我们在使用JPA对历史问题处理时，会涉及到组合主键的处理。处理的思路如下：

• 使用@Embeddabel定义一个组合组键ID
• ID实现SErializable接口
• 声明一个protected的构造方法
• 声明带有复合主键属性的构造方法
• 重写equals和hasCode方法

比如，我有一个使用studentId和courseId两个字段做为主键的复习主键表。

定义代码如下：

package com.mengyunzhi.english.entity;import javax.persistence.Embeddable;
import javax.persistence.EmbeddedId;
import javax.persistence.Entity;
import javax.persistence.Table;
import java.io.Serializable;
import java.util.Objects;/*** 学生单词学习节点* 记录学生学习某门课的单词进度* panjie*/
@Entity
@Table(name = "english_student_word_study_node")
public class StudentWordStudyNode {@EmbeddedIdprivate Id id;private Short maxIndex;protected StudentWordStudyNode() {}public StudentWordStudyNode(Id id) {this.id = id;}public Id getId() {return id;}public void setId(Id id) {this.id = id;}public java.lang.Short getMaxIndex() {return maxIndex;}public void setMaxIndex(java.lang.Short maxIndex) {this.maxIndex = maxIndex;}@Embeddablestatic public class Id implements Serializable {protected Integer studentId;protected Integer courseId;protected Id() {}public Id(Integer studentId, Integer courseId) {this.studentId = studentId;this.courseId = courseId;}public Integer getStudentId() {return studentId;}public void setStudentId(Integer studentId) {this.studentId = studentId;}public Integer getCourseId() {return courseId;}public void setCourseId(Integer courseId) {this.courseId = courseId;}@Overridepublic boolean equals(Object o) {if (this == o) return true;if (!(o instanceof Id)) return false;Id id = (Id) o;return Objects.equals(studentId, id.studentId) &&Objects.equals(courseId, id.courseId);}@Overridepublic int hashCode() {return Objects.hash(studentId, courseId);}}
}

注意：内部类ID的static不能少。这是由于：我们将主键ID声明为了内部类，而其做为复合组件出现，是需要直接暴露无参的构造函数的，否则将出现：No default constructor for entity