servletConfig和servletContext的应用

    一.ServletConfig的使用

    在学习servlet生命周期时我们讲过，Servlet在被web容器创建时要先调用其init方法传入一个ServletConfig类型的参数，什么是ServletConfig呢？ServletConfig顾名思义，就是Servlet的配置，我们在web.xml中定义的init-param标签的内容就是保存在ServletConfig这个对象中的。ServletConfig是一个接口，里面定义了如下的几个方法：

public interface ServletConfig {
   
    public String getServletName();
   
    public ServletContext getServletContext();

    public String getInitParameter(String name);

    public Enumeration<String> getInitParameterNames();

}

其中getServletName用于获取Servlet的名字；getInitParameter(String name)表示获取init-param的值；getInitParameterNames()表示获取所有配置在web.xml中的值，这三个方法都是比较的容易理解。这里我要着重介绍的是getServletContext()方法表示返回一个ServletContext对象,他代表应用的本身，那么在ServletConetxt中设置参数就会被所有的Servlet所共享而在initParamter参数只可以在配置的servlet中使用。下面我们来尝试获取一下配置在web.xml中的参数：

（1）定义web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"         
    xmlns="http://java.sun.com/xml/ns/javaee" 
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
                        http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" 
    version="3.0">
    <context-param>
       <param-name>test</param-name>
       <param-value>ServletContext</param-value>
    </context-param>
    
    <servlet>
      <servlet-name>testServletConfig</servlet-name>
      <servlet-class>com.web.test.TestServletConfig</servlet-class>
      <init-param>
         <param-name>testServletConfig</param-name>
         <param-value>getFromServletConfig</param-value>
      </init-param>
       <init-param>
         <param-name>encode</param-name>
         <param-value>utf-8</param-value>
         
      </init-param>
    </servlet>
    
    <servlet-mapping>
       <servlet-name>testServletConfig</servlet-name>
       <url-pattern>/testServletConfig</url-pattern>
    </servlet-mapping>
</web-app>

（2）获取参数

public class TestServletConfig extends HttpServlet{

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        doPost(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        ServletConfig servletConfig =  this.getServletConfig();
        String initParam =  servletConfig.getInitParameter("testServletConfig");
        System.out.println(initParam);
        Enumeration<String> inits = servletConfig.getInitParameterNames();
        while(inits.hasMoreElements()){
            System.out.println(servletConfig.getInitParameter(inits.nextElement()));
        }
    }
     
}


输出：
getFromServletConfig
getFromServletConfig
utf-8

     二.ServletContext的使用

    当web容器启动后会为每一个web容器创建一个ServletContext对象表示这个web应用本身，在我们刚才介绍的ServletConfig中保存着一个对ServletContext对象的引用。在同一个web应用中定义的Servlet共享一个ServletContext，所以我们也称ServletContext为域对象，使用ServletContext对象可以实现在不同的Servlet之间传递数据。

    在ServletContext接口中定义了如下的几个方法：

    public String getInitParameter(String name);
  
    public boolean setInitParameter(String name, String value);
    
    public Enumeration<String> getAttributeNames();
    
    public void setAttribute(String name, Object object);
    
    public void removeAttribute(String name);

    public Object getAttribute(String name);

其中getInitParameter表示获取在web.xml中使用标签context-param定义的初始化参数，setInitParameter表示设置初始化参数，getAttribute和setAttribute分别表示从ServletContext获取值和向ServletContext设置值，removeAttribute表示移除ServletContext中对应的值。首先我们来尝试获取一下ServletContext的初始化参数：

<?xml version="1.0" encoding="UTF-8"?>
<web-app 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"         
    xmlns="http://java.sun.com/xml/ns/javaee" 
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
                        http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" 
    version="3.0">
    <context-param>
       <param-name>test</param-name>
       <param-value>ServletContext</param-value>
    </context-param>
    
    <servlet>
       <servlet-name>testServletContext</servlet-name>
       <servlet-class>com.web.test.TestServletContext</servlet-class>
    </servlet>
</web-app>

先在web.xml中配置好servletContext的初始化参数，如上所示。在web.xml中定义的servlet中获取初始化参数：

package com.web.test;

import java.io.IOException;
import java.io.PrintWriter;

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 TestServletContext extends HttpServlet{

    private static final long serialVersionUID = 1L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        doPost(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();
        String initName = servletContext.getInitParameter("test");
        PrintWriter write =  resp.getWriter();
        write.print("initName:"+initName);
        write.flush();
        write.close();
    }
}
浏览器页面输出：

initName:ServletContext

在ServletContext中设置值会被同一个web应用的其他Servlet所共享。首先我需要定义两个Servlet:

<?xml version="1.0" encoding="UTF-8"?>
<web-app 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"         
    xmlns="http://java.sun.com/xml/ns/javaee" 
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
                        http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" 
    version="3.0">
    
    <servlet>
       <servlet-name>testServletContext1</servlet-name>
       <servlet-class>com.web.test.TestServletContext1</servlet-class>
    </servlet>
    
    <servlet-mapping>
       <servlet-name>testServletContext1</servlet-name>
       <url-pattern>/testServletContext1</url-pattern>
    </servlet-mapping>
    
    <servlet>
       <servlet-name>testServletContext2</servlet-name>
       <servlet-class>com.web.test.TestServletContext2</servlet-class>
    </servlet>
    
    <servlet-mapping>
       <servlet-name>testServletContext2</servlet-name>
       <url-pattern>/testServletContex2</url-pattern>
    </servlet-mapping>

</web-app>

分别是TestServletContext1和TestServletContext2。在TestServletContext1中我们获取了ServletContext对象并且使用setAttribute方法往ServletContext中设置了值然后转发到了TestServletContext2,关于转发我在介绍HttpServletRequest对象在来介绍。在TestServletContext2中我们使用getAttribute方法来获取存放在ServletContext中的值。

public class TestServletContext1 extends HttpServlet{

    private static final long serialVersionUID = 1L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        doPost(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
          ServletContext servletContext = getServletContext();
          servletContext.setAttribute("test","getFromServletContext");
          servletContext.getRequestDispatcher("/testServletContex2").forward(req, resp);
    }
    
    
    
}

public class TestServletContext2 extends HttpServlet{

    private static final long serialVersionUID = 1L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        doPost(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
         ServletContext servletContext = getServletContext();
         String value = (String) servletContext.getAttribute("test");
         PrintWriter writer =  resp.getWriter();
         writer.print("this is TestServletContext2
");
         writer.print("test:"+value);
         writer.flush();
         writer.close();
    }
        
}

浏览器输出：