java.io.writer API 以及 源码解读

声明 我看的是java7的API文档。

如下图所示，java.io.writer 继承了java.lang.Object，实现的接口有Closeable, Flushable, Appendable, AutoCloseable。

所有直接继承它的子类有BufferedWriter CharArrayWriter FilterWriter OutputStreamWriter PipedWriter PrintWriter StringWriter。

Writer是用来操作字符流的抽象类。所有继承它的子类必须要重写的方法有write(char[], int, int), flush(), and close().

下面是java.io.Writer的源码。

package java.io;

public abstract class Writer implements Appendable,Closebale,Flushable{

    private char[] writeBuffer;
    private static final int WRITE_BUFFER_SIZE = 1024;
    projected 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) throw IOException{
        syschronized (lock){
            if (writeBuffer == null){
                writeBuffer = new char[WRITE_BUFFER_SIZE];
            }
            writeBuffer[0] = (char) c;
            write(writeBuffer,0,1);
        }
    }

    public void write(char cbuf[]) throw IOException{
        write(cbuf, 0, cbuf.length);
    }

    abstract public void write(char buf[], int off, int len) throw IOException;

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

    public void write(String str, int off, int len) throw IOException{
        syschronized(lock){
            char cbuf[];
            if(len <= WRITE_BUFFER_SIZE){
                if(writeBuffer == null){
                    writeBuffer = new char[WRITE_BUFFER_SIZE];
                }
                cbuf = writeBuffer;
            }else{
                cbuf = new char[len];
            }
            str.getChars(off, (off + len), cbuf, 0);
            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) throw IOException{
        CharSequence cs = (csq == null ? "null" : csq);
        write(cs.subSequence(start,end).toString());
        return this;
    }

    public Writer append(char c) throw IOException{
        write(c);
        return this;
    }

    abstract public void flush() throw IOException;

    abstract public void close() throw IOException;
}

可以看到在Writer类中子类必须重写的类有三个，

1、abstract public void write(char buf[], int off, int len) throw IOException;

2、abstract public void flush() throw IOException;

3、abstract public void close() throw IOException;

其中，下面三个方法是实现Appendable接口必须实现的方法

1、public Writer append(CharSequence csq) throws IOException

2、public Writer append(CharSequence csq, int start, int end) throw IOException

3、public Writer append(char c) throw IOException

实现 Flushable接口必须实现的方法是

abstract public void flush() throw IOException;

实现Closeable接口必须实现的方法是

abstract public void close() throw IOException;