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 .


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));

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 .


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 .


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> {
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 .


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 {
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) {
return this;

Usage method :

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


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


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

maven To configure :


hypothesis , We have the following Domain class :

public class Customer {
@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));

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

As simple as :

QCustomer customer = QCustomer.customer;
Customer bob = queryFactory.selectFrom(customer)

Multi-table query :

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

Multiple conditions

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


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

Corresponding JPQL

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

Another example

.leftJoin(cat.kittens, kitten)

JPQL version

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


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




QDepartment department = QDepartment.department;
QDepartment d = new QDepartment("d");


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
Your support is a great encouragement to bloggers , Thank you for your careful reading .
The copyright of this article belongs to the author , Welcome to reprint , However, this statement must be retained without the consent of the author , And in the article page obvious position gives the original link , Otherwise, the right to pursue legal responsibility is reserved .


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