What are the commonly used built-in objects in Servlets?

What are the built-in objects commonly used in Servlet development?

In the development of Java Servlet, there are many built-in objects that can be used to handle client requests and generate responses. These built-in objects are automatically created and managed by Servlet containers (such as Tomcat), and developers can use them directly in Servlets to complete various tasks. Some commonly used built-in objects and usage examples are introduced below.

1. HttpServletRequest
   The HttpServletRequest object encapsulates all information of the HTTP request, including the requested URL, request parameters, request headers, etc. Using the HttpServletRequest object, we can obtain and process the request data sent by the client. The following are examples of some common methods of the HttpServletRequest object:

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class MyServlet extends HttpServlet {
  
  protected void doGet(HttpServletRequest request, HttpServletResponse response) {
    // 获取请求的URL
    String url = request.getRequestURL().toString();
    
    // 获取请求的参数
    String param = request.getParameter("param");
    
    // 获取请求头
    String header = request.getHeader("User-Agent");
    
    // ... 其他操作
  }
  
}

2. HttpServletResponse
   The HttpServletResponse object is used to generate a response and send data to the client. Through the HttpServletResponse object, we can set the response status code, content type, header information, etc. The following are examples of some common methods of the HttpServletResponse object:

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class MyServlet extends HttpServlet {
  
  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
    // 设置响应的状态码
    response.setStatus(HttpServletResponse.SC_OK);
    
    // 设置响应的内容类型
    response.setContentType("text/plain");
    
    // 设置响应的头部信息
    response.setHeader("Cache-Control", "no-cache");
    
    // 向客户端发送响应数据
    response.getWriter().write("Hello world!");
  }
  
}

3. HttpSession
   The HttpSession object is used to track the user's session state. Through the HttpSession object, we can store and obtain the user's session data. The following are examples of some common methods of the HttpSession object:

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class MyServlet extends HttpServlet {
  
  protected void doGet(HttpServletRequest request, HttpServletResponse response) {
    // 获取或创建HttpSession对象
    HttpSession session = request.getSession();
    
    // 向HttpSession对象中存储数据
    session.setAttribute("username", "admin");
    
    // 从HttpSession对象中获取数据
    String username = (String) session.getAttribute("username");
    
    // 移除HttpSession对象中的数据
    session.removeAttribute("username");
  }
  
}

4. ServletContext
   The ServletContext object represents the context of the current Servlet application. Through the ServletContext object, we can access application-wide shared data and configuration information. The following are examples of some common methods of the ServletContext object:

import javax.servlet.ServletContext;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class MyServlet extends HttpServlet {

  protected void doGet(HttpServletRequest request, HttpServletResponse response) {
    // 获取ServletContext对象
    ServletContext context = getServletContext();

    // 设置应用程序范围内的共享数据
    context.setAttribute("count", 0);

    // 从应用程序范围内获取共享数据
    int count = (int) context.getAttribute("count");

    // ... 其他操作
  }

}

In Servlet development, these built-in objects are very useful tools to efficiently handle client requests and generate responses. Using these built-in objects, we can more easily develop efficient, flexible and easy-to-maintain Servlet applications. However, there are other built-in objects (such as ServletContextAttributeEvent, ServletRequestEvent, etc.) that also provide developers with more extensibility and flexibility and can be used according to specific needs.

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