java - JsonMappingException : No suitable constructor found

Question

我正在实现他们文档中给出的 firebase 示例。我面临这个错误:

com.fasterxml.jackson.databind.JsonMappingException: No suitable constructor found for type [simple type, class com.XYZ.$BlogPost]: can not instantiate from JSON object (need to add/enable type information?)

这是我的代码:

    public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_user);
        Firebase.setAndroidContext(this);
    }

    @Override
    protected void onStart() {
        super.onStart();
        // Get a reference to our posts
        Firebase ref = new Firebase("https://docs-examples.firebaseio.com/web/saving-data/fireblog/posts");
        // Attach an listener to read the data at our posts reference
        ref.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot snapshot) {
                System.out.println("There are " + snapshot.getChildrenCount() + " blog posts");
                for (DataSnapshot postSnapshot: snapshot.getChildren()) {
                    BlogPost post = postSnapshot.getValue(BlogPost.class);
                    System.out.println(post.getAuthor() + " - " + post.getTitle());
                }
            }
            @Override
            public void onCancelled(FirebaseError firebaseError) {
                System.out.println("The read failed: " + firebaseError.getMessage());
            }
        });
    }
public class BlogPost {
        private String author;
        private String title;
        public BlogPost() {
            // empty default constructor, necessary for Firebase to be able to deserialize blog posts
        }
        public String getAuthor() {
            return author;
        }
        public String getTitle() {
            return title;
        }
    }
}

我已经回答了很多关于同一件事的问题，说要包含反序列化 JSON 所必需的空构造函数。我已经包括在内，但我仍然无法解决问题。这是我要反序列化的 JSON:

{  
   "-JRHTHaIs-jNPLXOQivY":{  
      "author":"gracehop",
      "title":"Announcing COBOL, a New Programming Language"
   },
   "-JRHTHaKuITFIhnj02kE":{  
      "author":"alanisawesome",
      "title":"The Turing Machine"
   }
}

我不知道我的代码中缺少什么。感谢任何对此进行重新分级的帮助。

Answer 1

我猜您的 BlogPost 类是您 Activity 的内部类。在这种情况下，Java 会为每个引用包含对象的对象添加一个隐藏字段。而这个隐式字段当然不会出现在您的 JSON 数据中。

要解决这个问题，您应该将 BlogPost 类保存在单独的文件中(这样它就不是内部类)或将其标记为静态类(这会删除隐式字段):

public static class BlogPost {