BufferedInputStream

public class BufferedInputStream extends FilterInputStream {

	private static int defaultBufferSize = 8192;

	// 内部用来存储数据的缓冲数组
	protected volatile byte buf[];
	
	private static final AtomicReferenceFieldUpdater<BufferedInputStream, byte[]> bufUpdater = AtomicReferenceFieldUpdater
			.newUpdater(BufferedInputStream.class, byte[].class, "buf");

	protected int count;

	protected int pos;

	protected int markpos = -1;

	protected int marklimit;
	
	//构造器，传入一个输入流，为内部的字节数组开辟空间
	public BufferedInputStream(InputStream in) {
		this(in, defaultBufferSize);
	}

	public BufferedInputStream(InputStream in, int size) {
		super(in);
		if (size <= 0) {
			throw new IllegalArgumentException("Buffer size <= 0");
		}
		buf = new byte[size];
	}

	//获取输入流
	private InputStream getInIfOpen() throws IOException {
		InputStream input = in;//in继承自FilterInputStream
		if (input == null)
			throw new IOException("Stream closed");
		return input;
	}

	//获取缓冲数组首地址
	private byte[] getBufIfOpen() throws IOException {
		byte[] buffer = buf;
		if (buffer == null)
			throw new IOException("Stream closed");
		return buffer;
	}

	

	/**
	 * 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;
	}

	/**
	 * See the general contract of the <code>read</code> method of
	 * <code>InputStream</code>.
	 * 
	 * @return the next byte of data, or <code>-1</code> if the end of the
	 *         stream is reached.
	 * @exception IOException
	 *                if this input stream has been closed by invoking its
	 *                {@link #close()} method, or an I/O error occurs.
	 * @see java.io.FilterInputStream#in
	 */
	public synchronized int read() throws IOException {
		if (pos >= count) {
			fill();
			if (pos >= count)
				return -1;
		}
		return getBufIfOpen()[pos++] & 0xff;
	}

	/**
	 * Read characters into a portion of an array, reading from the underlying
	 * stream at most once if necessary.
	 */
	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;
	}

	/**
	 * Reads bytes from this byte-input stream into the specified byte array,
	 * starting at the given offset.
	 * 
	 * <p>
	 * This method implements the general contract of the corresponding
	 * <code>{@link InputStream#read(byte[], int, int) read}</code> method of
	 * the <code>{@link InputStream}</code> class. As an additional convenience,
	 * it attempts to read as many bytes as possible by repeatedly invoking the
	 * <code>read</code> method of the underlying stream. This iterated
	 * <code>read</code> continues until one of the following conditions becomes
	 * true:
	 * <ul>
	 * 
	 * <li>The specified number of bytes have been read,
	 * 
	 * <li>The <code>read</code> method of the underlying stream returns
	 * <code>-1</code>, indicating end-of-file, or
	 * 
	 * <li>The <code>available</code> method of the underlying stream returns
	 * zero, indicating that further input requests would block.
	 * 
	 * </ul>
	 * If the first <code>read</code> on the underlying stream returns
	 * <code>-1</code> to indicate end-of-file then this method returns
	 * <code>-1</code>. Otherwise this method returns the number of bytes
	 * actually read.
	 * 
	 * <p>
	 * Subclasses of this class are encouraged, but not required, to attempt to
	 * read as many bytes as possible in the same fashion.
	 * 
	 * @param b
	 *            destination buffer.
	 * @param off
	 *            offset at which to start storing bytes.
	 * @param len
	 *            maximum number of bytes to read.
	 * @return the number of bytes read, or <code>-1</code> if the end of the
	 *         stream has been reached.
	 * @exception IOException
	 *                if this input stream has been closed by invoking its
	 *                {@link #close()} method, or an I/O error occurs.
	 */
	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;
		}
	}

	/**
	 * See the general contract of the <code>skip</code> method of
	 * <code>InputStream</code>.
	 * 
	 * @exception IOException
	 *                if the stream does not support seek, or if this input
	 *                stream has been closed by invoking its {@link #close()}
	 *                method, or an I/O error occurs.
	 */
	public synchronized long skip(long n) throws IOException {
		getBufIfOpen(); // Check for closed stream
		if (n <= 0) {
			return 0;
		}
		long avail = count - pos;

		if (avail <= 0) {
			// If no mark position set then don't keep in buffer
			if (markpos < 0)
				return getInIfOpen().skip(n);

			// Fill in buffer to save bytes for reset
			fill();
			avail = count - pos;
			if (avail <= 0)
				return 0;
		}

		long skipped = (avail < n) ? avail : n;
		pos += skipped;
		return skipped;
	}

	/**
	 * Returns an estimate of the number of bytes that can be read (or skipped
	 * over) from this input stream without blocking by the next invocation of a
	 * method for this input stream. The next invocation might be the same
	 * thread or another thread. A single read or skip of this many bytes will
	 * not block, but may read or skip fewer bytes.
	 * <p>
	 * This method returns the sum of the number of bytes remaining to be read
	 * in the buffer (<code>count&nbsp;- pos</code>) and the result of calling
	 * the {@link java.io.FilterInputStream#in in}.available().
	 * 
	 * @return an estimate of the number of bytes that can be read (or skipped
	 *         over) from this input stream without blocking.
	 * @exception IOException
	 *                if this input stream has been closed by invoking its
	 *                {@link #close()} method, or an I/O error occurs.
	 */
	public synchronized int available() throws IOException {
		return getInIfOpen().available() + (count - pos);
	}

	/**
	 * See the general contract of the <code>mark</code> method of
	 * <code>InputStream</code>.
	 * 
	 * @param readlimit
	 *            the maximum limit of bytes that can be read before the mark
	 *            position becomes invalid.
	 * @see java.io.BufferedInputStream#reset()
	 */
	public synchronized void mark(int readlimit) {
		marklimit = readlimit;
		markpos = pos;
	}

	/**
	 * See the general contract of the <code>reset</code> method of
	 * <code>InputStream</code>.
	 * <p>
	 * If <code>markpos</code> is <code>-1</code> (no mark has been set or the
	 * mark has been invalidated), an <code>IOException</code> is thrown.
	 * Otherwise, <code>pos</code> is set equal to <code>markpos</code>.
	 * 
	 * @exception IOException
	 *                if this stream has not been marked or, if the mark has
	 *                been invalidated, or the stream has been closed by
	 *                invoking its {@link #close()} method, or an I/O error
	 *                occurs.
	 * @see java.io.BufferedInputStream#mark(int)
	 */
	public synchronized void reset() throws IOException {
		getBufIfOpen(); // Cause exception if closed
		if (markpos < 0)
			throw new IOException("Resetting to invalid mark");
		pos = markpos;
	}

	/**
	 * Tests if this input stream supports the <code>mark</code> and
	 * <code>reset</code> methods. The <code>markSupported</code> method of
	 * <code>BufferedInputStream</code> returns <code>true</code>.
	 * 
	 * @return a <code>boolean</code> indicating if this stream type supports
	 *         the <code>mark</code> and <code>reset</code> methods.
	 * @see java.io.InputStream#mark(int)
	 * @see java.io.InputStream#reset()
	 */
	public boolean markSupported() {
		return true;
	}

	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()
		}
	}
}