BufferedInputStream 深入研究。

1. BufferedInputStream的基本原理

BuffredInputStream存在的意义在于其提供了一个内部缓冲区，当read的的时候先一次性的把byte读取到内部的缓冲区，以后每次调用read(byte[])实际是从缓冲区 copy数据。缓冲区的大小可以通过BufferedInputStream(InputStream in, int size)构造函数设置，默认大小8192。

2.BufferedInputStream的实现

注意read方法是同步的:

  public synchronized int read(byte b[], int off, int len)
	throws IOException
    {
        getBufIfOpen(); // Check for closed stream
        if ((off | len | (off + len) | (b.length - (off + len))) < 0) {
	    throw new IndexOutOfBoundsException();
	} else if (len == 0) {
            return 0;
        }

	int n = 0;
        for (;;) {
            int nread = read1(b, off + n, len - n);
            if (nread <= 0) 
                return (n == 0) ? nread : n;
            n += nread;
            if (n >= len)
                return n;
            // if not closed but no bytes available, return
            InputStream input = in;
            if (input != null && input.available() <= 0)
                return n;
        }
    }

 实现都委托给里read1()

   private int read1(byte[] b, int off, int len) throws IOException {
	int avail = count - pos;
	if (avail <= 0) {
	    /* If the requested length is at least as large as the buffer, and
	       if there is no mark/reset activity, do not bother to copy the
	       bytes into the local buffer.  In this way buffered streams will
	       cascade harmlessly. */
	    if (len >= getBufIfOpen().length && markpos < 0) {
		return getInIfOpen().read(b, off, len);
	    }
	    fill();
	    avail = count - pos;
	    if (avail <= 0) return -1;
	}
	int cnt = (avail < len) ? avail : len;
	System.arraycopy(getBufIfOpen(), pos, b, off, cnt);
	pos += cnt;
	return cnt;
    }

 fill:

/**
     * Fills the buffer with more data, taking into account
     * shuffling and other tricks for dealing with marks.
     * Assumes that it is being called by a synchronized method.
     * This method also assumes that all data has already been read in,
     * hence pos > count.
     */
    private void fill() throws IOException {
        byte[] buffer = getBufIfOpen();
	if (markpos < 0)
	    pos = 0;		/* no mark: throw away the buffer */
	else if (pos >= buffer.length)	/* no room left in buffer */
	    if (markpos > 0) {	/* can throw away early part of the buffer */
		int sz = pos - markpos;
		System.arraycopy(buffer, markpos, buffer, 0, sz);
		pos = sz;
		markpos = 0;
	    } else if (buffer.length >= marklimit) {
		markpos = -1;	/* buffer got too big, invalidate mark */
		pos = 0;	/* drop buffer contents */
	    } else {		/* grow buffer */
		int nsz = pos * 2;
		if (nsz > marklimit)
		    nsz = marklimit;
		byte nbuf[] = new byte[nsz];
		System.arraycopy(buffer, 0, nbuf, 0, pos);
                if (!bufUpdater.compareAndSet(this, buffer, nbuf)) {
                    // Can't replace buf if there was an async close.
                    // Note: This would need to be changed if fill()
                    // is ever made accessible to multiple threads.
                    // But for now, the only way CAS can fail is via close.
                    // assert buf == null;
                    throw new IOException("Stream closed");
                }
                buffer = nbuf;
	    }
        count = pos;
	int n = getInIfOpen().read(buffer, pos, buffer.length - pos);
        if (n > 0)
            count = n + pos;
    }

 3.BufferedInputStream对buffer的并发保护

buffreedInputStream是有状态的，需要考虑线程安全的问题，那么来看看它是如何保护buffer。首先

buf被定义为volatile,并且通过AtomicReferenceFieldUpdater来保护。

    /**
     * The internal buffer array where the data is stored. When necessary,
     * it may be replaced by another array of
     * a different size.
     */
    protected volatile byte buf[];

    /**
     * Atomic updater to provide compareAndSet for buf. This is
     * necessary because closes can be asynchronous. We use nullness
     * of buf[] as primary indicator that this stream is closed. (The
     * "in" field is also nulled out on close.)
     */
    private static final 
        AtomicReferenceFieldUpdater<BufferedInputStream, byte[]> bufUpdater = 
        AtomicReferenceFieldUpdater.newUpdater
        (BufferedInputStream.class,  byte[].class, "buf");

 AtomicReferenceFieldUpdater基于反射的实用工具，可以对指定类的指定 volatile 字段进行原子更新。用法： fill的时候：

System.arraycopy(buffer, 0, nbuf, 0, pos);
if (!bufUpdater.compareAndSet(this, buffer, nbuf)) {
	// Can't replace buf if there was an async close.
	// Note: This would need to be changed if fill()
	// is ever made accessible to multiple threads.
	// But for now, the only way CAS can fail is via close.
	// assert buf == null;
	throw new IOException("Stream closed");
}

close()的时候：

    public void close() throws IOException {
        byte[] buffer;
        while ( (buffer = buf) != null) {
            if (bufUpdater.compareAndSet(this, buffer, null)) {
                InputStream input = in;
                in = null;
                if (input != null)
                    input.close();
                return;
            }
            // Else retry in case a new buf was CASed in fill()
        }
    }

4.BufferedInputStream使用的注意点  这段代码有一隐含的bug，也是使用inputStream读取容易出错的地方。

	String str = "hellohellohelloh123";
		ByteArrayInputStream input = new ByteArrayInputStream(str.getBytes());
		BufferedInputStream buffer = new BufferedInputStream(input);
		ByteArrayOutputStream out = new ByteArrayOutputStream(8192);
		byte[] chunk = new byte[3];
		int readByte = -1;

		while ((readByte = buffer.read(chunk)) != -1) {
			
			out.write(chunk);
		}

		System.out.println(str);
		System.out.println(out.toString());

 这段代码的输出结果： hellohellohelloh123
hellohellohelloh12312 如果在while中加入System.out.println(readByte + ":" + new String(chunk));可以打印出：

3:hel
3:loh
3:ell
3:ohe
3:llo
3:h12
1:312

 最后一次read只读了1个byte，所以out.write(chunk)应该修改为

out.write(chunk, 0, readByte);