Happen Before原则

public class FutureTask<V> implements RunnableFuture<V> {
    // state transitions: NEW -> COMPLETING -> NORMAL NEW -> COMPLETING -> EXCEPTIONAL NEW -> CANCELLED NEW -> INTERRUPTING -> INTERRUPTED
    private volatile int state;
    private static final int NEW          = 0;
    private static final int COMPLETING   = 1;
    private static final int NORMAL       = 2;
    private static final int EXCEPTIONAL  = 3;
    private static final int CANCELLED    = 4;
    private static final int INTERRUPTING = 5;
    private static final int INTERRUPTED  = 6;

    /** The underlying callable; nulled out after running */
    private Callable<V> callable;
    /** The result to return or exception to throw from get() */
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    private volatile Thread runner;
    /** Treiber stack of waiting threads */
    private volatile WaitNode waiters;
}

protected void set(V v) {
    if (STATE.compareAndSet(this, NEW, COMPLETING)) {
        outcome = v;
        STATE.setRelease(this, NORMAL); // state变量写入之前会把outcome变量先写入到主存
        finishCompletion();
    }
}

public V get() throws InterruptedException, ExecutionException {
    int s = state;
    if (s <= COMPLETING)
        s = awaitDone(false, 0L);
    return report(s);  // 读取state变量之后，会读取到outcome的主存最新值
}

private V report(int s) throws ExecutionException {
    Object x = outcome;
    if (s == NORMAL)
        return (V)x;
    if (s >= CANCELLED)
        throw new CancellationException();
    throw new ExecutionException((Throwable)x);
}

public class HeavyObject {
    private HeavyObject() {  // 构造函数私有
    }
    
    /**
     * 多线程场景下，指令重排可能会导致新建对象先赋值给引用，但对象内部变量尚未初始化完成，其他线程拿到了未初始化完成的对象
     * 加上volatile关键字防止指令重排
     * */
    private static volatile HeavyObject heavyObject;  
    
    public static HeavyObject getInstance() {  // 获取单例
        if (heavyObject == null) {
            synchronized (HeavyObject.class) {
                if (heavyObject == null) {
                    heavyObject = new HeavyObject();
                }
            }
        }
        return heavyObject;
    }
}