Understanding Java Dynamic Proxy: How JDK Generates and Executes Proxy Classes

Static proxy classes become cumbersome as the number of methods grows, prompting the use of dynamic proxies which keep proxy code fixed and enable non‑intrusive code extension and AOP programming.

Key components of Java's dynamic proxy are Proxy and InvocationHandler. The InvocationHandler#invoke method receives three parameters: proxy (the proxy instance), method (the method to invoke), and args (method arguments).

JDK Dynamic Proxy Mechanism

The JDK creates a proxy class that implements the target interfaces and extends Proxy. This class (e.g., $Proxy0) contains method stubs that delegate to the InvocationHandler via invoke.

Import the target interface and related classes.

Instantiate the proxy by calling Proxy.newProxyInstance with a class loader, the target interfaces, and an InvocationHandler implementation.

In the invoke method, combine business logic and decide whether to call the original method.

Example Code

proxyObj.business();

When proxyObj.business() is called, the call is routed to InvocationHandler.invoke, where the method parameter is set.

Why It Is Called JDK Dynamic Proxy

The proxy class is generated at runtime by JDK code, not manually written, which is why it is termed a dynamic proxy.

Understanding Proxy Generation

The Proxy.newProxyInstance method performs the following steps:

Uses getProxyClass to locate or generate a proxy class ( $Proxy0) that implements the specified interfaces and extends Proxy.

Instantiates the proxy class via reflection, passing the InvocationHandler to the constructor.

The generated $Proxy0 source includes static Method fields for each method (e.g., m1 for equals, m3 for business) and overrides methods to delegate to the handler:

public final void business() throws ... {
    try {
        super.h.invoke(this, m3, (Object[])null);
    } ...
}

Finally, the proxy instance is cast to the target interface, and invoking its methods triggers the handler's invoke method, which controls execution of the original method.

Note: Proxy#getProxyClass returns the proxy class itself, not the class of the original target object.