Dynamic proxy

One 、 What is agency

sketch ： Buying on behalf of 、 The mediation 、 ladder 、 Businesses and so on

characteristic ： The agent and the target should do the same thing

Two 、 The proxy pattern

sketch ： Agency mode means , Provides a proxy for other objects to control access to this object . In some cases , An object Not suitable or cannot be quoted directly Another object , The proxy object can be in Customer class and Target audience Play a role in The mediation The role of . In short ： The target object is not accessible , Access... Through proxy object enhancements ;

effect ： Enhancements 【 Add extra features 】; Control access to the target object ;

classification ： Static proxy 【① The proxy class is implemented manually ② The target class needs to be certain 】; A dynamic proxy ;

3、 ... and 、 Static proxy

characteristic ： The target object and the proxy object implement the same business interface ; The target object must implement the interface 【 Compared with dynamic proxy 】; The proxy object already exists before the program runs ; It can flexibly switch the target objects , But it can't be function Flexible handling of

shortcoming ： In static proxy , Acting The function is dead , Unable to change dynamically , And this function already exists when the project starts ; If you want to add the function of agent , It needs to be in the interface 、 Target class and proxy class Add new code between , The impact is more , Dynamic proxy can automatically generate proxy classes .

Realization ：

0、 Project directory  1、 Create an interface , Define the method of selling goods , Show what your manufacturers and businesses do . 2、 Create factory class 【 Target class 】, Implementation steps 1 The interface of  3、 Create a merchant , It's the agent , You also need to implement steps 1 The interface of  4、 Create a client class , Call the merchant's method to buy something

Four 、 A dynamic proxy

sketch ： Proxy objects are dynamically built in memory during program execution , It can be carried out flexibly Business function Of Switch . let me put it another way ： When the program is running, the proxy class object JVM Generated dynamically according to the reflection mechanism , There is no need to define a proxy class .java object

1、JDK Dynamic proxy for ：

sketch ： Use java The classes and interfaces in the reflection package realize the function of dynamic agent

characteristic ： The target object must implement the business interface ;JDK Proxy objects do not need to implement business interfaces ;JDK The object of dynamic proxy does not exist before the program runs , Dynamically build in memory while the program is running ;JDK Dynamic agent can flexibly switch business functions ; Methods in this class （ Methods in non interface ） Can't be represented ; You can enhance your functional code in the agent

Parameters ： It is done using tool classes JDK Dynamic implementation of , Reflex package java.lang.reflect

1）Proxy class ： It is java.lang.reflect.Proxy Class under package , It has a way Proxy.newProxyInstance{....}, Specifically used to generate dynamic proxy objects

2）Method class ： Class for reflection , Used to make reflection calls to target object methods ,Method Object to catch the method we are calling

3）InvocationHandler Interface ： Used to implement agent and business functions , We use anonymous internal implementation when calling , We wrote it ourselves

Realization ：

0、 Project structure 1、 Create an interface , Define the functions to be completed by the target class 2、 Create the implementation interface of the target class 3、 establish InvocationHandler Implementation class of interface , stay invoke Method to complete the function of the proxy class         1） Calling the target method         2） Enhancements 4、 Use Proxy Class static methods , Create proxy object , And convert the return value to the interface type . ​​​​​​​ Actually ： Dynamic agents are better than static agents , Namely ： Automatically add the enhanced code to the front or back of the method , You don't have to write enhanced code for every method ; But static proxy can only add enhanced code one method at a time , Although the enhanced code is the same .

2、CGLib A dynamic proxy ：

sketch ： Referred to as subclass dynamic proxy , By dynamically building subclass objects in memory , Override the method of the parent class to enhance the proxy function . If the target object does not implement the interface , Only through CGLib Subclass proxy for function enhancement . Subclass proxies are object bytecode frameworks ASM To achieve ;CGLib stay MyBatis and Spring In the framework

Be careful ： The represented class cannot be final, Because it's inheritance