Servlet学习(二)：ServletConfig获取参数；ServletContext应用：请求转发，参数获取，资源读取；类装载器读取文件

时间  2020-12-21
标签 html java mysql 程序员 web sql 浏览器 缓存 服务器 app 栏目 Tomcat 繁體版
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转载：http://www.cnblogs.com/xdp-gacl/p/3763559.html

1、ServletConfig讲解

1.一、配置Servlet初始化参数

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

例如：java

<servlet>
    <servlet-name>ServletConfigDemo1</servlet-name>
    <servlet-class>gacl.servlet.study.ServletConfigDemo1</servlet-class>
    <!--配置ServletConfigDemo1的初始化参数 -->
    <init-param>
        <param-name>name</param-name>
        <param-value>gacl</param-value>
    </init-param>
     <init-param>
        <param-name>password</param-name>
        <param-value>123</param-value>
    </init-param>
    <init-param>
        <param-name>charset</param-name>
        <param-value>UTF-8</param-value>
    </init-param>
</servlet>

1.二、经过ServletConfig获取Servlet的初始化参数

　　当servlet配置了初始化参数后，web容器在建立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，咱们经过ServletConfig对象就能够获得当前servlet的初始化参数信息。mysql

例如：程序员

package gacl.servlet.study; import java.io.IOException; import java.util.Enumeration; import javax.servlet.ServletConfig; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class ServletConfigDemo1 extends HttpServlet { /** * 定义ServletConfig对象来接收配置的初始化参数 */
    private ServletConfig config; /** * 当servlet配置了初始化参数后，web容器在建立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，程序员经过ServletConfig对象就能够 * 获得当前servlet的初始化参数信息。 */ @Override public void init(ServletConfig config) throws ServletException { this.config = config; } public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { //获取在web.xml中配置的初始化参数
        String paramVal = this.config.getInitParameter("name");//获取指定的初始化参数
 response.getWriter().print(paramVal); response.getWriter().print("<hr/>"); //获取全部的初始化参数
        Enumeration<String> e = config.getInitParameterNames(); while(e.hasMoreElements()){ String name = e.nextElement(); String value = config.getInitParameter(name); response.getWriter().print(name + "=" + value + "<br/>"); } } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.doGet(request, response); } }

运行结果以下：web

　　

2、ServletContext对象

　　WEB容器在启动时，它会为每一个WEB应用程序都建立一个对应的ServletContext对象，它表明当前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，能够经过ServletConfig.getServletContext方法得到ServletContext对象。
　　因为一个WEB应用中的全部Servlet共享同一个ServletContext对象，所以Servlet对象之间能够经过ServletContext对象来实现通信。ServletContext对象一般也被称之为context域对象。sql

3、ServletContext的应用

　　3.一、多个Servlet经过ServletContext对象实现数据共享

　　范例：ServletContextDemo1和ServletContextDemo2经过ServletContext对象实现数据共享浏览器

package gacl.servlet.study; import java.io.IOException; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class ServletContextDemo1 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String data = "xdp_gacl"; /** * ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时， * 能够经过ServletConfig.getServletContext方法得到ServletContext对象。 */ ServletContext context = this.getServletConfig().getServletContext();//得到ServletContext对象
        context.setAttribute("data", data);  //将data存储到ServletContext对象中
 } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request, response); } }
package gacl.servlet.study; import java.io.IOException; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class ServletContextDemo2 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ServletContext context = this.getServletContext(); String data = (String) context.getAttribute("data");//从ServletContext对象中取出数据
        response.getWriter().print("data="+data); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request, response); } }

先运行ServletContextDemo1，将数据data存储到ServletContext对象中，而后运行ServletContextDemo2就能够从ServletContext对象中取出数据了，这样就实现了数据共享，以下图所示：缓存

　　

　　3.二、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，以下所示：服务器

<?xml version="1.0" encoding="UTF-8"?>
<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 
    http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
    <display-name></display-name>
    <!-- 配置WEB应用的初始化参数 -->
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/test</param-value>
    </context-param>

    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
</web-app>

获取Web应用的初始化参数，代码以下：app

package gacl.servlet.study; import java.io.IOException; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class ServletContextDemo3 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ServletContext context = this.getServletContext(); //获取整个web站点的初始化参数
        String contextInitParam = context.getInitParameter("url"); response.getWriter().print(contextInitParam); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request, response); } }

运行结果：

　　

　　3.三、用servletContext实现请求转发

ServletContextDemo4

package gacl.servlet.study; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.RequestDispatcher; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class ServletContextDemo4 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String data = "<h1><font color='red'>abcdefghjkl</font></h1>"; response.getOutputStream().write(data.getBytes()); ServletContext context = this.getServletContext();//获取ServletContext对象
        RequestDispatcher rd = context.getRequestDispatcher("/servlet/ServletContextDemo5");//获取请求转发对象(RequestDispatcher) rd.forward(request, response);//调用forward方法实现请求转发
 } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { } }
ServletContextDemo5

package gacl.servlet.study; 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 ServletContextDemo5 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.getOutputStream().write("servletDemo5".getBytes()); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.doGet(request, response); } }

运行结果：

　　

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

　　3.四、利用ServletContext对象读取资源文件

　　项目目录结构以下：

 　　

代码范例：使用servletContext读取资源文件

读取资源文件到输入流的方法：

InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db/config/db3.properties")

第二种：

String path = this.getServletContext().getRealPath("/WEB-INF/classes/db/config/db3.properties"); InputStream in = new FileInputStream(path);

例子：

package gacl.servlet.study; import java.io.FileInputStream; import java.io.FileNotFoundException; import java.io.IOException; import java.io.InputStream; import java.text.MessageFormat; import java.util.Properties; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; /** * 使用servletContext读取资源文件 * * @author gacl * */
public class ServletContextDemo6 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { /** * response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码； * 这样就不会出现中文乱码了 */ response.setHeader("content-type","text/html;charset=UTF-8"); readSrcDirPropCfgFile(response);//读取src目录下的properties配置文件
        response.getWriter().println("<hr/>"); readWebRootDirPropCfgFile(response);//读取WebRoot目录下的properties配置文件
        response.getWriter().println("<hr/>"); readPropCfgFile(response);//读取src目录下的db.config包中的db3.properties配置文件
        response.getWriter().println("<hr/>"); readPropCfgFile2(response);//读取src目录下的gacl.servlet.study包中的db4.properties配置文件
 } /** * 读取src目录下的gacl.servlet.study包中的db4.properties配置文件 * @param response * @throws IOException */
    private void readPropCfgFile2(HttpServletResponse response) throws IOException {
　　　　　//读取资源文件的路径不能/src/db4.properties方式，由于工程打包后src目录就没了，而会生成在一个名为classs的文件夹下。必须使用如下方式： InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/gacl/servlet/study/db4.properties"); Properties prop = new Properties();
　　　　 prop.load(in); String driver = prop.getProperty("driver"); String url = prop.getProperty("url"); String username = prop.getProperty("username"); String password = prop.getProperty("password"); response.getWriter().println("读取src目录下的gacl.servlet.study包中的db4.properties配置文件："); response.getWriter().println( MessageFormat.format( "driver={0},url={1},username={2},password={3}", driver,url, username, password)); } /** * 读取src目录下的db.config包中的db3.properties配置文件 * @param response * @throws FileNotFoundException * @throws IOException */
    private void readPropCfgFile(HttpServletResponse response) throws FileNotFoundException, IOException { //经过ServletContext的方法：getRealPath获取web资源的绝对路径
        String path = this.getServletContext().getRealPath("/WEB-INF/classes/db/config/db3.properties"); InputStream in = new FileInputStream(path); Properties prop = new Properties(); prop.load(in); String driver = prop.getProperty("driver"); String url = prop.getProperty("url"); String username = prop.getProperty("username"); String password = prop.getProperty("password"); response.getWriter().println("读取src目录下的db.config包中的db3.properties配置文件："); response.getWriter().println( MessageFormat.format( "driver={0},url={1},username={2},password={3}", driver,url, username, password)); } /** * 经过ServletContext对象读取WebRoot目录下的properties配置文件 * @param response * @throws IOException */
    private void readWebRootDirPropCfgFile(HttpServletResponse response) throws IOException { /** * 经过ServletContext对象读取WebRoot目录下的properties配置文件 * “/”表明的是项目根目录 */ InputStream in = this.getServletContext().getResourceAsStream("/db2.properties"); Properties prop = new Properties(); prop.load(in); String driver = prop.getProperty("driver"); String url = prop.getProperty("url"); String username = prop.getProperty("username"); String password = prop.getProperty("password"); response.getWriter().println("读取WebRoot目录下的db2.properties配置文件："); response.getWriter().print( MessageFormat.format( "driver={0},url={1},username={2},password={3}", driver,url, username, password)); } /** * 经过ServletContext对象读取src目录下的properties配置文件 * @param response * @throws IOException */
    private void readSrcDirPropCfgFile(HttpServletResponse response) throws IOException { /** * 经过ServletContext对象读取src目录下的db1.properties配置文件 */ InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db1.properties"); Properties prop = new Properties(); prop.load(in); String driver = prop.getProperty("driver"); String url = prop.getProperty("url"); String username = prop.getProperty("username"); String password = prop.getProperty("password"); response.getWriter().println("读取src目录下的db1.properties配置文件："); response.getWriter().println( MessageFormat.format( "driver={0},url={1},username={2},password={3}", driver,url, username, password)); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.doGet(request, response); } }

运行结果以下：

　　

 

代码范例：使用类装载器读取资源文件

若是读取资源文件的程序不是servlet的话，例如dao层，service层，没有servlet的地方，就只能经过类装载器去读取。

使用方式：

//获取到装载当前类的类装载器 ClassLoader loader = ServletContextDemo7.class.getClassLoader(); //用类装载器读取src目录下的db1.properties配置文件 InputStream in = loader.getResourceAsStream("db1.properties");

第二种：这种方式能读取到最新的properties文件内容，而上面读取到的永远是装载到类加载器里面的内容，若是手动修改了properties文件，

那么新的内容是读取不到的。

//获取到装载当前类的类装载器 ClassLoader loader = ServletContextDemo7.class.getClassLoader(); //用类装载器读取src目录下的db1.properties配置文件 String path = loader.getResource("db1.properties").getPath(); FillInputStream in=new FillInputStream(path);

实例：

package gacl.servlet.study; import java.io.FileOutputStream; import java.io.IOException; import java.io.InputStream; import java.io.OutputStream; import java.text.MessageFormat; import java.util.Properties; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; /** * 用类装载器读取资源文件 * 经过类装载器读取资源文件的注意事项:不适合装载大文件，不然会致使jvm内存溢出 * @author gacl * */
public class ServletContextDemo7 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { /** * response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码； * 这样就不会出现中文乱码了 */ response.setHeader("content-type","text/html;charset=UTF-8"); test1(response); response.getWriter().println("<hr/>"); test2(response); response.getWriter().println("<hr/>"); //test3();
 test4(); } /** * 读取类路径下的资源文件 * @param response * @throws IOException */
    private void test1(HttpServletResponse response) throws IOException { //获取到装载当前类的类装载器
        ClassLoader loader = ServletContextDemo7.class.getClassLoader(); //用类装载器读取src目录下的db1.properties配置文件
        InputStream in = loader.getResourceAsStream("db1.properties"); Properties prop = new Properties(); prop.load(in); String driver = prop.getProperty("driver"); String url = prop.getProperty("url"); String username = prop.getProperty("username"); String password = prop.getProperty("password"); response.getWriter().println("用类装载器读取src目录下的db1.properties配置文件："); response.getWriter().println( MessageFormat.format( "driver={0},url={1},username={2},password={3}", driver,url, username, password)); } /** * 读取类路径下面、包下面的资源文件 * @param response * @throws IOException */
    private void test2(HttpServletResponse response) throws IOException { //获取到装载当前类的类装载器
        ClassLoader loader = ServletContextDemo7.class.getClassLoader(); //用类装载器读取src目录下的gacl.servlet.study包中的db4.properties配置文件
        InputStream in = loader.getResourceAsStream("gacl/servlet/study/db4.properties"); Properties prop = new Properties(); prop.load(in); String driver = prop.getProperty("driver"); String url = prop.getProperty("url"); String username = prop.getProperty("username"); String password = prop.getProperty("password"); 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："); response.getWriter().println( MessageFormat.format( "driver={0},url={1},username={2},password={3}", driver,url, username, password)); } /** * 经过类装载器读取资源文件的注意事项:不适合装载大文件，不然会致使jvm内存溢出 */
    public void test3() { /** * 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致使内存溢出： * java.lang.OutOfMemoryError: Java heap space */ InputStream in = ServletContextDemo7.class.getClassLoader().getResourceAsStream("01.avi"); System.out.println(in); } /** * 读取01.avi,并拷贝到e:\根目录下 * 01.avi文件太大，只能用servletContext去读取 * @throws IOException */
    public void test4() throws IOException { // path=G:\Java学习视频\JavaWeb学习视频\JavaWeb\day05视频\01.avi // path=01.avi
        String path = this.getServletContext().getRealPath("/WEB-INF/classes/01.avi"); /** * path.lastIndexOf("\\") + 1是一个很是绝妙的写法 */ String filename = path.substring(path.lastIndexOf("\\") + 1);//获取文件名
        InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/01.avi"); byte buffer[] = new byte[1024]; int len = 0; OutputStream out = new FileOutputStream("e:\\" + filename); while ((len = in.read(buffer)) > 0) { out.write(buffer, 0, len); } out.close(); in.close(); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.doGet(request, response); } }

运行结果以下：

　　

4、在客户端缓存Servlet的输出

　　对于不常常变化的数据，在servlet中能够为其设置合理的缓存时间值，以免浏览器频繁向服务器发送请求，提高服务器的性能。例如：

package gacl.servlet.study; 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 ServletDemo5 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String data = "abcddfwerwesfasfsadf"; /** * 设置数据合理的缓存时间值，以免浏览器频繁向服务器发送请求，提高服务器的性能 * 这里是将数据的缓存时间设置为1天 */ response.setDateHeader("expires",System.currentTimeMillis() + 24 * 3600 * 1000); response.getOutputStream().write(data.getBytes()); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.doGet(request, response); } }

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