FrameDecorator 单例写法线程安全问题

[Shouheng88]

    public static FrameDecorator getInstance(final Context context) {
        if (instance == null) {
            if (Thread.currentThread() == Looper.getMainLooper().getThread()) {
                instance = new FrameDecorator(context, new FloatFrameView(context));
            } else {
                try {
                    synchronized (lock) { // 1
                        mainHandler.post(new Runnable() {
                            @Override
                            public void run() {
                                if (instance == null) { 
                                    instance = new FrameDecorator(context, new FloatFrameView(context));
                                }
                                synchronized (lock) {
                                    lock.notifyAll();
                                }
                            }
                        });
                        lock.wait(); // 2
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
        return instance;
    }

2 处释放锁的时候 block 到 1 处的子线程仍然有可能获取到锁并进入同步代码块，虽然实例化操作在主线程中线性执行，但是可能在消息队列中存在多条实例化消息导致单例非线程安全。

解决办法是在主线程 post 消息中增加一层判断，这样主线程线性执行，无线程安全问题，避免多次创建单例

[tencent-adm]

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