Jackson JSON包在从对象映射到Json字符串过程的循环依赖问题-分析与解决

比眉伴天荒 2022-03-06 13:52 225阅读 0赞

## 提出问题 ##

在基于Jackson类库将对象转化为json字符串时，如果对象中存在互相依赖，则会产生无限循环的情况，具体情况如下：  
BuyerEntity.java

@Table(name="t_buyer")
    @Entity
    @ToString(callSuper=true)
    @Data
    @EqualsAndHashCode(callSuper=false)
    public class BuyerEntity extends BaseEntity {
        @Column(nullable=false)
        private String name;
        
        @Column
        private Integer age;
        
        @Column(name="sex")
        private Integer sex;
        
        //@Enumerated(EnumType.ORDINAL)
        @Column(nullable=false, name="gender")
        @Convert(converter = GenderConverter.class)
        private Gender gender = Gender.Male;
        
        @Column
        private String email;
        
        @OneToMany(cascade= {CascadeType.ALL}, mappedBy="buyer")
        private List<OrderEntity> orders;
    }

OrderEntity.java

@Entity
    @Table(name="t_order")
    @ToString(callSuper=true)
    @Data
    public class OrderEntity extends BaseEntity {
         
    	//商品消费数据量
    	@Column
    	private int count;
    	
    	//消费记录的总价
    	@Column(name="total_fee")
    	private double totalFee;
    	
    	//折扣率
    	@Column(name="discount_rate")
    	private double discountRate;
    	
        @ManyToOne(cascade= {CascadeType.REFRESH})
    	@JoinColumn(name="buyer_id", referencedColumnName="id")
    	private BuyerEntity buyer;
    }

生成Json字符串的过程如下：

BuyerEntity buyer = new BuyerEntity();
        	buyer.setAge(12);
        	buyer.setCreatedTime(new Date());
        	buyer.setUpdatedTime(new Date());
        	buyer.setEmail("123@123.cn");
        	buyer.setGender(Gender.Female);
        	buyer.setName("zhangsan");
        	
        	OrderEntity order1 = new OrderEntity();
        	order1.setCount(12);
        	order1.setCreatedTime(new Date());
        	order1.setUpdatedTime(new Date());
        	order1.setTotalFee(123.3);
        	order1.setDiscountRate(0.80);
        	order1.setBuyer(buyer);
        	
        	OrderEntity order2 = new OrderEntity();
        	order2.setCount(9);
        	order2.setCreatedTime(new Date());
        	order2.setUpdatedTime(new Date());
        	order2.setTotalFee(35.3);
        	order2.setDiscountRate(0.60);
        	order1.setBuyer(buyer);
        	
        	List<OrderEntity> orders = Lists.newArrayList();
        	orders.add(order1);
        	orders.add(order2);
        	
        	buyer.setOrders(orders);
        	
      ObjectMapper objectMapper = new ObjectMapper();
      String jsonStr = objectMapper.writeValueAsString(buyer);
      System.out.println(jsonStr);

上述结果信息中则会出现如下的无限循环：

{"code":0,"message":"success","data":{"id":2,"createdTime":"2019-03-19T01:30:53.123+0000","updatedTime":"2019-03-19T01:30:53.123+0000","version":0,"name":"zhangsan","age":12,"sex":null,"gender":"Female","email":"123@123.cn","orders":[{"id":3,"createdTime":"2019-03-19T01:30:53.123+0000","updatedTime":"2019-03-19T01:30:53.123+0000","version":0,"count":12,"totalFee":123.3,"discountRate":0.8,"buyer":{"id":2,"createdTime":"2019-03-19T01:30:53.123+0000","updatedTime":"2019-03-19T01:30:53.123+0000","version":0,"name":"zhangsan","age":12,"sex":null,"gender":"Female","email":"123@123.cn","orders":[{"id":3,"createdTime":"2019-03-19T01:30:53.123+0000","updatedTime":"2019-03-19T01:30:53.123+0000","version":0,"count":12,"totalFee":123.3,"discountRate":0.8,"buyer":{"id":2,"createdTime":"2019-03-19T01:30:53.123+0000","updatedTime":"2019-03-19T01:30:53.123+0000","version":0,"name":"zhangsan","age":12,"sex":null,"gender":"Female","email":"123@123.cn","orders":[{"id":3,"createdTime":"2019-03-19T01:30:53.123+0000","updatedTime":"2019-03-19T01:30:53.123+0000","version":0,"count":12,"totalFee":123.3,"discountRate":0.8,"buyer":{"id":2,"createdTime":"2019-03-19T01:30:53.123+0000","updatedTime":"2019-03-19T01:30:53.123+0000","version":0,"name":"zhangsan","age":12,"sex":null,"gender":"Female","email":"123@123.cn","orders":[{"id":3,"createdTime":"2019-03-19T01:30:53.123+0000","updatedTime":"2019-03-19T01:30:53.123+0000","version":0,"count":12,"totalFee":123.3,"discountRate":0.8,"buyer":{"id":2,"createdTime":"2019-03-19T01:30:53.123+0000","updatedTime":"2019-03-19T01:30:53.123+0000","version":0,"name":"zhangsan","age":12,"sex":null,"gender":"Female","email":"123@123.cn","orders":[{"id":3,"createdTime":"2019-03-19T01:30:53.123+0000","updatedTime":"2019-03-19T01:30:53.123+0000","version":0,"count":12,"totalFee":123.3,"discountRate":0.8,"

## 问题解决 ##

那该问题该如何破解呢？

1.  使用 @JsonManagedReference @JsonBackReference  
    @JsonManagedReference： 正常输出Json的字符串  
    @JsonBackReference： 则会忽略输出json的字符串
2.  @JsonIgnore： 其本身是忽略json的字符串输出的。

## 解决方案 ##

于是将@JsonManagedReference 放到BuyerEntity.java中的List之上。

@JsonBackReference
      @ManyToOne(cascade= {CascadeType.REFRESH})
      @JoinColumn(name="buyer_id", referencedColumnName="id")
      private BuyerEntity buyer;

与之相对应的是另外一个：

@JsonManagedReference
      @OneToMany(cascade= {CascadeType.ALL}, mappedBy="buyer")
      private List<OrderEntity> orders;

## 测试用例 ##

public class JacksonCircularReferenceTest {
        private ObjectMapper objectMapper = new ObjectMapper();
        
    	@Before
    	public void setUp() throws Exception {
    	}
    
    	@After
    	public void tearDown() throws Exception {
    	}
    
    	@Test
    	public void test() throws JsonProcessingException {    
    		BuyerEntity buyer = new BuyerEntity();
        	buyer.setAge(12);
        	buyer.setCreatedTime(new Date());
        	buyer.setUpdatedTime(new Date());
        	buyer.setEmail("123@123.cn");
        	buyer.setGender(Gender.Female);
        	buyer.setName("zhangsan");
        	
        	OrderEntity order1 = new OrderEntity();
        	order1.setCount(12);
        	order1.setCreatedTime(new Date());
        	order1.setUpdatedTime(new Date());
        	order1.setTotalFee(123.3);
        	order1.setDiscountRate(0.80);
        	order1.setBuyer(buyer);
        	
        	OrderEntity order2 = new OrderEntity();
        	order2.setCount(9);
        	order2.setCreatedTime(new Date());
        	order2.setUpdatedTime(new Date());
        	order2.setTotalFee(35.3);
        	order2.setDiscountRate(0.60);
        	order1.setBuyer(buyer);
        	
        	List<OrderEntity> orders = Lists.newArrayList();
        	orders.add(order1);
        	orders.add(order2);
        	
        	buyer.setOrders(orders);
        	
        	String jsonStr = objectMapper.writeValueAsString(buyer);
        	System.out.println(jsonStr);
    	}
    }

## 参考资料 ##

1.  [https://www.baeldung.com/jackson-bidirectional-relationships-and-infinite-recursion][https_www.baeldung.com_jackson-bidirectional-relationships-and-infinite-recursion]

[https_www.baeldung.com_jackson-bidirectional-relationships-and-infinite-recursion]: https://www.baeldung.com/jackson-bidirectional-relationships-and-infinite-recursion