维护线程安全，同时防止可能的同步回调导致死锁

Question

我有一个如下所示的API，其中Baz是worker实现。 这个Bar需要是线程安全的，这在与 Baz 的回调交互时会变得棘手。

当前 baz 实例需要在回调中引用(可以在工作线程上调用或同步调用(。 注释应显示问题：

final class Bar {
final Lock lock = new ReentrantLock();
Baz baz; // Guarded by lock.
void run() { // Called by any thread.
lock.lock();
if (baz.isRunning()) {
lock.unlock();
return;
}
baz = new Baz();
// If it unlocks here, the next line may execute on the wrong Baz.
// If it doesn't unlock here, there will be a deadlock when done() is called synchronously.
// lock.unlock();
baz.run(new Baz.Callback() { // May be called synchronously or by Baz worker thread.
@Override
public void done() {
lock.lock();
baz = new Baz();
lock.unlock();
}
});
}
}

有没有一种好方法可以使它正常工作，同时又不会导致死锁？

编辑：更简洁：

final class Foo {
final Lock lock = new ReentrantLock();
void run() {
lock.lock();
worker.enqueue(new Callback() {
@Override void complete() {
lock.lock(); // Could cause deadlock.
}
});
lock.unlock();
}
}

Answer 1

不确定完全得到你想要实现的目标，但也许这就是你正在寻找的？

final class Bar {
final Lock lock = new ReentrantLock();
Baz baz = new Baz();
void run() {
if (!lock.tryLock()) {
return;
}
try {
CountdownLatch callbackFlag = new CountdownLatch(1);
baz.run(new Baz.Callback() {
@Override
public void done() {
callbackFlag.countDown();
}
});
try {
callbackFlag.await(); // better use overloaded method with max timeout waiting. you don't probably want to wait forever
baz = new Baz(); // do you really want to reinit Baz on each execution?
} catch (InterruptedException e) {
// decide what you want to happen here
}
} finally {
lock.unlock();
}
}
}