Servlets ServletContext#getResourceAsStream（）返回null

有人能回答这个问题吗？我不知道为什么在执行下面的程序时会出现NullPointerException

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException 
{
    response.setContentType("image/jpeg");
    ServletContext ctx = getServletContext();
    InputStream is = ctx.getResourceAsStream("C:/Users/Public/Pictures/Sample Pictures/Desert.jpg"); --> The picture exists at this location
    OutputStream os = response.getOutputStream();
    int read = 0;
    byte[] bytes = new byte[1024];
    System.out.println("bytes :" + bytes);
    while((read = is.read(bytes)) != -1) -- Error at this line
    {
    System.out.println("read :" +read);
    os.write(bytes, 0, read);
    }
    os.flush();
    os.close();
    }

在tomcat Apache中运行此命令时，出现以下错误

bytes :[B@158f9d3
Sep 19, 2013 7:23:04 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet Newest threw exception
java.lang.NullPointerException
        at com.examples.Newest.doPost(Newest.java:69)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:172)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:174)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:873)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:689)
at java.lang.Thread.run(Thread.java:662)

这个

不是您在文件系统上访问文件的方式。使用该方法调用，您正试图获取。而且，由于相对于上下文根，您显然没有名为

C:/Users/Public/Pictures/Sample Pictures/Desert.jpg的资源，因此它返回
null

使用Java7的NIO

Path path = Paths.get("C:/Users/Public/Pictures/Sample Pictures/Desert.jpg");
InputStream is = Files.newInputStream(path);

Path path = Paths.get("C:/Users/Public/Pictures/Sample Pictures/Desert.jpg");
InputStream is = Files.newInputStream(path);