JPA/Hibernate双向关联的同步策略：手动维护与字节码增强

import jakarta.persistence.*;
import java.util.ArrayList;
import java.util.List;

@Entity
public class Parent {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    // 非拥有方：mappedBy指向Child实体中的parent字段
    @OneToMany(mappedBy = "parent", cascade = { CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REMOVE }, orphanRemoval = true)
    private List<Child> children = new ArrayList<>(); // 初始化集合以避免NullPointerException

    // Getters and Setters
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public List<Child> getChildren() { return children; }
    public void setChildren(List<Child> children) { this.children = children; }

    // 辅助方法：确保双向关联同步
    public void addChild(Child child) {
        children.add(child);
        child.setParent(this); // 关键：设置Child的parent引用
    }

    public void removeChild(Child child) {
        children.remove(child);
        child.setParent(null); // 关键：解除Child的parent引用
    }
}

import jakarta.persistence.*;

@Entity
public class Child {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    // 拥有方：负责在数据库中维护外键
    @ManyToOne(optional = false) // optional=false 表示parent不能为空
    private Parent parent;

    private String name;

    // Getters and Setters
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public Parent getParent() { return parent; }
    public void setParent(Parent parent) { this.parent = parent; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
}

// Parent.java 中的辅助方法
public void addChild(Child child) {
    children.add(child);
    child.setParent(this); // 关键步骤：同步Child的parent引用
}

public void removeChild(Child child) {
    children.remove(child);
    child.setParent(null); // 关键步骤：解除Child的parent引用
}

// 创建并关联实体
Parent parent = new Parent();
parent.setName("Parent A"); // 假设Parent也有name字段

Child child1 = new Child();
child1.setName("Child 1");

Child child2 = new Child();
child2.setName("Child 2");

// 通过辅助方法添加子实体，确保双向同步
parent.addChild(child1);
parent.addChild(child2);

// 持久化Parent，Child也会被级联持久化
entityManager.persist(parent);

// 此时，child1和child2的parent字段都已正确设置
// 数据库中Child表的外键也会指向Parent的ID