How to load a jar file at runtime

Question

I was asked to build a java system that will have the ability to load new code  expansions  while running   How do I re-load a jar file while my code is running  or how do I load a new jar   Obviously  since constant up-time is important  I d like to add the ability to re-load existing classes while at it  if it does not complicate things too much    What are the things I should look out for   think of it as two different questions - one regarding reloading classes at runtime  the other regarding adding new classes

User · Answer

I googled a bit  and found this code here   File file   getJarFileToLoadFrom       String lcStr   getNameOfClassToLoad       URL jarfile   new URL  jar       file     file getAbsolutePath              URLClassLoader cl   URLClassLoader newInstance new URL    jarfile        Class loadedClass   cl loadClass lcStr        Can anyone share opinions comments answers regarding this approach

User · Answer

I was asked to build a java system that will have the ability to load new code while running   You might want to base your system on OSGi  or at least take a lot at it   which was made for exactly this situation   Messing with classloaders is really tricky business  mostly because of how class visibility works  and you do not want to run into hard-to-debug problems later on  For example  Class forName    which is widely used in many libraries does not work too well on a fragmented classloader space

User · Answer

This works for me   File file    new File  c   myjar jar     URL url   file toURL      URL   urls   new URL   url    ClassLoader cl   new URLClassLoader urls   Class cls   cl loadClass  com mypackage myclass

User · Answer

Reloading existing classes with existing data is likely to break things   You can load new code into new class loaders relatively easily   ClassLoader loader   URLClassLoader newInstance      new URL     yourURL        getClass   getClassLoader      Class lt   gt  clazz   Class forName  mypackage MyClass   true  loader   Class lt   extends Runnable gt  runClass   clazz asSubclass Runnable class      Avoid Class newInstance  for it is evil  Constructor lt   extends Runnable gt  ctor   runClass getConstructor    Runnable doRun   ctor newInstance    doRun run      Class loaders no longer used can be garbage collected  unless there is a memory leak  as is often the case with using ThreadLocal  JDBC drivers  java beans  etc    If you want to keep the object data  then I suggest a persistence mechanism such as Serialisation  or whatever you are used to   Of course debugging systems can do fancier things  but are more hacky and less reliable   It is possible to add new classes into a class loader  For instance  using URLClassLoader addURL  However  if a class fails to load  because  say  you haven t added it   then it will never load in that class loader instance

User · Answer

Reloading existing classes with existing data is likely to break things   You can load new code into new class loaders relatively easily   ClassLoader loader   URLClassLoader newInstance      new URL     yourURL        getClass   getClassLoader      Class lt   gt  clazz   Class forName  mypackage MyClass   true  loader   Class lt   extends Runnable gt  runClass   clazz asSubclass Runnable class      Avoid Class newInstance  for it is evil  Constructor lt   extends Runnable gt  ctor   runClass getConstructor    Runnable doRun   ctor newInstance    doRun run      Class loaders no longer used can be garbage collected  unless there is a memory leak  as is often the case with using ThreadLocal  JDBC drivers  java beans  etc    If you want to keep the object data  then I suggest a persistence mechanism such as Serialisation  or whatever you are used to   Of course debugging systems can do fancier things  but are more hacky and less reliable   It is possible to add new classes into a class loader  For instance  using URLClassLoader addURL  However  if a class fails to load  because  say  you haven t added it   then it will never load in that class loader instance

User · Answer

I googled a bit  and found this code here   File file   getJarFileToLoadFrom       String lcStr   getNameOfClassToLoad       URL jarfile   new URL  jar       file     file getAbsolutePath              URLClassLoader cl   URLClassLoader newInstance new URL    jarfile        Class loadedClass   cl loadClass lcStr        Can anyone share opinions comments answers regarding this approach

User · Answer

Reloading existing classes with existing data is likely to break things   You can load new code into new class loaders relatively easily   ClassLoader loader   URLClassLoader newInstance      new URL     yourURL        getClass   getClassLoader      Class lt   gt  clazz   Class forName  mypackage MyClass   true  loader   Class lt   extends Runnable gt  runClass   clazz asSubclass Runnable class      Avoid Class newInstance  for it is evil  Constructor lt   extends Runnable gt  ctor   runClass getConstructor    Runnable doRun   ctor newInstance    doRun run      Class loaders no longer used can be garbage collected  unless there is a memory leak  as is often the case with using ThreadLocal  JDBC drivers  java beans  etc    If you want to keep the object data  then I suggest a persistence mechanism such as Serialisation  or whatever you are used to   Of course debugging systems can do fancier things  but are more hacky and less reliable   It is possible to add new classes into a class loader  For instance  using URLClassLoader addURL  However  if a class fails to load  because  say  you haven t added it   then it will never load in that class loader instance

User · Answer

I googled a bit  and found this code here   File file   getJarFileToLoadFrom       String lcStr   getNameOfClassToLoad       URL jarfile   new URL  jar       file     file getAbsolutePath              URLClassLoader cl   URLClassLoader newInstance new URL    jarfile        Class loadedClass   cl loadClass lcStr        Can anyone share opinions comments answers regarding this approach

User · Answer

I was asked to build a java system that will have the ability to load new code while running   You might want to base your system on OSGi  or at least take a lot at it   which was made for exactly this situation   Messing with classloaders is really tricky business  mostly because of how class visibility works  and you do not want to run into hard-to-debug problems later on  For example  Class forName    which is widely used in many libraries does not work too well on a fragmented classloader space

User · Answer

I was asked to build a java system that will have the ability to load new code while running   You might want to base your system on OSGi  or at least take a lot at it   which was made for exactly this situation   Messing with classloaders is really tricky business  mostly because of how class visibility works  and you do not want to run into hard-to-debug problems later on  For example  Class forName    which is widely used in many libraries does not work too well on a fragmented classloader space

User · Answer

This works for me   File file    new File  c   myjar jar     URL url   file toURL      URL   urls   new URL   url    ClassLoader cl   new URLClassLoader urls   Class cls   cl loadClass  com mypackage myclass

User · Answer

Use org openide util Lookup and ClassLoader to dynamically load the Jar plugin  as shown here   public LoadEngine         Lookup ocrengineLookup      Collection lt OCREngine gt  ocrengines      Template ocrengineTemplate      Result ocrengineResults      try             ocrengineLookup   Lookup getDefault    this only load OCREngine in classpath of  application         ocrengineLookup   Lookups metaInfServices getClassLoaderForExtraModule      this load the OCREngine in the extra module as well         ocrengineTemplate   new Template OCREngine class           ocrengineResults   ocrengineLookup lookup ocrengineTemplate            ocrengines   ocrengineResults allInstances     all OCREngines must implement the defined interface in OCREngine  Reference to guideline of implement org openide util Lookup for more information        catch  Exception ex             public ClassLoader getClassLoaderForExtraModule   throws IOException        List lt URL gt  urls   new ArrayList lt URL gt  5         foreach  filepath  external file   JAR  with each external file   JAR  do as follows     File jar   new File filepath       JarFile jf   new JarFile jar       urls add jar toURI   toURL         Manifest mf   jf getManifest       If the jar has a class-path in it s manifest add it s entries     if  mf                null            String cp                   mf getMainAttributes   getValue  class-path            if  cp                    null                for  String cpe   cp split    s                       File lib                           new File jar getParentFile    cpe                   urls add lib toURI   toURL                                       ClassLoader cl   ClassLoader getSystemClassLoader        if  urls size    gt  0            cl   new URLClassLoader urls toArray new URL urls size      ClassLoader getSystemClassLoader               return cl

User · Answer

Reloading existing classes with existing data is likely to break things   You can load new code into new class loaders relatively easily   ClassLoader loader   URLClassLoader newInstance      new URL     yourURL        getClass   getClassLoader      Class lt   gt  clazz   Class forName  mypackage MyClass   true  loader   Class lt   extends Runnable gt  runClass   clazz asSubclass Runnable class      Avoid Class newInstance  for it is evil  Constructor lt   extends Runnable gt  ctor   runClass getConstructor    Runnable doRun   ctor newInstance    doRun run      Class loaders no longer used can be garbage collected  unless there is a memory leak  as is often the case with using ThreadLocal  JDBC drivers  java beans  etc    If you want to keep the object data  then I suggest a persistence mechanism such as Serialisation  or whatever you are used to   Of course debugging systems can do fancier things  but are more hacky and less reliable   It is possible to add new classes into a class loader  For instance  using URLClassLoader addURL  However  if a class fails to load  because  say  you haven t added it   then it will never load in that class loader instance

User · Answer

I googled a bit  and found this code here   File file   getJarFileToLoadFrom       String lcStr   getNameOfClassToLoad       URL jarfile   new URL  jar       file     file getAbsolutePath              URLClassLoader cl   URLClassLoader newInstance new URL    jarfile        Class loadedClass   cl loadClass lcStr        Can anyone share opinions comments answers regarding this approach

User · Answer

Use org openide util Lookup and ClassLoader to dynamically load the Jar plugin  as shown here   public LoadEngine         Lookup ocrengineLookup      Collection lt OCREngine gt  ocrengines      Template ocrengineTemplate      Result ocrengineResults      try             ocrengineLookup   Lookup getDefault    this only load OCREngine in classpath of  application         ocrengineLookup   Lookups metaInfServices getClassLoaderForExtraModule      this load the OCREngine in the extra module as well         ocrengineTemplate   new Template OCREngine class           ocrengineResults   ocrengineLookup lookup ocrengineTemplate            ocrengines   ocrengineResults allInstances     all OCREngines must implement the defined interface in OCREngine  Reference to guideline of implement org openide util Lookup for more information        catch  Exception ex             public ClassLoader getClassLoaderForExtraModule   throws IOException        List lt URL gt  urls   new ArrayList lt URL gt  5         foreach  filepath  external file   JAR  with each external file   JAR  do as follows     File jar   new File filepath       JarFile jf   new JarFile jar       urls add jar toURI   toURL         Manifest mf   jf getManifest       If the jar has a class-path in it s manifest add it s entries     if  mf                null            String cp                   mf getMainAttributes   getValue  class-path            if  cp                    null                for  String cpe   cp split    s                       File lib                           new File jar getParentFile    cpe                   urls add lib toURI   toURL                                       ClassLoader cl   ClassLoader getSystemClassLoader        if  urls size    gt  0            cl   new URLClassLoader urls toArray new URL urls size      ClassLoader getSystemClassLoader               return cl

User · Answer

I was asked to build a java system that will have the ability to load new code while running   You might want to base your system on OSGi  or at least take a lot at it   which was made for exactly this situation   Messing with classloaders is really tricky business  mostly because of how class visibility works  and you do not want to run into hard-to-debug problems later on  For example  Class forName    which is widely used in many libraries does not work too well on a fragmented classloader space

[java] How to load a jar file at runtime

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