spring aop proxy object

Question

I'm now reading a Spring book written in Korean language and my English is bad. Please understand it.

In the book, it says Spring's AOP initiates class using Dynamic proxy if it uses Interface and it uses CGLIB to initiate class in case of not using Interface. I don't clearly understand what it means. could you help me understand its deep meaning.

I don't know this question is silly or not. but I'm just curious THX.

Answer 1

A proxy is essentially a mediator between a client and an object such that it implements the object's non-final methods. Proxying an interface is relatively straightforward since an interface is simply a list of methods which need to be implemented, facilitating the interception of method invocations.

The Proxy class in Java is a class that implements a list of interfaces which are specified at runtime. A proxy then has an InvocationHandler associated with it, which delegates method calls made on the proxy to the object being proxied. It acts as a level of indirection such that methods are not invoked on the object itself but rather on its proxy. The InvocationHandler has but a single method which needs to be implemented:

public Object invoke(Object proxy, Method method, Object[] args)

Meanwhile, the client invoking the method can't tell the difference between a proxy and its underlying object representation, nor should it care.

Proxying a class dynamically, as opposed to an interface, is not quite as simple. While Java's Proxy is merely a runtime implementation of an interface or set of interfaces, objects do not have to implement an interface. Therefore, proxying classes requires bytecode generation, which is where libraries such as cglib come into play. cglib provides support for proxying classes because it can dynamically generate bytecode (i.e. class files), meaning it can extend classes at runtime in a way that Java's Proxy can implement an interface at runtime.

There are many uses of proxies. One such use is in lazy loading. Lazy loading allows objects in an object graph to be loaded only when they are needed. Rather than loading them all into memory immediately, which could be expensive and resource-intensive, we can load them on-the-fly when we need to access them, such as iterating over a collection of objects. Instead of loading the entire collection, we simply load a small set at a time. This can be achieved with proxies. The proxy represents the lazily-loaded object. The object itself, which might be loaded from a database, is not loaded until a method is invoked on its proxy. The proxy, which intercepts the method invocation, will then load the object into memory and delegate the method invocation to it.

Here is an example of a lazy-loading implementation:

public abstract class LazilyLoadedObject implements InvocationHandler {

    private Object target;

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if (target == null) {
            target = loadObject();
        }
        return method.invoke(target, args);
    }

    /**
     * Loads the proxied object. This might be an expensive operation
     * or loading lots of objects could consume a lot of memory, so
     * we only load the object when it's needed.
     */
     protected abstract Object loadObject();

}

The above InvocationHandler would be passed to a proxy so that methods invoked on the proxy would be handled by the InvocationHandler. The handler checks to see if the object has been loaded. If it hasn't, it will call loadObject(), which could be some sort of database query that retrieves the object.

Proxies are very powerful because they allow for the interception of method invocations. Such is the case in AOP.

Answer 2

Java Dynamic Proxy is a reflective element of the Java language that allows a user to create a proxy of an interface at runtime. Being a part of the reflection package, it is a part of Java and it ships with the JRE/JDK.

CGLIB is a code generation library which has the capability to extend Java classes at runtime. Because of this, Spring utilizes this functionality to proxy non-interfaces for its AOP library.