   new IWasDesignedPoorly   def hashTable   obj getStuffIWant

[java] How to read the value of a private field from a different class in Java?

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