@Jsonbackreference and @jsonmanagedreference (solve infinite recursion caused by bidirectional references in objects)

jackson Medium @JsonBackReference and @JsonManagedReference, as well as @JsonIgnore It is to solve the infinite recursion caused by the existence of two-way references in the object （infinite recursion） problem . These dimensions can be used in attributes or corresponding get、set In the method .  

@JsonBackReference and @JsonManagedReference： These two labels are usually used in pairs , Usually used in parent-child relationships [email protected] The attributes of the annotation are serialized （serialization, Convert the object to json data ） when , Will be ignored （ That is... In the result json The data does not contain the content of this attribute ）[email protected] The attributes of the annotation are serialized . At serialization ,@JsonBackReference Is equivalent to @JsonIgnore, There can be no @JsonManagedReference. But in deserialization （deserialization, namely json Data to object ） when , without @JsonManagedReference, Will not automatically inject @JsonBackReference The properties of the annotation （ Neglected parent or child ）; If there is @JsonManagedReference, Will be automatically injected @JsonBackReference The properties of the annotation .  

@JsonIgnore： Ignore an attribute directly , To break infinite recursion , Serialization or deserialization are ignored . Of course, if marked in get、set In the method , Can be controlled separately , Serialization corresponds to get Method , Deserialization corresponds to set Method . In a father son relationship , When deserializing ,@JsonIgnore Ignored attribute values are not automatically injected （ Parent or child ）, This is it with @JsonBackReference and @JsonManagedReference The biggest difference .  

Sample test code （ Pay attention to the deserialized TreeNode[readValue] Of children Inside parent）：  
TreeNode.java  

import java.util.ArrayList;  
import java.util.List;  
  
import org.codehaus.jackson.annotate.JsonBackReference;  
import org.codehaus.jackson.annotate.JsonManagedReference;  
  
public class TreeNode {  
    String name;  
    @JsonBackReference  
//  @JsonIgnore  
    TreeNode parent;  
    @JsonManagedReference  
    List<TreeNode> children;  
  
    public TreeNode() {  
    }  
  
    public TreeNode(String name) {  
        this.name = name;  
    }  
  
    public String getName() {  
        return name;  
    }  
  
    public void setName(String name) {  
        this.name = name;  
    }  
  
    public TreeNode getParent() {  
        return parent;  
    }  
  
    public void setParent(TreeNode parent) {  
        this.parent = parent;  
    }  
  
    public List<TreeNode> getChildren() {  
        return children;  
    }  
  
    public void setChildren(List<TreeNode> children) {  
        this.children = children;  
    }  
  
    public void addChild(TreeNode child) {  
        if (children == null)  
            children = new ArrayList<TreeNode>();  
        children.add(child);  
    }  
}  

JsonTest.java   
Java Code      Collection code 
import java.io.IOException;  
  
import org.codehaus.jackson.JsonGenerationException;  
import org.codehaus.jackson.map.JsonMappingException;  
import org.codehaus.jackson.map.ObjectMapper;  
import org.junit.AfterClass;  
import org.junit.BeforeClass;  
import org.junit.Test;  
  
public class JsonTest {  
    static TreeNode node;  
  
    @BeforeClass  
    public static void setUp() {  
        TreeNode node1 = new TreeNode("node1");  
        TreeNode node2 = new TreeNode("node2");  
        TreeNode node3 = new TreeNode("node3");  
        TreeNode node4 = new TreeNode("node4");  
        TreeNode node5 = new TreeNode("node5");  
        TreeNode node6 = new TreeNode("node6");  
  
        node1.addChild(node2);  
        node2.setParent(node1);  
        node2.addChild(node3);  
        node3.setParent(node2);  
        node2.addChild(node4);  
        node4.setParent(node2);  
        node3.addChild(node5);  
        node5.setParent(node3);  
        node5.addChild(node6);  
        node6.setParent(node5);  
  
        node = node3;  
    }  
  
    @Test   
    public void test() throws JsonGenerationException, JsonMappingException, IOException {  
        ObjectMapper mapper = new ObjectMapper();  
        String json = mapper.writeValueAsString(node);  
        System.out.println(json);  
        TreeNode readValue = mapper.readValue(json, TreeNode.class);  
        System.out.println(readValue.getName());  
    }  
  
    @AfterClass  
    public static void tearDown() {  
        node = null;  
    }  
}