解释下volatile

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;
}
}