What is the mechanism of dynamic proxy in Java?

The dynamic proxy mechanism in Java is a way to generate proxy classes and objects at runtime. With dynamic proxy, it is possible to create a proxy class that implements a given set of interfaces at runtime, and add some custom logic before and after method calls on the proxy class.

The dynamic proxy mechanism in Java mainly involves two classes: Proxy and InvocationHandler. Proxy is a utility class used to generate proxy classes, while InvocationHandler is an interface used to handle method calls in proxy classes.

To utilize dynamic proxy, the first step is to define a class that implements the InvocationHandler interface. Inside this class, the invoke method should be implemented, which will be called when a method is invoked on the proxy object. Within the invoke method, custom logic can be added, such as logging before and after method calls, or performing permission checks.

After that, a proxy class that implements the specified interface can be dynamically generated using the newProxyInstance method of the Proxy class. This method takes three parameters: a class loader, an array of interfaces to implement, and an InvocationHandler object. Upon calling this method, an instance of the proxy class will be returned.

By using an instance of a proxy class, we can invoke methods from the interface being proxied. When invoking a method, the invoke method of the InvocationHandler will be called first. Within this method, the actual method of the proxied object will then be invoked.

In summary, Java’s dynamic proxy mechanism can generate proxy classes and objects at runtime, allowing for custom logic to be added before and after method calls on the proxy object. This mechanism can be used to implement common cross-cutting concerns such as logging and permission checking.