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@JsonBackReference和@JsonManagedReference(解决对象中存在双向引用导致的无限递归)
2022-07-06 08:30:00 【XYLANCC】
jackson中的@JsonBackReference和@JsonManagedReference,以及@JsonIgnore均是为了解决对象中存在双向引用导致的无限递归(infinite recursion)问题。这些标注均可用在属性或对应的get、set方法中。
@JsonBackReference和@JsonManagedReference:这两个标注通常配对使用,通常用在父子关系中。@JsonBackReference标注的属性在序列化(serialization,即将对象转换为json数据)时,会被忽略(即结果中的json数据不包含该属性的内容)。@JsonManagedReference标注的属性则会被序列化。在序列化时,@JsonBackReference的作用相当于@JsonIgnore,此时可以没有@JsonManagedReference。但在反序列化(deserialization,即json数据转换为对象)时,如果没有@JsonManagedReference,则不会自动注入@JsonBackReference标注的属性(被忽略的父或子);如果有@JsonManagedReference,则会自动注入自动注入@JsonBackReference标注的属性。
@JsonIgnore:直接忽略某个属性,以断开无限递归,序列化或反序列化均忽略。当然如果标注在get、set方法中,则可以分开控制,序列化对应的是get方法,反序列化对应的是set方法。在父子关系中,当反序列化时,@JsonIgnore不会自动注入被忽略的属性值(父或子),这是它跟@JsonBackReference和@JsonManagedReference最大的区别。
示例测试代码(注意反序列化后的TreeNode[readValue]的children里的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代码 收藏代码
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;
}
}
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