Querydsl学习

Querydsl是jpa的扩展

（一）引入Querydsl的相关依赖和插件

        <dependency>
            <groupId>com.querydsl</groupId>
            <artifactId>querydsl-apt</artifactId>
            <version>${querydsl.version}</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>com.querydsl</groupId>
            <artifactId>querydsl-jpa</artifactId>
            <version>${querydsl.version}</version>
        </dependency>   

        <plugin>
                <groupId>com.mysema.maven</groupId>
                <artifactId>apt-maven-plugin</artifactId>
                <version>1.1.3</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>process</goal>
                        </goals>
                        <configuration>
                            <outputDirectory>target/generated-sources/java</outputDirectory>
                            <processor>com.querydsl.apt.jpa.JPAAnnotationProcessor</processor>
                        </configuration>
                    </execution>
                </executions>
            </plugin>   

（二）定义enetity（@Data在文末有说明）

@Entity
@Table(name = "school")
@Data
@ToString
public class School {

    @Id
    private Long id;

    private String name;

    private Integer age;

    private String title;
}

（三）定义Repository

public interface SchoolRepository extends QueryDslPredicateExecutor<School> {

}

（四）查询

我们先用spring data jpa官网给的使用方法

首先让我们自己的Repository继承QueryDslPredicateExecutor

public interface AuthorDslRepository extends JpaRepository<Author,Long>,QueryDslPredicateExecutor<Author>{

}

看下QueryDslPredicateExecutor接口，我们会看到这么几个定义好的类。

public interface QueryDslPredicateExecutor<T> {
    T findOne(Predicate var1);

    Iterable<T> findAll(Predicate var1);

    Iterable<T> findAll(Predicate var1, Sort var2);

    Iterable<T> findAll(Predicate var1, OrderSpecifier... var2);

    Iterable<T> findAll(OrderSpecifier... var1);

    Page<T> findAll(Predicate var1, Pageable var2);

    long count(Predicate var1);

    boolean exists(Predicate var1);
}

查询是通过JPAQuery实例来实现的。

下面看个使用案例

@RequestMapping("/dslTest")
    public String dslTest(){
        QSchool qSchool = QSchool.school;
        JPAQuery jpaQuery = new JPAQuery(entityManager);
        return jpaQuery.select(qSchool).from(qSchool).fetch().toString();
    }

不过querydsl官方推荐JPAQuery的实例的获取方式最好是通过JPAQueryFactory的方式获取（JPAQueryFactory should be the preferred option to obtain JPAQuery instances.）

下面看下使用JPAQueryFactory的案例

@RequestMapping("/factoryTest")
    public String factoryTest(){
        QSchool qSchool = QSchool.school;
        JPAQueryFactory queryFactory = new JPAQueryFactory(entityManager);
        return queryFactory.selectFrom(qSchool).fetch().toString();
    }

这边我们用的是selectFrom，其实就是上面的select().from()。

我们看下JPAQueryFactory的selectFrom()方法就明白了。

public <T> JPAQuery<T> selectFrom(EntityPath<T> from) {
        return (JPAQuery)this.select((Expression)from).from(from);
    }

加入查询条件

@RequestMapping("/whereTest")
    public String whereTest(){
        QSchool qSchool = QSchool.school;
        JPAQueryFactory queryFactory = new JPAQueryFactory(entityManager);
        List<School> schools = queryFactory.selectFrom(qSchool).where(qSchool.age.eq(74).or(qSchool.title.eq("sfd"))).fetch();
        List<School> schoolList = queryFactory.selectFrom(qSchool).where(qSchool.age.eq(74).and(qSchool.id.eq(1l))).fetch();
        List<School> list = new ArrayList<>();
        list.addAll(schoolList);
        list.addAll(schools);
        return list.toString();
    }

关于其他的一些的用法可以直接看官网给的这段描述

2.1.8. General usage

Use the the cascading methods of the JPQLQuery interface like this

select: Set the projection of the query. (Not necessary if created via query factory)

from: Add the query sources here.

innerJoin, join, leftJoin, rightJoin, on: Add join elements using these constructs. For the join methods the first argument is the join source and the second the target (alias).

where: Add query filters, either in varargs form separated via commas or cascaded via the and-operator.

groupBy: Add group by arguments in varargs form.

having: Add having filters of the "group by" grouping as an varags array of Predicate expressions.

orderBy: Add ordering of the result as an varargs array of order expressions. Use asc() and desc() on numeric, string and other comparable expression to access the OrderSpecifier instances.

limit, offset, restrict: Set the paging of the result. Limit for max results, offset for skipping rows and restrict for defining both in one call.

关于@Data是使用了lombok，关于lombok，下面会写一篇简单介绍下使用方法。