Dynamic proxy is a technology that dynamically generates proxy objects at runtime, and its core lies in the java.lang.reflect.Proxy class and the InvocationHandler interface. By implementing the InvocationHandler interface to define proxy behavior and using the Proxy.newProxyInstance() method to create proxy objects, you can intercept method calls and insert custom logic, such as logging, permission checking, etc. Application scenarios include Spring AOP, performance monitoring, remote call packaging, etc. It should be noted that JDK dynamic proxy only supports interface proxy, high-frequency calls have performance overhead, and complex logic may affect maintenance. Mastering dynamic proxying helps develop more flexible applications and understand mainstream framework principles.

Java's reflection mechanism is already very powerful, but when you start to touch Dynamic Proxy, you will truly appreciate its flexibility in handling objects and interfaces at runtime. If you are already familiar with basic reflection operations and want to further master more advanced applications in Java, understanding how to use reflection to generate dynamic proxy is a key step.

What is a dynamic proxy?

Dynamic proxy does not determine a good class at the compile time, but dynamically generates proxy objects based on interfaces or classes at runtime. Its core lies in java.lang.reflect.Proxy class and InvocationHandler interface. Through them, you can insert custom logic such as logs, transaction control, permission checking, etc. without modifying the original class.

To give a simple example: you want to record the method name and parameters before calling a service interface, and you can use a dynamic proxy to intercept the call and add additional behavior.

The basic structure of dynamic proxy

To create a dynamic proxy object, there are two main parts:

• Implementing the InvocationHandler interface : Responsible for defining the behavior of the proxy object when executing methods.
• Use the Proxy.newProxyInstance() method : Pass in the class loader, the target interface array, and the InvocationHandler instance to generate the proxy object.

 MyInterface proxy = (MyInterface) Proxy.newProxyInstance(
    target.getClass().getClassLoader(),
    new Class[]{MyInterface.class},
    new MyInvocationHandler(target)
);

InvocationHandler is the key in this process. Every time a method of the proxy object is called, it will enter its invoke() method. You can do various enhancements here.

The functions and techniques of InvocationHandler

InvocationHandler is the core logic of dynamic proxy. It has only one method invoke(Object proxy, Method method, Object[] args) , where:

• proxy is the proxy object itself (usually not available)
• method is the method object being called
• args is a method parameter

In this method, you can decide whether to call the original object's method, or directly return the simulation result, throw an exception, etc.

Some practical tips include:

• Intercept specific methods for enhancement (for example, add cache only for methods starting with "get")
• Recording the time spent invoking method
• Unified handling of exception packaging
• Control access rights

Sample code snippet:

 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if (method.getName().startsWith("get")) {
        System.out.println("Before calling: " method.getName());
        Object result = method.invoke(target, args);
        System.out.println("After calling: " method.getName());
        return result;
    } else {
        return method.invoke(target, args);
    }
}

Here we only process the get method before and after printing, and other methods remain called as-is.

Practical application scenarios of dynamic proxy

Dynamic proxy is widely used in many frameworks, such as Spring AOP, Hibernate lazy loading, Mock frameworks (such as Mockito), etc. Understanding these applications can help you better understand their value.

Common uses include:

• Logging and performance monitoring
• Security control and permission verification
• Remote Call Encapsulation (RMI)
• Mock object generation in unit tests
• Cache implementation (such as method-level cache)

Let me give you a practical example: Spring AOP is implemented based on dynamic proxy. When you use the @Transactional annotation, Spring creates a proxy for your bean, which automatically starts and commits transactions before and after the method is executed.

Precautions and limitations

Although dynamic proxy functions are powerful, you should also pay attention to the following points when using it:

• Only the proxy implements the interface class (JDK dynamic proxy). If you need to proxy classes without interfaces, you must use bytecode enhancement tools such as CGLIB.
• The performance overhead is slightly high, not suitable for high-frequency calling scenarios
• The stack information may not be intuitive enough during debugging
• It is not recommended to do too complex logic in the agent, which can easily affect maintainability.

Basically that's it. Mastering dynamic proxying will allow you to write more flexible and scalable Java applications, and will also help you better understand the working principles behind mainstream frameworks.

The above is the detailed content of Advanced Java Reflection for Dynamic Proxy Generation. For more information, please follow other related articles on the PHP Chinese website!