javaweb Servlet开发总结二<font color="red">abcdefghjkl</font>

这篇文章主要为大家详细介绍了javaweb Servlet开发总结的第二篇，感兴趣的小伙伴们可以参考一下

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标签为servlet配置一些初始化参数。

例如：

ServletConfigDemo1gacl.servlet.study.ServletConfigDemo1namegaclpassword123charsetUTF-8

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：

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，程序员通过ServletConfig对象就可以 * 得到当前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(""); //获取所有的初始化参数 Enumeration e = config.getinitParameterNames(); while(e.hasMoreElements()){ String name = e.nextElement(); String value = config.getinitParameter(name); response.getWriter().print(name + "=" + value + "

"); } } public void doPost(HttpServletRequest request, HttpServletResponse response) throws servletexception, IOException { this.doGet(request, response); } }

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

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， * 可以通过ServletConfig.getServletContext方法获得ServletContext对象。 */ ServletContext context = this.getServletConfig().getServletContext();//获得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，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文件中使用标签配置WEB应用的初始化参数，如下所示：

urljdbc:MysqL://localhost:3306/testindex.jsp

获取Web应用的初始化参数，代码如下：

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：

3.3、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 = "

abcdefghjkl

"; 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

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(""); readWebrootDirPropCfgFile(response);//读取Webroot目录下的properties配置文件 response.getWriter().println(""); readPropCfgFile(response);//读取src目录下的db.config包中的db3.properties配置文件 response.getWriter().println(""); 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 { 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 { //通过ServletContext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)); } /** * 通过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 { /** * 通过ServletContext对象读取Webroot目录下的properties配置文件 * “/”代表的是项目根目录 */ 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)); } /** * 通过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 { /** * 通过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); } }

运行结果如下：

代码范例：使用类装载器读取资源文件

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 * 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 * @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(""); test2(response); response.getWriter().println(""); //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)); } /** * 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 */ 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导致内存溢出： * 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JavaWebday05视频1.avi // path=01.avi String path = this.getServletContext().getRealPath("/WEB-INF/classes/01.avi"); /** * path.lastIndexOf("\") + 1是一个非常绝妙的写法 */ 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：

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，以避免浏览器频繁向服务器发送请求，提升服务器的性能 * 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); } }

以上就是本文的全部内容，希望对大家掌握javaweb Servlet开发技术有所帮助。