數據訪問層，所謂的CRUD是後端程式設計師的必修課程，Spring Data JPA 可以讓我們來簡化CRUD過程，本文由簡入深，從JPA的基本用法，到各種高級用法。
RepositorySpring Data JPA 可以用來簡化data access的實現，藉助JPA我們可以快速的實現一些簡單的查詢，分頁，排序不在話下。
public interface MovieRepository extends JpaRepository<Movie, Long> {
List<Movie> findByTitle(String title, Sort sort);
Page<Movie> findByYear(Int year, Pageable pageable);
}
JPA會根據方法命名，通過JPA 查詢生成器自動生成SQL，cool！
Criteria API但是，簡單並非萬能，有時候也需要面對一些複雜的查詢，不能享受JPA 查詢生成器帶來的便利。JPQ 提供了Criteria API 和
Criteria API 可以通過編程方式動態構建查詢，強類型檢查可以避免錯誤。核心原理就是構造一個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 可以很好的滿足一些複雜的查詢，但是他的問題在於不便於復用，因為你需要先構建CriteriaBuilder, CriteriaQuery, Root. 同時代碼可讀性也比較一般。
Specifications能不能定義可復用的Predicate呢？JPA 提供Specification接口來解決這個問題。
先來看這個接口定義：
public interface Specification<T> {
Predicate toPredicate(Root<T> root, CriteriaQuery query, CriteriaBuilder cb);
}
上文不是說需要先構建CriteriaBuilder, CriteriaQuery, Root嗎，那麼Specification接口就是給你提供這個三個參數，讓你自己構建Predicate，想什麼來什麼。
我們用Specifications來改寫代碼，先定義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));
};
}
}
然後改寫MovieRepository，為了讓Repository可以運行Specification，我們需要讓其繼承JpaSpecificationExecutor接口。
public interface MovieRepository extends JpaRepository<Movie, Long>, JpaSpecificationExecutor<Movie> {
// query methods here
}
然後我們就可以愉快的使用定義好的Specification了。
movieRepository.findAll(MovieSpecifications.isComedy());
movieRepository.findAll(MovieSpecifications.isReallyOld());
在這裡，repository 的代理類，會自動準備好CriteriaBuilder, CriteriaQuery, Root,是不是很爽？
從面向對象編程來講，MovieSpecifications並不是很優雅，你可以這樣做：
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);
}
聯合Specifications我們可以將多個predicates 合到一起使用，通過and，or來連接。
movieRepository.findAll(Specification.where(MovieSpecifications.isComedy())
.and(MovieSpecifications.isReallyOld()));
Specification 構造器產品定義的業務邏輯，有時候會很複雜，比如我們需要根據條件動態拼接查詢，我們可以定義一個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;
}
}
使用方法：
final MovieSpecificationsBuilder msb = new MovieSpecificationsBuilder();
// add SearchCriteria by invoking with()
final Specification<Movie> spec = msb.build();
movieRepository.findAll(spec);
QuerydslQuerydsl, 動態查詢語言，支持JPA。先引入：
<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會根據表結構，生成meta-model，需要引入APT插件
maven配置：
<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>
假設，我們有下面的Domain類：
@Entity
public class Customer {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
private String firstname;
private String lastname;
// … methods omitted
}
在這裡生成，會根據表結構生成查詢classes，比如QCustomer:
QCustomer customer = QCustomer.customer;
LocalDate today = new LocalDate();
BooleanExpression customerHasBirthday = customer.birthday.eq(today);
BooleanExpression isLongTermCustomer = customer.createdAt.lt(today.minusYears(2));
對比Specifications，這裡是BooleanExpression，基本上基於生成的代碼就可以構造了，更方便快捷。
現在我們到JPA使用,JPA 接口需要繼承QueryDslPredicateExecutor
public interface CustomerRepository extends JpaRepository<Customer>, QueryDslPredicateExecutor {
// Your query methods here
}
查詢代碼：
BooleanExpression customerHasBirthday = customer.birthday.eq(today);
BooleanExpression isLongTermCustomer = customer.createdAt.lt(today.minusYears(2));
customerRepository.findAll(customerHasBirthday.and(isLongTermCustomer));
同樣的，Queydsl 還有一些類似直接寫SQL的騷操作。
簡單如：
QCustomer customer = QCustomer.customer;
Customer bob = queryFactory.selectFrom(customer)
.where(customer.firstName.eq("Bob"))
.fetchOne();
多表查詢：QCustomer customer = QCustomer.customer;
QCompany company = QCompany.company;
query.from(customer, company);
多條件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")));
使用JOINQCat 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();
對應JPQL
inner join cat.mate as mate
left outer join cat.kittens as kitten
另外一個例子
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
OrderingQCustomer customer = QCustomer.customer;
queryFactory.selectFrom(customer)
.orderBy(customer.lastName.asc(), customer.firstName.desc())
.fetch();
GroupingqueryFactory.select(customer.lastName).from(customer)
.groupBy(customer.lastName)
.fetch();
子查詢QDepartment department = QDepartment.department;
QDepartment d = new QDepartment("d");
queryFactory.selectFrom(department)
.where(department.size.eq(
JPAExpressions.select(d.size.max()).from(d)))
.fetch();
小結本文簡單介紹了JPA的Repository，以及面向動態查詢的Querydsl和Specifications 的用法，使用JPA可以有效減少代碼編寫量，提升代碼易讀性和可維護性。