spring data jpa关联查询（一对一、一对多、多对多）

　　在实际过往的项目中，常用的查询操作有：1、单表查询，2、一对一查询（主表和详情表）3、一对多查询（一张主表，多张子表）4、多对多查询（如权限控制，用户、角色多对多）。做个总结，所以废话不多说。

　　使用idea构建springboot项目，引入依赖如下：

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

        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

　　使用h2数据库做测试用，application.yml配置如下：

spring:
  jpa:
    generate-ddl: true
    hibernate:
      ddl-auto: update
    properties:
      hibenate:
        format_sql: false
    show-sql: true

　　首先，一对一有好几种，这里举例的是常用的一对一双向外键关联（改造成单向很简单，在对应的实体类去掉要关联其它实体的属性即可），并且配置了级联删除和添加，相关类如下：

package io.powerx;

import lombok.*;

import javax.persistence.*;

/**
 * Created by Administrator on 2018/8/15.
 */
@Getter
@Setter
@Entity
public class Book {
    @Id
    @GeneratedValue
    private Integer id;

    private String name;

    @OneToOne(cascade = {CascadeType.PERSIST,CascadeType.REMOVE})
    @JoinColumn(name="detailId",referencedColumnName = "id")
    private BookDetail bookDetail;

    public Book(){
        super();
    }
    public Book(String name){
        super();
        this.name =name;
    }

    public Book(String name, BookDetail bookDetail) {
        super();
        this.name = name;
        this.bookDetail = bookDetail;
    }
    @Override
    public String toString() {
        if (null == bookDetail) {
            return String.format("Book [id=%s, name=%s, number of pages=%s]", id, name, "<EMPTY>");
        }

        return String.format("Book [id=%s, name=%s, number of pages=%s]", id, name, bookDetail.getNumberOfPages());
    }
}

package io.powerx;

import lombok.Getter;
import lombok.Setter;

import javax.persistence.*;

@Getter
@Setter
@Entity(name = "BOOK_DETAIL")
public class BookDetail {

    @Id
    @GeneratedValue
    private Integer id;

    @Column(name = "NUMBER_OF_PAGES")
    private Integer numberOfPages;

    @OneToOne(mappedBy = "bookDetail")
    private Book book;

    public BookDetail() {
        super();
    }

    public BookDetail(Integer numberOfPages) {
        super();
        this.numberOfPages = numberOfPages;
    }

    @Override
    public String toString() {
        if (null == book) {
            return String.format("Book [id=%s, name=%s, number of pages=%s]", id, "<EMPTY>");
        }

        return String.format("Book [id=%s, name=%s, number of pages=%s]", id,book.getId(),book.getName());
    }
}

package io.powerx;

import org.springframework.data.jpa.repository.JpaRepository;

/**
 * Created by Administrator on 2018/8/15.
 */
public interface BookRepository extends JpaRepository<Book,Integer> {
    Book findByName(String name);
}

package io.powerx;

import org.springframework.data.jpa.repository.JpaRepository;

/**
 * Created by Administrator on 2018/8/15.
 */
public interface BookDetailRepository  extends JpaRepository<BookDetail, Integer>{

    BookDetail findByNumberOfPages(Integer numberOfPages);
}

package io.powerx;

import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import java.util.Arrays;

import static org.junit.Assert.assertThat;

@RunWith(SpringRunner.class)
@SpringBootTest
public class OnetooneApplicationTests {

    @Autowired
    private BookRepository bookRepository;

    @Autowired
    private BookDetailRepository bookDetailRepository;

    @Before
    public void init() {
        Book bookA = new Book("Spring in Action", new BookDetail(208));
        Book bookB = new Book("Spring Data in Action", new BookDetail(235));
        Book bookC = new Book("Spring Boot in Action");
        bookRepository.saveAll(Arrays.asList(bookA, bookB, bookC));
    }

    @After
    public void clear() {
        bookRepository.deleteAll();
    }

    @Test
    public void find() {
        Book book = bookRepository.findByName("Spring in Action");
        System.err.println(book.toString());
    }

    @Test
    public void save() {
        Book book = new Book("springboot");
        BookDetail bookDetail = new BookDetail(124);
        book.setBookDetail(bookDetail);
        bookRepository.save(book);
    }

    @Test
    public void delete() {

        bookRepository.deleteById(31);
    }
    @Test
    public void findbook(){
        BookDetail bd = bookDetailRepository.findByNumberOfPages(235);
        System.err.println(bd.toString());

    }
}

　　一对多双向，相关类如下：

package io.powerx;

import lombok.Data;
import lombok.Getter;
import lombok.Setter;

import javax.persistence.*;

@Getter
@Setter
@Entity
public class Book {
    @Id
    @GeneratedValue
    private Integer id;

    private String name;

    @ManyToOne
    @JoinColumn(name="publishId")
    private Publisher publisher;

    @Override
    public String toString() {
        return "Book{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", publisher=" + publisher.getName() +
                '}';
    }

    public Book(String name) {
        this.name = name;
    }

    public Book() {
    }
}

package io.powerx;

import lombok.Data;
import lombok.Getter;
import lombok.Setter;

import javax.persistence.*;
import java.util.HashSet;
import java.util.Set;

/**
 * Created by Administrator on 2018/8/16.
 */
@Getter
@Setter
@Entity
public class Publisher {
    @Id
    @GeneratedValue
    private Integer id;

    private String name;

    @OneToMany(cascade = CascadeType.ALL,fetch = FetchType.EAGER)
    @JoinColumn(name="publishId",referencedColumnName = "id")
    private Set<Book> books;

    public Publisher() {
        super();
    }

    public Publisher(String name) {
        super();
        this.name = name;
    }

    @Override
    public String toString() {
        return "Publisher{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", books=" + books.size() +
                '}';
    }

}

package io.powerx;

import org.springframework.data.jpa.repository.JpaRepository;

/**
 * Created by Administrator on 2018/8/16.
 */
public interface BookRepository  extends JpaRepository<Book,Integer>{

    Book findByName(String name);

}

package io.powerx;

import org.springframework.data.jpa.repository.JpaRepository;

/**
 * Created by Administrator on 2018/8/16.
 */
public interface PublisherRepository extends JpaRepository<Publisher,Integer> {

    Publisher findByName(String name);
}

package io.powerx;

import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import java.util.HashSet;
import java.util.Set;

@RunWith(SpringRunner.class)
@SpringBootTest
public class OnetomanyApplicationTests {

    @Autowired
    private PublisherRepository publisherRepository;

    @Autowired
    private BookRepository bookRepository;

    @Before
    public void init() {

        Book book1 = new Book("spring");
        Book book2 = new Book("mvc");
        Book book3 = new Book("mybatis");
        Publisher publisher = new Publisher("zhonghua");
        Set<Book> set = new HashSet<Book>();
        set.add(book1);
        set.add(book2);
        set.add(book3);
        publisher.setBooks(set);
        publisherRepository.save(publisher);

    }

    @After
    public void clear() {
        publisherRepository.deleteAll();
    }

    @Test
    public void find() {
        Publisher publisher = publisherRepository.findByName("zhonghua");
        System.out.println(publisher);
    }

    @Test
    public void find2() {
        Book book = bookRepository.findByName("mvc");
        System.out.println(book);
    }
}

　　多对多双向，相关代码如下：

package io.powerx;

import lombok.Getter;
import lombok.Setter;
import lombok.ToString;

import javax.persistence.*;
import java.util.Set;

@Getter
@Setter
@Entity
public class Author {

    @Id
    @GeneratedValue
    private Integer id;

    private String name;

    @ManyToMany(mappedBy = "authors",fetch = FetchType.EAGER)
    private Set<Book> books;

    public Author() {
        super();
    }

    public Author(String name) {
        super();
        this.name = name;
    }

    @Override
    public String toString() {
        return "Author{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", books=" + books.size() +
                '}';
    }
}

package io.powerx;

import lombok.Getter;
import lombok.Setter;
import lombok.ToString;

import javax.persistence.*;
import java.util.HashSet;
import java.util.Set;

@Getter
@Setter
@Entity
public class Book {

    @Id
    @GeneratedValue
    private Integer id;

    private String name;

    @ManyToMany(cascade = CascadeType.ALL,fetch = FetchType.EAGER)
    @JoinTable(name = "BOOK_AUTHOR", joinColumns = {
            @JoinColumn(name = "BOOK_ID", referencedColumnName = "ID")}, inverseJoinColumns = {
            @JoinColumn(name = "AUTHOR_ID", referencedColumnName = "ID")})
    private Set<Author> authors;

    public Book() {
        super();
    }

    public Book(String name) {
        super();
        this.name = name;
        this.authors = new HashSet<>();
    }

    public Book(String name, Set<Author> authors) {
        super();
        this.name = name;
        this.authors = authors;
    }

    @Override
    public String toString() {
        return "Book{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", authors=" + authors.size() +
                '}';
    }
}

package io.powerx;

import org.springframework.data.jpa.repository.JpaRepository;

import java.util.List;

public interface AuthorRepository extends JpaRepository<Author, Integer> {

    Author findByName(String name);

    List<Author> findByNameContaining(String name);

}

package io.powerx;

import org.springframework.data.jpa.repository.JpaRepository;

import java.util.List;

public interface BookRepository extends JpaRepository<Book, Integer> {

    Book findByName(String name);

    List<Book> findByNameContaining(String name);

}

　　在调试过程中，注意实体类的tostring方法的重写，避免相互引用；此外如果超过两张表的关联查询，建议使用自定义sql，建立相应的pojo来接收查询结果。