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.

The following example shows how to selectively expose CRUD methods (findById and save, in this case):

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.

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:

The following example shows a repository that uses module-specific interfaces (JPA in this case):

The following example shows a repository that uses generic interfaces:

The following example shows a repository that uses domain classes with annotations:

The following bad example shows a repository that uses domain classes with mixed annotations:

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:

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.

The query builder mechanism built into the Spring Data repository infrastructure is useful for building constraining queries over entities of the repository.

The following example shows how to create a number of queries:

Parsing query method names is divided into subject and predicate. The first part (find…By, exists…By) defines the subject of the query, the second part forms the predicate. The introducing clause (subject) can contain further expressions. Any text between find (or other introducing keywords) and By is considered to be descriptive unless using one of the result-limiting keywords such as a Distinct to set a distinct flag on the query to be created or Top/First to limit query results.

The appendix contains the full list of query method subject keywords and query method predicate keywords including sorting and letter-casing modifiers. However, the first By acts as a delimiter to indicate the start of the actual criteria predicate. At a very basic level, you can define conditions on entity properties and concatenate them with And and Or.

The actual result of parsing the method depends on the persistence store for which you create the query. However, there are some general things to notice:

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:

Assume a Person has an Address with a ZipCode. In that case, the method creates the x.address.zipCode property traversal. The resolution algorithm starts by interpreting the entire part (AddressZipCode) as the property and checks the domain class for a property with that name (uncapitalized). If the algorithm succeeds, it uses that property. If not, the algorithm splits up the source at the camel-case parts from the right side into a head and a tail and tries to find the corresponding property — in our example, AddressZip and Code. If the algorithm finds a property with that head, it takes the tail and continues building the tree down from there, splitting the tail up in the way just described. If the first split does not match, the algorithm moves the split point to the left (Address, ZipCode) and continues.

Although this should work for most cases, it is possible for the algorithm to select the wrong property. Suppose the Person class has an addressZip property as well. The algorithm would match in the first split round already, choose the wrong property, and fail (as the type of addressZip probably has no code property).

To resolve this ambiguity you can use _ inside your method name to manually define traversal points. So our method name would be as follows:

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).

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:

The first method lets you pass an org.springframework.data.domain.Pageable instance to the query method to dynamically add paging to your statically defined query. A Page knows about the total number of elements and pages available. It does so by the infrastructure triggering a count query to calculate the overall number. As this might be expensive (depending on the store used), you can instead return a Slice. A Slice knows only about whether a next Slice is available, which might be sufficient when walking through a larger result set.

Sorting options are handled through the Pageable instance, too. If you need only sorting, add an org.springframework.data.domain.Sort parameter to your method. As you can see, returning a List is also possible. In this case, the additional metadata required to build the actual Page instance is not created (which, in turn, means that the additional count query that would have been necessary is not issued). Rather, it restricts the query to look up only the given range of entities.

You can limit the results of query methods by using the first or top keywords, which you can use interchangeably. You can append an optional numeric value to top or first to specify the maximum result size to be returned. If the number is left out, a result size of 1 is assumed. The following example shows how to limit the query size:

The limiting expressions also support the Distinct keyword for datastores that support distinct queries. Also, for the queries that limit the result set to one instance, wrapping the result into with the Optional keyword is supported.

If pagination or slicing is applied to a limiting query pagination (and the calculation of the number of available pages), it is applied within the limited result.

Query methods that return multiple results can use standard Java Iterable, List, and Set. Beyond that, we support returning Spring Data’s Streamable, a custom extension of Iterable, as well as collection types provided by Vavr. Refer to the appendix explaining all possible query method return types.

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:

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.

You can process the results of query methods incrementally by using a Java 8 Stream<T> as the return type. Instead of wrapping the query results in a Stream, data store-specific methods are used to perform the streaming, as shown in the following example:

You can run repository queries asynchronously by using Spring’s asynchronous method running capability. This means the method returns immediately upon invocation while the actual query occurs in a task that has been submitted to a Spring TaskExecutor. Asynchronous queries differ from reactive queries and should not be mixed. See the store-specific documentation for more details on reactive support. The following example shows a number of asynchronous queries:

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:

In the preceding example, Spring is instructed to scan com.acme.repositories and all its sub-packages for interfaces extending Repository or one of its sub-interfaces. For each interface found, the infrastructure registers the persistence technology-specific FactoryBean to create the appropriate proxies that handle invocations of the query methods. Each bean is registered under a bean name that is derived from the interface name, so an interface of UserRepository would be registered under userRepository. Bean names for nested repository interfaces are prefixed with their enclosing type name. The base-package attribute allows wildcards so that you can define a pattern of scanned packages.

You can also trigger the repository infrastructure by using a store-specific @Enable${store}Repositories annotation on a Java configuration class.For an introduction to Java-based configuration of the Spring container, see JavaConfig in the Spring reference documentation.

A sample configuration to enable Spring Data repositories resembles the following:

You can also use the repository infrastructure outside of a Spring container — for example, in CDI environments.You still need some Spring libraries in your classpath, but, generally, you can set up repositories programmatically as well.The Spring Data modules that provide repository support ship with a persistence technology-specific RepositoryFactory that you can use, as follows:

To enrich a repository with custom functionality, you must first define a fragment interface and an implementation for the custom functionality, as follows:

The implementation itself does not depend on Spring Data and can be a regular Spring bean.Consequently, you can use standard dependency injection behavior to inject references to other beans (such as a JdbcTemplate), take part in aspects, and so on.

Then you can let your repository interface extend the fragment interface, as follows:

Extending the fragment interface with your repository interface combines the CRUD and custom functionality and makes it available to clients.

Spring Data repositories are implemented by using fragments that form a repository composition.Fragments are the base repository, functional aspects (such as QueryDsl), and custom interfaces along with their implementations.Each time you add an interface to your repository interface, you enhance the composition by adding a fragment.The base repository and repository aspect implementations are provided by each Spring Data module.

The following example shows custom interfaces and their implementations:

The following example shows the interface for a custom repository that extends CrudRepository:

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:

The following example shows a repository that uses the preceding repository fragment:

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:

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:

A corresponding attribute is available in the XML namespace, as shown in the following example:

Querydsl is a framework that enables the construction of statically typed SQL-like queries through its fluent API.

Several Spring Data modules offer integration with Querydsl through QuerydslPredicateExecutor, as the following example shows:

To use the Querydsl support, extend QuerydslPredicateExecutor on your repository interface, as the following example shows:

The preceding example lets you write type-safe queries by using Querydsl Predicate instances, as the following example shows:

Spring Data modules that support the repository programming model ship with a variety of web support. The web related components require Spring MVC JARs to be on the classpath. Some of them even provide integration with Spring HATEOAS. In general, the integration support is enabled by using the @EnableSpringDataWebSupport annotation in your JavaConfig configuration class, as the following example shows:

The @EnableSpringDataWebSupport annotation registers a few components. We discuss those later in this section. It also detects Spring HATEOAS on the classpath and registers integration components (if present) for it as well.

Alternatively, if you use XML configuration, register either SpringDataWebConfiguration or HateoasAwareSpringDataWebConfiguration as Spring beans, as the following example shows (for SpringDataWebConfiguration):

If you work with the Spring JDBC module, you are probably familiar with the support for populating a DataSource with SQL scripts. A similar abstraction is available on the repositories level, although it does not use SQL as the data definition language because it must be store-independent. Thus, the populators support XML (through Spring’s OXM abstraction) and JSON (through Jackson) to define data with which to populate the repositories.

Assume you have a file called data.json with the following content:

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:

The preceding declaration causes the data.json file to be read and deserialized by a Jackson ObjectMapper.

The type to which the JSON object is unmarshalled is determined by inspecting the _class attribute of the JSON document. The infrastructure eventually selects the appropriate repository to handle the object that was deserialized.

To instead use XML to define the data the repositories should be populated with, you can use the unmarshaller-populator element. You configure it to use one of the XML marshaller options available in Spring OXM. See the Spring reference documentation for details. The following example shows how to unmarshall a repository populator with JAXB:

The following example shows an example of using Java-based bean metadata to register an instance of io.r2dbc.spi.ConnectionFactory:

This approach lets you use the standard io.r2dbc.spi.ConnectionFactory instance, with the container using Spring’s AbstractR2dbcConfiguration.As compared to registering a ConnectionFactory instance directly, the configuration support has the added advantage of also providing the container with an ExceptionTranslator implementation that translates R2DBC exceptions to exceptions in Spring’s portable DataAccessException hierarchy for data access classes annotated with the @Repository annotation.This hierarchy and the use of @Repository is described in Spring’s DAO support features.

AbstractR2dbcConfiguration also registers DatabaseClient, which is required for database interaction and for Repository implementation.

Spring Data R2DBC supports drivers through R2DBC’s pluggable SPI mechanism. You can use any driver that implements the R2DBC spec with Spring Data R2DBC. Since Spring Data R2DBC reacts to specific features of each database, it requires a Dialect implementation otherwise your application won’t start up. Spring Data R2DBC ships with dialect implementations for the following drivers:

Spring Data R2DBC reacts to database specifics by inspecting the ConnectionFactory and selects the appropriate database dialect accordingly. You need to configure your own R2dbcDialect if the driver you use is not yet known to Spring Data R2DBC.

There are several convenient methods on R2dbcEntityTemplate for saving and inserting your objects. To have more fine-grained control over the conversion process, you can register Spring converters with R2dbcCustomConversions — for example Converter<Person, OutboundRow> and Converter<Row, Person>.

The simple case of using the save operation is to save a POJO. In this case, the table name is determined by name (not fully qualified) of the class. You may also call the save operation with a specific collection name. You can use mapping metadata to override the collection in which to store the object.

When inserting or saving, if the Id property is not set, the assumption is that its value will be auto-generated by the database. Consequently, for auto-generation the type of the Id property or field in your class must be a Long, or Integer.

The following example shows how to insert a row and retrieving its contents:

The following insert and update operations are available:

A similar set of insert operations is also available:

Table names can be customized by using the fluent API.

The select(…) and selectOne(…) methods on R2dbcEntityTemplate are used to select data from a table. Both methods take a Query object that defines the field projection, the WHERE clause, the ORDER BY clause and limit/offset pagination. Limit/offset functionality is transparent to the application regardless of the underlying database. This functionality is supported by the R2dbcDialect abstraction to cater for differences between the individual SQL flavors.

This section explains the fluent API usage. Consider the following simple query:

The following example declares a more complex query that specifies the table name by name, a WHERE condition, and an ORDER BY clause:

You can switch between retrieving a single entity and retrieving multiple entities through the following terminating methods:

You can use the select() entry point to express your SELECT queries. The resulting SELECT queries support the commonly used clauses (WHERE and ORDER BY) and support pagination. The fluent API style let you chain together multiple methods while having easy-to-understand code. To improve readability, you can use static imports that let you avoid using the 'new' keyword for creating Criteria instances.

You can use the insert() entry point to insert data.

Consider the following simple typed insert operation:

You can use the update() entry point to update rows. Updating data starts by specifying the table to update by accepting Update specifying assignments. It also accepts Query to create a WHERE clause.

Consider the following simple typed update operation:

You can use the delete() entry point to delete rows. Removing data starts with a specification of the table to delete from and, optionally, accepts a Criteria to create a WHERE clause.

Consider the following simple insert operation:

The previous sections describe how to declare queries to access a given entity or collection of entities. Using keywords from the preceding table can be used in conjunction with delete…By or remove…By to create derived queries that delete matching rows.

As this approach is feasible for comprehensive custom functionality, you can modify queries that only need parameter binding by annotating the query method with @Modifying, as shown in the following example:

The result of a modifying query can be:

The @Modifying annotation is only relevant in combination with the @Query annotation. Derived custom methods do not require this annotation.

Alternatively, you can add custom modifying behavior by using the facilities described in Custom Implementations for Spring Data Repositories.

Query string definitions can be used together with SpEL expressions to create dynamic queries at runtime. SpEL expressions can provide predicate values which are evaluated right before running the query.

Expressions expose method arguments through an array that contains all the arguments. The following query uses [0] to declare the predicate value for lastname (which is equivalent to the :lastname parameter binding):

SpEL in query strings can be a powerful way to enhance queries. However, they can also accept a broad range of unwanted arguments. You should make sure to sanitize strings before passing them to the query to avoid unwanted changes to your query.

Expression support is extensible through the Query SPI: org.springframework.data.spel.spi.EvaluationContextExtension. The Query SPI can contribute properties and functions and can customize the root object. Extensions are retrieved from the application context at the time of SpEL evaluation when the query is built.

Spring Data R2DBC also lets you use Query By Example to fashion queries. This technique allows you to use a "probe" object. Essentially, any field that isn’t empty or null will be used to match.

Here’s an example:

This illustrates how to craft a simple probe using a domain object. In this case, it will query based on the Employee object’s name field being equal to Frodo. null fields are ignored.

It’s also possible to apply a withTransform() against any property, allowing you to transform a property before forming the query. For example, you can apply a toUpperCase() to a String -based property before the query is created.

Query By Example really shines when you you don’t know all the fields needed in a query in advance. If you were building a filter on a web page where the user can pick the fields, Query By Example is a great way to flexibly capture that into an efficient query.

The following table describes the strategies that Spring Data offers for detecting whether an entity is new:

Spring Data R2DBC uses the ID to identify entities. The ID of an entity must be annotated with Spring Data’s @Id annotation.

When your database has an auto-increment column for the ID column, the generated value gets set in the entity after inserting it into the database.

Spring Data R2DBC does not attempt to insert values of identifier columns when the entity is new and the identifier value defaults to its initial value. That is 0 for primitive types and null if the identifier property uses a numeric wrapper type such as Long.

One important constraint is that, after saving an entity, the entity must not be new anymore. 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.

The @Version annotation provides syntax similar to that of JPA in the context of R2DBC and makes sure updates are only applied to rows with a matching version. Therefore, the actual value of the version property is added to the update query in such a way that the update does not have any effect if another operation altered the row in the meantime. In that case, an OptimisticLockingFailureException is thrown. The following example shows these features:

Spring Data query methods usually return one or multiple instances of the aggregate root managed by the repository. However, it might sometimes be desirable to create projections based on certain attributes of those types. Spring Data allows modeling dedicated return types, to more selectively retrieve partial views of the managed aggregates.

Imagine a repository and aggregate root type such as the following example:

Now imagine that we want to retrieve the person’s name attributes only. What means does Spring Data offer to achieve this? The rest of this chapter answers that question.

An EntityCallback is directly associated with its domain type through its generic type argument. Each Spring Data module typically ships with a set of predefined EntityCallback interfaces covering the entity lifecycle.

Implement the interface suiting your application needs like shown in the example below:

EntityCallback beans are picked up by the store specific implementations in case they are registered in the ApplicationContext. Most template APIs already implement ApplicationContextAware and therefore have access to the ApplicationContext

The following example explains a collection of valid entity callback registrations:

Spring Data R2DBC uses the EntityCallback API for its auditing support and reacts on the following callbacks.

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.

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.

Auditing metadata does not necessarily need to live in the root level entity but can be added to an embedded one (depending on the actual store in use), as shown in the snipped below.

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.

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:

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.

When using reactive infrastructure you might want to make use of contextual information to provide @CreatedBy or @LastModifiedBy information. We provide an ReactiveAuditorAware<T> SPI interface that you have to implement to tell the infrastructure who the current user or system interacting with the application is. The generic type T defines what type the properties annotated with @CreatedBy or @LastModifiedBy have to be.

The following example shows an implementation of the interface that uses reactive Spring Security’s Authentication object:

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.

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:

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.

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:

Let’s have a look at the following entity:

Spring Data adapts specifics of Kotlin to allow object creation and mutation.

The following table explains how property types of an entity affect mapping:

The MappingR2dbcConverter can use metadata to drive the mapping of objects to rows. The following annotations are available:

The mapping metadata infrastructure is defined in the separate spring-data-commons project that is technology-agnostic. Specific subclasses are used in the R2DBC support to support annotation based metadata. Other strategies can also be put in place (if there is demand).

The mapping subsystem allows the customization of the object construction by annotating a constructor with the @PersistenceConstructor annotation.The values to be used for the constructor parameters are resolved in the following way:

When storing and querying your objects, it is often convenient to have a R2dbcConverter instance to handle the mapping of all Java types to OutboundRow instances. However, you may sometimes want the R2dbcConverter instances to do most of the work but let you selectively handle the conversion for a particular type — perhaps to optimize performance.

To selectively handle the conversion yourself, register one or more one or more org.springframework.core.convert.converter.Converter instances with the R2dbcConverter.

You can use the r2dbcCustomConversions method in AbstractR2dbcConfiguration to configure converters. The examples at the beginning of this chapter show how to perform the configuration with Java.

The following example of a Spring Converter implementation converts from a Row to a Person POJO:

Please note that converters get applied on singular properties. Collection properties (e.g. Collection<Person>) are iterated and converted element-wise. Collection converters (e.g. Converter<List<Person>>, OutboundRow) are not supported.

The following example converts from a Person to a OutboundRow:

Coroutines support is enabled when kotlinx-coroutines-core, kotlinx-coroutines-reactive and kotlinx-coroutines-reactor dependencies are in the classpath:

For return values, the translation from Reactive to Coroutines APIs is the following:

Flow is Flux equivalent in Coroutines world, suitable for hot or cold stream, finite or infinite streams, with the following main differences:

Read this blog post about Going Reactive with Spring, Coroutines and Kotlin Flow for more details, including how to run code concurrently with Coroutines.

Here is an example of a Coroutines repository:

Coroutines repositories are built on reactive repositories to expose the non-blocking nature of data access through Kotlin’s Coroutines. Methods on a Coroutines repository can be backed either by a query method or a custom implementation. Invoking a custom implementation method propagates the Coroutines invocation to the actual implementation method if the custom method is suspend-able without requiring the implementation method to return a reactive type such as Mono or Flux.

To ease migration, several deprecated types are now subtypes of their replacements provided by Spring R2DBC. Spring Data R2DBC has changes several methods or introduced new methods accepting Spring R2DBC types. Specifically the following classes are changed:

We recommend that you review and update your imports if you work with these types directly.

To make use of Spring R2DBC, make sure to include the following dependency: