Java Servlet初学入门教程

1. 简介

本文章基于:

Eclipse 4.6(NEON)
Servlet 3.0
Tomcat 9.0

2. Servlet是什么？

Java Servlet是在Web或应用程序服务器上运行的程序，并充当Web浏览器或其他HTTP客户端发出的请求与HTTP服务器上的数据库或应用程序之间的中间层。使用Servlet，可以通过网页表单收集用户的输入，从数据库或其他来源显示记录，并动态创建网页。

Servlet是什么

3. Servlet生命周期

下图说明了Servlet的生命周期。自创建以来，一直处理用户的请求，直到销毁为止。

Servlet生命周期

Servlet生命周期共有5个步骤:

第1步:加载Servlet类。
第2步:创建Servlet实例。
第3步:调用servlet init()方法。
第4步:调用servlets service()方法。
第5步:调用servlet destroy()方法。

最初加载Servlet时，第1、2和3步仅执行一次。默认情况下，只有在收到第一个请求后才加载servlet。不过，可以在容器启动时强制容器加载servlet。

第4步多次执行 – 对Servlet的每个HTTP请求一次都会执行第4步。当servlet容器销毁servlet时，执行第5步。

可以下图，以更好地了解Servlet的生命周期。

Servlet生命周期

当用户请求Servlet时，Servlet将调用service()方法来满足用户的需求，service()将调用doGet()或doPost()。
因此，当用户请求Servlet时，Servlet将在第一次请求时创建，并同时调用servlet的init()方法以对其进行初始化，而init()方法仅被调用一次。 destroy()方法用于销毁servlet，在删除Web应用程序的部署(取消部署)或停止Web服务器时，它将仅被调用一次。

4. 安装Tomcat Web服务器

要开始使用Servlet，需要下载Tomcat Web Server并在Eclipse中进行配置。参考以下网址:

http://www.xntutor.com/servlet/installing-and-configuring-tomcat-server-in-eclipse.html

5. 在Web项目中创建Servlet

打开Eclipse，选择:File/New/Other，创建一个动态Web项目，如下图所示:

创建Web项目

输入项目名称:ServletTutorial，如下图所示:

Java Servlet初学入门教程

点击下一步(Next>)，如下图所示:
Java Servlet初学入门教程

勾选生成web.xml，点击完成(Finish)，如下图所示:
Java Servlet初学入门教程

项目创建成功后，目录结构如下所示:
Java Servlet初学入门教程

接下来在WebContent目录中创建一个HTML文件:index.html，代码如下:

<!DOCTYPE html> <html> <head> <meta charset="UTF-8"> <title>Insert title here</title> </head> <body>     <h1>Hello World</h1>  </body> </html>

6. 配置Eclipse在Tomcat上运行应用

到目前为止，我们只是创建了一个动态Web项目。接下来要对项目进行配置，以便它可以运行Web应用程序。

在Eclipse中，右键单击项目名称:ServletTutorial，选择Properties:

Java Servlet初学入门教程

选择运行环境的版本，如下图所示:

运行环境的版本

填写服务器的名称，并选择运行服务器的安装路径，然后点击完成。如下图所示:

Java Servlet初学入门教程

返回运行时界面，选择上面配置的服务器(Apache Tomcat v9)，如下图所示:

选择服务器

右键单击项目ServletTutorial，选择“Run As / Run on Server”。如下图所示:

选择运行为...

弹出选择服务器，这里选择Tomcat V9 Server，如下图所示:

弹出选择服务器

选择发布项目，如下图所示:

网站已经在Eclipse浏览器上运行。如下图所示:

运行动态网站

工作原理

当您访问以下URL时:

http://localhost:8080/ServletTutorial/index.html

网站显示index.html页面的内容，这是可以理解的。但是，如果访问URL:

http://localhost:8080/ServletTutorial/

上面URL中并没有指定要访问的页面，因此Web服务器将查找在web.xml中声明的<welcome-file>标记中声明的默认页面以获取答案。

注意:/ServletTutorial称为上下文路径。每个网站都有一个上下文路径，可以为它配置另一个值或将其设置为空。如果为空，则可以通过以下方式访问网站:

http://localhost:8080

http://localhost:8080/index.html
当它在Tomcat中运行时，Context-Path命名为项目名称(本示例为:ServletTutorial)。

过程如下图中所示:

执行过程

7. 加入一些类

下面将向这个示例中加入一些Java类:

文件:Constants.java

package com.xntutor.tutorial.beans;  public class Constants {      public static final String ATTRIBUTE_USER_NAME_KEY = "ATTRIBUTE_USER_NAME_KEY";      public static final String SESSION_USER_KEY = "SESSION_USER_KEY";      public static final String CALLBACK_URL_KEY = "CALLBACK_URL_KEY";  }

文件:UserInfo.java

package com.xntutor.tutorial.beans;  public class UserInfo {      public String userName;     private int post;     private String country;      public UserInfo(String userName, String country, int post) {         this.userName = userName;         this.country = country;         this.post = post;     }      public int getPost() {         return post;     }      public void setPost(int post) {         this.post = post;     }      public String getCountry() {         return country;     }      public void setCountry(String country) {         this.country = country;     }      public void setUserName(String userName) {         this.userName = userName;     }      public UserInfo(String userName) {         this.userName = userName;     }      public String getUserName() {         return this.userName;     } }

8. 创建一个Servlet

项目的目录结构如下所示:

创建一个Servlet

文件:HelloServlet.java，完整代码如下:

package com.xntutor.tutorial.servlet;  import java.io.IOException;  import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;  public class HelloServlet extends HttpServlet {      private static final long serialVersionUID = 1L;      public HelloServlet() {     }      @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {          ServletOutputStream out = response.getOutputStream();          out.println("<html>");         out.println("<head><title>Hello Servlet</title></head>");          out.println("<body>");         out.println("<h3>Hello World</h3>");         out.println("This is my first Servlet");         out.println("</body>");         out.println("<html>");     }      @Override     protected void doPost(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {         this.doGet(request, response);     }  }

如果已创建的HelloServlet类代码提示有错误，可能是您还未加入Servlet库。在这里，因为使用Tomcat，因此需要对其进行声明。

在“项目”上单击鼠标右键，选择“属性”:

Java Servlet初学入门教程

然后，选择服务器运行时，如下图所示:

选择服务器运行时

选择对应的服务器版本，如下图所示:

选择服务器版本

选择服务器版本2

现在HelloServlet类代码应该没有错误提示了。
接下来，需要在Web.xml中声明HelloServlet和访问它的路径。需要添加以下配置:

<!-- Define servlet, named helloServlet --> <servlet>    <servlet-name>helloServlet</servlet-name>    <servlet-class>com.xntutor.tutorial.servlet.HelloServlet</servlet-class> </servlet>  <!-- Defines the path to access this Servlet --> <servlet-mapping>    <servlet-name>helloServlet</servlet-name>    <url-pattern>/hello</url-pattern> </servlet-mapping>

完整的web.xml文件中的代码如下所示:

<?xml version="1.0" encoding="UTF-8"?> <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"    xmlns="http://java.sun.com/xml/ns/javaee"    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"    id="WebApp_ID" version="3.0">    <display-name>ServletTutorial</display-name>      <servlet>        <servlet-name>helloServlet</servlet-name>        <servlet-class>com.xntutor.tutorial.servlet.HelloServlet</servlet-class>    </servlet>     <servlet-mapping>        <servlet-name>helloServlet</servlet-name>        <url-pattern>/hello</url-pattern>    </servlet-mapping>      <welcome-file-list>        <welcome-file>index.html</welcome-file>        <welcome-file>index.html</welcome-file>        <welcome-file>index.jsp</welcome-file>        <welcome-file>default.html</welcome-file>        <welcome-file>default.html</welcome-file>        <welcome-file>default.jsp</welcome-file>    </welcome-file-list>   </web-app>

右键单击项目(ServletTutorial)，然后选择:Run As/Run on Server，如下图所示:

右键单击项目

右键单击项目2

服务器启动完成后，使用浏览器访问以下URL:

http://localhost:8080/ServletTutorial/hello

访问结果

可以根据下面的执行流程图示来了解Servlet的工作原理。

Servlet的工作原理

调用Servlet时，根据客户端的请求方法来调用doGet(..)或doPost(..)。在doGet()或doPost()中，可以检索ServletOutputStream对象，这是将数据发送到用户浏览器的输出流。调用ServletOutputStream.println(..)将数据写入流中。如下示例代码:

// The output stream to send data to user's browser ServletOutputStream out = response.getOutputStream();

9. Servlet初始化参数

在web.xml中声明servlet时，可以为servlet设置初始化参数。参考以下Servlet实现代码。

文件:InitParamServlet.java

package com.xntutor.tutorial.servlet;  import java.io.IOException;  import javax.servlet.ServletConfig; import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;  public class InitParamServlet extends HttpServlet {      private static final long serialVersionUID = 1L;      private String emailSupport1;      public InitParamServlet() {     }      // This method is always called once after the Servlet object is created.     @Override     public void init(ServletConfig config) throws ServletException {         super.init(config);          // Get the value of the initialization parameter of the Servlet.         // (According to the Configuration of this Servlet in web.xml).         this.emailSupport1 = config.getInitParameter("emailSupport1");     }      @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {          // Get the initialization parameter's value in a different way.         String emailSupport2 = this.getServletConfig().getInitParameter("emailSupport2");          ServletOutputStream out = response.getOutputStream();          out.println("<html>");         out.println("<head><title>Init Param</title></head>");          out.println("<body>");         out.println("<h3>Init Param</h3>");         out.println("<p>emailSupport1 = " + this.emailSupport1 + "</p>");         out.println("<p>emailSupport2 = " + emailSupport2 + "</p>");         out.println("</body>");         out.println("<html>");     }      @Override     protected void doPost(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {         this.doGet(request, response);     }  }

接下来，向web.xml文件添加一些内容:

    <servlet>         <servlet-name>initParamServlet</servlet-name>         <servlet-class>com.xntutor.tutorial.servlet.InitParamServlet</servlet-class>          <init-param>             <param-name>emailSupport1</param-name>             <param-value>support1@xntutor.com</param-value>         </init-param>          <init-param>             <param-name>emailSupport2</param-name>             <param-value>support2@xntutor.com</param-value>         </init-param>      </servlet>

完整的web.xml文件内容，如下所示:

<?xml version="1.0" encoding="UTF-8"?> <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"     xmlns="http://java.sun.com/xml/ns/javaee"     xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"     id="WebApp_ID" version="3.0">     <display-name>ServletTutorial</display-name>     <!-- Define servlet, named helloServlet -->     <servlet>         <servlet-name>helloServlet</servlet-name>         <servlet-class>com.xntutor.tutorial.servlet.HelloServlet</servlet-class>     </servlet>     <servlet>         <servlet-name>initParamServlet</servlet-name>         <servlet-class>com.xntutor.tutorial.servlet.InitParamServlet</servlet-class>          <init-param>             <param-name>emailSupport1</param-name>             <param-value>support1@xntutor.com</param-value>         </init-param>          <init-param>             <param-name>emailSupport2</param-name>             <param-value>support2@xntutor.com</param-value>         </init-param>      </servlet>       <servlet-mapping>         <servlet-name>initParamServlet</servlet-name>         <url-pattern>/initParam</url-pattern>     </servlet-mapping>      <!-- Defines the path to access this Servlet -->     <servlet-mapping>         <servlet-name>helloServlet</servlet-name>         <url-pattern>/hello</url-pattern>     </servlet-mapping>     <welcome-file-list>         <welcome-file>index.html</welcome-file>         <welcome-file>index.html</welcome-file>         <welcome-file>index.jsp</welcome-file>         <welcome-file>default.html</welcome-file>         <welcome-file>default.html</welcome-file>         <welcome-file>default.jsp</welcome-file>     </welcome-file-list> </web-app>

重新运行Web应用程序并访问以下URL:

http://localhost:8080/ServletTutorial/initParam

应该会看到类似下面的结果:

运行结果

10. 使用注释配置Servlet

在Servlet 3.0或更高版本中，可以使用Annotation配置Servlet，在本项目中，我们使用Servlet 3.x版，以便可以使用Annotation进行配置。参考下面的一个例子。

文件:AnnotationExampleServlet.java

package com.xntutor.tutorial.servlet;  import java.io.IOException;  import javax.servlet.ServletConfig; import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.annotation.WebInitParam; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;  // You can configure one or multiple 'URL Patterns' can access this Servlet. @WebServlet(urlPatterns = { "/annotationExample", "/annExample" }, initParams = {         @WebInitParam(name = "emailSupport1", value = "support1@xntutor.com"),         @WebInitParam(name = "emailSupport2", value = "support2@xntutor.com") }) public class AnnotationExampleServlet extends HttpServlet {      private static final long serialVersionUID = 1L;      private String emailSupport1;      public AnnotationExampleServlet() {     }      // In any case, init() is guaranteed to be called     // before the Servlet handles its first request.     @Override     public void init(ServletConfig config) throws ServletException {         super.init(config);          this.emailSupport1 = config.getInitParameter("emailSupport1");     }      @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {          String emailSupport2 = this.getServletConfig().getInitParameter("emailSupport2");          ServletOutputStream out = response.getOutputStream();          out.println("<html>");         out.println("<head><title>Init Param</title></head>");          out.println("<body>");         out.println("<h3>Servlet with Annotation configuration</h3>");         out.println("<p>emailSupport1 = " + this.emailSupport1 + "</p>");         out.println("<p>emailSupport2 = " + emailSupport2 + "</p>");         out.println("</body>");         out.println("<html>");     }      @Override     protected void doPost(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {         this.doGet(request, response);     }  }

可以通过以下两个链接之一来访问此Servlet:

它们运行的结果相同:

使用注释配置Servlet

上面Servlet不需要在web.xml文件中配置。

11. Servlet URL模式

Servlet URL模式

有四种方法可以配置Servlet的路径:

第1种方式: /*，示例如下:

http://example.com/contextPath
http://example.com/contextPath/status/abc

第2种方式: /status/abc/*，示例如下:

http://example.com/contextPath/status/abc
http://example.com/contextPath/status/abc/mnp
http://example.com/contextPath/status/abc/mnp?date=today

第3种方式: *.map，示例如下:

http://example.com/contextPath/status/abc.map
http://example.com/contextPath/status.map?date=today

第4种方式: /，这是默认的Servlet。

当用户在浏览器上输入链接时，它将被发送到WebContainer。 WebContainer必须决定是否
Servlet将处理来自用户的请求。

下图说明了WebContainer如何决定使用Servlet来响应来自客户端的请求。

响应客户端的请求

例如，创建一个带有星号的Servlet url-pattern:

url-pattern = "/any/*";

文件:AsteriskServlet.java

package com.xntutor.tutorial.servlet;  import java.io.IOException;  import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;  @WebServlet(urlPatterns = { "/any/*" }) public class AsteriskServlet extends HttpServlet {      private static final long serialVersionUID = 1L;      public AsteriskServlet() {     }      @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {          ServletOutputStream out = response.getOutputStream();          out.println("<html>");         out.println("<head><title>Asterisk</title></head>");          out.println("<body>");          out.println("<h3>Hi, your URL match /any/*</h3>");          out.println("</body>");         out.println("<html>");     }      @Override     protected void doPost(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {         this.doGet(request, response);     }  }

以下URL由AsteriskServlet(/any/*)提供服务:

http://localhost:8080/ServletTutorial/any/path1/a.html
http://localhost:8080/ServletTutorial/any/something
http://localhost:8080/ServletTutorial/any

访问上面URL，可以看到以下结果:

默认的Servlet

默认Servlet

作为默认Servlet，Servlet将用于处理路径与应用程序中声明的Servlet的url-pattern不匹配的请求。下面来看看默认servlet的示例:

文件:MyDefaultServlet.java

package com.xntutor.tutorial.servlet;  import java.io.IOException;  import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;  @WebServlet(urlPatterns = { "/" }) public class MyDefaultServlet extends HttpServlet {     private static final long serialVersionUID = 1L;     public MyDefaultServlet() {    }     @Override    protected void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {         ServletOutputStream out = response.getOutputStream();         out.println("<html>");        out.println("<head><title>Page not found</title></head>");         out.println("<body>");        out.println("<h3>Sorry! Page not found</h3>");        out.println("<h1>404</h1>");        out.println("Message from servlet: " + this.getClass().getName());        out.println("</body>");        out.println("<html>");    }     @Override    protected void doPost(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        this.doGet(request, response);    }  }

重新运行Web应用程序，并访问URL:

http://localhost:8080/ServletTutorial/news/someonewho

上面的链接servletPath = /news/tomAndJerry，与声明的url模式不匹配。但是“默认Servlet”满足该请求。

默认Servlet

12. 获取Servlet的基本信息

有时因为某种需要，想了解Servlet实例提供的时间有关的信息，这些信息包括:

来自客户端的信息请求。
服务器信息。
客户资料。
标头信息按要求发送。
…

创建一个Servlet类文件:ExampleInfoServlet.java

package com.xntutor.tutorial.servlet;  import java.io.IOException; import java.util.Enumeration;  import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;  @WebServlet("/other/exampleInfo") public class ExampleInfoServlet extends HttpServlet {      private static final long serialVersionUID = 1L;      public ExampleInfoServlet() {         super();     }      @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {         ServletOutputStream out = response.getOutputStream();          out.println("<style> span {color:blue;} </style>");          String requestURL = request.getRequestURL().toString();         out.println("<br><span>requestURL:</span>");         out.println(requestURL);          String requestURI = request.getRequestURI();         out.println("<br><span>requestURI:</span>");         out.println(requestURI);          String contextPath = request.getContextPath();         out.println("<br><span>contextPath:</span>");         out.println(contextPath);          out.println("<br><span>servletPath:</span>");         String servletPath = request.getServletPath();         out.println(servletPath);          String queryString = request.getQueryString();         out.println("<br><span>queryString:</span>");         out.println(queryString);          String param1 = request.getParameter("text1");         out.println("<br><span>getParameter text1:</span>");         out.println(param1);          String param2 = request.getParameter("text2");         out.println("<br><span>getParameter text2:</span>");         out.println(param2);          // Server Infos         out.println("<br><br><b>Server info:</b>");          out.println("<br><span>serverName:</span>");         String serverName = request.getServerName();         out.println(serverName);          out.println("<br><span>serverPort:</span>");         int serverPort = request.getServerPort();         out.println(serverPort + "");          // Client Infos         out.println("<br><br><b>Client info:</b>");          out.println("<br><span>remoteAddr:</span>");         String remoteAddr = request.getRemoteAddr();         out.println(remoteAddr);          out.println("<br><span>remoteHost:</span>");         String remoteHost = request.getRemoteHost();         out.println(remoteHost);          out.println("<br><span>remoteHost:</span>");         int remotePort = request.getRemotePort();         out.println(remotePort + "");          out.println("<br><span>remoteUser:</span>");         String remoteUser = request.getRemoteUser();         out.println(remoteUser);          // Header Infos         out.println("<br><br><b>headers:</b>");          Enumeration<String> headers = request.getHeaderNames();         while (headers.hasMoreElements()) {             String header = headers.nextElement();             out.println("<br><span>" + header + "</span>: " + request.getHeader(header));         }          // Servlet Context info:         out.println("<br><br><b>Servlet Context info:</b>");         ServletContext servletContext = request.getServletContext();          // Location of web application in hard disk         out.println("<br><span>realPath:</span>");         String realPath = servletContext.getRealPath("");         out.println(realPath);     }      @Override     protected void doPost(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {         this.doGet(request, response);     }  }

请参阅下图以了解一些您感兴趣的信息:

重新运行Web应用程序并访问链接:

http://localhost:8080/ServletTutorial/other/exampleInfo?text1=Hello&text2=World

得到以下结果:

获取Servlet的基本信息

13. Servlet转发

转发(Forward):向Servlet发送请求时，它可以将请求转发到另一个页面(或另一个Servlet)。用户浏览器上的地址仍然是第一页的链接，但是内容是由转发的页面创建的。

请求的页面被转发到位于Web应用程序上的页面(或servlet)。使用转发，可以使用request.setAttribute()将数据从第1页传输到第2页。

Servlet转发

创建一个Servlet类文件:ForwardDemoServlet.java

import java.io.IOException;  import javax.servlet.RequestDispatcher; import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;  import com.xntutor.tutorial.beans.Constants;  @WebServlet("/other/forwardDemo") public class ForwardDemoServlet extends HttpServlet {     private static final long serialVersionUID = 1L;      // Request:     // http://localhost:8080/ServletTutorial/other/forwardDemo?forward=true     @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {          // Get value of parameter on URL.         String forward = request.getParameter("forward");          if ("true".equals(forward)) {             System.out.println("Forward to ShowMeServlet");              // Set data to attribute of the request.             request.setAttribute(Constants.ATTRIBUTE_USER_NAME_KEY, //                     "Hi, I'm Tom come from Walt Disney !");              RequestDispatcher dispatcher //                     = request.getServletContext().getRequestDispatcher("/showMe");             dispatcher.forward(request, response);              return;         }         ServletOutputStream out = response.getOutputStream();         out.println("<h3>Text of ForwardDemoServlet</h3>");         out.println("- servletPath=" + request.getServletPath());     }      @Override     protected void doPost(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {         this.doGet(request, response);     }  }

重新运行Web应用并分别访问以下2个URL:

情况1: 没有转发，页面的数据由ForwardDemoServlet生成。如下图所示:

情况2:向前转移到ShowMeServlet。在这种情况下，页面上的URL不变，而页面的数据是由ShowMeServlet生成的。如下图所示:

在某些情况下(例如，当用户请求Servlet-A时)通常使用转发。但是，此页面必须先登录。Servlet-A会检查用户的登录，如果尚未登录，它将移至Servlet-Login。回到RequestDispatcher，有两种获取RequestDispatcher对象的方法。

RequestDispatcher对象

request.getServletContext ().getRequestDispatcher(url)返回RequestDispatcher对象，它是位于相对于contextPath(相对于网站根目录的位置)。

request.getRequestDispatcher(url)返回RequestDispatcher对象，它是相对于当前页面位置。

http://localhost:8080/ServletTutorial/other/forwardDemo

注意:

重定向允许导航到网站以外的页面。

转发仅允许移动到网站内的页面，并且可以通过request.setAttribute在两个页面之间传输数据。

14. Servlet重定向

重定向:当用户向Servlet(页面A)发出请求时，Servlet可以将请求重定向到另一个页面(页面B)，并结束其任务。页面被重定向到应用程序中的页面，也可以Web应用以外是任何页面。

不同与转发，使用重定向不能使用request.setAttribute(..)将数据从页面A传输到页面B。

重定向

创建一个Servlet类文件:ShowMeServlet.java

package com.xntutor.tutorial.servlet.other;  import java.io.IOException;  import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;  import com.xntutor.tutorial.beans.Constants;   @WebServlet("/showMe") public class ShowMeServlet extends HttpServlet {     private static final long serialVersionUID = 1L;      @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {          // Get value of an attribute of the request.         String value = (String) request.getAttribute(Constants.ATTRIBUTE_USER_NAME_KEY);          ServletOutputStream out = response.getOutputStream();          out.println("<h1>ShowMeServlet</h1>");         out.println(value);     }      @Override     protected void doPost(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {         this.doGet(request, response);     }  }

创建另一个Servlet类文件:RedirectDemoServlet.java

package com.xntutor.tutorial.servlet.other;  import java.io.IOException;  import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;  @WebServlet("/other/redirectDemo") public class RedirectDemoServlet extends HttpServlet {      private static final long serialVersionUID = 1L;      // Request:     // http://localhost:8080/ServletTutorial/other/redirectDemo?redirect=true     @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {          // Get the value of parameter on the URL.         String redirect = request.getParameter("redirect");          if ("true".equals(redirect)) {             System.out.println("Redirect to ShowMeServlet");              // contextPath: Is an empty string "" or non-empty.             // If it is non-empty, it always starts with /             // and does not ends with /             String contextPath = request.getContextPath();              // ==> /ServletTutorial/showMe             response.sendRedirect(contextPath + "/showMe");             return;         }          ServletOutputStream out = response.getOutputStream();         out.println("<h3>Text of RedirectDemoServlet</h3>");         out.println("- servletPath=" + request.getServletPath());     }      @Override     protected void doPost(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {         this.doGet(request, response);     }  }

重新运行服务器并在浏览器中分别访问以下2个URL:

访问第一个URL显示内容如下:

访问第二个URL，请求将重定向到ShowMeServlet。在浏览器上看到的URL是Servlet ShowMeServlet的路径。显示内容如下:

第二个URL

15. Servlet会话

HttpSession对象代表一个用户会话。用户会话包含有关多个HTTP请求的用户信息。当用户首次进入网站时，Servlet会为用户提供唯一的ID来标识其会话依据。此ID通常存储在cookie或请求参数中。

下面是访问会话对象的方式:

protected void doGet(HttpServletRequest request,    HttpServletResponse response)        throws ServletException, IOException {     HttpSession session = request.getSession(); }

可以将值存储在会话对象中，并在以后检索(读取)它们。首先，让我们看看如何在会话对象中存储值:

// Get the HttpSession object. HttpSession session = request.getSession();  // Suppose user has successfully logged. UserInfo loginedInfo = new UserInfo("Maxsu", "China", 5);  // Store user information into a Session attribute. // You can retrieve this information using the getAttribute method. session.setAttribute(Constants.SESSION_USER_KEY, loginedInfo);

并检索(读取)存储在Session中特定页面的信息。

// Get the HttpSession object. HttpSession session = request.getSession();  // Get the UserInfo object stored to the session  // after the user login successfully. UserInfo loginedInfo      = (UserInfo) session.getAttribute(Constants.SESSION_USER_KEY);

下面通过一个完整的示例，演示创建使用会话。

Servlet类文件:LoginServlet.java

package com.xntutor.tutorial.servlet.session;  import java.io.IOException;  import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; 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.xntutor.tutorial.beans.Constants; import com.xntutor.tutorial.beans.UserInfo;  @WebServlet(urlPatterns = { "/login" }) public class LoginServlet extends HttpServlet {      private static final long serialVersionUID = 1L;      public LoginServlet() {     }      @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {          ServletOutputStream out = response.getOutputStream();          // Get HttpSession object         HttpSession session = request.getSession();          // Suppose a user has successfully logged.         UserInfo loginedInfo = new UserInfo("Maxsu", "China", 5);          // Storing user information in an attribute of Session.         session.setAttribute(Constants.SESSION_USER_KEY, loginedInfo);          out.println("<html>");         out.println("<head><title>Session example</title></head>");          out.println("<body>");         out.println("<h3>You are logined!, info stored in session</h3>");          out.println("<a href='userInfo'>View User Info</a>");         out.println("</body>");         out.println("<html>");     }  }

Servlet类文件:LoginServlet.java

package com.xntutor.tutorial.servlet.session;  import java.io.IOException;  import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; 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.xntutor.tutorial.beans.Constants; import com.xntutor.tutorial.beans.UserInfo;  @WebServlet(urlPatterns = { "/userInfo" }) public class UserInfoServlet extends HttpServlet {      private static final long serialVersionUID = 1L;      public UserInfoServlet() {     }      @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {          ServletOutputStream out = response.getOutputStream();          // Get HttpSession object.         HttpSession session = request.getSession();          // Get UserInfo object stored in session after user login successful.         UserInfo loginedInfo = (UserInfo) session.getAttribute(Constants.SESSION_USER_KEY);          // If not logined, redirect to login page (LoginServlet).         if (loginedInfo == null) {             // ==> /ServletTutorial/login             response.sendRedirect(this.getServletContext().getContextPath() + "/login");             return;         }          out.println("<html>");         out.println("<head><title>Session example</title></head>");          out.println("<body>");          out.println("<h3>User Info:</h3>");          out.println("<p>User Name:" + loginedInfo.getUserName() + "</p>");         out.println("<p>Country:" + loginedInfo.getCountry() + "</p>");         out.println("<p>Post:" + loginedInfo.getPost() + "</p>");          out.println("</body>");         out.println("<html>");     }  }

重新运行服务器并在浏览器中以下URL:

http://localhost:8080/ServletTutorial/login

运行结果

点击上面链接查看登录的用户信息，结果如下所示:

查看登录的用户信息

原创文章，作者：ItWorker，如若转载，请注明出处：https://blog.ytso.com/tech/pnotes/264052.html

Java Servlet初学入门教程

1. 简介

2. Servlet是什么？

3. Servlet生命周期

4. 安装Tomcat Web服务器

5. 在Web项目中创建Servlet

6. 配置Eclipse在Tomcat上运行应用

7. 加入一些类

8. 创建一个Servlet

9. Servlet初始化参数

10. 使用注释配置Servlet

11. Servlet URL模式

12. 获取Servlet的基本信息

13. Servlet转发

14. Servlet重定向

15. Servlet会话

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