Spring Data JPA

邵飞翔 2017-03-08

展开全文

This chapter will point out the specialties for repository support for JPA. This builds on the core repository support explained in Working with Spring Data Repositories. So make sure you’ve got a sound understanding of the basic concepts explained there.

5.1. Introduction

5.1.1. Spring namespace

The JPA module of Spring Data contains a custom namespace that allows defining repository beans. It also contains certain features and element attributes that are special to JPA. Generally the JPA repositories can be set up using the repositories element:

Example 41. Setting up JPA repositories using the namespace

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www./schema/beans"
  xmlns:xsi="http://www./2001/XMLSchema-instance"
  xmlns:jpa="http://www./schema/data/jpa"
  xsi:schemaLocation="http://www./schema/beans
    http://www./schema/beans/spring-beans.xsd
    http://www./schema/data/jpa
    http://www./schema/data/jpa/spring-jpa.xsd">

  <jpa:repositories base-package="com.acme.repositories" />

</beans>

Using this element looks up Spring Data repositories as described in Creating repository instances. Beyond that it activates persistence exception translation for all beans annotated with @Repository to let exceptions being thrown by the JPA persistence providers be converted into Spring’s DataAccessException hierarchy.

Custom namespace attributes

Beyond the default attributes of the repositories element the JPA namespace offers additional attributes to gain more detailed control over the setup of the repositories:

Table 2. Custom JPA-specific attributes of the repositories element
`entity-manager-factory-ref`	Explicitly wire the `EntityManagerFactory` to be used with the repositories being detected by the `repositories` element. Usually used if multiple `EntityManagerFactory` beans are used within the application. If not configured we will automatically lookup the `EntityManagerFactory` bean with the name `entityManagerFactory` in the `ApplicationContext`.
`transaction-manager-ref`	Explicitly wire the `PlatformTransactionManager` to be used with the repositories being detected by the `repositories` element. Usually only necessary if multiple transaction managers and/or `EntityManagerFactory` beans have been configured. Default to a single defined `PlatformTransactionManager` inside the current `ApplicationContext`.

Note that we require a PlatformTransactionManager bean named transactionManager to be present if no explicit transaction-manager-ref is defined.

5.1.2. Annotation based configuration

The Spring Data JPA repositories support cannot only be activated through an XML namespace but also using an annotation through JavaConfig.

Example 42. Spring Data JPA repositories using JavaConfig

@Configuration
@EnableJpaRepositories
@EnableTransactionManagement
class ApplicationConfig {

  @Bean
  public DataSource dataSource() {

    EmbeddedDatabaseBuilder builder = new EmbeddedDatabaseBuilder();
    return builder.setType(EmbeddedDatabaseType.HSQL).build();
  }

  @Bean
  public EntityManagerFactory entityManagerFactory() {

    HibernateJpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();
    vendorAdapter.setGenerateDdl(true);

    LocalContainerEntityManagerFactoryBean factory = new LocalContainerEntityManagerFactoryBean();
    factory.setJpaVendorAdapter(vendorAdapter);
    factory.setPackagesToScan("com.acme.domain");
    factory.setDataSource(dataSource());
    factory.afterPropertiesSet();

    return factory.getObject();
  }

  @Bean
  public PlatformTransactionManager transactionManager() {

    JpaTransactionManager txManager = new JpaTransactionManager();
    txManager.setEntityManagerFactory(entityManagerFactory());
    return txManager;
  }
}

The just shown configuration class sets up an embedded HSQL database using the EmbeddedDatabaseBuilder API of spring-jdbc. We then set up a EntityManagerFactory and use Hibernate as sample persistence provider. The last infrastructure component declared here is the JpaTransactionManager. We finally activate Spring Data JPA repositories using the @EnableJpaRepositories annotation which essentially carries the same attributes as the XML namespace does. If no base package is configured it will use the one the configuration class resides in.

5.2. Persisting entities

5.2.1. Saving entities

Saving an entity can be performed via the CrudRepository.save(…)-Method. It will persist or merge the given entity using the underlying JPA EntityManager. If the entity has not been persisted yet Spring Data JPA will save the entity via a call to the entityManager.persist(…) method, otherwise the entityManager.merge(…) method will be called.

Entity state detection strategies

Spring Data JPA offers the following strategies to detect whether an entity is new or not:

Table 3. Options for detection whether an entity is new in Spring Data JPA
Id-Property inspection (default)	By default Spring Data JPA inspects the identifier property of the given entity. If the identifier property is `null`, then the entity will be assumed as new, otherwise as not new.
Implementing `Persistable`	If an entity implements `Persistable`, Spring Data JPA will delegate the new detection to the `isNew(…)` method of the entity. See the JavaDoc for details.
Implementing `EntityInformation`	You can customize the `EntityInformation` abstraction used in the `SimpleJpaRepository` implementation by creating a subclass of `JpaRepositoryFactory` and overriding the `getEntityInformation(…)` method accordingly. You then have to register the custom implementation of `JpaRepositoryFactory` as a Spring bean. Note that this should be rarely necessary. See the JavaDoc for details.

5.3. Query methods

5.3.1. Query lookup strategies

The JPA module supports defining a query manually as String or have it being derived from the method name.

Declared queries

Although getting a query derived from the method name is quite convenient, one might face the situation in which either the method name parser does not support the keyword one wants to use or the method name would get unnecessarily ugly. So you can either use JPA named queries through a naming convention (see Using JPA NamedQueries for more information) or rather annotate your query method with @Query (see Using @Query for details).

5.3.2. Query creation

Generally the query creation mechanism for JPA works as described in Query methods. Here’s a short example of what a JPA query method translates into:

Example 43. Query creation from method names

public interface UserRepository extends Repository<User, Long> {

  List<User> findByEmailAddressAndLastname(String emailAddress, String lastname);
}

We will create a query using the JPA criteria API from this but essentially this translates into the following query: select u from User u where u.emailAddress = ?1 and u.lastname = ?2. Spring Data JPA will do a property check and traverse nested properties as described in Property expressions. Here’s an overview of the keywords supported for JPA and what a method containing that keyword essentially translates to.

Table 4. Supported keywords inside method names
Keyword	Sample	JPQL snippet
`And`	`findByLastnameAndFirstname`	`… where x.lastname = ?1 and x.firstname = ?2`
`Or`	`findByLastnameOrFirstname`	`… where x.lastname = ?1 or x.firstname = ?2`
`Is,Equals`	`findByFirstname`,`findByFirstnameIs`,`findByFirstnameEquals`	`… where x.firstname = ?1`
`Between`	`findByStartDateBetween`	`… where x.startDate between ?1 and ?2`
`LessThan`	`findByAgeLessThan`	`… where x.age < ?1`
`LessThanEqual`	`findByAgeLessThanEqual`	`… where x.age ? ?1`
`GreaterThan`	`findByAgeGreaterThan`	`… where x.age > ?1`
`GreaterThanEqual`	`findByAgeGreaterThanEqual`	`… where x.age >= ?1`
`After`	`findByStartDateAfter`	`… where x.startDate > ?1`
`Before`	`findByStartDateBefore`	`… where x.startDate < ?1`
`IsNull`	`findByAgeIsNull`	`… where x.age is null`
`IsNotNull,NotNull`	`findByAge(Is)NotNull`	`… where x.age not null`
`Like`	`findByFirstnameLike`	`… where x.firstname like ?1`
`NotLike`	`findByFirstnameNotLike`	`… where x.firstname not like ?1`
`StartingWith`	`findByFirstnameStartingWith`	`… where x.firstname like ?1` (parameter bound with appended `%`)
`EndingWith`	`findByFirstnameEndingWith`	`… where x.firstname like ?1` (parameter bound with prepended `%`)
`Containing`	`findByFirstnameContaining`	`… where x.firstname like ?1` (parameter bound wrapped in `%`)
`OrderBy`	`findByAgeOrderByLastnameDesc`	`… where x.age = ?1 order by x.lastname desc`
`Not`	`findByLastnameNot`	`… where x.lastname <> ?1`
`In`	`findByAgeIn(Collection<Age> ages)`	`… where x.age in ?1`
`NotIn`	`findByAgeNotIn(Collection<Age> age)`	`… where x.age not in ?1`
`True`	`findByActiveTrue()`	`… where x.active = true`
`False`	`findByActiveFalse()`	`… where x.active = false`
`IgnoreCase`	`findByFirstnameIgnoreCase`	`… where UPPER(x.firstame) = UPPER(?1)`

In and NotIn also take any subclass of Collection as parameter as well as arrays or varargs. For other syntactical versions of the very same logical operator check Repository query keywords.

5.3.3. Using JPA NamedQueries

The examples use simple <named-query /> element and @NamedQuery annotation. The queries for these configuration elements have to be defined in JPA query language. Of course you can use <named-native-query /> or @NamedNativeQuery too. These elements allow you to define the query in native SQL by losing the database platform independence.

XML named query definition

To use XML configuration simply add the necessary <named-query /> element to the orm.xml JPA configuration file located in META-INF folder of your classpath. Automatic invocation of named queries is enabled by using some defined naming convention. For more details see below.

Example 44. XML named query configuration

<named-query name="User.findByLastname">
  <query>select u from User u where u.lastname = ?1</query>
</named-query>

As you can see the query has a special name which will be used to resolve it at runtime.

Annotation configuration

Annotation configuration has the advantage of not needing another configuration file to be edited, probably lowering maintenance costs. You pay for that benefit by the need to recompile your domain class for every new query declaration.

Example 45. Annotation based named query configuration

@Entity
@NamedQuery(name = "User.findByEmailAddress",
  query = "select u from User u where u.emailAddress = ?1")
public class User {

}

Declaring interfaces

To allow execution of these named queries all you need to do is to specify the UserRepository as follows:

Example 46. Query method declaration in UserRepository

public interface UserRepository extends JpaRepository<User, Long> {

  List<User> findByLastname(String lastname);

  User findByEmailAddress(String emailAddress);
}

Spring Data will try to resolve a call to these methods to a named query, starting with the simple name of the configured domain class, followed by the method name separated by a dot. So the example here would use the named queries defined above instead of trying to create a query from the method name.

5.3.4. Using @Query

Using named queries to declare queries for entities is a valid approach and works fine for a small number of queries. As the queries themselves are tied to the Java method that executes them you actually can bind them directly using the Spring Data JPA @Query annotation rather than annotating them to the domain class. This will free the domain class from persistence specific information and co-locate the query to the repository interface.

Queries annotated to the query method will take precedence over queries defined using @NamedQuery or named queries declared in orm.xml.

Example 47. Declare query at the query method using @Query

public interface UserRepository extends JpaRepository<User, Long> {

  @Query("select u from User u where u.emailAddress = ?1")
  User findByEmailAddress(String emailAddress);
}

Using advanced LIKE expressionsThe query execution mechanism for manually defined queries using @Query allow the definition of advanced LIKE expressions inside the query definition.

Example 48. Advanced like-expressions in @Query

public interface UserRepository extends JpaRepository<User, Long> {

  @Query("select u from User u where u.firstname like %?1")
  List<User> findByFirstnameEndsWith(String firstname);
}

In the just shown sample LIKE delimiter character % is recognized and the query transformed into a valid JPQL query (removing the %). Upon query execution the parameter handed into the method call gets augmented with the previously recognized LIKE pattern.

Native queriesThe @Query annotation allows to execute native queries by setting the nativeQuery flag to true. Note, that we currently don’t support execution of pagination or dynamic sorting for native queries as we’d have to manipulate the actual query declared and we cannot do this reliably for native SQL.

Example 49. Declare a native query at the query method using @Query

public interface UserRepository extends JpaRepository<User, Long> {

  @Query(value = "SELECT * FROM USERS WHERE EMAIL_ADDRESS = ?1", nativeQuery = true)
  User findByEmailAddress(String emailAddress);
}

5.3.5. Using named parameters

By default Spring Data JPA will use position based parameter binding as described in all the samples above. This makes query methods a little error prone to refactoring regarding the parameter position. To solve this issue you can use @Param annotation to give a method parameter a concrete name and bind the name in the query.

Example 50. Using named parameters

public interface UserRepository extends JpaRepository<User, Long> {

  @Query("select u from User u where u.firstname = :firstname or u.lastname = :lastname")
  User findByLastnameOrFirstname(@Param("lastname") String lastname,
                                 @Param("firstname") String firstname);
}

Note that the method parameters are switched according to the occurrence in the query defined.

5.3.6. Using SpEL expressions

As of Spring Data JPA release 1.4 we support the usage of restricted SpEL template expressions in manually defined queries via @Query. Upon query execution these expressions are evaluated against a predefined set of variables. We support the following list of variables to be used in a manual query.

Table 5. Supported variables inside SpEL based query templates
Variable	Usage	Description
`entityName`	`select x from #{#entityName} x`	Inserts the `entityName` of the domain type associated with the given Repository. The `entityName` is resolved as follows: If the domain type has set the name property on the `@Entity` annotation then it will be used. Otherwise the simple class-name of the domain type will be used.

The following example demonstrates one use case for the #{#entityName} expression in a query string where you want to define a repository interface with a query method with a manually defined query. In order not to have to state the actual entity name in the query string of a @Query annotation one can use the #{#entityName} Variable.

The entityName can be customized via the @Entity annotation. Customizations via orm.xml are not supported for the SpEL expressions.

Example 51. Using SpEL expressions in repository query methods - entityName

@Entity
public class User {

  @Id
  @GeneratedValue
  Long id;

  String lastname;
}

public interface UserRepository extends JpaRepository<User,Long> {

  @Query("select u from #{#entityName} u where u.lastname = ?1")
  List<User> findByLastname(String lastname);
}

Of course you could have just used User in the query declaration directly but that would require you to change the query as well. The reference to #entityName will pick up potential future remappings of the User class to a different entity name (e.g. by using @Entity(name = "MyUser").

Another use case for the #{#entityName} expression in a query string is if you want to define a generic repository interface with specialized repository interfaces for a concrete domain type. In order not to have to repeat the definition of custom query methods on the concrete interfaces you can use the entity name expression in the query string of the @Query annotation in the generic repository interface.

Example 52. Using SpEL expressions in repository query methods - entityName with inheritance

@MappedSuperclass
public abstract class AbstractMappedType {
  …
  String attribute
}

@Entity
public class ConcreteType extends AbstractMappedType { … }

@NoRepositoryBean
public interface MappedTypeRepository<T extends AbstractMappedType>
  extends Repository<T, Long> {

  @Query("select t from #{#entityName} t where t.attribute = ?1")
  List<T> findAllByAttribute(String attribute);
}

public interface ConcreteRepository
  extends MappedTypeRepository<ConcreteType> { … }

In the example the interface MappedTypeRepository is the common parent interface for a few domain types extending AbstractMappedType. It also defines the generic method findAllByAttribute(…) which can be used on instances of the specialized repository interfaces. If you now invoke findByAllAttribute(…) on ConcreteRepository the query being executed will be select t from ConcreteType t where t.attribute = ?1.

5.3.7. Modifying queries

All the sections above describe how to declare queries to access a given entity or collection of entities. Of course you can add custom modifying behaviour by using facilities described in Custom implementations for Spring Data repositories. As this approach is feasible for comprehensive custom functionality, you can achieve the execution of modifying queries that actually only need parameter binding by annotating the query method with @Modifying:

Example 53. Declaring manipulating queries

@Modifying
@Query("update User u set u.firstname = ?1 where u.lastname = ?2")
int setFixedFirstnameFor(String firstname, String lastname);

This will trigger the query annotated to the method as updating query instead of a selecting one. As the EntityManager might contain outdated entities after the execution of the modifying query, we do not automatically clear it (see JavaDoc of EntityManager.clear() for details) since this will effectively drop all non-flushed changes still pending in the EntityManager. If you wish the EntityManager to be cleared automatically you can set @Modifying annotation’s clearAutomatically attribute to true.

5.3.8. Applying query hints

To apply JPA query hints to the queries declared in your repository interface you can use the @QueryHints annotation. It takes an array of JPA @QueryHint annotations plus a boolean flag to potentially disable the hints applied to the addtional count query triggered when applying pagination.

Example 54. Using QueryHints with a repository method

public interface UserRepository extends Repository<User, Long> {

  @QueryHints(value = { @QueryHint(name = "name", value = "value")},
              forCounting = false)
  Page<User> findByLastname(String lastname, Pageable pageable);
}

The just shown declaration would apply the configured @QueryHint for that actually query but omit applying it to the count query triggered to calculate the total number of pages.

5.3.9. Configuring Fetch- and LoadGraphs

The JPA 2.1 specification introduced support for specifiying Fetch- and LoadGraphs that we also support via the @EntityGraph annotation which allows to reference a @NamedEntityGraph definition, that can be annotated on an entity, to be used to configure the fetch plan of the resulting query. The type (Fetch / Load) of the fetching can be configured via the type attribute on the @EntityGraph annotation. Please have a look at the JPA 2.1 Spec 3.7.4 for further reference.

Example 55. Defining a named entity graph on an entity.

@Entity
@NamedEntityGraph(name = "GroupInfo.detail",
  attributeNodes = @NamedAttributeNode("members"))
public class GroupInfo {

  // default fetch mode is lazy.
  @ManyToMany
  List<GroupMember> members = new ArrayList<GroupMember>();

  …
}

Example 56. Referencing a named entity graph definition on an repository query method.

@Repository
public interface GroupRepository extends CrudRepository<GroupInfo, String> {

  @EntityGraph(value = "GroupInfo.detail", type = EntityGraphType.LOAD)
  GroupInfo getByGroupName(String name);

}

It is also possible to define ad-hoc entity graphs via @EntityGraph. The provided attributePaths will be translated into the according EntityGraph without the need of having to explicitly add @NamedEntityGraph to your domain types.

Example 57. Using AD-HOC entity graph definition on an repository query method.

@Repository
public interface GroupRepository extends CrudRepository<GroupInfo, String> {

  @EntityGraph(attributePaths = { "members" })
  GroupInfo getByGroupName(String name);

}

5.3.10. Projections

Spring Data Repositories usually return the domain model when using query methods. However, sometimes, you may need to alter the view of that model for various reasons. In this section, you will learn how to define projections to serve up simplified and reduced views of resources.

Look at the following domain model:

@Entity
public class Person {

  @Id @GeneratedValue
  private Long id;
  private String firstName, lastName;

  @OneToOne
  private Address address;
  …
}

@Entity
public class Address {

  @Id @GeneratedValue
  private Long id;
  private String street, state, country;

  …
}

This Person has several attributes:

id is the primary key
firstName and lastName are data attributes
address is a link to another domain object

Now assume we create a corresponding repository as follows:

interface PersonRepository extends CrudRepository<Person, Long> {

  Person findPersonByFirstName(String firstName);
}

Spring Data will return the domain object including all of its attributes. There are two options just to retrieve the address attribute. One option is to define a repository for Address objects like this:

interface AddressRepository extends CrudRepository<Address, Long> {}

In this situation, using PersonRepository will still return the whole Person object. Using AddressRepository will return just the Address.

However, what if you do not want to expose address details at all? You can offer the consumer of your repository service an alternative by defining one or more projections.

Example 58. Simple Projection

interface NoAddresses {  

  String getFirstName(); 

  String getLastName();  
}

This projection has the following details:

	A plain Java interface making it declarative.
2	Export the `firstName`.
3	Export the `lastName`.

The NoAddresses projection only has getters for firstName and lastName meaning that it will not serve up any address information. The query method definition returns in this case NoAdresses instead of Person.

interface PersonRepository extends CrudRepository<Person, Long> {

  NoAddresses findByFirstName(String firstName);
}

Projections declare a contract between the underlying type and the method signatures related to the exposed properties. Hence it is required to name getter methods according to the property name of the underlying type. If the underlying property is named firstName, then the getter method must be named getFirstName otherwise Spring Data is not able to look up the source property. This type of projection is also called closed projection. Closed projections expose a subset of properties hence they can be used to optimize the query in a way to reduce the selected fields from the data store. The other type is, as you might imagine, an open projection.

Remodelling data

So far, you have seen how projections can be used to reduce the information that is presented to the user. Projections can be used to adjust the exposed data model. You can add virtual properties to your projection. Look at the following projection interface:

Example 59. Renaming a property

interface RenamedProperty {    

  String getFirstName();       

  @Value("#{target.lastName}")
  String getName();            
}

This projection has the following details:

	A plain Java interface making it declarative.
2	Export the `firstName`.
3	Export the `name` property. Since this property is virtual it requires `@Value("#{target.lastName}")` to specify the property source.

The backing domain model does not have this property so we need to tell Spring Data from where this property is obtained. Virtual properties are the place where @Value comes into play. The name getter is annotated with @Value to use SpEL expressions pointing to the backing property lastName. You may have noticed lastName is prefixed with target which is the variable name pointing to the backing object. Using @Value on methods allows defining where and how the value is obtained.

Some applications require the full name of a person. Concatenating strings with String.format("%s %s", person.getFirstName(), person.getLastName()) would be one possibility but this piece of code needs to be called in every place the full name is required. Virtual properties on projections leverage the need for repeating that code all over.

interface FullNameAndCountry {

  @Value("#{target.firstName} #{target.lastName}")
  String getFullName();

  @Value("#{target.address.country}")
  String getCountry();
}

In fact, @Value gives full access to the target object and its nested properties. SpEL expressions are extremly powerful as the definition is always applied to the projection method. Let’s take SpEL expressions in projections to the next level.

Imagine you had the following domain model definition:

@Entity
public class User {

  @Id @GeneratedValue
  private Long id;
  private String name;

  private String password;
  …
}

This example may seem a bit contrived, but it is possible with a richer domain model and many projections, to accidentally leak such details. Since Spring Data cannot discern the sensitivity of such data, it is up to the developers to avoid such situations. Storing a password as plain-text is discouraged. You really should not do this. For this example, you could also replace password with anything else that is secret.

In some cases, you might keep the password as secret as possible and not expose it more than it should be. The solution is to create a projection using @Value together with a SpEL expression.

interface PasswordProjection {
  @Value("#{(target.password == null || target.password.empty) ? null : '******'}")
  String getPassword();
}

The expression checks whether the password is null or empty and returns null in this case, otherwise six asterisks to indicate a password was set.

5.4. Stored procedures

The JPA 2.1 specification introduced support for calling stored procedures via the JPA criteria query API. We Introduced the @Procedure annotation for declaring stored procedure metadata on a repository method.

Example 60. The definition of the pus1inout procedure in HSQL DB.

/;
DROP procedure IF EXISTS plus1inout
/;
CREATE procedure plus1inout (IN arg int, OUT res int)
BEGIN ATOMIC
 set res = arg + 1;
END
/;

Metadata for stored procedures can be configured via the NamedStoredProcedureQuery annotation on an entity type.

Example 61. StoredProcedure metadata definitions on an entity.

@Entity
@NamedStoredProcedureQuery(name = "User.plus1", procedureName = "plus1inout", parameters = {
  @StoredProcedureParameter(mode = ParameterMode.IN, name = "arg", type = Integer.class),
  @StoredProcedureParameter(mode = ParameterMode.OUT, name = "res", type = Integer.class) })
public class User {}

Stored procedures can be referenced from a repository method in multiple ways. The stored procedure to be called can either be defined directly via the value or procedureName attribute of the @Procedure annotation or indirectly via the name attribute. If no name is configured the name of the repository method is used as a fallback.

Example 62. Referencing explicitly mapped procedure with name "plus1inout" in database.

@Procedure("plus1inout")
Integer explicitlyNamedPlus1inout(Integer arg);

Example 63. Referencing implicitly mapped procedure with name "plus1inout" in database via procedureName alias.

@Procedure(procedureName = "plus1inout")
Integer plus1inout(Integer arg);

Example 64. Referencing explicitly mapped named stored procedure "User.plus1IO" in EntityManager.

@Procedure(name = "User.plus1IO")
Integer entityAnnotatedCustomNamedProcedurePlus1IO(@Param("arg") Integer arg);

Example 65. Referencing implicitly mapped named stored procedure "User.plus1" in EntityManager via method-name.

@Procedure
Integer plus1(@Param("arg") Integer arg);

5.5. Specifications

JPA 2 introduces a criteria API that can be used to build queries programmatically. Writing a criteria you actually define the where-clause of a query for a domain class. Taking another step back these criteria can be regarded as predicate over the entity that is described by the JPA criteria API constraints.

Spring Data JPA takes the concept of a specification from Eric Evans' book "Domain Driven Design", following the same semantics and providing an API to define such specifications using the JPA criteria API. To support specifications you can extend your repository interface with the JpaSpecificationExecutor interface:

public interface CustomerRepository extends CrudRepository<Customer, Long>, JpaSpecificationExecutor {
 …
}

The additional interface carries methods that allow you to execute specifications in a variety of ways. For example, the findAll method will return all entities that match the specification:

List<T> findAll(Specification<T> spec);

The Specification interface is defined as follows:

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

Okay, so what is the typical use case? Specifications can easily be used to build an extensible set of predicates on top of an entity that then can be combined and used with JpaRepository without the need to declare a query (method) for every needed combination. Here’s an example:

Example 66. Specifications for a Customer

public class CustomerSpecs {

  public static Specification<Customer> isLongTermCustomer() {
    return new Specification<Customer>() {
      public Predicate toPredicate(Root<Customer> root, CriteriaQuery<?> query,
            CriteriaBuilder builder) {

         LocalDate date = new LocalDate().minusYears(2);
         return builder.lessThan(root.get(_Customer.createdAt), date);
      }
    };
  }

  public static Specification<Customer> hasSalesOfMoreThan(MontaryAmount value) {
    return new Specification<Customer>() {
      public Predicate toPredicate(Root<T> root, CriteriaQuery<?> query,
            CriteriaBuilder builder) {

         // build query here
      }
    };
  }
}

Admittedly the amount of boilerplate leaves room for improvement (that will hopefully be reduced by Java 8 closures) but the client side becomes much nicer as you will see below. The _Customer type is a metamodel type generated using the JPA Metamodel generator (see the Hibernate implementation’s documentation for example). So the expression _Customer.createdAt is asuming the Customer having a createdAt attribute of type Date. Besides that we have expressed some criteria on a business requirement abstraction level and created executable Specifications. So a client might use a Specification as follows:

Example 67. Using a simple Specification

List<Customer> customers = customerRepository.findAll(isLongTermCustomer());

Okay, why not simply create a query for this kind of data access? You’re right. Using a single Specification does not gain a lot of benefit over a plain query declaration. The power of specifications really shines when you combine them to create new Specification objects. You can achieve this through the Specifications helper class we provide to build expressions like this:

Example 68. Combined Specifications

MonetaryAmount amount = new MonetaryAmount(200.0, Currencies.DOLLAR);
List<Customer> customers = customerRepository.findAll(
  where(isLongTermCustomer()).or(hasSalesOfMoreThan(amount)));

As you can see, Specifications offers some glue-code methods to chain and combine Specification instances. Thus extending your data access layer is just a matter of creating new Specification implementations and combining them with ones already existing.

5.6. Query by Example

5.6.1. Introduction

This chapter will give you an introduction to Query by Example and explain how to use Examples.

Query by Example (QBE) is a user-friendly querying technique with a simple interface. It allows dynamic query creation and does not require to write queries containing field names. In fact, Query by Example does not require to write queries using store-specific query languages at all.

5.6.2. Usage

The Query by Example API consists of three parts:

Probe: That is the actual example of a domain object with populated fields.
ExampleMatcher: The ExampleMatcher carries details on how to match particular fields. It can be reused across multiple Examples.
Example: An Example consists of the probe and the ExampleMatcher. It is used to create the query.

Query by Example is suited for several use-cases but also comes with limitations:

When to use

Querying your data store with a set of static or dynamic constraints
Frequent refactoring of the domain objects without worrying about breaking existing queries
Works independently from the underlying data store API

Limitations

Query predicates are combined using the AND keyword
No support for nested/grouped property constraints like firstname = ?0 or (firstname = ?1 and lastname = ?2)
Only supports starts/contains/ends/regex matching for strings and exact matching for other property types

Before getting started with Query by Example, you need to have a domain object. To get started, simply create an interface for your repository:

Example 69. Sample Person object

public class Person {

  @Id
  private String id;
  private String firstname;
  private String lastname;
  private Address address;

  // … getters and setters omitted
}

This is a simple domain object. You can use it to create an Example. By default, fields having null values are ignored, and strings are matched using the store specific defaults. Examples can be built by either using the of factory method or by using ExampleMatcher. Example is immutable.

Example 70. Simple Example

Person person = new Person();                         
person.setFirstname("Dave");                          

Example<Person> example = Example.of(person);

	Create a new instance of the domain object
2	Set the properties to query
3	Create the `Example`

Examples are ideally be executed with repositories. To do so, let your repository interface extend QueryByExampleExecutor<T>. Here’s an excerpt from the QueryByExampleExecutor interface:

Example 71. The QueryByExampleExecutor

public interface QueryByExampleExecutor<T> {

  <S extends T> S findOne(Example<S> example);

  <S extends T> Iterable<S> findAll(Example<S> example);

  // … more functionality omitted.
}

You can read more about Query by Example Execution below.

5.6.3. Example matchers

Examples are not limited to default settings. You can specify own defaults for string matching, null handling and property-specific settings using the ExampleMatcher.

Example 72. Example matcher with customized matching

Person person = new Person();                          
person.setFirstname("Dave");                           

ExampleMatcher matcher = ExampleMatcher.matching()     
  .withIgnorePaths("lastname")                         
  .withIncludeNullValues()                             
  .withStringMatcherEnding();                          

Example<Person> example = Example.of(person, matcher);

	Create a new instance of the domain object.
2	Set properties.
3	Create an `ExampleMatcher` which is usable at this stage even without further configuration.
4	Construct a new `ExampleMatcher` to ignore the property path `lastname`.
5	Construct a new `ExampleMatcher` to ignore the property path `lastname` and to include null values.
6	Construct a new `ExampleMatcher` to ignore the property path `lastname`, to include null values, and use perform suffix string matching.
7	Create a new `Example` based on the domain object and the configured `ExampleMatcher`.

You can specify behavior for individual properties (e.g. "firstname" and "lastname", "address.city" for nested properties). You can tune it with matching options and case sensitivity.

Example 73. Configuring matcher options

ExampleMatcher matcher = ExampleMatcher.matching()
  .withMatcher("firstname", endsWith())
  .withMatcher("lastname", startsWith().ignoreCase());
}

Another style to configure matcher options is by using Java 8 lambdas. This approach is a callback that asks the implementor to modify the matcher. It’s not required to return the matcher because configuration options are held within the matcher instance.

Example 74. Configuring matcher options with lambdas

ExampleMatcher matcher = ExampleMatcher.matching()
  .withMatcher("firstname", match -> match.endsWith())
  .withMatcher("firstname", match -> match.startsWith());
}

Queries created by Example use a merged view of the configuration. Default matching settings can be set at ExampleMatcher level while individual settings can be applied to particular property paths. Settings that are set on ExampleMatcher are inherited by property path settings unless they are defined explicitly. Settings on a property patch have higher precedence than default settings.

Table 6. Scope of `ExampleMatcher` settings
Setting	Scope
Null-handling	`ExampleMatcher`
String matching	`ExampleMatcher` and property path
Ignoring properties	Property path
Case sensitivity	`ExampleMatcher` and property path
Value transformation	Property path

5.6.4. Executing an example

In Spring Data JPA you can use Query by Example with Repositories.

Example 75. Query by Example using a Repository

public interface PersonRepository extends JpaRepository<Person, String> { … }

public class PersonService {

  @Autowired PersonRepository personRepository;

  public List<Person> findPeople(Person probe) {
    return personRepository.findAll(Example.of(probe));
  }
}

Only SingularAttribute properties can currently be used for property matching.

Property specifier accepts property names (e.g. "firstname" and "lastname"). You can navigate by chaining properties together with dots ("address.city"). You can tune it with matching options and case sensitivity.

Table 7. `StringMatcher` options
Matching	Logical result
`DEFAULT` (case-sensitive)	`firstname = ?0`
`DEFAULT` (case-insensitive)	`LOWER(firstname) = LOWER(?0)`
`EXACT` (case-sensitive)	`firstname = ?0`
`EXACT` (case-insensitive)	`LOWER(firstname) = LOWER(?0)`
`STARTING` (case-sensitive)	`firstname like ?0 + '%'`
`STARTING` (case-insensitive)	`LOWER(firstname) like LOWER(?0) + '%'`
`ENDING` (case-sensitive)	`firstname like '%' + ?0`
`ENDING` (case-insensitive)	`LOWER(firstname) like '%' + LOWER(?0)`
`CONTAINING` (case-sensitive)	`firstname like '%' + ?0 + '%'`
`CONTAINING` (case-insensitive)	`LOWER(firstname) like '%' + LOWER(?0) + '%'`

5.7. Transactionality

CRUD methods on repository instances are transactional by default. For reading operations the transaction configuration readOnly flag is set to true, all others are configured with a plain @Transactional so that default transaction configuration applies. For details see JavaDoc of CrudRepository. If you need to tweak transaction configuration for one of the methods declared in a repository simply redeclare the method in your repository interface as follows:

Example 76. Custom transaction configuration for CRUD

public interface UserRepository extends CrudRepository<User, Long> {

  @Override
  @Transactional(timeout = 10)
  public List<User> findAll();

  // Further query method declarations
}

This will cause the findAll() method to be executed with a timeout of 10 seconds and without the readOnly flag.

Another possibility to alter transactional behaviour is using a facade or service implementation that typically covers more than one repository. Its purpose is to define transactional boundaries for non-CRUD operations:

Example 77. Using a facade to define transactions for multiple repository calls

@Service
class UserManagementImpl implements UserManagement {

  private final UserRepository userRepository;
  private final RoleRepository roleRepository;

  @Autowired
  public UserManagementImpl(UserRepository userRepository,
    RoleRepository roleRepository) {
    this.userRepository = userRepository;
    this.roleRepository = roleRepository;
  }

  @Transactional
  public void addRoleToAllUsers(String roleName) {

    Role role = roleRepository.findByName(roleName);

    for (User user : userRepository.findAll()) {
      user.addRole(role);
      userRepository.save(user);
    }
}

This will cause call to addRoleToAllUsers(…) to run inside a transaction (participating in an existing one or create a new one if none already running). The transaction configuration at the repositories will be neglected then as the outer transaction configuration determines the actual one used. Note that you will have to activate <tx:annotation-driven /> or use @EnableTransactionManagement explicitly to get annotation based configuration at facades working. The example above assumes you are using component scanning.

5.7.1. Transactional query methods

To allow your query methods to be transactional simply use @Transactional at the repository interface you define.

Example 78. Using @Transactional at query methods

@Transactional(readOnly = true)
public interface UserRepository extends JpaRepository<User, Long> {

  List<User> findByLastname(String lastname);

  @Modifying
  @Transactional
  @Query("delete from User u where u.active = false")
  void deleteInactiveUsers();
}

Typically you will want the readOnly flag set to true as most of the query methods will only read data. In contrast to that deleteInactiveUsers() makes use of the @Modifying annotation and overrides the transaction configuration. Thus the method will be executed with readOnly flag set to false.

It’s definitely reasonable to use transactions for read only queries and we can mark them as such by setting the readOnly flag. This will not, however, act as check that you do not trigger a manipulating query (although some databases reject INSERT and UPDATE statements inside a read only transaction). The readOnly flag instead is propagated as hint to the underlying JDBC driver for performance optimizations. Furthermore, Spring will perform some optimizations on the underlying JPA provider. E.g. when used with Hibernate the flush mode is set to NEVER when you configure a transaction as readOnly which causes Hibernate to skip dirty checks (a noticeable improvement on large object trees).

5.8. Locking

To specify the lock mode to be used the @Lock annotation can be used on query methods:

Example 79. Defining lock metadata on query methods

interface UserRepository extends Repository<User, Long> {

  // Plain query method
  @Lock(LockModeType.READ)
  List<User> findByLastname(String lastname);
}

This method declaration will cause the query being triggered to be equipped with the LockModeType READ. You can also define locking for CRUD methods by redeclaring them in your repository interface and adding the @Lock annotation:

Example 80. Defining lock metadata on CRUD methods

interface UserRepository extends Repository<User, Long> {

  // Redeclaration of a CRUD method
  @Lock(LockModeType.READ);
  List<User> findAll();
}

5.9. Auditing

5.9.1. Basics

Spring Data provides sophisticated support to transparently keep track of who created or changed an entity and the point in time this happened. To benefit from that functionality you have to equip your entity classes with auditing metadata that can be defined either using annotations or by implementing an interface.

Annotation based auditing metadata

We provide @CreatedBy, @LastModifiedBy to capture the user who created or modified the entity as well as @CreatedDate and @LastModifiedDate to capture the point in time this happened.

Example 81. An audited entity

class Customer {

  @CreatedBy
  private User user;

  @CreatedDate
  private DateTime createdDate;

  // … further properties omitted
}

As you can see, the annotations can be applied selectively, depending on which information you’d like to capture. For the annotations capturing the points in time can be used on properties of type JodaTimes DateTime, legacy Java Date and Calendar, JDK8 date/time types as well as long/Long.

Interface-based auditing metadata

In case you don’t want to use annotations to define auditing metadata you can let your domain class implement the Auditable interface. It exposes setter methods for all of the auditing properties.

There’s also a convenience base class AbstractAuditable which you can extend to avoid the need to manually implement the interface methods. Be aware that this increases the coupling of your domain classes to Spring Data which might be something you want to avoid. Usually the annotation based way of defining auditing metadata is preferred as it is less invasive and more flexible.

AuditorAware

In case you use either @CreatedBy or @LastModifiedBy, the auditing infrastructure somehow needs to become aware of the current principal. To do so, we provide an AuditorAware<T> SPI interface that you have to implement to tell the infrastructure who the current user or system interacting with the application is. The generic type T defines of what type the properties annotated with @CreatedBy or @LastModifiedBy have to be.

Here’s an example implementation of the interface using Spring Security’s Authentication object:

Example 82. Implementation of AuditorAware based on Spring Security

class SpringSecurityAuditorAware implements AuditorAware<User> {

  public User getCurrentAuditor() {

    Authentication authentication = SecurityContextHolder.getContext().getAuthentication();

    if (authentication == null || !authentication.isAuthenticated()) {
      return null;
    }

    return ((MyUserDetails) authentication.getPrincipal()).getUser();
  }
}

The implementation is accessing the Authentication object provided by Spring Security and looks up the custom UserDetails instance from it that you have created in your UserDetailsService implementation. We’re assuming here that you are exposing the domain user through that UserDetails implementation but you could also look it up from anywhere based on the Authentication found.

5.10. JPA Auditing

5.10.1. General auditing configuration

Spring Data JPA ships with an entity listener that can be used to trigger capturing auditing information. So first you have to register the AuditingEntityListener inside your orm.xml to be used for all entities in your persistence contexts:

Example 83. Auditing configuration orm.xml

<persistence-unit-metadata>
  <persistence-unit-defaults>
    <entity-listeners>
      <entity-listener class="….data.jpa.domain.support.AuditingEntityListener" />
    </entity-listeners>
  </persistence-unit-defaults>
</persistence-unit-metadata>

You can also enable the AuditingEntityListener per entity using the @EntityListeners annotation:

@Entity
@EntityListeners(AuditingEntityListener.class)
public class MyEntity {

}

Note that the auditing feature requires spring-aspects.jar to be on the classpath.

With that in place, activating auditing functionality is just a matter of adding the Spring Data JPA auditing namespace element to your configuration:

Example 84. Activating auditing using XML configuration

<jpa:auditing auditor-aware-ref="yourAuditorAwareBean" />

As of Spring Data JPA 1.5, auditing can be enabled by annotating a configuration class with the @EnableJpaAuditing annotation.

Example 85. Activating auditing via Java configuration

@Configuration
@EnableJpaAuditing
class Config {

  @Bean
  public AuditorAware<AuditableUser> auditorProvider() {
    return new AuditorAwareImpl();
  }
}

If you expose a bean of type AuditorAware to the ApplicationContext, the auditing infrastructure will pick it up automatically and use it to determine the current user to be set on domain types. If you have multiple implementations registered in the ApplicationContext, you can select the one to be used by explicitly setting the auditorAwareRef attribute of @EnableJpaAuditing.