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  • httpServletRequest中的流只能读取一次的原因

    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 == '
    ' || 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。

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  • 原文地址:https://www.cnblogs.com/yepei/p/7011081.html
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