CAS机制的使用
AtomicInteger 就是对 unsafe 类进行封装
手动实现
import java.lang.reflect.Field;
import sun.misc.Unsafe;
public class CounterUnsafe {
volatile int i = 0;
private static Unsafe unsafe = null;
// i字段的偏移量
private static long valueOffset;
static {
try {
Field field = Unsafe.class.getDeclaredField("theUnsafe");
field.setAccessible(true);
unsafe = (Unsafe) field.get(null);
Field fieldi = CounterUnsafe.class.getDeclaredField("i");
valueOffset = unsafe.objectFieldOffset(fieldi);
} catch (NoSuchFieldException | IllegalAccessException e) {
e.printStackTrace();
}
}
public void add() {
for (;;) {
int current = unsafe.getIntVolatile(this, valueOffset);
if (unsafe.compareAndSwapInt(this, valueOffset, current, current + 1))
break;
}
}
}
运行
public class Demo1_CounterTest {
public static void main(String[] args) throws InterruptedException {
final CounterUnsafe ct = new CounterUnsafe();
for (int i = 0; i < 6; i++) {
new Thread(new Runnable() {
@Override
public void run() {
// TODO Auto-generated method stub
for (int j = 0; j < 1000; j++) {
ct.add();
System.out.println("dome...");
}
}
}).start();
Thread.sleep(6000L);
System.out.println(ct.i);
}
}
}