From dynamic proxy implementation to spring AOP

Huawei cloud developer community 2021-01-22 12:21:53
dynamic proxy implementation spring aop


Abstract : This article mainly talked about Spring Aop There are two ways to implement dynamic proxy .

1. Spring AOP

Spring It's a lightweight container ,Spring The core concept of the whole series is IoC、AOP. so AOP yes Spring One of the cores of the framework , It plays a very important role in application , It's also Spring The basis of other components .AOP(Aspect Oriented Programming), That is, face-to-face programming , Can be said to be OOP(Object Oriented Programming, object-oriented programming ) To complement and perfect .OOP Imported packaging 、 Inherit 、 The concept of polymorphism is used to establish an object hierarchy , A collection used to simulate public behavior . however OOP Allows developers to define vertical relationships , But it's not appropriate to define horizontal relationships , For example, log function .

About AOP Basic knowledge of , Is not the point of this article , Let's mainly look at AOP The underlying implementation mechanism of the core functions of : The principle of dynamic agent .AOP The interception function of is provided by java To implement the dynamic agent in . Add section logic to the target class , Generate enhanced target classes ( The facet logic is either before the target class function is executed , Or after the target class function is executed , Or when the target class function throws an exception . Different entry time corresponds to different Interceptor The type of , Such as BeforeAdviseInterceptor,AfterAdviseInterceptor as well as ThrowsAdviseInterceptor etc. ).

So how does the dynamic agent implement the section logic (advise) Woven into the target class method ? Now let's introduce and implement AOP Two dynamic proxies used in .

AOP Two kinds of dynamic agents are used in the source code of :jdk Dynamic proxy sum cglib A dynamic proxy . Two methods exist at the same time , Each have advantages and disadvantages .jdk Dynamic agents are created by java Internal reflection mechanism ,cglib The bottom layer of dynamic agent is based on asm To achieve . in general , Reflection mechanism is more efficient in the process of class generation , and asm It is more efficient in the related execution process after the class is generated ( It can be done by putting asm Generated classes for caching , Solve it like this asm Low efficiency of generating class process ).

Let's implement these two methods respectively .

2. JDK A dynamic proxy

2.1 Define interfaces and implementation classes

public interface OrderService {
public void save(UUID orderId, String name);
public void update(UUID orderId, String name);
public String getByName(String name);
}

The above code defines an intercepted object interface , Crosscutting concerns . The following code implements the intercepted object interface .

public class OrderServiceImpl implements OrderService {
private String user = null;
public OrderServiceImpl() {
}
public OrderServiceImpl(String user) {
this.setUser(user);
}
//...

@Override
public void save(UUID orderId, String name) {
System.out.println("call save() Method ,save:" + name);
}
@Override
public void update(UUID orderId, String name) {
System.out.println("call update() Method ");
}
@Override
public String getByName(String name) {
System.out.println("call getByName() Method ");
return "aoho";
}
}

2.2 JDK Dynamic proxy class

public class JDKProxy implements InvocationHandler {
// The target object that needs the proxy 
private Object targetObject;
public Object createProxyInstance(Object targetObject) {
this.targetObject = targetObject;
return Proxy.newProxyInstance(this.targetObject.getClass().getClassLoader(),
this.targetObject.getClass().getInterfaces(), this);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
// Proxied object 
OrderServiceImpl bean = (OrderServiceImpl) this.targetObject;
Object result = null;
// Tangent logic (advise), This is before the target class code is executed 
System.out.println("---before invoke----");
if (bean.getUser() != null) {
result = method.invoke(targetObject, args);
}
System.out.println("---after invoke----");
return result;
}
//...

}

The above code implements the dynamic proxy class JDKProxy, Realization InvocationHandler Interface , And implement the invoke Method . When the client calls the business method of the proxy object , Proxy object execution invoke Method ,invoke Method delegate the call to targetObject, Equivalent to calling the method of the target object , stay invoke Determine permissions before method delegation , Implementation method interception .

2.3 test

public class AOPTest {
public static void main(String[] args) {
JDKProxy factory = new JDKProxy();
//Proxy by InvocationHandler The implementation class dynamically creates a proxy instance that conforms to a certain interface 
OrderService orderService = (OrderService) factory.createProxyInstance(new OrderServiceImpl("aoho"));
// By dynamically generated proxy objects orderService Agent executor 
orderService.save(UUID.randomUUID(), "aoho");
}
}

give the result as follows :

---before invoke----
call save() Method ,save:aoho
---after invoke----

3. CGLIB Bytecode generation

3.1 Class to proxy

CGLIB You can generate a proxy for the class of the interface , You can also generate proxies for classes . Example , Implementing a proxy for a class .

public class OrderManager {
private String user = null;
public OrderManager() {
}
public OrderManager(String user) {
this.setUser(user);
}
//...
public void save(UUID orderId, String name) {
System.out.println("call save() Method ,save:" + name);
}
public void update(UUID orderId, String name) {
System.out.println("call update() Method ");
}
public String getByName(String name) {
System.out.println("call getByName() Method ");
return "aoho";
}
}

The implementation of this class is the same as the interface implementation above , In order to maintain unity .

3.2 CGLIB Dynamic proxy class

public class CGLibProxy implements MethodInterceptor {
// CGLib The target object that needs the proxy 
private Object targetObject;
public Object createProxyObject(Object obj) {
this.targetObject = obj;
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(obj.getClass());
// The parameter of the callback method is the proxy class object CglibProxy, Finally, the enhanced target class calls the proxy class object CglibProxy Medium intercept Method 
enhancer.setCallback(this);
// Enhanced target class 
Object proxyObj = enhancer.create();
// Return proxy object 
return proxyObj;
}
@Override
public Object intercept(Object proxy, Method method, Object[] args,
MethodProxy methodProxy) throws Throwable {
Object obj = null;
// Tangent logic (advise), This is before the target class code is executed 
System.out.println("---before intercept----");
obj = method.invoke(targetObject, args);
System.out.println("---after intercept----");
return obj;
}
}

The above implementation of the creation of subclasses and proxy methods .getProxy(SuperClass.class) Method through the input parameter, that is, the bytecode of the parent class , Extend the parent's class To create a proxy object .intercept() Method to intercept all calls to methods of the target class ,obj Represents an instance of the target class ,method Is the reflection object of the target class method ,args Is the dynamic input parameter of the method ,methodProxy For proxy class instances .method.invoke(targetObject, args) Calling methods in the parent class through the proxy class .

3.3 test

public class AOPTest {
public static void main(String[] args) {
OrderManager order = (OrderManager) new CGLibProxy().createProxyObject(new OrderManager("aoho"));
order.save(UUID.randomUUID(), "aoho");
}

give the result as follows :

---before intercept----
call save() Method ,save:aoho
---after intercept----

4. summary

This article mainly talked about Spring Aop There are two ways to implement dynamic proxy , The advantages and disadvantages are introduced .jdk The premise of dynamic proxy application is that the target class is based on a unified interface . Without that premise ,jdk Dynamic agent cannot be applied . From this we can see that ,jdk Dynamic agent has some limitations ,cglib This kind of third-party class library realizes the application of dynamic agent more widely , And more efficient .

JDK Dynamic agent mechanism is principal mechanism , No need for third-party libraries , As long as you want to JDK The environment can act as a proxy , Dynamic implementation interface class , In the dynamically generated implementation class, the delegate is hanlder To call the original implementation class method ;CGLib Must depend on CGLib Class library of , Using the inheritance mechanism , Is the relationship between the proxy class and the inheritance of the proxy class , So the proxy class can be assigned to the proxy class , If the proxy class has an interface , Then the proxy class can also be assigned to the interface .

Reference resources

  1. jdk Dynamic agent and cglib The principle of agency
  2. AOP The underlying implementation of -CGLIB Dynamic proxy sum JDK A dynamic proxy

 

 

This article is shared from Huawei cloud community 《 Also don't understand, Spring AOP? This article takes you to understand dynamic agent 》, Original author :aoho .

 

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版权声明
本文为[Huawei cloud developer community]所创,转载请带上原文链接,感谢
https://javamana.com/2021/01/20210122114803522z.html

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