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深入了解Java ServletContext

更新時(shí)間：2020年08月03日 08:37:36 作者：一只胡說八道的猴子

這篇文章主要介紹了Java ServletContext的相關(guān)資料，文中講解非常細(xì)致，代碼幫助大家更好的理解和學(xué)習(xí)，感興趣的朋友可以了解下

什么是ServletContext？

根據(jù)字面意思即Servlet上下文

服務(wù)器會(huì)為每一個(gè)工程創(chuàng)建一個(gè)對象，這個(gè)對象就是ServletContext對象，這個(gè)對象是
全局唯一的，工程內(nèi)部的所有servlet都共享這個(gè)對象，所有應(yīng)用程序共享對象

下面用一段簡單的代碼來證明一下

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樣

ServletContext對象的生命周期

誕生：web部署到服務(wù)器上，啟動(dòng)服務(wù)器，此時(shí)上下文環(huán)境對象創(chuàng)建，只要tomcat服務(wù)器不關(guān)閉，上下文環(huán)境對象就一直
存在

銷毀：關(guān)閉服務(wù)器或者是從web服務(wù)器上卸載該程序的時(shí)候，該對象銷毀

怎么創(chuàng)建ServletContext對象？

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對象的作用與相關(guān)方法

1.是一個(gè)域?qū)ο?

什么是域：域其實(shí)就是一個(gè)map結(jié)構(gòu)，及存入一個(gè)鍵值對，來存儲(chǔ)獲取數(shù)據(jù)
作用范圍：本工程內(nèi)部
域?qū)ο罂梢员槐竟こ虄?nèi)所有的servlet對象共享，因?yàn)橐粋€(gè)工程只有一個(gè)servletcontext對象

方法介紹：
存儲(chǔ)數(shù)據(jù)： void setAttribute(String var1, Object var2);

移除數(shù)據(jù)： void removeAttribute(String var1);

獲取數(shù)據(jù)： Object getAttribute(String var1);
1.由于取出的是一個(gè)Object類型，需要強(qiáng)轉(zhuǎn)
2.如果指定的屬性名不存在，則返回null

2.可以獲取文件的真實(shí)路徑

真實(shí)路徑：即部署在服務(wù)器是的路徑
String getRealPath（）獲取文件的真實(shí)路徑

不同目錄下訪問資源的方式不同

//src下的資源訪問
    String realPath = servletContext.getRealPath("/WEB-INF/classes/a.text");
    System.out.println(realPath);
//WEB-INF下的資源訪問
    String realPath1 = servletContext.getRealPath("/WEB-INF/b.text");
    System.out.println(realPath1);
 //web目錄下的資源訪問
    String realPath2 = servletContext.getRealPath("/c.text");
    System.out.println(realPath2);

代碼

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下的資源訪問
    String realPath = servletContext.getRealPath("/WEB-INF/classes/a.text");
    System.out.println(realPath);
    //WEB-INF下的資源訪問
    String realPath1 = servletContext.getRealPath("/WEB-INF/b.text");
    System.out.println(realPath1);
    //web目錄下的資源訪問
    String realPath2 = servletContext.getRealPath("/c.text");
    System.out.println(realPath2);
  }
}

結(jié)果

3.獲得MIME類型

s什么是MIME類型，是在互聯(lián)網(wǎng)通信過程中定義等等一種文件數(shù)據(jù)類型
格式：大類型/小類型 image/jpeg
獲?。?String getMimeType（String file)

代碼如下

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);
  }
}

結(jié)果