Что такое Динамические Прокси-классы и почему я использовал бы тот?

A dynamic proxy class is a class that implements a list of интерфейсы, указанные во время выполнения, так что вызов метода через один из интерфейсов экземпляра класса будет закодирован и отправляется другому объекту через единый интерфейс. Может быть используется для создания типобезопасного прокси-объекта для списка интерфейсов без необходимости предварительного создания прокси-класса. Динамический прокси классы полезны для приложения или библиотеки, которые должны предоставлять типобезопасная отражающая отправка вызовов к объектам, которые представляют интерфейсов API.

Динамические прокси-классы

One use case is hibernate - it gives you objects implementing your model classes interface but under getters and setters there resides db related code. I.e. you use them as if they are just simple POJO, but actually there is much going on under cover.

For example - you just call a getter of lazily loaded property, but really the property (probably whole big object structure) gets fetched from the database.

You should check cglib library for more info.

The class java.lang.reflect.Proxy allows you to implement interfaces dynamically by handling method calls in an InvocationHandler. It is considered part of Java's reflection facility, but has nothing to do with bytecode generation.

Sun has a tutorial about the use of the Proxy class. Google helps, too.

I just came up with an interesting use for a dynamic proxy.

We were having some trouble a non-critical service that is coupled with another dependant service and wanted to explore ways of being fault-tolerant when that dependant service becomes unavailable.

So I wrote a LoadSheddingProxy that takes two delegates - one is the remote impl for the 'normal' service (after the JNDI lookup). The other object is a 'dummy' load-shedding impl. There is simple logic surrounding each method invoke that catches timeouts and diverts to the dummy for a certain length of time before retrying. Here's how I use it:

// This is part of your ServiceLocator class
public static MyServiceInterface getMyService() throws Exception
{
    MyServiceInterface loadShedder = new MyServiceInterface() {
        public Thingy[] getThingys(Stuff[] whatever) throws Exception {
            return new Thingy[0];
        }
        //... etc - basically a dummy version of your service goes here
    }           
    Context ctx = JndiUtil.getJNDIContext(MY_CLUSTER);
    try {
        MyServiceInterface impl = ((MyServiceHome) PortableRemoteObject.narrow(
                ctx.lookup(MyServiceHome.JNDI_NAME), 
                MyServiceHome.class)).create();
        // Here's where the proxy comes in
        return (MyService) Proxy.newProxyInstance(
            MyServiceHome.class.getClassLoader(),
        new Class[] { MyServiceInterface.class },
        new LoadSheddingProxy(MyServiceHome.JNDI_NAME, impl, loadShedder, 60000));  // 10 minute retry
    } catch (RemoteException e) {    // If we can't even look up the service we can fail by shedding load too
        logger.warn("Shedding load");
        return loadShedder;
    } finally {
        if (ctx != null) {
        ctx.close();
        }
    }
}

And here's the proxy:

public class LoadSheddingProxy implements InvocationHandler {

static final Logger logger = ApplicationLogger.getLogger(LoadSheddingProxy.class);

Object primaryImpl, loadDumpingImpl;
long retry;
String serviceName;
// map is static because we may have many instances of a proxy around repeatedly looked-up remote objects
static final Map<String, Long> servicesLastTimedOut = new HashMap<String, Long>();

public LoadSheddingProxy(String serviceName, Object primaryImpl, Object loadDumpingImpl, long retry)
{
    this.serviceName = serviceName;
    this.primaryImpl = primaryImpl;
    this.loadDumpingImpl = loadDumpingImpl;
    this.retry = retry;
}

public Object invoke(Object obj, Method m, Object[] args) throws Throwable
{
    try
    {
        if (!servicesLastTimedOut.containsKey(serviceName) || timeToRetry()) {
            Object ret = m.invoke(primaryImpl, args);
            servicesLastTimedOut.remove(serviceName);
            return ret;
        } 
        return m.invoke(loadDumpingImpl, args);
    }
    catch (InvocationTargetException e)
    {
        Throwable targetException = e.getTargetException();

        // DETECT TIMEOUT HERE SOMEHOW - not sure this is the way to do it???
        if (targetException instanceof RemoteException) {
            servicesLastTimedOut.put(serviceName, Long.valueOf(System.currentTimeMillis()));
        }
        throw targetException;
    }                    
}

private boolean timeToRetry() {
    long lastFailedAt = servicesLastTimedOut.get(serviceName).longValue();
    return (System.currentTimeMillis() - lastFailedAt) > retry;
}
}