我在下面的代碼中遇到了一個問題,該代碼幾乎逐字地從Firebase SDK Java文檔中復制到了工作中.我是一個真正的語言的新手,比如來自PHP和JavaScript的webdev背景的Java.

基本上,addListenerForSingleValueEvent沒有觸發以返回數據.我注意到了這一點,因為系統打印輸出沒有觸發,因此我認為監聽事件沒有觸發.

我懷疑這與我有限的知識如何與函數本身的工作方式有關,就像我缺少一些關于類和函數如何相互作用的結構知識.

任何幫助將不勝感激,謝謝.

class FireBase {

public static void main(String[] args) throws IOException {

// Fetch the service account key JSON file contents

FileInputStream serviceAccount = new FileInputStream("key.json");

// Initialize the app with a service account, granting admin privileges

FirebaseOptions options = new FirebaseOptions.Builder()

.setCredential(FirebaseCredentials.fromCertificate(serviceAccount))

.setDatabaseUrl("https://ssworks-adwords.firebaseio.com/")

.build();

FirebaseApp.initializeApp(options);

// As an admin, the app has access to read and write all data, regardless of Security Rules

DatabaseReference ref = FirebaseDatabase

.getInstance()

.getReference("petitions");

ref.addListenerForSingleValueEvent(new ValueEventListener() {

@Override

public void onDataChange(DataSnapshot dataSnapshot) {

System.out.println("Before Get Value");

Object document = dataSnapshot.getValue();

System.out.println(document);

}

@Override

public void onCancelled(DatabaseError arg0) {

// TODO Auto-generated method stub

System.out.println("This didn't work");

}

});

}

}