【Java IO】 四、Writer和Reader源码阅读

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1.Writer

官方描述: 写入字符流的抽象类。子类必须实现的方法仅有 write(char[], int, int)、flush() 和 close()。但是，多数子类将重写此处定义的一些方法，以提供更高的效率和/或其他功能。

继承了三个接口

public abstract class Writer implements Appendable, Closeable, Flushable {


//缓存区 ，如欲写入String/CharSequence将使用该buffer
    private char[] writeBuffer;
//默认的大小，如写入的String大于该缓冲区，则会新建相应大小的buffer
    private static final int WRITE_BUFFER_SIZE = 1024;

  
    protected Object lock;

    protected Writer() {
        this.lock = this;
    }

    protected Writer(Object lock) {
        if (lock == null) {
            throw new NullPointerException();
        }
        this.lock = lock;
    }

    public void write(int c) throws IOException {
//加锁
        synchronized (lock) {
//如果buffer未初始化，则初始化
            if (writeBuffer == null){
                writeBuffer = new char[WRITE_BUFFER_SIZE];
            }
//将 写入的int c 的char值写入到 char数组中
            writeBuffer[0] = (char) c;
            write(writeBuffer, 0, 1);
        }
    }
//调用抽象的方法
    public void write(char cbuf[]) throws IOException {
        write(cbuf, 0, cbuf.length);
    }

//**核心方法**，将char数组中的内容写入流中，需子类实现
    abstract public void write(char cbuf[], int off, int len) throws IOException;


    public void write(String str) throws IOException {
        write(str, 0, str.length());
    }

    public void write(String str, int off, int len) throws IOException {
//加锁
        synchronized (lock) {
//一个chars数组，用来将String放入，再调用write(char cbuf[])
            char cbuf[];
//欲写入的字符串长度如果小于buffer，就使用默认的buffer，如果大于，就创
//建len长度的char[]
            if (len <= WRITE_BUFFER_SIZE) {
                if (writeBuffer == null) {
                    writeBuffer = new char[WRITE_BUFFER_SIZE];
                }
                cbuf = writeBuffer;
            } else {    // Don't permanently allocate very large buffers.
                cbuf = new char[len];
            }
            str.getChars(off, (off + len), cbuf, 0);
//调用抽象的方法，需子类进行实现
            write(cbuf, 0, len);
        }
    }


    public Writer append(CharSequence csq) throws IOException {
        if (csq == null)
            write("null");
        else
            write(csq.toString());
        return this;
    }

    public Writer append(CharSequence csq, int start, int end) throws IOException {
        CharSequence cs = (csq == null ? "null" : csq);
        write(cs.subSequence(start, end).toString());
        return this;
    }

    public Writer append(char c) throws IOException {
        write(c);
        return this;
    }
//子类需实现
    abstract public void flush() throws IOException;

    abstract public void close() throws IOException;

1.Reader

public abstract class Reader implements Readable, Closeable {


    protected Object lock;

//将new出的对象的引用赋值给lock
    protected Reader() {
        this.lock = this;
    }

 //用一个给定的对象来实现同步
    protected Reader(Object lock) {
        if (lock == null) {
            throw new NullPointerException();
        }
        this.lock = lock;
    }

 //将流读入到buffer中
    public int read(java.nio.CharBuffer target) throws IOException {
        int len = target.remaining();
        char[] cbuf = new char[len];
        int n = read(cbuf, 0, len);
        if (n > 0)
            target.put(cbuf, 0, n);
        return n;
    }

 //读一个单独的字符
    public int read() throws IOException {
        char cb[] = new char[1];
        if (read(cb, 0, 1) == -1)
            return -1;
        else
            return cb[0];
    }


    public int read(char cbuf[]) throws IOException {
        return read(cbuf, 0, cbuf.length);
    }

//**核心方法**，将流读入字符数组，返回读入的字符个数， 需子类实现
    abstract public int read(char cbuf[], int off, int len) throws IOException;


    private static final int maxSkipBufferSize = 8192;

    private char skipBuffer[] = null;


    public long skip(long n) throws IOException {
        if (n < 0L)
            throw new IllegalArgumentException("skip value is negative");
        int nn = (int) Math.min(n, maxSkipBufferSize);
        synchronized (lock) {
            if ((skipBuffer == null) || (skipBuffer.length < nn))
                skipBuffer = new char[nn];
            long r = n;
            while (r > 0) {
                int nc = read(skipBuffer, 0, (int)Math.min(r, nn));
                if (nc == -1)
                    break;
                r -= nc;
            }
            return n - r;
        }
    }

//这个流是否准备好被读
    public boolean ready() throws IOException {
        return false;
    }

 //是否支持mark操作
    public boolean markSupported() {
        return false;
    }


    public void mark(int readAheadLimit) throws IOException {
        throw new IOException("mark() not supported");
    }

    public void reset() throws IOException {
        throw new IOException("reset() not supported");
    }

//关闭该流，释放占用的资源
     abstract public void close() throws IOException;

}