掌握Servlet核心技术从Hello World开始构建你的第一个Java Web应用

威震华夏关云长 · 发表于 2025-9-2 00:30:16

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1. Servlet技术概述

Servlet（Server Applet）是Java EE规范中用于扩展Web服务器功能的组件，它运行在服务器端，用于处理客户端请求并生成响应。作为Java Web开发的核心技术，Servlet提供了一种基于组件的、平台无关的方法来构建Web应用程序。

Servlet的主要优势包括：

• 高性能：Servlet在初始化后常驻内存，能够快速响应请求
• 可扩展性：可以轻松地与其他Java技术（如JDBC、JNDI等）集成
• 安全性：受益于Java的安全特性
• 平台无关性：一次编写，到处运行

2. 开发环境准备

在开始Servlet开发之前，我们需要准备以下开发环境：

2.1 安装JDK

确保你的系统上安装了Java Development Kit (JDK) 8或更高版本。你可以通过以下命令检查Java版本：

java -version

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2.2 安装IDE

推荐使用以下IDE之一进行Servlet开发：

• Eclipse IDE for Enterprise Java Developers
• IntelliJ IDEA Ultimate Edition
• Apache NetBeans

2.3 安装Web服务器

Servlet需要一个Web容器（也称为Servlet容器）来运行。最常用的选择是Apache Tomcat，你可以从官方网站下载并安装。

2.4 配置Maven

Maven是一个项目管理工具，可以帮助我们管理项目依赖。确保你的IDE中已配置Maven。

3. 创建第一个Servlet “Hello World”项目

让我们通过创建一个简单的”Hello World”Servlet项目来开始我们的学习之旅。

3.1 创建Maven项目

在IDE中创建一个新的Maven项目，并选择maven-archetype-webapp模板。

3.2 配置pom.xml

在pom.xml文件中添加Servlet API依赖：

<dependencies>
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<version>4.0.1</version>
<scope>provided</scope>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.1</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
</build>

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3.3 创建HelloWorld Servlet

在src/main/java目录下创建一个包，例如com.example.servlet，然后在该包下创建HelloWorldServlet.java文件：

package com.example.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class HelloWorldServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// 设置响应内容类型
response.setContentType("text/html");
// 获取PrintWriter对象
PrintWriter out = response.getWriter();
// 编写HTML响应
out.println("<html>");
out.println("<head>");
out.println("<title>Hello World Servlet</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1>Hello World!</h1>");
out.println("<p>This is my first Servlet application.</p>");
out.println("</body>");
out.println("</html>");
}
}

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3.4 配置web.xml

在src/main/webapp/WEB-INF目录下创建或编辑web.xml文件，添加Servlet映射：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
version="4.0">
<servlet>
<servlet-name>HelloWorldServlet</servlet-name>
<servlet-class>com.example.servlet.HelloWorldServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>HelloWorldServlet</servlet-name>
<url-pattern>/hello</url-pattern>
</servlet-mapping>
</web-app>

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3.5 部署和运行

将项目部署到Tomcat服务器并启动。然后，在浏览器中访问以下URL：

http://localhost:8080/your-webapp-context/hello

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（注意：your-webapp-context是你的Web应用程序的上下文路径，通常是项目名称）

你将看到浏览器显示”Hello World!“消息，这标志着你的第一个Servlet应用程序成功运行！

4. Servlet生命周期详解

Servlet生命周期指的是Servlet从创建到销毁的整个过程。理解Servlet生命周期对于开发高效、可靠的Web应用程序至关重要。

Servlet生命周期主要包括三个阶段：

4.1 初始化阶段

当Servlet第一次被请求时，或者服务器启动时（如果配置了load-on-startup），Servlet容器会创建Servlet实例并调用其init()方法进行初始化。

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

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init()方法只会在Servlet生命周期中被调用一次，可以在这里执行一些一次性的初始化操作，如加载配置文件、建立数据库连接等。

4.2 处理请求阶段

初始化完成后，Servlet可以处理客户端请求。对于每个请求，Servlet容器会调用Servlet的service()方法：

@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// 处理请求的代码
}

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在HttpServlet中，service()方法会根据请求类型（GET、POST、PUT等）调用相应的doGet()、doPost()、doPut()等方法。我们通常只需要重写这些方法，而不是直接重写service()方法。

4.3 销毁阶段

当Servlet容器需要卸载Servlet时（如服务器关闭或应用被移除），会调用Servlet的destroy()方法：

@Override
public void destroy() {
// 清理代码
System.out.println("Servlet is being destroyed");
}

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destroy()方法也只会在Servlet生命周期中被调用一次，可以在这里执行一些清理操作，如关闭数据库连接、释放资源等。

4.4 完整的生命周期示例

下面是一个完整的Servlet生命周期示例：

package com.example.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class LifeCycleServlet extends HttpServlet {
private int visitCount = 0;
@Override
public void init() throws ServletException {
super.init();
System.out.println("LifeCycleServlet: 初始化");
visitCount = 0;
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
visitCount++;
out.println("<html>");
out.println("<head>");
out.println("<title>Servlet生命周期示例</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1>Servlet生命周期示例</h1>");
out.println("<p>这是第 " + visitCount + " 次访问此Servlet</p>");
out.println("</body>");
out.println("</html>");
System.out.println("LifeCycleServlet: 处理GET请求，访问次数: " + visitCount);
}
@Override
public void destroy() {
System.out.println("LifeCycleServlet: 销毁，总访问次数: " + visitCount);
super.destroy();
}
}

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5. Servlet API核心接口和类

Servlet API提供了一系列接口和类，用于开发Servlet应用程序。了解这些核心接口和类对于掌握Servlet技术至关重要。

5.1 核心接口

javax.servlet.Servlet接口是所有Servlet必须直接或间接实现的核心接口。它定义了以下方法：

public interface Servlet {
void init(ServletConfig config) throws ServletException;
ServletConfig getServletConfig();
void service(ServletRequest request, ServletResponse response)
throws ServletException, IOException;
String getServletInfo();
void destroy();
}

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javax.servlet.ServletConfig接口用于获取Servlet的配置信息：

public interface ServletConfig {
String getServletName();
ServletContext getServletContext();
String getInitParameter(String name);
Enumeration<String> getInitParameterNames();
}

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javax.servlet.ServletContext接口表示Servlet应用程序的环境：

public interface ServletContext {
Object getAttribute(String name);
Enumeration<String> getAttributeNames();
void setAttribute(String name, Object object);
void removeAttribute(String name);
String getInitParameter(String name);
Enumeration<String> getInitParameterNames();
String getServletContextName();
String getContextPath();
// 还有许多其他方法...
}

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javax.servlet.ServletRequest接口封装了客户端请求的信息：

public interface ServletRequest {
Object getAttribute(String name);
Enumeration<String> getAttributeNames();
String getCharacterEncoding();
void setCharacterEncoding(String env)
throws UnsupportedEncodingException;
int getContentLength();
String getContentType();
ServletInputStream getInputStream() throws IOException;
String getParameter(String name);
Enumeration<String> getParameterNames();
String[] getParameterValues(String name);
Map<String, String[]> getParameterMap();
String getProtocol();
String getScheme();
String getServerName();
int getServerPort();
BufferedReader getReader() throws IOException;
String getRemoteAddr();
String getRemoteHost();
void setAttribute(String name, Object o);
void removeAttribute(String name);
Locale getLocale();
Enumeration<Locale> getLocales();
boolean isSecure();
RequestDispatcher getRequestDispatcher(String path);
String getRealPath(String path);
int getRemotePort();
String getLocalName();
String getLocalAddr();
int getLocalPort();
ServletContext getServletContext();
AsyncContext startAsync() throws IllegalStateException;
AsyncContext startAsync(ServletRequest servletRequest,
ServletResponse servletResponse)
throws IllegalStateException;
boolean isAsyncStarted();
boolean isAsyncSupported();
AsyncContext getAsyncContext();
DispatcherType getDispatcherType();
}

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javax.servlet.ServletResponse接口用于向客户端发送响应：

public interface ServletResponse {
String getCharacterEncoding();
String getContentType();
ServletOutputStream getOutputStream() throws IOException;
PrintWriter getWriter() throws IOException;
void setCharacterEncoding(String charset);
void setContentLength(int len);
void setContentLengthLong(long len);
void setContentType(String type);
void setBufferSize(int size);
int getBufferSize();
void flushBuffer() throws IOException;
void resetBuffer();
boolean isCommitted();
void reset();
void setLocale(Locale loc);
Locale getLocale();
}

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5.2 核心类

javax.servlet.GenericServlet是一个抽象类，实现了Servlet接口和ServletConfig接口，提供了Servlet接口中除service()方法外的所有方法的默认实现：

public abstract class GenericServlet
implements Servlet, ServletConfig, java.io.Serializable {
private transient ServletConfig config;
public GenericServlet() {}
@Override
public void destroy() {
// 默认实现为空
}
@Override
public String getInitParameter(String name) {
return getServletConfig().getInitParameter(name);
}
@Override
public Enumeration<String> getInitParameterNames() {
return getServletConfig().getInitParameterNames();
}
@Override
public ServletConfig getServletConfig() {
return config;
}
@Override
public ServletContext getServletContext() {
return getServletConfig().getServletContext();
}
@Override
public String getServletInfo() {
return "";
}
@Override
public void init(ServletConfig config) throws ServletException {
this.config = config;
this.init();
}
public void init() throws ServletException {
// 默认实现为空
}
@Override
public abstract void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException;
@Override
public String getServletName() {
return config.getServletName();
}
public void log(String msg) {
getServletContext().log(getServletName() + ": " + msg);
}
public void log(String message, Throwable t) {
getServletContext().log(getServletName() + ": " + message, t);
}
}

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javax.servlet.http.HttpServlet是一个抽象类，继承自GenericServlet类，专门用于处理HTTP请求：

public abstract class HttpServlet extends GenericServlet {
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
String protocol = req.getProtocol();
String msg = lStrings.getString("http.method_get_not_supported");
if (protocol.endsWith("1.1")) {
resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
} else {
resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
}
}
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
String protocol = req.getProtocol();
String msg = lStrings.getString("http.method_post_not_supported");
if (protocol.endsWith("1.1")) {
resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
} else {
resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
}
}
// 其他doXxx方法...
@Override
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException {
HttpServletRequest request;
HttpServletResponse response;
try {
request = (HttpServletRequest) req;
response = (HttpServletResponse) res;
} catch (ClassCastException e) {
throw new ServletException("non-HTTP request or response");
}
service(request, response);
}
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
String method = req.getMethod();
if (method.equals(METHOD_GET)) {
long lastModified = getLastModified(req);
if (lastModified == -1) {
doGet(req, resp);
} else {
long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
if (ifModifiedSince < lastModified) {
maybeSetLastModified(resp, lastModified);
doGet(req, resp);
} else {
resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
}
}
} else if (method.equals(METHOD_HEAD)) {
long lastModified = getLastModified(req);
maybeSetLastModified(resp, lastModified);
doHead(req, resp);
} else if (method.equals(METHOD_POST)) {
doPost(req, resp);
} else if (method.equals(METHOD_PUT)) {
doPut(req, resp);
} else if (method.equals(METHOD_DELETE)) {
doDelete(req, resp);
} else if (method.equals(METHOD_OPTIONS)) {
doOptions(req, resp);
} else if (method.equals(METHOD_TRACE)) {
doTrace(req, resp);
} else {
String errMsg = lStrings.getString("http.method_not_implemented");
Object[] errArgs = new Object[1];
errArgs[0] = method;
errMsg = MessageFormat.format(errMsg, errArgs);
resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
}
}
// 其他方法...
}

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6. 处理HTTP请求和响应

在Servlet开发中，处理HTTP请求和响应是最常见的任务。HttpServletRequest和HttpServletResponse接口提供了处理HTTP请求和响应的方法。

6.1 获取请求参数

HttpServletRequest提供了多种方法来获取客户端提交的参数：

package com.example.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class ParameterServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<html>");
out.println("<head>");
out.println("<title>请求参数示例</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1>请求参数示例</h1>");
out.println("<form method='post'>");
out.println("姓名: <input type='text' name='name'><br>");
out.println("年龄: <input type='text' name='age'><br>");
out.println("性别: ");
out.println("<input type='radio' name='gender' value='male'>男");
out.println("<input type='radio' name='gender' value='female'>女<br>");
out.println("爱好: ");
out.println("<input type='checkbox' name='hobbies' value='reading'>阅读");
out.println("<input type='checkbox' name='hobbies' value='sports'>运动");
out.println("<input type='checkbox' name='hobbies' value='music'>音乐<br>");
out.println("<input type='submit' value='提交'>");
out.println("</form>");
out.println("</body>");
out.println("</html>");
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// 设置请求和响应的字符编码，防止中文乱码
request.setCharacterEncoding("UTF-8");
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
// 获取单个参数值
String name = request.getParameter("name");
String age = request.getParameter("age");
String gender = request.getParameter("gender");
// 获取多个参数值（如复选框）
String[] hobbies = request.getParameterValues("hobbies");
out.println("<html>");
out.println("<head>");
out.println("<title>请求参数结果</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1>请求参数结果</h1>");
out.println("<p>姓名: " + name + "</p>");
out.println("<p>年龄: " + age + "</p>");
out.println("<p>性别: " + gender + "</p>");
out.println("<p>爱好: ");
if (hobbies != null) {
for (String hobby : hobbies) {
out.println(hobby + " ");
}
}
out.println("</p>");
out.println("<a href='parameter'>返回</a>");
out.println("</body>");
out.println("</html>");
}
}

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6.2 处理HTTP头信息

HTTP头信息包含了请求和响应的元数据。HttpServletRequest和HttpServletResponse提供了处理头信息的方法：

package com.example.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class HeaderServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
// 设置响应头
response.setHeader("Custom-Header", "CustomValue");
response.addHeader("Another-Header", "AnotherValue");
out.println("<html>");
out.println("<head>");
out.println("<title>HTTP头信息示例</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1>HTTP头信息示例</h1>");
// 显示请求头信息
out.println("<h2>请求头信息:</h2>");
out.println("<table border='1'>");
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>");
// 显示特定请求头信息
out.println("<h2>特定请求头信息:</h2>");
out.println("<p>User-Agent: " + request.getHeader("User-Agent") + "</p>");
out.println("<p>Accept: " + request.getHeader("Accept") + "</p>");
out.println("<p>Accept-Language: " + request.getHeader("Accept-Language") + "</p>");
out.println("</body>");
out.println("</html>");
}
}

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6.3 请求转发和重定向

在Web应用中，经常需要将请求转发到其他资源或者重定向到其他URL。

请求转发是在服务器端进行的，客户端浏览器不知道发生了转发，URL不会改变。

package com.example.servlet;
import java.io.IOException;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class ForwardServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// 设置请求属性
request.setAttribute("message", "这是通过请求转发传递的消息");
// 获取请求分发器
RequestDispatcher dispatcher = request.getRequestDispatcher("/target.jsp");
// 转发请求
dispatcher.forward(request, response);
}
}

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重定向是告诉客户端浏览器去请求另一个URL，URL会改变。

package com.example.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class RedirectServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// 设置会话属性
request.getSession().setAttribute("message", "这是通过重定向传递的消息");
// 重定向到目标URL
response.sendRedirect(request.getContextPath() + "/target.jsp");
}
}

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7. 使用注解配置Servlet

从Servlet 3.0开始，我们可以使用注解来配置Servlet，而无需在web.xml中进行配置。这种方式简化了开发过程。

7.1 @WebServlet注解

@WebServlet注解用于声明一个Servlet：

package com.example.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet(
name = "AnnotationServlet",
urlPatterns = {"/annotation", "/anno"},
loadOnStartup = 1,
initParams = {
@WebInitParam(name = "adminEmail", value = "admin@example.com"),
@WebInitParam(name = "adminPhone", value = "123456789")
}
)
public class AnnotationServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
// 获取初始化参数
String adminEmail = getInitParameter("adminEmail");
String adminPhone = getInitParameter("adminPhone");
out.println("<html>");
out.println("<head>");
out.println("<title>注解配置Servlet示例</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1>注解配置Servlet示例</h1>");
out.println("<p>管理员邮箱: " + adminEmail + "</p>");
out.println("<p>管理员电话: " + adminPhone + "</p>");
out.println("</body>");
out.println("</html>");
}
}

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7.2 @WebFilter注解

@WebFilter注解用于声明一个过滤器：

package com.example.filter;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
@WebFilter(
urlPatterns = {"/*"},
initParams = {
@WebInitParam(name = "encoding", value = "UTF-8")
}
)
public class EncodingFilter implements Filter {
private String encoding;
@Override
public void init(FilterConfig filterConfig) throws ServletException {
encoding = filterConfig.getInitParameter("encoding");
}
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
request.setCharacterEncoding(encoding);
response.setCharacterEncoding(encoding);
chain.doFilter(request, response);
}
@Override
public void destroy() {
// 清理代码
}
}

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7.3 @WebListener注解

@WebListener注解用于声明一个监听器：

package com.example.listener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
@WebListener
public class AppListener implements ServletContextListener, HttpSessionListener {
@Override
public void contextInitialized(ServletContextEvent sce) {
System.out.println("应用程序启动");
sce.getServletContext().setAttribute("appStartTime", System.currentTimeMillis());
}
@Override
public void contextDestroyed(ServletContextEvent sce) {
System.out.println("应用程序关闭");
}
@Override
public void sessionCreated(HttpSessionEvent se) {
System.out.println("会话创建: " + se.getSession().getId());
}
@Override
public void sessionDestroyed(HttpSessionEvent se) {
System.out.println("会话销毁: " + se.getSession().getId());
}
}

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8. 会话管理

在Web应用中，HTTP协议是无状态的，这意味着服务器不会记录之前请求的任何信息。为了在多个请求之间保持用户状态，Servlet提供了会话管理机制。

8.1 HttpSession基础

HttpSession接口提供了会话管理的方法：

package com.example.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Date;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
public class SessionServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
// 获取或创建会话
HttpSession session = request.getSession();
// 获取会话ID
String sessionId = session.getId();
// 检查是否是新会话
boolean isNew = session.isNew();
// 获取会话创建时间
long creationTime = session.getCreationTime();
// 获取最后访问时间
long lastAccessedTime = session.getLastAccessedTime();
// 设置会话属性
session.setAttribute("username", "JohnDoe");
session.setAttribute("loginTime", new Date());
// 获取会话属性
String username = (String) session.getAttribute("username");
Date loginTime = (Date) session.getAttribute("loginTime");
out.println("<html>");
out.println("<head>");
out.println("<title>会话管理示例</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1>会话管理示例</h1>");
out.println("<p>会话ID: " + sessionId + "</p>");
out.println("<p>是否是新会话: " + isNew + "</p>");
out.println("<p>会话创建时间: " + new Date(creationTime) + "</p>");
out.println("<p>最后访问时间: " + new Date(lastAccessedTime) + "</p>");
out.println("<p>用户名: " + username + "</p>");
out.println("<p>登录时间: " + loginTime + "</p>");
out.println("<p><a href='invalidate'>使会话失效</a></p>");
out.println("</body>");
out.println("</html>");
}
}

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8.2 会话超时和失效

可以设置会话的超时时间，或者手动使会话失效：

package com.example.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
public class SessionInvalidateServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
HttpSession session = request.getSession(false);
if (session != null) {
// 使会话失效
session.invalidate();
response.getWriter().println("会话已失效");
} else {
response.getWriter().println("没有活动的会话");
}
}
}

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8.3 会话配置

可以通过web.xml或注解配置会话超时时间：

<session-config>
<session-timeout>30</session-timeout>
</session-config>

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或者在Servlet中使用注解：

@WebServlet(
urlPatterns = {"/sessionConfig"},
initParams = {
@WebInitParam(name = "timeout", value = "15")
}
)
public class SessionConfigServlet extends HttpServlet {
@Override
public void init() throws ServletException {
String timeout = getInitParameter("timeout");
if (timeout != null) {
getServletContext().setSessionTimeout(Integer.parseInt(timeout) * 60);
}
}
// ...
}

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9. Servlet过滤器

Servlet过滤器可以用于拦截请求和响应，进行预处理和后处理。过滤器可以用于日志记录、安全检查、数据压缩、编码转换等场景。

9.1 创建过滤器

创建一个过滤器需要实现javax.servlet.Filter接口：

package com.example.filter;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
@WebFilter("/*")
public class LoggingFilter implements Filter {
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("LoggingFilter初始化");
}
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
HttpServletRequest httpRequest = (HttpServletRequest) request;
// 记录请求信息
System.out.println("请求URL: " + httpRequest.getRequestURL());
System.out.println("请求方法: " + httpRequest.getMethod());
System.out.println("远程地址: " + httpRequest.getRemoteAddr());
System.out.println("查询字符串: " + httpRequest.getQueryString());
long startTime = System.currentTimeMillis();
try {
// 继续处理请求
chain.doFilter(request, response);
} finally {
// 记录处理时间
long endTime = System.currentTimeMillis();
System.out.println("请求处理时间: " + (endTime - startTime) + "ms");
}
}
@Override
public void destroy() {
System.out.println("LoggingFilter销毁");
}
}

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9.2 多个过滤器的执行顺序

当有多个过滤器应用于同一个URL时，它们的执行顺序由web.xml中的配置顺序或注解的类名顺序决定。

package com.example.filter;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
@WebFilter("/*")
public class FirstFilter implements Filter {
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("FirstFilter初始化");
}
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
System.out.println("FirstFilter - 请求预处理");
// 继续处理请求
chain.doFilter(request, response);
System.out.println("FirstFilter - 响应后处理");
}
@Override
public void destroy() {
System.out.println("FirstFilter销毁");
}
}

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package com.example.filter;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
@WebFilter("/*")
public class SecondFilter implements Filter {
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("SecondFilter初始化");
}
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
System.out.println("SecondFilter - 请求预处理");
// 继续处理请求
chain.doFilter(request, response);
System.out.println("SecondFilter - 响应后处理");
}
@Override
public void destroy() {
System.out.println("SecondFilter销毁");
}
}

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9.3 过滤器配置

除了使用注解，还可以在web.xml中配置过滤器：

<filter>
<filter-name>LoggingFilter</filter-name>
<filter-class>com.example.filter.LoggingFilter</filter-class>
<init-param>
<param-name>logLevel</param-name>
<param-value>DEBUG</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>LoggingFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<filter>
<filter-name>AuthenticationFilter</filter-name>
<filter-class>com.example.filter.AuthenticationFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>AuthenticationFilter</filter-name>
<url-pattern>/secure/*</url-pattern>
</filter-mapping>

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10. Servlet监听器

Servlet监听器用于监听Web应用中的事件，如Servlet上下文的创建和销毁、会话的创建和销毁、请求的初始化等。

10.1 ServletContextListener

ServletContextListener用于监听Servlet上下文的创建和销毁事件：

package com.example.listener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
@WebListener
public class MyServletContextListener implements ServletContextListener {
@Override
public void contextInitialized(ServletContextEvent sce) {
System.out.println("Servlet上下文初始化");
// 在应用程序启动时执行初始化操作
sce.getServletContext().setAttribute("appStartTime", System.currentTimeMillis());
// 加载配置文件
// 初始化数据库连接池
// 等等
}
@Override
public void contextDestroyed(ServletContextEvent sce) {
System.out.println("Servlet上下文销毁");
// 在应用程序关闭时执行清理操作
// 关闭数据库连接池
// 保存统计数据
// 等等
}
}

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10.2 HttpSessionListener

HttpSessionListener用于监听会话的创建和销毁事件：

package com.example.listener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
import javax.servlet.annotation.WebListener;
@WebListener
public class MyHttpSessionListener implements HttpSessionListener {
private int activeSessions = 0;
@Override
public void sessionCreated(HttpSessionEvent se) {
activeSessions++;
System.out.println("会话创建: " + se.getSession().getId());
System.out.println("活动会话数: " + activeSessions);
se.getSession().getServletContext().setAttribute("activeSessions", activeSessions);
}
@Override
public void sessionDestroyed(HttpSessionEvent se) {
activeSessions--;
System.out.println("会话销毁: " + se.getSession().getId());
System.out.println("活动会话数: " + activeSessions);
se.getSession().getServletContext().setAttribute("activeSessions", activeSessions);
}
}

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10.3 ServletRequestListener

ServletRequestListener用于监听请求的创建和销毁事件：

package com.example.listener;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpServletRequest;
@WebListener
public class MyServletRequestListener implements ServletRequestListener {
@Override
public void requestInitialized(ServletRequestEvent sre) {
HttpServletRequest request = (HttpServletRequest) sre.getServletRequest();
System.out.println("请求初始化: " + request.getRequestURI());
}
@Override
public void requestDestroyed(ServletRequestEvent sre) {
HttpServletRequest request = (HttpServletRequest) sre.getServletRequest();
System.out.println("请求销毁: " + request.getRequestURI());
}
}

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10.4 属性监听器

属性监听器用于监听作用域中属性的添加、删除和替换事件：

package com.example.listener;
import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;
import javax.servlet.annotation.WebListener;
@WebListener
public class MyAttributeListener implements ServletContextAttributeListener {
@Override
public void attributeAdded(ServletContextAttributeEvent event) {
System.out.println("属性添加: " + event.getName() + " = " + event.getValue());
}
@Override
public void attributeRemoved(ServletContextAttributeEvent event) {
System.out.println("属性移除: " + event.getName() + " = " + event.getValue());
}
@Override
public void attributeReplaced(ServletContextAttributeEvent event) {
System.out.println("属性替换: " + event.getName() +
" 旧值 = " + event.getValue() +
" 新值 = " + event.getServletContext().getAttribute(event.getName()));
}
}

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11. 实际应用示例

让我们通过一个完整的用户登录示例来综合运用前面学到的Servlet技术。

11.1 数据库准备

首先，我们创建一个用户表：

CREATE DATABASE servlet_demo;
USE servlet_demo;
CREATE TABLE users (
id INT AUTO_INCREMENT PRIMARY KEY,
username VARCHAR(50) NOT NULL UNIQUE,
password VARCHAR(100) NOT NULL,
email VARCHAR(100),
created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);
INSERT INTO users (username, password, email) VALUES
('admin', 'admin123', 'admin@example.com'),
('user1', 'password1', 'user1@example.com'),
('user2', 'password2', 'user2@example.com');

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11.2 数据库连接工具类

创建一个数据库连接工具类：

package com.example.util;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class DatabaseUtil {
private static final String URL = "jdbc:mysql://localhost:3306/servlet_demo?useSSL=false&serverTimezone=UTC";
private static final String USERNAME = "root";
private static final String PASSWORD = "root";
static {
try {
Class.forName("com.mysql.cj.jdbc.Driver");
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
public static Connection getConnection() throws SQLException {
return DriverManager.getConnection(URL, USERNAME, PASSWORD);
}
public static void closeConnection(Connection connection) {
if (connection != null) {
try {
connection.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
}
}

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11.3 用户模型类

创建一个用户模型类：

package com.example.model;
import java.sql.Timestamp;
public class User {
private int id;
private String username;
private String password;
private String email;
private Timestamp createdAt;
public User() {}
public User(int id, String username, String password, String email, Timestamp createdAt) {
this.id = id;
this.username = username;
this.password = password;
this.email = email;
this.createdAt = createdAt;
}
// Getters and Setters
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public Timestamp getCreatedAt() {
return createdAt;
}
public void setCreatedAt(Timestamp createdAt) {
this.createdAt = createdAt;
}
@Override
public String toString() {
return "User [id=" + id + ", username=" + username + ", email=" + email + ", createdAt=" + createdAt + "]";
}
}

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11.4 用户DAO类

创建一个用户数据访问对象（DAO）类：

package com.example.dao;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
import com.example.model.User;
import com.example.util.DatabaseUtil;
public class UserDao {
public User findByUsername(String username) {
String sql = "SELECT * FROM users WHERE username = ?";
Connection connection = null;
PreparedStatement statement = null;
ResultSet resultSet = null;
User user = null;
try {
connection = DatabaseUtil.getConnection();
statement = connection.prepareStatement(sql);
statement.setString(1, username);
resultSet = statement.executeQuery();
if (resultSet.next()) {
user = new User();
user.setId(resultSet.getInt("id"));
user.setUsername(resultSet.getString("username"));
user.setPassword(resultSet.getString("password"));
user.setEmail(resultSet.getString("email"));
user.setCreatedAt(resultSet.getTimestamp("created_at"));
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
try {
if (resultSet != null) resultSet.close();
if (statement != null) statement.close();
if (connection != null) DatabaseUtil.closeConnection(connection);
} catch (SQLException e) {
e.printStackTrace();
}
}
return user;
}
public List<User> findAll() {
String sql = "SELECT * FROM users";
Connection connection = null;
PreparedStatement statement = null;
ResultSet resultSet = null;
List<User> users = new ArrayList<>();
try {
connection = DatabaseUtil.getConnection();
statement = connection.prepareStatement(sql);
resultSet = statement.executeQuery();
while (resultSet.next()) {
User user = new User();
user.setId(resultSet.getInt("id"));
user.setUsername(resultSet.getString("username"));
user.setPassword(resultSet.getString("password"));
user.setEmail(resultSet.getString("email"));
user.setCreatedAt(resultSet.getTimestamp("created_at"));
users.add(user);
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
try {
if (resultSet != null) resultSet.close();
if (statement != null) statement.close();
if (connection != null) DatabaseUtil.closeConnection(connection);
} catch (SQLException e) {
e.printStackTrace();
}
}
return users;
}
public boolean save(User user) {
String sql = "INSERT INTO users (username, password, email) VALUES (?, ?, ?)";
Connection connection = null;
PreparedStatement statement = null;
int result = 0;
try {
connection = DatabaseUtil.getConnection();
statement = connection.prepareStatement(sql);
statement.setString(1, user.getUsername());
statement.setString(2, user.getPassword());
statement.setString(3, user.getEmail());
result = statement.executeUpdate();
} catch (SQLException e) {
e.printStackTrace();
} finally {
try {
if (statement != null) statement.close();
if (connection != null) DatabaseUtil.closeConnection(connection);
} catch (SQLException e) {
e.printStackTrace();
}
}
return result > 0;
}
}

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11.5 认证过滤器

创建一个认证过滤器，用于保护需要登录才能访问的资源：

package com.example.filter;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
@WebFilter("/secure/*")
public class AuthenticationFilter implements Filter {
@Override
public void init(FilterConfig filterConfig) throws ServletException {
// 初始化代码
}
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
HttpServletRequest httpRequest = (HttpServletRequest) request;
HttpServletResponse httpResponse = (HttpServletResponse) response;
HttpSession session = httpRequest.getSession(false);
boolean isLoggedIn = (session != null && session.getAttribute("user") != null);
if (isLoggedIn) {
// 用户已登录，继续处理请求
chain.doFilter(request, response);
} else {
// 用户未登录，重定向到登录页面
httpResponse.sendRedirect(httpRequest.getContextPath() + "/login");
}
}
@Override
public void destroy() {
// 清理代码
}
}

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11.6 登录Servlet

创建一个处理用户登录的Servlet：

package com.example.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import com.example.dao.UserDao;
import com.example.model.User;
@WebServlet("/login")
public class LoginServlet extends HttpServlet {
private UserDao userDao;
@Override
public void init() throws ServletException {
userDao = new UserDao();
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// 如果用户已经登录，重定向到欢迎页面
HttpSession session = request.getSession(false);
if (session != null && session.getAttribute("user") != null) {
response.sendRedirect(request.getContextPath() + "/secure/welcome");
return;
}
request.getRequestDispatcher("/WEB-INF/view/login.jsp").forward(request, response);
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String username = request.getParameter("username");
String password = request.getParameter("password");
User user = userDao.findByUsername(username);
if (user != null && user.getPassword().equals(password)) {
// 登录成功
HttpSession session = request.getSession();
session.setAttribute("user", user);
// 重定向到欢迎页面
response.sendRedirect(request.getContextPath() + "/secure/welcome");
} else {
// 登录失败
request.setAttribute("errorMessage", "用户名或密码错误");
request.getRequestDispatcher("/WEB-INF/view/login.jsp").forward(request, response);
}
}
}

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11.7 登出Servlet

创建一个处理用户登出的Servlet：

package com.example.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
@WebServlet("/logout")
public class LogoutServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
HttpSession session = request.getSession(false);
if (session != null) {
session.invalidate();
}
response.sendRedirect(request.getContextPath() + "/login");
}
}

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11.8 欢迎Servlet

创建一个欢迎页面Servlet：

package com.example.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet("/secure/welcome")
public class WelcomeServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.getRequestDispatcher("/WEB-INF/view/welcome.jsp").forward(request, response);
}
}

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11.9 用户列表Servlet

创建一个显示用户列表的Servlet：

package com.example.servlet;
import java.io.IOException;
import java.util.List;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.example.dao.UserDao;
import com.example.model.User;
@WebServlet("/secure/users")
public class UserListServlet extends HttpServlet {
private UserDao userDao;
@Override
public void init() throws ServletException {
userDao = new UserDao();
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
List<User> users = userDao.findAll();
request.setAttribute("users", users);
request.getRequestDispatcher("/WEB-INF/view/userList.jsp").forward(request, response);
}
}

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11.10 注册Servlet

创建一个处理用户注册的Servlet：

package com.example.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.example.dao.UserDao;
import com.example.model.User;
@WebServlet("/register")
public class RegisterServlet extends HttpServlet {
private UserDao userDao;
@Override
public void init() throws ServletException {
userDao = new UserDao();
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.getRequestDispatcher("/WEB-INF/view/register.jsp").forward(request, response);
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String username = request.getParameter("username");
String password = request.getParameter("password");
String confirmPassword = request.getParameter("confirmPassword");
String email = request.getParameter("email");
// 验证密码是否匹配
if (!password.equals(confirmPassword)) {
request.setAttribute("errorMessage", "两次输入的密码不匹配");
request.getRequestDispatcher("/WEB-INF/view/register.jsp").forward(request, response);
return;
}
// 检查用户名是否已存在
User existingUser = userDao.findByUsername(username);
if (existingUser != null) {
request.setAttribute("errorMessage", "用户名已存在");
request.getRequestDispatcher("/WEB-INF/view/register.jsp").forward(request, response);
return;
}
// 创建新用户
User newUser = new User();
newUser.setUsername(username);
newUser.setPassword(password);
newUser.setEmail(email);
boolean success = userDao.save(newUser);
if (success) {
// 注册成功，重定向到登录页面
response.sendRedirect(request.getContextPath() + "/login?registered=true");
} else {
// 注册失败
request.setAttribute("errorMessage", "注册失败，请稍后再试");
request.getRequestDispatcher("/WEB-INF/view/register.jsp").forward(request, response);
}
}
}

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11.11 JSP视图

创建登录页面（login.jsp）：

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>登录</title>
<style>
.error {
color: red;
}
</style>
</head>
<body>
<h1>用户登录</h1>
<%-- 显示注册成功消息 --%>
<c:if test="${not empty param.registered}">
<p style="color: green;">注册成功，请登录！</p>
</c:if>
<%-- 显示错误消息 --%>
<c:if test="${not empty errorMessage}">
<p class="error">${errorMessage}</p>
</c:if>
<form action="${pageContext.request.contextPath}/login" method="post">
<div>
<label for="username">用户名:</label>
<input type="text" id="username" name="username" required>
</div>
<div>
<label for="password">密码:</label>
<input type="password" id="password" name="password" required>
</div>
<div>
<input type="submit" value="登录">
</div>
</form>
<p>还没有账号？<a href="${pageContext.request.contextPath}/register">立即注册</a></p>
</body>
</html>

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创建注册页面（register.jsp）：

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>注册</title>
<style>
.error {
color: red;
}
</style>
</head>
<body>
<h1>用户注册</h1>
<%-- 显示错误消息 --%>
<c:if test="${not empty errorMessage}">
<p class="error">${errorMessage}</p>
</c:if>
<form action="${pageContext.request.contextPath}/register" method="post">
<div>
<label for="username">用户名:</label>
<input type="text" id="username" name="username" required>
</div>
<div>
<label for="email">邮箱:</label>
<input type="email" id="email" name="email" required>
</div>
<div>
<label for="password">密码:</label>
<input type="password" id="password" name="password" required>
</div>
<div>
<label for="confirmPassword">确认密码:</label>
<input type="password" id="confirmPassword" name="confirmPassword" required>
</div>
<div>
<input type="submit" value="注册">
</div>
</form>
<p>已有账号？<a href="${pageContext.request.contextPath}/login">立即登录</a></p>
</body>
</html>

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创建欢迎页面（welcome.jsp）：

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>欢迎</title>
</head>
<body>
<h1>欢迎, ${user.username}!</h1>
<p>您的邮箱: ${user.email}</p>
<p>注册时间: ${user.createdAt}</p>
<p>
<a href="${pageContext.request.contextPath}/secure/users">查看用户列表</a> |
<a href="${pageContext.request.contextPath}/logout">退出登录</a>
</p>
</body>
</html>

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创建用户列表页面（userList.jsp）：

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>用户列表</title>
<style>
table {
border-collapse: collapse;
width: 100%;
}
th, td {
border: 1px solid #ddd;
padding: 8px;
text-align: left;
}
th {
background-color: #f2f2f2;
}
tr:nth-child(even) {
background-color: #f9f9f9;
}
</style>
</head>
<body>
<h1>用户列表</h1>
<table>
<tr>
<th>ID</th>
<th>用户名</th>
<th>邮箱</th>
<th>注册时间</th>
</tr>
<c:forEach items="${users}" var="user">
<tr>
<td>${user.id}</td>
<td>${user.username}</td>
<td>${user.email}</td>
<td>${user.createdAt}</td>
</tr>
</c:forEach>
</table>
<p>
<a href="${pageContext.request.contextPath}/secure/welcome">返回首页</a> |
<a href="${pageContext.request.contextPath}/logout">退出登录</a>
</p>
</body>
</html>

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11.12 配置web.xml

最后，配置web.xml文件：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
version="4.0">
<display-name>Servlet Demo</display-name>
<welcome-file-list>
<welcome-file>index.html</welcome-file>
</welcome-file-list>
<session-config>
<session-timeout>30</session-timeout>
</session-config>
<error-page>
<error-code>404</error-code>
<location>/WEB-INF/view/error404.jsp</location>
</error-page>
<error-page>
<error-code>500</error-code>
<location>/WEB-INF/view/error500.jsp</location>
</error-page>
</web-app>

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创建一个简单的首页（index.html）：

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Servlet Demo</title>
</head>
<body>
<h1>欢迎访问Servlet Demo</h1>
<p>
<a href="login">登录</a> |
<a href="register">注册</a>
</p>
</body>
</html>

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12. 最佳实践和常见问题

12.1 最佳实践

在Servlet开发中，推荐使用Model-View-Controller（MVC）架构模式：

• Model：负责业务逻辑和数据访问（如User类和UserDao类）
• View：负责显示数据（如JSP页面）
• Controller：接收用户输入并调用Model和View（如Servlet类）

Servlet应该只作为控制器，负责接收请求、调用业务逻辑、选择视图并返回响应。业务逻辑应该放在单独的类中。

使用Data Access Object（DAO）模式将数据库访问逻辑与业务逻辑分离，提高代码的可维护性和可测试性。

使用数据库连接池（如HikariCP、Apache DBCP等）来管理数据库连接，提高性能和资源利用率。

在执行SQL查询时，使用PreparedStatement而不是Statement，以防止SQL注入攻击。

// 不安全的方式
String sql = "SELECT * FROM users WHERE username = '" + username + "' AND password = '" + password + "'";
Statement statement = connection.createStatement();
ResultSet resultSet = statement.executeQuery(sql);
// 安全的方式
String sql = "SELECT * FROM users WHERE username = ? AND password = ?";
PreparedStatement statement = connection.prepareStatement(sql);
statement.setString(1, username);
statement.setString(2, password);
ResultSet resultSet = statement.executeQuery();

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使用try-with-resources语句自动关闭资源，避免资源泄漏。

// 传统方式
Connection connection = null;
PreparedStatement statement = null;
ResultSet resultSet = null;
try {
connection = DatabaseUtil.getConnection();
statement = connection.prepareStatement(sql);
// ...
resultSet = statement.executeQuery();
// ...
} catch (SQLException e) {
e.printStackTrace();
} finally {
try {
if (resultSet != null) resultSet.close();
if (statement != null) statement.close();
if (connection != null) connection.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
// 使用try-with-resources
try (Connection connection = DatabaseUtil.getConnection();
PreparedStatement statement = connection.prepareStatement(sql)) {
// ...
try (ResultSet resultSet = statement.executeQuery()) {
// ...
}
} catch (SQLException e) {
e.printStackTrace();
}

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使用日志框架（如Log4j、SLF4J等）记录应用程序的运行信息，便于调试和问题排查。

使用编码过滤器统一设置请求和响应的字符编码，避免中文乱码问题。

@WebFilter("/*")
public class EncodingFilter implements Filter {
private String encoding;
@Override
public void init(FilterConfig filterConfig) throws ServletException {
encoding = filterConfig.getInitParameter("encoding");
if (encoding == null) {
encoding = "UTF-8";
}
}
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
request.setCharacterEncoding(encoding);
response.setCharacterEncoding(encoding);
chain.doFilter(request, response);
}
@Override
public void destroy() {
// 清理代码
}
}

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使用Servlet的异常处理机制统一处理异常，提供友好的错误页面。

<error-page>
<exception-type>java.lang.Exception</exception-type>
<location>/WEB-INF/view/error.jsp</location>
</error-page>
<error-page>
<error-code>404</error-code>
<location>/WEB-INF/view/error404.jsp</location>
</error-page>
<error-page>
<error-code>500</error-code>
<location>/WEB-INF/view/error500.jsp</location>
</error-page>

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12.2 常见问题

中文乱码是Servlet开发中常见的问题，主要原因是字符编码不一致。解决方法：

1. 设置请求的字符编码：

request.setCharacterEncoding("UTF-8");

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1. 设置响应的字符编码：

response.setContentType("text/html;charset=UTF-8");

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1. 使用编码过滤器统一设置字符编码。

Servlet路径映射不正确会导致404错误。解决方法：

1. 检查web.xml或@WebServlet注解中的URL模式是否正确。
2. 检查请求URL是否包含正确的上下文路径。
3. 使用绝对路径（以”/“开头）而不是相对路径。

会话过早失效会导致用户需要频繁登录。解决方法：

1. 设置合适的会话超时时间：

<session-config>
<session-timeout>30</session-timeout>
</session-config>

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1. 在需要长时间保持登录的场景，使用Cookie或令牌（Token）机制。

Servlet默认是单例多线程的，如果Servlet类中包含可变的实例变量，可能会导致并发问题。解决方法：

1. 避免在Servlet中使用实例变量存储请求特定的数据。
2. 使用局部变量而不是实例变量。
3. 如果必须使用实例变量，确保它们是线程安全的（如使用同步块或并发集合）。

未正确关闭数据库连接、文件流等资源会导致资源泄漏。解决方法：

1. 使用try-with-resources语句自动关闭资源。
2. 在finally块中关闭资源。
3. 使用连接池管理数据库连接。

Servlet应用程序可能会面临性能问题。解决方法：

1. 使用缓存减少数据库访问。
2. 使用连接池管理数据库连接。
3. 优化SQL查询。
4. 使用异步处理提高吞吐量。
5. 使用CDN加速静态资源。

总结

通过本文的学习，我们从Servlet的Hello World示例开始，逐步深入了解了Servlet的核心技术，包括Servlet生命周期、Servlet API、请求和响应处理、会话管理、过滤器和监听器等内容。我们还通过一个完整的用户登录示例，综合运用了这些技术，并分享了一些最佳实践和常见问题的解决方法。

Servlet作为Java Web开发的基础技术，虽然现在有许多更高级的框架（如Spring MVC），但理解Servlet的核心原理对于掌握Java Web开发仍然至关重要。希望本文能够帮助你更好地理解和应用Servlet技术，构建出高效、可靠的Java Web应用程序。

	通知：来制造点氛围！（增加进阶任务）	03-01 22:34
	通知：本站资源由网友上传分享，如有违规等问题请到版务模块进行投诉，资源失效请在帖子内回复要求补档，会尽快处理！	10-23 09:31

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