Explore spring data JPA from repository to specifications and querydsl

JadePeng 2020-11-10 12:04:34
explore spring data jpa repository


Data access layer , So-called CRUD It's a required course for back-end programmers ,Spring Data JPA Let's simplify CRUD The process , This paper goes from simple to deep , from JPA The basic usage of , To various advanced uses .

Repository

Spring Data JPA Can be used to simplify data access The implementation of the , With the help of JPA We can quickly implement some simple queries , Pagination , It's not a matter of order .

public interface MovieRepository extends JpaRepository<Movie, Long> {
List<Movie> findByTitle(String title, Sort sort);
Page<Movie> findByYear(Int year, Pageable pageable);
}

JPA It will be named according to the method , adopt JPA The query generator generates automatically SQL,cool!

Criteria API

however , Simplicity is not everything , Sometimes we have to face some complex queries , Cannot enjoy JPA The convenience of query builder .JPQ Provides Criteria API and

Criteria API Queries can be dynamically built programmatically , Strong type checking can avoid errors . The core principle is to construct a Predicate

LocalDate today = new LocalDate();
CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<Movie> query = builder.createQuery(Movie.class);
Root<Movie> root = query.from(Movie.class);
Predicate isComedy = builder.equal(root.get(Movie.genre), Genre.Comedy);
Predicate isReallyOld = builder.lessThan(root.get(Movie.createdAt), today.minusYears(25));
query.where(builder.and(isComedy, isReallyOld));
em.createQuery(query.select(root)).getResultList();

Predicate Can be very good to meet some complex queries , But his problem is that it's not easy to reuse , Because you need to build first CriteriaBuilder, CriteriaQuery, Root. At the same time, the readability of the code is relatively common .

Specifications

Can we define reusable Predicate Well ? JPA Provide Specification Interface to solve this problem .

Let's look at the interface definition first :

public interface Specification<T> {
Predicate toPredicate(Root<T> root, CriteriaQuery query, CriteriaBuilder cb);
}

This is not to say that you need to build CriteriaBuilder, CriteriaQuery, Root Do you , that Specification The interface is to provide you with these three parameters , Let yourself build Predicate, What do you want .

We use it Specifications To rewrite the code , First define Specification

public MovieSpecifications {
public static Specification<Movie> isComedy() {
return (root, query, cb) -> {
return cb.equal(root.get(Movie_.genre), Genre.Comedy);
};
}
public static Specification<Movie> isReallyOld() {
return (root, query, cb) -> {
return cb.lessThan(root.get(Movie_.createdAt), new LocalDate.now().minusYears(25));
};
}
}

And then rewrite MovieRepository , In order to make Repository Can run Specification , We need to let it inherit JpaSpecificationExecutor Interface .

public interface MovieRepository extends JpaRepository<Movie, Long>, JpaSpecificationExecutor<Movie> {
// query methods here
}

Then we can happily use the defined Specification 了 .

movieRepository.findAll(MovieSpecifications.isComedy());
movieRepository.findAll(MovieSpecifications.isReallyOld());

ad locum ,repository Proxy class , It'll be ready automatically CriteriaBuilder, CriteriaQuery, Root, Is it cool ?

In terms of object-oriented programming ,MovieSpecifications It's not very elegant , You can do that :

public MovieComedySpecification implements Specification<Movie> {
@Override
public Predicate toPredicate(Root<Movie> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
return cb.equal(root.get(Movie_.genre), Genre.Comedy);
}

union Specifications

We can have multiple predicates Put it together , adopt and,or To connect .

movieRepository.findAll(Specification.where(MovieSpecifications.isComedy())
.and(MovieSpecifications.isReallyOld()));

Specification Constructors

The business logic of product definition , Sometimes it's complicated , For example, we need to dynamically join queries according to conditions , We can define one SpecificationBuilder.

public enum SearchOperation {
EQUALITY, NEGATION, GREATER_THAN, LESS_THAN, LIKE;
public static final String[] SIMPLE_OPERATION_SET =
{ ":", "!", ">", "<", "~" };
public static SearchOperation getSimpleOperation(final char input)
{
switch (input) {
case ':': return EQUALITY;
case '!': return NEGATION;
case '>': return GREATER_THAN;
case '<': return LESS_THAN;
case '~': return LIKE;
default: return null;
}
}
}
public class SearchCriteria {
private String key;
private Object value;
private SearchOperation operation;
}
public final class MovieSpecificationsBuilder {
private final List<SearchCriteria> params;
public MovieSpecificationsBuilder() {
params = new ArrayList<>();
}
public Specification<Movie> build() {
// convert each of SearchCriteria params to Specification and construct combined specification based on custom rules
}
public final MovieSpecificationsBuilder with(final SearchCriteria criteria) {
params.add(criteria);
return this;
}
}

Usage method :

final MovieSpecificationsBuilder msb = new MovieSpecificationsBuilder();
// add SearchCriteria by invoking with()
final Specification<Movie> spec = msb.build();
movieRepository.findAll(spec);

Querydsl

Querydsl, Dynamic query language , Support JPA. First introduced :

<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>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-log4j12</artifactId>
<version>1.6.1</version>
</dependency>

Querydsl According to the table structure , Generate meta-model, Need to introduce APT plug-in unit

maven To configure :

<project>
<build>
<plugins>
...
<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>
...
</plugins>
</build>
</project>

hypothesis , We have the following Domain class :

@Entity
public class Customer {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
private String firstname;
private String lastname;
// … methods omitted
}

Generate... Here , The query is generated according to the table structure classes, such as QCustomer :

QCustomer customer = QCustomer.customer;
LocalDate today = new LocalDate();
BooleanExpression customerHasBirthday = customer.birthday.eq(today);
BooleanExpression isLongTermCustomer = customer.createdAt.lt(today.minusYears(2));

contrast Specifications, Here is BooleanExpression, Basically based on the generated code, you can construct , More convenient and quick .

Now let's go to JPA Use ,JPA Interface needs to inherit QueryDslPredicateExecutor

public interface CustomerRepository extends JpaRepository<Customer>, QueryDslPredicateExecutor {
// Your query methods here
}

Query code :

BooleanExpression customerHasBirthday = customer.birthday.eq(today);
BooleanExpression isLongTermCustomer = customer.createdAt.lt(today.minusYears(2));
customerRepository.findAll(customerHasBirthday.and(isLongTermCustomer));

alike ,Queydsl There are some things like writing directly SQL Operation .

As simple as :

QCustomer customer = QCustomer.customer;
Customer bob = queryFactory.selectFrom(customer)
.where(customer.firstName.eq("Bob"))
.fetchOne();

Multi-table query :

QCustomer customer = QCustomer.customer;
QCompany company = QCompany.company;
query.from(customer, company);

Multiple conditions

queryFactory.selectFrom(customer)
.where(customer.firstName.eq("Bob"), customer.lastName.eq("Wilson"));
queryFactory.selectFrom(customer)
.where(customer.firstName.eq("Bob").and(customer.lastName.eq("Wilson")));

Use JOIN

QCat cat = QCat.cat;
QCat mate = new QCat("mate");
QCat kitten = new QCat("kitten");
queryFactory.selectFrom(cat)
.innerJoin(cat.mate, mate)
.leftJoin(cat.kittens, kitten)
.fetch();

Corresponding JPQL

inner join cat.mate as mate
left outer join cat.kittens as kitten

Another example

queryFactory.selectFrom(cat)
.leftJoin(cat.kittens, kitten)
.on(kitten.bodyWeight.gt(10.0))
.fetch();

JPQL version

select cat from Cat as cat
left join cat.kittens as kitten
on kitten.bodyWeight > 10.0

Ordering

QCustomer customer = QCustomer.customer;
queryFactory.selectFrom(customer)
.orderBy(customer.lastName.asc(), customer.firstName.desc())
.fetch();

Grouping

queryFactory.select(customer.lastName).from(customer)
.groupBy(customer.lastName)
.fetch();

Subquery

QDepartment department = QDepartment.department;
QDepartment d = new QDepartment("d");
queryFactory.selectFrom(department)
.where(department.size.eq(
JPAExpressions.select(d.size.max()).from(d)))
.fetch();

Summary

This article briefly introduces JPA Of Repository, And dynamic query oriented Querydsl and Specifications Usage of , Use JPA Can effectively reduce the amount of code written , Improve code readability and maintainability .

Reference resources


author :Jadepeng
Source :jqpeng Technical notepad --http://www.cnblogs.com/xiaoqi
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