© 2018 The original authors.
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Preface
Project Metadata
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Version control: http://github.com/spring-projects/spring-data-jdbc
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Bugtracker: https://jira.spring.io/browse/DATAJDBC
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Release repository: https://repo.spring.io/libs-release
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Milestone repository: https://repo.spring.io/libs-milestone
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Snapshot repository: https://repo.spring.io/libs-snapshot
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-releasetrain</artifactId>
<version>Lovelace-SR3</version>
<scope>import</scope>
<type>pom</type>
</dependency>
</dependencies>
</dependencyManagement>The current release train version is Lovelace-SR3. The train names ascend alphabetically and the currently available trains are listed here. The version name follows the following pattern: ${name}-${release}, where release can be one of the following:
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BUILD-SNAPSHOT: Current snapshots -
M1,M2, and so on: Milestones -
RC1,RC2, and so on: Release candidates -
RELEASE: GA release -
SR1,SR2, and so on: Service releases
A working example of using the BOMs can be found 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 Spring Data modules for you. If you still want to upgrade to a newer version, configure the property spring-data-releasetrain.version to the train name and iteration you would like to use.
3. 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.
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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 Java Persistence API (JPA) module. 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 supporting 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. |
3.1. Core concepts
The central interface in the Spring Data repository abstraction is Repository. It takes the domain class to manage as well as the ID 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 provides sophisticated CRUD functionality for the entity class that is being managed.
CrudRepository interfacepublic interface CrudRepository<T, ID extends Serializable>
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. |
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.
|
On top of the CrudRepository, there is a PagingAndSortingRepository abstraction that adds additional methods to ease paginated access to entities:
PagingAndSortingRepository interfacepublic interface PagingAndSortingRepository<T, ID extends Serializable>
extends CrudRepository<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(new PageRequest(1, 20));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 list shows the interface definition for a derived delete query:
interface UserRepository extends CrudRepository<User, Long> {
long deleteByLastname(String lastname);
List<User> removeByLastname(String lastname);
}3.2. Query methods
Standard CRUD functionality repositories usually have queries on the underlying datastore. With Spring Data, declaring those queries becomes a four-step process:
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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.
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To use Java configuration, create a class similar to the following:
import org.springframework.data.jpa.repository.config.EnableJpaRepositories; @EnableJpaRepositories class Config {} -
To use XML configuration, define a bean 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:jpa="http://www.springframework.org/schema/data/jpa" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/data/jpa http://www.springframework.org/schema/data/jpa/spring-jpa.xsd"> <jpa: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
jpain favor of, for example,mongodb.+ Also, 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@Enable${store}Repositories-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:
3.3. Defining Repository Interfaces
First, 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, extend CrudRepository instead of Repository.
3.3.1. Fine-tuning Repository Definition
Typically, your repository interface extends Repository, CrudRepository, or PagingAndSortingRepository. Alternatively, if you do not want to extend Spring Data interfaces, you can also annotate your repository interface with @RepositoryDefinition. Extending CrudRepository 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 CrudRepository into your domain repository.
| Doing so lets you define your own abstractions on top of the provided Spring Data Repositories functionality. |
The following example shows how to selectively expose CRUD methods (findById and save, in this case):
@NoRepositoryBean
interface MyBaseRepository<T, ID extends Serializable> 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.
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3.3.2. 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:
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com.google.common.base.Optional -
scala.Option -
io.vavr.control.Option -
javaslang.control.Option(deprecated as Javaslang is deprecated)
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:
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@NonNullApi: Used on the package level to declare that the default behavior for parameters and return values is to not accept or producenullvalues. -
@NonNull: Used on a parameter or return value that must not benull(not needed on a parameter and return value where@NonNullApiapplies). -
@Nullable: Used on a parameter or return value that can benull.
Spring annotations are meta-annotated with JSR 305 annotations (a dormant but widely spread 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 execution 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 the repository resides in).
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)
import org.springframework.lang.Nullable;
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 executed does not produce a result. Throws an IllegalArgumentException when the emailAddress handed to the method is null. |
| 3 | Returns null when the query executed does not produce a result. Also accepts null as the value for emailAddress. |
| 4 | Returns Optional.empty() when the query executed 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 execution yields an empty result, an EmptyResultDataAccessException is thrown. |
| 2 | This method accepts null for the firstname parameter and returns null if the query execution does not produce a result. |
3.3.3. 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, then it is a valid candidate for the particular Spring Data module.
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If the domain class is annotated with the module-specific type annotation, then 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@Documentfor 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 Serializable> 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 Serializable> 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 perfectly 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")
interface Configuration { }3.4. Defining Query Methods
The repository proxy has two ways to derive a store-specific query from the method name:
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By deriving the query from the method name directly.
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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.
3.4.1. 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 Enable${store}Repositories annotation. Some strategies may not be supported for particular datastores.
-
CREATEattempts 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_QUERYtries to find a declared query and throws an exception if cannot find one. The query can be defined by an annotation somewhere or declared by other means. Consult 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(default) combinesCREATEandUSE_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.
3.4.2. Query Creation
The query builder mechanism built into Spring Data repository infrastructure is useful for building constraining queries over entities of the repository. The mechanism strips the prefixes find…By, read…By, query…By, count…By, and get…By from the method and starts parsing the rest of it. The introducing clause can contain further expressions, such as a Distinct to set a distinct flag on the query to be created. However, the first By acts as delimiter to indicate the start of the actual criteria. At a very basic level, you can define conditions on entity properties and concatenate them with And and Or. The following example shows how to create a number of queries:
interface PersonRepository extends Repository<User, 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);
}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
ANDandOR. You also get support for operators such asBetween,LessThan,GreaterThan, andLikefor 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
IgnoreCaseflag for individual properties (for example,findByLastnameIgnoreCase(…)) or for all properties of a type that supports ignoring case (usuallyStringinstances — 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
OrderByclause to the query method that references a property and by providing a sorting direction (AscorDesc). To create a query method that supports dynamic sorting, see “Special parameter handling”.
3.4.3. 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 property traversal x.address.zipCode. 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).
3.4.4. Special parameter handling
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 and Sort, to apply pagination and sorting to your queries dynamically. The following example demonstrates these features:
Pageable, Slice, and Sort in query methodsPage<User> findByLastname(String lastname, Pageable pageable);
Slice<User> findByLastname(String lastname, Pageable pageable);
List<User> findByLastname(String lastname, Sort sort);
List<User> findByLastname(String lastname, Pageable pageable);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 only knows 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 only need 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. |
3.4.5. Limiting Query Results
The results of query methods can be limited by using the first or top keywords, which can be used interchangeably. An optional numeric value can be appended 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 FirstUser 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. Also, for the queries limiting 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 pages available), 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.
|
3.4.6. Streaming query results
The results of query methods can be processed incrementally by using a Java 8 Stream<T> as 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.
|
3.4.7. Async query results
Repository queries can be run asynchronously by using Spring’s asynchronous method execution capability. This means the method returns immediately upon invocation while the actual query execution occurs in a task that has been submitted to a Spring TaskExecutor. Asynchronous query execution is different from reactive query execution and should not be mixed. Refer to 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)
@Async
ListenableFuture<User> findOneByLastname(String lastname); (3)| 1 | Use java.util.concurrent.Future as the return type. |
| 2 | Use a Java 8 java.util.concurrent.CompletableFuture as the return type. |
| 3 | Use a org.springframework.util.concurrent.ListenableFuture as the return type. |
3.5. Creating Repository Instances
In this section, you create instances and bean definitions for the defined repository interfaces. One way to do so is by using the Spring namespace that is shipped with each Spring Data module that supports the repository mechanism, although we generally recommend using Java configuration.
3.5.1. 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
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/data/jpa
http://www.springframework.org/schema/data/jpa/spring-jpa.xsd">
<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. The base-package attribute allows wildcards so that you can define a pattern of scanned packages.
Using filters
By default, the infrastructure picks up every interface extending 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 <include-filter /> and <exclude-filter /> elements inside the <repositories /> element. The semantics are exactly equivalent to the elements in Spring’s context namespace. 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:
<repositories base-package="com.acme.repositories">
<context:exclude-filter type="regex" expression=".*SomeRepository" />
</repositories>The preceding example excludes all interfaces ending in SomeRepository from being instantiated.
3.5.2. JavaConfig
The repository infrastructure can also be triggered by using a store-specific @Enable${store}Repositories annotation on a JavaConfig class. For an introduction into 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.
|
3.5.3. 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 a persistence technology-specific RepositoryFactory that you can use as follows:
RepositoryFactorySupport factory = … // Instantiate factory here
UserRepository repository = factory.getRepository(UserRepository.class);3.6. Custom Implementations for Spring Data Repositories
This section covers repository customization and how fragments form a composite repository.
When a query method requires a different behavior or cannot be implemented by query derivation, then it is necessary to provide a custom implementation. Spring Data repositories let you provide custom repository code and integrate it with generic CRUD abstraction and query method functionality.
3.6.1. 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 shown in the following example:
interface CustomizedUserRepository {
void someCustomMethod(User user);
}Then you can let your repository interface additionally extend from the fragment interface, as shown in the following example:
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.
You can let your repository interface extend the fragment interface, as shown in the following example:
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 implementation. 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
If you use namespace 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 the namespace element’s repository-impl-postfix attribute to the fragment interface name. This postfix defaults to Impl. The following example shows a repository that uses the default postfix and a repository that sets a custom value for the postfix:
<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 lookup 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:
<repositories base-package="com.acme.repository" />
<beans:bean id="userRepositoryImpl" class="…">
<!-- further configuration -->
</beans:bean>3.6.2. 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 Serializable>
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 Java configuration, you can do so by using the repositoryBaseClass attribute of the @Enable${store}Repositories annotation, as shown in the following example:
@Configuration
@EnableJpaRepositories(repositoryBaseClass = MyRepositoryImpl.class)
class ApplicationConfiguration { … }A corresponding attribute is available in the XML namespace, as shown in the following example:
<repositories base-package="com.acme.repository"
base-class="….MyRepositoryImpl" />3.7. 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 using @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. It can be used to potentially clean the list of events to be published (among other uses). |
The methods are called every time one of a Spring Data repository’s save(…) methods is called.
3.8. 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.
3.8.1. 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 shown in the following example:
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 make use of Querydsl support, extend QuerydslPredicateExecutor on your repository interface, as shown in the following example
interface UserRepository extends CrudRepository<User, Long>, QuerydslPredicateExecutor<User> {
}The preceding example lets you write typesafe queries using Querydsl Predicate instances, as shown in the following example:
Predicate predicate = user.firstname.equalsIgnoreCase("dave")
.and(user.lastname.startsWithIgnoreCase("mathews"));
userRepository.findAll(predicate);3.8.2. Web support
| This section contains the documentation for the Spring Data web support as it is implemented in the current (and later) versions of Spring Data Commons. As the newly introduced support changes many things, we kept the documentation of the former behavior in [web.legacy]. |
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 shown in the following example:
@Configuration
@EnableWebMvc
@EnableSpringDataWebSupport
class WebConfiguration {}The @EnableSpringDataWebSupport annotation registers a few components we will discuss in a bit. It will also detect Spring HATEOAS on the classpath and register integration components for it as well if present.
Alternatively, if you use XML configuration, register either SpringDataWebConfiguration or HateoasAwareSpringDataWebConfiguration as Spring beans, as shown in the following example (for SpringDataWebConfiguration):
<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" />Basic Web Support
The configuration shown in the previous section registers a few basic components:
-
A
DomainClassConverterto let Spring MVC resolve instances of repository-managed domain classes from request parameters or path variables. -
HandlerMethodArgumentResolverimplementations to let Spring MVC resolvePageableandSortinstances from request parameters.
DomainClassConverter
The DomainClassConverter 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 shown in the following example:
@Controller
@RequestMapping("/users")
class UserController {
@RequestMapping("/{id}")
String showUserForm(@PathVariable("id") User user, Model model) {
model.addAttribute("user", user);
return "userForm";
}
}As you can see, 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 shown in the following example:
@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 implementing the PageableHandlerMethodArgumentResolverCustomizer interface or the SortHandlerMethodArgumentResolverCustomizer interface, respectively. Its customize() method gets called, letting you change settings, as shown in the following example:
@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 followig example shows the resulting method signature:
String showUsers(Model model,
@Qualifier("thing1") Pageable first,
@Qualifier("thing2") Pageable second) { … }you have to populate thing1_page and thing2_page and so on.
The default Pageable passed into the method is equivalent to a new PageRequest(0, 20) but can be customized by using the @PageableDefault annotation on the Pageable parameter.
Hypermedia Support for Pageables
Spring HATEOAS ships with a representation model class (PagedResources) that allows enriching the content of a Page instance with the necessary Page metadata as well as links to let the clients easily navigate the pages. The conversion of a Page to a PagedResources is done by an implementation of the Spring HATEOAS ResourceAssembler interface, called the PagedResourcesAssembler. The following example shows how to use a PagedResourcesAssembler as a controller method argument:
@Controller
class PersonController {
@Autowired PersonRepository repository;
@RequestMapping(value = "/persons", method = RequestMethod.GET)
HttpEntity<PagedResources<Person>> persons(Pageable pageable,
PagedResourcesAssembler assembler) {
Page<Person> persons = repository.findAll(pageable);
return new ResponseEntity<>(assembler.toResources(persons), HttpStatus.OK);
}
}Enabling the configuration as shown in the preceding example lets the PagedResourcesAssembler be used as a controller method argument. Calling toResources(…) on it has the following effects:
-
The content of the
Pagebecomes the content of thePagedResourcesinstance. -
The
PagedResourcesobject gets aPageMetadatainstance attached, and it is populated with information from thePageand the underlyingPageRequest. -
The
PagedResourcesmay getprevandnextlinks 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 thePageableHandlerMethodArgumentResolverto 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/persons) 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
}
}You see that 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 that can be customized by handing in a custom Link to be used as base to build the pagination links, which overloads the PagedResourcesAssembler.toResource(…) method.
Web Databinding Support
Spring Data projections (described in [projections]) can be used to bind incoming request payloads by either using JSONPath expressions (requires Jayway JsonPath or XPath expressions (requires XmlBeam), as shown in the following example:
@ProjectedPayload
public interface UserPayload {
@XBRead("//firstname")
@JsonPath("$..firstname")
String getFirstname();
@XBRead("/lastname")
@JsonPath({ "$.lastname", "$.user.lastname" })
String getLastname();
}The type shown in the preceding example can be used as a Spring MVC handler method argument or by using ParameterizedTypeReference on one of RestTemplate's methods.
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 having QueryDSL integration, it is possible to derive queries from the attributes contained in a Request query string.
Consider the following query string:
?firstname=Dave&lastname=MatthewsGiven the User object from previous examples, a query string can be resolved to the following value by using the QuerydslPredicateArgumentResolver.
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 can be 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 exampe 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:
-
Objecton simple properties aseq. -
Objecton collection like properties ascontains. -
Collectionon simple properties asin.
Those bindings can be customized through the bindings attribute of @QuerydslPredicate or by making use of Java 8 default methods and adding the QuerydslBinderCustomizer method to the repository interface.
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. |
3.8.3. Repository Populators
If you work with the Spring JDBC module, you are probably familiar with the support to populate 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 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
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/data/repository
http://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 unmarshal 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
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/data/repository
http://www.springframework.org/schema/data/repository/spring-repository.xsd
http://www.springframework.org/schema/oxm
http://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>Reference Documentation
4. JDBC Repositories
This chapter points out the specialties for repository support for JDBC. This builds on the core repository support explained in Working with Spring Data Repositories. You should have a sound understanding of the basic concepts explained there.
4.1. Why Spring Data JDBC?
The main persistence API for relational databases in the Java world is certainly JPA, which has its own Spring Data module. Why is there another one?
JPA does a lot of things in order to help the developer. Among other things, it tracks changes to entities. It does lazy loading for you. It lets you map a wide array of object constructs to an equally wide array of database designs.
This is great and makes a lot of things really easy. Just take a look at a basic JPA tutorial. But it often gets really confusing as to why JPA does a certain thing. Also, things that are really simple conceptually get rather difficult with JPA.
Spring Data JDBC aims to be much simpler conceptually, by embracing the following design decisions:
-
If you load an entity, SQL statements get executed. Once this is done, you have a completely loaded entity. No lazy loading or caching is done.
-
If you save an entity, it gets saved. If you do not, it does not. There is no dirty tracking and no session.
-
There is a simple model of how to map entities to tables. It probably only works for rather simple cases. If you do not like that, you should code your own strategy. Spring Data JDBC offers only very limited support for customizing the strategy with annotations.
4.2. Domain Driven Design and Relational Databases.
All Spring Data modules are inspired by the concepts of “repository”, “aggregate”, and “aggregate root” from Domain Driven Design. These are possibly even more important for Spring Data JDBC, because they are, to some extent, contrary to normal practice when working with relational databases.
An aggregate is a group of entities that is guaranteed to be consistent between atomic changes to it.
A classic example is an Order with OrderItems.
A property on Order (for example, numberOfItems is consistent with the actual number of OrderItems) remains consistent as changes are made.
References across aggregates are not guaranteed to be consistent at all times. They are guaranteed to become consistent eventually.
Each aggregate has exactly one aggregate root, which is one of the entities of the aggregate. The aggregate gets manipulated only through methods on that aggregate root. These are the atomic changes mentioned earlier.
A repository is an abstraction over a persistent store that looks like a collection of all the aggregates of a certain type.
For Spring Data in general, this means you want to have one Repository per aggregate root.
In addition, for Spring Data JDBC this means that all entities reachable from an aggregate root are considered to be part of that aggregate root.
Spring Data JDBC assumes that only the aggregate has a foreign key to a table storing non-root entities of the aggregate and no other entity points toward non-root entities.
| In the current implementation, entities referenced from an aggregate root are deleted and recreated by Spring Data JDBC. |
You can overwrite the repository methods with implementations that match your style of working and designing your database.
4.3. Annotation-based Configuration
The Spring Data JDBC repositories support can be activated by an annotation through Java configuration, as the following example shows:
@Configuration
@EnableJdbcRepositories
class ApplicationConfig {
@Bean
public DataSource dataSource() {
EmbeddedDatabaseBuilder builder = new EmbeddedDatabaseBuilder();
return builder.setType(EmbeddedDatabaseType.HSQL).build();
}
}The configuration class in the preceding example sets up an embedded HSQL database by using the EmbeddedDatabaseBuilder API of spring-jdbc.
We activate Spring Data JDBC repositories by using the @EnableJdbcRepositories.
If no base package is configured, it uses the package in which the configuration class resides.
4.4. Persisting Entities
Saving an aggregate can be performed with the CrudRepository.save(…) method.
If the aggregate is new, this results in an insert for the aggregate root, followed by insert statements for all directly or indirectly referenced entities.
If the aggregate root is not new, all referenced entities get deleted, the aggregate root gets updated, and all referenced entities get inserted again. Note that whether an instance is new is part of the instance’s state.
| This approach has some obvious downsides. If only few of the referenced entities have been actually changed, the deletion and insertion is wasteful. While this process could and probably will be improved, there are certain limitations to what Spring Data JDBC can offer. It does not know the previous state of an aggregate. So any update process always has to take whatever it finds in the database and make sure it converts it to whatever is the state of the entity passed to the save method. |
4.4.1. 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.
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’s a no-argument constructor, it will be used. Other constructors will be ignored.
-
If there’s a single constructor taking arguments, it will be used.
-
If there are multiple constructors taking arguments, the one to be used by Spring Data will have to be annotated with
@PersistenceConstructor.
The value resolution assumes constructor 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.
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 wither method (see below), we use the wither 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.
-
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);
}
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. |
| 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 wither being present. |
| 4 | The comment property is mutable is populated by setting its field directly. |
| 5 | The remarks properties are mutable and populated by setting the comment field directly or by invoking the setter method for |
| 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 @PersistenceConstructor.
Instead, defaulting of properties is handled within the factory method. |
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
@PersistenceConstructor— 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 a wither 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.
Kotlin support
Spring Data adapts specifics of Kotlin to allow object creation and mutation.
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. 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 @PersistenceConstructor to indicate a constructor preference:
data class Person(var id: String, val name: String) {
@PersistenceConstructor
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.
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 to create 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.
4.4.2. Supported Types in Your Entity
The properties of the following types are currently supported:
-
All primitive types and their boxed types (
int,float,Integer,Float, and so on) -
Enums get mapped to their name.
-
String -
java.util.Date,java.time.LocalDate,java.time.LocalDateTime, andjava.time.LocalTime -
Anything your database driver accepts.
-
References to other entities. They are considered a one-to-one relationship. It is optional for such entities to have an
idattribute. The table of the referenced entity is expected to have an additional column named the same as the table of the referencing entity. You can change this name by implementingNamingStrategy.getReverseColumnName(RelationalPersistentProperty property). -
Set<some entity>is considered a one-to-many relationship. The table of the referenced entity is expected to have an additional column named the same as the table of the referencing entity. You can change this name by implementingNamingStrategy.getReverseColumnName(RelationalPersistentProperty property). -
Map<simple type, some entity>is considered a qualified one-to-many relationship. The table of the referenced entity is expected to have two additional columns: One named the same as the table of the referencing entity for the foreign key and one with the same name and an additional_keysuffix for the map key. You can change this behavior by implementingNamingStrategy.getReverseColumnName(RelationalPersistentProperty property)andNamingStrategy.getKeyColumn(RelationalPersistentProperty property), respectively. Alternatively you may annotate the attribute with@Column(value="your_column_name", keyColumn="your_key_column_name") -
List<some entity>is mapped as aMap<Integer, some entity>.
The handling of referenced entities is limited. This is based on the idea of aggregate roots as described above. If you reference another entity, that entity is, by definition, part of your aggregate. So, if you remove the reference, the previously referenced entity gets deleted. This also means references are 1-1 or 1-n, but not n-1 or n-m.
If you have n-1 or n-m references, you are, by definition, dealing with two separate aggregates.
References between those should be encoded as simple id values, which should map properly with Spring Data JDBC.
4.4.3. NamingStrategy
When you use the standard implementations of CrudRepository that Spring Data JDBC provides, they expect a certain table structure.
You can tweak that by providing a NamingStrategy in your application context.
4.4.4. Entity State Detection Strategies
The following table describes the strategies that Spring Data JDBC offers for detecting whether an entity is new:
Id-Property inspection (the default) |
By default, Spring Data JDBC inspects the identifier property of the given entity.
If the identifier property is |
Implementing |
If an entity implements |
Implementing |
You can customize the |
4.4.5. ID Generation
Spring Data JDBC uses the ID to identify entities.
The ID of an entity must be annotated with Spring Data’s @Id annotation.
When your data base has an auto-increment column for the ID column, the generated value gets set in the entity after inserting it into the database.
One important constraint is that, after saving an entity, the entity must not be new any more.
Note that whether an entity is new is part of the entity’s state.
With auto-increment columns, this happens automatically, because the ID gets set by Spring Data with the value from the ID column.
If you are not using auto-increment columns, you can use a BeforeSave listener, which sets the ID of the entity (covered later in this document).
4.5. Query Methods
This section offers some specific information about the implementation and use of Spring Data JDBC.
4.5.1. Query Lookup Strategies
The JDBC module supports defining a query manually only as a String in a @Query annotation.
Deriving a query from the name of the method is currently not supported.
4.5.2. Using @Query
The following example shows how to use @Query to declare a query method:
public interface UserRepository extends CrudRepository<User, Long> {
@Query("select firstName, lastName from User u where u.emailAddress = :email")
User findByEmailAddress(@Param("email") String email);
}
Spring fully supports Java 8’s parameter name discovery based on the -parameters compiler flag. By using this flag in your build as an alternative to debug information, you can omit the @Param annotation for named parameters.
|
| Spring Data JDBC supports only named parameters. |
Custom RowMapper
You can configure which RowMapper to use, either by using the @Query(rowMapperClass = ….) or by registering a RowMapperMap bean and registering a RowMapper per method return type. The following example shows how to register RowMappers:
@Bean
RowMapperMap rowMappers() {
return new ConfigurableRowMapperMap() //
.register(Person.class, new PersonRowMapper()) //
.register(Address.class, new AddressRowMapper());
}When determining which RowMapper to use for a method, the following steps are followed, based on the return type of the method:
-
If the type is a simple type, no
RowMapperis used.Instead, the query is expected to return a single row with a single column, and a conversion to the return type is applied to that value.
-
The entity classes in the
RowMapperMapare iterated until one is found that is a superclass or interface of the return type in question. TheRowMapperregistered for that class is used.Iterating happens in the order of registration, so make sure to register more general types after specific ones.
If applicable, wrapper types such as collections or Optional are unwrapped.
Thus, a return type of Optional<Person> uses the Person type in the preceding process.
Modifying Query
You can mark a query as being a modifying query by using the @Modifying on query method, as the following example shows:
@Modifying
@Query("UPDATE DUMMYENTITY SET name = :name WHERE id = :id")
boolean updateName(@Param("id") Long id, @Param("name") String name);You can specify the following return types:
-
void -
int(updated record count) -
boolean(whether a record was updated)
4.6. MyBatis Integration
For each operation in CrudRepository, Spring Data JDBC runs multiple statements.
If there is a SqlSessionFactory in the application context, Spring Data checks, for each step, whether the SessionFactory offers a statement.
If one is found, that statement (including its configured mapping to an entity) is used.
The name of the statement is constructed by concatenating the fully qualified name of the entity type with Mapper. and a String determining the kind of statement.
For example, if an instance of org.example.User is to be inserted, Spring Data JDBC looks for a statement named org.example.UserMapper.insert.
When the statement is run, an instance of [MyBatisContext] gets passed as an argument, which makes various arguments available to the statement.
The following table describes the available MyBatis statements:
| Name | Purpose | CrudRepository methods that might trigger this statement | Attributes available in the MyBatisContext |
|---|---|---|---|
|
Inserts a single entity. This also applies for entities referenced by the aggregate root. |
|
|
|
Updates a single entity. This also applies for entities referenced by the aggregate root. |
|
|
|
Deletes a single entity. |
|
|
|
Deletes all entities referenced by any aggregate root of the type used as prefix with the given property path. Note that the type used for prefixing the statement name is the name of the aggregate root, not the one of the entity to be deleted. |
|
|
|
Deletes all aggregate roots of the type used as the prefix |
|
|
|
Deletes all entities referenced by an aggregate root with the given propertyPath |
|
|
|
Selects an aggregate root by ID |
|
|
|
Select all aggregate roots |
|
|
|
Select a set of aggregate roots by ID values |
|
|
|
Select a set of entities that is referenced by another entity. The type of the referencing entity is used for the prefix. The referenced entities type is used as the suffix. |
All |
|
|
Count the number of aggregate root of the type used as prefix |
|
|
4.7. Events
Spring Data JDBC triggers events that get published to any matching ApplicationListener in the application context.
For example, the following listener gets invoked before an aggregate gets saved:
@Bean
public ApplicationListener<BeforeSave> timeStampingSaveTime() {
return event -> {
Object entity = event.getEntity();
if (entity instanceof Category) {
Category category = (Category) entity;
category.timeStamp();
}
};
}The following table describes the available events:
| Event | When It Is Published |
|---|---|
Before an aggregate root gets deleted. |
|
After an aggregate root gets deleted. |
|
Before an aggregate root gets saved (that is, inserted or updated but after the decision about whether if it gets updated or deleted was made).
The event has a reference to an |
|
After an aggregate root gets saved (that is, inserted or updated). |
|
After an aggregate root gets created from a database |
4.8. Logging
Spring Data JDBC does little to no logging on its own.
Instead, the mechanics of JdbcTemplate to issue SQL statements provide logging.
Thus, if you want to inspect what SQL statements are executed, activate logging for Spring’s NamedParameterJdbcTemplate or MyBatis.
4.9. Transactionality
CRUD methods on repository instances are transactional by default.
For reading operations, the transaction configuration readOnly flag is set to true. All others are configured with a plain @Transactional annotation so that default transaction configuration applies.
For details, see the Javadoc of SimpleJdbcRepository. If you need to tweak transaction configuration for one of the methods declared in a repository, redeclare the method in your repository interface, as follows:
public interface UserRepository extends CrudRepository<User, Long> {
@Override
@Transactional(timeout = 10)
public List<User> findAll();
// Further query method declarations
}The preceding causes the findAll() method to be executed with a timeout of 10 seconds and without the readOnly flag.
Another way to alter transactional behavior is by using a facade or service implementation that typically covers more than one repository. Its purpose is to define transactional boundaries for non-CRUD operations. The following example shows how to create such a facade:
@Service
class UserManagementImpl implements UserManagement {
private final UserRepository userRepository;
private final RoleRepository roleRepository;
@Autowired
public UserManagementImpl(UserRepository userRepository,
RoleRepository roleRepository) {
this.userRepository = userRepository;
this.roleRepository = roleRepository;
}
@Transactional
public void addRoleToAllUsers(String roleName) {
Role role = roleRepository.findByName(roleName);
for (User user : userRepository.findAll()) {
user.addRole(role);
userRepository.save(user);
}
}The preceding example causes calls to addRoleToAllUsers(…) to run inside a transaction (participating in an existing one or creating a new one if none are already running). The transaction configuration for the repositories is neglected, as the outer transaction configuration determines the actual repository to be used. Note that you have to explicitly activate <tx:annotation-driven /> or use @EnableTransactionManagement to get annotation-based configuration for facades working. Note that the preceding example assumes you use component scanning.
4.9.1. Transactional Query Methods
To let your query methods be transactional, use @Transactional at the repository interface you define, as the following example shows:
@Transactional(readOnly = true)
public interface UserRepository extends CrudRepository<User, Long> {
List<User> findByLastname(String lastname);
@Modifying
@Transactional
@Query("delete from User u where u.active = false")
void deleteInactiveUsers();
}Typically, you want the readOnly flag to be set to true, because most of the query methods only read data. In contrast to that, deleteInactiveUsers() uses the @Modifying annotation and overrides the transaction configuration. Thus, the method is with the readOnly flag set to false.
It is definitely reasonable to use transactions for read-only queries, and we can mark them as such by setting the readOnly flag. This does not, however, act as a check that you do not trigger a manipulating query (although some databases reject INSERT and UPDATE statements inside a read-only transaction). Instead, the readOnly flag is propagated as a hint to the underlying JDBC driver for performance optimizations.
|
4.10. Auditing
4.10.1. 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.
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 DateTime createdDate;
// … further properties omitted
}As you can see, the annotations can be applied selectively, depending on which information you want to capture. The annotations capturing when changes were made can be used on properties of type Joda-Time, DateTime, legacy Java Date and Calendar, JDK8 date and time types, and long or Long.
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.
There is also a convenience base class, AbstractAuditable, which you can extend to avoid the need to manually implement the interface methods. Doing so increases the coupling of your domain classes to Spring Data, which might be something you want to avoid. Usually, the annotation-based way of defining auditing metadata is preferred as it is less invasive and more flexible.
AuditorAware
In case you use either @CreatedBy or @LastModifiedBy, the auditing infrastructure somehow needs to become aware of the current principal. To do so, we provide an AuditorAware<T> SPI interface that you have to implement to tell the infrastructure who the current user or system interacting with the application is. The generic type T defines 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:
class SpringSecurityAuditorAware implements AuditorAware<User> {
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.
4.11. JDBC Auditing
In order to activate auditing, add @EnableJdbcAuditing to your configuration, as the following example shows:
@Configuration
@EnableJdbcAuditing
class Config {
@Bean
public AuditorAware<AuditableUser> auditorProvider() {
return new AuditorAwareImpl();
}
}If you expose a bean of type AuditorAware to the ApplicationContext, the auditing infrastructure automatically picks it up and uses it to determine the current user to be set on domain types. If you have multiple implementations registered in the ApplicationContext, you can select the one to be used by explicitly setting the auditorAwareRef attribute of @EnableJdbcAuditing.
Appendix
Appendix B: Glossary
- AOP
-
Aspect-Oriented Programming
- CRUD
-
Create, Read, Update, Delete - Basic persistence operations
- Dependency Injection
-
Pattern to hand a component’s dependency to the component from outside, freeing the component to lookup the dependent itself. For more information, see http://en.wikipedia.org/wiki/Dependency_Injection.
- JPA
-
Java Persistence API
- Spring
-
Java application framework — http://projects.spring.io/spring-framework
Appendix C: 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 |
|---|---|
|
Defines the package to be scanned for repository interfaces that extend |
|
Defines the postfix to autodetect custom repository implementations. Classes whose names end with the configured postfix are considered as candidates. Defaults to |
|
Determines the strategy to be used to create finder queries. See “Query Lookup Strategies” for details. Defaults to |
|
Defines the location to search for a Properties file containing externally defined queries. |
|
Whether nested repository interface definitions should be considered. Defaults to |
Appendix D: Populators namespace reference
The <populator /> element
The <populator /> element allows to populate the 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 F: 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.
|
| Return type | Description |
|---|---|
|
Denotes no return value. |
Primitives |
Java primitives. |
Wrapper types |
Java wrapper types. |
|
An unique entity. Expects the query method to return one result at most. If no result is found, |
|
An |
|
A |
|
A |
|
A Java 8 or Guava |
|
Either a Scala or Javaslang |
|
A Java 8 |
|
A |
|
A Java 8 |
|
A |
|
A sized chunk of data with an indication of whether there is more data available. Requires a |
|
A |
|
A result entry with additional information, such as the distance to a reference location. |
|
A list of |
|
A |
|
A Project Reactor |
|
A Project Reactor |
|
A RxJava |
|
A RxJava |
|
A RxJava |