Java中的異步執行流程

Question

我正在編寫一個方法，我想在其中查詢數據庫並在查詢完成后執行另一種方法。 但是，由於對數據庫的調用是異步的，我似乎無法弄清楚如何在數據庫調用完成之前停止主要執行流程，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
}

我習慣於在 Nodejs 中使用異步回調，但我不知道在 Java 中是否有任何等效的方法來控制執行流程。 有什么簡單的方法可以實現我想做的事情嗎？

Answer 1

你不能停止主流程，主線程會繼續。 （除非您以其他方式停止它，但這會導致“應用程序無響應”，所以不要）。

只需在回調中執行操作即可。

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
}

如果您來自 Node 背景，那么這可能看起來很相似。 將函數傳遞給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);
}

Answer 2

如果你真的想使用鎖，你可以控制流程。 例如使用CountdownLatch ：

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();
}

然而，這種方法會阻塞執行getUserFromEmail的線程，直到 Firebase 返回一些數據。 特別是，如果您從 UI 線程調用getUserFromEmail ，那么這將阻塞 UI 線程並導致 ANR。

最好遵循這樣的回調方法：

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) {
        }
    });
}