Java中的异步执行流

我正在编写一个方法，其中我希望查询数据库，并在查询完成后执行另一个方法。但是，由于对数据库的调用是异步的，在数据库调用完成之前，我似乎不知道如何停止主执行流，如下所示：

public void getUserFromEmail(String email) {
    Firebase usersRef = rootRef.child("users");

    //Asynchronous database call, want to stop everything else 
    usersRef.orderByKey().equalTo(email).addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            System.out.println(firebaseError.getMessage());
        }
    });
    //Perform some more operations after database call is finished
}

---

我习惯于在Nodejs中使用异步回调，但我不知道在Java中是否有任何等效的方法来控制执行流。有什么简单的方法可以实现我想做的吗？

你不能停止主流程，主线程将继续。（除非您另外停止它，但这将导致“应用程序没有响应”，所以不要）

只需在回调中执行操作

public void getUserFromEmail(String email) {
    Firebase usersRef = rootRef.child("users");

    //Asynchronous database call, want to stop everything else 
    usersRef.orderByKey().equalTo(email).addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            // Perform some more operations after database call is finished
            // OR... callSomeMethod();
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            System.out.println(firebaseError.getMessage());
        }
    });

}

public void callSomeMethod() {
    // Perform some more operations after database call is finished
}

如果您来自节点背景，则可能与此类似。将函数传递到

getUserFromEmail

public void otherMethod() {
    getUserFromEmail("steve@example.com", new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            // Perform some more operations after database call is finished
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            System.out.println(firebaseError.getMessage());
        }
    });
}

public void getUserFromEmail(String email, ValueEventListener listener) {
    rootRef.child("users")
        .orderByKey()
        .equalTo(email)
        .addListenerForSingleValueEvent(listener);
}

---

如果真的想使用锁，可以控制流。例如，使用：

---

如果我想在回调完成后从原始meyhod返回一个值，该怎么办？方法是

void

。你不能回来。你也不应该。Async不能很好地处理

return

语句您可以直接从上次我检查导致ANRs的回调中从

dataSnapshot

获取的数据更新UI。如果在UI线程中执行我的更新，请参阅中的“我的更新”，然后选择“是”，否则选择“否”。添加到回答中，以警告人们从UI线程使用该方法。

public DataSnapshot getUserFromEmail(String email) throws InterruptedException {
    AtomicReference<DataSnapshot> dataSnapshotRef = new AtomicReference<DataSnapshot>();
    CountDownLatch completeSignal = new CountDownLatch(1);

    Firebase usersRef = rootRef.child("users");

    //Asynchronous database call, want to stop everything else 
    usersRef.orderByKey().equalTo(email).addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            dataSnapshotRef.set(dataSnapshot);
            // Causes execution of the waiting thread to continue
            // from the `await` call
            completeSignal.countDown();
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            completeSignal.countDown();
        }
    });

    // Execution on this thread pauses at the `await` call until
    // countDown has been called from another thread.
    completeSignal.await();

    // Do stuff after onDataChanged or onCancelled has been called
    DataSnapshot dataSnapshot = dataSnapshotRef.get();
}

public DataSnapshot getUserFromEmail(String email, ValueEventListener listener) {
    Firebase usersRef = rootRef.child("users");

    //Asynchronous database call, want to stop everything else 
    usersRef.orderByKey().equalTo(email).addListenerForSingleValueEvent(listener);
}

public void getUserAndThenDoSomething() {
    getUserFromEmail("bob@example.com", new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            // Do stuff after getting the email here
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
        }
    });
}