Common Servlet Methods

Servlets are core components in Java web development for handling requests, operating under a strict "create once, use multiple times" lifecycle model. Understanding this mechanism is fundamental to mastering servlet programming. The servlet container (such as Tomcat) manages the creation, loading, initialization, request servicing, and final destruction of Servlet instances.

Before a Servlet is invoked for the first time, the container executes an initialization phase. During this stage, the init() method is called once to complete necessary preparatory work. Developers typically do not handle specific business logic directly in this phase but rather perform global configurations or resource preparations. For example, loading configuration files, initializing connection pools, or setting response header information. Since init() executes only once, it is ideal for storing data that remains constant throughout the entire Servlet lifecycle.
When a client initiates an HTTP request, the servlet container forwards the request to the corresponding Servlet instance. At this point, the service() method is triggered; it serves as the bridge connecting the Servlet with the web container. Internally, the service() method automatically calls the appropriate handler based on the request type: if it is a GET request, it invokes doGet(), and if it is a POST request, it invokes doPost(), and so on.

When writing business logic, developers typically override specific HTTP methods rather than directly overriding the service() method. This approach not only results in clearer code but also avoids the tedious work of repeatedly judging the request type. Within the doGet() or doPost() methods, the primary tasks include: reading request parameters, processing business logic, generating response content (such as HTML, JSON, etc.), and returning data to the client via the response object.

Detailed Analysis and Application Scenarios of Common Methods

The Servlet API provides a series of standard methods, each with specific uses and best practice scenarios. Below is an in-depth analysis of several key methods:

init() Method
This method executes once after the Servlet instance is created but before it is called for the first time. It receives a ServletConfig object as a parameter, which contains initialization parameters (such as content defined by <init-param> tags).
```
public void init(ServletConfig config) throws ServletException {
    String dbUrl = config.getInitParameter("database.url");
    // Initialize database connection pool or other one-time configurations here
}
```

service() Method
This method is called every time a request is received. It first checks the request type and then dispatches to specific handler functions.

public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    if ("GET".equals(request.getMethod())) {
        doGet(request, response);
    } else if ("POST".equals(request.getMethod())) {
        doPost(request, response);
    }
}

doGet() and doPost() Methods
These methods are used to handle specific HTTP requests. doGet() is typically used for retrieving resources (such as displaying pages), while doPost() is commonly used for submitting data or triggering state changes.
- Example: Receiving form data in doPost() and saving it to a database.
destroy() Method
When the servlet container decides to destroy this instance, it calls this method. At this point, all resources initialized by init() but not yet released (such as database connections, thread pools, etc.) must be cleaned up here to prevent memory leaks.
```
public void destroy() {
    // Close resources, such as disconnecting from the database
}
```

Best Practices and Precautions

In actual development, adhering to certain standards can significantly improve code maintainability and robustness. First, ensure that all non-static resources are correctly released in the destroy() method, especially those objects occupying system resources (such as JDBC connections, file streams, etc.). Second, neither the init() nor the destroy() methods should contain any business logic capable of throwing exceptions, as they are primarily used for lifecycle management and may not be executed multiple times under extreme circumstances.

Furthermore, to enhance code reusability and testability, it is recommended to extract general-purpose business logic into independent classes or utility methods rather than writing them directly within the Servlet's lifecycle methods. Additionally, pay attention to using try-catch-finally blocks to ensure resources are properly closed, preventing resource leaks caused by exceptions. By strictly following these guidelines, developers can build more stable and efficient web applications.