Try with resource Context Classloaders

Gerard Davison

Jun. 09, 12 · Interview

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For various reasons I seem to end up writing a lot of code that fiddles with the context class loader in order to get non-module code running in the OSGi environment that JDeveloper runs in. This leads to a whole bunch of code that looks like this:

 public void doSomething() {

       ClassLoader context = Thread.currentThread().getContextClassLoader();

       try {
           Thread.currentThread().setContextClassLoader(Example.class.getClassLoader());

           // Class class that required the context class loader
           Endpoint.publish(null, null);            
       }
       finally {
           Thread.currentThread().setContextClassLoader(context);

       }  
  }

Now it occurred to me that the try/with/resources feature in JDK 7 isn't just for the nasty things in life, well resources, you can use it for any operation that might previously used a try/finally for.

   public void doSomething() {

       try (CloseableContext c = contextClassLoader(Example.class)) {

           // Class class that required the context class loader
           Endpoint.publish(null, null);            
       }
   }

It would have been nice to just call the method an not to allocate any variables as in the following example but it isn't allowable in the spec.

   public void doSomething() {

       try (contextClassLoader(Example.class)) { // Compilation errors

           // Class class that required the context class loader
           Endpoint.publish(null, null);            
       }
   }

Still the implementation of this is rather trivial and does still tidy up the original code. The only wrinkle is the need to have a public class/ interface subtype of AutoCloseable so that we can narrow the throws clause on the close() operation. If you don't do this then the original code has to deal with "throws Exception".

   public static CloseableContext contextClassLoader(Class loader) {
       return contextClassLoader(loader.getClassLoader());
   }

   public static CloseableContext contextClassLoader(ClassLoader loader) {
       final Thread currentThread = Thread.currentThread();
       final ClassLoader ocl = currentThread.getContextClassLoader();
       currentThread.setContextClassLoader(loader);
       return new CloseableContext(currentThread, ocl);
   }

   public static class CloseableContext implements AutoCloseable {
       private Thread _currentThread;
       private ClassLoader _ocl;

       private CloseableContext(Thread currentThread, ClassLoader ocl) {
           this._currentThread = currentThread;
           this._ocl = ocl;
       }

       @Override
       public void close() {
           this._currentThread.setContextClassLoader(this._ocl);
       }
   }

Java Development Kit Java (programming language) Loader (equipment) Interface (computing) JDeveloper Implementation

Published at DZone with permission of Gerard Davison, DZone MVB. See the original article here.

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Try with resource Context Classloaders

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