ジャクソン–双方向の関係

投稿日: 2019-10-18 2022-11-09
タグ: Data, Jackson, JSON

1. 概要

In this tutorial, we’ll examine the best ways to deal with bidirectional relationships in Jackson.

First, we’ll discuss the Jackson JSON infinite recursion problem. Then we’ll see how to serialize entities with bidirectional relationships. Finally, we’ll deserialize them.

2. 無限再帰

Let’s take a look at the Jackson infinite recursion problem. In the following example, we have two entities, “User” and “Item,” with a simple one-to-many relationship:

「User」エンティティ：

public class User {
    public int id;
    public String name;
    public List<Item> userItems;
}

「Item」エンティティ：

public class Item {
    public int id;
    public String itemName;
    public User owner;
}

When we try to serialize an instance of “Item,” Jackson will throw a JsonMappingException exception:

@Test(expected = JsonMappingException.class)
public void givenBidirectionRelation_whenSerializing_thenException()
  throws JsonProcessingException {
 
    User user = new User(1, "John");
    Item item = new Item(2, "book", user);
    user.addItem(item);

    new ObjectMapper().writeValueAsString(item);
}

完全な例外は次のとおりです。

com.fasterxml.jackson.databind.JsonMappingException:
Infinite recursion (StackOverflowError) 
(through reference chain: 
org.baeldung.jackson.bidirection.Item["owner"]
->org.baeldung.jackson.bidirection.User["userItems"]
->java.util.ArrayList[0]
->org.baeldung.jackson.bidirection.Item["owner"]
->…..

We’ll see over the course of the next few sections how to solve this problem.

**3. @JsonManagedReference 、@JsonBackReferenceを使用します**

First, let’s annotate the relationship with @JsonManagedReference, and @JsonBackReference to allow Jackson to better handle the relation:

「User」エンティティは次のとおりです。

public class User {
    public int id;
    public String name;

    @JsonManagedReference
    public List<Item> userItems;
}

そして「アイテム」：

public class Item {
    public int id;
    public String itemName;

    @JsonBackReference
    public User owner;
}

Now let’s test out the new entities:

@Test
public void givenBidirectionRelation_whenUsingJacksonReferenceAnnotationWithSerialization_thenCorrect() throws JsonProcessingException {
    final User user = new User(1, "John");
    final Item item = new Item(2, "book", user);
    user.addItem(item);

    final String itemJson = new ObjectMapper().writeValueAsString(item);
    final String userJson = new ObjectMapper().writeValueAsString(user);

    assertThat(itemJson, containsString("book"));
    assertThat(itemJson, not(containsString("John")));

    assertThat(userJson, containsString("John"));
    assertThat(userJson, containsString("userItems"));
    assertThat(userJson, containsString("book"));
}

Here’s the output of serializing the Item object:

{
 "id":2,
 "itemName":"book"
}

And here’s the output of serializing the User object:

{
 "id":1,
 "name":"John",
 "userItems":[{
   "id":2,
   "itemName":"book"}]
}

ご了承ください：

@JsonManagedReference is the forward part of reference, the one that gets serialized normally.
@JsonBackReference is the back part of reference; it’ll be omitted from serialization.
The serialized Item object doesn’t contain a reference to the User object.

Also note that we can’t switch around the annotations. シリアル化には以下が機能します。

@JsonBackReference
public List<Item> userItems;

@JsonManagedReference
public User owner;

But when we attempt to deserialize the object, it’ll throw an exception, as @JsonBackReference can’t be used on a collection.

シリアル化されたItemオブジェクトにUserへの参照を含める場合は、@JsonIdentityInfoを使用する必要があります。 We’ll look at this in the next section.

**4. @JsonIdentityInfoを使用します**

Now let’s learn how we can help with the serialization of entities with bidirectional relationships using @JsonIdentityInfo.

We’ll add the class level annotation to our “User” entity:

@JsonIdentityInfo(
  generator = ObjectIdGenerators.PropertyGenerator.class, 
  property = "id")
public class User { ... }

そして、「 Item 」エンティティに：

@JsonIdentityInfo(
  generator = ObjectIdGenerators.PropertyGenerator.class, 
  property = "id")
public class Item { ... }

テストの時間：

@Test
public void givenBidirectionRelation_whenUsingJsonIdentityInfo_thenCorrect()
  throws JsonProcessingException {
 
    User user = new User(1, "John");
    Item item = new Item(2, "book", user);
    user.addItem(item);

    String result = new ObjectMapper().writeValueAsString(item);

    assertThat(result, containsString("book"));
    assertThat(result, containsString("John"));
    assertThat(result, containsString("userItems"));
}

Here’s the output of serialization:

{
 "id":2,
 "itemName":"book",
 "owner":
    {
        "id":1,
        "name":"John",
        "userItems":[2]
    }
}

**5. @JsonIgnoreを使用します**

Alternatively, we can use the @JsonIgnore annotation to simply ignore one of the sides of the relationship, thus breaking the chain.

In the following example, we’ll prevent the infinite recursion by ignoring the “User” property “userItems” from serialization:

「User」エンティティは次のとおりです。

public class User {
    public int id;
    public String name;

    @JsonIgnore
    public List<Item> userItems;
}

And here’s our test:

@Test
public void givenBidirectionRelation_whenUsingJsonIgnore_thenCorrect()
  throws JsonProcessingException {
 
    User user = new User(1, "John");
    Item item = new Item(2, "book", user);
    user.addItem(item);

    String result = new ObjectMapper().writeValueAsString(item);

    assertThat(result, containsString("book"));
    assertThat(result, containsString("John"));
    assertThat(result, not(containsString("userItems")));
}

Finally, here’s the output of serialization:

{
 "id":2,
 "itemName":"book",
 "owner":
    {
        "id":1,
        "name":"John"
    }
}

**6. @JsonViewを使用してください**

新しい@JsonViewアノテーションを使用して、関係の片側を除外することもできます。

In the following example, we’ll use two JSON Views, Public and Internal, where Internal extends Public:

public class Views {
    public static class Public {}

    public static class Internal extends Public {}
}

We’ll include all User and Item fields in the Public View except the User field userItems, which will be included in the Internal View:

Here’s our “User” entity:

public class User {
    @JsonView(Views.Public.class)
    public int id;

    @JsonView(Views.Public.class)
    public String name;

    @JsonView(Views.Internal.class)
    public List<Item> userItems;
}

And here’s our “Item” entity:

public class Item {
    @JsonView(Views.Public.class)
    public int id;

    @JsonView(Views.Public.class)
    public String itemName;

    @JsonView(Views.Public.class)
    public User owner;
}

When we serialize using the Public view, it works correctly because we excluded userItems from being serialized:

@Test
public void givenBidirectionRelation_whenUsingPublicJsonView_thenCorrect() 
  throws JsonProcessingException {
 
    User user = new User(1, "John");
    Item item = new Item(2, "book", user);
    user.addItem(item);

    String result = new ObjectMapper().writerWithView(Views.Public.class)
      .writeValueAsString(item);

    assertThat(result, containsString("book"));
    assertThat(result, containsString("John"));
    assertThat(result, not(containsString("userItems")));
}

But if we serialize using an Internal view, JsonMappingException is thrown because all the fields are included:

@Test(expected = JsonMappingException.class)
public void givenBidirectionRelation_whenUsingInternalJsonView_thenException()
  throws JsonProcessingException {
 
    User user = new User(1, "John");
    Item item = new Item(2, "book", user);
    user.addItem(item);

    new ObjectMapper()
      .writerWithView(Views.Internal.class)
      .writeValueAsString(item);
}

7. カスタムシリアライザーを使用する

Next, we’ll see how to serialize entities with bidirectional relationships using a custom serializer.

In the following example, we’ll use a custom serializer to serialize the “User” property “userItems:”

「User」エンティティは次のとおりです。

public class User {
    public int id;
    public String name;

    @JsonSerialize(using = CustomListSerializer.class)
    public List<Item> userItems;
}

And here’s the “CustomListSerializer:”

public class CustomListSerializer extends StdSerializer<List<Item>>{

   public CustomListSerializer() {
        this(null);
    }

    public CustomListSerializer(Class<List> t) {
        super(t);
    }

    @Override
    public void serialize(
      List<Item> items, 
      JsonGenerator generator, 
      SerializerProvider provider) 
      throws IOException, JsonProcessingException {
        
        List<Integer> ids = new ArrayList<>();
        for (Item item : items) {
            ids.add(item.id);
        }
        generator.writeObject(ids);
    }
}

Now let’s test out the serializer. As we can see, the right kind of output is being produced:

@Test
public void givenBidirectionRelation_whenUsingCustomSerializer_thenCorrect()
  throws JsonProcessingException {
    User user = new User(1, "John");
    Item item = new Item(2, "book", user);
    user.addItem(item);

    String result = new ObjectMapper().writeValueAsString(item);

    assertThat(result, containsString("book"));
    assertThat(result, containsString("John"));
    assertThat(result, containsString("userItems"));
}

Here’s the final output of the serialization with the custom serializer:

{
 "id":2,
 "itemName":"book",
 "owner":
    {
        "id":1,
        "name":"John",
        "userItems":[2]
    }
}

**8. @JsonIdentityInfoで逆シリアル化**

Now let’s see how to deserialize entities with bidirectional relationships using @JsonIdentityInfo.

「User」エンティティは次のとおりです。

@JsonIdentityInfo(
  generator = ObjectIdGenerators.PropertyGenerator.class, 
  property = "id")
public class User { ... }

そして、「 Item 」エンティティ：

@JsonIdentityInfo(
  generator = ObjectIdGenerators.PropertyGenerator.class, 
  property = "id")
public class Item { ... }

We’ll write a quick test, starting with some manual JSON data we want to parse, and finishing with the correctly constructed entity:

@Test
public void givenBidirectionRelation_whenDeserializingWithIdentity_thenCorrect() 
  throws JsonProcessingException, IOException {
    String json = 
      "{\"id\":2,\"itemName\":\"book\",\"owner\":{\"id\":1,\"name\":\"John\",\"userItems\":[2]}}";

    ItemWithIdentity item
      = new ObjectMapper().readerFor(ItemWithIdentity.class).readValue(json);
    
    assertEquals(2, item.id);
    assertEquals("book", item.itemName);
    assertEquals("John", item.owner.name);
}

9. カスタムデシリアライザーを使用する

最後に、カスタムデシリアライザーを使用して、双方向の関係でエンティティをデシリアライズしましょう。

In the following example, we’ll use a custom deserializer to parse the “User” property “userItems:”

「User」エンティティは次のとおりです。

public class User {
    public int id;
    public String name;

    @JsonDeserialize(using = CustomListDeserializer.class)
    public List<Item> userItems;
}

And here’s our “CustomListDeserializer:”

public class CustomListDeserializer extends StdDeserializer<List<Item>>{

    public CustomListDeserializer() {
        this(null);
    }

    public CustomListDeserializer(Class<?> vc) {
        super(vc);
    }

    @Override
    public List<Item> deserialize(
      JsonParser jsonparser, 
      DeserializationContext context) 
      throws IOException, JsonProcessingException {
        
        return new ArrayList<>();
    }
}

Finally, here’s the simple test:

@Test
public void givenBidirectionRelation_whenUsingCustomDeserializer_thenCorrect()
  throws JsonProcessingException, IOException {
    String json = 
      "{\"id\":2,\"itemName\":\"book\",\"owner\":{\"id\":1,\"name\":\"John\",\"userItems\":[2]}}";

    Item item = new ObjectMapper().readerFor(Item.class).readValue(json);
 
    assertEquals(2, item.id);
    assertEquals("book", item.itemName);
    assertEquals("John", item.owner.name);
}

10. 結論

In this article, we illustrated how to serialize/deserialize entities with bidirectional relationships using Jackson.

The implementation of all of these examples and code snippets can be found in our GitHub project. This is a Maven-based project, so it should be easy to import and run as it is.

getdocs

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