Call a method before any method invocation of that class in Java

Question

Is there a way that I can have a method in a class executed every time when any method of that class is invoked.

I'll give a brief of my scenario here:

Class Util{

   private isConnected(){
      if(!xxxapi.connected())
            throw new MyException(....)
}
   public createFile(){....}
   public removeFile(){....}
}

So, anytime I call new Util.createFile() I want that isConnected() is invoked before createFile() actually starts. Obviously I can call isConnected() everytime in the starting of each method, but I was wondering if I can have another solution.

Is there any other suggestion/solution for such a scenario.

Answer 1

You should write a InvocationHandler (http://docs.oracle.com/javase/7/docs/api/java/lang/reflect/InvocationHandler.html) that would intercept calls to your objects, and then reflectively (using reflection API) invoke first the isConnected() method followed by the method to which the call was made.

Sample: Util Interface:

public interface Util {

    void isConnected();

    void createFile();

    void removeFile();

}

Util invocation handler:

public class UtilInvocationHandler implements InvocationHandler {

    private Util util = new UtilImpl();

    @Override
    public Object invoke(Object proxy, Method method, Object[] args)
            throws Throwable {

        // look up isConnectedMethod
        Method isConnectedMethod = util.getClass().getMethod("isConnected");

        // invoke the method
        isConnectedMethod.invoke(util);

        // process result of the above call here

        // invoke the method to which the call was made
        Object returnObj = method.invoke(util, args);

        return returnObj;
    }

    private static class UtilImpl implements Util {
        public void isConnected(){
            System.out.println("isConnected()");
        }

        public void createFile(){
            System.out.println("createFile()");
        }

        public void removeFile(){
            System.out.println("removeFile()");
        }
    }
}

Object initialization:

    Util util = (Util) Proxy.newProxyInstance(
                                   Util.class.getClassLoader(), 
                                   new Class[] {Util.class}, 
                                   new UtilInvocationHandler());