浅谈request.getinputstream只能读取一次的问题

作者：袖梨 2022-06-29

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

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

代码如下 复制代码
/*
* 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 readLine method for reading data one line
* at a time. With some protocols, such as HTTP POST and PUT, a
* ServletInputStream object can be used to read data sent from the
* client.
*


* A ServletInputStream object is normally retrieved via the
* {@link ServletRequest#getInputStream} method.
*


* This is an abstract class that a servlet container implements. Subclasses of
* this class must implement the java.io.InputStream.read() 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.
*


* 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
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 true if all the data has been read from the stream,
* else false.
*
* @since Servlet 3.1
*/
public abstract boolean isFinished();
/**
* Returns true if data can be read without blocking, else
* false. 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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