What is Java dynamic proxy and how to implement it

1. Proxy mode

When we need to call a method of a certain class (specific implementation class), we do not directly create an object of that class, but get the proxy class object of the class and use it through the proxy Object, calling the function of the specific implementation class. Both the concrete implementation class and the proxy class implement the same interface, and the proxy class holds the object of the implementation class. This creates a layer of isolation between the calling end and the specific implementation end to avoid direct interaction.

There are many similar examples of the agency model in reality. For example, when we buy or rent a house, we have to go through an intermediary. This intermediary is equivalent to an agent.

2. Static proxy implementation

1) Define interface:

public interface IHouse {
    void sallHouse();
    int sallHouse2();
}

2) Specific implementation class:

public class Andy implements IHouse {
    @Override
    public void sallHouse() {
        System.out.println("andy sall house..");
    }
    @Override
    public int sallHouse2() {
        return 100;
    }
}

3) Proxy class:

public class HouseProxy implements IHouse {
    Andy andy;
    public HouseProxy(Andy andy) {
        this.andy = andy;
    }
    @Override
    public void sallHouse() {
        andy.sallHouse();
    }
    @Override
    public int sallHouse2() {
        return andy.sallHouse2();
    }
}

4) Client call:

        //1.创建被代理对象
        Andy andy = new Andy();
        //2.创建代理对象，代理对象持有被代理对象的引用
        HouseProxy proxy = new HouseProxy(andy);
        //3.客户端通过代理对象调用。
        proxy.sallHouse();

3. Dynamic proxy implementation

  //1.被代理对象
        final Andy andy = new Andy();
        //2.创建动态代理，Java在运行时动态生成的。
        ClassLoader classLoader = andy.getClass().getClassLoader();
        Class[] interfaces = andy.getClass().getInterfaces();
        IHouse iHouse = (IHouse) Proxy.newProxyInstance(classLoader, interfaces, new             InvocationHandler() {
            @Override
            public Object invoke(Object o, Method method, Object[] objects) throws Throwable {
                //通过反射调用被代理对象的方法
                return method.invoke(andy, objects);
            }
        });
        //3.客户端通过代理对象调用被代理方法。
        iHouse.sallHouse();

Dynamic proxy analysis:

1) IHouse iHouse = (IHouse) Proxy.newProxyInstance();Create a dynamic proxy object,

has three parameters:

1.ClassLoader class loader

2.Class class of the proxy interface ,

3.InvocationHandler interface implementation class

2) After getting the iHouse dynamic proxy, call the interface method iHouse.sallHouse();

As soon as this method is called, The invoke method in the InvocationHandler class will be executed.

@Override

public Object invoke(Object o, Method method, Object[] objects) throws Throwable {

//Invoke the proxy object through reflection Method

return method.invoke(andy, objects);

}

The invoke method has three parameters:

method, which is the proxy class The called method name (sallHouse)

objects is the parameter passed when the proxy class calls the method.

Object obj = method.invoke(andy, objects); Invoke the andy object through the reflection mechanism, the corresponding method in the specific implementer.

His return value can be received when the proxy object calls the interface. Whatever type it is, what type is returned. This is what Retrofit does

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