浅谈request.getinputstream只能读取一次的问题

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
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * 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);
}

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