Maven-Java Web App无法从Firebase实时数据库检索数据

Question

    public static class Post {

              public String author;
              public String title;

              public Post(String author, String title) {
                // ...
              }

            }
...
FirebaseOptions options = new FirebaseOptions.Builder()
                .setCredentials(GoogleCredentials.fromStream(serviceAccount))
                .setDatabaseUrl(DATABASE_URL)
                .build();
        FirebaseApp.initializeApp(options);
        System.out.println("0");

        final FirebaseDatabase database = FirebaseDatabase.getInstance(DATABASE_URL);

        DatabaseReference ref = database.getReference("server/saving-data/fireblog/posts");
        System.out.println("1");

        // Attach a listener to read the data at our posts reference
        ref.addChildEventListener(new ChildEventListener() {
              @Override
              public void onChildAdded(DataSnapshot dataSnapshot, String prevChildKey) {
                  System.out.println("2");
                Post newPost = dataSnapshot.getValue(Post.class);
                System.out.println("3");
                System.out.println("Author: " + newPost.author);
                System.out.println("Title: " + newPost.title);
                System.out.println("Previous Post ID: " + prevChildKey);
              }

              @Override
              public void onChildChanged(DataSnapshot dataSnapshot, String prevChildKey) {}

              @Override
              public void onChildRemoved(DataSnapshot dataSnapshot) {}

              @Override
              public void onChildMoved(DataSnapshot dataSnapshot, String prevChildKey) {}

              @Override
              public void onCancelled(DatabaseError databaseError) {}
            });

该代码是从Firebase网站的教程中复制的。在“ 2”之后停止打印。 Eclipse控制台上没有出现错误。我已经更改了很多次代码，但仍然无法正常工作。顺便说一句，我可以将数据写入数据库。

这是数据结构：