Java Web——文件下载getResourceAsStream()返回NULL解决方案

问题描述

严重: Servlet.service() for servlet [club.zstuca.demo.DownloadDemo] in context with path [/DEMOWeb] threw exception
java.lang.NullPointerExceptionat club.zstuca.demo.DownloadDemo.doGet(DownloadDemo.java:48)at javax.servlet.http.HttpServlet.service(HttpServlet.java:634)at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:493)at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:137)at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:660)at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:798)at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:808)at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1498)at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)at java.lang.Thread.run(Unknown Source)

问题分析

文件不在WebAPP根目录下，导致找不到资源。

API

Class.getResourceAsStream(String path)

JDK设置这样的规则，是很好理解的，path不以“/”开头时，我们就能获取与当前类所在的路径相同的资源文件，而以“/”开头时可以获取classPath根下任意路径的资源。

path不以“/”开头时，默认是从当前类所在的包下面获取资源

path以“/”开头时，则是从项目的classPath根下获取资源

ClassLoader.getResourceAsStream(String path)

默认是从classpath根下获取，path不能以“/”开头，path是指类加载器的加载范围，在资源加载的过程中，使用逐级向上的委托的形式加载的，“/”表示Boot ClassLoader中的加载范围，因为这个类加载器是C++实现的，所以加载范围为null。

ServletContext.getResourceAsStream(String path)

默认从WebAPP根目录下取资源，Tomcat下path是否以“/”开头无所谓，当然这和具体的容器实现有关。

解决方案

1、在WebAPP根目录下创建res文件夹，将资源移入此文件夹

2、修改代码

源代码

package club.zstuca.demo;import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;/*** Servlet implementation class DownloadDemo*/
@WebServlet("/DownloadDemo")
public class DownloadDemo extends HttpServlet {private static final long serialVersionUID = 1L;/*** @see HttpServlet#HttpServlet()*/public DownloadDemo() {super();// TODO Auto-generated constructor stub}/*** @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)*/protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubrequest.setCharacterEncoding("UTF-8");//response.setContentType("application/octet-stream");response.addHeader("content-Type", "application/octet-stream" );String filename = request.getParameter("filename");response.addHeader("content-Disposition", "attachment; filename=" + filename);//response.getWriter().append("Served at: ").append(request.getContextPath());InputStream in = this.getServletContext().getResourceAsStream("/res/" + filename);System.out.println("res/" + filename);System.out.println(in);ServletOutputStream out = response.getOutputStream();byte[] bs = new byte[1024];int len = -1;while((len=in.read(bs))!=-1) {out.write(bs,0,len);}out.close();   }/*** @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)*/protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}

参考文章

http://www.imooc.com/wenda/detail/593313

https://www.cnblogs.com/lebo0425/p/6607804.html

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Java Web——文件下载getResourceAsStream()返回NULL解决方案

问题描述

问题分析

API

解决方案

源代码

参考文章

Java Web——文件下载getResourceAsStream()返回NULL解决方案相关推荐

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