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2 つのエンティティ クラスがあります。User質問のリストを持つQuestionクラスと、ユーザー属性を持つクラスです。

ユーザークラスの宣言:

 @Entity
    @Table(schema="test",name="so_user")
    public class User {
    private List<Question> questions;
    //other attributes
            @OneToMany(cascade=CascadeType.ALL)
        @PrimaryKeyJoinColumn   
        public List<Question> getQuestions() {
            return questions;
        }
    //.. other methods
    }

質問クラスの宣言:

@Entity
@Table(schema="test")
public class Question {
private User askedBy;
//other attributes
  @ManyToOne(cascade=CascadeType.ALL,fetch=FetchType.EAGER)
  @PrimaryKeyJoinColumn
  public User getAskedBy() {
    return askedBy;
  }

//.. other methods
}

を使用してユーザー オブジェクトを Json に変換しようとするとGson library、stackoverflow error. 理由: ユーザーには質問のリストがあり、各質問にはユーザー属性があります (休止状態で双方向マッピングを使用していました)。

テストコード:

public class MappingTest {
    public static void main(String[] args) {
        List<User> users = null;
        User user = null;
        //populate some users
        for (int i = 0; i < 10; i++) {
            if (i == 0)
                users = new ArrayList<User>();
            user = new User();
            List<Question> questions = null;
            Question question = null;
            //create dummy questions
            for (int questionCount = 0; questionCount < 5; questionCount++) {
                if(questionCount==0)
                    questions = new ArrayList<Question>();
                question = new Question();
                question.setAskedBy(user);
                System.out.println("question added is "+question);
                questions.add(question);
            }
            user.setQuestions(questions);
            users.add(user);
            System.out.println("user added is "+user);
        }
        //TODO
        System.out.println(users);
        Gson gson = new Gson();
        System.out.println(gson.toJson(users));

    }
}

コンソール ログ:

Exception in thread "main" java.lang.StackOverflowError
    at java.lang.AbstractStringBuilder.append(Unknown Source)
    at java.lang.StringBuffer.append(Unknown Source)
    at java.io.StringWriter.write(Unknown Source)
    at com.google.gson.stream.JsonWriter.string(JsonWriter.java:534)
    at com.google.gson.stream.JsonWriter.writeDeferredName(JsonWriter.java:402)
    at com.google.gson.stream.JsonWriter.value(JsonWriter.java:495)
    at com.google.gson.internal.bind.TypeAdapters$8.write(TypeAdapters.java:268)
    at com.google.gson.internal.bind.TypeAdapters$8.write(TypeAdapters.java:253)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:96)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:60)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
    at com.google.gson.Gson$FutureTypeAdapter.write(Gson.java:892)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:96)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:60)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
    at com.google.gson.Gson$FutureTypeAdapter.write(Gson.java:892)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:96)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:60)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
    at com.google.gson.Gson$FutureTypeAdapter.write(Gson.java:892)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:96)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:60)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
    at com.google.gson.Gson$FutureTypeAdapter.write(Gson.java:892)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:96)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:60)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
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1 に答える 1

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( com.fasterxml.jackson ) アプリケーションに Jackson API を配置し、この注釈を実際に使用します。

逆クラスへの @JsonBackReference (1 対 N の場合はリストまたはセット)。

@JsonManagedReference への mappedBy クラス (1 対 N の場合は単一オブジェクト)。

あなたの場合

@OneToMany(cascade=CascadeType.ALL)
@PrimaryKeyJoinColumn
@JsonManagedReference   
public List<Question> getQuestions() {
     return questions;
}

______________________________________


@ManyToOne(cascade=CascadeType.ALL,fetch=FetchType.EAGER)
@PrimaryKeyJoinColumn
@JsonBackReference
public User getAskedBy() {
    return askedBy;
}

json を Jackson (gson ではなく) で出力する必要があります。

シリアライゼーションの双方向再帰で動作します。

于 2016-01-27T13:06:42.780 に答える