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
/* * Licensed to the Apache Software Foundation (ASF) under one or more * contributor license agreements. See the NOTICE file distributed with * this work for additional information regarding copyright ownership. * The ASF licenses this file to You under the Apache License, Version 2.0 * (the "License"); you may not use this file except in compliance with * the License. You may obtain a copy of the License at * * * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package javax.servlet; import java.io.IOException; import java.io.InputStream; /** * 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. * * @see ServletRequest */ public abstract class ServletInputStream extends InputStream { /** * Does nothing, because this is an abstract class. */ protected ServletInputStream() { // NOOP } /** * 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; } /** * Returns <code>true</code> if all the data has been read from the stream, * else <code>false</code>. * * @since Servlet 3.1 */ public abstract boolean isFinished(); /** * Returns <code>true</code> if data can be read without blocking, else * <code>false</code>. If this method is called and returns false, the * container will invoke {@link ReadListener#onDataAvailable()} when data is * available. * * @since Servlet 3.1 */ public abstract boolean isReady(); /** * Sets the {@link ReadListener} for this {@link ServletInputStream} and * thereby switches to non-blocking IO. It is only valid to switch to * non-blocking IO within async processing or HTTP upgrade processing. * * @param listener The non-blocking IO read listener * * @throws IllegalStateException If this method is called if neither * async nor HTTP upgrade is in progress or * if the {@link ReadListener} has already * been set * @throws NullPointerException If listener is null * * @since Servlet 3.1 */ public abstract void setReadListener(ReadListener listener); }
以上这篇浅谈request.getinputstream只能读取一次的问题就是小编分享给大家的全部内容了，希望能给大家一个参考，也希望大家多多支持脚本之家。