Java IO流源码学习之二（Buffered字节流）

本文记录BufferedInputStream，BufferedOutputStream两个带缓冲的输入输出流。

BufferedInputStream继承于FilterInputStream，BufferedOutputStream继承于FilterOutputStream，这两个类为具体的装饰类，而FilterInputStream与FilterOutputStream为抽象装饰者类，但是该两个类不是抽象类。

BufferedInputStream

下面是BufferedInputStream的源码：

public class BufferedInputStream extends FilterInputStream {

	//默认缓冲区大小
    private static int DEFAULT_BUFFER_SIZE = 8192;

    //最大缓冲区大小
    private static int MAX_BUFFER_SIZE = Integer.MAX_VALUE - 8;

    //内部缓冲数组
    protected volatile byte buf[];

    //为buf缓冲区提供原子的更新器
    private static final
        AtomicReferenceFieldUpdater<BufferedInputStream, byte[]> bufUpdater =
        AtomicReferenceFieldUpdater.newUpdater
        (BufferedInputStream.class,  byte[].class, "buf");

   	//该值表示缓冲区下一个接收Input Stream字节的位置
    protected int count;

    //缓冲区的当前位置，也就是下一个从缓冲区读数据的位置，该值小于等于count
    protected int pos;

    //Mark标记的位置
    protected int markpos = -1;

    //pos - markpos > marklimit 时，Mark标记清除
    protected int marklimit;

    //获取该对象相应的InputStream对象
    private InputStream getInIfOpen() throws IOException {
        InputStream input = in;
        if (input == null)
            throw new IOException("Stream closed");
        return input;
    }
	//获取buf缓冲区
    private byte[] getBufIfOpen() throws IOException {
        byte[] buffer = buf;
        if (buffer == null)
            throw new IOException("Stream closed");
        return buffer;
    }
	//构造方法，调用父类方法，用于对InputStream 对象进行增强
    public BufferedInputStream(InputStream in) {
        this(in, DEFAULT_BUFFER_SIZE);
    }
	
    public BufferedInputStream(InputStream in, int size) {
        super(in);
        if (size <= 0) {
            throw new IllegalArgumentException("Buffer size <= 0");
        }
        buf = new byte[size];
    }
	//读入字节填充缓冲区
    private void fill() throws IOException {
        byte[] buffer = getBufIfOpen();
        //如果标记位无效，则直接填充缓冲区，并且覆盖原来的字节
        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 if (buffer.length >= MAX_BUFFER_SIZE) {
                throw new OutOfMemoryError("Required array size too large");
            } else {            /* grow buffer */
                int nsz = (pos <= MAX_BUFFER_SIZE - pos) ?
                        pos * 2 : MAX_BUFFER_SIZE;
                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;
    }

    //向缓冲区读取一个数据，若缓冲区没有数据，则使用fill方法填充缓冲区
    public synchronized int read() throws IOException {
        if (pos >= count) {
            fill();
            if (pos >= count)
                return -1;
        }
        return getBufIfOpen()[pos++] & 0xff;
    }

    //读取字节至相应数组（private方法）
    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;
    }

    //读取相应字节至数组，为一个同步方法，线程安全
    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;
        }
    }

    //跳过n个字节
    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;
    }

    public synchronized int available() throws IOException {
        int n = count - pos;
        int avail = getInIfOpen().available();
        return n > (Integer.MAX_VALUE - avail)
                    ? Integer.MAX_VALUE
                    : n + avail;
    }

    public synchronized void mark(int readlimit) {
        marklimit = readlimit;
        markpos = pos;
    }

    public synchronized void reset() throws IOException {
        getBufIfOpen(); // Cause exception if closed
        if (markpos < 0)
            throw new IOException("Resetting to invalid mark");
        pos = markpos;
    }

    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()
        }
    }
}

BufferedOutputStream

public
class BufferedOutputStream extends FilterOutputStream {
    //buffer数组
    protected byte buf[];

    //buffer中有效值
    protected int count;

    public BufferedOutputStream(OutputStream out) {
        this(out, 8192);
    }

    public BufferedOutputStream(OutputStream out, int size) {
        super(out);
        if (size <= 0) {
            throw new IllegalArgumentException("Buffer size <= 0");
        }
        buf = new byte[size];
    }

    //清空缓冲区
    private void flushBuffer() throws IOException {
        if (count > 0) {
            out.write(buf, 0, count);
            count = 0;
        }
    }
	//写入一个字节至缓冲区
    public synchronized void write(int b) throws IOException {
        if (count >= buf.length) {
            flushBuffer();
        }
        buf[count++] = (byte)b;
    }
	//写入一个字节数组
    public synchronized void write(byte b[], int off, int len) throws IOException {
    	//如果需要写入的长度大于缓冲区大小，则先清空缓冲区（将缓冲区所有字节写出），再写出该字节。
        if (len >= buf.length) {
            /* If the request length exceeds the size of the output buffer,
               flush the output buffer and then write the data directly.
               In this way buffered streams will cascade harmlessly. */
            flushBuffer();
            out.write(b, off, len);
            return;
        }
        //如果剩下的缓冲区容量放不下需要写入的字节长度，则把缓冲区字节写出
        if (len > buf.length - count) {
            flushBuffer();
        }
        //将相应字节数组复制至缓冲区
        System.arraycopy(b, off, buf, count, len);
        count += len;
    }

    //强制刷新缓冲，将缓冲内的数据写出至Output Stream
    public synchronized void flush() throws IOException {
        flushBuffer();
        out.flush();
    }
}