标签：servletcontext web应用 tomcat服务器 servlet
ServletContext对象是Servlet三大域对象之一，每个Web应用程序都拥有一个ServletContext对象，该对象是Web应用程序的全局对象或者上下文。Tomcat服务器在启动时，会自动创建一个ServletContext对象，在关闭时，会自动销毁这个ServletContext对象。每个Web应用程序只拥有一个ServletContext对象，ServletContext对象可以在整个Web应用中共享数据资源。
下列是ServletContext提供的方法列表：
| Method Summary | |
|---|---|
| Object | getAttribute(String name) |
| Enumeration | getAttributeNames() |
| String | getInitParameter(String name) |
| Enumeration | getInitParameterNames() |
| String | getMimeType(String file) |
| String | getRealPath(String path) |
| String | getServletContextName() |
| Enumeration | getServletNames() |
| void | log(String msg) |
| void | removeAttribute(String name) |
| void | setAttribute(String name, Object object) |
在自定义Servlet中有以下几种方式获取到ServletContext对象：
我们通过一个案例来讨论一下。
public class AServlet extends GenericServlet {
@Override
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException {
ServletConfig config = getServletConfig();
ServletContext context1 = config.getServletContext();
context1.log("这是通过ServletConfig对象获取到的ServletContext对象.");
ServletContext context2 = getServletContext();
context2.log("这是通过继承GenericServlet类获取到的ServletContext对象.");
}
}
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5"
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_2_5.xsd">
<servlet>
<servlet-name>AServlet</servlet-name>
<servlet-class>app.java.context.AServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>AServlet</servlet-name>
<url-pattern>/servlet/AServlet</url-pattern>
</servlet-mapping>
</web-app>
在web.xml文件中，使用定义的初始化参数，只能在当前Servlet中使用，而其他Servlet是无权限访问当前Servlet下配置的初始化参数的。而可以使用ServletContext在web.xml文件中配置全局初始化参数，这样当前Web应用程序中的所有Servlet都可以访问。
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5"
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_2_5.xsd">
<context-param>
<param-name>weixin</param-name>
<param-value>longestory</param-value>
</context-param>
<servlet>
<servlet-name>AServlet</servlet-name>
<servlet-class>app.java.context.AServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>AServlet</servlet-name>
<url-pattern>/servlet/AServlet</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>BServlet</servlet-name>
<servlet-class>app.java.context.BServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>BServlet</servlet-name>
<url-pattern>/servlet/BServlet</url-pattern>
</servlet-mapping>
</web-app>
public class BServlet extends GenericServlet {
@Override
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException {
ServletContext context = getServletContext();
String weixin = context.getInitParameter("weixin");
System.out.println(weixin);
}
}
在自定义Servlet中，可以通过ServletContext对象的getInitParameter(String name)方法获取对应参数名称的全局初始化参数值，也可以通过ServletContext对象的getInitParameterNames()方法获取所有全局初始化参数的名称。
还可以通过ServletContext对象的getMineType(String file)方法根据文件扩展名获取文件MIME类型。
public class BServlet extends GenericServlet {
@Override
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException {
ServletContext context = getServletContext();
String html = context.getMimeType("1.html");
String css = context.getMimeType("2.css");
String javascript = context.getMimeType("3.js");
System.out.println("HTML的文件类型为"+html+", CSS的文件类型为"+css+", javascript的文件类型为"+javascript);
}
}
发布Web应用程序，并启动Tomcat服务器，在控制台中打印：
HTML的扩展名为text/html, CSS的扩展名为text/css, javascript的扩展名为application/javascriptServletContext对象的getMineType(String file)方法会自动读取Tomcat安装目录中conf目录中的web.xml文件。
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5"
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_2_5.xsd">
<mime-mapping>
<extension>html</extension>
<mime-type>text/html</mime-type>
</mime-mapping>
<mime-mapping>
<extension>css</extension>
<mime-type>text/css</mime-type>
</mime-mapping>
<mime-mapping>
<extension>js</extension>
<mime-type>application/javascript</mime-type>
</mime-mapping>
</web-app>
在同一个Web应用程序中，多个Servlet之间可以共享ServletContext对象中的数据信息。主要是通过ServletContext对象的setAttribute(String name, Object object)方法和getAttribute(String name)方法完成，下面我们来实现统计网站访问次数的案例。
public class VisitServlet extends HttpServlet {
@Override
public void init() throws ServletException {
ServletContext context = getServletContext();
context.setAttribute("times", 0);
}
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
ServletContext context = getServletContext();
int times = (Integer)context.getAttribute("times");
times ++;
context.setAttribute("times", times);
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
}
public class ShowTimeServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
ServletContext context = getServletContext();
int times = (Integer)context.getAttribute("times");
response.setContentType("text/html;charset=utf-8");
PrintWriter out = response.getWriter();
out.println("<h1>VisitServlet共被访问了"+times+"次</h1>");
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
}
<web-app version="2.5"
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_2_5.xsd">
<servlet>
<servlet-name>VisitServlet</servlet-name>
<servlet-class>app.java.context.VisitServlet</servlet-class>
</servlet>
<servlet>
<servlet-name>ShowTimeServlet</servlet-name>
<servlet-class>app.java.context.ShowTimeServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>VisitServlet</servlet-name>
<url-pattern>/visit</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>ShowTimeServlet</servlet-name>
<url-pattern>/show</url-pattern>
</servlet-mapping>
</web-app>
读取工程中的资源文件，Java中的IO流其实就可以完成，下面使用Java中的IO流完成读取资源文件。
public class ReaderFileTest {
// 编写readfile()方法完成资源文件的读取工作.
public static void readfile(String fileName) throws Exception{
BufferedReader reader = new BufferedReader(new FileReader(fileName));
String line;
while ((line = reader.readLine()) != null) {
System.out.println(line);
}
reader.close();
}
public static void main(String[] args) throws Exception {
// 读取1.txt
String filename1 = "1.txt";
readfile(filename1);
// 读取2.txt
String filename2 = "WebRoot/2.txt";
readfile(filename2);
// 读取3.txt
String filename3 = "WebRoot/WEB-INF/3.txt";
readfile(filename3);
// 读取4.txt
String filename4 = "src/4.txt";
readfile(filename4);
}
}
如果要想利用Servlet API的内容来读取Web工程中的资源文件，又要如何来做呢？ServletContext对象的getRealPath()方法可以来完成此项工作。
public class ReadFileServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/html;charset=utf-8");
OutputStream out = response.getOutputStream();
/*
* 读取1.txt
* * 因为1.txt资源文件在Web工程的根目录.
* * Web工程的WebRoot目录发布到Tomcat服务器.
* * 所以,1.txt资源文件是不会发布到Tomcat服务器的,Servlet无法读取.
*/
// 读取2.txt
String filename2 = getServletContext().getRealPath("/2.txt");
InputStream in2 = new FileInputStream(new File(filename2));
IOUtils.copy(in2, out);
// 读取3.txt
String filename3 = getServletContext().getRealPath("/WEB-INF/3.txt");
InputStream in3 = new FileInputStream(new File(filename3));
IOUtils.copy(in3, out);
// 读取4.txt
String filename4 = getServletContext().getRealPath("/WEB-INF/classes/4.txt");
InputStream in4 = new FileInputStream(new File(filename4));
IOUtils.copy(in4, out);
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
}
除了可以使用ServletContext对象的getRealPath()方法之外，还可以使用ServletContext对象的getResourceAsStream()方法来完成读取资源文件的工作。
public class ReadFileServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse
response) throws ServletException, IOException {
InputStream in = getServletContext().getResourceAsStream("/WEB-INF/classes/4.txt");
IOUtils.copy(in, out);
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
}
还有一种通用的方法：利用Class类的getResource()方法也可以完成读取资源文件的工作。
public class ReadFileServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// 利用类加载器读取Web工程的资源文件
String filename = ReadFileServlet.class.getResource("/4.txt").getFile();
InputStream in = new FileInputStream(new File(filename));
IOUtils.copy(in, out);
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
}
转载说明：请注明作者及原文链接，谢谢！
标签：servletcontext web应用 tomcat服务器 servlet
原文地址：http://blog.csdn.net/longestory/article/details/46404277
