首先，我们复习一下InputStream read方法的基础知识，

java InputStream read方法内部有一个，postion，标志当前流读取到的位置，每读取一次，位置就会移动一次，如果读到最后，InputStream.read方法会返回-1，标志已经读取完了，如果想再次读取，可以调用inputstream.reset方法，position就会移动到上次调用mark的位置，mark默认是0，所以就能从头再读了。

当然，能否reset是有条件的，它取决于markSupported,markSupported() 方法返回是否可以mark/reset

我们再回头看request.getInputStream

request.getInputStream返回的值是ServletInputStream,查看ServletInputStream源码发现，没有重写reset方法，所以查看InputStream源码发现marksupported 返回false，并且reset方法，直接抛出异常。

InputStream.java 

public boolean markSupported() {   return false; }  public synchronized void reset() throws IOException {   throw new IOException("mark/reset not supported"); }

综上所述，在request.getinputstream读取一次后position到了文件末尾，第二次就读取不到数据，由于无法reset()，所以，request.getinputstream只能读取一次。

 

总结：

这个问题最根本还是对java IO的read、reset方法的深入理解，尤其是read方法的内部实现原理。

附ServletInputStream.java源码

/* * The contents of this file are subject to the terms * of the Common Development and Distribution License * (the "License").  You may not use this file except * in compliance with the License. * * You can obtain a copy of the license at * glassfish/bootstrap/legal/CDDLv1.0.txt or * https://glassfish.dev.java.net/public/CDDLv1.0.html. * See the License for the specific language governing * permissions and limitations under the License. * * When distributing Covered Code, include this CDDL * HEADER in each file and include the License file at * glassfish/bootstrap/legal/CDDLv1.0.txt.  If applicable, * add the following below this CDDL HEADER, with the * fields enclosed by brackets "[]" replaced with your * own identifying information: Portions Copyright [yyyy] * [name of copyright owner] * * Copyright 2005 Sun Microsystems, Inc. All rights reserved. * * Portions Copyright Apache Software Foundation. */ package javax.servlet;import java.io.InputStream;import java.io.IOException;/** *  * Provides an input stream for reading binary data from a client * request, including an efficient readLine method * for reading data one line at a time. With some protocols, such * as HTTP POST and PUT, a ServletInputStream * object can be used to read data sent from the client. * * 
A ServletInputStream object is normally retrieved via * the {
    @link ServletRequest#getInputStream} method. * * * 
This is an abstract class that a servlet container implements. * Subclasses of this class * must implement the java.io.InputStream.read() method. * * * @author     Various * * @see        ServletRequest  * */public abstract class ServletInputStream extends InputStream {    /**     * Does nothing, because this is an abstract class.     *     */    protected ServletInputStream() { }            /**     *     * Reads the input stream, one line at a time. Starting at an     * offset, reads bytes into an array, until it reads a certain number     * of bytes or reaches a newline character, which it reads into the     * array as well.     *     * 
This method returns -1 if it reaches the end of the input     * stream before reading the maximum number of bytes.     *     *     *     * @param b         an array of bytes into which data is read     *     * @param off         an integer specifying the character at which     *                this method begins reading     *     * @param len        an integer specifying the maximum number of      *                bytes to read     *     * @return            an integer specifying the actual number of bytes      *                read, or -1 if the end of the stream is reached     *     * @exception IOException    if an input or output exception has occurred     *     */         public int readLine(byte[] b, int off, int len) throws IOException {    if (len <= 0) {        return 0;    }    int count = 0, c;    while ((c = read()) != -1) {        b[off++] = (byte)c;        count++;        if (c == '\n' || count == len) {        break;        }    }    return count > 0 ? count : -1;    }}