Java实现动态代理

注意点：UserDao userDao = (UserDao) hello.createProxyInstance(new UserDaoImpl());中接收的userDao 必须是接口，如果是类，会报类型转换错误：Exception in thread "main" java.lang.ClassCastException: com.sun.proxy.$Proxy0 cannot be cast to proxy.UserDao

具体实现：

package proxy;

public interface UserDao {
	void addUser();
}

package proxy;

public class UserDaoImpl implements UserDao {
	@Override
	public void addUser() {
		User user = new User();
		user.setId(1);
		user.setUsername("zhangsan");
		user.setPassword("123456");
	}
	
}

package proxy;

public class User {
	private int id;
	private String username;
	private String password;
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getUsername() {
		return username;
	}
	public void setUsername(String username) {
		this.username = username;
	}
	public String getPassword() {
		return password;
	}
	public void setPassword(String password) {
		this.password = password;
	}
	
	
}

package proxy;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class HelloInvocationHandler implements InvocationHandler {
	private Object targetObject;
	
	public Object createProxyInstance(Object targetObject) {
		this.targetObject = targetObject;
		return Proxy.newProxyInstance(targetObject.getClass().getClassLoader(), targetObject.getClass().getInterfaces(), this);
	}

	@Override
	public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
		helloWorld();
		Object object = method.invoke(targetObject, args);
		return object;
	}
	
	public void helloWorld() {
		System.out.println("Hello world HelloInvocationHandler111");
	}

}

main方法：

package proxy;

public class Demo {
	public static void main(String[] args) {
		HelloInvocationHandler hello = new HelloInvocationHandler();
		UserDao userDao = (UserDao) hello.createProxyInstance(new UserDaoImpl());
		userDao.addUser();
	}
}

关键是实现InvocationHandler接口。