# Spring Data JPA
### 1. pom.xml 設定
> 加入spring-boot-starter-data-jpa依賴

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>

### 2. 建立Model
* @Entity的Bean是告訴Spring這是數據模型層的宣告
* @Table name: Table的name對映到資料庫中的資料表名稱
* @Column name: 對應到Table的欄位中的欄位名稱
* @Id : 是此資料表的Primary Key
* @GeneratedValue : 告訴此Column的生成方式 ,如果設定成GenerationType.AUTO讓容器來自動產生
> <font color='red'>需對應資料庫裡的course_student資料表</font>
> 可以省略@Table，但是要確保資料庫內有與此Model類別一樣名稱的資料表

package eyes.media.student.vo;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.IdClass;
import javax.persistence.Table;

@Entity
@Table(name = "course_student")
@IdClass(CourseStudentId.class)
public class CourseStudent {
	@Id
	private String courseid;
	@Id
	private String studentid;
	private String gradelevel;

	public String getCourseid() {
		return courseid;
	}

	public void setCourseid(String courseid) {
		this.courseid = courseid;
	}

	public String getStudentid() {
		return studentid;
	}

	public void setStudentid(String studentid) {
		this.studentid = studentid;
	}

	public String getGradelevel() {
		return gradelevel;
	}

	public void setGradelevel(String gradelevel) {
		this.gradelevel = gradelevel;
	}

	@Override
	public String toString() {
		return "CourseStudent [courseid=" + courseid + ", studentid=" + studentid + ", gradelevel=" + gradelevel + "]";
	}
}

### 3-1. 組合鍵設定方式 Composite Primary Keys
>須遵循下列規則
> * The composite primary key class must be public
> * It must have a no-arg constructor
> * It must define equals() and hashCode() methods
> * It must be Serializable

package eyes.media.student.vo;

import java.io.Serializable;
import javax.persistence.Id;

public class CourseStudentId implements Serializable {
	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;
	@Id
	private String courseid;
	@Id
	private String studentid;

	public CourseStudentId() {

	}

	public CourseStudentId(String courseid, String studentid) {
		this.courseid = courseid;
		this.studentid = studentid;
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((courseid == null) ? 0 : courseid.hashCode());
		result = prime * result + ((studentid == null) ? 0 : studentid.hashCode());
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		CourseStudentId other = (CourseStudentId) obj;
		if (courseid == null) {
			if (other.courseid != null)
				return false;
		} else if (!courseid.equals(other.courseid))
			return false;
		if (studentid == null) {
			if (other.studentid != null)
				return false;
		} else if (!studentid.equals(other.studentid))
			return false;
		return true;
	}
}

### 3-2. 組合鍵設定方式 Composite Primary Keys

@Entity
public class YourEntity {
    @EmbeddedId
    private MyKey myKey;

    @Column(name = "ColumnA")
    private String columnA;

    /** Your getters and setters **/
}

@Embeddable
public class MyKey implements Serializable {

    @Column(name = "Id", nullable = false)
    private int id;

    @Column(name = "Version", nullable = false)
    private int version;

    /** getters and setters **/
}

### 3-2. UUID主鍵設定方式

    @Id
    @GeneratedValue(generator = "uuidGenerator")    
    @GenericGenerator(name = "uuidGenerator", strategy = "uuid")
    @Column(length = 32)
    private String workId;

### 4. 建立Interface
> 建立一個繼承 CrudRepository 的介面：
> CrudRepository 這個介面，定義了 save、findById、delete 等方法，繼承的時候，只要指定對應表格的類型與 id 類型，剩下的就交給 Spring Data JDBC 來處理了。

### 5. 客製化SQL語法
> 可以使用 @Query 與 @Modifying 來定義，@Query 中可以撰寫 SQL，相關的參數可以使用 @Param 來標註對應，如果是個更新操作，額外標註 @Modifying 就可以了。
> 非HQL原生語法需加上 <font color='red'>**nativeQuery = true**</font>

package eyes.media.student.dao;

import java.util.List;
import org.springframework.data.jpa.repository.Query;
import org.springframework.data.repository.CrudRepository;
import org.springframework.data.repository.query.Param;
import org.springframework.stereotype.Repository;
import eyes.media.student.vo.CourseStudent;

@Repository
public interface CourseStudentDao extends CrudRepository<CourseStudent, String> {

	@Query(value = "SELECT * FROM course_student cs WHERE cs.courseid = :courseid", nativeQuery = true)
	List<CourseStudent> findCouresStuByCourseId(@Param("courseid") String courseid);

}

### 6. 回傳結果處裡

    //You will get null when Student not found
    public Student getStudent(String studentId) {
		return studentRepository.findById(studentId).orElse(null);
    }
    
    public Student getStudent(String studentId) {
		return studentRepository.findById(studentId).get();
    }
    
    public List<Student> findAll() {
        List<Student> students = new ArrayList<>();
        studentRepository.findAll().forEach(students::add);
        return students;
    }
    
    //insert
    @Transactional
	public void add(Student vo) {
		studentDao.save(vo);
	}

資料來源:
[Day12-Spring Boot-什麼是Spring Data JPA](https://ithelp.ithome.com.tw/articles/10194906)
[Composite Primary Keys in JPA](https://www.baeldung.com/jpa-composite-primary-keys)
[簡介 Spring Data JDBC](https://openhome.cc/Gossip/Spring/SpringDataJDBC.html)
[Spring Boot中CrudRepository与JpaRepository](https://blog.csdn.net/xuemengrui12/article/details/80525227)
[理解JPA注解@GeneratedValue](https://blog.csdn.net/canot/article/details/51455967)
###### tags: `Spring boot` `Spring Data JPA`