Java Web 开发实战

作者: 钱老师更新: 2024-03-20 阅读: 42367 难度: 中级

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2. Servlet入门与Web开发基础

Servlet 是 Java Web 开发的核心技术，用于处理客户端请求并生成动态 Web 内容。本章将介绍 Servlet 的基本概念、生命周期和开发流程。

Maven 依赖配置

首先需要在 pom.xml 中添加 Servlet API 依赖：

pom.xml

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
    <scope>provided</scope>
</dependency>
    

第一个 Servlet 程序

下面是一个简单的 Servlet 示例：

HelloServlet.java

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {

    @Override
    public void init() throws ServletException {
        // Servlet 初始化代码
        System.out.println("HelloServlet 初始化");
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        // 设置响应内容类型
        response.setContentType("text/html;charset=UTF-8");

        // 获取请求参数
        String name = request.getParameter("name");
        if (name == null || name.trim().isEmpty()) {
            name = "访客";
        }

        // 获取输出流
        PrintWriter out = response.getWriter();

        // 生成 HTML 响应
        out.println("<!DOCTYPE html>");
        out.println("<html>");
        out.println("<head>");
        out.println("<title>Hello Servlet</title>");
        out.println("</head>");
        out.println("<body>");
        out.println("<h1>Hello, " + name + "!</h1>");
        out.println("<p>当前时间: " + new java.util.Date() + "</p>");
        out.println("<a href=\"index.jsp\">返回首页</a>");
        out.println("</body>");
        out.println("</html>");
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }

    @Override
    public void destroy() {
        // Servlet 销毁代码
        System.out.println("HelloServlet 销毁");
    }
}
    

Servlet 生命周期详解

Servlet 生命周期包括初始化、服务和销毁三个阶段：

Servlet 生命周期示例

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;

@WebServlet("/lifecycle")
public class LifecycleServlet extends HttpServlet {
    
    private int requestCount;
    
    // 1. 初始化方法
    @Override
    public void init() throws ServletException {
        super.init();
        requestCount = 0;
        System.out.println("Servlet 初始化完成，请求计数器已重置");
        
        // 获取初始化参数
        ServletConfig config = getServletConfig();
        String initParam = config.getInitParameter("appName");
        System.out.println("初始化参数 appName: " + initParam);
    }
    
    // 2. 服务方法 - 处理所有请求
    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        // 在服务方法开始前执行
        long startTime = System.currentTimeMillis();
        
        // 调用父类的service方法，它会根据请求方法调用相应的doXxx方法
        super.service(request, response);
        
        // 在服务方法结束后执行
        long endTime = System.currentTimeMillis();
        long processingTime = endTime - startTime;
        
        System.out.println("请求处理完成，耗时: " + processingTime + "ms");
    }
    
    // 3. 处理GET请求
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        // 增加请求计数
        synchronized(this) {
            requestCount++;
        }
        
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        
        out.println("<!DOCTYPE html>");
        out.println("<html>");
        out.println("<head><title>Servlet 生命周期</title></head>");
        out.println("<body>");
        out.println("<h1>Servlet 生命周期演示</h1>");
        out.println("<p>总请求次数: " + requestCount + "</p>");
        out.println("<p>Servlet 信息: " + getServletInfo() + "</p>");
        out.println("<p>服务器信息: " + getServletContext().getServerInfo() + "</p>");
        out.println("</body>");
        out.println("</html>");
    }
    
    // 4. 处理POST请求
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        // 获取表单数据
        String username = request.getParameter("username");
        String email = request.getParameter("email");
        
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        
        out.println("<!DOCTYPE html>");
        out.println("<html>");
        out.println("<head><title>表单处理结果</title></head>");
        out.println("<body>");
        out.println("<h1>表单提交成功</h1>");
        out.println("<p>用户名: " + username + "</p>");
        out.println("<p>邮箱: " + email + "</p>");
        out.println("<a href=\"lifecycle\">返回</a>");
        out.println("</body>");
        out.println("</html>");
    }
    
    // 5. 销毁方法
    @Override
    public void destroy() {
        super.destroy();
        System.out.println("Servlet 销毁，总请求次数: " + requestCount);
        System.out.println("释放资源...");
    }
    
    // 6. 获取Servlet信息
    @Override
    public String getServletInfo() {
        return "LifecycleServlet - 演示Servlet生命周期";
    }
}
    

请求和响应对象

HttpServletRequest 和 HttpServletResponse 是 Servlet 开发中最重要的两个对象：

请求和响应处理示例

import java.io.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;

@WebServlet("/request-info")
public class RequestInfoServlet extends HttpServlet {
    
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        
        out.println("<!DOCTYPE html>");
        out.println("<html>");
        out.println("<head>");
        out.println("<title>请求信息</title>");
        out.println("<style>");
        out.println("table { border-collapse: collapse; width: 100%; }");
        out.println("th, td { border: 1px solid #ddd; padding: 8px; text-align: left; }");
        out.println("th { background-color: #f2f2f2; }");
        out.println("</style>");
        out.println("</head>");
        out.println("<body>");
        out.println("<h1>HTTP 请求信息</h1>");
        
        // 基本请求信息
        out.println("<h2>基本请求信息</h2>");
        out.println("<table>");
        out.println("<tr><th>项目</th><th>值</th></tr>");
        out.println("<tr><td>请求方法</td><td>" + request.getMethod() + "</td></tr>");
        out.println("<tr><td>请求URL</td><td>" + request.getRequestURL() + "</td></tr>");
        out.println("<tr><td>请求URI</td><td>" + request.getRequestURI() + "</td></tr>");
        out.println("<tr><td>协议</td><td>" + request.getProtocol() + "</td></tr>");
        out.println("<tr><td>服务器名</td><td>" + request.getServerName() + "</td></tr>");
        out.println("<tr><td>服务器端口</td><td>" + request.getServerPort() + "</td></tr>");
        out.println("<tr><td>客户端IP</td><td>" + request.getRemoteAddr() + "</td></tr>");
        out.println("<tr><td>客户端主机</td><td>" + request.getRemoteHost() + "</td></tr>");
        out.println("<tr><td>查询字符串</td><td>" + request.getQueryString() + "</td></tr>");
        out.println("</table>");
        
        // 请求头信息
        out.println("<h2>请求头信息</h2>");
        out.println("<table>");
        out.println("<tr><th>头名称</th><th>头值</th></tr>");
        
        Enumeration<String> headerNames = request.getHeaderNames();
        while (headerNames.hasMoreElements()) {
            String headerName = headerNames.nextElement();
            String headerValue = request.getHeader(headerName);
            out.println("<tr><td>" + headerName + "</td><td>" + headerValue + "</td></tr>");
        }
        out.println("</table>");
        
        // 请求参数
        out.println("<h2>请求参数</h2>");
        out.println("<table>");
        out.println("<tr><th>参数名</th><th>参数值</th></tr>");
        
        Map<String, String[]> parameterMap = request.getParameterMap();
        for (Map.Entry<String, String[]> entry : parameterMap.entrySet()) {
            String paramName = entry.getKey();
            String[] paramValues = entry.getValue();
            out.println("<tr><td>" + paramName + "</td><td>" + 
                       String.join(", ", paramValues) + "</td></tr>");
        }
        out.println("</table>");
        
        // 属性信息
        out.println("<h2>请求属性</h2>");
        out.println("<table>");
        out.println("<tr><th>属性名</th><th>属性值</th></tr>");
        
        Enumeration<String> attributeNames = request.getAttributeNames();
        while (attributeNames.hasMoreElements()) {
            String attrName = attributeNames.nextElement();
            Object attrValue = request.getAttribute(attrName);
            out.println("<tr><td>" + attrName + "</td><td>" + attrValue + "</td></tr>");
        }
        out.println("</table>");
        
        out.println("</body>");
        out.println("</html>");
    }
}
    

Servlet 配置和注解

Servlet 3.0 引入了注解配置，简化了部署描述符的使用：

Servlet 配置示例

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;

// 使用注解配置Servlet
@WebServlet(
    name = "ConfigServlet",
    urlPatterns = { "/config", "/configure" },
    loadOnStartup = 1,
    initParams = {
        @WebInitParam(name = "databaseURL", value = "jdbc:mysql://localhost:3306/mydb"),
        @WebInitParam(name = "maxConnections", value = "100"),
        @WebInitParam(name = "timeout", value = "30")
    }
)
public class ConfigServlet extends HttpServlet {
    
    private String databaseURL;
    private int maxConnections;
    private int timeout;
    
    @Override
    public void init() throws ServletException {
        super.init();
        
        // 获取初始化参数
        ServletConfig config = getServletConfig();
        databaseURL = config.getInitParameter("databaseURL");
        maxConnections = Integer.parseInt(config.getInitParameter("maxConnections"));
        timeout = Integer.parseInt(config.getInitParameter("timeout"));
        
        System.out.println("数据库URL: " + databaseURL);
        System.out.println("最大连接数: " + maxConnections);
        System.out.println("超时时间: " + timeout);
    }
    
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        
        out.println("<!DOCTYPE html>");
        out.println("<html>");
        out.println("<head><title>Servlet 配置</title></head>");
        out.println("<body>");
        out.println("<h1>Servlet 配置信息</h1>");
        out.println("<ul>");
        out.println("<li>数据库URL: " + databaseURL + "</li>");
        out.println("<li>最大连接数: " + maxConnections + "</li>");
        out.println("<li>超时时间: " + timeout + "秒</li>");
        out.println("<li>Servlet名称: " + getServletName() + "</li>");
        out.println("</ul>");
        out.println("</body>");
        out.println("</html>");
    }
}
    

Servlet 上下文和会话管理

ServletContext 和 HttpSession 用于在多个请求间共享数据：

上下文和会话管理示例

import java.io.*;
import java.util.concurrent.atomic.AtomicInteger;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;

@WebServlet("/session-demo")
public class SessionDemoServlet extends HttpServlet {
    
    private static final String VISIT_COUNT = "visitCount";
    private static final String LAST_VISIT_TIME = "lastVisitTime";
    
    // 应用级别的访问计数器
    private AtomicInteger totalVisits;
    
    @Override
    public void init() throws ServletException {
        super.init();
        
        // 初始化应用级别的计数器
        ServletContext context = getServletContext();
        totalVisits = (AtomicInteger) context.getAttribute("totalVisits");
        if (totalVisits == null) {
            totalVisits = new AtomicInteger(0);
            context.setAttribute("totalVisits", totalVisits);
        }
    }
    
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        
        // 获取或创建会话
        HttpSession session = request.getSession(true);
        
        // 会话级别的访问计数
        Integer visitCount = (Integer) session.getAttribute(VISIT_COUNT);
        if (visitCount == null) {
            visitCount = 1;
        } else {
            visitCount++;
        }
        session.setAttribute(VISIT_COUNT, visitCount);
        session.setAttribute(LAST_VISIT_TIME, new java.util.Date());
        
        // 应用级别的访问计数
        int appVisitCount = totalVisits.incrementAndGet();
        
        out.println("<!DOCTYPE html>");
        out.println("<html>");
        out.println("<head>");
        out.println("<title>会话管理</title>");
        out.println("<style>");
        out.println(".info { background-color: #f0f8ff; padding: 10px; margin: 10px 0; }");
        out.println("</style>");
        out.println("</head>");
        out.println("<body>");
        out.println("<h1>Servlet 会话管理演示</h1>");
        
        // 会话信息
        out.println("<div class=\"info\">");
        out.println("<h2>会话信息</h2>");
        out.println("<p>会话ID: " + session.getId() + "</p>");
        out.println("<p>创建时间: " + new java.util.Date(session.getCreationTime()) + "</p>");
        out.println("<p>最后访问时间: " + new java.util.Date(session.getLastAccessedTime()) + "</p>");
        out.println("<p>最大不活动间隔: " + session.getMaxInactiveInterval() + "秒</p>");
        out.println("<p>是否新会话: " + session.isNew() + "</p>");
        out.println("</div>");
        
        // 访问统计
        out.println("<div class=\"info\">");
        out.println("<h2>访问统计</h2>");
        out.println("<p>本次会话访问次数: " + visitCount + "</p>");
        out.println("<p>最后访问时间: " + session.getAttribute(LAST_VISIT_TIME) + "</p>");
        out.println("<p>应用总访问次数: " + appVisitCount + "</p>");
        out.println("</div>");
        
        // 操作按钮
        out.println("<div>");
        out.println("<form method=\"post\" action=\"session-demo\">");
        out.println("<button type=\"submit\" name=\"action\" value=\"refresh\">刷新页面</button>");
        out.println("<button type=\"submit\" name=\"action\" value=\"invalidate\">销毁会话</button>");
        out.println("</form>");
        out.println("</div>");
        
        out.println("</body>");
        out.println("</html>");
    }
    
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        String action = request.getParameter("action");
        
        if ("invalidate".equals(action)) {
            HttpSession session = request.getSession(false);
            if (session != null) {
                session.invalidate();
            }
            response.sendRedirect("session-demo");
        } else {
            doGet(request, response);
        }
    }
}
    

过滤器 (Filter) 示例

过滤器用于在请求到达 Servlet 之前或响应发送到客户端之前进行处理：

日志过滤器示例

import java.io.*;
import javax.servlet.*;
import javax.servlet.annotation.*;
import javax.servlet.http.*;

@WebFilter("/*")
public class LoggingFilter implements Filter {
    
    private FilterConfig filterConfig;
    
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        this.filterConfig = filterConfig;
        System.out.println("LoggingFilter 初始化");
    }
    
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, 
                       FilterChain chain) throws IOException, ServletException {
        
        HttpServletRequest httpRequest = (HttpServletRequest) request;
        HttpServletResponse httpResponse = (HttpServletResponse) response;
        
        long startTime = System.currentTimeMillis();
        
        // 记录请求信息
        String clientIP = httpRequest.getRemoteAddr();
        String requestURI = httpRequest.getRequestURI();
        String method = httpRequest.getMethod();
        
        System.out.println(String.format("请求开始 - IP: %s, 方法: %s, URI: %s, 时间: %s", 
            clientIP, method, requestURI, new java.util.Date()));
        
        // 继续处理请求
        chain.doFilter(request, response);
        
        long endTime = System.currentTimeMillis();
        long processingTime = endTime - startTime;
        
        // 记录响应信息
        System.out.println(String.format("请求完成 - URI: %s, 处理时间: %dms, 状态: %d", 
            requestURI, processingTime, httpResponse.getStatus()));
    }
    
    @Override
    public void destroy() {
        System.out.println("LoggingFilter 销毁");
    }
}
    

关键知识点总结

Servlet 生命周期：init() → service() → destroy()
请求处理：doGet(), doPost(), doPut(), doDelete() 等方法
注解配置：@WebServlet, @WebFilter, @WebListener
会话管理：HttpSession 用于跟踪用户状态
上下文对象：ServletContext 用于应用级别数据共享
过滤器：用于请求和响应的预处理和后处理

最佳实践

使用注解配置简化部署
合理管理会话数据，避免存储大量数据
及时释放资源，特别是在 destroy() 方法中
使用过滤器处理通用功能（如日志、认证、编码设置）
设置合适的字符编码防止中文乱码
合理使用重定向和请求转发

注意：在实际项目中，Servlet 通常与 JSP、MVC 框架（如 Spring MVC）结合使用，以更好地分离表现层和业务逻辑。

提示： 这是一个重要的概念，需要特别注意理解和掌握。

注意： 这是一个常见的错误点，请避免犯同样的错误。

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