ide output

一般的，如果多个线程协作存、取某个变量时，一般需要用到synchronized关键字进行同步操作，如：
public class MyTestThread extends MyTest implements Runnable {
private boolean _done = false;
public synchronized boolean getDone()
{
return _done;
}
public synchronized void setDone(boolean b)
{
_done = b;
}

public void run( ) {
boolean done;
done = getDone();
while (!done) {
repaint( );
try {
Thread.sleep(100);
} catch (InterruptedException ie) {
return;
}
}
}
}
或者：
public class MyTestThread extends MyTest implements Runnable {
private boolean _done = false;
public void setDone(boolean b)
{
synchronized(this)
{
_done = b;
}
}

public void run( ) {
boolean done;
synchronized(this)
{
done = _done;
}

while (!done) {
repaint( );
try {
Thread.sleep(100);
} catch (InterruptedException ie) {
return;
}
}
}
}
但是，通过volatile关键字，我们可以大大简化：
public class MyTestThread extends MyTest implements Runnable {
private volatile boolean done = false;
public void run( ) {
while (!done) {
repaint( );
try {
Thread.sleep(100);
} catch (InterruptedException ie) {
return;
}
}
}
public void setDone(boolean b) {
done = b;
}
}

 

volatile只是声明变量被拒绝优化而已。。。也就是说使得存取变量变为了原子操作(atom operation) 你上面的最后一段代码可以正确的在单CPU上运行但是如果在多CPU计算机上（甚至是现在的多核CPU上）就会发生操作，因为即使是原子操作也只是针对单个CPU无法被打断而已但是对于多个并行的CPU还是会发生错误但是上面的两段代码却可以正确的在多CPU环境被运行