JsonMappingException:找不到合适的构造函数

Question

我正在实现他们的文档中给出的firebase示例.我正面临这个错误:

com.fasterxml.jackson.databind.JsonMappingException:[简单类型,类com.XYZ $博文]发现型没有合适的构造函数:不能从JSON对象实例化(需要添加/启用类型的信息？)

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;
        }
    }
}

我在同一件事上经历了许多问题,说要包括反序列化JSON所必需的空构造函数.我已经包括了但仍然无法解决问题.这是我试图反序列化的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.任何有助于重塑这一点的帮助表示赞赏.

Answer 1

我猜你的BlogPost班级是你活动的内在阶级.在这种情况下,Java会为引用包含对象的每个对象添加一个隐藏字段.而这个隐含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类保留在单独的文件中(以便它不是内部类)或将其标记为静态类(删除隐式字段):

public static class BlogPost {