catalogue
(2) Simple implementation of static agent
(2) Simple implementation of dynamic agent
4, Principle analysis of dynamic agent
5, InvocationHandler interface and Proxy class
6, Comparison and summary of JDK dynamic agent and CGLIB dynamic agent code examples
(1) Define and create user management interface
(3) JDK agent implementation: JDK dynamic agent implements InvocationHandler interface
(5) Client call test and results
(6) JDK and CGLIB dynamic agent summary
Reference books, literature and materials
1, Agent mode
Proxy mode is a commonly used java design mode. Its feature is that the proxy class has the same interface with the delegate class. The proxy class is mainly responsible for preprocessing messages for the delegate class, filtering messages, forwarding messages to the delegate class, and post-processing messages. There is usually an association relationship between proxy class and delegate class. The object of a proxy class is associated with the object of a delegate class. The object of the proxy class itself does not really realize the service, but provides a specific service by calling the relevant methods of the object of the delegate class. In short, when accessing the actual object, we access it through the proxy object. The proxy mode introduces a certain degree of indirectness when accessing the actual object, because this indirectness can be added for a variety of purposes.
2, Static proxy
(1) Static proxy
Static proxy: it is created by programmers or automatically generated by specific tools, that is, the interface, proxy class and proxy class have been determined at compile time. The name of the proxy class before the program runs The class file has been generated.
(2) Simple implementation of static proxy
According to the class diagram of the agent mode above, write a simple example of static agent: if a classmate wants to pay the class fee to the teacher, but they all pass their money to the teacher through the monitor. Here, the monitor acts as an agent for students to pay class fees, and the monitor is the agent of students.
- 1. Determine the specific behavior of creating an interface
First, we create a Person interface. This interface is the public interface for students (proxy class) and monitor (proxy class). They all have the behavior of paying shift fees. In this way, the class fee for students can be implemented by the monitor.
- 2. The proxy object implements the interface and completes the specific business logic
The student class implements the Person interface. Student can specifically implement the action of shift handover fee:
- 3. The agent class implements the interface to complete the pre-processing messages, filtering messages, forwarding messages to the delegate class, and post-processing messages of the delegate class.
StudentsProxy class, which also implements the Person interface, but also holds a student class object. Since the Peson interface is implemented and a student object is held at the same time, he can execute the shift fee (execute the giveMoney() method) on behalf of the student object.
- 4. Client operation and analysis
Instead of directly implementing the shift fee through Zhang San (the agent), it is implemented through the monitor (the agent). This is the agent mode.
The most important thing of proxy mode is to have a public interface (Person), a specific class (Student) and a proxy class (StudentsProxy). The proxy class holds the instance of the specific class and executes the instance method of the specific class on its behalf. As mentioned above, the proxy mode introduces a certain degree of indirection when accessing actual objects, because this indirection can be used for a variety of purposes. The indirectness here means that we do not directly call the methods of the actual object, so we can add some other uses in the proxy process. For example, before joining the monitor to help Zhang San pay the shift fee, I want to reflect that Zhang San has made great progress in his recent study, which can be easily achieved through the agent mode:
You only need to perform other operations before helping Zhang San pay the shift fee in the agent class. This operation is also a great advantage of using proxy mode. The most straightforward is aspect oriented programming (AOP) in Spring. We can perform some operations before and after a pointcut. This pointcut is a method. The class of these methods must be proxied, and some other operations are cut in during the proxy process. In fact, it is also the main idea of cross-sectional thinking. A blog will be published later, and examples of how to use it. The idea is to verify the access parameters of the controller layer and necessary operations. There are many such examples in the Spring source code, and there will be time to sort them out later.
3, Dynamic agent
(1) Dynamic agent
The proxy method created by the proxy class when the program is running is called dynamic proxy.
In our static proxy example above, the proxy class (studentProxy) is self-defined and has been compiled before the program runs. However, the dynamic proxy class is not defined in Java code, but dynamically generated at runtime according to our "instructions" in Java code. Compared with static proxy, the advantage of dynamic proxy is that it can easily handle the functions of proxy classes in a unified way without modifying the methods in each proxy class. For example, you want to add a processing method before each agent's method:
There is only one giveMoney method here. Write the beforeMethod method method once, but if there are many other methods besides giveMoney, you need to write the beforeMethod method method many times, which is troublesome. Therefore, it is recommended to use dynamic proxy implementation.
(2) Simple implementation of dynamic agent
In java Lang.reflect package provides a Proxy class and an InvocationHandler interface, through which JDK dynamic Proxy classes and dynamic Proxy objects can be generated.
- 1. Determine the specific behavior of creating an interface
First, we create a Person interface. This interface is the public interface for students (proxy class) and monitor (proxy class). They all have the behavior of paying shift fees. In this way, the class fee for students can be implemented by the monitor.
- 2. The proxy object implements the interface and completes the specific business logic (some methods are added here for detection, and dynamic proxy is used later for differentiation)
The Student class implements the Person interface.
A tool class for detecting execution time is defined. The start method is invoked before any method is executed. After the execution of the finsh method, the running time of the method can be calculated. This is also the simplest method of execution time detection.
- 3. The agent class implements the interface to complete the pre-processing messages, filtering messages, forwarding messages to the delegate class, and post-processing messages of the delegate class.
Create StuInvocationHandler class and implement InvocationHandler interface. This class holds an instance target of the proxy object. There is an invoke method in InvocationHandler. All methods that execute proxy objects will be replaced with the invoke method. Execute the corresponding method of the proxy object target in the invoke method. In the proxy process, we add our own other processing before actually executing the methods of the proxy object. This is also the main principle of AOP implementation in Spring. It also involves a very important basic knowledge about java reflection.
- 4. Client operation and analysis
Zhang San created a student who needs to be proxied and passed the zhangsan object to stuHandler. When we created the proxy object stuProxy, we took stuHandler as a parameter. As mentioned above, all methods executing the proxy object will be replaced with the invoke method, that is, the invoke method in StuInvocationHandler will be executed finally.
The advantage of dynamic proxy is that it can easily handle the functions of proxy classes uniformly without modifying the methods in each proxy class. Because all the methods executed by the proxy are called through the invoke method in the InvocationHandler, we can perform the same operation on all the proxy methods as long as we handle them uniformly in the invoke method. For example, in the method timing here, all the methods executed by the proxy object will be timed. However, I only made a small amount of code.
The process of dynamic proxy and the relationship between proxy object and proxy object are not as clear and clear as static proxy. Because in the process of dynamic proxy, we don't actually see the proxy class, nor do we clearly see the specific appearance of the proxy class. In addition, the proxy object and proxy object in dynamic proxy are completed through InvocationHandler. What's the specific operation, Why do all methods executed by proxy objects execute through the invoke method in InvocationHandler. With these problems, we need to briefly analyze the source code of java Dynamic agent to find out the reason.
4, Principle analysis of dynamic agent
We use the newProxyInstance method of Proxy class to create a dynamic Proxy object. Looking at the source code of this method, we find that it only encapsulates the steps of creating a dynamic Proxy class:
The focus is on these four locations:
The most important thing to pay attention to is "class <? > cl = getProxyClass0(loader, intfs); In this sentence, the proxy class is generated here, and the constructor in the following code is also obtained through the class generated here. It can be seen that the generation of this class is the key to the whole dynamic proxy. Because it is a dynamically generated class file, I will not specifically analyze how to generate this class file here, but only need to know that this class file is stored in the java virtual machine, We can print it into the file by the following methods to see the true face:
Decompile this class file. Let's see what jdk generates for us:
jdk generates a proxy class named $Proxy0 (0 after this name is the number, and multiple proxy classes will increase at one time). This class file is placed in memory. When we create a proxy object, we obtain the construction method of this class through reflection, and then create a proxy instance. Through the view of the generated proxy class source code, we can easily see the specific process of dynamic proxy implementation.
We can think of InvocationHandler as an intermediary class, and the intermediary class holds a proxy object and invokes the corresponding method of the proxy object in the invoke method. Hold the reference of the proxy object through aggregation, and finally turn the external call to invoke into the call to the proxy object.
When the proxy class calls its own method, it calls the invoke method of the mediation class object through its own mediation class object, so as to achieve the proxy to execute the method of the proxy object. In other words, the dynamic agent realizes the specific agent function through the intermediary class.
Summary: the generated Proxy class: $Proxy0 extends Proxy implements Person. We can see that the Proxy class inherits the Proxy class, so it determines that the Java dynamic Proxy can only Proxy the interface. The inheritance mechanism of Java doomed these dynamic Proxy classes to be unable to implement the dynamic Proxy to the class. The above example of dynamic agent is actually a simple implementation of AOP. It is processed before and after the method of the target object is executed, and the time consumption of the method is counted. Spring's AOP implementation actually uses Proxy and InvocationHandler.
5, InvocationHandler interface and Proxy class
InvocationHandler interface is an interface implemented by the calling handler of proxy proxy instance. Each proxy instance has an associated calling handler; When a method is called by a proxy instance, the method call is encoded and dispatched to the invoke method of the calling handler.
See the description of InvocationHandler interface in the official document:
When we call a method through a dynamic proxy object, the call of this method will be forwarded to the invoke method implementing the InvocationHandler interface class to call, as shown in the following invoke method:
Proxy class is a class used to create a proxy object. It provides many methods, but the most commonly used method is newProxyInstance.
The function of this method is to create a proxy class object, which receives three parameters. Let's look at the meaning of the following parameters:
- loader: a classloader object that defines which classloader object loads the generated proxy class
- Interfaces: an array of interface objects, which indicates what kind of interfaces we will provide to our proxy object. If we provide such an array of interface objects, then we declare that the proxy class implements these interfaces, and the proxy class can call all the methods declared in the interface.
- h: An InvocationHandler object indicates which InvocationHandler object will be associated with when the method is called by the dynamic proxy object, and finally called by it.
6, Comparison and summary of JDK dynamic agent and CGLIB dynamic agent code examples
(1) Define and create user management interface
(2) User management implementation class, which implements the user management interface (the implementation class of the proxy)
(3) JDK agent implementation: JDK dynamic agent implements InvocationHandler interface
(4) Adopt CGLIB Proxy implementation: the asm version package needs to be imported to implement the MethodInterceptor interface
(5) Client call test and results
Operation results:
(6) JDK and CGLIB dynamic agent summary
- JDK dynamic agent can only generate agents for classes that implement interfaces, not for classes. It is implemented by Java reflection technology, and the process of generating classes is relatively efficient.
- CGLIB implements a proxy for a class. It mainly generates a subclass of a specified class and covers its methods. It is implemented using the asm bytecode framework. The related execution process is more efficient. The process of generating a class can be compensated by the cache. Because it is inheritance, it is better not to declare this class or method as final
- JDK proxy does not need the support of a third-party library. It can be used only in the JDK environment. Usage conditions: implement InvocationHandler + use proxy Newproxyinstance generates proxy object + the proxied object must implement the interface
- CGLib must rely on CGLib's class library, but it needs classes to implement any interface proxy. It generates a subclass from the specified class and overrides the methods in it. It is an inherited proxy, but it is recommended to use JDK in the environment of interface programming;
Reference books, literature and materials
1.https://blog.csdn.net/zxzzxzzxz123/article/details/69941910
2.https://www.cnblogs.com/liubin1988/p/8909610.html
3.https://blog.csdn.net/weixin_38327420/article/details/85068641 Examples of dynamic agents
4.https://www.cnblogs.com/wangenxian/p/10885309.html
5.https://www.cnblogs.com/gonjan-blog/p/6685611.html 2. Main learning ideas and sources
Welcome to the Markdown editor
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Markdown Text-to- HTML conversion tool Authors John LukeHow to create a footnote
A text with footnotes. 2
Annotations are also essential
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Gamma formula display Γ ( n ) = ( n − 1 ) ! ∀ n ∈ N \Gamma(n) = (n-1)!\quad\forall n\in\mathbb N Γ (n)=(n−1)! ∀ N ∈ N is through Euler integral
Γ ( z ) = ∫ 0 ∞ t z − 1 e − t d t . \Gamma(z) = \int_0^\infty t^{z-1}e^{-t}dt\,. Γ(z)=∫0∞tz−1e−tdt.
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Explanation of footnotes ↩︎