java web 项目总结_javaweb Servlet开发总结（二）

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：

ServletConfigDemo1

gacl.servlet.study.ServletConfigDemo1

name

gacl

password

123

charset

UTF-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：

url

jdbc:mysql://localhost:3306/test

index.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\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。