Analyze the working principle of Struts2 framework

Exploring the internal mechanism of the Struts2 framework

Struts2 is a popular Java Web application framework that is widely used to develop Web applications based on the MVC architecture. It has made a lot of improvements and optimizations based on Struts1, providing more powerful and flexible functions.

Before we delve into the internal mechanisms of the Struts2 framework, we need to understand some basic concepts. The core of the Struts2 framework is the MVC (Model-View-Controller) pattern, which improves code maintainability and reusability by separating concerns.

1. Core components

1. Action (action)
   Action is the most basic component of the Struts2 framework and is responsible for processing user requests. Each Action class represents a specific user request, and it contains one or more methods for handling specific requests. An Action class must inherit from the ActionSupport class and implement the execute() method, which is the entry method called by the Struts2 framework.
2. Interceptor (Interceptor)
   Interceptor is one of the very important components in the Struts2 framework. It pre-processes or post-processes requests by intercepting before, after, or at some point in the middle of the execution of the Action method. The main function of the interceptor is to extract some common functions to avoid duplication of code. The Struts2 framework has many commonly used interceptors built in, such as parameter interceptors, verification interceptors, internationalization interceptors, etc.
3. Result (result)
   Result is used to present the execution results of Action to the user. In the Struts2 framework, an Action method can return different types of Result, such as jumping to a JSP page, returning JSON data, etc. The framework will display the results to the user based on the Result configuration.
4. ValueStack (value stack)
   ValueStack is a very important core component in the Struts2 framework. It is used to store data in Action. ValueStack is a stack structure, and each Action class corresponds to a ValueStack instance. During the request processing, the Struts2 framework will push the data in the Action into the ValueStack, and take the data out of the ValueStack when displaying the results.

2. Request processing flow

1. User initiates a request
   When the user enters a URL in the browser or clicks a link, the request is sent to the server.
2. Container processing request
   Web container (such as Tomcat) will forward the request to the core controller ActionServlet of the Struts2 framework according to the configuration file.
3. ActionServlet handles requests
   ActionServlet is the core controller of the Struts2 framework. It finds the Action class and method corresponding to the request according to the configuration file, and distributes the request to the corresponding Action.
4. Interceptor interception
   Before the Action is executed, the Struts2 framework will intercept the request according to the interceptor chain defined in the configuration file. Interceptors can perform some common functions before and after Action execution, such as security verification, logging, etc.
5. Action execution
   After the interceptor chain is executed, the framework will call the execute() method of the corresponding Action class to execute specific business logic. During execution, Action can access the requested parameters, session, etc.
6. Result processing
   After the Action is executed, the framework selects the corresponding Result for result processing based on the return type of the Action method. Result can display results to users, such as jumping to JSP pages, returning JSON data, etc.
7. The results are displayed to the user
   According to the path, page and other information configured in the Result, the framework displays the results to the user.

3. Sample code

1. Action class example

public class HelloWorldAction extends ActionSupport {
    private String name;

    public String execute() throws Exception {
        return SUCCESS;
    }

    // getter和setter方法
    // ...
}

2. Configuration file example (struts.xml)

<struts>
    <package name="default" extends="struts-default">
        <action name="hello" class="com.example.HelloWorldAction">
            <result name="success">/hello.jsp</result>
        </action>
    </package>
</struts>

In this example, when the user accesses the "/hello" path, the Struts2 framework will call the execute() method of HelloWorldAction, then display the result to the user and jump to the page named hello.jsp.

Through the above examples, we briefly introduced the core components of the Struts2 framework, the request processing process, and provided some code examples. Of course, the internal mechanism of the Struts2 framework is far more than these, and there are many other functions and features that require further in-depth study and exploration. But I hope this article can give readers a basic understanding and introduction, and lay a foundation for subsequent study and practice.

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