all and sundry

Java Dynamic proxy mechanism provides an interesting way to create proxy instances. The steps to create a dynamic proxy is a little tedious though, consider a proxy to be used for auditing the time taken for a method call for a service instance -

public interface InventoryService {
    public Inventory create(Inventory inventory);
    public List<Inventory> list();
    public Inventory findByVin(String vin);
    public Inventory update(Inventory inventory);
    public boolean delete(Long id);
    public Inventory compositeUpdateService(String vin, String newMake);
}

The steps to create a dynamic proxy for instances of this interface is along these lines:
1. Create an instance of a java.lang.reflect.InvocationHandler, this will be responsible for handling the method calls on behalf of the actual service instance, a sample Invocation handler for auditing is the following:

...
public class AuditProxy implements java.lang.reflect.InvocationHandler {

    private Object obj;

    public static Object newInstance(Object obj) {
        return java.lang.reflect.Proxy.newProxyInstance(obj.getClass().getClassLoader(), obj
                .getClass().getInterfaces(), new AuditProxy(obj));
    }

    private AuditProxy(Object obj) {
        this.obj = obj;
    }

    public Object invoke(Object proxy, Method m, Object[] args) throws Throwable {
        Object result;
        try {
            logger.info("before method " + m.getName());
            long start = System.nanoTime();
            result = m.invoke(obj, args);
            long end = System.nanoTime();
            logger.info(String.format("%s took %d ns", m.getName(), (end-start)) );
        } catch (InvocationTargetException e) {
            throw e.getTargetException(); 
        } catch (Exception e) {
            throw new RuntimeException("unexpected invocation exception: " + e.getMessage());
        } finally {
            logger.info("after method " + m.getName());
        }
        return result;
    }
}

2. When creating instances of InventoryService, return a proxy which in this case is the AuditProxy, composing instances of InventoryService, which can be better explained using a UML:




This is how it would look in code:

InventoryService inventoryService = (InventoryService)AuditProxy.newInstance(new DefaultInventoryService());

Now, any calls to inventoryService will be via the AuditProxy instance, which would measure the time taken in the method while delegating the actual method call to the InventoryService instance.



So what are proxies used for:
1. Spring AOP uses it extensively - it internally creates a dynamic proxy for different AOP constructs
2. As in this example, for any class decoration - AOP will definitely be a better fit for such a use case though
3. For any frameworks needing to support interface and annotation based features - A real proxied instance need not even exist, a dynamic proxy can recreate the behavior expected of an interface, based on some meta-data provided through annotations.