This guide walks you through the process of using Spring Data Neo4j to build an application that stores data in and retrieves it from Neo4j, a graph-based database.

What You Will Build

You will use Neo4j’s NoSQL graph-based data store to build an embedded Neo4j server, store entities and relationships, and develop queries.

What You Need

How to complete this guide

Like most Spring Getting Started guides, you can start from scratch and complete each step or you can bypass basic setup steps that are already familiar to you. Either way, you end up with working code.

To start from scratch, move on to Starting with Spring Initializr.

To skip the basics, do the following:

When you finish, you can check your results against the code in gs-accessing-data-neo4j/complete.

Starting with Spring Initializr

For all Spring applications, you should start with the Spring Initializr. The Initializr offers a fast way to pull in all the dependencies you need for an application and does a lot of the set up for you. This example needs the Spring Web Starter, Spring Data JPA, and MySQL Driver dependencies. The following image shows the Initializr set up for this sample project:

initializr
The preceding image shows the Initializr with Maven chosen as the build tool. You can also use Gradle. It also shows values of com.example and accessing-data-mongodb as the Group and Artifact, respectively. You will use those values throughout the rest of this sample.

The following listing shows the pom.xml file created when you choose Maven:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.1.7.RELEASE</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>
	<groupId>com.example</groupId>
	<artifactId>accessing-data-neo4j</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>accessing-data-neo4j</name>
	<description>Demo project for Spring Boot</description>

	<properties>
		<java.version>1.8</java.version>
	</properties>

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

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
		</dependency>
	</dependencies>

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
		</plugins>
	</build>

</project>

The following listing shows the build.gradle file created when you choose Gradle:

plugins {
	id 'org.springframework.boot' version '2.1.7.RELEASE'
	id 'io.spring.dependency-management' version '1.0.8.RELEASE'
	id 'java'
}

group = 'com.example'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '1.8'

repositories {
	mavenCentral()
}

dependencies {
	implementation 'org.springframework.boot:spring-boot-starter-data-neo4j'
	testImplementation 'org.springframework.boot:spring-boot-starter-test'
}

Standing up a Neo4j Server

Before you can build this application, you need to set up a Neo4j server.

Neo4j has an open source server you can install for free.

On a Mac that has Homebrew installed, run the following command:

$ brew install neo4j

Once installed, launch it with its default settings by running the following command:

$ neo4j start

You should see output similar to the following:

Starting Neo4j.
Started neo4j (pid 96416). By default, it is available at http://localhost:7474/
There may be a short delay until the server is ready.
See /usr/local/Cellar/neo4j/3.0.6/libexec/logs/neo4j.log for current status.

By default, Neo4j has a username and password of neo4j and neo4j. However, it requires that the new account password be changed. To do so, run the following command:

curl -v -u neo4j:neo4j POST localhost:7474/user/neo4j/password -H "Content-type:application/json" -d "{\"password\":\"secret\"}"

This changes the password from neo4j to secret — something to NOT do in production! With that step completed, you should be ready to run the rest of this guide.

Define a Simple Entity

Neo4j captures entities and their relationships, with both aspects being of equal importance. Imagine you are modeling a system where you store a record for each person. However, you also want to track a person’s co-workers (teammates in this example). With Spring Data Neo4j, you can capture all that with some simple annotations, as the following listing (in src/main/java/com/example/accessingdataneo4j/Person.java) shows:

package com.example.accessingdataneo4j;

import java.util.Collections;
import java.util.HashSet;
import java.util.Optional;
import java.util.Set;
import java.util.stream.Collectors;

import org.neo4j.ogm.annotation.GeneratedValue;
import org.neo4j.ogm.annotation.Id;
import org.neo4j.ogm.annotation.NodeEntity;
import org.neo4j.ogm.annotation.Relationship;

@NodeEntity
public class Person {

  @Id @GeneratedValue private Long id;

  private String name;

  private Person() {
    // Empty constructor required as of Neo4j API 2.0.5
  };

  public Person(String name) {
    this.name = name;
  }

  /**
   * Neo4j doesn't REALLY have bi-directional relationships. It just means when querying
   * to ignore the direction of the relationship.
   * https://dzone.com/articles/modelling-data-neo4j
   */
  @Relationship(type = "TEAMMATE", direction = Relationship.UNDIRECTED)
  public Set<Person> teammates;

  public void worksWith(Person person) {
    if (teammates == null) {
      teammates = new HashSet<>();
    }
    teammates.add(person);
  }

  public String toString() {

    return this.name + "'s teammates => "
      + Optional.ofNullable(this.teammates).orElse(
          Collections.emptySet()).stream()
            .map(Person::getName)
            .collect(Collectors.toList());
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }
}

Here you have a Person class that has only one attribute: name.

The Person class is annotated with @NodeEntity. When Neo4j stores it, a new node is created. This class also has an id marked @GraphId. Neo4j uses @GraphId internally to track the data.

The next important piece is the set of teammates. It is a simple Set<Person> but is marked as @Relationship. This means that every member of this set is expected to also exist as a separate Person node. Note how the direction is set to UNDIRECTED. This means that when you query the TEAMMATE relationship, Spring Data Neo4j ignores the direction of the relationship.

With the worksWith() method, you can easily link people together.

Finally, you have a convenient toString() method to print out the person’s name and that person’s co-workers.

Create Simple Queries

Spring Data Neo4j is focused on storing data in Neo4j. But it inherits functionality from the Spring Data Commons project, including the ability to derive queries. Essentially, you need not learn the query language of Neo4j. Instead, you can write a handful of methods and let the queries be written for you.

To see how this works, create an interface that queries Person nodes. The following listing (in src/main/java/com/example/accessingdataneo4j/PersonRepository.java) shows such a query:

Unresolved directive in <stdin> - include::complete/src/main/java/hello/PersonRepository.java[]

PersonRepository extends the GraphRepository interface and plugs in the type on which it operates: Person. This interface comes with many operations, including standard CRUD (create, read, update, and delete) operations.

But you can define other queries by declaring their method signatures. In this case, you added findByName, which seeks nodes of type Person and finds the one that matches on name. You also have findByTeammatesName, which looks for a Person node, drills into each entry of the teammates field, and matches based on the teammate’s name.

Permissions to Access Neo4j

Neo4j Community Edition requires credentials to access it. You can configure these credential by setting a couple of properties (in src/main/resources/application.properties), as the following listing shows:

spring.data.neo4j.username=neo4j
spring.data.neo4j.password=secret

This includes the default username (neo4j) and the newly set password we picked earlier (secret).

Do NOT store real credentials in your source repository. Instead, configure them in your runtime using Spring Boot’s property overrides.

With this in place, you can wire this up and see what it looks like!

Create an Application Class

Spring Initializr creates a simple class for the application. The following listing shows the class that Initializr created for this example (in src/main/java/com/example/accessingdatamysql/AccessingDataMysqlApplication.java):

package com.example.accessingdataneo4j;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class AccessingDataNeo4jApplication {

  public static void main(String[] args) {
    SpringApplication.run(AccessingDataNeo4jApplication.class, args);
  }

}

@SpringBootApplication is a convenience annotation that adds all of the following:

  • @Configuration: Tags the class as a source of bean definitions for the application context.

  • @EnableAutoConfiguration: Tells Spring Boot to start adding beans based on classpath settings, other beans, and various property settings. For example, if spring-webmvc is on the classpath, this annotation flags the application as a web application and activates key behaviors, such as setting up a DispatcherServlet.

  • @ComponentScan: Tells Spring to look for other components, configurations, and services in the com/example package, letting it find the controllers.

The main() method uses Spring Boot’s SpringApplication.run() method to launch an application. Did you notice that there was not a single line of XML? There is no web.xml file, either. This web application is 100% pure Java and you did not have to deal with configuring any plumbing or infrastructure.

Spring Boot automatically handles those repositories as long as they are included in the same package (or a sub-package) of your @SpringBootApplication class. For more control over the registration process, you can use the @EnableNeo4jRepositories annotation.

By default, @EnableNeo4jRepositories scans the current package for any interfaces that extend one of Spring Data’s repository interfaces. You can use its basePackageClasses=MyRepository.class to safely tell Spring Data Neo4j to scan a different root package by type if your project layout has multiple projects and it does not find your repositories.

Logging output is displayed. The service should be up and running within a few seconds.

Now autowire the instance of PersonRepository that you defined earlier. Spring Data Neo4j dynamically implements that interface and plugs in the needed query code to meet the interface’s obligations.

The main method uses Spring Boot’s SpringApplication.run() to launch the application and invoke the CommandLineRunner that builds the relationships.

In this case, you create three local Person instances: Greg, Roy, and Craig. Initially, they only exist in memory. Note that no one is a teammate of anyone (yet).

At first, you find Greg, indicate that he works with Roy and Craig, and then persist him again. Remember, the teammate relationship was marked as UNDIRECTED (that is, bidirectional). That means that Roy and Craig have been updated as well.

That is why when you need to update Roy. It is critical that you fetch that record from Neo4j first. You need the latest status on Roy’s teammates before adding Craig to the list.

Why is there no code that fetches Craig and adds any relationships? Because you already have it! Greg earlier tagged Craig as a teammate, and so did Roy. That means there is no need to update Craig’s relationships again. You can see it as you iterate over each team member and print their information to the console.

Finally, check out that other query where you look backwards, answering the question of "Who works with whom?"

Build an executable JAR

You can run the application from the command line with Gradle or Maven. You can also build a single executable JAR file that contains all the necessary dependencies, classes, and resources and run that. Building an executable jar so makes it easy to ship, version, and deploy the service as an application throughout the development lifecycle, across different environments, and so forth.

If you use Gradle, you can run the application by using ./gradlew bootRun. Alternatively, you can build the JAR file by using ./gradlew build and then run the JAR file, as follows:

java -jar build/libs/gs-accessing-data-neo4j-0.1.0.jar

If you use Maven, you can run the application by using ./mvnw spring-boot:run. Alternatively, you can build the JAR file with ./mvnw clean package and then run the JAR file, as follows:

java -jar target/gs-accessing-data-neo4j-0.1.0.jar
The steps described here create a runnable JAR. You can also build a classic WAR file.

You should see something similar to the following listing (with other stuff, such as queries, as well):

Before linking up with Neo4j...
	Greg's teammates => []
	Roy's teammates => []
	Craig's teammates => []

Lookup each person by name...
	Greg's teammates => [Roy, Craig]
	Roy's teammates => [Greg, Craig]
	Craig's teammates => [Roy, Greg]

You can see from the output that (initially) no one is connected by any relationship. Then, after you add people, they are tied together. Finally, you can see the handy query that looks up people based on teammate.

Summary

Congratulations! You just set up an embedded Neo4j server, stored some simple related entities, and developed some quick queries.

If you want to expose Neo4j repositories with a hypermedia-based RESTful front end with little effort, read Accessing Neo4j Data with REST.

See Also

The following guides may also be helpful:

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All guides are released with an ASLv2 license for the code, and an Attribution, NoDerivatives creative commons license for the writing.