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Preface
1. Project Metadata
-
Version control: https://github.com/spring-projects/spring-data-commons
-
Bugtracker: https://github.com/spring-projects/spring-data-commons/issues
-
Release repository: https://repo1.maven.org/maven2/
-
Milestone repository: https://repo.spring.io/milestone/
-
Snapshot repository: https://repo.spring.io/snapshot/
Reference Documentation
2. Dependencies
Due to the different inception dates of individual Spring Data modules, most of them carry different major and minor version numbers. The easiest way to find compatible ones is to rely on the Spring Data Release Train BOM that we ship with the compatible versions defined. In a Maven project, you would declare this dependency in the <dependencyManagement />
section of your POM as follows:
<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework.data</groupId>
<artifactId>spring-data-bom</artifactId>
<version>2023.1.0-RC1</version>
<scope>import</scope>
<type>pom</type>
</dependency>
</dependencies>
</dependencyManagement>
The current release train version is 2023.1.0-RC1
. The train version uses calver with the pattern YYYY.MINOR.MICRO
.
The version name follows ${calver}
for GA releases and service releases and the following pattern for all other versions: ${calver}-${modifier}
, where modifier
can be one of the following:
-
SNAPSHOT
: Current snapshots -
M1
,M2
, and so on: Milestones -
RC1
,RC2
, and so on: Release candidates
You can find a working example of using the BOMs in our Spring Data examples repository. With that in place, you can declare the Spring Data modules you would like to use without a version in the <dependencies />
block, as follows:
<dependencies>
<dependency>
<groupId>org.springframework.data</groupId>
<artifactId>spring-data-jpa</artifactId>
</dependency>
<dependencies>
2.1. Dependency Management with Spring Boot
Spring Boot selects a recent version of the Spring Data modules for you. If you still want to upgrade to a newer version,
set the spring-data-bom.version
property to the train version and iteration
you would like to use.
See Spring Boot’s documentation (search for "Spring Data Bom") for more details.
3. Object Mapping Fundamentals
This section covers the fundamentals of Spring Data object mapping, object creation, field and property access, mutability and immutability. Note, that this section only applies to Spring Data modules that do not use the object mapping of the underlying data store (like JPA). Also be sure to consult the store-specific sections for store-specific object mapping, like indexes, customizing column or field names or the like.
Core responsibility of the Spring Data object mapping is to create instances of domain objects and map the store-native data structures onto those. This means we need two fundamental steps:
-
Instance creation by using one of the constructors exposed.
-
Instance population to materialize all exposed properties.
3.1. Object creation
Spring Data automatically tries to detect a persistent entity’s constructor to be used to materialize objects of that type. The resolution algorithm works as follows:
-
If there is a single static factory method annotated with
@PersistenceCreator
then it is used. -
If there is a single constructor, it is used.
-
If there are multiple constructors and exactly one is annotated with
@PersistenceCreator
, it is used. -
If the type is a Java
Record
the canonical constructor is used. -
If there’s a no-argument constructor, it is used. Other constructors will be ignored.
The value resolution assumes constructor/factory method argument names to match the property names of the entity, i.e. the resolution will be performed as if the property was to be populated, including all customizations in mapping (different datastore column or field name etc.).
This also requires either parameter names information available in the class file or an @ConstructorProperties
annotation being present on the constructor.
The value resolution can be customized by using Spring Framework’s @Value
value annotation using a store-specific SpEL expression.
Please consult the section on store specific mappings for further details.
3.2. Property population
Once an instance of the entity has been created, Spring Data populates all remaining persistent properties of that class. Unless already populated by the entity’s constructor (i.e. consumed through its constructor argument list), the identifier property will be populated first to allow the resolution of cyclic object references. After that, all non-transient properties that have not already been populated by the constructor are set on the entity instance. For that we use the following algorithm:
-
If the property is immutable but exposes a
with…
method (see below), we use thewith…
method to create a new entity instance with the new property value. -
If property access (i.e. access through getters and setters) is defined, we’re invoking the setter method.
-
If the property is mutable we set the field directly.
-
If the property is immutable we’re using the constructor to be used by persistence operations (see Object creation) to create a copy of the instance.
-
By default, we set the field value directly.
Let’s have a look at the following entity:
class Person {
private final @Id Long id; (1)
private final String firstname, lastname; (2)
private final LocalDate birthday;
private final int age; (3)
private String comment; (4)
private @AccessType(Type.PROPERTY) String remarks; (5)
static Person of(String firstname, String lastname, LocalDate birthday) { (6)
return new Person(null, firstname, lastname, birthday,
Period.between(birthday, LocalDate.now()).getYears());
}
Person(Long id, String firstname, String lastname, LocalDate birthday, int age) { (6)
this.id = id;
this.firstname = firstname;
this.lastname = lastname;
this.birthday = birthday;
this.age = age;
}
Person withId(Long id) { (1)
return new Person(id, this.firstname, this.lastname, this.birthday, this.age);
}
void setRemarks(String remarks) { (5)
this.remarks = remarks;
}
}
1 | The identifier property is final but set to null in the constructor.
The class exposes a withId(…) method that’s used to set the identifier, e.g. when an instance is inserted into the datastore and an identifier has been generated.
The original Person instance stays unchanged as a new one is created.
The same pattern is usually applied for other properties that are store managed but might have to be changed for persistence operations.
The wither method is optional as the persistence constructor (see 6) is effectively a copy constructor and setting the property will be translated into creating a fresh instance with the new identifier value applied. |
2 | The firstname and lastname properties are ordinary immutable properties potentially exposed through getters. |
3 | The age property is an immutable but derived one from the birthday property.
With the design shown, the database value will trump the defaulting as Spring Data uses the only declared constructor.
Even if the intent is that the calculation should be preferred, it’s important that this constructor also takes age as parameter (to potentially ignore it) as otherwise the property population step will attempt to set the age field and fail due to it being immutable and no with… method being present. |
4 | The comment property is mutable and is populated by setting its field directly. |
5 | The remarks property is mutable and is populated by invoking the setter method. |
6 | The class exposes a factory method and a constructor for object creation.
The core idea here is to use factory methods instead of additional constructors to avoid the need for constructor disambiguation through @PersistenceCreator .
Instead, defaulting of properties is handled within the factory method.
If you want Spring Data to use the factory method for object instantiation, annotate it with @PersistenceCreator . |
3.3. General recommendations
-
Try to stick to immutable objects — Immutable objects are straightforward to create as materializing an object is then a matter of calling its constructor only. Also, this avoids your domain objects to be littered with setter methods that allow client code to manipulate the objects state. If you need those, prefer to make them package protected so that they can only be invoked by a limited amount of co-located types. Constructor-only materialization is up to 30% faster than properties population.
-
Provide an all-args constructor — Even if you cannot or don’t want to model your entities as immutable values, there’s still value in providing a constructor that takes all properties of the entity as arguments, including the mutable ones, as this allows the object mapping to skip the property population for optimal performance.
-
Use factory methods instead of overloaded constructors to avoid
@PersistenceCreator
— With an all-argument constructor needed for optimal performance, we usually want to expose more application use case specific constructors that omit things like auto-generated identifiers etc. It’s an established pattern to rather use static factory methods to expose these variants of the all-args constructor. -
Make sure you adhere to the constraints that allow the generated instantiator and property accessor classes to be used —
-
For identifiers to be generated, still use a final field in combination with an all-arguments persistence constructor (preferred) or a
with…
method — -
Use Lombok to avoid boilerplate code — As persistence operations usually require a constructor taking all arguments, their declaration becomes a tedious repetition of boilerplate parameter to field assignments that can best be avoided by using Lombok’s
@AllArgsConstructor
.
3.3.1. Overriding Properties
Java’s allows a flexible design of domain classes where a subclass could define a property that is already declared with the same name in its superclass. Consider the following example:
public class SuperType {
private CharSequence field;
public SuperType(CharSequence field) {
this.field = field;
}
public CharSequence getField() {
return this.field;
}
public void setField(CharSequence field) {
this.field = field;
}
}
public class SubType extends SuperType {
private String field;
public SubType(String field) {
super(field);
this.field = field;
}
@Override
public String getField() {
return this.field;
}
public void setField(String field) {
this.field = field;
// optional
super.setField(field);
}
}
Both classes define a field
using assignable types. SubType
however shadows SuperType.field
.
Depending on the class design, using the constructor could be the only default approach to set SuperType.field
.
Alternatively, calling super.setField(…)
in the setter could set the field
in SuperType
.
All these mechanisms create conflicts to some degree because the properties share the same name yet might represent two distinct values.
Spring Data skips super-type properties if types are not assignable.
That is, the type of the overridden property must be assignable to its super-type property type to be registered as override, otherwise the super-type property is considered transient.
We generally recommend using distinct property names.
Spring Data modules generally support overridden properties holding different values. From a programming model perspective there are a few things to consider:
-
Which property should be persisted (default to all declared properties)? You can exclude properties by annotating these with
@Transient
. -
How to represent properties in your data store? Using the same field/column name for different values typically leads to corrupt data so you should annotate least one of the properties using an explicit field/column name.
-
Using
@AccessType(PROPERTY)
cannot be used as the super-property cannot be generally set without making any further assumptions of the setter implementation.
3.4. Kotlin support
Spring Data adapts specifics of Kotlin to allow object creation and mutation.
3.4.1. Kotlin object creation
Kotlin classes are supported to be instantiated, all classes are immutable by default and require explicit property declarations to define mutable properties.
Spring Data automatically tries to detect a persistent entity’s constructor to be used to materialize objects of that type. The resolution algorithm works as follows:
-
If there is a constructor that is annotated with
@PersistenceCreator
, it is used. -
If the type is a Kotlin data cass the primary constructor is used.
-
If there is a single static factory method annotated with
@PersistenceCreator
then it is used. -
If there is a single constructor, it is used.
-
If there are multiple constructors and exactly one is annotated with
@PersistenceCreator
, it is used. -
If the type is a Java
Record
the canonical constructor is used. -
If there’s a no-argument constructor, it is used. Other constructors will be ignored.
Consider the following data
class Person
:
data class Person(val id: String, val name: String)
The class above compiles to a typical class with an explicit constructor.We can customize this class by adding another constructor and annotate it with @PersistenceCreator
to indicate a constructor preference:
data class Person(var id: String, val name: String) {
@PersistenceCreator
constructor(id: String) : this(id, "unknown")
}
Kotlin supports parameter optionality by allowing default values to be used if a parameter is not provided.
When Spring Data detects a constructor with parameter defaulting, then it leaves these parameters absent if the data store does not provide a value (or simply returns null
) so Kotlin can apply parameter defaulting.Consider the following class that applies parameter defaulting for name
data class Person(var id: String, val name: String = "unknown")
Every time the name
parameter is either not part of the result or its value is null
, then the name
defaults to unknown
.
3.4.2. Property population of Kotlin data classes
In Kotlin, all classes are immutable by default and require explicit property declarations to define mutable properties.
Consider the following data
class Person
:
data class Person(val id: String, val name: String)
This class is effectively immutable.
It allows creating new instances as Kotlin generates a copy(…)
method that creates new object instances copying all property values from the existing object and applying property values provided as arguments to the method.
3.4.3. Kotlin Overriding Properties
Kotlin allows declaring property overrides to alter properties in subclasses.
open class SuperType(open var field: Int)
class SubType(override var field: Int = 1) :
SuperType(field) {
}
Such an arrangement renders two properties with the name field
.
Kotlin generates property accessors (getters and setters) for each property in each class.
Effectively, the code looks like as follows:
public class SuperType {
private int field;
public SuperType(int field) {
this.field = field;
}
public int getField() {
return this.field;
}
public void setField(int field) {
this.field = field;
}
}
public final class SubType extends SuperType {
private int field;
public SubType(int field) {
super(field);
this.field = field;
}
public int getField() {
return this.field;
}
public void setField(int field) {
this.field = field;
}
}
Getters and setters on SubType
set only SubType.field
and not SuperType.field
.
In such an arrangement, using the constructor is the only default approach to set SuperType.field
.
Adding a method to SubType
to set SuperType.field
via this.SuperType.field = …
is possible but falls outside of supported conventions.
Property overrides create conflicts to some degree because the properties share the same name yet might represent two distinct values.
We generally recommend using distinct property names.
Spring Data modules generally support overridden properties holding different values. From a programming model perspective there are a few things to consider:
-
Which property should be persisted (default to all declared properties)? You can exclude properties by annotating these with
@Transient
. -
How to represent properties in your data store? Using the same field/column name for different values typically leads to corrupt data so you should annotate least one of the properties using an explicit field/column name.
-
Using
@AccessType(PROPERTY)
cannot be used as the super-property cannot be set.
3.4.4. Kotlin Value Classes
Kotlin Value Classes are designed for a more expressive domain model to make underlying concepts explicit. Spring Data can read and write types that define properties using Value Classes.
Consider the following domain model:
@JvmInline
value class EmailAddress(val theAddress: String) (1)
data class Contact(val id: String, val name:String, val emailAddress: EmailAddress) (2)
1 | A simple value class with a non-nullable value type. |
2 | Data class defining a property using the EmailAddress value class. |
Non-nullable properties using non-primitive value types are flattened in the compiled class to the value type. Nullable primitive value types or nullable value-in-value types are represented with their wrapper type and that affects how value types are represented in the database. |
4. Working with Spring Data Repositories
The goal of the Spring Data repository abstraction is to significantly reduce the amount of boilerplate code required to implement data access layers for various persistence stores.
Spring Data repository documentation and your module This chapter explains the core concepts and interfaces of Spring Data repositories. The information in this chapter is pulled from the Spring Data Commons module. It uses the configuration and code samples for the Jakarta Persistence API (JPA) module. If you want to use XML configuration you should adapt the XML namespace declaration and the types to be extended to the equivalents of the particular module that you use. “Namespace reference” covers XML configuration, which is supported across all Spring Data modules that support the repository API. “Repository query keywords” covers the query method keywords supported by the repository abstraction in general. For detailed information on the specific features of your module, see the chapter on that module of this document. |
4.1. Core concepts
The central interface in the Spring Data repository abstraction is Repository
.
It takes the domain class to manage as well as the identifier type of the domain class as type arguments.
This interface acts primarily as a marker interface to capture the types to work with and to help you to discover interfaces that extend this one.
The CrudRepository
and ListCrudRepository
interfaces provide sophisticated CRUD functionality for the entity class that is being managed.
CrudRepository
Interfacepublic interface CrudRepository<T, ID> extends Repository<T, ID> {
<S extends T> S save(S entity); (1)
Optional<T> findById(ID primaryKey); (2)
Iterable<T> findAll(); (3)
long count(); (4)
void delete(T entity); (5)
boolean existsById(ID primaryKey); (6)
// … more functionality omitted.
}
1 | Saves the given entity. |
2 | Returns the entity identified by the given ID. |
3 | Returns all entities. |
4 | Returns the number of entities. |
5 | Deletes the given entity. |
6 | Indicates whether an entity with the given ID exists. |
The methods declared in this interface are commonly referred to as CRUD methods.
ListCrudRepository
offers equivalent methods, but they return List
where the CrudRepository
methods return an Iterable
.
We also provide persistence technology-specific abstractions, such as JpaRepository or MongoRepository .
Those interfaces extend CrudRepository and expose the capabilities of the underlying persistence technology in addition to the rather generic persistence technology-agnostic interfaces such as CrudRepository .
|
Additional to the CrudRepository
, there is a PagingAndSortingRepository
abstraction that adds additional methods to ease paginated access to entities:
PagingAndSortingRepository
interfacepublic interface PagingAndSortingRepository<T, ID> {
Iterable<T> findAll(Sort sort);
Page<T> findAll(Pageable pageable);
}
To access the second page of User
by a page size of 20, you could do something like the following:
PagingAndSortingRepository<User, Long> repository = // … get access to a bean
Page<User> users = repository.findAll(PageRequest.of(1, 20));
In addition to pagination, scrolling provides a more fine-grained access to iterate through chunks of larger result sets.
In addition to query methods, query derivation for both count and delete queries is available. The following list shows the interface definition for a derived count query:
interface UserRepository extends CrudRepository<User, Long> {
long countByLastname(String lastname);
}
The following listing shows the interface definition for a derived delete query:
interface UserRepository extends CrudRepository<User, Long> {
long deleteByLastname(String lastname);
List<User> removeByLastname(String lastname);
}
4.2. Query Methods
Standard CRUD functionality repositories usually have queries on the underlying datastore. With Spring Data, declaring those queries becomes a four-step process:
-
Declare an interface extending Repository or one of its subinterfaces and type it to the domain class and ID type that it should handle, as shown in the following example:
interface PersonRepository extends Repository<Person, Long> { … }
-
Declare query methods on the interface.
interface PersonRepository extends Repository<Person, Long> { List<Person> findByLastname(String lastname); }
-
Set up Spring to create proxy instances for those interfaces, either with JavaConfig or with XML configuration.
Java@EnableJpaRepositories class Config { … }
XML<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:jpa="http://www.springframework.org/schema/data/jpa" xsi:schemaLocation="http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/data/jpa https://www.springframework.org/schema/data/jpa/spring-jpa.xsd"> <repositories base-package="com.acme.repositories"/> </beans>
The JPA namespace is used in this example. If you use the repository abstraction for any other store, you need to change this to the appropriate namespace declaration of your store module. In other words, you should exchange
jpa
in favor of, for example,mongodb
.Note that the JavaConfig variant does not configure a package explicitly, because the package of the annotated class is used by default. To customize the package to scan, use one of the
basePackage…
attributes of the data-store-specific repository’s@EnableJpaRepositories
-annotation. -
Inject the repository instance and use it, as shown in the following example:
class SomeClient { private final PersonRepository repository; SomeClient(PersonRepository repository) { this.repository = repository; } void doSomething() { List<Person> persons = repository.findByLastname("Matthews"); } }
The sections that follow explain each step in detail:
4.3. Defining Repository Interfaces
To define a repository interface, you first need to define a domain class-specific repository interface.
The interface must extend Repository
and be typed to the domain class and an ID type.
If you want to expose CRUD methods for that domain type, you may extend CrudRepository
, or one of its variants instead of Repository
.
4.3.1. Fine-tuning Repository Definition
There are a few variants how you can get started with your repository interface.
The typical approach is to extend CrudRepository
, which gives you methods for CRUD functionality.
CRUD stands for Create, Read, Update, Delete.
With version 3.0 we also introduced ListCrudRepository
which is very similar to the CrudRepository
but for those methods that return multiple entities it returns a List
instead of an Iterable
which you might find easier to use.
If you are using a reactive store you might choose ReactiveCrudRepository
, or RxJava3CrudRepository
depending on which reactive framework you are using.
If you are using Kotlin you might pick CoroutineCrudRepository
which utilizes Kotlin’s coroutines.
Additional you can extend PagingAndSortingRepository
, ReactiveSortingRepository
, RxJava3SortingRepository
, or CoroutineSortingRepository
if you need methods that allow to specify a Sort
abstraction or in the first case a Pageable
abstraction.
Note that the various sorting repositories no longer extended their respective CRUD repository as they did in Spring Data Versions pre 3.0.
Therefore, you need to extend both interfaces if you want functionality of both.
If you do not want to extend Spring Data interfaces, you can also annotate your repository interface with @RepositoryDefinition
.
Extending one of the CRUD repository interfaces exposes a complete set of methods to manipulate your entities.
If you prefer to be selective about the methods being exposed, copy the methods you want to expose from the CRUD repository into your domain repository.
When doing so, you may change the return type of methods.
Spring Data will honor the return type if possible.
For example, for methods returning multiple entities you may choose Iterable<T>
, List<T>
, Collection<T>
or a VAVR list.
If many repositories in your application should have the same set of methods you can define your own base interface to inherit from.
Such an interface must be annotated with @NoRepositoryBean
.
This prevents Spring Data to try to create an instance of it directly and failing because it can’t determine the entity for that repository, since it still contains a generic type variable.
The following example shows how to selectively expose CRUD methods (findById
and save
, in this case):
@NoRepositoryBean
interface MyBaseRepository<T, ID> extends Repository<T, ID> {
Optional<T> findById(ID id);
<S extends T> S save(S entity);
}
interface UserRepository extends MyBaseRepository<User, Long> {
User findByEmailAddress(EmailAddress emailAddress);
}
In the prior example, you defined a common base interface for all your domain repositories and exposed findById(…)
as well as save(…)
.These methods are routed into the base repository implementation of the store of your choice provided by Spring Data (for example, if you use JPA, the implementation is SimpleJpaRepository
), because they match the method signatures in CrudRepository
.
So the UserRepository
can now save users, find individual users by ID, and trigger a query to find Users
by email address.
The intermediate repository interface is annotated with @NoRepositoryBean .
Make sure you add that annotation to all repository interfaces for which Spring Data should not create instances at runtime.
|
4.3.2. Using Repositories with Multiple Spring Data Modules
Using a unique Spring Data module in your application makes things simple, because all repository interfaces in the defined scope are bound to the Spring Data module. Sometimes, applications require using more than one Spring Data module. In such cases, a repository definition must distinguish between persistence technologies. When it detects multiple repository factories on the class path, Spring Data enters strict repository configuration mode. Strict configuration uses details on the repository or the domain class to decide about Spring Data module binding for a repository definition:
-
If the repository definition extends the module-specific repository, it is a valid candidate for the particular Spring Data module.
-
If the domain class is annotated with the module-specific type annotation, it is a valid candidate for the particular Spring Data module. Spring Data modules accept either third-party annotations (such as JPA’s
@Entity
) or provide their own annotations (such as@Document
for Spring Data MongoDB and Spring Data Elasticsearch).
The following example shows a repository that uses module-specific interfaces (JPA in this case):
interface MyRepository extends JpaRepository<User, Long> { }
@NoRepositoryBean
interface MyBaseRepository<T, ID> extends JpaRepository<T, ID> { … }
interface UserRepository extends MyBaseRepository<User, Long> { … }
MyRepository
and UserRepository
extend JpaRepository
in their type hierarchy.
They are valid candidates for the Spring Data JPA module.
The following example shows a repository that uses generic interfaces:
interface AmbiguousRepository extends Repository<User, Long> { … }
@NoRepositoryBean
interface MyBaseRepository<T, ID> extends CrudRepository<T, ID> { … }
interface AmbiguousUserRepository extends MyBaseRepository<User, Long> { … }
AmbiguousRepository
and AmbiguousUserRepository
extend only Repository
and CrudRepository
in their type hierarchy.
While this is fine when using a unique Spring Data module, multiple modules cannot distinguish to which particular Spring Data these repositories should be bound.
The following example shows a repository that uses domain classes with annotations:
interface PersonRepository extends Repository<Person, Long> { … }
@Entity
class Person { … }
interface UserRepository extends Repository<User, Long> { … }
@Document
class User { … }
PersonRepository
references Person
, which is annotated with the JPA @Entity
annotation, so this repository clearly belongs to Spring Data JPA. UserRepository
references User
, which is annotated with Spring Data MongoDB’s @Document
annotation.
The following bad example shows a repository that uses domain classes with mixed annotations:
interface JpaPersonRepository extends Repository<Person, Long> { … }
interface MongoDBPersonRepository extends Repository<Person, Long> { … }
@Entity
@Document
class Person { … }
This example shows a domain class using both JPA and Spring Data MongoDB annotations.
It defines two repositories, JpaPersonRepository
and MongoDBPersonRepository
.
One is intended for JPA and the other for MongoDB usage.
Spring Data is no longer able to tell the repositories apart, which leads to undefined behavior.
Repository type details and distinguishing domain class annotations are used for strict repository configuration to identify repository candidates for a particular Spring Data module. Using multiple persistence technology-specific annotations on the same domain type is possible and enables reuse of domain types across multiple persistence technologies. However, Spring Data can then no longer determine a unique module with which to bind the repository.
The last way to distinguish repositories is by scoping repository base packages. Base packages define the starting points for scanning for repository interface definitions, which implies having repository definitions located in the appropriate packages. By default, annotation-driven configuration uses the package of the configuration class. The base package in XML-based configuration is mandatory.
The following example shows annotation-driven configuration of base packages:
@EnableJpaRepositories(basePackages = "com.acme.repositories.jpa")
@EnableMongoRepositories(basePackages = "com.acme.repositories.mongo")
class Configuration { … }
4.4. Defining Query Methods
The repository proxy has two ways to derive a store-specific query from the method name:
-
By deriving the query from the method name directly.
-
By using a manually defined query.
Available options depend on the actual store. However, there must be a strategy that decides what actual query is created. The next section describes the available options.
4.4.1. Query Lookup Strategies
The following strategies are available for the repository infrastructure to resolve the query.
With XML configuration, you can configure the strategy at the namespace through the query-lookup-strategy
attribute.
For Java configuration, you can use the queryLookupStrategy
attribute of the EnableJpaRepositories
annotation.
Some strategies may not be supported for particular datastores.
-
CREATE
attempts to construct a store-specific query from the query method name. The general approach is to remove a given set of well known prefixes from the method name and parse the rest of the method. You can read more about query construction in “Query Creation”. -
USE_DECLARED_QUERY
tries to find a declared query and throws an exception if it cannot find one. The query can be defined by an annotation somewhere or declared by other means. See the documentation of the specific store to find available options for that store. If the repository infrastructure does not find a declared query for the method at bootstrap time, it fails. -
CREATE_IF_NOT_FOUND
(the default) combinesCREATE
andUSE_DECLARED_QUERY
. It looks up a declared query first, and, if no declared query is found, it creates a custom method name-based query. This is the default lookup strategy and, thus, is used if you do not configure anything explicitly. It allows quick query definition by method names but also custom-tuning of these queries by introducing declared queries as needed.
4.4.2. Query Creation
The query builder mechanism built into the Spring Data repository infrastructure is useful for building constraining queries over entities of the repository.
The following example shows how to create a number of queries:
interface PersonRepository extends Repository<Person, Long> {
List<Person> findByEmailAddressAndLastname(EmailAddress emailAddress, String lastname);
// Enables the distinct flag for the query
List<Person> findDistinctPeopleByLastnameOrFirstname(String lastname, String firstname);
List<Person> findPeopleDistinctByLastnameOrFirstname(String lastname, String firstname);
// Enabling ignoring case for an individual property
List<Person> findByLastnameIgnoreCase(String lastname);
// Enabling ignoring case for all suitable properties
List<Person> findByLastnameAndFirstnameAllIgnoreCase(String lastname, String firstname);
// Enabling static ORDER BY for a query
List<Person> findByLastnameOrderByFirstnameAsc(String lastname);
List<Person> findByLastnameOrderByFirstnameDesc(String lastname);
}
Parsing query method names is divided into subject and predicate.
The first part (find…By
, exists…By
) defines the subject of the query, the second part forms the predicate.
The introducing clause (subject) can contain further expressions.
Any text between find
(or other introducing keywords) and By
is considered to be descriptive unless using one of the result-limiting keywords such as a Distinct
to set a distinct flag on the query to be created or Top
/First
to limit query results.
The appendix contains the full list of query method subject keywords and query method predicate keywords including sorting and letter-casing modifiers.
However, the first By
acts as a delimiter to indicate the start of the actual criteria predicate.
At a very basic level, you can define conditions on entity properties and concatenate them with And
and Or
.
The actual result of parsing the method depends on the persistence store for which you create the query. However, there are some general things to notice:
-
The expressions are usually property traversals combined with operators that can be concatenated. You can combine property expressions with
AND
andOR
. You also get support for operators such asBetween
,LessThan
,GreaterThan
, andLike
for the property expressions. The supported operators can vary by datastore, so consult the appropriate part of your reference documentation. -
The method parser supports setting an
IgnoreCase
flag for individual properties (for example,findByLastnameIgnoreCase(…)
) or for all properties of a type that supports ignoring case (usuallyString
instances — for example,findByLastnameAndFirstnameAllIgnoreCase(…)
). Whether ignoring cases is supported may vary by store, so consult the relevant sections in the reference documentation for the store-specific query method. -
You can apply static ordering by appending an
OrderBy
clause to the query method that references a property and by providing a sorting direction (Asc
orDesc
). To create a query method that supports dynamic sorting, see “Paging, Iterating Large Results, Sorting & Limiting”.
4.4.3. Property Expressions
Property expressions can refer only to a direct property of the managed entity, as shown in the preceding example. At query creation time, you already make sure that the parsed property is a property of the managed domain class. However, you can also define constraints by traversing nested properties. Consider the following method signature:
List<Person> findByAddressZipCode(ZipCode zipCode);
Assume a Person
has an Address
with a ZipCode
.
In that case, the method creates the x.address.zipCode
property traversal.
The resolution algorithm starts by interpreting the entire part (AddressZipCode
) as the property and checks the domain class for a property with that name (uncapitalized).
If the algorithm succeeds, it uses that property.
If not, the algorithm splits up the source at the camel-case parts from the right side into a head and a tail and tries to find the corresponding property — in our example, AddressZip
and Code
.
If the algorithm finds a property with that head, it takes the tail and continues building the tree down from there, splitting the tail up in the way just described.
If the first split does not match, the algorithm moves the split point to the left (Address
, ZipCode
) and continues.
Although this should work for most cases, it is possible for the algorithm to select the wrong property.
Suppose the Person
class has an addressZip
property as well.
The algorithm would match in the first split round already, choose the wrong property, and fail (as the type of addressZip
probably has no code
property).
To resolve this ambiguity you can use _
inside your method name to manually define traversal points.
So our method name would be as follows:
List<Person> findByAddress_ZipCode(ZipCode zipCode);
Because we treat the underscore character as a reserved character, we strongly advise following standard Java naming conventions (that is, not using underscores in property names but using camel case instead).
4.4.4. Paging, Iterating Large Results, Sorting & Limiting
To handle parameters in your query, define method parameters as already seen in the preceding examples.
Besides that, the infrastructure recognizes certain specific types like Pageable
, Sort
and Limit
, to apply pagination, sorting and limiting to your queries dynamically.
The following example demonstrates these features:
Pageable
, Slice
, ScrollPosition
, Sort
and Limit
in query methodsPage<User> findByLastname(String lastname, Pageable pageable);
Slice<User> findByLastname(String lastname, Pageable pageable);
Window<User> findTop10ByLastname(String lastname, ScrollPosition position, Sort sort);
List<User> findByLastname(String lastname, Sort sort);
List<User> findByLastname(String lastname, Sort sort, Limit limit);
List<User> findByLastname(String lastname, Pageable pageable);
APIs taking Sort , Pageable and Limit expect non-null values to be handed into methods.
If you do not want to apply any sorting or pagination, use Sort.unsorted() , Pageable.unpaged() and Limit.unlimited() .
|
The first method lets you pass an org.springframework.data.domain.Pageable
instance to the query method to dynamically add paging to your statically defined query.
A Page
knows about the total number of elements and pages available.
It does so by the infrastructure triggering a count query to calculate the overall number.
As this might be expensive (depending on the store used), you can instead return a Slice
.
A Slice
knows only about whether a next Slice
is available, which might be sufficient when walking through a larger result set.
Sorting options are handled through the Pageable
instance, too.
If you need only sorting, add an org.springframework.data.domain.Sort
parameter to your method.
As you can see, returning a List
is also possible.
In this case, the additional metadata required to build the actual Page
instance is not created (which, in turn, means that the additional count query that would have been necessary is not issued).
Rather, it restricts the query to look up only the given range of entities.
To find out how many pages you get for an entire query, you have to trigger an additional count query. By default, this query is derived from the query you actually trigger. |
Special parameters may only be used once within a query method.
The |
Which Method is Appropriate?
The value provided by the Spring Data abstractions is perhaps best shown by the possible query method return types outlined in the following table below. The table shows which types you can return from a query method
Method | Amount of Data Fetched | Query Structure | Constraints |
---|---|---|---|
All results. |
Single query. |
Query results can exhaust all memory. Fetching all data can be time-intensive. |
|
All results. |
Single query. |
Query results can exhaust all memory. Fetching all data can be time-intensive. |
|
Chunked (one-by-one or in batches) depending on |
Single query using typically cursors. |
Streams must be closed after usage to avoid resource leaks. |
|
|
Chunked (one-by-one or in batches) depending on |
Single query using typically cursors. |
Store module must provide reactive infrastructure. |
|
|
One to many queries fetching data starting at |
A
|
Offset-based |
|
One to many queries fetching data starting at |
A
|
|
|
One to many queries starting at |
Often times,
|
Keyset-based |
|
One to many queries fetching data starting at |
A
|
Paging and Sorting
You can define simple sorting expressions by using property names. You can concatenate expressions to collect multiple criteria into one expression.
Sort sort = Sort.by("firstname").ascending()
.and(Sort.by("lastname").descending());
For a more type-safe way to define sort expressions, start with the type for which to define the sort expression and use method references to define the properties on which to sort.
TypedSort<Person> person = Sort.sort(Person.class);
Sort sort = person.by(Person::getFirstname).ascending()
.and(person.by(Person::getLastname).descending());
TypedSort.by(…) makes use of runtime proxies by (typically) using CGlib, which may interfere with native image compilation when using tools such as Graal VM Native.
|
If your store implementation supports Querydsl, you can also use the generated metamodel types to define sort expressions:
QSort sort = QSort.by(QPerson.firstname.asc())
.and(QSort.by(QPerson.lastname.desc()));
Scrolling
Scrolling is a more fine-grained approach to iterate through larger results set chunks.
Scrolling consists of a stable sort, a scroll type (Offset- or Keyset-based scrolling) and result limiting.
You can define simple sorting expressions by using property names and define static result limiting using the Top
or First
keyword through query derivation.
You can concatenate expressions to collect multiple criteria into one expression.
Scroll queries return a Window<T>
that allows obtaining the scroll position to resume to obtain the next Window<T>
until your application has consumed the entire query result.
Similar to consuming a Java Iterator<List<…>>
by obtaining the next batch of results, query result scrolling lets you access the a ScrollPosition
through Window.positionAt(…)
.
Window<User> users = repository.findFirst10ByLastnameOrderByFirstname("Doe", OffsetScrollPosition.initial());
do {
for (User u : users) {
// consume the user
}
// obtain the next Scroll
users = repository.findFirst10ByLastnameOrderByFirstname("Doe", users.positionAt(users.size() - 1));
} while (!users.isEmpty() && users.hasNext());
WindowIterator
provides a utility to simplify scrolling across Window
s by removing the need to check for the presence of a next Window
and applying the ScrollPosition
.
WindowIterator<User> users = WindowIterator.of(position -> repository.findFirst10ByLastnameOrderByFirstname("Doe", position))
.startingAt(OffsetScrollPosition.initial());
while (users.hasNext()) {
User u = users.next();
// consume the user
}
Scrolling using Offset
Offset scrolling uses similar to pagination, an Offset counter to skip a number of results and let the data source only return results beginning at the given Offset. This simple mechanism avoids large results being sent to the client application. However, most databases require materializing the full query result before your server can return the results.
OffsetScrollPosition
with Repository Query Methodsinterface UserRepository extends Repository<User, Long> {
Window<User> findFirst10ByLastnameOrderByFirstname(String lastname, OffsetScrollPosition position);
}
WindowIterator<User> users = WindowIterator.of(position -> repository.findFirst10ByLastnameOrderByFirstname("Doe", position))
.startingAt(OffsetScrollPosition.initial()); (1)
1 | Start from the initial offset at position 0 . |
Scrolling using Keyset-Filtering
Offset-based requires most databases require materializing the entire result before your server can return the results. So while the client only sees the portion of the requested results, your server needs to build the full result, which causes additional load.
Keyset-Filtering approaches result subset retrieval by leveraging built-in capabilities of your database aiming to reduce the computation and I/O requirements for individual queries. This approach maintains a set of keys to resume scrolling by passing keys into the query, effectively amending your filter criteria.
The core idea of Keyset-Filtering is to start retrieving results using a stable sorting order.
Once you want to scroll to the next chunk, you obtain a ScrollPosition
that is used to reconstruct the position within the sorted result.
The ScrollPosition
captures the keyset of the last entity within the current Window
.
To run the query, reconstruction rewrites the criteria clause to include all sort fields and the primary key so that the database can leverage potential indexes to run the query.
The database needs only constructing a much smaller result from the given keyset position without the need to fully materialize a large result and then skipping results until reaching a particular offset.
Keyset-Filtering requires the keyset properties (those used for sorting) to be non-nullable.
This limitation applies due to the store specific |
KeysetScrollPosition
with Repository Query Methodsinterface UserRepository extends Repository<User, Long> {
Window<User> findFirst10ByLastnameOrderByFirstname(String lastname, KeysetScrollPosition position);
}
WindowIterator<User> users = WindowIterator.of(position -> repository.findFirst10ByLastnameOrderByFirstname("Doe", position))
.startingAt(KeysetScrollPosition.initial()); (1)
1 | Start at the very beginning and do not apply additional filtering. |
Keyset-Filtering works best when your database contains an index that matches the sort fields, hence a static sort works well. Scroll queries applying Keyset-Filtering require to the properties used in the sort order to be returned by the query, and these must be mapped in the returned entity.
You can use interface and DTO projections, however make sure to include all properties that you’ve sorted by to avoid keyset extraction failures.
When specifying your Sort
order, it is sufficient to include sort properties relevant to your query;
You do not need to ensure unique query results if you do not want to.
The keyset query mechanism amends your sort order by including the primary key (or any remainder of composite primary keys) to ensure each query result is unique.
4.4.5. Limiting Query Results
You can limit the results of query methods by using the first
or top
keywords, which you can use interchangeably.
You can append an optional numeric value to top
or first
to specify the maximum result size to be returned.
If the number is left out, a result size of 1 is assumed.
The following example shows how to limit the query size:
Top
and First
User findFirstByOrderByLastnameAsc();
User findTopByOrderByAgeDesc();
Page<User> queryFirst10ByLastname(String lastname, Pageable pageable);
Slice<User> findTop3ByLastname(String lastname, Pageable pageable);
List<User> findFirst10ByLastname(String lastname, Sort sort);
List<User> findTop10ByLastname(String lastname, Pageable pageable);
The limiting expressions also support the Distinct
keyword for datastores that support distinct queries.
Also, for the queries that limit the result set to one instance, wrapping the result into with the Optional
keyword is supported.
If pagination or slicing is applied to a limiting query pagination (and the calculation of the number of available pages), it is applied within the limited result.
Limiting the results in combination with dynamic sorting by using a Sort parameter lets you express query methods for the 'K' smallest as well as for the 'K' biggest elements.
|
4.4.6. Repository Methods Returning Collections or Iterables
Query methods that return multiple results can use standard Java Iterable
, List
, and Set
.
Beyond that, we support returning Spring Data’s Streamable
, a custom extension of Iterable
, as well as collection types provided by Vavr.
Refer to the appendix explaining all possible query method return types.
Using Streamable as Query Method Return Type
You can use Streamable
as alternative to Iterable
or any collection type.
It provides convenience methods to access a non-parallel Stream
(missing from Iterable
) and the ability to directly ….filter(…)
and ….map(…)
over the elements and concatenate the Streamable
to others:
interface PersonRepository extends Repository<Person, Long> {
Streamable<Person> findByFirstnameContaining(String firstname);
Streamable<Person> findByLastnameContaining(String lastname);
}
Streamable<Person> result = repository.findByFirstnameContaining("av")
.and(repository.findByLastnameContaining("ea"));
Returning Custom Streamable Wrapper Types
Providing dedicated wrapper types for collections is a commonly used pattern to provide an API for a query result that returns multiple elements. Usually, these types are used by invoking a repository method returning a collection-like type and creating an instance of the wrapper type manually. You can avoid that additional step as Spring Data lets you use these wrapper types as query method return types if they meet the following criteria:
-
The type implements
Streamable
. -
The type exposes either a constructor or a static factory method named
of(…)
orvalueOf(…)
that takesStreamable
as an argument.
The following listing shows an example:
class Product { (1)
MonetaryAmount getPrice() { … }
}
@RequiredArgsConstructor(staticName = "of")
class Products implements Streamable<Product> { (2)
private final Streamable<Product> streamable;
public MonetaryAmount getTotal() { (3)
return streamable.stream()
.map(Priced::getPrice)
.reduce(Money.of(0), MonetaryAmount::add);
}
@Override
public Iterator<Product> iterator() { (4)
return streamable.iterator();
}
}
interface ProductRepository implements Repository<Product, Long> {
Products findAllByDescriptionContaining(String text); (5)
}
1 | A Product entity that exposes API to access the product’s price. |
2 | A wrapper type for a Streamable<Product> that can be constructed by using Products.of(…) (factory method created with the Lombok annotation).
A standard constructor taking the Streamable<Product> will do as well. |
3 | The wrapper type exposes an additional API, calculating new values on the Streamable<Product> . |
4 | Implement the Streamable interface and delegate to the actual result. |
5 | That wrapper type Products can be used directly as a query method return type.
You do not need to return Streamable<Product> and manually wrap it after the query in the repository client. |
Support for Vavr Collections
Vavr is a library that embraces functional programming concepts in Java. It ships with a custom set of collection types that you can use as query method return types, as the following table shows:
Vavr collection type | Used Vavr implementation type | Valid Java source types |
---|---|---|
|
|
|
|
|
|
|
|
|
You can use the types in the first column (or subtypes thereof) as query method return types and get the types in the second column used as implementation type, depending on the Java type of the actual query result (third column).
Alternatively, you can declare Traversable
(the Vavr Iterable
equivalent), and we then derive the implementation class from the actual return value.
That is, a java.util.List
is turned into a Vavr List
or Seq
, a java.util.Set
becomes a Vavr LinkedHashSet
Set
, and so on.
4.4.7. Streaming Query Results
You can process the results of query methods incrementally by using a Java 8 Stream<T>
as the return type.
Instead of wrapping the query results in a Stream
, data store-specific methods are used to perform the streaming, as shown in the following example:
Stream<T>
@Query("select u from User u")
Stream<User> findAllByCustomQueryAndStream();
Stream<User> readAllByFirstnameNotNull();
@Query("select u from User u")
Stream<User> streamAllPaged(Pageable pageable);
A Stream potentially wraps underlying data store-specific resources and must, therefore, be closed after usage.
You can either manually close the Stream by using the close() method or by using a Java 7 try-with-resources block, as shown in the following example:
|
Stream<T>
result in a try-with-resources
blocktry (Stream<User> stream = repository.findAllByCustomQueryAndStream()) {
stream.forEach(…);
}
Not all Spring Data modules currently support Stream<T> as a return type.
|
4.4.8. Null Handling of Repository Methods
As of Spring Data 2.0, repository CRUD methods that return an individual aggregate instance use Java 8’s Optional
to indicate the potential absence of a value.
Besides that, Spring Data supports returning the following wrapper types on query methods:
-
com.google.common.base.Optional
-
scala.Option
-
io.vavr.control.Option
Alternatively, query methods can choose not to use a wrapper type at all.
The absence of a query result is then indicated by returning null
.
Repository methods returning collections, collection alternatives, wrappers, and streams are guaranteed never to return null
but rather the corresponding empty representation.
See “Repository query return types” for details.
Nullability Annotations
You can express nullability constraints for repository methods by using Spring Framework’s nullability annotations.
They provide a tooling-friendly approach and opt-in null
checks during runtime, as follows:
-
@NonNullApi
: Used on the package level to declare that the default behavior for parameters and return values is, respectively, neither to accept nor to producenull
values. -
@NonNull
: Used on a parameter or return value that must not benull
(not needed on a parameter and return value where@NonNullApi
applies). -
@Nullable
: Used on a parameter or return value that can benull
.
Spring annotations are meta-annotated with JSR 305 annotations (a dormant but widely used JSR).
JSR 305 meta-annotations let tooling vendors (such as IDEA, Eclipse, and Kotlin) provide null-safety support in a generic way, without having to hard-code support for Spring annotations.
To enable runtime checking of nullability constraints for query methods, you need to activate non-nullability on the package level by using Spring’s @NonNullApi
in package-info.java
, as shown in the following example:
package-info.java
@org.springframework.lang.NonNullApi
package com.acme;
Once non-null defaulting is in place, repository query method invocations get validated at runtime for nullability constraints.
If a query result violates the defined constraint, an exception is thrown.
This happens when the method would return null
but is declared as non-nullable (the default with the annotation defined on the package in which the repository resides).
If you want to opt-in to nullable results again, selectively use @Nullable
on individual methods.
Using the result wrapper types mentioned at the start of this section continues to work as expected: an empty result is translated into the value that represents absence.
The following example shows a number of the techniques just described:
package com.acme; (1)
interface UserRepository extends Repository<User, Long> {
User getByEmailAddress(EmailAddress emailAddress); (2)
@Nullable
User findByEmailAddress(@Nullable EmailAddress emailAdress); (3)
Optional<User> findOptionalByEmailAddress(EmailAddress emailAddress); (4)
}
1 | The repository resides in a package (or sub-package) for which we have defined non-null behavior. |
2 | Throws an EmptyResultDataAccessException when the query does not produce a result.
Throws an IllegalArgumentException when the emailAddress handed to the method is null . |
3 | Returns null when the query does not produce a result.
Also accepts null as the value for emailAddress . |
4 | Returns Optional.empty() when the query does not produce a result.
Throws an IllegalArgumentException when the emailAddress handed to the method is null . |
Nullability in Kotlin-based Repositories
Kotlin has the definition of nullability constraints baked into the language.
Kotlin code compiles to bytecode, which does not express nullability constraints through method signatures but rather through compiled-in metadata.
Make sure to include the kotlin-reflect
JAR in your project to enable introspection of Kotlin’s nullability constraints.
Spring Data repositories use the language mechanism to define those constraints to apply the same runtime checks, as follows:
interface UserRepository : Repository<User, String> {
fun findByUsername(username: String): User (1)
fun findByFirstname(firstname: String?): User? (2)
}
1 | The method defines both the parameter and the result as non-nullable (the Kotlin default).
The Kotlin compiler rejects method invocations that pass null to the method.
If the query yields an empty result, an EmptyResultDataAccessException is thrown. |
2 | This method accepts null for the firstname parameter and returns null if the query does not produce a result. |
4.4.9. Asynchronous Query Results
You can run repository queries asynchronously by using Spring’s asynchronous method running capability.
This means the method returns immediately upon invocation while the actual query occurs in a task that has been submitted to a Spring TaskExecutor
.
Asynchronous queries differ from reactive queries and should not be mixed.
See the store-specific documentation for more details on reactive support.
The following example shows a number of asynchronous queries:
@Async
Future<User> findByFirstname(String firstname); (1)
@Async
CompletableFuture<User> findOneByFirstname(String firstname); (2)
1 | Use java.util.concurrent.Future as the return type. |
2 | Use a Java 8 java.util.concurrent.CompletableFuture as the return type. |
4.5. Creating Repository Instances
This section covers how to create instances and bean definitions for the defined repository interfaces.
4.5.1. Java Configuration
Use the store-specific @EnableJpaRepositories
annotation on a Java configuration class to define a configuration for repository activation.
For an introduction to Java-based configuration of the Spring container, see JavaConfig in the Spring reference documentation.
A sample configuration to enable Spring Data repositories resembles the following:
@Configuration
@EnableJpaRepositories("com.acme.repositories")
class ApplicationConfiguration {
@Bean
EntityManagerFactory entityManagerFactory() {
// …
}
}
The preceding example uses the JPA-specific annotation, which you would change according to the store module you actually use. The same applies to the definition of the EntityManagerFactory bean. See the sections covering the store-specific configuration.
|
4.5.2. XML Configuration
Each Spring Data module includes a repositories
element that lets you define a base package that Spring scans for you, as shown in the following example:
<?xml version="1.0" encoding="UTF-8"?>
<beans:beans xmlns:beans="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://www.springframework.org/schema/data/jpa"
xsi:schemaLocation="http://www.springframework.org/schema/beans
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/data/jpa
https://www.springframework.org/schema/data/jpa/spring-jpa.xsd">
<jpa:repositories base-package="com.acme.repositories" />
</beans:beans>
In the preceding example, Spring is instructed to scan com.acme.repositories
and all its sub-packages for interfaces extending Repository
or one of its sub-interfaces.
For each interface found, the infrastructure registers the persistence technology-specific FactoryBean
to create the appropriate proxies that handle invocations of the query methods.
Each bean is registered under a bean name that is derived from the interface name, so an interface of UserRepository
would be registered under userRepository
.
Bean names for nested repository interfaces are prefixed with their enclosing type name.
The base package attribute allows wildcards so that you can define a pattern of scanned packages.
4.5.3. Using Filters
By default, the infrastructure picks up every interface that extends the persistence technology-specific Repository
sub-interface located under the configured base package and creates a bean instance for it.
However, you might want more fine-grained control over which interfaces have bean instances created for them.
To do so, use filter elements inside the repository declaration.
The semantics are exactly equivalent to the elements in Spring’s component filters.
For details, see the Spring reference documentation for these elements.
For example, to exclude certain interfaces from instantiation as repository beans, you could use the following configuration:
@Configuration
@EnableJpaRepositories(basePackages = "com.acme.repositories",
includeFilters = { @Filter(type = FilterType.REGEX, pattern = ".*SomeRepository") },
excludeFilters = { @Filter(type = FilterType.REGEX, pattern = ".*SomeOtherRepository") })
class ApplicationConfiguration {
@Bean
EntityManagerFactory entityManagerFactory() {
// …
}
}
<repositories base-package="com.acme.repositories">
<context:exclude-filter type="regex" expression=".*SomeRepository" />
<context:include-filter type="regex" expression=".*SomeOtherRepository" />
</repositories>
The preceding example excludes all interfaces ending in SomeRepository
from being instantiated and includes those ending with SomeOtherRepository
.
4.5.4. Standalone Usage
You can also use the repository infrastructure outside of a Spring container — for example, in CDI environments. You still need some Spring libraries in your classpath, but, generally, you can set up repositories programmatically as well. The Spring Data modules that provide repository support ship with a persistence technology-specific RepositoryFactory
that you can use, as follows:
RepositoryFactorySupport factory = … // Instantiate factory here
UserRepository repository = factory.getRepository(UserRepository.class);
4.6. Custom Implementations for Spring Data Repositories
Spring Data provides various options to create query methods with little coding. But when those options don’t fit your needs you can also provide your own custom implementation for repository methods. This section describes how to do that.
4.6.1. Customizing Individual Repositories
To enrich a repository with custom functionality, you must first define a fragment interface and an implementation for the custom functionality, as follows:
interface CustomizedUserRepository {
void someCustomMethod(User user);
}
class CustomizedUserRepositoryImpl implements CustomizedUserRepository {
public void someCustomMethod(User user) {
// Your custom implementation
}
}
The most important part of the class name that corresponds to the fragment interface is the Impl postfix.
|
The implementation itself does not depend on Spring Data and can be a regular Spring bean.
Consequently, you can use standard dependency injection behavior to inject references to other beans (such as a JdbcTemplate
), take part in aspects, and so on.
Then you can let your repository interface extend the fragment interface, as follows:
interface UserRepository extends CrudRepository<User, Long>, CustomizedUserRepository {
// Declare query methods here
}
Extending the fragment interface with your repository interface combines the CRUD and custom functionality and makes it available to clients.
Spring Data repositories are implemented by using fragments that form a repository composition. Fragments are the base repository, functional aspects (such as QueryDsl), and custom interfaces along with their implementations. Each time you add an interface to your repository interface, you enhance the composition by adding a fragment. The base repository and repository aspect implementations are provided by each Spring Data module.
The following example shows custom interfaces and their implementations:
interface HumanRepository {
void someHumanMethod(User user);
}
class HumanRepositoryImpl implements HumanRepository {
public void someHumanMethod(User user) {
// Your custom implementation
}
}
interface ContactRepository {
void someContactMethod(User user);
User anotherContactMethod(User user);
}
class ContactRepositoryImpl implements ContactRepository {
public void someContactMethod(User user) {
// Your custom implementation
}
public User anotherContactMethod(User user) {
// Your custom implementation
}
}
The following example shows the interface for a custom repository that extends CrudRepository
:
interface UserRepository extends CrudRepository<User, Long>, HumanRepository, ContactRepository {
// Declare query methods here
}
Repositories may be composed of multiple custom implementations that are imported in the order of their declaration. Custom implementations have a higher priority than the base implementation and repository aspects. This ordering lets you override base repository and aspect methods and resolves ambiguity if two fragments contribute the same method signature. Repository fragments are not limited to use in a single repository interface. Multiple repositories may use a fragment interface, letting you reuse customizations across different repositories.
The following example shows a repository fragment and its implementation:
save(…)
interface CustomizedSave<T> {
<S extends T> S save(S entity);
}
class CustomizedSaveImpl<T> implements CustomizedSave<T> {
public <S extends T> S save(S entity) {
// Your custom implementation
}
}
The following example shows a repository that uses the preceding repository fragment:
interface UserRepository extends CrudRepository<User, Long>, CustomizedSave<User> {
}
interface PersonRepository extends CrudRepository<Person, Long>, CustomizedSave<Person> {
}
Configuration
The repository infrastructure tries to autodetect custom implementation fragments by scanning for classes below the package in which it found a repository.
These classes need to follow the naming convention of appending a postfix defaulting to Impl
.
The following example shows a repository that uses the default postfix and a repository that sets a custom value for the postfix:
@EnableJpaRepositories(repositoryImplementationPostfix = "MyPostfix")
class Configuration { … }
<repositories base-package="com.acme.repository" />
<repositories base-package="com.acme.repository" repository-impl-postfix="MyPostfix" />
The first configuration in the preceding example tries to look up a class called com.acme.repository.CustomizedUserRepositoryImpl
to act as a custom repository implementation.
The second example tries to look up com.acme.repository.CustomizedUserRepositoryMyPostfix
.
Resolution of Ambiguity
If multiple implementations with matching class names are found in different packages, Spring Data uses the bean names to identify which one to use.
Given the following two custom implementations for the CustomizedUserRepository
shown earlier, the first implementation is used.
Its bean name is customizedUserRepositoryImpl
, which matches that of the fragment interface (CustomizedUserRepository
) plus the postfix Impl
.
package com.acme.impl.one;
class CustomizedUserRepositoryImpl implements CustomizedUserRepository {
// Your custom implementation
}
package com.acme.impl.two;
@Component("specialCustomImpl")
class CustomizedUserRepositoryImpl implements CustomizedUserRepository {
// Your custom implementation
}
If you annotate the UserRepository
interface with @Component("specialCustom")
, the bean name plus Impl
then matches the one defined for the repository implementation in com.acme.impl.two
, and it is used instead of the first one.
Manual Wiring
If your custom implementation uses annotation-based configuration and autowiring only, the preceding approach shown works well, because it is treated as any other Spring bean. If your implementation fragment bean needs special wiring, you can declare the bean and name it according to the conventions described in the preceding section. The infrastructure then refers to the manually defined bean definition by name instead of creating one itself. The following example shows how to manually wire a custom implementation:
class MyClass {
MyClass(@Qualifier("userRepositoryImpl") UserRepository userRepository) {
…
}
}
<repositories base-package="com.acme.repository" />
<beans:bean id="userRepositoryImpl" class="…">
<!-- further configuration -->
</beans:bean>
4.6.2. Customize the Base Repository
The approach described in the preceding section requires customization of each repository interfaces when you want to customize the base repository behavior so that all repositories are affected. To instead change behavior for all repositories, you can create an implementation that extends the persistence technology-specific repository base class. This class then acts as a custom base class for the repository proxies, as shown in the following example:
class MyRepositoryImpl<T, ID>
extends SimpleJpaRepository<T, ID> {
private final EntityManager entityManager;
MyRepositoryImpl(JpaEntityInformation entityInformation,
EntityManager entityManager) {
super(entityInformation, entityManager);
// Keep the EntityManager around to used from the newly introduced methods.
this.entityManager = entityManager;
}
@Transactional
public <S extends T> S save(S entity) {
// implementation goes here
}
}
The class needs to have a constructor of the super class which the store-specific repository factory implementation uses.
If the repository base class has multiple constructors, override the one taking an EntityInformation plus a store specific infrastructure object (such as an EntityManager or a template class).
|
The final step is to make the Spring Data infrastructure aware of the customized repository base class.
In configuration, you can do so by using the repositoryBaseClass
, as shown in the following example:
@Configuration
@EnableJpaRepositories(repositoryBaseClass = MyRepositoryImpl.class)
class ApplicationConfiguration { … }
<repositories base-package="com.acme.repository"
base-class="….MyRepositoryImpl" />
4.7. Publishing Events from Aggregate Roots
Entities managed by repositories are aggregate roots.
In a Domain-Driven Design application, these aggregate roots usually publish domain events.
Spring Data provides an annotation called @DomainEvents
that you can use on a method of your aggregate root to make that publication as easy as possible, as shown in the following example:
class AnAggregateRoot {
@DomainEvents (1)
Collection<Object> domainEvents() {
// … return events you want to get published here
}
@AfterDomainEventPublication (2)
void callbackMethod() {
// … potentially clean up domain events list
}
}
1 | The method that uses @DomainEvents can return either a single event instance or a collection of events.
It must not take any arguments. |
2 | After all events have been published, we have a method annotated with @AfterDomainEventPublication .
You can use it to potentially clean the list of events to be published (among other uses). |
The methods are called every time one of the following a Spring Data repository methods are called:
-
save(…)
,saveAll(…)
-
delete(…)
,deleteAll(…)
,deleteAllInBatch(…)
,deleteInBatch(…)
Note, that these methods take the aggregate root instances as arguments.
This is why deleteById(…)
is notably absent, as the implementations might choose to issue a query deleting the instance and thus we would never have access to the aggregate instance in the first place.
4.8. Spring Data Extensions
This section documents a set of Spring Data extensions that enable Spring Data usage in a variety of contexts. Currently, most of the integration is targeted towards Spring MVC.
4.8.1. Querydsl Extension
Querydsl is a framework that enables the construction of statically typed SQL-like queries through its fluent API.
Several Spring Data modules offer integration with Querydsl through QuerydslPredicateExecutor
, as the following example shows:
public interface QuerydslPredicateExecutor<T> {
Optional<T> findById(Predicate predicate); (1)
Iterable<T> findAll(Predicate predicate); (2)
long count(Predicate predicate); (3)
boolean exists(Predicate predicate); (4)
// … more functionality omitted.
}
1 | Finds and returns a single entity matching the Predicate . |
2 | Finds and returns all entities matching the Predicate . |
3 | Returns the number of entities matching the Predicate . |
4 | Returns whether an entity that matches the Predicate exists. |
To use the Querydsl support, extend QuerydslPredicateExecutor
on your repository interface, as the following example shows:
interface UserRepository extends CrudRepository<User, Long>, QuerydslPredicateExecutor<User> {
}
The preceding example lets you write type-safe queries by using Querydsl Predicate
instances, as the following example shows:
Predicate predicate = user.firstname.equalsIgnoreCase("dave")
.and(user.lastname.startsWithIgnoreCase("mathews"));
userRepository.findAll(predicate);
4.8.2. Web support
Spring Data modules that support the repository programming model ship with a variety of web support.
The web related components require Spring MVC JARs to be on the classpath.
Some of them even provide integration with Spring HATEOAS.
In general, the integration support is enabled by using the @EnableSpringDataWebSupport
annotation in your JavaConfig configuration class, as the following example shows:
@Configuration
@EnableWebMvc
@EnableSpringDataWebSupport
class WebConfiguration {}
<bean class="org.springframework.data.web.config.SpringDataWebConfiguration" />
<!-- If you use Spring HATEOAS, register this one *instead* of the former -->
<bean class="org.springframework.data.web.config.HateoasAwareSpringDataWebConfiguration" />
The @EnableSpringDataWebSupport
annotation registers a few components.
We discuss those later in this section.
It also detects Spring HATEOAS on the classpath and registers integration components (if present) for it as well.
Basic Web Support
The configuration shown in the previous section registers a few basic components:
-
A Using the
DomainClassConverter
Class to let Spring MVC resolve instances of repository-managed domain classes from request parameters or path variables. -
HandlerMethodArgumentResolver
implementations to let Spring MVC resolvePageable
andSort
instances from request parameters. -
Jackson Modules to de-/serialize types like
Point
andDistance
, or store specific ones, depending on the Spring Data Module used.
Using the DomainClassConverter
Class
The DomainClassConverter
class lets you use domain types in your Spring MVC controller method signatures directly so that you need not manually lookup the instances through the repository, as the following example shows:
@Controller
@RequestMapping("/users")
class UserController {
@RequestMapping("/{id}")
String showUserForm(@PathVariable("id") User user, Model model) {
model.addAttribute("user", user);
return "userForm";
}
}
The method receives a User
instance directly, and no further lookup is necessary.
The instance can be resolved by letting Spring MVC convert the path variable into the id
type of the domain class first and eventually access the instance through calling findById(…)
on the repository instance registered for the domain type.
Currently, the repository has to implement CrudRepository to be eligible to be discovered for conversion.
|
HandlerMethodArgumentResolvers for Pageable and Sort
The configuration snippet shown in the previous section also registers a PageableHandlerMethodArgumentResolver
as well as an instance of SortHandlerMethodArgumentResolver
.
The registration enables Pageable
and Sort
as valid controller method arguments, as the following example shows:
@Controller
@RequestMapping("/users")
class UserController {
private final UserRepository repository;
UserController(UserRepository repository) {
this.repository = repository;
}
@RequestMapping
String showUsers(Model model, Pageable pageable) {
model.addAttribute("users", repository.findAll(pageable));
return "users";
}
}
The preceding method signature causes Spring MVC try to derive a Pageable
instance from the request parameters by using the following default configuration:
|
Page you want to retrieve. 0-indexed and defaults to 0. |
|
Size of the page you want to retrieve. Defaults to 20. |
|
Properties that should be sorted by in the format |
To customize this behavior, register a bean that implements the PageableHandlerMethodArgumentResolverCustomizer
interface or the SortHandlerMethodArgumentResolverCustomizer
interface, respectively.
Its customize()
method gets called, letting you change settings, as the following example shows:
@Bean SortHandlerMethodArgumentResolverCustomizer sortCustomizer() {
return s -> s.setPropertyDelimiter("<-->");
}
If setting the properties of an existing MethodArgumentResolver
is not sufficient for your purpose, extend either SpringDataWebConfiguration
or the HATEOAS-enabled equivalent, override the pageableResolver()
or sortResolver()
methods, and import your customized configuration file instead of using the @Enable
annotation.
If you need multiple Pageable
or Sort
instances to be resolved from the request (for multiple tables, for example), you can use Spring’s @Qualifier
annotation to distinguish one from another.
The request parameters then have to be prefixed with ${qualifier}_
.
The following example shows the resulting method signature:
String showUsers(Model model,
@Qualifier("thing1") Pageable first,
@Qualifier("thing2") Pageable second) { … }
You have to populate thing1_page
, thing2_page
, and so on.
The default Pageable
passed into the method is equivalent to a PageRequest.of(0, 20)
, but you can customize it by using the @PageableDefault
annotation on the Pageable
parameter.
Hypermedia Support for Page
and Slice
Spring HATEOAS ships with a representation model class (PagedModel
/SlicedModel
) that allows enriching the content of a Page
or Slice
instance with the necessary Page
/Slice
metadata as well as links to let the clients easily navigate the pages.
The conversion of a Page
to a PagedModel
is done by an implementation of the Spring HATEOAS RepresentationModelAssembler
interface, called the PagedResourcesAssembler
.
Similarly Slice
instances can be converted to a SlicedModel
using a SlicedResourcesAssembler
.
The following example shows how to use a PagedResourcesAssembler
as a controller method argument, as the SlicedResourcesAssembler
works exactly the same:
@Controller
class PersonController {
private final PersonRepository repository;
// Constructor omitted
@GetMapping("/people")
HttpEntity<PagedModel<Person>> people(Pageable pageable,
PagedResourcesAssembler assembler) {
Page<Person> people = repository.findAll(pageable);
return ResponseEntity.ok(assembler.toModel(people));
}
}
Enabling the configuration, as shown in the preceding example, lets the PagedResourcesAssembler
be used as a controller method argument.
Calling toModel(…)
on it has the following effects:
-
The content of the
Page
becomes the content of thePagedModel
instance. -
The
PagedModel
object gets aPageMetadata
instance attached, and it is populated with information from thePage
and the underlyingPageable
. -
The
PagedModel
may getprev
andnext
links attached, depending on the page’s state. The links point to the URI to which the method maps. The pagination parameters added to the method match the setup of thePageableHandlerMethodArgumentResolver
to make sure the links can be resolved later.
Assume we have 30 Person
instances in the database.
You can now trigger a request (GET http://localhost:8080/people
) and see output similar to the following:
{ "links" : [
{ "rel" : "next", "href" : "http://localhost:8080/persons?page=1&size=20" }
],
"content" : [
… // 20 Person instances rendered here
],
"pageMetadata" : {
"size" : 20,
"totalElements" : 30,
"totalPages" : 2,
"number" : 0
}
}
The JSON envelope format shown here doesn’t follow any formally specified structure and it’s not guaranteed stable and we might change it at any time.
It’s highly recommended to enable the rendering as a hypermedia-enabled, official media type, supported by Spring HATEOAS, like HAL.
Those can be activated by using its @EnableHypermediaSupport annotation.
Find more information in the Spring HATEOAS reference documentation.
|
The assembler produced the correct URI and also picked up the default configuration to resolve the parameters into a Pageable
for an upcoming request.
This means that, if you change that configuration, the links automatically adhere to the change.
By default, the assembler points to the controller method it was invoked in, but you can customize that by passing a custom Link
to be used as base to build the pagination links, which overloads the PagedResourcesAssembler.toModel(…)
method.
Spring Data Jackson Modules
The core module, and some of the store specific ones, ship with a set of Jackson Modules for types, like org.springframework.data.geo.Distance
and org.springframework.data.geo.Point
, used by the Spring Data domain.
Those Modules are imported once web support is enabled and com.fasterxml.jackson.databind.ObjectMapper
is available.
During initialization SpringDataJacksonModules
, like the SpringDataJacksonConfiguration
, get picked up by the infrastructure, so that the declared com.fasterxml.jackson.databind.Module
s are made available to the Jackson ObjectMapper
.
Data binding mixins for the following domain types are registered by the common infrastructure.
org.springframework.data.geo.Distance org.springframework.data.geo.Point org.springframework.data.geo.Box org.springframework.data.geo.Circle org.springframework.data.geo.Polygon
The individual module may provide additional |
Web Databinding Support
You can use Spring Data projections (described in Projections) to bind incoming request payloads by using either JSONPath expressions (requires Jayway JsonPath) or XPath expressions (requires XmlBeam), as the following example shows:
@ProjectedPayload
public interface UserPayload {
@XBRead("//firstname")
@JsonPath("$..firstname")
String getFirstname();
@XBRead("/lastname")
@JsonPath({ "$.lastname", "$.user.lastname" })
String getLastname();
}
You can use the type shown in the preceding example as a Spring MVC handler method argument or by using ParameterizedTypeReference
on one of methods of the RestTemplate
.
The preceding method declarations would try to find firstname
anywhere in the given document.
The lastname
XML lookup is performed on the top-level of the incoming document.
The JSON variant of that tries a top-level lastname
first but also tries lastname
nested in a user
sub-document if the former does not return a value.
That way, changes in the structure of the source document can be mitigated easily without having clients calling the exposed methods (usually a drawback of class-based payload binding).
Nested projections are supported as described in Projections.
If the method returns a complex, non-interface type, a Jackson ObjectMapper
is used to map the final value.
For Spring MVC, the necessary converters are registered automatically as soon as @EnableSpringDataWebSupport
is active and the required dependencies are available on the classpath.
For usage with RestTemplate
, register a ProjectingJackson2HttpMessageConverter
(JSON) or XmlBeamHttpMessageConverter
manually.
For more information, see the web projection example in the canonical Spring Data Examples repository.
Querydsl Web Support
For those stores that have QueryDSL integration, you can derive queries from the attributes contained in a Request
query string.
Consider the following query string:
?firstname=Dave&lastname=Matthews
Given the User
object from the previous examples, you can resolve a query string to the following value by using the QuerydslPredicateArgumentResolver
, as follows:
QUser.user.firstname.eq("Dave").and(QUser.user.lastname.eq("Matthews"))
The feature is automatically enabled, along with @EnableSpringDataWebSupport , when Querydsl is found on the classpath.
|
Adding a @QuerydslPredicate
to the method signature provides a ready-to-use Predicate
, which you can run by using the QuerydslPredicateExecutor
.
Type information is typically resolved from the method’s return type.
Since that information does not necessarily match the domain type, it might be a good idea to use the root attribute of QuerydslPredicate .
|
The following example shows how to use @QuerydslPredicate
in a method signature:
@Controller
class UserController {
@Autowired UserRepository repository;
@RequestMapping(value = "/", method = RequestMethod.GET)
String index(Model model, @QuerydslPredicate(root = User.class) Predicate predicate, (1)
Pageable pageable, @RequestParam MultiValueMap<String, String> parameters) {
model.addAttribute("users", repository.findAll(predicate, pageable));
return "index";
}
}
1 | Resolve query string arguments to matching Predicate for User . |
The default binding is as follows:
-
Object
on simple properties aseq
. -
Object
on collection like properties ascontains
. -
Collection
on simple properties asin
.
You can customize those bindings through the bindings
attribute of @QuerydslPredicate
or by making use of Java 8 default methods
and adding the QuerydslBinderCustomizer
method to the repository interface, as follows:
interface UserRepository extends CrudRepository<User, String>,
QuerydslPredicateExecutor<User>, (1)
QuerydslBinderCustomizer<QUser> { (2)
@Override
default void customize(QuerydslBindings bindings, QUser user) {
bindings.bind(user.username).first((path, value) -> path.contains(value)) (3)
bindings.bind(String.class)
.first((StringPath path, String value) -> path.containsIgnoreCase(value)); (4)
bindings.excluding(user.password); (5)
}
}
1 | QuerydslPredicateExecutor provides access to specific finder methods for Predicate . |
2 | QuerydslBinderCustomizer defined on the repository interface is automatically picked up and shortcuts @QuerydslPredicate(bindings=…) . |
3 | Define the binding for the username property to be a simple contains binding. |
4 | Define the default binding for String properties to be a case-insensitive contains match. |
5 | Exclude the password property from Predicate resolution. |
You can register a QuerydslBinderCustomizerDefaults bean holding default Querydsl bindings before applying specific bindings from the repository or @QuerydslPredicate .
|
4.8.3. Repository Populators
If you work with the Spring JDBC module, you are probably familiar with the support for populating a DataSource
with SQL scripts.
A similar abstraction is available on the repositories level, although it does not use SQL as the data definition language because it must be store-independent.
Thus, the populators support XML (through Spring’s OXM abstraction) and JSON (through Jackson) to define data with which to populate the repositories.
Assume you have a file called data.json
with the following content:
[ { "_class" : "com.acme.Person",
"firstname" : "Dave",
"lastname" : "Matthews" },
{ "_class" : "com.acme.Person",
"firstname" : "Carter",
"lastname" : "Beauford" } ]
You can populate your repositories by using the populator elements of the repository namespace provided in Spring Data Commons.
To populate the preceding data to your PersonRepository
, declare a populator similar to the following:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:repository="http://www.springframework.org/schema/data/repository"
xsi:schemaLocation="http://www.springframework.org/schema/beans
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/data/repository
https://www.springframework.org/schema/data/repository/spring-repository.xsd">
<repository:jackson2-populator locations="classpath:data.json" />
</beans>
The preceding declaration causes the data.json
file to be read and deserialized by a Jackson ObjectMapper
.
The type to which the JSON object is unmarshalled is determined by inspecting the _class
attribute of the JSON document.
The infrastructure eventually selects the appropriate repository to handle the object that was deserialized.
To instead use XML to define the data the repositories should be populated with, you can use the unmarshaller-populator
element.
You configure it to use one of the XML marshaller options available in Spring OXM. See the Spring reference documentation for details.
The following example shows how to unmarshall a repository populator with JAXB:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:repository="http://www.springframework.org/schema/data/repository"
xmlns:oxm="http://www.springframework.org/schema/oxm"
xsi:schemaLocation="http://www.springframework.org/schema/beans
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/data/repository
https://www.springframework.org/schema/data/repository/spring-repository.xsd
http://www.springframework.org/schema/oxm
https://www.springframework.org/schema/oxm/spring-oxm.xsd">
<repository:unmarshaller-populator locations="classpath:data.json"
unmarshaller-ref="unmarshaller" />
<oxm:jaxb2-marshaller contextPath="com.acme" />
</beans>
5. Projections
Spring Data query methods usually return one or multiple instances of the aggregate root managed by the repository. However, it might sometimes be desirable to create projections based on certain attributes of those types. Spring Data allows modeling dedicated return types, to more selectively retrieve partial views of the managed aggregates.
Imagine a repository and aggregate root type such as the following example:
class Person {
@Id UUID id;
String firstname, lastname;
Address address;
static class Address {
String zipCode, city, street;
}
}
interface PersonRepository extends Repository<Person, UUID> {
Collection<Person> findByLastname(String lastname);
}
Now imagine that we want to retrieve the person’s name attributes only. What means does Spring Data offer to achieve this? The rest of this chapter answers that question.
5.1. Interface-based Projections
The easiest way to limit the result of the queries to only the name attributes is by declaring an interface that exposes accessor methods for the properties to be read, as shown in the following example:
interface NamesOnly {
String getFirstname();
String getLastname();
}
The important bit here is that the properties defined here exactly match properties in the aggregate root. Doing so lets a query method be added as follows:
interface PersonRepository extends Repository<Person, UUID> {
Collection<NamesOnly> findByLastname(String lastname);
}
The query execution engine creates proxy instances of that interface at runtime for each element returned and forwards calls to the exposed methods to the target object.
Declaring a method in your Repository that overrides a base method (e.g. declared in CrudRepository , a store-specific repository interface, or the Simple…Repository ) results in a call to the base method regardless of the declared return type. Make sure to use a compatible return type as base methods cannot be used for projections. Some store modules support @Query annotations to turn an overridden base method into a query method that then can be used to return projections.
|
Projections can be used recursively. If you want to include some of the Address
information as well, create a projection interface for that and return that interface from the declaration of getAddress()
, as shown in the following example:
interface PersonSummary {
String getFirstname();
String getLastname();
AddressSummary getAddress();
interface AddressSummary {
String getCity();
}
}
On method invocation, the address
property of the target instance is obtained and wrapped into a projecting proxy in turn.
5.1.1. Closed Projections
A projection interface whose accessor methods all match properties of the target aggregate is considered to be a closed projection. The following example (which we used earlier in this chapter, too) is a closed projection:
interface NamesOnly {
String getFirstname();
String getLastname();
}
If you use a closed projection, Spring Data can optimize the query execution, because we know about all the attributes that are needed to back the projection proxy. For more details on that, see the module-specific part of the reference documentation.
5.1.2. Open Projections
Accessor methods in projection interfaces can also be used to compute new values by using the @Value
annotation, as shown in the following example:
interface NamesOnly {
@Value("#{target.firstname + ' ' + target.lastname}")
String getFullName();
…
}
The aggregate root backing the projection is available in the target
variable.
A projection interface using @Value
is an open projection.
Spring Data cannot apply query execution optimizations in this case, because the SpEL expression could use any attribute of the aggregate root.
The expressions used in @Value
should not be too complex — you want to avoid programming in String
variables.
For very simple expressions, one option might be to resort to default methods (introduced in Java 8), as shown in the following example:
interface NamesOnly {
String getFirstname();
String getLastname();
default String getFullName() {
return getFirstname().concat(" ").concat(getLastname());
}
}
This approach requires you to be able to implement logic purely based on the other accessor methods exposed on the projection interface. A second, more flexible, option is to implement the custom logic in a Spring bean and then invoke that from the SpEL expression, as shown in the following example:
@Component
class MyBean {
String getFullName(Person person) {
…
}
}
interface NamesOnly {
@Value("#{@myBean.getFullName(target)}")
String getFullName();
…
}
Notice how the SpEL expression refers to myBean
and invokes the getFullName(…)
method and forwards the projection target as a method parameter.
Methods backed by SpEL expression evaluation can also use method parameters, which can then be referred to from the expression.
The method parameters are available through an Object
array named args
. The following example shows how to get a method parameter from the args
array:
interface NamesOnly {
@Value("#{args[0] + ' ' + target.firstname + '!'}")
String getSalutation(String prefix);
}
Again, for more complex expressions, you should use a Spring bean and let the expression invoke a method, as described earlier.
5.1.3. Nullable Wrappers
Getters in projection interfaces can make use of nullable wrappers for improved null-safety. Currently supported wrapper types are:
-
java.util.Optional
-
com.google.common.base.Optional
-
scala.Option
-
io.vavr.control.Option
interface NamesOnly {
Optional<String> getFirstname();
}
If the underlying projection value is not null
, then values are returned using the present-representation of the wrapper type.
In case the backing value is null
, then the getter method returns the empty representation of the used wrapper type.
5.2. Class-based Projections (DTOs)
Another way of defining projections is by using value type DTOs (Data Transfer Objects) that hold properties for the fields that are supposed to be retrieved. These DTO types can be used in exactly the same way projection interfaces are used, except that no proxying happens and no nested projections can be applied.
If the store optimizes the query execution by limiting the fields to be loaded, the fields to be loaded are determined from the parameter names of the constructor that is exposed.
The following example shows a projecting DTO:
record NamesOnly(String firstname, String lastname) {
}
Java Records are ideal to define DTO types since they adhere to value semantics:
All fields are private final
and equals(…)
/hashCode()
/toString()
methods are created automatically.
Alternatively, you can use any class that defines the properties you want to project.
5.3. Dynamic Projections
So far, we have used the projection type as the return type or element type of a collection. However, you might want to select the type to be used at invocation time (which makes it dynamic). To apply dynamic projections, use a query method such as the one shown in the following example:
interface PersonRepository extends Repository<Person, UUID> {
<T> Collection<T> findByLastname(String lastname, Class<T> type);
}
This way, the method can be used to obtain the aggregates as is or with a projection applied, as shown in the following example:
void someMethod(PersonRepository people) {
Collection<Person> aggregates =
people.findByLastname("Matthews", Person.class);
Collection<NamesOnly> aggregates =
people.findByLastname("Matthews", NamesOnly.class);
}
Query parameters of type Class are inspected whether they qualify as dynamic projection parameter.
If the actual return type of the query equals the generic parameter type of the Class parameter, then the matching Class parameter is not available for usage within the query or SpEL expressions.
If you want to use a Class parameter as query argument then make sure to use a different generic parameter, for example Class<?> .
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6. Query by Example
6.1. Introduction
This chapter provides an introduction to Query by Example and explains how to use it.
Query by Example (QBE) is a user-friendly querying technique with a simple interface. It allows dynamic query creation and does not require you to write queries that contain field names. In fact, Query by Example does not require you to write queries by using store-specific query languages at all.
6.2. Usage
The Query by Example API consists of four parts:
-
Probe: The actual example of a domain object with populated fields.
-
ExampleMatcher
: TheExampleMatcher
carries details on how to match particular fields. It can be reused across multiple Examples. -
Example
: AnExample
consists of the probe and theExampleMatcher
. It is used to create the query. -
FetchableFluentQuery
: AFetchableFluentQuery
offers a fluent API, that allows further customization of a query derived from anExample
. Using the fluent API lets you to specify ordering projection and result processing for your query.
Query by Example is well suited for several use cases:
-
Querying your data store with a set of static or dynamic constraints.
-
Frequent refactoring of the domain objects without worrying about breaking existing queries.
-
Working independently from the underlying data store API.
Query by Example also has several limitations:
-
No support for nested or grouped property constraints, such as
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, create an interface for your repository, as shown in the following example:
public class Person {
@Id
private String id;
private String firstname;
private String lastname;
private Address address;
// … getters and setters omitted
}
The preceding example shows a simple domain object.
You can use it to create an Example
.
By default, fields having null
values are ignored, and strings are matched by using the store specific defaults.
Inclusion of properties into a Query by Example criteria is based on nullability.
Properties using primitive types (int , double , …) are always included unless the ExampleMatcher ignores the property path.
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Examples can be built by either using the of
factory method or by using ExampleMatcher
. Example
is immutable.
The following listing shows a simple Example:
Person person = new Person(); (1)
person.setFirstname("Dave"); (2)
Example<Person> example = Example.of(person); (3)
1 | Create a new instance of the domain object. |
2 | Set the properties to query. |
3 | Create the Example . |
You can run the example queries by using repositories.
To do so, let your repository interface extend QueryByExampleExecutor<T>
.
The following listing shows an excerpt from the QueryByExampleExecutor
interface:
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.
}
6.3. Example Matchers
Examples are not limited to default settings.
You can specify your own defaults for string matching, null handling, and property-specific settings by using the ExampleMatcher
, as shown in the following example:
Person person = new Person(); (1)
person.setFirstname("Dave"); (2)
ExampleMatcher matcher = ExampleMatcher.matching() (3)
.withIgnorePaths("lastname") (4)
.withIncludeNullValues() (5)
.withStringMatcher(StringMatcher.ENDING); (6)
Example<Person> example = Example.of(person, matcher); (7)
1 | Create a new instance of the domain object. |
2 | Set properties. |
3 | Create an ExampleMatcher to expect all values to match.
It is usable at this stage even without further configuration. |
4 | Construct a new ExampleMatcher to ignore the lastname property path. |
5 | Construct a new ExampleMatcher to ignore the lastname property path and to include null values. |
6 | Construct a new ExampleMatcher to ignore the lastname property path, to include null values, and to perform suffix string matching. |
7 | Create a new Example based on the domain object and the configured ExampleMatcher . |
By default, the ExampleMatcher
expects all values set on the probe to match.
If you want to get results matching any of the predicates defined implicitly, use ExampleMatcher.matchingAny()
.
You can specify behavior for individual properties (such as "firstname" and "lastname" or, for nested properties, "address.city"). You can tune it with matching options and case sensitivity, as shown in the following example:
ExampleMatcher matcher = ExampleMatcher.matching()
.withMatcher("firstname", endsWith())
.withMatcher("lastname", startsWith().ignoreCase());
}
Another way to configure matcher options is to use lambdas (introduced in Java 8). This approach creates a callback that asks the implementor to modify the matcher. You need not return the matcher, because configuration options are held within the matcher instance. The following example shows a matcher that uses 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 the 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.
The following table describes the scope of the various ExampleMatcher
settings:
Setting | Scope |
---|---|
Null-handling |
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String matching |
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Ignoring properties |
Property path |
Case sensitivity |
|
Value transformation |
Property path |
6.4. Fluent API
QueryByExampleExecutor
offers one more method, which we did not mention so far: <S extends T, R> R findBy(Example<S> example, Function<FluentQuery.FetchableFluentQuery<S>, R> queryFunction)
.
As with other methods, it executes a query derived from an Example
.
However, with the second argument, you can control aspects of that execution that you cannot dynamically control otherwise.
You do so by invoking the various methods of the FetchableFluentQuery
in the second argument.
sortBy
lets you specify an ordering for your result.
as
lets you specify the type to which you want the result to be transformed.
project
limits the queried attributes.
first
, firstValue
, one
, oneValue
, all
, page
, stream
, count
, and exists
define what kind of result you get and how the query behaves when more than the expected number of results are available.
Optional<Person> match = repository.findBy(example,
q -> q
.sortBy(Sort.by("lastname").descending())
.first()
);
7. Auditing
7.1. Basics
Spring Data provides sophisticated support to transparently keep track of who created or changed an entity and when the change 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. Additionally, auditing has to be enabled either through Annotation configuration or XML configuration to register the required infrastructure components. Please refer to the store-specific section for configuration samples.
Applications that only track creation and modification dates are not required do make their entities implement |
7.1.1. Annotation-based Auditing Metadata
We provide @CreatedBy
and @LastModifiedBy
to capture the user who created or modified the entity as well as @CreatedDate
and @LastModifiedDate
to capture when the change happened.
class Customer {
@CreatedBy
private User user;
@CreatedDate
private Instant createdDate;
// … further properties omitted
}
As you can see, the annotations can be applied selectively, depending on which information you want to capture.
The annotations, indicating to capture when changes are made, can be used on properties of type JDK8 date and time types, long
, Long
, and legacy Java Date
and Calendar
.
Auditing metadata does not necessarily need to live in the root level entity but can be added to an embedded one (depending on the actual store in use), as shown in the snippet below.
class Customer {
private AuditMetadata auditingMetadata;
// … further properties omitted
}
class AuditMetadata {
@CreatedBy
private User user;
@CreatedDate
private Instant createdDate;
}
7.1.2. Interface-based Auditing Metadata
In case you do not 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.
7.1.3. 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 what type the properties annotated with @CreatedBy
or @LastModifiedBy
have to be.
The following example shows an implementation of the interface that uses Spring Security’s Authentication
object:
AuditorAware
based on Spring Securityclass SpringSecurityAuditorAware implements AuditorAware<User> {
@Override
public Optional<User> getCurrentAuditor() {
return Optional.ofNullable(SecurityContextHolder.getContext())
.map(SecurityContext::getAuthentication)
.filter(Authentication::isAuthenticated)
.map(Authentication::getPrincipal)
.map(User.class::cast);
}
}
The implementation accesses the Authentication
object provided by Spring Security and looks up the custom UserDetails
instance that you have created in your UserDetailsService
implementation. We assume here that you are exposing the domain user through the UserDetails
implementation but that, based on the Authentication
found, you could also look it up from anywhere.
7.1.4. ReactiveAuditorAware
When using reactive infrastructure you might want to make use of contextual information to provide @CreatedBy
or @LastModifiedBy
information.
We provide an ReactiveAuditorAware<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 what type the properties annotated with @CreatedBy
or @LastModifiedBy
have to be.
The following example shows an implementation of the interface that uses reactive Spring Security’s Authentication
object:
ReactiveAuditorAware
based on Spring Securityclass SpringSecurityAuditorAware implements ReactiveAuditorAware<User> {
@Override
public Mono<User> getCurrentAuditor() {
return ReactiveSecurityContextHolder.getContext()
.map(SecurityContext::getAuthentication)
.filter(Authentication::isAuthenticated)
.map(Authentication::getPrincipal)
.map(User.class::cast);
}
}
The implementation accesses the Authentication
object provided by Spring Security and looks up the custom UserDetails
instance that you have created in your UserDetailsService
implementation. We assume here that you are exposing the domain user through the UserDetails
implementation but that, based on the Authentication
found, you could also look it up from anywhere.
Appendices
Appendix A: Namespace reference
The <repositories />
Element
The <repositories />
element triggers the setup of the Spring Data repository infrastructure. The most important attribute is base-package
, which defines the package to scan for Spring Data repository interfaces. See “XML Configuration”. The following table describes the attributes of the <repositories />
element:
Name | Description |
---|---|
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Defines the package to be scanned for repository interfaces that extend |
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Defines the postfix to autodetect custom repository implementations. Classes whose names end with the configured postfix are considered as candidates. Defaults to |
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Determines the strategy to be used to create finder queries. See “Query Lookup Strategies” for details. Defaults to |
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Defines the location to search for a Properties file containing externally defined queries. |
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Whether nested repository interface definitions should be considered. Defaults to |
Appendix B: Populators namespace reference
The <populator /> element
The <populator />
element allows to populate a data store via the Spring Data repository infrastructure.[1]
Name | Description |
---|---|
|
Where to find the files to read the objects from the repository shall be populated with. |
Appendix C: Repository query keywords
Supported query method subject keywords
The following table lists the subject keywords generally supported by the Spring Data repository query derivation mechanism to express the predicate. Consult the store-specific documentation for the exact list of supported keywords, because some keywords listed here might not be supported in a particular store.
Keyword | Description |
---|---|
|
General query method returning typically the repository type, a |
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Exists projection, returning typically a |
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Count projection returning a numeric result. |
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Delete query method returning either no result ( |
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Limit the query results to the first |
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Use a distinct query to return only unique results. Consult the store-specific documentation whether that feature is supported. This keyword can occur in any place of the subject between |
Supported query method predicate keywords and modifiers
The following table lists the predicate keywords generally supported by the Spring Data repository query derivation mechanism. However, consult the store-specific documentation for the exact list of supported keywords, because some keywords listed here might not be supported in a particular store.
Logical keyword | Keyword expressions |
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In addition to filter predicates, the following list of modifiers is supported:
Keyword | Description |
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Used with a predicate keyword for case-insensitive comparison. |
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Ignore case for all suitable properties. Used somewhere in the query method predicate. |
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Specify a static sorting order followed by the property path and direction (e. g. |
Appendix D: Repository query return types
Supported Query Return Types
The following table lists the return types generally supported by Spring Data repositories. However, consult the store-specific documentation for the exact list of supported return types, because some types listed here might not be supported in a particular store.
Geospatial types (such as GeoResult , GeoResults , and GeoPage ) are available only for data stores that support geospatial queries.
Some store modules may define their own result wrapper types.
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Return type | Description |
---|---|
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Denotes no return value. |
Primitives |
Java primitives. |
Wrapper types |
Java wrapper types. |
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A unique entity. Expects the query method to return one result at most. If no result is found, |
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An |
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A |
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A |
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A Java 8 or Guava |
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Either a Scala or Vavr |
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A Java 8 |
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A convenience extension of |
Types that implement |
Types that expose a constructor or |
Vavr |
Vavr collection types. See Support for Vavr Collections for details. |
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A |
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A Java 8 |
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A sized chunk of data with an indication of whether there is more data available. Requires a |
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A |
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A result entry with additional information, such as the distance to a reference location. |
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A list of |
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A |
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A Project Reactor |
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A Project Reactor |
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A RxJava |
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A RxJava |
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A RxJava |