Java ServletContext详解
什么是ServletContext？
根据字面意思即Servlet上下文
服务器会为每一个工程创建一个对象，这个对象就是ServletContext对象，这个对象是
全局唯一的，工程内部的所有servlet都共享这个对象，所有应用程序共享对象
下面用一段简单的代码来证明一下
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet("/ServletContext2")
public class ServletContext2 extends HttpServlet{
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ServletContext servletContext = this.getServletContext();
System.out.println("ServletContext2的地址：");
System.out.println(servletContext);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
this.doGet(req, resp);
}
}
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet("/ServletContext")
public class ServletContext extends HttpServlet{
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
this.doPost(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
javax.servlet.ServletContext servletContext = this.getServletContext();
System.out.println("ServletContext2的地址：");
System.out.println(servletContext);
RequestDispatcher servletContext2 = req.getRequestDispatcher("ServletContext2");
servletContext2.forward(req,resp);
}
}
可以看到二者的地址一模一样
ServletContext对象的生命周期
诞生：web部署到服务器上，启动服务器，此时上下文环境对象创建，只要tomcat服务器不关闭，上下文环境对象就一直
存在
销毁：关闭服务器或者是从web服务器上卸载该程序的时候，该对象销毁
怎么创建ServletContext对象？
方法一：
javax.servlet.ServletContext servletContext = this.getServletContext();
方法二:
javax.servlet.ServletContext servletContext1 = req.getServletContext();
方法三：
javax.servlet.ServletContext servletContext3 = this.getServletConfig().getServletContext();
ServletContext对象的作用与相关方法
1.是一个域对象*
什么是域：域其实就是一个map结构，及存入一个键值对，来存储获取数据
作用范围：本工程内部
域对象可以被本工程内所有的servlet对象共享，因为一个工程只有一个servletcontext对象
方法介绍：
存储数据： void setAttribute(String var1, Object var2);
移除数据： void removeAttribute(String var1);
获取数据： Object getAttribute(String var1);
1.由于取出的是一个Object类型，需要强转
2.如果指定的属性名不存在，则返回null
2.可以获取文件的真实路径
真实路径：即部署在服务器是的路径
String getRealPath（） 获取文件的真实路径
不同目录下访问资源的方式不同
//src下的资源访问
String realPath = servletContext.getRealPath("/WEB-INF/classes/a.text");
System.out.println(realPath);
//WEB-INF下的资源访问
String realPath1 = servletContext.getRealPath("/WEB-INF/b.text");
System.out.println(realPath1);
//web目录下的资源访问
String realPath2 = servletContext.getRealPath("/c.text");
System.out.println(realPath2);
代码
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet("/ServletContext")
public class ServletContext extends HttpServlet{
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
this.doPost(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
javax.servlet.ServletContext servletContext = this.getServletContext();
//src下的资源访问
String realPath = servletContext.getRealPath("/WEB-INF/classes/a.text");
System.out.println(realPath);
//WEB-INF下的资源访问
String realPath1 = servletContext.getRealPath("/WEB-INF/b.text");
System.out.println(realPath1);
//web目录下的资源访问
String realPath2 = servletContext.getRealPath("/c.text");
System.out.println(realPath2);
}
}
结果
3.获得MIME类型
s什么是MIME类型，是在互联网通信过程中定义等等一种文件数据类型
格式：大类型/小类型 image/jpeg
获取： String getMimeType（String file)
代码如下
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet("/ServletContext")
public class ServletContext extends HttpServlet{
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
this.doPost(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
javax.servlet.ServletContext servletContext = this.getServletContext();
String filename="a.jpg";
String mimeType = servletContext.getMimeType(filename);
System.out.println(mimeType);
}
}
结果