httpServletRequest中的流只能讀取一次的原因
阿新 • • 發佈：2017-06-14
eve puts object c row led 實現原理 see bject maximum
/* */ 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 [email protected] ReadListener#onDataAvailable()} when data is
* available.
*
* @since Servlet 3.1
*/
public abstract boolean isReady();
/**
* Sets the [email protected] ReadListener} for this [email protected] 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 [email protected] ReadListener} has already
* been set
* @throws NullPointerException If listener is null
*
* @since Servlet 3.1
*/
public abstract void setReadListener(ReadListener listener);
}
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，標誌當前流讀取到的位置，每讀取一次，位置就會移動一次，如果讀到最後，InputStream.read方法會返回-1，標誌已經讀取完了，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讀取一次後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 * [email protected]
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。
httpServletRequest中的流只能讀取一次的原因