Java 7源码分析第13篇 - 字节输入输出流（1）

上一篇介绍了关于字节输入输出流的Java类框架，同时也简单介绍了一下各个类的作用，下面就来具体看一下这些类是怎么实现这些功能的。

1、InputStream和OutputStream

InputStream类的源代码如下：

public abstract class InputStream implements Closeable {

    private static final int MAX_SKIP_BUFFER_SIZE = 2048;//最多可以跳过字节的数量

    // 获取下一个字节数据并返回int值（范围0~255），如果流结束，返回-1
    public abstract int read() throws IOException;

    public int read(byte b[]) throws IOException {//读取一个字节，返回值为所读得字节
        return read(b, 0, b.length);
    }
    //读取len个字节，放置到以下标off开始字节数组b中，返回值为实际 读取的字节的数量
    public int read(byte b[], int off, int len) throws IOException {
        if (b == null) {
            throw new NullPointerException();
        } else if (off < 0 || len < 0 || len > b.length - off) {
            throw new IndexOutOfBoundsException();
        } else if (len == 0) {
            return 0;
        }
        int c = read();
        if (c == -1) {
            return -1;
        }
        b[off] = (byte)c;
        int i = 1;
        try {
            for (; i < len ; i++) {
                c = read();
                if (c == -1) {
                    break;
                }
                b[off + i] = (byte)c;
            }
        } catch (IOException ee) {
        }
        return i;
    }
    //读指针跳过n个字节不读，返回值为实际跳过的字节数量
    public long skip(long n) throws IOException {
        long remaining = n;
        int nr;
        if (n <= 0) {
            return 0;
        }
        int size = (int)Math.min(MAX_SKIP_BUFFER_SIZE, remaining);
        byte[] skipBuffer = new byte[size];
        while (remaining > 0) {
            nr = read(skipBuffer, 0, (int)Math.min(size, remaining));
            if (nr < 0) {
                break;
            }
            remaining -= nr;
        }
        return n - remaining;
    }

    // 返回值为流中尚未读取的字节的数量,这个方法应该被子类覆写
    public int available() throws IOException {
        return 0;
    }

    public void close() throws IOException {}
    // 纪录当前指针的所在位置.
    // readlimit参数表示读指针读出的readlimit个字节后 所标记的指针位置才实效。
    public synchronized void mark(int readlimit) {}
    //把读指针重新指向用mark方法所记录的位置
    public synchronized void reset() throws IOException {
        throw new IOException("mark/reset not supported");
    }
    //当前的流是否支持读指针的记录功能
    public boolean markSupported() {
        return false;
    }

}

来解释一下mark()和reset()方法，如下图所示。

讲到具体的类时会进行详细的解说。

下面来看InputStream的源代码，如下：

public abstract class OutputStream implements Closeable, Flushable {
    /**
       The byte to be written is the eight low-order bits of the argument b. The 24
     * high-order bits of b are ignored.
     */
    public abstract void write(int b) throws IOException;
    public void write(byte b[]) throws IOException {
        write(b, 0, b.length);
    }

    public void write(byte b[], int off, int len) throws IOException {
        if (b == null) {
            throw new NullPointerException();
        } else if ((off < 0) || (off > b.length) || (len < 0) ||
                   ((off + len) > b.length) || ((off + len) < 0)) {
            throw new IndexOutOfBoundsException();
        } else if (len == 0) {
            return;
        }
        for (int i = 0 ; i < len ; i++) {
            write(b[off + i]);
        }
    }

    public void flush() throws IOException {    }
    public void close() throws IOException {    }

}

如上主要就是将字节写入到输出流中，具体的写入方法write(int b)是一个抽象方法，取决于具体的实现类的实现。

2、PipedInputStream和PipedOutputStream

PipedInputStream类与PipedOutputStream类用于在应用程序中创建管道通信.一个PipedInputStream实例对象必须和一个PipedOutputStream实例对象进行连接而产生一个通信管道.PipedOutputStream可以向管道中写入数据,PipedIntputStream可以读取PipedOutputStream向管道中写入的数据.这两个类主要用来完成线程之间的通信.一个线程的PipedInputStream对象能够从另外一个线程的PipedOutputStream对象中读取数据.
         PipedInputStream和PipedOutputStream的实现原理类似于"生产者-消费者"原理,PipedOutputStream是生产者,PipedInputStream是消费者,在PipedInputStream中有一个buffer字节数组,默认大小为1024,作为缓冲区,存放"生产者"生产出来的东西.还有两个变量in和out。in是用来记录"生产者"生产了多少,out是用来记录"消费者"消费了多少,in为-1表示消费完了,in==out表示生产满了.当消费者没东西可消费的时候,也就是当in为-1的时候,消费者会一直等待,直到有东西可消费.
    在两者的构造函数中，都相互提供了连接的构造方法，分别用于接收对方的管道实例，然后调用各自的connect()方法进行连接，如PipedInputStream：

    //  PipedInputStream
    public PipedInputStream(PipedOutputStream src, int pipeSize)
            throws IOException {
         initPipe(pipeSize);
         connect(src);
    }
    private void initPipe(int pipeSize) {
         if (pipeSize <= 0) {
            throw new IllegalArgumentException("Pipe Size <= 0");
         }
         buffer = new byte[pipeSize];
    }
    public void connect(PipedOutputStream src) throws IOException {
        src.connect(this);
    }

同时还可以指定缓冲区的大小，看PipedOutputStream：

private PipedInputStream sink;
    public PipedOutputStream(PipedInputStream snk)  throws IOException {
        connect(snk);
    }
    public PipedOutputStream() {    }
    //  PipedOutputStream
    public synchronized void connect(PipedInputStream snk) throws IOException {
        if (snk == null) {
            throw new NullPointerException();
        } else if (sink != null || snk.connected) {
            throw new IOException("Already connected");
        }
        sink = snk;
        snk.in = -1;
        snk.out = 0;
        snk.connected = true;
    }

没有谁连接谁的规定，只要连接上，效果是一样的。来看输出管道中的write()方法，如下：

 public void write(int b)  throws IOException {
        if (sink == null) {
            throw new IOException("Pipe not connected");
        }
        sink.receive(b);
    }
    public void write(byte b[], int off, int len) throws IOException {
        if (sink == null) {
            throw new IOException("Pipe not connected");
        } else if (b == null) {
            throw new NullPointerException();
        } else if ((off < 0) || (off > b.length) || (len < 0) ||
                   ((off + len) > b.length) || ((off + len) < 0)) {
            throw new IndexOutOfBoundsException();
        } else if (len == 0) {
            return;
        }
        sink.receive(b, off, len);
    }

方法在写入到byte[]数组缓存区数据后，就会调用输出管道中的receive方法。输出管道中的receive()方法如下：

/**
     * Receives a byte of data.  This method will block if no input is
     * available.
     */
    protected synchronized void receive(int b) throws IOException {
        checkStateForReceive();
        writeSide = Thread.currentThread();
        if (in == out)//in==out implies the buffer is full
            awaitSpace();
        if (in < 0) {//输入管道无数据
            in = 0;
            out = 0;
        }
        buffer[in++] = (byte)(b & 0xFF);
        if (in >= buffer.length) {
            in = 0;// 缓冲区已经满了，等待下一次从头写入
        }
    }

    /**
     * Receives data into an array of bytes.  This method will
     * block until some input is available.
     */
    synchronized void receive(byte b[], int off, int len)  throws IOException {
        checkStateForReceive();
        writeSide = Thread.currentThread();
        int bytesToTransfer = len;
        while (bytesToTransfer > 0) {
            if (in == out)
                awaitSpace();
            int nextTransferAmount = 0;
            if (out < in) {
                nextTransferAmount = buffer.length - in;
            } else if (in < out) {
                if (in == -1) {
                    in = out = 0;
                    nextTransferAmount = buffer.length - in;
                } else {
                    nextTransferAmount = out - in;
                }
            }
            if (nextTransferAmount > bytesToTransfer)
                nextTransferAmount = bytesToTransfer;
            assert(nextTransferAmount > 0);
            System.arraycopy(b, off, buffer, in, nextTransferAmount);
            bytesToTransfer -= nextTransferAmount;
            off += nextTransferAmount;
            in += nextTransferAmount;
            if (in >= buffer.length) {
                in = 0;
            }
        }
    }

输入管理通过如上的对应方法接收到数据并保存到输入缓冲区后，下面就可以使用read()方法读出这些数据了，如下：

    public synchronized int read()  throws IOException {
        if (!connected) {
            throw new IOException("Pipe not connected");
        } else if (closedByReader) {
            throw new IOException("Pipe closed");
        } else if (writeSide != null && !writeSide.isAlive()
                   && !closedByWriter && (in < 0)) {
            throw new IOException("Write end dead");
        }

        readSide = Thread.currentThread();
        int trials = 2;
        while (in < 0) {
            if (closedByWriter) {
                /* closed by writer, return EOF */
                return -1;
            }
            if ((writeSide != null) && (!writeSide.isAlive()) && (--trials < 0)) {
                throw new IOException("Pipe broken");
            }
            /* might be a writer waiting */
            notifyAll();
            try {
                wait(1000);
            } catch (InterruptedException ex) {
                throw new java.io.InterruptedIOException();
            }
        }
        int ret = buffer[out++] & 0xFF;
        if (out >= buffer.length) {
            out = 0;
        }
        if (in == out) {
            /* now empty */
            in = -1;
        }
        return ret;
    }

    /**
     * Reads up to len bytes of data from this piped input stream into an array of bytes. Less than len bytes
     * will be read if the end of the data stream is reached or if len exceeds the pipe's buffer size.
     * If len  is zero, then no bytes are read and 0 is returned;otherwise, the method blocks until
     * at least 1 byte of input is available, end of the stream has been detected, or an exception is
     */
    public synchronized int read(byte b[], int off, int len)  throws IOException {
        if (b == null) {
            throw new NullPointerException();
        } else if (off < 0 || len < 0 || len > b.length - off) {
            throw new IndexOutOfBoundsException();
        } else if (len == 0) {
            return 0;
        }
        /* possibly wait on the first character */
        int c = read();
        if (c < 0) {
            return -1;
        }
        b[off] = (byte) c;
        int rlen = 1;
        while ((in >= 0) && (len > 1)) {
            int available;
            if (in > out) {
                available = Math.min((buffer.length - out), (in - out));
            } else {
                available = buffer.length - out;
            }
            // A byte is read beforehand outside the loop
            if (available > (len - 1)) {
                available = len - 1;
            }
            System.arraycopy(buffer, out, b, off + rlen, available);
            out += available;
            rlen += available;
            len -= available;

            if (out >= buffer.length) {
                out = 0;
            }
            if (in == out) {
                /* now empty */
                in = -1;
            }
        }
        return rlen;
    }

下面来具体举一个例子，如下：

public class test04 {
    public static void main(String [] args) {
        Sender sender = new Sender();
        Receiver receiver = new Receiver();  

        PipedOutputStream outStream = sender.getOutStream();
        PipedInputStream inStream = receiver.getInStream();
        try {
            //inStream.connect(outStream); // 与下一句一样
            outStream.connect(inStream);
        } catch (Exception e) {
            e.printStackTrace();
        }
        sender.start();
        receiver.start();
    }
}  

class Sender extends Thread {
    private PipedOutputStream outStream = new PipedOutputStream();
    public PipedOutputStream getOutStream() {
        return outStream;
    }
    public void run() {
        String info = "hello, receiver";
        try {
            outStream.write(info.getBytes());
            outStream.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}  

class Receiver extends Thread {
    private PipedInputStream inStream = new PipedInputStream();
    public PipedInputStream getInStream() {
        return inStream;
    }
    public void run() {
        byte[] buf = new byte[1024];
        try {
            int len = inStream.read(buf);
            System.out.println("receive message from sender : " + new String(buf, 0, len));
            inStream.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}  

最后运行后输出的结果如下：receive message from sender : hello, receiver

3、ByteArrayInputStream和ByteArrayOutputStream

先来看ByteArrayOutputStream类中write()方法：

 // Writes the specified byte to this byte array output stream.
    public synchronized void write(int b) {
        ensureCapacity(count + 1);
        buf[count] = (byte) b;
        count += 1;
    }
    /**
     * Writes len bytes from the specified byte array
     * starting at offset off to this byte array output stream.
     */
    public synchronized void write(byte b[], int off, int len) {
        if ((off < 0) || (off > b.length) || (len < 0) ||
            ((off + len) - b.length > 0)) {
            throw new IndexOutOfBoundsException();
        }
        ensureCapacity(count + len);
        System.arraycopy(b, off, buf, count, len);
        count += len;
    }

这个类也是通过向数组中定入值来进行数值传递的，而字节数组的大小可以在创建ByteArrayOutputStream时通过构造函数指定，默认的大小为32.如果在调用write()方法时，都会确保字节数组的容量。如果过小，会自动进行扩容操作。这样就可以把需要的数据写到字节数组中去了。还可以通过调用writeTo()方法可以写入到其他输出流中，源代码如下：

 /**
     * Writes the complete contents of this byte array output stream to
     * the specified output stream argument, as if by calling the output
     * stream's write method using out.write(buf, 0, count).
     */
    public synchronized void writeTo(OutputStream out) throws IOException {
        out.write(buf, 0, count);
    }

继续来看ByteArrayInputStream类，这个类可以从字节数组中读出数据，具体的源代码如下：

 public synchronized int read() {
        return (pos < count) ? (buf[pos++] & 0xff) : -1;
    }

    public synchronized int read(byte b[], int off, int len) {
        if (b == null) {
            throw new NullPointerException();
        } else if (off < 0 || len < 0 || len > b.length - off) {
            throw new IndexOutOfBoundsException();
        }

        if (pos >= count) {
            return -1;
        }

        int avail = count - pos;
        if (len > avail) {
            len = avail;
        }
        if (len <= 0) {
            return 0;
        }
        System.arraycopy(buf, pos, b, off, len);
        pos += len;
        return len;
    }

同时也提供了其他的一些方法，如可以跳读字节的skip()方法、查看剩余有效字节的avaible()方法等等，有兴趣的可以自己去看。下面来举一个具体应用的例子，如下：

byte[] bytes = { 0,2, 3, 4, 5 };
		try (ByteArrayOutputStream out = new ByteArrayOutputStream();
			 ByteArrayInputStream in = new ByteArrayInputStream(bytes);){
			out.write(bytes);
			System.out.println(out.size());//5
			System.out.println(in.read());//解
			in.skip(1);//2
			in.mark(4);
			System.out.println(in.read());//3
			in.reset();// 从索引为2的地方重新开始读
			System.out.println(in.read());//3
			System.out.println(in.read());

		} catch (IOException e) {
			e.printStackTrace();
		}

4、StringBufferInputStream

这个类现在已经不提倡使用了，个人觉得是因为编码的原因吧。查看这个类后的源代码，如下：

 public synchronized int read() {
        return (pos < count) ? (buffer.charAt(pos++) & 0xFF) : -1;
    }

     public synchronized int read(byte b[], int off, int len) {
        if (b == null) {
            throw new NullPointerException();
        } else if ((off < 0) || (off > b.length) || (len < 0) ||
                   ((off + len) > b.length) || ((off + len) < 0)) {
            throw new IndexOutOfBoundsException();
        }
        if (pos >= count) {
            return -1;
        }
        if (pos + len > count) {
            len = count - pos;
        }
        if (len <= 0) {
            return 0;
        }
        String  s = buffer;
        int cnt = len;
        while (--cnt >= 0) {
            b[off++] = (byte)s.charAt(pos++);
        }

        return len;
    }

发现，其实这个类是将字符串中的字符转换为字节进行读取的，如果这个字符串的字符全部为ISO-8859-1编码所能表示的，那肯定能正常读取。但是通常Java都是Unicode编码，两个字符，所以如果其中出现了Unicode字符的时候，例如中文，读取就会不准确。如下举例：

String str = "马智AB";
		StringBufferInputStream st = new StringBufferInputStream(str);
		byte[] j = new byte[16];
		st.read(j);
		System.out.println(new String(j)); //lzAB

原因可能大家也知道了，两个read()方法在获取到这个字符串的字符(s.charAt())后，强制转换为byte或与0xff相与，这样的结果只能导致取到低8位的编码，而对于两个字节编码的汉字来说，肯定会产生错误。