關於JAVA中流的flush問題

目前在學習Socket，因爲和IO關係緊密，於是順便也學了下IO

發現有這樣一句話：

The flush method is valid on any output stream,but has no effect unless the stream is buffered

這句話意思是所有的輸出流都有flush方法，但是僅對緩衝流有效

看到這裏，筆者想到了自己寫的serversocket類，部分代碼如下，沒有用buffered流也調用了flush方法啊？於是進行了一番探討

private OutputStream ous;
ous = socket.getOutputStream();
public void sendMsg(String msg){
		PrintWriter out = new PrintWriter(ous);
		out.println("消息:"+msg);
		out.flush();
}

看到這裏用PrintWriter包裝了OutputStream流，OutputStream肯定爲非緩衝流，所以問題一定出在PrintWriter上

讓我們來看一下PrintWriter的底層代碼，部分相關代碼如下

public class PrintWriter extends Writer {
    protected Writer out;
    private final boolean autoFlush;

     /**
     * Creates a new PrintWriter, without automatic line flushing.
     *
     * @param  out        A character-output stream
     */
    public PrintWriter (Writer out) {
        this(out, false);
    }
     /**
     * Creates a new PrintWriter.
     *
     * @param  out        A character-output stream
     * @param  autoFlush  A boolean; if true, the <tt>println</tt>,
     *                    <tt>printf</tt>, or <tt>format</tt> methods will
     *                    flush the output buffer
     */
    public PrintWriter(Writer out,
                       boolean autoFlush) {
        super(out);
        this.out = out;
        this.autoFlush = autoFlush;
        lineSeparator = java.security.AccessController.doPrivileged(
            new sun.security.action.GetPropertyAction("line.separator"));
    }
    public PrintWriter(OutputStream out) {
        this(out, false);
    }
}

如上所示，這樣我們就很清楚了，當調用包裝OutputStream的構造方法時，autoFlush參數是falus