Problem Description

Critical: Servlet.service() for servlet [club.zstuca.demo.DownloadDemo] in context with path [/DEMOWeb] threw exception
java.lang.NullPointerException
	at 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)

problem analysis

The file is not in the root directory of the WebAPP, causing the resource to be found.

API

Class.getResourceAsStream(String path)

JDK sets up such rules, which is well understood. When path does not start with "/", we can get the same resource file as the path where the current class is located, and when it starts with "/", we can get any path under the root of classPath. resource of.

When the path does not start with "/", the default is to obtain resources from the package where the current class is located

When the path starts with "/", resources are obtained from the project's classPath root

ClassLoader.getResourceAsStream(String path)

The default is to get from the root of the classpath,path cannot start with "/", Path refers to the loading range of the class loader. In the process of resource loading, it is loaded in the form of step-by-step delegation. "/" means the loading range in Boot ClassLoader, because this class loader is implemented in C++, So the loading range is null.

ServletContext.getResourceAsStream(String path)

By default, resources are fetched from the root directory of WebAPP. It doesn't matter whether the path under Tomcat starts with "/", of course this is related to the specific container implementation.

solution

1. Create a res folder in the root directory of WebAPP and move resources into this folder

2. Modify the code

Source code

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 stub
		request.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 stub
		doGet(request, response);
	}

}

Reference article