在講atomic原子類之前先看一個小例子:
public class UseAtomic {
public static void main(String[] args) {
AtomicInteger atomicInteger=new AtomicInteger();
for(int i=0;i<10;i++){
Thread t=new Thread(new AtomicTest(atomicInteger));
t.start();
try {
t.join(0);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println(atomicInteger.get());
}
}
class AtomicTest implements Runnable{
AtomicInteger atomicInteger;
public AtomicTest(AtomicInteger atomicInteger){
this.atomicInteger=atomicInteger;
}
@Override
public void run() {
atomicInteger.addAndGet(1);
atomicInteger.addAndGet(2);
atomicInteger.addAndGet(3);
atomicInteger.addAndGet(4);
}
}
最終的輸出結果為100,可見這個程式是線程安全的。如果把AtomicInteger換成變量i的話,那最終結果就不确定了。
打開AtomicInteger的源碼可以看到:
// setup to use Unsafe.compareAndSwapInt for updates
private static final Unsafe unsafe = Unsafe.getUnsafe();
private volatile int value;
CAS簡要
/**
* Atomically sets the value to the given updated value
* if the current value {@code ==} the expected value.
*
* @param expect the expected value
* @param update the new value
* @return {@code true} if successful. False return indicates that
* the actual value was not equal to the expected value.
*/
public final boolean compareAndSet(int expect, int update) {
return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
}
作者:zxin1
連結:https://www.jianshu.com/p/a2f3c46d4783
來源:簡書
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