Java Web学习总结（3）——Servlet详解

比眉伴天荒 2022-01-13 04:39 232阅读 0赞

[2019独角兽企业重金招聘Python工程师标准>>> ][2019_Python_] ![hot3.png][]

## 一、ServletConfig讲解 ##

### 1.1、配置Servlet初始化参数 ###

在Servlet的配置文件web.xml中，可以使用一个或多个<init-param>标签为servlet配置一些初始化参数。

例如：

![复制代码][20140736_ztTg.gif]

1 <servlet>
     2     <servlet-name>ServletConfigDemo1</servlet-name>
     3     <servlet-class>gacl.servlet.study.ServletConfigDemo1</servlet-class>
     4     
     5     <init-param>
     6         <param-name>name</param-name>
     7         <param-value>gacl</param-value>
     8     </init-param>
     9      <init-param>
    10         <param-name>password</param-name>
    11         <param-value>123</param-value>
    12     </init-param>
    13     <init-param>
    14         <param-name>charset</param-name>
    15         <param-value>UTF-8</param-value>
    16     </init-param>
    17 </servlet>

![复制代码][20140736_ztTg.gif]

### 1.2、通过ServletConfig获取Servlet的初始化参数 ###

当servlet配置了初始化参数后，web容器在创建servlet实例对象时，会自动将这些初始化参数封装到ServletConfig对象中，并在调用servlet的init方法时，将ServletConfig对象传递给servlet。进而，我们通过ServletConfig对象就可以得到当前servlet的初始化参数信息。

例如：

![复制代码][20140736_ztTg.gif]

1 package gacl.servlet.study; 2 
     3 import java.io.IOException; 4 import java.util.Enumeration; 5 import javax.servlet.ServletConfig; 6 import javax.servlet.ServletException; 7 import javax.servlet.http.HttpServlet; 8 import javax.servlet.http.HttpServletRequest; 9 import javax.servlet.http.HttpServletResponse; 10 
    11 public class ServletConfigDemo1 extends HttpServlet { 12 
    13     /**
    14 * 定义ServletConfig对象来接收配置的初始化参数 15      */
    16     private ServletConfig config; 17     
    18     /**
    19 * 当servlet配置了初始化参数后，web容器在创建servlet实例对象时， 20 * 会自动将这些初始化参数封装到ServletConfig对象中，并在调用servlet的init方法时， 21 * 将ServletConfig对象传递给servlet。进而，程序员通过ServletConfig对象就可以 22 * 得到当前servlet的初始化参数信息。 23      */
    24 @Override 25     public void init(ServletConfig config) throws ServletException { 26         this.config = config; 27 } 28 
    29     public void doGet(HttpServletRequest request, HttpServletResponse response) 30             throws ServletException, IOException { 31         //获取在web.xml中配置的初始化参数
    32         String paramVal = this.config.getInitParameter("name");//获取指定的初始化参数
    33 response.getWriter().print(paramVal); 34         
    35         response.getWriter().print("<hr/>"); 36         //获取所有的初始化参数
    37         Enumeration<String> e = config.getInitParameterNames(); 38         while(e.hasMoreElements()){ 39             String name = e.nextElement(); 40             String value = config.getInitParameter(name); 41             response.getWriter().print(name + "=" + value + "<br/>"); 42 } 43 } 44 
    45     public void doPost(HttpServletRequest request, HttpServletResponse response) 46             throws ServletException, IOException { 47         this.doGet(request, response); 48 } 49 
    50 }

![复制代码][20140736_ztTg.gif]

运行结果如下：

## 二、ServletContext对象 ##

WEB容器在启动时，它会为每个WEB应用程序都创建一个对应的ServletContext对象，它代表当前web应用。  
　　ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，可以通过ServletConfig.getServletContext方法获得ServletContext对象。  
　　由于一个WEB应用中的所有Servlet共享同一个ServletContext对象，因此Servlet对象之间可以通过ServletContext对象来实现通讯。ServletContext对象通常也被称之为context域对象。

## 三、ServletContext的应用 ##

### 　　3.1、多个Servlet通过ServletContext对象实现数据共享 ###

范例：ServletContextDemo1和ServletContextDemo2通过ServletContext对象实现数据共享

![复制代码][20140736_ztTg.gif]

1 package gacl.servlet.study; 2 
     3 import java.io.IOException; 4 import javax.servlet.ServletContext; 5 import javax.servlet.ServletException; 6 import javax.servlet.http.HttpServlet; 7 import javax.servlet.http.HttpServletRequest; 8 import javax.servlet.http.HttpServletResponse; 9 
    10 public class ServletContextDemo1 extends HttpServlet { 11 
    12     public void doGet(HttpServletRequest request, HttpServletResponse response) 13             throws ServletException, IOException { 14         String data = "xdp_gacl"; 15         /**
    16 * ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时， 17 * 可以通过ServletConfig.getServletContext方法获得ServletContext对象。 18          */
    19         ServletContext context = this.getServletConfig().getServletContext();//获得ServletContext对象
    20         context.setAttribute("data", data);  //将data存储到ServletContext对象中
    21 } 22 
    23     public void doPost(HttpServletRequest request, HttpServletResponse response) 24             throws ServletException, IOException { 25 doGet(request, response); 26 } 27 }

![复制代码][20140736_ztTg.gif]

1 package gacl.servlet.study; 2 
     3 import java.io.IOException; 4 import javax.servlet.ServletContext; 5 import javax.servlet.ServletException; 6 import javax.servlet.http.HttpServlet; 7 import javax.servlet.http.HttpServletRequest; 8 import javax.servlet.http.HttpServletResponse; 9 
    10 public class ServletContextDemo2 extends HttpServlet { 11 
    12     public void doGet(HttpServletRequest request, HttpServletResponse response) 13             throws ServletException, IOException { 14         ServletContext context = this.getServletContext(); 15         String data = (String) context.getAttribute("data");//从ServletContext对象中取出数据
    16         response.getWriter().print("data="+data); 17 } 18 
    19     public void doPost(HttpServletRequest request, HttpServletResponse response) 20             throws ServletException, IOException { 21 doGet(request, response); 22 } 23 }

![复制代码][20140736_ztTg.gif]

先运行ServletContextDemo1，将数据data存储到ServletContext对象中，然后运行ServletContextDemo2就可以从ServletContext对象中取出数据了，这样就实现了数据共享，如下图所示：

### 　　3.2、获取WEB应用的初始化参数 ###

在web.xml文件中使用<context-param>标签配置WEB应用的初始化参数，如下所示：

![复制代码][20140736_ztTg.gif]

1 <?xml version="1.0" encoding="UTF-8"?>
     2 <web-app version="3.0" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 3 http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
     4     <display-name></display-name>
     5     
     6     <context-param>
     7         <param-name>url</param-name>
     8         <param-value>jdbc:mysql://localhost:3306/test</param-value>
     9     </context-param>
    10 
    11     <welcome-file-list>
    12         <welcome-file>index.jsp</welcome-file>
    13     </welcome-file-list>
    14 </web-app>

![复制代码][20140736_ztTg.gif]

获取Web应用的初始化参数，代码如下：

![复制代码][20140736_ztTg.gif]

1 package gacl.servlet.study; 2 
     3 import java.io.IOException; 4 import javax.servlet.ServletContext; 5 import javax.servlet.ServletException; 6 import javax.servlet.http.HttpServlet; 7 import javax.servlet.http.HttpServletRequest; 8 import javax.servlet.http.HttpServletResponse; 9 
    10 
    11 public class ServletContextDemo3 extends HttpServlet { 12 
    13     public void doGet(HttpServletRequest request, HttpServletResponse response) 14             throws ServletException, IOException { 15 
    16         ServletContext context = this.getServletContext(); 17         //获取整个web站点的初始化参数
    18         String contextInitParam = context.getInitParameter("url"); 19 response.getWriter().print(contextInitParam); 20 } 21 
    22     public void doPost(HttpServletRequest request, HttpServletResponse response) 23             throws ServletException, IOException { 24 doGet(request, response); 25 } 26 
    27 }

![复制代码][20140736_ztTg.gif]

运行结果：

### 　　3.3、用servletContext实现请求转发 ###

ServletContextDemo4

![复制代码][20140736_ztTg.gif]

1 package gacl.servlet.study; 2 
     3 import java.io.IOException; 4 import java.io.PrintWriter; 5 import javax.servlet.RequestDispatcher; 6 import javax.servlet.ServletContext; 7 import javax.servlet.ServletException; 8 import javax.servlet.http.HttpServlet; 9 import javax.servlet.http.HttpServletRequest; 10 import javax.servlet.http.HttpServletResponse; 11 
    12 public class ServletContextDemo4 extends HttpServlet { 13 
    14     public void doGet(HttpServletRequest request, HttpServletResponse response) 15             throws ServletException, IOException { 16         String data = "<h1><font color='red'>abcdefghjkl</font></h1>"; 17 response.getOutputStream().write(data.getBytes()); 18         ServletContext context = this.getServletContext();//获取ServletContext对象
    19         RequestDispatcher rd = context.getRequestDispatcher("/servlet/ServletContextDemo5");//获取请求转发对象(RequestDispatcher)
    20         rd.forward(request, response);//调用forward方法实现请求转发
    21 } 22 
    23     public void doPost(HttpServletRequest request, HttpServletResponse response) 24             throws ServletException, IOException { 25 } 26 }

![复制代码][20140736_ztTg.gif]

ServletContextDemo5

![复制代码][20140736_ztTg.gif]

1 package gacl.servlet.study; 2 
     3 import java.io.IOException; 4 import javax.servlet.ServletException; 5 import javax.servlet.http.HttpServlet; 6 import javax.servlet.http.HttpServletRequest; 7 import javax.servlet.http.HttpServletResponse; 8 
     9 public class ServletContextDemo5 extends HttpServlet { 10 
    11     public void doGet(HttpServletRequest request, HttpServletResponse response) 12             throws ServletException, IOException { 13         response.getOutputStream().write("servletDemo5".getBytes()); 14 } 15 
    16     public void doPost(HttpServletRequest request, HttpServletResponse response) 17             throws ServletException, IOException { 18         this.doGet(request, response); 19 } 20 
    21 }

![复制代码][20140736_ztTg.gif]

运行结果：

访问的是ServletContextDemo4，浏览器显示的却是ServletContextDemo5的内容，这就是使用ServletContext实现了请求转发

### 　　3.4、利用ServletContext对象读取资源文件 ###

项目目录结构如下：

### 代码范例：使用servletContext读取资源文件 ###

![复制代码][20140736_ztTg.gif]

1 package gacl.servlet.study; 2 
     3 import java.io.FileInputStream; 4 import java.io.FileNotFoundException; 5 import java.io.IOException; 6 import java.io.InputStream; 7 import java.text.MessageFormat; 8 import java.util.Properties; 9 import javax.servlet.ServletException; 10 import javax.servlet.http.HttpServlet; 11 import javax.servlet.http.HttpServletRequest; 12 import javax.servlet.http.HttpServletResponse; 13 
     14 /**
     15 * 使用servletContext读取资源文件 16 * 17 * @author gacl 18 * 19  */
     20 public class ServletContextDemo6 extends HttpServlet { 21 
     22     public void doGet(HttpServletRequest request, HttpServletResponse response) 23             throws ServletException, IOException { 24         /**
     25 * response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码； 26 * 这样就不会出现中文乱码了 27          */
     28         response.setHeader("content-type","text/html;charset=UTF-8"); 29         readSrcDirPropCfgFile(response);//读取src目录下的properties配置文件
     30         response.getWriter().println("<hr/>"); 31         readWebRootDirPropCfgFile(response);//读取WebRoot目录下的properties配置文件
     32         response.getWriter().println("<hr/>"); 33         readPropCfgFile(response);//读取src目录下的db.config包中的db3.properties配置文件
     34         response.getWriter().println("<hr/>"); 35         readPropCfgFile2(response);//读取src目录下的gacl.servlet.study包中的db4.properties配置文件
     36         
     37 } 38 
     39     /**
     40 * 读取src目录下的gacl.servlet.study包中的db4.properties配置文件 41 * @param response 42 * @throws IOException 43      */
     44     private void readPropCfgFile2(HttpServletResponse response) 45             throws IOException { 46         InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/gacl/servlet/study/db4.properties"); 47         Properties prop = new Properties(); 48 prop.load(in); 49         String driver = prop.getProperty("driver"); 50         String url = prop.getProperty("url"); 51         String username = prop.getProperty("username"); 52         String password = prop.getProperty("password"); 53         response.getWriter().println("读取src目录下的gacl.servlet.study包中的db4.properties配置文件："); 54 response.getWriter().println( 55 MessageFormat.format( 56                         "driver={0},url={1},username={2},password={3}", 57 driver,url, username, password)); 58 } 59 
     60     /**
     61 * 读取src目录下的db.config包中的db3.properties配置文件 62 * @param response 63 * @throws FileNotFoundException 64 * @throws IOException 65      */
     66     private void readPropCfgFile(HttpServletResponse response) 67             throws FileNotFoundException, IOException { 68         //通过ServletContext获取web资源的绝对路径
     69         String path = this.getServletContext().getRealPath("/WEB-INF/classes/db/config/db3.properties"); 70         InputStream in = new FileInputStream(path); 71         Properties prop = new Properties(); 72 prop.load(in); 73         String driver = prop.getProperty("driver"); 74         String url = prop.getProperty("url"); 75         String username = prop.getProperty("username"); 76         String password = prop.getProperty("password"); 77         response.getWriter().println("读取src目录下的db.config包中的db3.properties配置文件："); 78 response.getWriter().println( 79 MessageFormat.format( 80                         "driver={0},url={1},username={2},password={3}", 81 driver,url, username, password)); 82 } 83 
     84     /**
     85 * 通过ServletContext对象读取WebRoot目录下的properties配置文件 86 * @param response 87 * @throws IOException 88      */
     89     private void readWebRootDirPropCfgFile(HttpServletResponse response) 90             throws IOException { 91         /**
     92 * 通过ServletContext对象读取WebRoot目录下的properties配置文件 93 * “/”代表的是项目根目录 94          */
     95         InputStream in = this.getServletContext().getResourceAsStream("/db2.properties"); 96         Properties prop = new Properties(); 97 prop.load(in); 98         String driver = prop.getProperty("driver"); 99         String url = prop.getProperty("url"); 100         String username = prop.getProperty("username"); 101         String password = prop.getProperty("password"); 102         response.getWriter().println("读取WebRoot目录下的db2.properties配置文件："); 103 response.getWriter().print( 104 MessageFormat.format( 105                         "driver={0},url={1},username={2},password={3}", 106 driver,url, username, password)); 107 } 108 
    109     /**
    110 * 通过ServletContext对象读取src目录下的properties配置文件 111 * @param response 112 * @throws IOException 113      */
    114     private void readSrcDirPropCfgFile(HttpServletResponse response) throws IOException { 115         /**
    116 * 通过ServletContext对象读取src目录下的db1.properties配置文件 117          */
    118         InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db1.properties"); 119         Properties prop = new Properties(); 120 prop.load(in); 121         String driver = prop.getProperty("driver"); 122         String url = prop.getProperty("url"); 123         String username = prop.getProperty("username"); 124         String password = prop.getProperty("password"); 125         response.getWriter().println("读取src目录下的db1.properties配置文件："); 126 response.getWriter().println( 127 MessageFormat.format( 128                         "driver={0},url={1},username={2},password={3}", 129 driver,url, username, password)); 130 } 131 
    132     public void doPost(HttpServletRequest request, HttpServletResponse response) 133             throws ServletException, IOException { 134         this.doGet(request, response); 135 } 136 
    137 }

![复制代码][20140736_ztTg.gif]

运行结果如下：

### **代码范例：使用类装载器读取资源文件** ###

![复制代码][20140736_ztTg.gif]

1 package gacl.servlet.study; 2 
     3 import java.io.FileOutputStream; 4 import java.io.IOException; 5 import java.io.InputStream; 6 import java.io.OutputStream; 7 import java.text.MessageFormat; 8 import java.util.Properties; 9 
     10 import javax.servlet.ServletException; 11 import javax.servlet.http.HttpServlet; 12 import javax.servlet.http.HttpServletRequest; 13 import javax.servlet.http.HttpServletResponse; 14 
     15 /**
     16 * 用类装载器读取资源文件 17 * 通过类装载器读取资源文件的注意事项:不适合装载大文件，否则会导致jvm内存溢出 18 * @author gacl 19 * 20  */
     21 public class ServletContextDemo7 extends HttpServlet { 22 
     23     public void doGet(HttpServletRequest request, HttpServletResponse response) 24             throws ServletException, IOException { 25         /**
     26 * response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码； 27 * 这样就不会出现中文乱码了 28          */
     29         response.setHeader("content-type","text/html;charset=UTF-8"); 30 test1(response); 31         response.getWriter().println("<hr/>"); 32 test2(response); 33         response.getWriter().println("<hr/>"); 34         //test3();
     35 test4(); 36         
     37 } 38     
     39     /**
     40 * 读取类路径下的资源文件 41 * @param response 42 * @throws IOException 43      */
     44     private void test1(HttpServletResponse response) throws IOException { 45         //获取到装载当前类的类装载器
     46         ClassLoader loader = ServletContextDemo7.class.getClassLoader(); 47         //用类装载器读取src目录下的db1.properties配置文件
     48         InputStream in = loader.getResourceAsStream("db1.properties"); 49         Properties prop = new Properties(); 50 prop.load(in); 51         String driver = prop.getProperty("driver"); 52         String url = prop.getProperty("url"); 53         String username = prop.getProperty("username"); 54         String password = prop.getProperty("password"); 55         response.getWriter().println("用类装载器读取src目录下的db1.properties配置文件："); 56 response.getWriter().println( 57 MessageFormat.format( 58                         "driver={0},url={1},username={2},password={3}", 59 driver,url, username, password)); 60 } 61 
     62     /**
     63 * 读取类路径下面、包下面的资源文件 64 * @param response 65 * @throws IOException 66      */
     67     private void test2(HttpServletResponse response) throws IOException { 68         //获取到装载当前类的类装载器
     69         ClassLoader loader = ServletContextDemo7.class.getClassLoader(); 70         //用类装载器读取src目录下的gacl.servlet.study包中的db4.properties配置文件
     71         InputStream in = loader.getResourceAsStream("gacl/servlet/study/db4.properties"); 72         Properties prop = new Properties(); 73 prop.load(in); 74         String driver = prop.getProperty("driver"); 75         String url = prop.getProperty("url"); 76         String username = prop.getProperty("username"); 77         String password = prop.getProperty("password"); 78         response.getWriter().println("用类装载器读取src目录下的gacl.servlet.study包中的db4.properties配置文件："); 79 response.getWriter().println( 80 MessageFormat.format( 81                         "driver={0},url={1},username={2},password={3}", 82 driver,url, username, password)); 83 } 84     
     85     /**
     86 * 通过类装载器读取资源文件的注意事项:不适合装载大文件，否则会导致jvm内存溢出 87      */
     88     public void test3() { 89         /**
     90 * 01.avi是一个150多M的文件，使用类加载器去读取这个大文件时会导致内存溢出： 91 * java.lang.OutOfMemoryError: Java heap space 92          */
     93         InputStream in = ServletContextDemo7.class.getClassLoader().getResourceAsStream("01.avi"); 94 System.out.println(in); 95 } 96     
     97     /**
     98 * 读取01.avi,并拷贝到e:\根目录下 99 * 01.avi文件太大，只能用servletContext去读取 100 * @throws IOException 101      */
    102     public void test4() throws IOException { 103         // path=G:\Java学习视频\JavaWeb学习视频\JavaWeb\day05视频\01.avi 104         // path=01.avi
    105         String path = this.getServletContext().getRealPath("/WEB-INF/classes/01.avi"); 106         /**
    107 * path.lastIndexOf("\\") + 1是一个非常绝妙的写法 108          */
    109         String filename = path.substring(path.lastIndexOf("\\") + 1);//获取文件名
    110         InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/01.avi"); 111         byte buffer[] = new byte[1024]; 112         int len = 0; 113         OutputStream out = new FileOutputStream("e:\\" + filename); 114         while ((len = in.read(buffer)) > 0) { 115             out.write(buffer, 0, len); 116 } 117 out.close(); 118 in.close(); 119 } 120 
    121     public void doPost(HttpServletRequest request, HttpServletResponse response) 122             throws ServletException, IOException { 123 
    124         this.doGet(request, response); 125 } 126 
    127 }

![复制代码][20140736_ztTg.gif]

运行结果如下：

## 四、在客户端缓存Servlet的输出 ##

对于不经常变化的数据，在servlet中可以为其设置合理的缓存时间值，以避免浏览器频繁向服务器发送请求，提升服务器的性能。例如：

![复制代码][20140736_ztTg.gif]

1 package gacl.servlet.study; 2 
     3 import java.io.IOException; 4 
     5 import javax.servlet.ServletException; 6 import javax.servlet.http.HttpServlet; 7 import javax.servlet.http.HttpServletRequest; 8 import javax.servlet.http.HttpServletResponse; 9 
    10 public class ServletDemo5 extends HttpServlet { 11 
    12     public void doGet(HttpServletRequest request, HttpServletResponse response) 13             throws ServletException, IOException { 14         String data = "abcddfwerwesfasfsadf"; 15         /**
    16 * 设置数据合理的缓存时间值，以避免浏览器频繁向服务器发送请求，提升服务器的性能 17 * 这里是将数据的缓存时间设置为1天 18          */
    19         response.setDateHeader("expires",System.currentTimeMillis() + 24 * 3600 * 1000); 20 response.getOutputStream().write(data.getBytes()); 21 } 22 
    23     public void doPost(HttpServletRequest request, HttpServletResponse response) 24             throws ServletException, IOException { 25 
    26         this.doGet(request, response); 27 } 28 
    29 }

![复制代码][20140736_ztTg.gif]

转载于:https://my.oschina.net/zhanghaiyang/blog/606536

[2019_Python_]: https://my.oschina.net/u/2663968/blog/3061697
[hot3.png]: /images/20220113/a46bda28e4bb40e0ad46d3f72d31ddfe.png
[20140736_ztTg.gif]: /images/20220113/1e603669aa44421b992691cd3f23308f.png