Java annotation for wrapping a method

Question

I have a lot of boilerplate code that basically follows this pattern:

function doSomething() {
  try {
    [implementation]
    [implementation]
    [implementation]
    [implementation]
  } catch (Exception e) {
    MyEnv.getLogger().log(e);
  } finally {
    genericCleanUpMethod();
  }
}

I'd love to create my own annotation to clean my code up a bit:

@TryCatchWithLoggingAndCleanUp
function doSomething() {
  [implementation]
  [implementation]
  [implementation]
  [implementation]
}

The method signatures vary wildly (depending on the actual implementation of the method), but the boilerplate try/catch/finally part is always the same.

The annotation I have in mind would automatically wrap the contents of the annotated method with the whole try...catch...finally hoopla.

I've searched high and low for a straightforward way to do this, but have found nothing. I don't know, maybe I just can't see the woods for all the annotated trees.

Any pointers on how I might implement such an annotation would be greatly appreciated.

Answer 1

To do this, you would need some AOP framework that would use a proxy around your method. This proxy would catch the exception and execute the finally block. Quite frankly, if you don't use a framework supporting AOP already, I'm not sure I would use one just to save these few lines od code.

You could use the following pattern to do this in a more elegant way, though:

public void doSomething() {
    logAndCleanup(new Callable<Void>() {
        public Void call() throws Exception {
            implementationOfDoSomething();
            return null;
        }
    });
}

private void logAndCleanup(Callable<Void> callable) {
    try {
        callable.call();
    } 
    catch (Exception e) {
        MyEnv.getLogger().log(e);
    } 
    finally {
        genericCleanUpMethod();
    }
}

I just used Callable<Void> as an interface, but you could define your own Command interface:

public interface Command {
    public void execute() throws Exception;
}

and thus avoid the need to use a generic Callable<Void> and return null from the Callable.

EDIT: in case you want to return something from your methods, then make the logAndCleanup() method generic. Here's a complete example:

public class ExceptionHandling {
    public String doSomething(final boolean throwException) {
        return logAndCleanup(new Callable<String>() {
            public String call() throws Exception {
                if (throwException) {
                    throw new Exception("you asked for it");
                }
                return "hello";
            }
        });
    }

    public Integer doSomethingElse() {
        return logAndCleanup(new Callable<Integer>() {
            public Integer call() throws Exception {
                return 42;
            }
        });
    }

    private <T> T logAndCleanup(Callable<T> callable) {
        try {
            return callable.call();
        }
        catch (Exception e) {
            System.out.println("An exception has been thrown: " + e);
            throw new RuntimeException(e); // or return null, or whatever you want
        }
        finally {
            System.out.println("doing some cleanup...");
        }
    }

    public static void main(String[] args) {
        ExceptionHandling eh = new ExceptionHandling();

        System.out.println(eh.doSomething(false));
        System.out.println(eh.doSomethingElse());
        System.out.println(eh.doSomething(true));
    }
}

EDIT : And with Java 8, the wrapped code can be a bit prettier :

public String doSomething(final boolean throwException) {
    return logAndCleanup(() -> {                
        if (throwException) {
            throw new Exception("you asked for it");
        }
        return "hello";                
    });
}

Answer 2

You could use dynamic proxies to implement this. It takes a bit of setting up, but once done, is pretty straightforward.

First, you define an interface and place the annotation on the interface.

public interface MyInterface {
    @TryCatchWithLogging
    public void doSomething();
}

Now, when you want to provide an implementation of the interface to a consumer, dont provide with him with the actual implementation, but instead a Proxy to it.

MyInterface impl = new java.lang.reflect.Proxy.newProxyInstance(
                         Impl.class.getClassLoader(), 
                         Impl.class.getInterfaces(), YourProxy(new Impl());

Then implement YourProxy.

public class YourProxy implements InvocationHandler {
....

     public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
         if ( method.isAnnotationPresent(TryCatchLogging.class) ) {
              // Enclose with try catch
}

Answer 3

you can implement annotation and annotation processor yourself and instrument code everytime when you do compilation (javac -processor). Other way is to use AOP, say AspectJ or Spring AOP (If you use Spring).