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 <code>readLine</code> method * for reading data one line at a time. With some protocols, such * as HTTP POST and PUT, a <code>ServletInputStream</code> * object can be used to read data sent from the client. * * <p>A <code>ServletInputStream</code> object is normally retrieved via * the {@link ServletRequest#getInputStream} method. * * * <p>This is an abstract class that a servlet container implements. * Subclasses of this class * must implement the <code>java.io.InputStream.read()</code> 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. ????* ????* <p>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; ???}}httpServletRequest中的流只能读取一次的原因
原文地址：https://www.cnblogs.com/zhaoyibing/p/9132513.html