JPA 應該都熟悉了，我就不多說了什麼是JPA了。目前JPA主要實現由hibernate和openJPA等。

Spring Data JPA 是Spring Data 的一個子專案，它通過提供基於JPA的Repository極大了減少了操作JPA的程式碼。筆者覺得這個由SpringBoot 提供的JPARepository真的是非常爽。基本上大部分的業務都可以滿足了。

在Spring環境中需要配置大量了XML配置，但是SpringBoot基本上幫助我們配置好了，我們只需要簡單地配置一下DataSource和幾項jpa額外的配置就完成了整個配置持久化層的工作。EntityManagerFactory 那些都不用配置了。

常規貼出一下Spring-data-JPA的MAVEN

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

一、資料來源和JPA配置

當然我們還是需要在application.properties配置資料來源，SpringBoot預設是使用tomcat的資料來源（intellij有自動提示，spring.datasource.tomcat開頭的配置）：

#datasource
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver
spring.datasource.url=jdbc:mysql://localhost:3306/mybatis_db?charset=utf8mb4&serverTimezone=UTC
spring.datasource.username=root
spring.datasource.password=tonyyan
spring.datasource.tomcat.max-active=20
spring.datasource.tomcat.test-while-idle
 
=true
spring.datasource.tomcat.validation-query=select 1
spring.datasource.tomcat.default-auto-commit=false
spring.datasource.tomcat.min-idle=15
spring.datasource.tomcat.initial-size=15

spring.jpa.hibernate.ddl-auto=update
spring.jpa.show-sql=true
spring.jackson.serialization.indent-output=true

<dependency>
   <groupId>mysql</groupId>
   <artifactId>mysql-connector-java</artifactId>
   <version>6.0.6</version>
</dependency>

二、Entity配置

開始之前我先提供一下我的entity配置，便於理解後面的查詢操作，我們使用模仿微博的資料實體：

User類：

@Entity
@Table(name = "users")
public class User {

    public User() {
    }

    public User(String username, String userpwd) {
        this.username = username;
        this.userpwd = userpwd;
}

    private long userId;
    private String username;
    private String userpwd;
@JsonIgnore
private Set<Weibo> weibos;
@Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    public long getUserId() {
        return userId;
}

    public void setUserId(long userId) {
        this.userId = userId;
}

    @Column(name="username")
    public String getUsername() {
        return username;
}

    public void setUsername(String username) {
        this.username = username;
}

    @Column(name = "userpwd")
    public String getUserpwd() {
        return userpwd;
}

    public void setUserpwd(String userpwd) {
        this.userpwd = userpwd;
}

    @OneToMany(fetch = FetchType.LAZY,cascade = {CascadeType.REMOVE},mappedBy = "user")
    public Set<Weibo> getWeibos() {
        return weibos;
}

    public void setWeibos(Set<Weibo> weibos) {
        this.weibos = weibos;
}

    @Override
public String toString() {
        return "User{" +
                "userId=" + userId +
                ", username='" + username + '\'' +
                ", userpwd='" + userpwd + '\'' +
                '}';
}
}

Weibo類：

@Entity
@Table(name="weibo")
public class Weibo {

    public Weibo() {
    }

    public Weibo(User user, String weiboText, Date createDate) {
        this.user = user;
        this.weiboText = weiboText;
        this.createDate = createDate;
}

    private long weiboId;
    private User user;
    private String weiboText;
    private Date createDate;
    private Set<Comment> comments;
@Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    public long getWeiboId() {
        return weiboId;
}

    public void setWeiboId(long weiboId) {
        this.weiboId = weiboId;
}

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "user_id")
    public User getUser() {
        return user;
}

    public void setUser(User user) {
        this.user = user;
}

    @Column(name = "weibo_text")
    public String getWeiboText() {
        return weiboText;
}

    public void setWeiboText(String weiboText) {
        this.weiboText = weiboText;
}

    @Column(name = "create_date")
    public Date getCreateDate() {
        return createDate;
}

    public void setCreateDate(Date createDate) {
        this.createDate = createDate;
}

    @OneToMany(fetch = FetchType.LAZY,cascade = {CascadeType.REMOVE},mappedBy = "weibo")
    public Set<Comment> getComments() {
        return comments;
}

    public void setComments(Set<Comment> comments) {
        this.comments = comments;
}

    @Override
public String toString() {
        return "Weibo{" +
                "weiboId=" + weiboId +
                ", user=" + user +
                ", weiboText='" + weiboText + '\'' +
                ", createDate=" + createDate +
                '}';
}
}

comment評論類：

@Entity
@Table(name = "comments")
public class Comment {

    public Comment() {
    }

    public Comment(User user, Weibo weibo, String commentText, Date commentDate) {
        this.user = user;
        this.weibo = weibo;
        this.commentText = commentText;
        this.commentDate = commentDate;
}

    private long commentId;
    private User user;
@JsonIgnore
private Weibo weibo;
    private String commentText;
    private Date commentDate;
@Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    public long getCommentId() {
        return commentId;
}

    public void setCommentId(long commentId) {
        this.commentId = commentId;
}

    @ManyToOne
    @JoinColumn(name = "user_id")
    public User getUser() {
        return user;
}

    public void setUser(User user) {
        this.user = user;
}

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "weibo_id")
    public Weibo getWeibo() {
        return weibo;
}

    public void setWeibo(Weibo weibo) {
        this.weibo = weibo;
}

    @Column(name = "comment_text")
    public String getCommentText() {
        return commentText;
}

    public void setCommentText(String commentText) {
        this.commentText = commentText;
}

    @Column(name = "comment_date")
    public Date getCommentDate() {
        return commentDate;
}

    public void setCommentDate(Date commentDate) {
        this.commentDate = commentDate;
}

    @Override
public String toString() {
        return "Comment{" +
                "commentId=" + commentId +
                ", user=" + user +
                ", weibo=" + weibo +
                ", commentText='" + commentText + '\'' +
                ", commentDate=" + commentDate +
                '}';
}
}

三、強大的Spring JpaRepository

SpringBoot建立DAO層很多種方法其中japrepository是最強大的而且最有特色的一種，我們可以針對不同的實體建立repository介面。Spring會根據方法名稱的規則進行自動生成實現，強大的不要不要的。在SpringBoot中預設已經提供了非常多的常規CRUD操作的repository，以下就是Spring為我們提供的repository介面：

@NoRepositoryBean
public interface JpaRepository<T, ID extends Serializable> extends PagingAndSortingRepository<T, ID>, QueryByExampleExecutor<T> {
    List<T> findAll();
List<T> findAll(Sort var1);
List<T> findAll(Iterable<ID> var1);
<S extends T> List<S> save(Iterable<S> var1);
    void flush();
<S extends T> S saveAndFlush(S var1);
    void deleteInBatch(Iterable<T> var1);
    void deleteAllInBatch();
T getOne(ID var1);
<S extends T> List<S> findAll(Example<S> var1);
<S extends T> List<S> findAll(Example<S> var1, Sort var2);
}

@NoRepositoryBean
public interface PagingAndSortingRepository<T, ID extends Serializable> extends CrudRepository<T, ID> {
    Iterable<T> findAll(Sort var1);
Page<T> findAll(Pageable var1);
}

@NoRepositoryBean
public interface CrudRepository<T, ID extends Serializable> extends Repository<T, ID> {
    <S extends T> S save(S var1);
<S extends T> Iterable<S> save(Iterable<S> var1);
T findOne(ID var1);
    boolean exists(ID var1);
Iterable<T> findAll();
Iterable<T> findAll(Iterable<ID> var1);
    long count();
    void delete(ID var1);
    void delete(T var1);
    void delete(Iterable<? extends T> var1);
    void deleteAll();
}

這些操作完全不用我們去實現，這些不是我們以往在普通的Spring專案中自己所定義的BaseDao嗎？SpringBoot真的是非常體貼，大大減低了我們的工作量。但是更為強大的還在後面。我們通過繼承JpaRepository介面，除了可以獲得上面的基礎CRUD操作方法之外，還可以通過Spring規定的介面命名方法自動建立複雜的CRUD操作，以下是我在Spring Data JPA 文件中找到的命名規則表：