request.getInputStream() 流只能读取一次问题

问题： 一次开发过程中同事在 sptring interceptor 中获取 request body 中值，以对数据的校验和预处理等操作 、导致之后spring 在读取request body 值做数据映射时一直报 request body is null 、以此在这通过源码回顾了一下 InputStream read 的方法中的基础知

首先来看看inputStream 中 read() 源码 实现以 ByteArrayInputStream 中源码来查看：

1: inputStream read()

 /**
     * Reads the next byte of data from the input stream. The value byte is
     * returned as an <code>int</code> in the range <code>0</code> to
     * <code>255</code>. If no byte is available because the end of the stream
     * has been reached, the value <code>-1</code> is returned. This method
     * blocks until input data is available, the end of the stream is detected,
     * or an exception is thrown.
     *
     * <p> A subclass must provide an implementation of this method.
     *
     * @return     the next byte of data, or <code>-1</code> if the end of the
     *             stream is reached.
     * @exception  IOException  if an I/O error occurs.
     */
    public abstract int read() throws IOException;

大致意思从输入流中读取下一个字节、如果以达到末尾侧返回-1

2: ByteArrayInputStream 中实现

 /**
     * Reads the next byte of data from this input stream. The value
     * byte is returned as an <code>int</code> in the range
     * <code>0</code> to <code>255</code>. If no byte is available
     * because the end of the stream has been reached, the value
     * <code>-1</code> is returned.
     * <p>
     * This <code>read</code> method
     * cannot block.
     *
     * @return  the next byte of data, or <code>-1</code> if the end of the
     *          stream has been reached.
     */
    public synchronized int read() {
        return (pos < count) ? (buf[pos++] & 0xff) : -1;
    }

这个实现看起来很好理解、 方法内pos 标识当前流每次流读取的位置、 每读取一次pos 做一次位移、直至结束返回-1: 看看 pos 的 值定义：

/**
     * Creates a <code>ByteArrayInputStream</code>
     * so that it  uses <code>buf</code> as its
     * buffer array.
     * The buffer array is not copied.
     * The initial value of <code>pos</code>
     * is <code>0</code> and the initial value
     * of  <code>count</code> is the length of
     * <code>buf</code>.
     *
     * @param   buf   the input buffer.
     */
    public ByteArrayInputStream(byte buf[]) {
        this.buf = buf;
        this.pos = 0;
        this.count = buf.length;
    }

在这可以看到在类实例化时 给 pos 初始化 0 默认从流的的起始位置开始读！ 当然如果想从固定位置读区可以看read的其它构造方法在这就不多说了。

到这之后当流读取完成后 pos 变量已经达到流的末尾处。这是如果在读取就会直接返回 -1 、 现在很明白了、如果想在此读取流中的值只需要把 pos 的值 rest 到初始位置就可以、 OK 没问题在 inputStrean 也提供了 rest() 方法 我们一起来看看它的实现：

/**
     * Resets the buffer to the marked position.  The marked position
     * is 0 unless another position was marked or an offset was specified
     * in the constructor.
     */
    public synchronized void reset() {
        pos = mark;
    }

 /**
     * The currently marked position in the stream.
     * ByteArrayInputStream objects are marked at position zero by
     * default when constructed.  They may be marked at another
     * position within the buffer by the <code>mark()</code> method.
     * The current buffer position is set to this point by the
     * <code>reset()</code> method.
     * <p>
     * If no mark has been set, then the value of mark is the offset
     * passed to the constructor (or 0 if the offset was not supplied).
     *
     * @since   JDK1.1
     */
    protected int mark = 0;

从这两段中不难看出rest方法是将 pos 值从新初始化为0、 当然到这还没有结束、并不是所有的流都有权限实现 rest() 方法的取决条件在 markSupported() 方法中 请看下面源码的介绍

 /**
     * Tests if this <code>InputStream</code> supports mark/reset. The
     * <code>markSupported</code> method of <code>ByteArrayInputStream</code>
     * always returns <code>true</code>.
     *
     * @since   JDK1.1
     */
    public boolean markSupported() {
        return true;
    }

从方法的注释上很容易看出markSupported是 mark/reset 方法的标识变量、由它来决定 mark/reset 是否可调用。

到这我们基本很清楚inputStrean 中 read() 方法了。

回头我们再来看看 request.getInputStream

/**
     * Retrieves the body of the request as binary data using a
     * {@link ServletInputStream}. Either this method or {@link #getReader} may
     * be called to read the body, not both.
     *
     * @return a {@link ServletInputStream} object containing the body of the
     *         request
     * @exception IllegalStateException
     *                if the {@link #getReader} method has already been called
     *                for this request
     * @exception IOException
     *                if an input or output exception occurred
     */
    public ServletInputStream getInputStream() throws IOException;

从源码中可以看出 request.getInputStream 方法返回的是 ServletInputStream 对象 我们在来看看 ServletInputStream 中源码的实现

/*
 * 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;
    }

    /**
     * Has the end of this InputStream been reached?
     *
     * @return <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();

    /**
     * Can data be read from this InputStream without blocking?
     * Returns  If this method is called and returns false, the container will
     * invoke {@link ReadListener#onDataAvailable()} when data is available.
     *
     * @return <code>true</code> if data can be read without blocking, else
     * <code>false</code>
     *
     * @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);
}

从源码中查看 ServletInputStream 并没有重写 rest() 方法、我们在 到 InputStream 中去查看

/**
     * Repositions this stream to the position at the time the
     * <code>mark</code> method was last called on this input stream.
     *
     * <p> The general contract of <code>reset</code> is:
     *
     * <ul>
     * <li> If the method <code>markSupported</code> returns
     * <code>true</code>, then:
     *
     *     <ul><li> If the method <code>mark</code> has not been called since
     *     the stream was created, or the number of bytes read from the stream
     *     since <code>mark</code> was last called is larger than the argument
     *     to <code>mark</code> at that last call, then an
     *     <code>IOException</code> might be thrown.
     *
     *     <li> If such an <code>IOException</code> is not thrown, then the
     *     stream is reset to a state such that all the bytes read since the
     *     most recent call to <code>mark</code> (or since the start of the
     *     file, if <code>mark</code> has not been called) will be resupplied
     *     to subsequent callers of the <code>read</code> method, followed by
     *     any bytes that otherwise would have been the next input data as of
     *     the time of the call to <code>reset</code>. </ul>
     *
     * <li> If the method <code>markSupported</code> returns
     * <code>false</code>, then:
     *
     *     <ul><li> The call to <code>reset</code> may throw an
     *     <code>IOException</code>.
     *
     *     <li> If an <code>IOException</code> is not thrown, then the stream
     *     is reset to a fixed state that depends on the particular type of the
     *     input stream and how it was created. The bytes that will be supplied
     *     to subsequent callers of the <code>read</code> method depend on the
     *     particular type of the input stream. </ul></ul>
     *
     * <p>The method <code>reset</code> for class <code>InputStream</code>
     * does nothing except throw an <code>IOException</code>.
     *
     * @exception  IOException  if this stream has not been marked or if the
     *               mark has been invalidated.
     * @see     java.io.InputStream#mark(int)
     * @see     java.io.IOException
     */
    public synchronized void reset() throws IOException {
        throw new IOException("mark/reset not supported");
    }

    /**
     * Tests if this input stream supports the <code>mark</code> and
     * <code>reset</code> methods. Whether or not <code>mark</code> and
     * <code>reset</code> are supported is an invariant property of a
     * particular input stream instance. The <code>markSupported</code> method
     * of <code>InputStream</code> returns <code>false</code>.
     *
     * @return  <code>true</code> if this stream instance supports the mark
     *          and reset methods; <code>false</code> otherwise.
     * @see     java.io.InputStream#mark(int)
     * @see     java.io.InputStream#reset()
     */
    public boolean markSupported() {
        return false;
    }

至此我们基本搞清楚了 为什么 reqeust.getInputStream 方法只能读取一次、 因在读取一次后 pos 值已经达到文件末尾、而 ServletInputStream 没有重写 rest() 方法、从而导致request.getInputStream 只能读取一次。

解决方案其实很简单 可以使用 HttpServletRequest 的装饰器 HttpServletRequestWrapper 来解决。写一个简单例子仅供参考： 这里装饰器模式不了解的同学可以去了解下。

import java.io.BufferedReader;
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.nio.charset.StandardCharsets;

import javax.servlet.ReadListener;
import javax.servlet.ServletInputStream;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class RequestWrapper extends HttpServletRequestWrapper {

	private byte[] body;

	public RequestWrapper(HttpServletRequest request) {
		super(request);
	}

	public RequestWrapper(HttpServletRequest request, String body) {
		super(request);
		this.body = body.getBytes(StandardCharsets.UTF_8);
	}

	@Override
	public ServletInputStream getInputStream() throws IOException {
		final ByteArrayInputStream bais = new ByteArrayInputStream(body);

		return new ServletInputStream() {

			@Override
			public boolean isFinished() {
				// Auto-generated method stub
				return false;
			}

			@Override
			public boolean isReady() {
				// Auto-generated method stub
				return false;
			}

			@Override
			public void setReadListener(ReadListener listener) {
				// Auto-generated method stub

			}

			@Override
			public int read() throws IOException {
				return bais.read();
			}
		};
	}

	@Override
	public BufferedReader getReader() throws IOException {
		return new BufferedReader(new InputStreamReader(this.getInputStream()));
	}

	public String getBody() {
	    return new String(body, StandardCharsets.UTF_8);
	}

}

这是一个简单的列子、 具体使用根据自己业务来实现。

本章就介绍到这！如有不对的地方请多多指教、大家互相学习、谢谢！