java多線程間斷輸出刨根問底

今天早上在公交車看《java多線程設計模式》的時候，看到一個java多線程簡單輸出hello,world的問題，原始碼如下：

public class Mythread {public static void main(String[] args) {new ThreadTest().start();for (int i = 0; i < 100; i++) {System.out.print("world" + " ");}}}class ThreadTest extends Thread {public void run() {for (int i = 0; i < 100; i++) {System.out.print("hello" + " ");}}}

在console裡面輸出間斷的的 hello world···········world hello沒有問題，我突然想到為什麼不能輸出 hel wor......這樣間斷的輸出呢？

於是果斷在eclipse中進入debug模式，採用單步調試就看到了問題的實質

當運行到system.out.println()函數時候，單步就進入代碼如下

    public void print(String s) {if (s == null) {    s = "null";}write(s);    }

我們再來看看write(s)方法

 

private void write(String s) {try {    synchronized (this) {ensureOpen();textOut.write(s);textOut.flushBuffer();charOut.flushBuffer();if (autoFlush && (s.indexOf('\n') >= 0))    out.flush();    }}catch (InterruptedIOException x) {    Thread.currentThread().interrupt();}catch (IOException x) {    trouble = true;}    }

  public void write(String s, int off, int len) throws IOException {synchronized (lock) {    ensureOpen();    int b = off, t = off + len;    while (b < t) {int d = min(nChars - nextChar, t - b);s.getChars(b, b + d, cb, nextChar);b += d;nextChar += d;if (nextChar >= nChars)    flushBuffer();    }}

看到synchronized就一目瞭然。