httpServletRequest中的流只能讀取一次的原因

首先，我們復習一下InputStream read方法的基礎知識，

java InputStream read方法內部有一個，postion，標志當前流讀取到的位置，每讀取一次，位置就會移動一次，如果讀到最后，InputStream.read方法會返回-1，標志已經讀取完了，如果想再次讀取，可以調用inputstream.reset方法，position就會移動到上次調用mark的位置，mark默認是0，所以就能從頭再讀了。

當然，能否reset是有條件的，它取決於markSupported,markSupported() 方法返回是否可以mark/reset

我們再回頭看request.getInputStream

request.getInputStream返回的值是ServletInputStream,查看ServletInputStream源碼發現，沒有重寫reset方法，所以查看InputStream源碼發現marksupported 返回false，並且reset方法，直接拋出異常。

InputStream.java 

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

　

綜上所述，在request.getinputstream讀取一次后position到了文件末尾，第二次就讀取不到數據，由於無法reset()，所以，request.getinputstream只能讀取一次。

 

總結：

這個問題最根本還是對java IO的read、reset方法的深入理解，尤其是read方法的內部實現原理。

附ServletInputStream.java源碼

/*
 * 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);
}

　　

以上這篇淺談request.getinputstream只能讀取一次的問題就是小編分享給大家的全部內容了，希望能給大家一個參考。