© 2010-2017 Graph Aware Ltd - Neo Technology, Inc. - Pivotal Software, Inc.

Spring + Neo4j
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Table of Contents

Preface

The Spring Data Neo4J project applies core Spring concepts to the development of solutions using the Neo4j graph data store. We provide "repositories" as a high-level abstraction for storing and querying documents. You will notice similarities to the JPA/Hibernate support in the Spring Framework.

This document is the reference guide for Spring Data - Graph Support. It explains Graph module concepts and semantics and the syntax for various store namespaces.

This section provides some basic introduction to Spring and Graph databases. The rest of the document refers only to Spring Data Neo4j features and assumes the user is familiar with the Neo4j graph database as well as Spring concepts.

1. Spring and Spring Data

Spring Data uses Spring framework’s core functionality, such as the IoC container, type conversion system, expression language, JMX integration, and portable DAO exception hierarchy. While it is not important to know the Spring APIs, understanding the concepts behind them is. At a minimum, the idea behind IoC should be familiar for whatever IoC container you choose to use.

The core functionality of the Neo4J support can be used directly, with no need to invoke the IoC services of the Spring Container. This is much like Hibernate Session or JPA EntityManager which can be used 'standalone' without any other services of the Spring container. To leverage all the features of Spring Data Neo4j, such as the repository support, you will need to configure some parts of the library using Spring.

To learn more about Spring, you can refer to the comprehensive (and sometimes disarming) documentation that explains in detail the Spring Framework. There are a lot of articles, blog entries and books on the matter - take a look at the Spring framework home page for more information.

2. NoSQL and Graph databases

A graph database is a storage engine that is specialised in storing and retrieving vast networks of information. It efficiently stores data as nodes and relationships and allows high performance retrieval and querying of those structures. Properties can be added to both nodes and relationships. Nodes can be labelled by zero or more labels, relationships are always directed and named.

Graph databases are well suited for storing most kinds of domain models. In almost all domains, there are certain things connected to other things. In most other modelling approaches, the relationships between things are reduced to a single link without identity and attributes. Graph databases allow to keep the rich relationships that originate from the domain equally well-represented in the database without resorting to also modelling the relationships as "things". There is very little "impedance mismatch" when putting real-life domains into a graph database.

2.1. Introducing Neo4j

Neo4j is an open source NOSQL graph database. It is a fully transactional database (ACID) that stores data structured as graphs consisting of nodes, connected by relationships. Inspired by the structure of the real world, it allows for high query performance on complex data, while remaining intuitive and simple for the developer.

Neo4j is very well-established. It has been in commercial development for 15 years and in production for over 12 years. Most importantly, it has an active and contributing community surrounding it, but it also:

  • has an intuitive, rich graph-oriented model for data representation. Instead of tables, rows, and columns, you work with a graph consisting of nodes, relationships, and properties.

  • has a disk-based, native storage manager optimised for storing graph structures with maximum performance and scalability.

  • is scalable. Neo4j can handle graphs with many billions of nodes/relationships/properties on a single machine, but can also be scaled out across multiple machines for high availability.

  • has a powerful graph query language called Cypher, which allows users to efficiently read/write data by expressing graph patterns.

  • has a powerful traversal framework and query languages for traversing the graph.

  • can be deployed as a standalone server, which is the recommended way of using Neo4j

  • can be deployed as an embedded (in-process) database, giving developers access to its core Java API

In addition, Neo4j provides ACID transactions, durable persistence, concurrency control, transaction recovery, high availability, and more. Neo4j is released under a dual free software/commercial licence model.

The jumping off ground for learning about Neo4J is neo4j.com. Here is a list of other useful resources:

3. Requirements

Spring Data Neo4j 5.0.x at minimum, requires:

  • JDK Version 8 and above.

  • Neo4j Graph Database 3.0.x / 3.1.x and above.

  • Spring Framework 5.0.0.RC2 and above.

If you plan on altering the version of the OGM make sure it is only in the 3.0.0+ release family.

4. Additional Resources

4.2. Getting Help

If you encounter issues or you are just looking for advice, feel free to use one of the links below:

To learn more refer to:

If you are new to Spring as well as to Spring Data, look for information about Spring projects.

4.3. Got feedback?

Whenever you look for the means to employ the full power of the Spring Data Neo4j library, you should be able to find your answers in this document. Hopefully we’ve created a guide that is well-received by our peers in the development community but sometimes things aren’t detailed enough or have enough documentation. We are a very quick turnaround development team so if you don’t see what, please inform us about missing or incorrect content.

If you have any feedback on Spring Data Neo4j or this book, please provide it via:

5. New & Noteworthy

5.1. What’s new in Spring Data Neo4j 5.0 M2

  • SDN 5.x is designed to work with Spring 5 and Spring Boot 2.x.

  • Java 8 support : derived / custom queries can now return stream results and Optional.

  • @Async support.

  • Annotations are now only supported on attributes.

  • Configuration is now best done as code with the new configuration builder.

  • Internal metadata handling has been refactored for better reliability.

  • Upgrade to Neo4j OGM 3.0.0-M01.

  • Support for Neo4j 3.1.2.

  • Bolt is now the default database protocol.

  • When migrating from 4.x, please see the migration guide below. = Dependencies

Due to 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 by relying on the Spring Data Release Train BOM we ship with the compatible versions defined. In a Maven project you’d declare this dependency in the <dependencyManagement /> section of your POM:

Example 1. Using the Spring Data release train BOM
<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>org.springframework.data</groupId>
      <artifactId>spring-data-releasetrain</artifactId>
      <version>${release-train}</version>
      <scope>import</scope>
      <type>pom</type>
    </dependency>
  </dependencies>
</dependencyManagement>

The current release train version is Kay-BUILD-SNAPSHOT. The train names are ascending alphabetically and currently available ones are listed here. The version name follows the following pattern: ${name}-${release} where release can be one of the following:

  • BUILD-SNAPSHOT - current snapshots

  • M1, M2 etc. - milestones

  • RC1, RC2 etc. - release candidates

  • RELEASE - GA release

  • SR1, SR2 etc. - service releases

A working example of using the BOMs can be found in our Spring Data examples repository. If that’s in place declare the Spring Data modules you’d like to use without a version in the <dependencies /> block.

Example 2. Declaring a dependency to a Spring Data module
<dependencies>
  <dependency>
    <groupId>org.springframework.data</groupId>
    <artifactId>spring-data-jpa</artifactId>
  </dependency>
<dependencies>

5.2. Dependency management with Spring Boot

Spring Boot already selects a very recent version of Spring Data modules for you. In case you want to upgrade to a newer version nonetheless, simply configure the property spring-data-releasetrain.version to the train name and iteration you’d like to use.

5.3. Spring Framework

The current version of Spring Data modules require Spring Framework in version 5.0.0.RC2 or better. The modules might also work with an older bugfix version of that minor version. However, using the most recent version within that generation is highly recommended.

6. Working with Spring Data Repositories

The goal of Spring Data repository abstraction is to significantly reduce the amount of boilerplate code required to implement data access layers for various persistence stores.

Spring Data repository documentation and your module

This chapter explains the core concepts and interfaces of Spring Data repositories. The information in this chapter is pulled from the Spring Data Commons module. It uses the configuration and code samples for the Java Persistence API (JPA) module. Adapt the XML namespace declaration and the types to be extended to the equivalents of the particular module that you are using. 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, consult the chapter on that module of this document.

6.1. Core concepts

The central interface in Spring Data repository abstraction is Repository (probably not that much of a surprise). 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.

Example 3. CrudRepository interface
public interface CrudRepository<T, ID extends Serializable>
    extends Repository<T, ID> {

    <S extends T> S save(S entity); (1)

    T findOne(ID primaryKey);       (2)

    Iterable<T> findAll();          (3)

    Long count();                   (4)

    void delete(T entity);          (5)

    boolean exists(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 like e.g. 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 like e.g. CrudRepository.

On top of the CrudRepository there is a PagingAndSortingRepository abstraction that adds additional methods to ease paginated access to entities:

Example 4. PagingAndSortingRepository
public interface PagingAndSortingRepository<T, ID extends Serializable>
  extends CrudRepository<T, ID> {

  Iterable<T> findAll(Sort sort);

  Page<T> findAll(Pageable pageable);
}

Accessing the second page of User by a page size of 20 you could simply do something like this:

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.

Example 5. Derived Count Query
public interface UserRepository extends CrudRepository<User, Long> {

  Long countByLastname(String lastname);
}
Example 6. Derived Delete Query
public interface UserRepository extends CrudRepository<User, Long> {

  Long deleteByLastname(String lastname);

  List<User> removeByLastname(String lastname);

}

6.2. Query methods

Standard CRUD functionality repositories usually have queries on the underlying datastore. With Spring Data, declaring those queries becomes a four-step process:

  1. Declare an interface extending Repository or one of its subinterfaces and type it to the domain class and ID type that it will handle.

    interface PersonRepository extends Repository<Person, Long> { … }
  2. Declare query methods on the interface.

    interface PersonRepository extends Repository<Person, Long> {
      List<Person> findByLastname(String lastname);
    }
  3. Set up Spring to create proxy instances for those interfaces. Either via JavaConfig:

    import org.springframework.data.jpa.repository.config.EnableJpaRepositories;
    
    @EnableJpaRepositories
    class Config {}
    <?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 are using the repository abstraction for any other store, you need to change this to the appropriate namespace declaration of your store module which should be exchanging jpa in favor of, for example, mongodb.

    Also, note that the JavaConfig variant doesn’t configure a package explictly as the package of the annotated class is used by default. To customize the package to scan use one of the basePackage… attribute of the data-store specific repository @Enable…-annotation.

  4. Get the repository instance injected and use it.

    public class SomeClient {
    
      @Autowired
      private PersonRepository repository;
    
      public void doSomething() {
        List<Person> persons = repository.findByLastname("Matthews");
      }
    }

The sections that follow explain each step in detail.

6.3. Defining repository interfaces

As a first step you 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.

6.3.1. Fine-tuning repository definition

Typically, your repository interface will extend 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, simply copy the ones you want to expose from CrudRepository into your domain repository.

This allows you to define your own abstractions on top of the provided Spring Data Repositories functionality.
Example 7. Selectively exposing CRUD methods
@NoRepositoryBean
interface MyBaseRepository<T, ID extends Serializable> extends Repository<T, ID> {

  T findOne(ID id);

  T save(T entity);
}

interface UserRepository extends MyBaseRepository<User, Long> {
  User findByEmailAddress(EmailAddress emailAddress);
}

In this first step you defined a common base interface for all your domain repositories and exposed findOne(…) as well as save(…).These methods will be routed into the base repository implementation of the store of your choice provided by Spring Data ,e.g. in the case if JPA SimpleJpaRepository, because they are matching the method signatures in CrudRepository. So the UserRepository will now be able to save users, and find single ones by id, as well as triggering a query to find Users by their email address.

Note, that the intermediate repository interface is annotated with @NoRepositoryBean. Make sure you add that annotation to all repository interfaces that Spring Data should not create instances for at runtime.

6.3.2. Using Repositories with multiple Spring Data modules

Using a unique Spring Data module in your application makes things simple hence, 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 case, it’s required for a repository definition to distinguish between persistence technologies. Spring Data enters strict repository configuration mode because it detects multiple repository factories on the class path. Strict configuration requires details on the repository or the domain class to decide about Spring Data module binding for a repository definition:

  1. If the repository definition extends the module-specific repository, then it’s a valid candidate for the particular Spring Data module.

  2. If the domain class is annotated with the module-specific type annotation, then it’s a valid candidate for the particular Spring Data module. Spring Data modules accept either 3rd party annotations (such as JPA’s @Entity) or provide own annotations such as @Document for Spring Data MongoDB/Spring Data Elasticsearch.

Example 8. Repository definitions using Module-specific Interfaces
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.

Example 9. Repository definitions using 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 using a unique Spring Data module, multiple modules cannot distinguish to which particular Spring Data these repositories should be bound.

Example 10. Repository definitions using Domain Classes with Annotations
interface PersonRepository extends Repository<Person, Long> {
 …
}

@Entity
public class Person {
  …
}

interface UserRepository extends Repository<User, Long> {
 …
}

@Document
public class User {
  …
}

PersonRepository references Person which is annotated with the JPA annotation @Entity so this repository clearly belongs to Spring Data JPA. UserRepository uses User annotated with Spring Data MongoDB’s @Document annotation.

Example 11. Repository definitions using Domain Classes with mixed Annotations
interface JpaPersonRepository extends Repository<Person, Long> {
 …
}

interface MongoDBPersonRepository extends Repository<Person, Long> {
 …
}

@Entity
@Document
public 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 identifying domain class annotations are used for strict repository configuration identify repository candidates for a particular Spring Data module. Using multiple persistence technology-specific annotations on the same domain type is possible to reuse domain types across multiple persistence technologies, but then Spring Data is no longer able to determine a unique module to bind the repository.

The last way to distinguish repositories is scoping repository base packages. Base packages define the starting points for scanning for repository interface definitions which implies to have 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.

Example 12. Annotation-driven configuration of base packages
@EnableJpaRepositories(basePackages = "com.acme.repositories.jpa")
@EnableMongoRepositories(basePackages = "com.acme.repositories.mongo")
interface Configuration { }

6.4. Defining query methods

The repository proxy has two ways to derive a store-specific query from the method name. It can derive the query from the method name directly, or by using a manually defined query. Available options depend on the actual store. However, there’s got to be a strategy that decides what actual query is created. Let’s have a look at the available options.

6.4.1. Query lookup strategies

The following strategies are available for the repository infrastructure to resolve the query. You can configure the strategy at the namespace through the query-lookup-strategy attribute in case of XML configuration or via the queryLookupStrategy attribute of the Enable${store}Repositories annotation in case of Java config. Some strategies may not be supported for particular datastores.

  • CREATE attempts to construct a store-specific query from the query method name. The general approach is to remove a given set of well-known prefixes from the method name and parse the rest of the method. Read more about query construction in Query creation.

  • USE_DECLARED_QUERY tries to find a declared query and will throw an exception in case it can’t 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) combines CREATE and USE_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 will be 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.

6.4.2. 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.

Example 13. Query creation from method names
public 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 AND and OR. You also get support for operators such as Between, LessThan, GreaterThan, Like for the property expressions. The supported operators can vary by datastore, so consult the appropriate part of your reference documentation.

  • The method parser supports setting an IgnoreCase flag for individual properties (for example, findByLastnameIgnoreCase(…)) or for all properties of a type that support ignoring case (usually String instances, for example, findByLastnameAndFirstnameAllIgnoreCase(…)). Whether ignoring cases is supported may vary by store, so consult the relevant sections in the reference documentation for the store-specific query method.

  • You can apply static ordering by appending an OrderBy clause to the query method that references a property and by providing a sorting direction (Asc or Desc). To create a query method that supports dynamic sorting, see Special parameter handling.

6.4.3. 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. Assume a Person has an Address with a ZipCode. In that case a method name of

List<Person> findByAddressZipCode(ZipCode zipCode);

creates the property traversal x.address.zipCode. The resolution algorithm starts with 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 continue building the tree down from there, splitting the tail up in the way just described. If the first split does not match, the algorithm move 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 and essentially choose the wrong property and finally 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 end up like so:

List<Person> findByAddress_ZipCode(ZipCode zipCode);

As we treat underscore as a reserved character we strongly advise to follow standard Java naming conventions (i.e. not using underscores in property names but camel case instead).

6.4.4. Special parameter handling

To handle parameters in your query you simply define method parameters as already seen in the examples above. Besides that the infrastructure will recognize certain specific types like Pageable and Sort to apply pagination and sorting to your queries dynamically.

Example 14. Using Pageable, Slice and Sort in query methods
Page<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 allows you to 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, Slice can be used as return instead. A Slice only knows about whether there’s a next Slice available which might be just sufficient when walking through a larger result set.

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

To find out how many pages you get for a query entirely you have to trigger an additional count query. By default this query will be derived from the query you actually trigger.

6.4.5. Limiting query results

The results of query methods can be limited via the keywords first or top, which can be used interchangeably. An optional numeric value can be appended to top/first to specify the maximum result size to be returned. If the number is left out, a result size of 1 is assumed.

Example 15. Limiting the result size of a query with Top and First
User findFirstByOrderByLastnameAsc();

User findTopByOrderByAgeDesc();

Page<User> queryFirst10ByLastname(String lastname, Pageable pageable);

Slice<User> findTop3ByLastname(String lastname, Pageable pageable);

List<User> findFirst10ByLastname(String lastname, Sort sort);

List<User> findTop10ByLastname(String lastname, Pageable pageable);

The limiting expressions also support the Distinct keyword. Also, for the queries limiting the result set to one instance, wrapping the result into an Optional is supported.

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

Note that limiting the results in combination with dynamic sorting via a Sort parameter allows to express query methods for the 'K' smallest as well as for the 'K' biggest elements.

6.4.6. Streaming query results

The results of query methods can be processed incrementally by using a Java 8 Stream<T> as return type. Instead of simply wrapping the query results in a Stream data store specific methods are used to perform the streaming.

Example 16. Stream the result of a query with Java 8 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 using the close() method or by using a Java 7 try-with-resources block.
Example 17. Working with a Stream<T> result in a try-with-resources block
try (Stream<User> stream = repository.findAllByCustomQueryAndStream()) {
  stream.forEach(…);
}
Not all Spring Data modules currently support Stream<T> as a return type.

6.4.7. Async query results

Repository queries can be executed asynchronously using Spring’s asynchronous method execution capability. This means the method will return immediately upon invocation and the actual query execution will occur in a task that has been submitted to a Spring TaskExecutor.

@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 return type.
2 Use a Java 8 java.util.concurrent.CompletableFuture as return type.
3 Use a org.springframework.util.concurrent.ListenableFuture as return type.

6.5. Creating repository instances

In this section you create instances and bean definitions for the repository interfaces defined. One way to do so is using the Spring namespace that is shipped with each Spring Data module that supports the repository mechanism although we generally recommend to use the Java-Config style configuration.

6.5.1. XML configuration

Each Spring Data module includes a repositories element that allows you to simply define a base package that Spring scans for you.

Example 18. Enabling Spring Data repositories via XML
<?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 bean instances get created for. To do this you use <include-filter /> and <exclude-filter /> elements inside <repositories />. The semantics are exactly equivalent to the elements in Spring’s context namespace. For details, see Spring reference documentation on these elements.

For example, to exclude certain interfaces from instantiation as repository, you could use the following configuration:

Example 19. Using exclude-filter element
<repositories base-package="com.acme.repositories">
  <context:exclude-filter type="regex" expression=".*SomeRepository" />
</repositories>

This example excludes all interfaces ending in SomeRepository from being instantiated.

6.5.2. JavaConfig

The repository infrastructure can also be triggered using a store-specific @Enable${store}Repositories annotation on a JavaConfig class. For an introduction into Java-based configuration of the Spring container, see the reference documentation.[1]

A sample configuration to enable Spring Data repositories looks something like this.

Example 20. Sample annotation based repository configuration
@Configuration
@EnableJpaRepositories("com.acme.repositories")
class ApplicationConfiguration {

  @Bean
  public EntityManagerFactory entityManagerFactory() {
    // …
  }
}
The sample 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. Consult the sections covering the store-specific configuration.

6.5.3. Standalone usage

You can also use the repository infrastructure outside of a Spring container, e.g. 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.

Example 21. Standalone usage of repository factory
RepositoryFactorySupport factory = … // Instantiate factory here
UserRepository repository = factory.getRepository(UserRepository.class);

6.6. Custom implementations for Spring Data repositories

Often it is necessary to provide a custom implementation for a few repository methods. Spring Data repositories easily allow you to provide custom repository code and integrate it with generic CRUD abstraction and query method functionality.

6.6.1. Adding custom behavior to single repositories

To enrich a repository with custom functionality you first define an interface and an implementation for the custom functionality. Use the repository interface you provided to extend the custom interface.

Example 22. Interface for custom repository functionality
interface UserRepositoryCustom {
  public void someCustomMethod(User user);
}
Example 23. Implementation of custom repository functionality
class UserRepositoryImpl implements UserRepositoryCustom {

  public void someCustomMethod(User user) {
    // Your custom implementation
  }
}
The most important bit for the class to be found is the Impl postfix of the name on it compared to the core repository interface (see below).

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

Example 24. Changes to the your basic repository interface
interface UserRepository extends CrudRepository<User, Long>, UserRepositoryCustom {

  // Declare query methods here
}

Let your standard repository interface extend the custom one. Doing so combines the CRUD and custom functionality and makes it available to clients.

Configuration

If you use namespace configuration, the repository infrastructure tries to autodetect custom implementations by scanning for classes below the package we found a repository in. These classes need to follow the naming convention of appending the namespace element’s attribute repository-impl-postfix to the found repository interface name. This postfix defaults to Impl.

Example 25. Configuration example
<repositories base-package="com.acme.repository" />

<repositories base-package="com.acme.repository" repository-impl-postfix="FooBar" />

The first configuration example will try to look up a class com.acme.repository.UserRepositoryImpl to act as custom repository implementation, whereas the second example will try to lookup com.acme.repository.UserRepositoryFooBar.

Resolution of ambiguity

If multiple implementations with matching class names get found in different packages, Spring Data uses the bean names to identify the correct one to use.

Given the following two custom implementations for the UserRepository introduced above the first implementation will get picked. Its bean name is userRepositoryImpl matches that of the repository interface (userRepository) plus the postfix Impl.

Example 26. Resolution of amibiguous implementations
package com.acme.impl.one;

class UserRepositoryImpl implements UserRepositoryCustom {

  // Your custom implementation
}
package com.acme.impl.two;

@Component("specialCustomImpl")
class UserRepositoryImpl implements UserRepositoryCustom {

  // Your custom implementation
}

If you annotate the UserRepository interface with @Component("specialCustom") the bean name plus Impl matches the one defined for the repository implementation in com.acme.impl.two and it will be picked instead of the first one.

Manual wiring

The approach just shown works well if your custom implementation uses annotation-based configuration and autowiring only, as it will be treated as any other Spring bean. If your custom implementation bean needs special wiring, you simply declare the bean and name it after the conventions just described. The infrastructure will then refer to the manually defined bean definition by name instead of creating one itself.

Example 27. Manual wiring of custom implementations
<repositories base-package="com.acme.repository" />

<beans:bean id="userRepositoryImpl" class="…">
  <!-- further configuration -->
</beans:bean>

6.6.2. Adding custom behavior to all repositories

The preceding approach is not feasible when you want to add a single method to all your repository interfaces. To add custom behavior to all repositories, you first add an intermediate interface to declare the shared behavior.

Example 28. An interface declaring custom shared behavior
@NoRepositoryBean
public interface MyRepository<T, ID extends Serializable>
  extends PagingAndSortingRepository<T, ID> {

  void sharedCustomMethod(ID id);
}

Now your individual repository interfaces will extend this intermediate interface instead of the Repository interface to include the functionality declared. Next, create an implementation of the intermediate interface that extends the persistence technology-specific repository base class. This class will then act as a custom base class for the repository proxies.

Example 29. Custom repository base class
public class MyRepositoryImpl<T, ID extends Serializable>
  extends SimpleJpaRepository<T, ID> implements MyRepository<T, ID> {

  private final EntityManager entityManager;

  public MyRepositoryImpl(JpaEntityInformation entityInformation,
                          EntityManager entityManager) {
    super(entityInformation, entityManager);

    // Keep the EntityManager around to used from the newly introduced methods.
    this.entityManager = entityManager;
  }

  public void sharedCustomMethod(ID id) {
    // implementation goes here
  }
}
The class needs to have a constructor of the super class which the store-specific repository factory implementation is using. In case the repository base class has multiple constructors, override the one taking an EntityInformation plus a store specific infrastructure object (e.g. an EntityManager or a template class).

The default behavior of the Spring <repositories /> namespace is to provide an implementation for all interfaces that fall under the base-package. This means that if left in its current state, an implementation instance of MyRepository will be created by Spring. This is of course not desired as it is just supposed to act as an intermediary between Repository and the actual repository interfaces you want to define for each entity. To exclude an interface that extends Repository from being instantiated as a repository instance, you can either annotate it with @NoRepositoryBean (as seen above) or move it outside of the configured base-package.

The final step is to make the Spring Data infrastructure aware of the customized repository base class. In JavaConfig this is achieved by using the repositoryBaseClass attribute of the @Enable…Repositories annotation:

Example 30. Configuring a custom repository base class using JavaConfig
@Configuration
@EnableJpaRepositories(repositoryBaseClass = MyRepositoryImpl.class)
class ApplicationConfiguration { … }

A corresponding attribute is available in the XML namespace.

Example 31. Configuring a custom repository base class using XML
<repositories base-package="com.acme.repository"
     base-class="….MyRepositoryImpl" />

6.7. 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 @DomainEvents you can use on a method of your aggregate root to make that publication as easy as possible.

Example 32. Exposing domain events from an aggregate root
class AnAggregateRoot {

    @DomainEvents (1)
    Collection<Object> domainEvents() {
        // … return events you want to get published here
    }

    @AfterDomainEventsPublication (2)
    void callbackMethod() {
       // … potentially clean up domain events list
    }
}
1 The method using @DomainEvents can either return a single event instance or a collection of events. It must not take any arguments.
2 After all events have been published, a method annotated with @AfterDomainEventsPublication. It e.g. can be used to potentially clean the list of events to be published.

The methods will be called every time one of a Spring Data repository’s save(…) methods is called.

6.8. 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.

6.8.1. Querydsl Extension

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

Several Spring Data modules offer integration with Querydsl via QueryDslPredicateExecutor.

Example 33. QueryDslPredicateExecutor interface
public interface QueryDslPredicateExecutor<T> {

    T findOne(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 if an entity that matches the Predicate exists.

To make use of Querydsl support simply extend QueryDslPredicateExecutor on your repository interface.

Example 34. Querydsl integration on repositories
interface UserRepository extends CrudRepository<User, Long>, QueryDslPredicateExecutor<User> {

}

The above enables to write typesafe queries using Querydsl Predicate s.

Predicate predicate = user.firstname.equalsIgnoreCase("dave")
	.and(user.lastname.startsWithIgnoreCase("mathews"));

userRepository.findAll(predicate);

6.8.2. Web support

This section contains the documentation for the Spring Data web support as it is implemented as of Spring Data Commons in the 1.6 range. As it the newly introduced support changes quite a lot of things we kept the documentation of the former behavior in Legacy web support.

Spring Data modules ships with a variety of web support if the module supports the repository programming model. The web related stuff requires Spring MVC JARs on the classpath, some of them even provide integration with Spring HATEOAS [2]. In general, the integration support is enabled by using the @EnableSpringDataWebSupport annotation in your JavaConfig configuration class.

Example 35. Enabling Spring Data web support
@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 are using XML configuration, register either SpringDataWebSupport or HateoasAwareSpringDataWebSupport as Spring beans:

Example 36. Enabling Spring Data web support in XML
<bean class="org.springframework.data.web.config.SpringDataWebConfiguration" />

<!-- If you're using Spring HATEOAS as well register this one *instead* of the former -->
<bean class="org.springframework.data.web.config.HateoasAwareSpringDataWebConfiguration" />
Basic web support

The configuration setup shown above will register a few basic components:

  • A DomainClassConverter to enable Spring MVC to resolve instances of repository managed domain classes from request parameters or path variables.

  • HandlerMethodArgumentResolver implementations to let Spring MVC resolve Pageable and Sort instances from request parameters.

DomainClassConverter

The DomainClassConverter allows you to use domain types in your Spring MVC controller method signatures directly, so that you don’t have to manually lookup the instances via the repository:

Example 37. A Spring MVC controller using domain types in method signatures
@Controller
@RequestMapping("/users")
public class UserController {

  @RequestMapping("/{id}")
  public 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 findOne(…) 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 above also registers a PageableHandlerMethodArgumentResolver as well as an instance of SortHandlerMethodArgumentResolver. The registration enables Pageable and Sort being valid controller method arguments

Example 38. Using Pageable as controller method argument
@Controller
@RequestMapping("/users")
public class UserController {

  @Autowired UserRepository repository;

  @RequestMapping
  public String showUsers(Model model, Pageable pageable) {

    model.addAttribute("users", repository.findAll(pageable));
    return "users";
  }
}

This method signature will cause Spring MVC try to derive a Pageable instance from the request parameters using the following default configuration:

Table 1. Request parameters evaluated for Pageable instances

page

Page you want to retrieve, 0 indexed and defaults to 0.

size

Size of the page you want to retrieve, defaults to 20.

sort

Properties that should be sorted by in the format property,property(,ASC|DESC). Default sort direction is ascending. Use multiple sort parameters if you want to switch directions, e.g. ?sort=firstname&sort=lastname,asc.

To customize this behavior register a bean implementing the interface PageableHandlerMethodArgumentResolverCustomizer or SortHandlerMethodArgumentResolverCustomizer respectively. It’s customize() method will get called allowing you to change settings. Like in the following example.

@Bean SortHandlerMethodArgumentResolverCustomizer sortCustomizer() {
    return s -> s.setPropertyDelimiter("<-->");
}

If setting the properties of an existing MethodArgumentResolver isn’t sufficient for your purpose extend either SpringDataWebConfiguration or the HATEOAS-enabled equivalent and override the pageableResolver() or sortResolver() methods and import your customized configuration file instead of using the @Enable-annotation.

In case 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}_. So for a method signature like this:

public String showUsers(Model model,
      @Qualifier("foo") Pageable first,
      @Qualifier("bar") Pageable second) { … }

you have to populate foo_page and bar_page etc.

The default Pageable handed into the method is equivalent to a new PageRequest(0, 20) but can be customized using the @PageableDefaults 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, the PagedResourcesAssembler.

Example 39. Using a PagedResourcesAssembler as 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 above allows the PagedResourcesAssembler to be used as controller method argument. Calling toResources(…) on it will cause the following:

  • The content of the Page will become the content of the PagedResources instance.

  • The PagedResources will get a PageMetadata instance attached populated with information form the Page and the underlying PageRequest.

  • The PagedResources gets prev and next links attached depending on the page’s state. The links will point to the URI the method invoked is mapped to. The pagination parameters added to the method will match the setup of the PageableHandlerMethodArgumentResolver to make sure the links can be resolved later on.

Assume we have 30 Person instances in the database. You can now trigger a request GET http://localhost:8080/persons and you’ll see something similar to this:

{ "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 picks up the default configuration present to resolve the parameters into a Pageable for an upcoming request. This means, if you change that configuration, the links will 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 to overloads of the PagedResourcesAssembler.toResource(…) method.

Querydsl web support

For those stores having QueryDSL integration it is possible to derive queries from the attributes contained in a Request query string.

This means that given the User object from previous samples a query string

?firstname=Dave&lastname=Matthews

can be resolved to

QUser.user.firstname.eq("Dave").and(QUser.user.lastname.eq("Matthews"))

using the QuerydslPredicateArgumentResolver.

The feature will be automatically enabled along @EnableSpringDataWebSupport when Querydsl is found on the classpath.

Adding a @QuerydslPredicate to the method signature will provide a ready to use Predicate which can be executed via the QueryDslPredicateExecutor.

Type information is typically resolved from the methods return type. Since those information does not necessarily match the domain type it might be a good idea to use the root attribute of QuerydslPredicate.
@Controller
class UserController {

  @Autowired UserRepository repository;

  @RequestMapping(value = "/", method = RequestMethod.GET)
  String index(Model model, @QuerydslPredicate(root = User.class) Predicate predicate,    (1)
          Pageable pageable, @RequestParam MultiValueMap<String, String> parameters) {

    model.addAttribute("users", repository.findAll(predicate, pageable));

    return "index";
  }
}
1 Resolve query string arguments to matching Predicate for User.

The default binding is as follows:

  • Object on simple properties as eq.

  • Object on collection like properties as contains.

  • Collection on simple properties as in.

Those bindings can be customized via the bindings attribute of @QuerydslPredicate or by making use of Java 8 default methods adding the QuerydslBinderCustomizer to the repository interface.

interface UserRepository extends CrudRepository<User, String>,
                                 QueryDslPredicateExecutor<User>,                (1)
                                 QuerydslBinderCustomizer<QUser> {               (2)

  @Override
  default public 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 will be 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.

6.8.3. Repository populators

If you work with the Spring JDBC module, you probably are familiar with the support to populate a DataSource using 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:

Example 40. Data defined in JSON
[ { "_class" : "com.acme.Person",
 "firstname" : "Dave",
  "lastname" : "Matthews" },
  { "_class" : "com.acme.Person",
 "firstname" : "Carter",
  "lastname" : "Beauford" } ]

You can easily 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 , do the following:

Example 41. Declaring a Jackson repository populator
<?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>

This declaration causes the data.json file to be read and deserialized via a Jackson ObjectMapper.

The type to which the JSON object will be unmarshalled to will be determined by inspecting the _class attribute of the JSON document. The infrastructure will eventually select the appropriate repository to handle the object just deserialized.

To rather use XML to define the data the repositories shall be populated with, you can use the unmarshaller-populator element. You configure it to use one of the XML marshaller options Spring OXM provides you with. See the Spring reference documentation for details.

Example 42. Declaring an unmarshalling repository populator (using 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>

6.8.4. Legacy web support

Domain class web binding for Spring MVC

Given you are developing a Spring MVC web application you typically have to resolve domain class ids from URLs. By default your task is to transform that request parameter or URL part into the domain class to hand it to layers below then or execute business logic on the entities directly. This would look something like this:

@Controller
@RequestMapping("/users")
public class UserController {

  private final UserRepository userRepository;

  @Autowired
  public UserController(UserRepository userRepository) {
    Assert.notNull(repository, "Repository must not be null!");
    this.userRepository = userRepository;
  }

  @RequestMapping("/{id}")
  public String showUserForm(@PathVariable("id") Long id, Model model) {

    // Do null check for id
    User user = userRepository.findOne(id);
    // Do null check for user

    model.addAttribute("user", user);
    return "user";
  }
}

First you declare a repository dependency for each controller to look up the entity managed by the controller or repository respectively. Looking up the entity is boilerplate as well, as it’s always a findOne(…) call. Fortunately Spring provides means to register custom components that allow conversion between a String value to an arbitrary type.

PropertyEditors

For Spring versions before 3.0 simple Java PropertyEditors had to be used. To integrate with that, Spring Data offers a DomainClassPropertyEditorRegistrar, which looks up all Spring Data repositories registered in the ApplicationContext and registers a custom PropertyEditor for the managed domain class.

<bean class="….web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter">
  <property name="webBindingInitializer">
    <bean class="….web.bind.support.ConfigurableWebBindingInitializer">
      <property name="propertyEditorRegistrars">
        <bean class="org.springframework.data.repository.support.DomainClassPropertyEditorRegistrar" />
      </property>
    </bean>
  </property>
</bean>

If you have configured Spring MVC as in the preceding example, you can configure your controller as follows, which reduces a lot of the clutter and boilerplate.

@Controller
@RequestMapping("/users")
public class UserController {

  @RequestMapping("/{id}")
  public String showUserForm(@PathVariable("id") User user, Model model) {

    model.addAttribute("user", user);
    return "userForm";
  }
}

Reference Documentation

7. Introduction

Spring Data Neo4j has been rewritten between versions 3.5 and 4. In order to understand what Spring Data Neo4j can do it’s important to understand how an SDN application is structured and could have implications in how you design your application.

7.1. Components

SDN 3.x was monolithic in a sense that all mapping code, database drivers and Spring integrations were all in one. SDN 4 has decided to break these components up into:

  • Drivers: At the moment these come in 3 variants: Embedded, HTTP and the binary protocol Bolt.

  • The Object Graph Mapper (OGM): This is similar to an ORM in that it maps database nodes to java objects. This library is agnostic of any framework (including Spring).

  • Spring Data Neo4j 4: Provides syntactic sugar and code on top of the OGM to help quickly build Spring Based Neo4j/OGM apps.

Those coming from other Spring Data projects or are familiar with ORM products like JPA or Hibernate may quickly recognise this architecture. A bulk of the heavy lifting has been moved into the OGM.

It’s therefore worth noting that there will be backward compatibility issues when migrating to version 4.x, so be sure to check the Migration Guide to avoid any unwanted surprises.
Spring Data Neo4j Architecture

7.2. How to use this reference

Spring Data Neo4j is largely broken up into two main components:

It is recommended SDN developers also familiarise themselves with the OGM. The OGM reference documentation has been reproduced after this section for convenience.

8. Getting started

Depending on what type of project you are doing there are several options when it comes to creating a new SDN project:

If you plan on using Neo4j in server mode, you will also need a running instance. Refer to the Getting Started section of the Neo4j Developer manual on how to get that up and running.

8.1. Using Boot

To create a Spring Boot project simply go to http://start.spring.io and specify a group and artifact like: org.spring.neo4j.example and demo. In the Dependencies box type: "Neo4J". You can also add any other Spring support like "Web" etc. Once you are satisfied with your dependencies hit the generate button, download the zip and unzip into your workspace.

8.2. Using STS

To create a Spring project in STS go to File → New → Spring Template Project → Simple Spring Utility Project → press Yes when prompted. Then enter a project and a package name such as org.spring.neo4j.example.

Then add the following to pom.xml dependencies section.

<dependencies>

  <!-- other dependency elements omitted -->

  <dependency>
    <groupId>org.springframework.data</groupId>
    <artifactId>spring-data-neo4j</artifactId>
    <version>{version}</version>
  </dependency>

</dependencies>

Also change the version of Spring in the pom.xml to be

<spring.framework.version>{springVersion}</spring.framework.version>

8.3. Using Dependency Management

Spring Data Neo4j projects can be built using Maven, Gradle or any other tool that supports Maven’s repository system.

For more in depth configuration details please consult the Configuration section of the OGM Reference Manual.

8.3.1. Maven

By default, SDN will use the BOLT driver to connect to Neo4j and you don’t need to declare it as a separate dependency in your pom. If you want to use the embedded or HTTP drivers in your production application, you must add the following dependencies as well. (This dependency on the embedded driver is not required if you only want to use the embedded driver for testing. See the section on Testing below for more information).

<dependency>
    <groupId>org.springframework.data</groupId>
    <artifactId>spring-data-neo4j</artifactId>
    <version>{version}</version>
</dependency>

<!-- add this dependency if you want to use the embedded driver -->
<dependency>
    <groupId>org.neo4j</groupId>
    <artifactId>neo4j-ogm-embedded-driver</artifactId>
    <version>{ogm-version}</version>
</dependency>

<!-- add this dependency if you want to use the HTTP driver -->
<dependency>
    <groupId>org.neo4j</groupId>
    <artifactId>neo4j-ogm-http-driver</artifactId>
    <version>{ogm-version}</version>
</dependency>
Testing
Maven dependencies for testing SDN 5 applications
       <dependency>
           <groupId>org.springframework.data</groupId>
           <artifactId>spring-data-neo4j</artifactId>
           <version>${sdn.version}</version>
           <type>test-jar</type>
       </dependency>

       <!-- the neo4j-ogm-test jar provides access to the http and embedded drivers for testing purposes -->
       <dependency>
          <groupId>org.neo4j</groupId>
          <artifactId>neo4j-ogm-test</artifactId>
          <version>${neo4j-ogm.version}</version>
          <type>test-jar</type>
          <scope>test</scope>
      </dependency>

       <dependency>
           <groupId>org.neo4j</groupId>
            <artifactId>neo4j-kernel</artifactId>
            <version>${neo4j.version}</version>
            <type>test-jar</type>
       </dependency>

       <dependency>
            <groupId>org.neo4j.app</groupId>
            <artifactId>neo4j-server</artifactId>
            <version>${neo4j.version}</version>
            <type>test-jar</type>
       </dependency>

      <dependency>
          <groupId>org.neo4j.test</groupId>
          <artifactId>neo4j-harness</artifactId>
          <version>${neo4j.version}</version>
          <scope>test</scope>
      </dependency>

The InProcessServer has been deprecated. This class was used in previous versions to set up an in-memory Http server so that you could run your tests. This is no longer appropriate given the new Driver mechanism, and we recommend you configure an Embedded Driver (impermanent data store) for your integration tests instead.

8.3.2. Gradle

Gradle dependencies are basically the same as Maven:

dependencies {
    compile 'org.springframework.data:spring-data-neo4j:{version}'

    # add this dependency if you want to use the embedded driver
    compile 'org.neo4j:neo4j-ogm-embedded-driver:{ogm-version}'

    # add this dependency if you want to use the Http driver
    compile 'org.neo4j:neo4j-ogm-http-driver:{ogm-version}'

}

8.4. Examples

There is an github repository with several examples that you can download and play around with to get a feel for how the library works.

8.5. Configuration

Right now SDN only supports JavaConfig. There is no XML based support but this may change in future.

For those not familiar with how to configure the Spring container using Java based bean metadata instead of XML based metadata see the high level introduction in the reference docs here as well as the detailed documentation here.

For most applications the following configuration is all that’s needed to get up and running.

Spring Data Neo4j repositories using JavaConfig
@Configuration
@EnableNeo4jRepositories(basePackages = "org.neo4j.example.repository")
@EnableTransactionManagement
public class MyConfiguration {

    @Bean
    public SessionFactory sessionFactory() {
        // with domain entity base package(s)
        return new SessionFactory(configuration(), "org.neo4j.example.domain");
    }

    @Bean
    public org.neo4j.ogm.config.Configuration configuration() {
        ConfigurationSource properties = new ClasspathConfigurationSource("ogm.properties");
        org.neo4j.ogm.config.Configuration configuration = new org.neo4j.ogm.config.Configuration.Builder(properties)
        return configuration;
    }

    @Bean
    public Neo4jTransactionManager transactionManager() {
        return new Neo4jTransactionManager(sessionFactory());
    }

}

Here we wire up a SessionFactory configured from defaults. We can change these defaults by providing an ogm.properties file at the root of the classpath or by passing in a org.neo4j.ogm.config.Configuration object. The last infrastructure component declared here is the Neo4jTransactionManager. We finally activate Spring Data Neo4j repositories using the @EnableNeo4jRepositories annotation. If no base package is configured it will use the one the configuration class resides in.

Note that you will have to activate @EnableTransactionManagement explicitly to get annotation based configuration at facades working as well as define an instance of this Neo4jTransactionManager with the bean name transactionManager. The example above assumes you are using component scanning.

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

8.5.1. Driver Configuration

SDN provides support for connecting to Neo4j using different drivers.

The following drivers are available.

  • Http driver

  • Embedded driver

  • Bolt driver

Java Configuration

To configure the Driver programmatically, create a Configuration bean and pass it as the first argument to the SessionFactory constructor in your Spring configuration:

@Bean
public org.neo4j.ogm.config.Configuration configuration() {
    org.neo4j.ogm.config.Configuration configuration = new org.neo4j.ogm.config.Configuration.Builder()
            .uri("bolt://localhost")
            .credentials("user", "secret")
            .build();
    return configuration;
}


@Bean
public SessionFactory sessionFactory() {
    return new SessionFactory(configuration(), <packages> ); (1)
}
1 packages is a list of java packages containing the annotated domain model.

Configuration can also be initialized from an external file like this.

@Bean
public org.neo4j.ogm.config.Configuration configuration() {
    ConfigurationSource properties = new ClasspathConfigurationSource("db.properties");
    org.neo4j.ogm.config.Configuration configuration = new org.neo4j.ogm.config.Configuration.Builder(properties)
    return configuration;
}
Fallback configuration

Please note that this way of configuration might be removed in the future.

Java configuration is the preferred way of initiallizing SDN. However, if no configuration is given to the SessionFactory, SDN will try to configure the driver from a file ogm.properties, which it expects to find on the root of the classpath.

SDN expects the properties file to be called "ogm.properties". If you want to configure your application using a different properties file, you must either set a System property or Environment variable called "ogm.properties" pointing to the alternative configuration file you want to use.

For more detail about this properties file, please refer to the configuration section of the OGM reference.

The driver is automatically inferred from the URI scheme.
To set up authentication, TLS or other advanced options please see the Configuration section of the OGM Reference.
As of 4.2.0 the Neo4j OGM embedded driver no longer ships with the Neo4j kernel. Users are expected to provide this dependency through their dependency management system.

8.5.2. Spring Boot Applications

Spring Boot 2.0 works straight out of the box with Spring Data Neo4j 5.0.0.

To do that update the Spring Boot properties to use the current SDN version. Update your Spring Boot Maven POM with the following. You may need to add <repositories> depending on versioning.

    ...
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-neo4j</artifactId>
        </dependency>
    </dependencies>
    ...
Configuring Events with Boot

When defining a Spring EventListener. Simply defining a @Bean will automatically register it with the SessionFactory.

8.6. Connecting to Neo4j

The SessionFactory is needed by SDN to create instances of org.neo4j.ogm.session.Session as required. When constructed, it sets up the object-graph mapping metadata, which is then used across all Session objects that it creates. As seen in the above example, the packages to scan for domain object metadata should be provided to the SessionFactory constructor.

There should typically be only one SessionFactory per application.

9. Neo4j OGM Support

To get started, you need only your domain model and the annotations provided by the OGM library. You use annotations to mark domain objects to be reflected by nodes and relationships of the graph database. For individual fields the annotations allow you to declare how they should be processed and mapped to the graph. For property fields and references to other entities this is straightforward.

Refer to the OGM documentation for more details.

9.1. What is an OGM?

An OGM (Object Graph Mapper) maps nodes and relationships in the graph to objects and references in your domain model. Object instances are mapped to nodes while object references are mapped using relationships, or serialized to properties (e.g. references to a Date). JVM primitives are mapped to node or relationship properties. An OGM abstracts the database and provides a convenient way to persist your domain model in the graph and query it without using low level drivers. It also provides the flexibility to the developer to supply custom queries where the queries generated by the OGM are insufficient.

The OGM can be thought of as analogous to Hibernate or JPA. It is expected users have a working understanding of the OGM when using this guide.

Session now replaces Neo4jTemplate functionality as all functionality can be found on the OGM Session object.

SDN now allows you to wire up the OGM Session directly into your Spring managed beans.

While SDN Repository will cover a majority of user scenarios sometimes it doesn’t offer enough options. The OGM’s Session offers a convenient API to interact more tightly with a Neo4j graph database.

9.1.1. Understanding the Session

A Session is used to drive the object-graph mapping framework. All repository implementations are driven by the Session. It keeps track of the changes that have been made to entities and their relationships. The reason it does this is so that only entities and relationships that have changed get persisted on save, which is particularly efficient when working with large graphs.

Sessions are usually bound to a thread by default and rely on the garbage collector to clean it up once it is out of scope of processing. For most users this means there is nothing to configure. Request/response type applications SDN will take care of Session management for you (as defined in the Configuration section above). If you have a batch or long running desktop type application you may want to know how you can control using the session a bit more.

Design Consideration: Session caching

Once an entity is tracked by the session, reloading this entity within the scope of the same session will result in the session cache returning the previously loaded entity. However, the subgraph in the session will expand if the entity or its related entities retrieve additional relationships from the graph.

If you want to fetch fresh data from the graph, then this can be achieved by using a new session or clearing the current sessions context using org.neo4j.ogm.session.Session.clear().

The lifetime of the Session can be managed in code. For example, associated with single fetch-update-save cycle or unit of work.

If your application relies on long-running sessions then you may not see changes made from other users and find yourself working with outdated objects. On the other hand, if your sessions have too narrow a scope then your save operations can be unnecessarily expensive, as updates will be made to all objects if the session isn’t aware of the those that were originally loaded.

There’s therefore a trade off between the two approaches. In general, the scope of a Session should correspond to a "unit of work" in your application.

9.2. Basic Operations

For Spring Data Neo4j 4, low level operations are handled by the OGM Session. Basic operations are now entirely limited to CRUD operations on entities and executing arbitrary Cypher queries; more low-level manipulation of the graph database is not possible.

There is no longer a way to manipulate relationship- and node-objects directly.

Given that the latest version of the framework is driven by Cypher queries alone, there’s no way to work directly with Node and Relationship objects any more in remote server mode. Similarly, the traverse() method has disappeared, again because the underlying query-driven model doesn’t handle it in an efficient way.

If you find yourself in trouble because of the omission of these features, then your best options are:

  1. Write a Cypher query to perform the operations on the nodes/relationships instead

  2. Write a Neo4j server extension and call it over REST from your application

Of course, there are pros and cons to both of these approaches, but these are largely outside the scope of this document. In general, for low-level, very high-performance operations like complex graph traversals you’ll get the best performance by writing a server-side extension. For most purposes, though, Cypher will be performant and expressive enough to perform the operations that you need.

9.3. Entity Persistence

Session allows you to save, load, loadAll and delete entities. The eagerness with which objects are retrieved is controlled by specifying the 'depth' argument to any of the load methods.

All of these basic CRUD methods just call onto the underlying methods of Session, albeit with transaction handling and exception translation managed for you by SDN’s Transaction Manager bean.

9.4. Cypher Queries

The Session also allows execution of arbitrary Cypher queries via its query, queryForObject and queryForObjects methods. Cypher queries that return tabular results should be passed into the query method. An org.neo4j.ogm.session.result.Result is returned. This consists of org.neo4j.ogm.session.result.QueryStatistics representing statistics of modifying cypher statements if applicable, and an Iterable<Map<String,Object>> containing the raw data, of which nodes and relationships are mapped to domain entities if possible. The keys in each Map correspond to the names listed in the return clause of the executed Cypher query.

Modifications made to the graph via Cypher queries directly will not be reflected in your domain objects within the session.

9.5. Transactions

If you configured the Neo4jTransactionManager bean, any Session that is managed by Spring will automatically take part in Thread contextual Transactions. In order to do this you will need to wrap your service code using @Transactional or the TransactionTemplate.

It is important to know that if you enable Transactions ALL code that uses the Session or a Repository must be enclosed in a @Transactional annotation.

For more details see Transactions

10. Neo4J Repositories

10.1. Introduction

This chapter will point out the specialties for repository support for Neo4J and the Neo4J OGM. This builds on the core repository support explained in Working with Spring Data Repositories. So make sure you’ve got a sound understanding of the basic concepts explained there.

The following table outlines the repositories functionality currently either supported, partially supported or not supported in SDN:

Feature Supported in SDN Notes

CrudRepository support

PagingAndSortingRepository support

Derived Count Queries

JavaConfig annotation based configuration

XML based configuration

Multi Spring Data module support

Configurable Query Lookup Strategy

Derived Query support

See Supported keywords for query methods below

Derived Query Property expressions support

Paging and Slice support

Derived query paging limit support

Java 8 Streaming and Optional support

@Async support

Custom behaviour on repositories

QueryDslPredicateExecutor support

Web support (incl Spring Data REST)

Partial: QueryDSL not supported.

Repository populators

10.2. Usage

The Repository instances are only created through Spring and can be auto-wired into your Spring beans as required.

Using basic Neo4jRepository CRUD-methods
@Repository
public interface PersonRepository extends Neo4jRepository<Person, Long> {}

public class MySpringBean {
   @Autowired
   private PersonRepository repo;
   ...
}

// then you can use the repository as you would any other object
Person michael = repo.save(new Person("Michael", 36));

Person dave = repo.load(123);

long numberOfPeople = repo.count();

The recommended way of providing repositories is to define a repository interface per domain class. The underlying Spring repository infrastructure will automatically detect these repositories, along with additional implementation classes, and create an injectable repository implementation to be used in services or other spring beans.

The repositories provided by Spring Data Neo4j build on the composable repository infrastructure in Spring Data Commons. These allow for interface-based composition of repositories consisting of provided default implementations for certain interfaces and additional custom implementations for other methods.

Spring Data Neo4j comes with a single org.springframework.data.repository.PagingAndSortingRepository specialisation called Neo4jRepository<T. ID> used for all object-graph mapping repositories. This sub-interface also adds specific finder methods that take a depth argument to control the horizon with which related entities are fetched and saved. Generally, it provides all the desired repository methods. If other operations are required then the additional repository interfaces should be added to the individual interface declaration.

10.3. Query Methods

10.3.1. Query and Finder Methods

Most of the data access operations you usually trigger on a repository result a query being executed against the Neo4j database. Defining such a query is just a matter of declaring a method on the repository interface

Example 43. PersonRepository with query methods
public interface PersonRepository extends PagingAndSortingRepository<Person, String> {

    List<Person> findByLastname(String lastname);                      (1)

    Page<Person> findByFirstname(String firstname, Pageable pageable); (2)

    Person findByShippingAddresses(Address address);                   (3)

    Stream<Person> findAllBy();                                        (4)
}
1 The method shows a query for all people with the given lastname. The query will be derived parsing the method name for constraints which can be concatenated with And and Or. Thus the method name will result in a query expression of {"lastname" : lastname}.
2 Applies pagination to a query. Just equip your method signature with a Pageable parameter and let the method return a Page instance and we will automatically page the query accordingly.
3 Shows that you can query based on properties which are not a primitive type.
4 Uses a Java 8 Stream which reads and converts individual elements while iterating the stream.
Table 2. Supported keywords for query methods
Logical keyword Keyword expressions Restrictions

AND

And

AFTER

After, IsAfter

BEFORE

Before, IsBefore

CONTAINING

Containing, IsContaining, Contains

BETWEEN

Between, IsBetween

ENDING_WITH

EndingWith, IsEndingWith, EndsWith

EXISTS

Exists

FALSE

False, IsFalse

GREATER_THAN

GreaterThan, IsGreaterThan

GREATER_THAN_EQUALS

GreaterThanEqual, IsGreaterThanEqual

IN

In, IsIn

IS

Is, Equals

IS_NOT_NULL

NotNull, IsNotNull

IS_NULL

Null, IsNull

LESS_THAN

LessThan, IsLessThan

LESS_THAN_EQUAL

LessThanEqual, IsLessThanEqual

LIKE

Like, IsLike

NEAR

Near, IsNear

NOT

Not, IsNot

NOT_IN

NotIn, IsNotIn

NOT_LIKE

NotLike, IsNotLike

REGEX

Regex, MatchesRegex, Matches

OR

Or

Cannot be used to OR nested properties

NEAR

Near, IsNear

STARTING_WITH

StartingWith, IsStartingWith, StartsWith

TRUE

True, IsTrue

10.3.2. Annotated queries

Queries using the Cypher graph query language can be supplied with the @Query annotation.

That means a repository method annotated with
@Query("MATCH (:Actor {name:{name}})-[:ACTED_IN]->(m:Movie) return m")
will use the supplied query query to retrieve data from Neo4j.

The named or indexed parameter {param} will be substituted by the actual method parameter. Node and Relationship-Entities are handled directly and converted into their respective ids. All other parameters types are provided directly (i.e. Strings, Longs, etc).

There is special support for the Pageable parameter from Spring Data Commons, which is supported to add programmatic paging and slicing(alternatively static paging and sorting can be supplied in the query string itself).

If it is required that paged results return the correct total count, the @Query annotation can be supplied with a count query in the countQuery attribute. This query is executed separately after the result query and its result is used to populate the number of elements on the Page.

Custom queries do not support a custom depth. Additionally, @Query does not support mapping a path to domain entities, as such, a path should not be returned from a Cypher query. Instead, return nodes and relationships to have them mapped to domain entities.

10.3.3. Query results

Typical results for queries are Iterable<Type>, Iterable<Map<String,Object>> or simply Type. Nodes and relationships are converted to their respective entities (if they exist). Other values are converted using the registered conversion services (e.g. enums).

10.3.4. Cypher examples

MATCH (n) WHERE id(n)=9 RETURN n

returns the node with id 9

MATCH (movie:Movie {title:'Matrix'}) RETURN movie

returns the nodes which are indexed with title equal to 'Matrix'

MATCH (movie:Movie {title:'Matrix'})←[:ACTS_IN]-(actor) RETURN actor.name

returns the names of the actors that have a ACTS_IN relationship to the movie node for 'Matrix'

MATCH (movie:Movie {title:'Matrix'})←[r:RATED]-(user) WHERE r.stars > 3 RETURN user.name, r.stars, r.comment

returns users names and their ratings (>3) of the movie titled 'Matrix'

MATCH (user:User {name='Michael'})-[:FRIEND]-(friend)-[r:RATED]->(movie) RETURN movie.title, AVG(r.stars), COUNT(*) ORDER BY AVG(r.stars) DESC, COUNT(*) DESC

returns the movies rated by the friends of the user 'Michael', aggregated by movie.title, with averaged ratings and rating-counts sorted by both

Examples of Cypher queries placed on repository methods with @Query where values are replaced with method parameters, as described in the Annotated queries) section.

public interface MovieRepository extends Neo4jRepository<Movie, Long> {

    // returns the node with id equal to idOfMovie parameter
    @Query("MATCH (n) WHERE id(n)={0} RETURN n")
    Movie getMovieFromId(Integer idOfMovie);

    // returns the nodes which have a title according to the movieTitle parameter
    @Query("MATCH (movie:Movie {title={0}}) RETURN movie")
    Movie getMovieFromTitle(String movieTitle);

    // same with optional result
    @Query("MATCH (movie:Movie {title={0}}) RETURN movie")
    Optional<Movie> getMovieFromTitle(String movieTitle);

    // returns a Page of Actors that have a ACTS_IN relationship to the movie node with the title equal to movieTitle parameter.
    @Query(value = "MATCH (movie:Movie {title={0}})<-[:ACTS_IN]-(actor) RETURN actor", countQuery= "MATCH (movie:Movie {title={0}})<-[:ACTS_IN]-(actor) RETURN count(actor)")
    Page<Actor> getActorsThatActInMovieFromTitle(String movieTitle, PageRequest page);

    // returns a Page of Actors that have a ACTS_IN relationship to the movie node with the title equal to movieTitle parameter with an accurate total count
    @Query(value = "MATCH (movie:Movie {title={0}})<-[:ACTS_IN]-(actor) RETURN actor", countQuery = "MATCH (movie:Movie {title={0}})<-[:ACTS_IN]-(actor) RETURN count(*)")
    Page<Actor> getActorsThatActInMovieFromTitle(String movieTitle, Pageable page);

    // returns a Slice of Actors that have a ACTS_IN relationship to the movie node with the title equal to movieTitle parameter.
    @Query("MATCH (movie:Movie {title={0}})<-[:ACTS_IN]-(actor) RETURN actor")
    Slice<Actor> getActorsThatActInMovieFromTitle(String movieTitle, Pageable page);

    // returns users who rated a movie (movie parameter) higher than rating (rating parameter)
    @Query("MATCH (movie:Movie)<-[r:RATED]-(user) " +
           "WHERE id(movie)={movieId} AND r.stars > {rating} " +
           "RETURN user")
    Iterable<User> getUsersWhoRatedMovieFromTitle(@Param("movieId") Movie movie, @Param("rating") Integer rating);

    // returns users who rated a movie based on movie title (movieTitle parameter) higher than rating (rating parameter)
    @Query("MATCH (movie:Movie {title:{0}})<-[r:RATED]-(user) " +
           "WHERE r.stars > {1} " +
           "RETURN user")
    Iterable<User> getUsersWhoRatedMovieFromTitle(String movieTitle, Integer rating);

    @Query(value = "MATCH (movie:Movie) RETURN movie;")
    Stream<Movie> getAllMovies();
}

10.3.5. Queries derived from finder-method names

Using the metadata infrastructure in the underlying object-graph mapper, a finder method name can be split into its semantic parts and converted into a cypher query. Navigation along relationships will be reflected in the generated MATCH clause and properties with operators will end up as expressions in the WHERE clause. The parameters will be used in the order they appear in the method signature so they should align with the expressions stated in the method name.

Some examples of methods and corresponding Cypher queries of a PersonRepository
public interface PersonRepository extends Neo4jRepository<Person, Long> {

    // MATCH (person:Person {name={0}}) RETURN person
    Person findByName(String name);

    // MATCH (person:Person)
    // WHERE person.age = {0} AND person.married = {1}
    // RETURN person
    Iterable<Person> findByAgeAndMarried(int age, boolean married);

    // MATCH (person:Person)
    // WHERE person.age = {0}
    // RETURN person ORDER BY person.name SKIP {skip} LIMIT {limit}
    Page<Person> findByAge(int age, Pageable pageable);

    // MATCH (person:Person)
    // WHERE person.age = {0}
    // RETURN person ORDER BY person.name
    List<Person> findByAge(int age, Sort sort);

    //Allow a custom depth as a parameter
    Person findByName(String name, @Depth int depth);

    //Fix the depth for the query
    @Depth(value = 0)
    Person findBySurname(String surname);

}

10.3.6. Mapping Query Results

For queries executed via @Query repository methods, it’s possible to specify a conversion of complex query results to POJOs. These result objects are then populated with the query result data and can be serialized and sent to a different part of the application, e.g. a frontend-ui. To take advantage of this feature, use a class annotated with @QueryResult as the method return type.

Example of query result mapping
public interface MovieRepository extends Neo4jRepository<Movie, Long> {

    @Query("MATCH (movie:Movie)-[r:RATING]\->(), (movie)<-[:ACTS_IN]-(actor:Actor) " +
           "WHERE movie.id={0} " +
           "RETURN movie as movie, COLLECT(actor) AS 'cast', AVG(r.stars) AS 'averageRating'")
    MovieData getMovieData(String movieId);

    @QueryResult
    public class MovieData {
        Movie movie;
        Double averageRating;
        Set<Actor> cast;
    }

}

10.3.7. Sorting and Paging

Spring Data Neo4j supports sorting and paging of results when using Spring Data’s Pageable and Sort interfaces.

Repository-based paging
Pageable pageable = new PageRequest(0, 3);
Page<World> page = worldRepository.findAll(pageable, 0);
Repository-based sorting
Sort sort = new Sort(Sort.Direction.ASC, "name");
Iterable<World> worlds = worldRepository.findAll(sort, 0)) {
Repository-based sorting with paging
Pageable pageable = new PageRequest(0, 3, Sort.Direction.ASC, "name");
Page<World> page = worldRepository.findAll(pageable, 0);

The total number of pages reported by the PagingAndSortingRepository findAll methods are estimates and should not be relied upon for accuracy

10.4. Transactions

Neo4j is a transactional database, only allowing operations to be performed within transaction boundaries. Spring Data Neo4j integrates nicely with both the declarative transaction support with @Transactional as well as the manual transaction handling with TransactionTemplate.

Demarcating @Transactional is required for all methods that interact with SDN. CRUD methods on Repository instances are transactional by default. If you are simply just looking up an object through a repository for example, then you do not need to define anything else: SDN will take of everything for you. That said, it is strongly recommended that you always annotate any service boundaries to the database with a @Transactional annotation. This way all your code for that method will always run in one transaction, even if you add a write operation later on.

More standard behaviour with Transactions is using a facade or service implementation that typically covers more than one repository or database call as part of a 'Unit of Work'. Its purpose is to define transactional boundaries for non-CRUD operations:

SDN only supports PROPAGATION_REQUIRED and ISOLATION_DEFAULT type transactions.
Using a facade to define transactions for multiple repository calls
@Service
class UserManagementImpl implements UserManagement {

  private final UserRepository userRepository;
  private final RoleRepository roleRepository;

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

  @Transactional
  public void addRoleToAllUsers(String roleName) {

    Role role = roleRepository.findByName(roleName);

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

This will cause call to addRoleToAllUsers(…) to run inside a transaction (participating in an existing one or create a new one if none already running). The transaction configuration at the repositories will be neglected then as the outer transaction configuration determines the actual one used.

It is highly recommended that users understand how Spring Transactions work. Below are some excellent resources:

10.4.1. Read only Transactions

You can start a read only transaction by marking a class or method with @Transactional(readOnly=true).

Note that if you open a read only transaction from, for example a service method, and then call a mutating method that is marked as read/write your transaction semantics will always be defined by the outermost transaction. Be wary!

10.4.2. Transaction Bound Events

SDN provides the ability to bind the listener of an event to a phase of the transaction. The typical example is to handle the event when the transaction has completed successfully: this allows events to be used with more flexibility when the outcome of the current transaction actually matters to the listener.

Spring Framework is currently structured in such a way that the context is not aware of the transaction support and has an open infrastructure to allow additional components to be registered and influence the way event listeners are created.

The transaction module implements an EventListenerFactory that looks for the new @TransactionalEventListener annotation. When this one is present, an extended event listener that is aware of the transaction is registered instead of the default.

Example: An order creation listener.
@Component
public class MyComponent {

  @TransactionalEventListener(condition = "#creationEvent.awesome")
  public void handleOrderCreatedEvent(CreationEvent<Order> creationEvent) {
    ...
  }

}

@TransactionalEventListener is a regular @EventListener and also exposes a TransactionPhase, the default being AFTER_COMMIT. You can also hook other phases of the transaction (BEFORE_COMMIT, AFTER_ROLLBACK and AFTER_COMPLETION that is just an alias for AFTER_COMMIT and AFTER_ROLLBACK).

By default, if no transaction is running the event isn’t sent at all as we can’t obviously honor the requested phase, but there is a fallbackExecution attribute in @TransactionalEventListener that tells Spring to invoke the listener immediately if there is no transaction.

Only public methods in a managed bean can be annotated with @EventListener to consume events. @TransactionalEventListener is the annotation that provides transaction-bound event support described here.

To find out more about Spring’s Event listening capabilities see the Spring reference manual and How to build Transaction aware Eventing with Spring 4.2.

10.5. Miscellaneous

10.5.1. CDI integration

Instances of the repository interfaces are usually created by a container, which Spring is the most natural choice when working with Spring Data. There’s sophisticated support to easily set up Spring to create bean instances documented in Creating repository instances. Spring Data Neo4j ships with a custom CDI extension that allows using the repository abstraction in CDI environments. The extension is part of the JAR so all you need to do to activate it is dropping the Spring Data Neo4j JAR into your classpath.

You can now set up the infrastructure by implementing a CDI Producer for the SessionFactory and Session:

class sessionFactoryProducer {

  @Produces
  @ApplicationScoped
  public SessionFactory createSessionFactory() {
    return new SessionFactory("package");
  }

  public void close(@Disposes SessionFactory sessionFactory) {
    sessionFactory.close();
  }
}

The necessary setup can vary depending on the JavaEE environment you run in. It might also just be enough to redeclare a session as CDI bean as follows:

class CdiConfig {

  @Produces
  @RequestScoped
  @PersistenceContext
  public session session;
}

In this example, the container has to be capable of creating OGM Sessions itself. All the configuration does is re-exporting the OGM Session as CDI bean.

The Spring Data Neo4J CDI extension will pick up all sessions availables as CDI beans and create a proxy for a Spring Data repository whenever an bean of a repository type is requested by the container. Thus obtaining an instance of a Spring Data repository is a matter of declaring an @Injected property:

class RepositoryClient {

  @Inject
  PersonRepository repository;

  public void businessMethod() {
    List<Person> people = repository.findAll();
  }
}

10.5.2. JSR-303 (Bean Validation) Support

Spring Data Neo4J allows developers to use JSR-303 annotations like @NotNull etc. on their domain models. While this is provided it’s not a best practice. It is highly recommended to create JSR-303 annotations on actual Java Beans, similar to things like Data Transfer Objects (DTOs).

10.5.3. Conversion Service

It is possible to have Spring Data Neo4j 4 use converters registered with Spring’s ConversionService. In order to do this, provide org.springframework.data.neo4j.conversion.MetaDataDrivenConversionService as a Spring bean.

Provide MetaDataDrivenConversionService as a Spring bean
@Bean
public ConversionService conversionService() {
    return new MetaDataDrivenConversionService(getSessionFactory().metaData());
}

Then, instead of defining an implementation of org.neo4j.ogm.typeconversion.AttributeConverter on the @Convert annotation, use the graphPropertyType attribute to define the type to convert to.

Using graphPropertyType
@NodeEntity
public class MyEntity {

    @Convert(graphPropertyType = Integer.class)
    private DecimalCurrencyAmount fundValue;

}

Spring Data Neo4j 4 will look for converters registered with Spring’s ConversionService that can convert both to and from the type specified by graphPropertyType and use them if they exist.

Default converters and those defined explicitly via an implementation of org.neo4j.ogm.typeconversion.AttributeConverter will take precedence over converters registered with Spring’s ConversionService.

As of SDN 4, this Neo4jRepository<T, ID> should be the interface from which your entity repository interfaces inherit, with T being specified as the domain entity type to persist. ID is defined by the field type annotated with @Index(unique=true,primary=true).

Examples of methods you get for free out of Neo4jRepository are as follows. For all of these examples the ID parameter is a Long that matches the graph ID:

Load an entity instance via an id

Optional<T> findOne(id)

Check for existence of an id in the graph

boolean exists(id)

Iterate over all nodes of a node entity type

Iterable<T> findAll() Iterable<T> findAll(Sort …​) Page<T> findAll(Pageable …​)

Count the instances of the repository entity type

Long count()

Save entities

T save(T) and Iterable<T> save(Iterable<T>)

Delete graph entities

void delete(T), void delete(Iterable<T>), and void deleteAll()

For users coming from versions before 4.2.x, Neo4jRepository has replaced GraphRepository but essentially has the same features.

Neo4j OGM Reference Documentation

This chapter is taken from the Official Neo4j OGM Reference Documentation.

11. Introduction

Neo4j OGM is a fast object-graph mapping library for Neo4j, optimised for server-based installations utilising Cypher.

It aims to simplify development with the Neo4j graph database and like JPA, it uses annotations on simple POJO domain objects to do so.

With a focus on performance, the OGM introduces a number of innovations, including:

  • non-reflection based classpath scanning for much faster startup times;

  • variable-depth persistence to allow you to fine-tune requests according to the characteristics of your graph;

  • smart object-mapping to reduce redundant requests to the database, improve latency and minimise wasted CPU cycles; and

  • user-definable session lifetimes, helping you to strike a balance between memory-usage and server request efficiency in your applications.

11.1. Overview

This reference documentation is broken down into sections to help the user understand specifics of how the OGM works.

Getting started

Getting started can sometimes be a chore. What versions of the OGM do you need? Where do you get them from? What build tool should you use? Getting Started is the perfect place to well…​ get started!

Configuration

Drivers, Logging; properties, configuration via Java. How to make sense of all the options? Configuration has got you covered.

Annotating your Domain Objects

To get started with your OGM application, you need only your domain model and the annotations provided by the library. You use annotations to mark domain objects to be reflected by nodes and relationships of the graph database. For individual fields the annotations allow you to declare how they should be processed and mapped to the graph. For property fields and references to other entities this is straightforward. Because Neo4j is a schema-free database, the OGM uses a simple mechanism to map Java types to Neo4j nodes using labels. Relationships between entities are first class citizens in a graph database and therefore worth a section of it’s own describing their usage in Neo4j OGM.

Connecting to the Database

Managing how you connect to the database is important. Connecting to the Database has all the details on what needs to happen to get you up and running.

Indexing and Primary Constraints

Indexing is an important part of any database. The Neo4j OGM provides a variety of features to support the management of Indexes as well as the ability to query your domain objects by something other than the internal Neo4j ID. Indexing has everything you will want to know when it comes to getting that working.

Interacting with the Graph Model

Neo4j OGM offers a session for interacting with the mapped entities and the Neo4j graph database. Neo4j uses transactions to guarantee the integrity of your data and Neo4j OGM supports this fully. The implications of this are described here. To use advanced functionality like Cypher queries, a basic understanding of the graph data model is required. The graph data model is explained in the chapter about Neo4j.

Type Conversion

The OGM provides support for default and bespoke type conversions, which allow you to configure how certain data types are mapped to nodes or relationships in Neo4j. See Type Converstion for more details.

Filtering your domain objects

Filters provides a simple API to append criteria to your stock Session.loadX() behaviour. This is covered in more detail in Filters

Reacting to Persistence events

The Events mechanism allows users to register event listeners for handling persistence events related both to top-level objects being saved as well as connected objects. Event handling discusses all the aspects of working with events.

Testing in your application

Sometimes you want to be able to run your tests against an in-memory version of the OGM. Testing goes into more detail of how to set that up.

Support for High Availability

For those using Neo4j Enterprise, support for high availability is extremely important. The chapter on High Availability goes into all the options the OGM provides to support this.

12. Getting Started

12.1. Versions

Consult the version table to determine which version of the OGM to use with a particular version of Neo4j and related technologies.

12.1.1. Compatibility

Neo4j OGM Version Neo4j Version Bolt Version# Spring Data Neo4j Version Spring Boot Version

3.0.0+

3.0.x, 3.1.x

1.1.0+

5.0.0+

2.0.0+

2.1.0+

2.3.x, 3.0.x, 3.1.x

1.1.0+

4.2.0+

1.5.0+

2.0.2+

2.3.x, 3.0.x

1.0.0+

4.1.2 - 4.1.6+

1.4.x

2.0.1*

2.2.x, 2.3.x

1.0.0-RC1

4.1.0 - 4.1.1

1.4.x

1.1.5*

2.1.x, 2.2.x, 2.3.x

N/A

4.0.0+

1.4.x

* These versions are no longer actively developed or supported.

# Not applicable to Embedded and HTTP drivers

12.1.2. Transitive dependencies

The following table list transitive dependencies between specific versions of projects related to OGM. When reporting issues or asking for help on StackOverflow or neo4j-users slack channel always verify versions used (e.g through mvn dependency:tree) and report them as well.

Spring Boot Version Spring Data Neo4j Version Neo4j OGM Version Bolt Version

1.5.3

4.2.3

2.1.2

1.1.0

1.4.6

4.1.7

2.0.5

1.0.6

These versions can be overridden manually in pom.xml or build.gradle files.

12.2. Dependency Management

For building an application, your build automation tool needs to be configured to include the Neo4j OGM dependencies.

The OGM dependencies consist of neo4j-ogm-core, together with the relevant dependency declarations on the driver you want to use. OGM 2.1 provides support for connecting to Neo4j by configuring one of the following Drivers:

  • neo4j-ogm-http-driver - Uses HTTP to communicate between the OGM and a remote Neo4j instance.

  • neo4j-ogm-embedded-driver - Connects directly to the Neo4j database engine.

  • neo4j-ogm-bolt-driver - Uses native Bolt protocol to communicate between the OGM and a remote Neo4j instance.

If you’re not using a particular driver, you don’t need to declare it.

Neo4j OGM projects can be built using Maven, Gradle or any other build system that utilises Maven’s artifact repository structure.

12.2.1. Maven

In the <dependencies> section of your pom.xml add the following:

Maven dependencies
<dependency>
    <groupId>org.neo4j</groupId>
    <artifactId>neo4j-ogm-core</artifactId>
    <version>5.0.0.BUILD-SNAPSHOT</version>
    <scope>compile</scope>
</dependency>

<!-- Only add if you're using the HTTP driver -->
<dependency>
    <groupId>org.neo4j</groupId>
    <artifactId>neo4j-ogm-http-driver</artifactId>
    <version>5.0.0.BUILD-SNAPSHOT</version>
    <scope>runtime</scope>
</dependency>

<!-- Only add if you're using the Embedded driver -->
<dependency>
    <groupId>org.neo4j</groupId>
    <artifactId>neo4j-ogm-embedded-driver</artifactId>
    <version>5.0.0.BUILD-SNAPSHOT</version>
    <scope>runtime</scope>
</dependency>

<!-- Only add if you're using the Bolt driver -->
<dependency>
    <groupId>org.neo4j</groupId>
    <artifactId>neo4j-ogm-bolt-driver</artifactId>
    <version>5.0.0.BUILD-SNAPSHOT</version>
    <scope>runtime</scope>
</dependency>

If you plan on using a development (i.e. SNAPSHOT) version of the OGM you will need to add the following to the <repositories> section of your pom.xml:

Neo4j Snapshot Repository
<repository>
    <id>neo4j-snapshot-repository</id>
    <name>Neo4j Maven 2 snapshot repository</name>
    <url>http://m2.neo4j.org/content/repositories/snapshots</url>
</repository>

12.2.2. Gradle

Ensure the following dependencies are added to you build.gradle:

Gradle dependencies
dependencies {
    compile 'org.neo4j:neo4j-ogm-core:5.0.0.BUILD-SNAPSHOT'
    runtime 'org.neo4j:neo4j-ogm-http-driver:5.0.0.BUILD-SNAPSHOT' // Only add if you're using the HTTP driver
    runtime 'org.neo4j:neo4j-ogm-embedded-driver:5.0.0.BUILD-SNAPSHOT' //  Only add if you're using the Embedded driver
    runtime 'org.neo4j:neo4j-ogm-bolt-driver:5.0.0.BUILD-SNAPSHOT' // Only add if you're using the Bolt driver
}

If you plan on using a development (i.e. SNAPSHOT) version of the OGM you will need to add the following section of your build.gradle:

Neo4j Snapshot Repository
repositories {
    maven { url "http://m2.neo4j.org/content/repositories/snapshots" }
}

13. Configuration

13.1. Configuration method

There are two ways to supply configuration to the OGM:

  • using an ogm.properties file; and

  • programmatically using Java.

13.1.1. Using ogm.properties

Unless you supply an explicit Configuration object to the SessionFactory (see below), the OGM will attempt to auto-configure itself using a file called ogm.properties, which it expects to find on the root of the classpath.

If you want to configure the OGM using a properties file, but with a different filename, you must set a System property or Environment variable called 'ogm.properties' pointing to the alternative configuration file you want to use.

13.1.2. Programmatically using Java

In cases where you are not be able to provide configuration via a properties file you can configure the OGM programmatically instead.

The Configuration object provides a fluent API to set various configuration options. This object then needs to be supplied to the SessionFactory constructor in order to be configured.

13.2. Driver Configuration

13.2.1. HTTP Driver

Table 3. Basic HTTP Driver Configuration
ogm.properties Java Configuration
driver=org.neo4j.ogm.drivers.http.driver.HttpDriver
URI=http://user:[email protected]:7474
Configuration configuration = new Configuration()
        .setDriverClassName("org.neo4j.ogm.drivers.http.driver.HttpDriver")
        .setURI("http://user:[email protected]:7474")

13.2.2. Bolt Driver

Note that for the URI, if no port is specified, the default Bolt port of 7687 is used. Otherwise, a port can be specified with bolt://neo4j:[email protected]:1234

Also, the bolt driver allows you to define a connection pool size, which refers to the maximum number of sessions per URL. This property is optional and defaults to 50

Table 4. Basic Bolt Driver Configuration
ogm.properties Java Configuration
driver=org.neo4j.ogm.drivers.bolt.driver.BoltDriver
URI=bolt://neo4j:[email protected]
connection.pool.size=150
Configuration configuration = new Configuration();
        configuration
        .setDriverClassName("org.neo4j.ogm.drivers.bolt.driver.BoltDriver")
        .setURI("bolt://neo4j:[email protected]")
        .setConnectionPoolSize(150)

A timeout to the database with the Bolt driver can be set by updating your Database’s neo4j.conf. The exact setting to change can be found here.

13.2.3. Embedded Driver

You should use the Embedded driver if you don’t want to use a client-server model, or if your application is running as a Neo4j Unmanaged Extension. You can specify a permanent data store location to provide durability of your data after your application shuts down, or you can use an impermanent data store, which will only exist while your application is running.

As of 2.1.0 the Neo4j OGM embedded driver no longer ships with the Neo4j kernel. Users are expected to provide this dependency through their dependency management system. See Getting Started for more details.
Table 5. Permanent Data Store Embedded Driver Configuration
ogm.properties Java Configuration
driver=org.neo4j.ogm.drivers.embedded.driver.EmbeddedDriver
URI=file:///var/tmp/neo4j.db
Configuration configuration = new Configuration()
             .setDriverClassName("org.neo4j.ogm.drivers.embedded.driver.EmbeddedDriver")
             .setURI("file:///var/tmp/neo4j.db");

To use an impermanent data store which will be deleted on shutdown of the JVM, you just omit the URI attribute.

Table 6. Impermanent Data Store Embedded Driver Configuration
ogm.properties Java Configuration
driver=org.neo4j.ogm.drivers.embedded.driver.EmbeddedDriver
Configuration configuration = new Configuration()
             .setDriverClassName("org.neo4j.ogm.drivers.embedded.driver.EmbeddedDriver");
Embedded Driver in an Unmanaged Extension

When your application is running as unmanaged extension inside the Neo4j server itself, you will need to set up the Driver configuration slightly differently. In this situation, an existing GraphDatabaseService will already be available via a @Context annotation, and you must configure the Components framework to enable the OGM to use the provided instance. Note your application should typically do this only once before the call to set up a Configuration object.

    Components.setDriver(new EmbeddedDriver(graphDatabaseService));

13.2.4. Credentials

If you are using the HTTP or Bolt Driver you have a number of different ways to supply credentials to the Driver Configuration.

ogm.properties Java Configuration
# embedded
URI=http://user:[email protected]:7474

# separately
username="user"
password="password"
Configuration configuration = new Configuration()
             .setURI("bolt://user:[email protected]");

Configuration configuration = new Configuration()
             .setCredentials("user", "password);

Credentials credentials = new UsernameAndPasswordCredentials("user", "password");
Configuration configuration = new Configuration()
             .setCredentials(credentials);

Note: Currently only Basic Authentication is supported by Neo4j, so the only Credentials implementation supplied by the OGM is `UsernameAndPasswordCredentials`

13.2.5. Transport Layer Security (TLS/SSL)

The Bolt and Http drivers also allow you to connect to Neo4j over a secure channel. These rely on Transport Layer Security (aka SSL) and require the installation of a signed certificate on the server.

In certain situations (e.g. some cloud environments) it may not be possible to install a signed certificate even though you still want to use an encrypted connection.

To support this, both drivers have configuration settings allowing you to bypass certificate checking, although they differ in their implementation.

Both these strategies leave you vulnerable to a MITM attack. You should probably not use them unless your servers are behind a secure firewall.
Bolt
ogm.properties Java Configuration
#Encryption level (TLS), optional, defaults to REQUIRED.
#Valid values are NONE,REQUIRED
encryption.level=REQUIRED

#Trust strategy, optional, not used if not specified.
#Valid values are TRUST_ON_FIRST_USE,TRUST_SIGNED_CERTIFICATES
trust.strategy=TRUST_ON_FIRST_USE

#Trust certificate file, required if trust.strategy is specified
trust.certificate.file=/tmp/cert

# interval, in milliseconds, to check for stale db connections (test-on-borrow)
connection.liveness.check.timeout=1000
Configuration config = new Configuration.Builder()
    ...
    .encryptionLevel("REQUIRED")
    .trustStrategy("TRUST_ON_FIRST_USE")
    .trustCertFile("/tmp/cert")
    .connectionLivenessCheckTimeout(1000)
    .build();

TRUST_ON_FIRST_USE means that the Bolt Driver will trust the first connection to a host to be safe and intentional. On subsequent connections, the driver will verify that the host is the same as on that first connection.

HTTP
ogm.properties Java Configuration
trust.strategy = ACCEPT_UNSIGNED
Configuration configuration = new Configuration();
        configuration
        .setTrustStrategy("ACCEPT_UNSIGNED")

The ACCEPT_UNSIGNED strategy permits the Http Driver to accept Neo4j’s default snakeoil.cert (and any other) unsigned certificate when connecting over HTTPS.

13.3. Logging

Neo4j OGM uses SLF4J to log statements. In production, you can set the log level in a file called logback.xml to be found at the root of the classpath. Please see the Logback manual for further details.

14. Annotating Entities

14.1. @NodeEntity: The basic building block

The @NodeEntity annotation is used to declare that a POJO class is an entity backed by a node in the graph database. Entities handled by the OGM must have one empty public constructor to allow the library to construct the objects.

Fields on the entity are by default mapped to properties of the node. Fields referencing other node entities (or collections thereof) are linked with relationships.

@NodeEntity annotations are inherited from super-types and interfaces. It is not necessary to annotate your domain objects at every inheritance level.

If the label attribute is set then this will replace the default label applied to the node in the database. The default label is just the simple class name of the annotated entity. All parent classes (excluding java.lang.Object) are also added as labels so that retrieving a collection of nodes via a parent type is supported.

Entity fields can be annotated with @Property, @GraphId, @Transient or @Relationship. All annotations live in the org.neo4j.ogm.annotation package. Marking a field with the transient modifier has the same effect as annotating it with @Transient; it won’t be persisted to the graph database.

Persisting an annotated entity
@NodeEntity
public class Actor extends DomainObject {

   @GraphId
   private Long id;

   @Property(name="name")
   private String fullName;

   @Relationship(type="ACTED_IN", direction=Relationship.OUTGOING)
   private List<Movie> filmography;

}

@NodeEntity(label="Film")
public class Movie {

   @GraphId Long id;

   @Property(name="title")
   private String name;

}

Saving a simple object graph containing one actor and one film using the above annotated objects would result in the following being persisted in Neo4j.

(:Actor:DomainObject {name:'Tom Cruise'})-[:ACTED_IN]->(:Film {title:'Mission Impossible'})

When annotating your objects, you can apply the annotations to either the fields or their accessor methods, but bear in mind the aforementioned EntityAccessStrategy ordering when annotating your domain model.

Persisting a non-annotated entity
public class Actor extends DomainObject {

   private Long id;
   private String fullName;
   private List<Movie> filmography;

}

public class Movie {

   private Long id;
   private String name;

}

In this case, a graph similar to the following would be persisted.

(:Actor:DomainObject {fullName:'Tom Cruise'})-[:FILMOGRAPHY]->(:Movie {name:'Mission Impossible'})

While this will map successfully to the database, it’s important to understand that the names of the properties and relationship types are tightly coupled to the class’s member names. Renaming any of these fields will cause parts of the graph to map incorrectly, hence the recommendation to use annotations.

14.1.1. @Properties: dynamically mapping properties to graph

A @Properties annotation tells OGM to map values of a Map field in a node or relationship entity to properties of a node or a relationship in the graph.

The property names are derived from field name or prefix(), delimiter and keys in the Map. For example Map field with name address containing following entries:

"street" => "Downing Street"
"number" => 10

will map to following node/relationship properties

address.street=Downing Street
address.number=10

Supported types for keys in the Map are String and Enum.

The values in the Map can be of any Java type equivalent to Cypher types. If full type information is provided other Java types are also supported.

If {@link #allowCast()} is set to true then types that can be cast to corresponding Cypher types are allowed as well. Note that the original type cannot be deduced and the value will be deserialized to corresponding type - e.g. when Integer instance is put to {@code Map<String, Object>} it will be deserialized as Long.

@NodeEntity
public class Student {

    @Properties
    private Map<String, Integer> properties = new HashMap<>();

    @Properties
    private Map<String, Object> properties = new HashMap<>();

}

14.1.2. Runtime managed labels

As stated above, the label applied to a node is the contents of the @NodeEntity label property, or if not specified, it will default to the simple class name of the entity. Sometimes it might be necessary to add and remove additional labels to a node at runtime. We can do this using the @Labels annotation. Let’s provide a facility for adding additional labels to the Student entity:

@NodeEntity
public class Student {

    @Labels
    private List<String> labels = new ArrayList<>();

}

Now, upon save, the node’s labels will correspond to the entity’s class hierarchy plus whatever the contents of the backing field are. We can use one @Labels field per class hierarchy - it should be exposed or hidden from sub-classes as appropriate.

14.2. @Relationship: Connecting node entities

Every field of an entity that references one or more other node entities is backed by relationships in the graph. These relationships are managed by the OGM automatically.

The simplest kind of relationship is a single object reference pointing to another entity (1:1). In this case, the reference does not have to be annotated at all, although the annotation may be used to control the direction and type of the relationship. When setting the reference, a relationship is created when the entity is persisted. If the field is set to null, the relationship is removed.

Single relationship field
@NodeEntity
public class Movie {
    ...
    private Actor topActor;
}

It is also possible to have fields that reference a set of entities (1:N). Neo4j OGM supports the following types of entity collections:

  • java.util.Vector

  • java.util.List, backed by a java.util.ArrayList

  • java.util.SortedSet, backed by a java.util.TreeSet

  • java.util.Set, backed by a java.util.HashSet

  • Arrays

Node entity with relationships
@NodeEntity
public class Actor {
    ...
    @Relationship(type = "TOP_ACTOR", direction = Relationship.INCOMING)
    private Set<Movie> topActorIn;

    @Relationship(type = "ACTS_IN")
    private Set<Movie> movies;
}

For graph to object mapping, the automatic transitive loading of related entities depends on the depth of the horizon specified on the call to Session.load(). The default depth of 1 implies that related node or relationship entities will be loaded and have their properties set, but none of their related entities will be populated.

If this Set of related entities is modified, the changes are reflected in the graph once the root object (Actor, in this case) is saved. Relationships are added, removed or updated according to the differences between the root object that was loaded and the corresponding one that was saved..

Neo4j OGM ensures by default that there is only one relationship of a given type between any two given entities. The exception to this rule is when a relationship is specified as either OUTGOING or INCOMING between two entities of the same type. In this case, it is possible to have two relationships of the given type between the two entities, one relationship in either direction.

If you don’t care about the direction then you can specify direction=Relationship.UNDIRECTED which will guarantee that the path between two node entities is navigable from either side.

For example, consider the PARTNER relationship between two companies, where (A)-[:PARTNER_OF]→(B) implies (B)-[:PARTNER_OF]→(A). The direction of the relationship does not matter; only the fact that a PARTNER_OF relationship exists between these two companies is of importance. Hence an UNDIRECTED relationship is the correct choice, ensuring that there is only one relationship of this type between two partners and navigating between them from either entity is possible.

The direction attribute on a @Relationship defaults to OUTGOING. Any fields or methods backed by an INCOMING relationship must be explicitly annotated with an INCOMING direction.

14.2.1. Using more than one relationship of the same type

In some cases, you want to model two different aspects of a conceptual relationship using the same relationship type. Here is a canonical example:

Clashing Relationship Type
@NodeEntity
class Person {
    private Long id;
    @Relationship(type="OWNS")
    private Car car;

    @Relationship(type="OWNS")
    private Pet pet;
...
}

This will work just fine, however, please be aware that this is only because the end node types (Car and Pet) are different types. If you wanted a person to own two cars, for example, then you’d have to use a Collection of cars or use differently-named relationship types.

14.2.2. Ambiguity in relationships

In cases where the relationship mappings could be ambiguous, the recommendation is that:

  • The objects be navigable in both directions.

  • The @Relationship annotations are explicit. This means if the entity has setter methods, they must be annotated

Examples of ambiguous relationship mappings are multiple relationship types that resolve to the same types of entities, in a given direction, but whose domain objects are not navigable in both directions.

14.2.3. Ordering

Neo4j doesn’t have any ordering on relationships, so the relationships are fetched without any specific ordering. If you want to impose order on collections of relationships you have several options:

  • use a -SortedSet- and implement Comparable

  • sort relationships in @PostLoad method

You can sort either by a property of a related node or by relationship property. To sort by relationship property you need to use relationship entity. See @RelationshipEntity: Rich relationships.

14.3. @RelationshipEntity: Rich relationships

To access the full data model of graph relationships, POJOs can also be annotated with @RelationshipEntity, making them relationship entities. Just as node entities represent nodes in the graph, relationship entities represent relationships. Such POJOs allow you to access and manage properties on the underlying relationships in the graph.

Fields in relationship entities are similar to node entities, in that they’re persisted as properties on the relationship. For accessing the two endpoints of the relationship, two special annotations are available: @StartNode and @EndNode. A field annotated with one of these annotations will provide access to the corresponding endpoint, depending on the chosen annotation.

For controlling the relationship-type a String attribute called type is available on the @RelationshipEntity annotation. Like the simple strategy for labelling node entities, if this is not provided then the name of the class is used to derive the relationship type, although it’s converted into SNAKE_CASE to honour the naming conventions of Neo4j relationships As of the current version of the OGM, the type must be specified on the @RelationshipEntity annotation as well as its corresponding @Relationship annotations.

You must include @RelationshipEntity plus exactly one @StartNode field and one @EndNode field on your relationship entity classes or the OGM will throw a MappingException when reading or writing. It is not possible to use relationship entities in a non-annotated domain model.

A simple Relationship entity
@NodeEntity
public class Actor {
    Long id;
    private Role playedIn;
}

@RelationshipEntity(type="PLAYED_IN")
public class Role {
    @GraphId   private Long relationshipId;
    @Property  private String title;
    @StartNode private Actor actor;
    @EndNode   private Movie movie;
}

@NodeEntity
public class Movie {
    private Long id;
    private String title;
}

Note that the Actor also contains a reference to a Role. This is important for persistence, even when saving the Role directly, because paths in the graph are written starting with nodes first and then relationships are created between them. Therefore, you need to structure your domain models so that relationship entities are reachable from node entities for this to work correctly.

Additionally, the OGM will not persist a relationship entity that doesn’t have any properties defined. If you don’t want to include properties in your relationship entity then you should use a plain @Relationship instead. Multiple relationship entities which have the same property values and relate the same nodes are indistinguishable from each other and are represented as a single relationship by the OGM.

The @RelationshipEntity annotation must appear on all leaf subclasses if they are part of a class hierarchy representing relationship entities. This annotation is optional on superclasses.

14.4. @GraphId: Neo4j id field

This is a required field which must be of type java.lang.Long. It is used by Neo4j OGM to store the node or relationship-id to re-connect the entity to the graph. As such, user code should never assign a value to it.

It must not be a primitive type because then an object in a transient state cannot be represented, as the default value 0 would point to the reference node.

Do not rely on this ID for long running applications. Neo4j will reuse deleted node ID’s. It is recommended users come up with their own unique identifier for their domain objects (or use a UUID).

If the field is simply named 'id' then it is not necessary to annotate it with @GraphId as the OGM will use it automatically.

14.4.1. Entity Equality

Entity equality can be a grey area. There are many debatable issues, such as whether natural keys or database identifiers best describe equality and the effects of versioning over time. Neo4j OGM does not impose a dependency upon a particular style of equals() or hashCode() implementation. The graph-id field is directly checked to see if two entities represent the same node and a 64-bit hash code is used for dirty checking, so you’re not forced to write your code in a certain way!

You are free to write your equals and hashcode in a domain specific way for managed entities. However, we strongly advise developers to not use the @GraphId field in these implementations. This is because when you first persist an entity, its hashcode changes because the OGM populates the database ID on save. This causes problems if you had inserted the newly created entity into a hash-based collection before saving.

14.5. @Property: Optional annotation for property fields

As we touched on earlier, it is not necessary to annotate property fields as they are persisted by default. Fields that are annotated as @Transient or with transient are exempted from persistence. All fields that contain primitive values are persisted directly to the graph. All fields convertible to a String using the conversion services will be stored as a string. Neo4j OGM includes default type converters that deal with the following types:

  • java.util.Date to a String in the ISO 8601 format: "yyyy-MM-dd’T’HH:mm:ss.SSSXXX"

  • java.math.BigInteger to a String property

  • java.math.BigDecimal to a String property

  • binary data (as byte[] or Byte[]) to base-64 String

  • java.lang.Enum types using the enum’s name() method and Enum.valueOf()

Collections of primitive or convertible values are stored as well. They are converted to arrays of their type or strings respectively. Custom converters are also specified by using @Convert - this is discussed in detail later on.

Node property names can be explicitly assigned by setting the name attribute. For example @Property(name="last_name") String lastName. The node property name defaults to the field name when not specified.

Property fields to be persisted to the graph must not be declared final.

14.6. Object Read/Write Ordering

Neo4j OGM supports mapping annotated and non-annotated objects models. It’s possible to save any POJO without annotations to the graph, as the framework applies conventions to decide what to do. This is useful in cases when you don’t have control over the classes that you want to persist. The recommended approach, however, is to use annotations wherever possible, since this gives greater control and means that code can be refactored safely without risking breaking changes to the labels and relationships in your graph.

Annotated and non-annotated objects can be used within the same project without issue. There is an EntityAccessStrategy used to control how objects are read from or written to. The default implementation of this uses the following convention:

  1. Annotated method (getter/setter).

  2. Annotated field.

  3. Plain method (getter/setter).

  4. Plain field.

The object graph mapping comes into play whenever an entity is constructed from a node or relationship. This could be done explicitly during the lookup or create operations of the Session but also implicitly while executing any graph operation that returns nodes or relationships and expecting mapped entities to be returned.

Entities handled by the OGM must have one empty public constructor to allow the library to construct the objects.

Unless annotations are used to specify otherwise, the framework will attempt to map any of an object’s "simple" fields to node properties and any rich composite objects to related nodes. A "simple" field is any primitive, boxed primitive or String or arrays thereof, essentially anything that naturally fits into a Neo4j node property. For related entities the type of a relationship is inferred by the bean property name.

15. Indexing

Indexing is used in Neo4j to quickly find nodes and relationships from which to start graph operations. Indexes are also employed to ensure uniqueness of elements with certain labels and properties.

Please note that the lucene-based manual indexes are deprecated with Neo4j 2.0. The default index is now based on labels and schema indexes and the related old APIs have been deprecated as well. The "legacy" index framework should only be used for fulltext and spatial indexes which are not currently supported via schema-based indexes.

Previous version of the OGM have restricted developers to only be able to find unique Objects the internal Neo4j ID. This can be a problem for users when running Neo4j across restarts as this can result in key duplication. In order to support querying for unique objects via business or synthetic (e.g. UUID) properties, the OGM can now enforce uniqueness constraints on certain properties to make lookups easier. On top of this, many users have the use case of wanting to save their objects via this same property and in effect have the OGM run the MERGE operation using the same property to identify the object.

15.1. Indexes and Constraints

Indexes and unique constraints based on labels and properties are supported as of version 2.1.0 with the @Index annotation. Any property field annotated with @Index will use have an appropriate schema index created. For @Index(unique=true) a constraint is created.

You may add as many indexes or constraints as you like to your class. If you annotate a field in a class that is part of an inheritance hierarchy then the index or constraint will only be added to that class’s label.

15.2. Index Creation

By default index management is set to None.

If you would like the OGM to manage your schema creation there are several ways to go about it.

Only classes marked with @Index will be used. Indexes will always be generated with the containing class’s label and the annotated property’s name. Index generation behaviour can be defined in ogm.properties by defining a property called: indexes.auto and providing a value of:

Below is a table of all options available for configuring Auto-Indexing.

Option Description Properties Example Java Example

none (default)

Nothing is done with index and constraint annotations.

-

-

validate

Make sure the connected database has all indexes and constraints in place before starting up

indexes.auto=validate

config.setAutoIndex("validate");

assert

Drops all constraints and indexes on startup then builds indexes based on whatever is represented in OGM by @Index. Handy during development

indexes.auto=assert

config.setAutoIndex("assert");

dump

Dumps the generated constraints and indexes to a file. Good for setting up environments. none: Default. Simply marks the field as using an index.

indexes.auto=dump indexes.auto.dump.dir=<a directory> indexes.auto.dump.filename=<a filename>

config.setAutoIndex("dump"); config.setDumpDir("XXX"); config.setDumpFilename("XXX");

15.3. Primary Constraints

The OGM can now be used to look up or merge entities based on a unique constraint. This unique constraint is known as a primary constraint. It can be enabled through the index annotation like so: Index(unique=true, primary=true).

Fields marked with unique=false, primary=true together will be ignored.

Improvements have also been made to Session to allow you to load a particular object via this primary constraint.

T Session.load(Class<T>, U id), where T is the highest class in a class hierarchy and a primary constraint of type U is defined on a field in that same class.

There may only be one primary=true index per Class hierarchy.

15.3.1. Merging

When an object is created via a call to Session.save(Object) a check is made to see if that object’s class has a primary constraint associated with it. If it does that constraint will always as part of a Cypher MERGE operation, otherwise a regular CREATE will be used.

16. Connecting to the Graph

In order to interact with mapped entities and the Neo4j graph, your application will require a Session, which is provided by the SessionFactory.

16.1. SessionFactory

The SessionFactory is needed by OGM to create instances of Session as required. This also sets up the object-graph mapping metadata when constructed, which is then used across all Session objects that it creates. The packages to scan for domain object metadata should be provided to the SessionFactory constructor. Multiple packages may be provided as well. If you would rather just pass in specific classes you can also do that via an overloaded constructor. The SessionFactory must also be configured. There are two ways this can be done. Please see the section below on Configuration for further details.

Multiple packages
SessionFactory sessionFactory = new SessionFactory("first.package.domain", "second.package.domain",...);

Note that the SessionFactory should typically be set up once during life of your application.

17. Using the OGM Session

The Session provides the core functionality to persist objects to the graph and load them in a variety of ways.

17.1. Session Configuration

A Session is used to drive the object-graph mapping framework. It keeps track of the changes that have been made to entities and their relationships. The reason it does this is so that only entities and relationships that have changed get persisted on save, which is particularly efficient when working with large graphs. Once an entity is tracked by the session, reloading this entity within the scope of the same session will result in the session cache returning the previously loaded entity. However, the subgraph in the session will expand if the entity or its related entities retrieve additional relationships from the graph.

If you want to fetch fresh data from the graph, then this can be achieved by using a new session or clearing the current sessions context using Session.clear().

The lifetime of the Session can be managed in code. For example, associated with single fetch-update-save cycle or unit of work.

If your application relies on long-running sessions then you may not see changes made from other users and find yourself working with outdated objects. On the other hand, if your sessions have too narrow a scope then your save operations can be unnecessarily expensive, as updates will be made to all objects if the session isn’t aware of the those that were originally loaded.

There’s therefore a trade off between the two approaches. In general, the scope of a Session should correspond to a "unit of work" in your application.

17.2. Basic operations

Basic operations are limited to CRUD operations on entities and executing arbitrary Cypher queries; more low-level manipulation of the graph database is not possible.

There is no way to manipulate relationship- and node-objects directly.

Given that the Neo4j OGM framework is driven by Cypher queries alone, there’s no way to work directly with Node and Relationship objects in remote server mode. Similarly, Traversal Framework operations are not supported, again because the underlying query-driven model doesn’t handle it in an efficient way.

If you find yourself in trouble because of the omission of these features, then your best options are:

  1. Write a Cypher query to perform the operations on the nodes/relationships instead.

  2. Write a Neo4j server extension and call it over REST from your application.

Of course, there are pros and cons to both of these approaches, but these are largely outside the scope of this document. In general, for low-level, very high-performance operations like complex graph traversals you’ll get the best performance by writing a server-side extension. For most purposes, though, Cypher will be performant and expressive enough to perform the operations that you need.

17.3. Persisting entities

Session allows to save, load, loadAll and delete entities with transaction handling and exception translation managed for you. The eagerness with which objects are retrieved is controlled by specifying the 'depth' argument to any of the load methods.

Entity persistence is performed through the save() method on the underlying Session object.

Under the bonnet, the implementation of Session has access to the MappingContext that keeps track of the data that has been loaded from Neo4j during the lifetime of the session. Upon invocation of save() with an entity, it checks the given object graph for changes compared with the data that was loaded from the database. The differences are used to construct a Cypher query that persists the deltas to Neo4j before repopulating it’s state based on the response from the database server.

The OGM doesn’t automatically commit when a transaction closes, so an explicit call to save(…​) is required in order to persist changes to the database.

Example 44. Persisting entities
@NodeEntity
public class Person {
   private String name;
   public Person(String name) {
      this.name = name;
   }
}

// Store Michael in the database.
Person p = new Person("Michael");
session.save(p);

17.3.1. Save depth

As mentioned previously, save(entity) is overloaded as save(entity, depth), where depth dictates the number of related entities to save starting from the given entity. The default depth, -1, will persist properties of the specified entity as well as every modified entity in the object graph reachable from it. This means that all affected objects in the entity model that are reachable from the root object being persisted will be modified in the graph. This is the recommended approach because it means you can persist all your changes in one request. The OGM is able to detect which objects and relationships require changing, so you won’t flood Neo4j with a bunch of objects that don’t require modification. You can change the persistence depth to any value, but you should not make it less than the value used to load the corresponding data or you run the risk of not having changes you expect to be made actually being persisted in the graph. A depth of 0 will persist only the properties of the specified entity to the database.

Specifying the save depth is handy when it comes to dealing with complex collections, that could potentially be very expensive to load.

Example 45. Relationship save cascading
@NodeEntity
class Movie {
    String title;
    Actor topActor;
    public void setTopActor(Actor actor) {
        topActor = actor;
    }
}

@NodeEntity
class Actor {
    String name;
}

Movie movie = new Movie("Polar Express");
Actor actor = new Actor("Tom Hanks");

movie.setTopActor(actor);

Neither the actor nor the movie has been assigned a node in the graph. If we were to call session.save(movie), then the OGM would first create a node for the movie. It would then note that there is a relationship to an actor, so it would save the actor in a cascading fashion. Once the actor has been persisted, it will create the relationship from the movie to the actor. All of this will be done atomically in one transaction.

The important thing to note here is that if session.save(actor) is called instead, then only the actor will be persisted. The reason for this is that the actor entity knows nothing about the movie entity - it is the movie entity that has the reference to the actor. Also note that this behaviour is not dependent on any configured relationship direction on the annotations. It is a matter of Java references and is not related to the data model in the database.

In the following example, the actor and the movie are both managed entities, having both been previously persisted to the graph:

Example 46. Cascade for modified fields
actor.setBirthyear(1956);
session.save(movie);

In this case, even though the movie has a reference to the actor, the property change on the actor will be persisted by the call to save(movie). The reason for this is, as mentioned above, that cascading will be done for fields that have been modified and reachable from the root object being saved.

In the example below, session.save(user,1) will persist all modified objects reachable from user up to one level deep. This includes posts and groups but not entities related to them, namely author, comments, members or location. A persistence depth of 0 i.e. session.save(user,0) will save only the properties on the user, ignoring any related entities. In this case, fullName is persisted but not friends, posts or groups.

Persistence Depth
public class User  {

   private Long id;
   private String fullName;
   private List<Post> posts;
   private List<Group> groups;

}

public class Post {

   private Long id;
   private String name;
   private String content;
   private User author;
   private List<Comment> comments;

}

public class Group {

   private Long id;
   private String name;
   private List<User> members;
   private Location location;

}

17.4. Loading Entities

Entities can be loaded from the OGM through the use of the session.loadXXX() methods or via session.query()/session.queryForObject() which will accept your own Cypher queries (See section below on cypher queries).

Neo4j OGM includes the concept of persistence horizon (depth). On any individual request, the persistence horizon indicates how many relationships should be traversed in the graph when loading or saving data. A horizon of zero means that only the root object’s properties will be loaded or saved, a horizon of 1 will include the root object and all its immediate neighbours, and so on. This attribute is enabled via a depth argument available on all session methods, but the OGM chooses sensible defaults so that you don’t have to specify the depth attribute unless you want change the default values.

17.4.1. Load depth

By default, loading an instance will map that object’s simple properties and its immediately-related objects (i.e. depth = 1). This helps to avoid accidentally loading the entire graph into memory, but allows a single request to fetch not only the object of immediate interest, but also its closest neighbours, which are likely also to be of interest. This strategy attempts to strike a balance between loading too much of the graph into memory and having to make repeated requests for data.

If parts of your graph structure are deep and not broad (for example a linked-list), you can increase the load horizon for those nodes accordingly. Finally, if your graph will fit into memory, and you’d like to load it all in one go, you can set the depth to -1.

On the other hand when fetching structures which are potentially very "bushy" (e.g. lists of things that themselves have many relationships), you may want to set the load horizon to 0 (depth = 0) to avoid loading thousands of objects most of which you won’t actually inspect.

When loading entities with a custom depth less than the one used previously to load the entity within the session, existing relationships will not be flushed from the session; only new entities and relationships are added. This means that reloading entities will always result in retaining related objects loaded at the highest depth within the session for those entities. If it is required to load entities with a lower depth than previously requested, this must be done on a new session, or after clearing your current session with Session.clear().

17.4.2. Cypher queries

Cypher is Neo4j’s powerful query language. It is understood by all the different drivers in the OGM which means that your application code should run identically, whichever driver you choose to use. This makes application development much easier: you can use the Embedded Driver for your integration tests, and then plug in the Http Driver or the Bolt Driver when deploying your code into a production client-server environment.

The Session also allows execution of arbitrary Cypher queries via its query, queryForObject and queryForObjects methods. Cypher queries that return tabular results should be passed into the query method which returns an Result. This consists of QueryStatistics representing statistics of modifying cypher statements if applicable, and an Iterable<Map<String,Object>> containing the raw data, which can be either used as-is or converted into a richer type if needed. The keys in each Map correspond to the names listed in the return clause of the executed Cypher query.

queryForObject specifically queries for entities and as such, queries supplied to this method must return nodes and not individual properties.

In the current version, custom queries do not support paging, sorting or a custom depth. In addition, it does not support mapping a path to domain entities, as such, a path should not be returned from a Cypher query. Instead, return nodes and relationships to have them mapped to domain entities.

Modifications made to the graph via Cypher queries directly will not be reflected in your domain objects within the session.

17.4.3. Sorting and paging

Neo4j OGM supports Sorting and Paging of results when using the Session object. The Session object methods take independent arguments for Sorting and Pagination

Paging
Iterable<World> worlds = session.loadAll(World.class,
                                        new Pagination(pageNumber,itemsPerPage), depth)
Sorting
Iterable<World> worlds = session.loadAll(World.class,
                                        new SortOrder().add("name"), depth)
Sort in descending order
Iterable<World> worlds = session.loadAll(World.class,
                                        new SortOrder().add(SortOrder.Direction.DESC,"name"))
Sorting with paging
Iterable<World> worlds = session.loadAll(World.class,
                                        new SortOrder().add("name"), new Pagination(pageNumber,itemsPerPage))

Neo4j OGM does not yet support sorting and paging on custom queries.

17.5. Transactions

Neo4j is a transactional database, only allowing operations to be performed within transaction boundaries.

Transactions can be managed explicitly by calling the beginTransaction() method on the Session followed by a commit() or rollback() as required.

Transaction management
Transaction tx = session.beginTransaction();
Person person = session.load(Person.class,personId);
Concert concert= session.load(Concert.class,concertId);
Hotel hotel = session.load(Hotel.class,hotelId);

try {
    buyConcertTicket(person,concert);
    bookHotel(person, hotel);
    tx.commit();
}
catch (SoldOutException e) {
    tx.rollback();
}
tx.close();

In the example above, the transaction is committed only when both a concert ticket and hotel room is available, otherwise, neither booking is made.

If you do not manage a transaction in this manner, auto commit transactions are provided implicitly for Session methods such as save, load, delete, execute and so on.

18. Type Conversion

The object-graph mapping framework provides support for default and bespoke type conversions, which allow you to configure how certain data types are mapped to nodes or relationships in Neo4j.

18.1. Built-in type conversions

Neo4j OGM will automatically perform the following type conversions:

  • java.util.Date to a String in the ISO 8601 format: "yyyy-MM-dd’T’HH:mm:ss.SSSXXX"

  • java.time.Instant to a String in the ISO 8601 with timezone format: "yyyy-MM-dd’T’HH:mm:ss.SSSXXXZ"

  • java.time.LocalDate to a String in the ISO 8601 with format: "yyyy-MM-dd"

  • Any object that extends java.lang.Number to a String property

  • binary data (as byte[] or Byte[]) to base-64 String as Cypher does not support byte arrays

  • java.lang.Enum types using the enum’s name() method and Enum.valueOf()

Two Date converters are provided "out of the box":

  1. @DateString

  2. @DateLong

By default, the OGM will use the @DateString converter as described above. However if you want to use a different date format, you can annotate your entity attribute accordingly:

Example of user-defined date format
public class MyEntity {

    @DateString("yy-MM-dd")
    private Date entityDate;
}

Alternatively, if you want to store java.util.Date or java.time.Instant as long values, use the @DateLong annotation:

Example of date stored as a long value
public class MyEntity {

    @DateLong
    private Date entityDate;
}

java.time.Instant dates are stored in the database using UTC.

Collections of primitive or convertible values are also automatically mapped by converting them to arrays of their type or strings respectively.

Collections are not supported for java.time.Instant and java.time.LocalDate

18.2. Custom Type Conversion

In order to define bespoke type conversions for particular members, you can annotate a field or method with @Convert. One of either two convert implementations can be used. For simple cases where a single property maps to a single field, with type conversion, specify an implementation of AttributeConverter.

Example of mapping a single property to a field
public class MoneyConverter implements AttributeConverter<DecimalCurrencyAmount, Integer> {

   @Override
   public Integer toGraphProperty(DecimalCurrencyAmount value) {
       return value.getFullUnits() * 100 + value.getSubUnits();
   }

   @Override
   public DecimalCurrencyAmount toEntityAttribute(Integer value) {
       return new DecimalCurrencyAmount(value / 100, value % 100);
   }

}

You could then apply this to your class as follows:

@NodeEntity
public class Invoice {

   @Convert(MoneyConverter.class)
   private DecimalCurrencyAmount value;
   ...
}

When more than one node property is to be mapped to a single field, use: CompositeAttributeConverter.

Example of mapping multiple node entity properties onto a single instance of a type
/**
* This class maps latitude and longitude properties onto a Location type that encapsulates both of these attributes.
*/
public class LocationConverter implements CompositeAttributeConverter<Location> {

    @Override
    public Map<String, ?> toGraphProperties(Location location) {
        Map<String, Double> properties = new HashMap<>();
        if (location != null)  {
            properties.put("latitude", location.getLatitude());
            properties.put("longitude", location.getLongitude());
        }
        return properties;
    }

    @Override
    public Location toEntityAttribute(Map<String, ?> map) {
        Double latitude = (Double) map.get("latitude");
        Double longitude = (Double) map.get("longitude");
        if (latitude != null && longitude != null) {
            return new Location(latitude, longitude);
        }
        return null;
    }

}

And just as with an AttributeConverter, a CompositeAttributeConverter could be applied to your class as follows:

@NodeEntity
public class Person {

   @Convert(LocationConverter.class)
   private Location location;
   ...
}

19. Filters

Filters provide a mechanism for customising the where clause of CYPHER generated by OGM. They can be chained together with boolean operators, and associated with a comparison operator. Additionally, each filter contains a FilterFunction. A filter function can be provided when the filter is instantiated, otherwise, by default a PropertyComparison is used.

In the example below, we’re return a collection containing any satellites that are manned.

Example of using a default property comparison Filter
Collection<Satellite> satellites = session.loadAll(Satellite.class, new Filter("manned", true));

20. Events

As of version 2.0.4, Neo4j OGM supports persistence events.

20.1. Event types

There are four types of events:

Event.LIFECYCLE.PRE_SAVE
Event.LIFECYCLE.POST_SAVE
Event.LIFECYCLE.PRE_DELETE
Event.LIFECYCLE.POST_DELETE

Events are fired for every @NodeEntity or @RelationshipEntity object that is created, updated or deleted, or otherwise affected by a save or delete request. This includes:

  • The top-level objects or objects being created, modified or deleted.

  • Any connected objects that have been modified, created or deleted.

  • Any objects affected by the creation, modification or removal of a relationship in the graph.

Events will only fire when one of the session.save() or session.delete() methods is invoked. Directly executing Cypher queries against the database using session.query() will not trigger any events.

20.2. Interfaces

The Events mechanism introduces two new interfaces, Event and EventListener.

The Event interface

The Event interface is implemented by PersistenceEvent. Whenever an application wishes to handle an event it will be given an instance of Event, which exposes the following methods:

public interface Event {

    Object getObject();
    LIFECYCLE getLifeCycle();

    enum LIFECYCLE {
        PRE_SAVE, POST_SAVE, PRE_DELETE, POST_DELETE
    }
}

The Event Listener interface

The EventListener interface provides methods allowing implementing classes to handle each of the different Event types:

public interface EventListener {

    void onPreSave(Event event);
    void onPostSave(Event event);
    void onPreDelete(Event event);
    void onPostDelete(Event event);

}

Although the Event interface allows you to retrieve the event type, in most cases, your code won’t need it because the EventListener provides methods to capture each type of event explicitly.

20.3. Registering an EventListener

There are two way to register an event listener:

  • on an individual Session

  • across multiple sessions by using a SessionFactory

In this example we register an anonymous EventListener to inject a UUID onto new objects before they’re saved

class AddUuidPreSaveEventListener implements EventListener {

    void onPreSave(Event event) {
        DomainEntity entity = (DomainEntity) event.getObject():
        if (entity.getId() == null) {
            entity.setUUID(UUID.randomUUID());
        }
    }
    void onPostSave(Event event) {
    }
    void onPreDelete(Event event) {
    }
    void onPostDelete(Event event) {
}

EventListener eventListener = new AddUuidPreSaveEventListener();

// register it on an individual session
session.register(eventListener);

// remove it.
session.dispose(eventListener);

// register it across multiple sessions
sessionFactory.register(eventListener);

// remove it.
sessionFactory.deregister(eventListener);

It’s possible and sometimes desirable to add several EventListener objects to the session, depending on the application’s requirements. For example, our business logic might require us to add a UUID to a new object, as well as manage wider concerns such as ensuring that a particular persistence event won’t leave our domain model in a logically inconsistent state. It’s usually a good idea to separate these concerns into different objects with specific responsibilities, rather than having one single object try to do everything.

20.4. Using the EventListenerAdapter

The EventListener above is fine, but we’ve had to create three methods for events we don’t intend to handle. It would be preferable if we didn’t have to do this each time we needed an EventListener.

The EventListenerAdapter is an abstract class providing a no-op implementation of the EventListener interface. If you don’t need to handle all the different types of persistence event you can create a subclass of EventListenerAdapter instead and override just the methods for the event types you’re interested in.

For example:

class PreSaveEventListener extends EventListenerAdaper {
    @Override
    void onPreSave(Event event) {
        DomainEntity entity = (DomainEntity) event.getObject();
        if (entity.id == null) {
            entity.UUID = UUID.randomUUID();
        }
    }
}

20.5. Disposing of an EventListener

Something to bear in mind is that once an EventListener has been registered it will continue to respond to any and all persistence events. Sometimes you may want only to handle events for a short period of time, rather than for the duration of the entire session.

If you’re done with an EventListener you can stop it from firing any more events by invoking session.dispose(…​), passing in the EventListener to be disposed of.

The process of collecting persistence events prior to dispatching them to any EventListeners adds a small performance overhead to the persistence layer. Consequently, the OGM is configured to suppress the event collection phase if there are no EventListeners registered with the Session. Using dispose() when you’re finished with an EventListener is good practice!

To remove an event listener across multiple sessions use the degresiter method on the SessionFactory.

20.6. Connected objects

As mentioned previously, events are not only fired for the top-level objects being saved but for all their connected objects as well.

Connected objects are any objects reachable in the domain model from the top-level object being saved. Connected objects can be many levels deep in the domain model graph.

In this way, the Events mechanism allows us to capture events for objects that we didn’t explicitly save ourselves.

// initialise the graph
Folder folder = new Folder("folder");
Document a = new Document("a");
Document b = new Document("b");
folder.addDocuments(a, b);

session.save(folder);

// change the names of both documents and save one of them
a.setName("A");
b.setName("B");

// because `b` is reachable from `a` (via the common shared folder) they will both be persisted,
// with PRE_SAVE and POST_SAVE events being fired for each of them
session.save(a);

20.7. Events and types

When we delete a Type, all the nodes with a label corresponding to that Type are deleted in the graph. The affected objects are not enumerated by the Events mechanism (they may not even be known). Instead, _DELETE events will be raised for the Type:

    // 2 events will be fired when the type is deleted.
    // - PRE_DELETE Document.class
    // - POST_DELETE Document.class
    session.delete(Document.class);

20.8. Events and collections

When saving or deleting a collection of objects, separate events are fired for each object in the collection, rather than for the collection itself.

Document a = new Document("a");
Document b = new Document("b");

// 4 events will be fired when the collection is saved.
// - PRE_SAVE a
// - PRE_SAVE b
// - POST_SAVE a
// - POST_SAVE b

session.save(Arrays.asList(a, b));

20.9. Event ordering

Events are partially ordered. PRE_ events are guaranteed to fire before any POST_ event within the same save or delete request. However, the internal ordering of the PRE_ events and POST_ events with the request is undefined.

Example: Partial ordering of events
Document a = new Document("a");
Document b = new Document("b");

// Although the save order of objects is implied by the request, the PRE_SAVE event for `b`
// may be fired before the PRE_SAVE event for `a`, and similarly for the POST_SAVE events.
// However, all PRE_SAVE events will be fired before any POST_SAVE event.

session.save(Arrays.asList(a, b));

20.10. Relationship events

The previous examples show how events fire when the underlying node representing an entity is updated or deleted in the graph. Events are also fired when a save or delete request results in the modification, addition or deletion of a relationship in the graph.

For example, if you delete a Document object that is a member of a Folder’s documents collection, events will be fired for the Document as well as the Folder, to reflect the fact that the relationship between the folder and the document has been removed in the graph

Example: Deleting a Document attached to a Folder
Folder folder = new Folder();
Document a = new Document("a");
folder.addDocuments(a);
session.save(folder);

// When we delete the document, the following events will be fired
// - PRE_DELETE a
// - POST_DELETE a
// - PRE_SAVE folder  (1)
// - POST_SAVE folder
session.delete(a);
1 Note that the folder events are _SAVE events, not _DELETE events. The folder was not deleted.

The event mechanism does not try to synchronise your domain model. In this example, the folder is still holding a reference to the Document, even though it no longer exists in the graph. As always, your code must take care of domain model synchronisation.

20.11. Event uniqueness

The event mechanism guarantees to not fire more than one event of the same type for an object in a save or delete request.

Example: Multiple changes, single event of each type
 // Even though we're making changes to both the folder node, and its relationships,
 // only one PRE_SAVE and one POST_SAVE event will be fired.
 folder.removeDocument(a);
 folder.setName("newFolder");
 session.save(folder);

21. Testing

In 2.0, the Neo4jIntegrationTestRule class has been removed from the test-jar.

In previous versions this class provided access to an underlying GraphDatabaseService instance, allowing you to independently verify your code was working correctly. However it is incompatible with the Driver interfaces in 2.0, as it always requires you to connect using HTTP.

The recommended approach is to configure an Embedded Driver for testing as described above, although you can still use an in-process HTTP server if you wish (see below). Please note that if you’re just using the Embedded Driver for your tests you do not need to include any additional test jars in your pom.

21.1. Log levels

When running unit tests, it can be useful to see what the OGM is doing, and in particular to see the Cypher requests being transferred between your application and the database. The OGM uses slf4j along with Logback as its logging framework and by default the log level for all the OGM components is set to WARN, which does not include any Cypher output. To change the OGM log level, create a file logback-test.xml in your test resources folder, configured as shown below:

logback-test.xml
<?xml version="1.0" encoding="UTF-8"?>
<configuration>

    <appender name="console" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%d %5p %40.40c:%4L - %m%n</pattern>
        </encoder>
    </appender>

    <!--
      ~ Set the required log level for the OGM components here.
      ~ To just see Cypher statements set the level to "info"
      ~ For finer-grained diagnostics, set the level to "debug".
    -->
    <logger name="org.neo4j.ogm" level="info" />

    <root level="warn">
        <appender-ref ref="console" />
    </root>

</configuration>

21.2. Using an in-process server for testing

If you want don’t want to use the Embedded Driver to run your tests, it is still possible to create an in-process HTTP server instead. Just like the Embedded Driver, a TestServer exposes a GraphDatabaseService instance which you can use in your tests. You should always close the server when you’re done with it.

You’ll first need to add the OGM test dependency to your pom:

<dependency>
    <groupId>org.neo4j</groupId>
    <artifactId>neo4j-ogm-test</artifactId>
    <version>5.0.0.BUILD-SNAPSHOT</version>
    <scope>test</scope>
</dependency>

Next, create a TestServer instance:

testServer = new TestServer.Builder()
                .enableAuthentication(true)    // defaults to false
                .transactionTimeoutSeconds(10) // defaults to 30 seconds
                .port(2222)                    // defaults to a random non-privileged port
                .build();

A TestServer is backed by an impermanent database store, and configures the OGM to use an HttpDriver. The driver authenticates automatically if you have requested an authenticating server so you don’t have to do provide additional credentials.

Example test class using an in-process HTTP server
private static TestServer testServer;

@BeforeClass
public static setupTestServer() {
    testServer = new TestServer.Builder().build();
}

@AfterClass
public static teardownTestServer() {
    testServer.close();
}

@Test
public void shouldCreateUser() {

    session.save(new User("Bilbo Baggins"));

    GraphDatabaseService db = testServer.getGraphDatabaseService();
    try (Transaction tx = db.beginTx()) {
        Result r = db.execute("MATCH (u:User {name: 'Bilbo Baggins'}) RETURN u");
        assertTrue(r.hasNext());
        tx.success();
    }
}

22. High Availability (HA) Support

The clustering features are available in Neo4j Enterprise Edition.

Neo4j offers two separate solutions for ensuring redundancy and performance in a high-demand production environment:

  • Causal Clustering

  • Highly Available (HA) Cluster

Neo4j 3.1 introduced Causal Clustering – a brand-new architecture using the state-of-the-art Raft protocol – that enables support for ultra-large clusters and a wider range of cluster topologies for data center and cloud.

A Neo4j HA cluster is comprised of a single master instance and zero or more slave instances. All instances in the cluster have full copies of the data in their local database files. The basic cluster configuration usally consists of three instances.

22.1. Causal Clustering

To find out more about Causal Clustering architecture please see: https://neo4j.com/docs/operations-manual/beta/clustering/causal-clustering/.

Causal Clustering only works with the Neo4j Bolt Driver (1.1.0 onwards). Trying to set this up with the HTTP or Embedded Driver will not work. The bolt driver will fully handle any load balancing, which operate in concert with the Causal Cluster to spread the workload. New cluster-aware sessions, managed on the client-side by the Bolt drivers, alleviate complex infrastructure concerns for developers.

22.1.1. Configuring the OGM

To use clustering, simply configure your bolt URI (in ogm.properties or through Components.driver().setURI()) to use the bolt routing protocol:

URI=bolt+routing://instance0

instance0 must be one of your core cluster group (that accepts reads and writes).

22.1.2. Using OGM Sessions.

By default all Session 's Transaction s are set to read/write. This means reads and writes will always hit the core cluster. In order to break this up you can call session.beginTransaction(Transaction.Type) with READ to hit the replica servers.

22.1.3. Bookmarks

Causal consistency allows you to specify guarantees around query ordering, including the ability to read your own writes, view the last data you read, and later on, committed writes from other users. The Bolt drivers collaborate with the core servers to ensure that all transactions are applied in the same order using a concept of a bookmark.

The cluster returns a bookmark when it commits an update transaction, so then the driver links a bookmark to the user’s next transaction. The server that received query starts this new bookmarked transaction only when its internal state reached the desired bookmark. This ensures that the view of related data is always consistent, that all servers are eventually updated, and that users reading and re-reading data always see the same — and the latest — data.

If you have multiple application tier JVM instances you will need to manage this state across them. The Session object allows you to set and retrieve bookmarks through the use of: Session.withBookmark() and Session.getLastBookmark().

22.2. Highly Available (HA) Cluster

A typical Neo4j HA cluster will consist of a master node and a couple of slave nodes for providing failover capability and optionally for handling reads. (Although it is possible to write to slaves, this is uncommon because it requires additional effort to synchronise a slave with the master node)

Typical HA Cluster

22.2.1. Transaction binding in HA mode

When operating in HA mode, Neo4j does not make open transactions available across all nodes in the cluster. This means we must bind every request within a specific transaction to the same node in the cluster, or the commit will fail with 404 Not Found.

22.2.2. Read-only transactions

As of Version 2.0.5 read-only transactions are supported by the OGM.

API changes

There is new API method session.beginTransaction(Transaction.Type) where Transaction.Type is one of:

  • READ_WRITE

  • READ_ONLY

In the case that your code doesn’t declare an explicit transaction, autocommit transactions (or their logical equivalent) will be created by the various drivers. These will be READ_WRITE.

The previous API semantics have not changed, i.e. session.beginTransaction() creates a READ_WRITE transaction.

Drivers

The Drivers have been updated to transmit additional information about the transaction type of the current transaction to the server.

  • The HttpDriver implementation sets a Http Header "X-WRITE" to "1" for READ_WRITE transactions (the default) or to "0" for READ_ONLY ones.

  • The Embedded Driver can support both READ_ONLY and READ_WRITE (as of version 2.1.0).

  • The native Bolt Driver can support both READ_ONLY and READ_WRITE (as of version 2.1.0).

22.2.3. Dynamic binding via a load balancer

In the Neo4j HA architecture, a cluster is typically fronted by a load balancer.

The following example shows how to configure your application and set up HAProxy as a load balancer to route write requests to whichever machine in the cluster is currently identified as the master, with read requests being distributed to any available machine in the cluster on a round-robin basis.

This configuration will also ensure that requests against a specific transaction are directed to the server where the transaction was created.

Example cluster fronted by HAProxy
  1. haproxy: 10.0.2.200

  2. neo4j-server1: 10.0.1.10

  3. neo4j-server2: 10.0.1.11

  4. neo4j-server3: 10.0.1.12

OGM Binding via HAProxy
Components.driver().setURI("http://10.0.2.200");
Sample haproxy.cfg
global
    daemon
    maxconn 256

defaults
    mode http
    timeout connect 5000ms
    timeout client 50000ms
    timeout server 50000ms

frontend http-in
    bind *:80
    acl write_hdr hdr_val(X-WRITE) eq 1
    use_backend neo4j-master if write_hdr
    default_backend neo4j-cluster

backend neo4j-cluster
    balance roundrobin
    # create a sticky table so that requests with a transaction id are always sent to the correct server
    stick-table type integer size 1k expire 70s
    stick match path,word(4,/)
    stick store-response hdr(Location),word(6,/)
    option httpchk GET /db/manage/server/ha/available
    server s1 10.0.1.10:7474 maxconn 32
    server s2 10.0.1.11:7474 maxconn 32
    server s3 10.0.1.12:7474 maxconn 32

backend neo4j-master
    option httpchk GET /db/manage/server/ha/master
    server s1 10.0.1.10:7474 maxconn 32
    server s2 10.0.1.11:7474 maxconn 32
    server s3 10.0.1.12:7474 maxconn 32

listen admin
    bind *:8080
    stats enable

Appendix

Appendix A: Namespace reference

The <repositories /> element

The <repositories /> element triggers the setup of the Spring Data repository infrastructure. The most important attribute is base-package which defines the package to scan for Spring Data repository interfaces.[3]

Table 7. Attributes
Name Description

base-package

Defines the package to be used to be scanned for repository interfaces extending *Repository (actual interface is determined by specific Spring Data module) in auto detection mode. All packages below the configured package will be scanned, too. Wildcards are allowed.

repository-impl-postfix

Defines the postfix to autodetect custom repository implementations. Classes whose names end with the configured postfix will be considered as candidates. Defaults to Impl.

query-lookup-strategy

Determines the strategy to be used to create finder queries. See Query lookup strategies for details. Defaults to create-if-not-found.

named-queries-location

Defines the location to look for a Properties file containing externally defined queries.

consider-nested-repositories

Controls whether nested repository interface definitions should be considered. Defaults to false.

Appendix B: Populators namespace reference

The <populator /> element

The <populator /> element allows to populate the a data store via the Spring Data repository infrastructure.[4]

Table 8. Attributes
Name Description

locations

Where to find the files to read the objects from the repository shall be populated with.

Appendix C: Repository query keywords

Supported query keywords

The following table lists the keywords generally supported by the Spring Data repository query derivation mechanism. However, consult the store-specific documentation for the exact list of supported keywords, because some listed here might not be supported in a particular store.

Table 9. Query keywords
Logical keyword Keyword expressions

AND

And

OR

Or

AFTER

After, IsAfter

BEFORE

Before, IsBefore

CONTAINING

Containing, IsContaining, Contains

BETWEEN

Between, IsBetween

ENDING_WITH

EndingWith, IsEndingWith, EndsWith

EXISTS

Exists

FALSE

False, IsFalse

GREATER_THAN

GreaterThan, IsGreaterThan

GREATER_THAN_EQUALS

GreaterThanEqual, IsGreaterThanEqual

IN

In, IsIn

IS

Is, Equals, (or no keyword)

IS_EMPTY

IsEmpty, Empty

IS_NOT_EMPTY

IsNotEmpty, NotEmpty

IS_NOT_NULL

NotNull, IsNotNull

IS_NULL

Null, IsNull

LESS_THAN

LessThan, IsLessThan

LESS_THAN_EQUAL

LessThanEqual, IsLessThanEqual

LIKE

Like, IsLike

NEAR

Near, IsNear

NOT

Not, IsNot

NOT_IN

NotIn, IsNotIn

NOT_LIKE

NotLike, IsNotLike

REGEX

Regex, MatchesRegex, Matches

STARTING_WITH

StartingWith, IsStartingWith, StartsWith

TRUE

True, IsTrue

WITHIN

Within, IsWithin

Appendix D: Repository query return types

Supported query return types

The following table lists the return types generally supported by Spring Data repositories. However, consult the store-specific documentation for the exact list of supported return types, because some listed here might not be supported in a particular store.

Geospatial types like (GeoResult, GeoResults, GeoPage) are only available for data stores that support geospatial queries.
Table 10. Query return types
Return type Description

void

Denotes no return value.

Primitives

Java primitives.

Wrapper types

Java wrapper types.

T

An unique entity. Expects the query method to return one result at most. In case no result is found null is returned. More than one result will trigger an IncorrectResultSizeDataAccessException.

Iterator<T>

An Iterator.

Collection<T>

A Collection.

List<T>

A List.

Optional<T>

A Java 8 or Guava Optional. Expects the query method to return one result at most. In case no result is found Optional.empty()/Optional.absent() is returned. More than one result will trigger an IncorrectResultSizeDataAccessException.

Option<T>

An either Scala or JavaSlang Option type. Semantically same behavior as Java 8’s Optional described above.

Stream<T>

A Java 8 Stream.

Future<T>

A Future. Expects method to be annotated with @Async and requires Spring’s asynchronous method execution capability enabled.

CompletableFuture<T>

A Java 8 CompletableFuture. Expects method to be annotated with @Async and requires Spring’s asynchronous method execution capability enabled.

ListenableFuture

A org.springframework.util.concurrent.ListenableFuture. Expects method to be annotated with @Async and requires Spring’s asynchronous method execution capability enabled.

Slice

A sized chunk of data with information whether there is more data available. Requires a Pageable method parameter.

Page<T>

A Slice with additional information, e.g. the total number of results. Requires a Pageable method parameter.

GeoResult<T>

A result entry with additional information, e.g. distance to a reference location.

GeoResults<T>

A list of GeoResult<T> with additional information, e.g. average distance to a reference location.

GeoPage<T>

A Page with GeoResult<T>, e.g. average distance to a reference location.

Appendix E: Migration Guide

Migrating from 4.2 → 5.0

  • Base class for repositories GraphRepository has been renamed Neo4jRepository and parameter types change from <T> to <T, ID>.

  • Annotations are now only supported at attribute level.

  • @PostLoad annotation has bean added to support use cases.

  • Repository methods can return Streams.

  • findOne repository methods now return Optional<T>.

  • Paged custom queries no longer accept queries without countQuery attribute.

Migrating from 4.0/4.1 → 4.2

Spring Data Neo4j 4.2 significantly reduces complexity of configuration for application developers. There is no longer a need to extend from Neo4jConfiguration or define a Session bean. Configuration for various types of applications are described here

  1. Remove any subclassing of Neo4jConfiguration

  2. Define the sessionFactory bean with an instance of SessionFactory and the transactionManager bean with an instance of Neo4jTransactionManager. Be sure to pass the SessionFactory into the constructor for the transaction manager.

Migrating from pre 4.0 → 4.2

Package Changes

Because the Neo4j Object Graph Mapper can be used independently of Spring Data Neo4j, the core annotations have been moved out of the spring framework packages:

org.springframework.data.neo4j.annotationorg.neo4j.ogm.annotation

The @Query and @QueryResult annotations are only supported in the Spring modules, and are not used by the core mapping framework. These annotations have not changed.

Annotation Changes

There have been some changes to the annotations that were used in previous versions of Spring Data Neo4j. Wherever possible we have tried to maintain the previous annotations verbatim, but in a few cases this has not been possible, usually for technical reasons but sometimes for aesthetic ones. Our goal has been to minimise the number of annotations you need to use as well as trying to make them more self-explanatory. The following annotations have been changed.

Old

New

@RelatedTo

@Relationship

@RelatedToVia

@Relationship

@GraphProperty

@Property

@MapResult

@QueryResult

@ResultColumn

@Property

Relationship Direction.BOTH

Relationship.UNDIRECTED

Custom Type Conversion

SDN provides automatic type conversion for the obvious candidates: byte[] and Byte[] arrays, Dates, BigDecimal and BigInteger types. In order to define bespoke type conversions for particular entity attribute, you can annotate a field or method with @Convert to specify your own implementation of org.neo4j.ogm.typeconversion.AttributeConverter.

You can find out more about type conversions here: Custom Converters

Date Format Changes

The default Date converter is @DateString.

SDN 3.x and earlier represented Dates as a String value consisting of the number of milliseconds since January 1, 1970, 00:00:00 GMT.

If you are upgrading to SDN 4.x from these versions and your application used the default, then you need to annotate your Date properties with @DateLong. Moreover, the property values in the graph need to be converted to numbers.

Upgrade Date properties to numbers
MATCH (n:Foo) //All nodes which contain date properties to be migrated
WHERE NOT HAS(n.migrated)// Take the first 10k nodes that haven't been migrated yet
WITH n LIMIT 10000
SET n.dateProperty = toInt(n.dateProperty),n.migrated=1 //where dateProperty is the date with a String value to be migrated
RETURN count(n); //Run until the statement returns zero records
//Similar process to remove the migrated flag

However, if your application already represented Dates as @GraphProperty(propertyType = Long.class) then simply changing this to @DateLong is sufficient.

Indexing

The best way to retrieve start nodes for traversals and queries is by using Neo4j’s integrated index facilities. SDN supports Index and Constraint management but differs in how it does this to previous versions.

Obsolete Annotations

The following annotations are no longer used, either because they are no longer needed, or cannot be supported via Cypher.

  • @GraphTraversal

  • @RelatedToVia

  • @RelatedTo

  • @TypeAlias

  • @Fetch

Features No Longer Supported

Some features of the previous annotations have been dropped.

Overriding @Property Types

Support for overriding property types via arguments to @Property has been dropped. If your attribute requires a non-default conversion to and from a database property, you can use a Custom Converter instead.

@Relationship enforceTargetType

In previous versions of Spring Data Neo4j, you would have to add an enforceTargetType attribute into every clashing @Relationship annotation. Thanks to changes in the underlying object-graph mapping mechanism, this is no longer necessary.

Clashing Relationship Types
@NodeEntity
class Person {
    @Relationship(type="OWNS")
    private Car car;

    @Relationship(type="OWNS")
    private Pet pet;
...
}
Cross-store Persistence

Neo4j is dropping XA support and therefore SDN does not provide any capability for cross-store persistence

TypeRepresentationStrategy

SDN 4 replaces the existing TypeRepresentionStrategy configuration with a straightforward convention based on simple class-names or entities using @NodeEntity(label=…​)

AspectJ Support

Support for AspectJ-based persistence has been removed from SDN 4 as the write-and-read-through approach only works with an integrated, embedded database, not Neo4j server. The performance improvements in SDN 4 should make their use as a performance optimisation unnecessary anyway.

Deprecation of Neo4jTemplate

It is highly recommended for users starting new SDN projects to use the OGM Session directly. Neo4jTemplate has been kept to give upgrading users a better experience.

The Neo4jTemplate has been slimmed-down significantly for SDN 4. It contains the exact same methods as Session. In fact Neo4jTemplate is just a very thin wrapper with an ability to support SDN Exception Translation. Many of the operations are no longer needed or can be expressed with a straightforward Cypher query.

If you do use Neo4jTemplate, then you should code against its Neo4jOperations interface instead of the template class.

The following table shows the Neo4jTemplate functions that have been retained for version 4 of Spring Data Neo4j. In some cases the method names have changed but the same functionality is offered under the new version.

Table 11. Neo4j Template Method Migration
Old Method Name New Method Name Notes

findOne

load

Overloaded to take optional depth parameter

findAll

loadAll

Overloaded to take optional depth parameter, also now returns a Collection rather than a Result

query

query

Return type changed from Result to be Iterable

save

save

delete

delete

count

count

No longer defines generic type parameters

findByIndexedValue

loadByProperty

Indexes are not supported natively, but you can index node properties in your database setup and use this method to find by them

To achieve the old template.fetch(entity) equivalent behaviour, you should call one of the load methods specifying the fetch depth as a parameter.

It’s also worth noting that exec(GraphCallback) and the create…​() methods have been made obsolete by Cypher. Instead, you should now issue a Cypher query to the new execute method to create the nodes or relationships that you need.

Dynamic labels, properties and relationship types are not supported as of this version, server extensions should be considered instead.

Built-In Query DSL Support

Previous versions of SDN allowed you to use a DSL to generate Cypher queries. There are many different DSL libraries available and you’re free to use which of these - or none - that you want. With Cypher changing on a regular basis, avoiding a DSL implementation in SDN means less ongoing maintenance and less likelihood of your code being incompatible with future versions of Neo4j.

Graph Traversal and Node/Relationship Manipulation

These features cannot be supported by Cypher and have therefore been dropped from Neo4jTemplate.

Please provide feedback on the new APIs of SDN 5 and the migration needs to [email protected] or via a JIRA issue

Appendix F: Frequently asked questions

  1. What is the difference between Neo4j OGM and Spring Data Neo4j (SDN)?

    Spring Data Neo4j (SDN) uses the OGM under the covers. It’s like Spring Data JPA, where JPA/Hibernate underly it. Most of the power of SDN actually comes from the OGM.

  2. How do I set up my Spring Configuration with Spring WebMVC projects?

    If you are using a Spring WebMVC application, the following configuration is all that’s required:

@Configuration
@EnableWebMvc
@ComponentScan({"org.neo4j.example.web"})
@EnableNeo4jRepositories("org.neo4j.example.repository")
@EnableTransactionManagement
public class MyWebAppConfiguration extends WebMvcConfigurerAdapter {

    @Bean
    public OpenSessionInViewInterceptor openSessionInViewInterceptor() {
        OpenSessionInViewInterceptor openSessionInViewInterceptor =
            new OpenSessionInViewInterceptor();
        openSessionInViewInterceptor.setSessionFactory(sessionFactory());
        return openSessionInViewInterceptor;
    }

    @Override
        public void addInterceptors(InterceptorRegistry registry) {
        registry.addWebRequestInterceptor(openSessionInViewInterceptor());
    }

    @Bean
    public SessionFactory sessionFactory() {
        // with domain entity base package(s)
        return new SessionFactory("org.neo4j.example.domain");
    }

    @Bean
    public Neo4jTransactionManager transactionManager() throws Exception {
        return new Neo4jTransactionManager(sessionFactory());
    }
}
How do I set up my Spring Configuration with a Java Servlet 3.x+ Container project?

If you are using a Java Servlet 3.x+ Container, you can configure a Servlet filter with Spring’s AbstractAnnotationConfigDispatcherServletInitializer. The configuration below will open a new session for every web request then automatically close it on completion. SDN provides the org.springframework.data.neo4j.web.support.OpenSessionInViewFilter to do this:

public class MyAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

  @Override
  protected void customizeRegistration(ServletRegistration.Dynamic registration) {
      registration.setInitParameter("throwExceptionIfNoHandlerFound", "true");
  }

  @Override
  protected Class<?>[] getRootConfigClasses() {
      return new Class[] {ApplicationConfiguration.class} // if you have broken up your configuration, this points to your non web application config/s.
  }

  @Override
  protected Class<?>[] getServletConfigClasses() {
      throw new Class[] {WebConfiguration.class}; // a configuration that extends the WebMvcConfigurerAdapter as seen above.
  }

  @Override
  protected String[] getServletMappings() {
    return new String[] {"/"};
  }

  protected Filter[] getServletFilters() {
    return return new Filter[] {new OpenSessionInViewFilter()};
  }

}

1. JavaConfig in the Spring reference documentation
2. Spring HATEOAS - https://github.com/SpringSource/spring-hateoas
3. see XML configuration
4. see XML configuration