baeldung - Coding and Testing Stuff: A Guide to Neo4J with Java

1. Introduction

This article is about Neo4j one of the most mature and full-featured graph databases on the markettoday.Graph databases approach the task of data modeling with the view that many things in life lend themselves to being represented as a collection of nodes (V) and connections between them called edges (E).

2.Embedded Neo4j

The easiest way to getstarted with Neo4j is to use the embedded version in which Neo4j runs in the same JVM as yourapplication.

First, we need toadd a Maven dependency:

<dependency>
<groupId>org.neo4j</groupId>
<artifactId>neo4j</artifactId>
<version>3.1.0</version>
</dependency>

You can check this link to download the latest version.

Next, let’screate a factory:

GraphDatabaseFactory graphDbFactory = new GraphDatabaseFactory();

Finally, we create an embedded database:

GraphDatabaseService graphDb = graphDbFactory.newEmbeddedDatabase(
new File("data/cars"));

Now the real action can begin! First, we need to create some nodes in our graph and for that, we need to start a transaction since Neo4j will reject any destructive operation unless a transaction has been started:

graphDb.beginTx();

Once we have a transaction in progress, we can start adding nodes:

Node car = graphDb.createNode(Label.label("Car"));
car.setProperty("make", "tesla");
car.setProperty("model", "model3");
Node owner = graphDb.createNode(Label.label("Person"));
owner.setProperty("firstName", "baeldung");
owner.setProperty("lastName", "baeldung");

Here we added a node Car with properties make and model as well as node Person with properties firstName and lastName

Nowwe can add a relationship:

owner.createRelationshipTo(car, RelationshipType.withName("owner"));

The statement above added an edge joining the two nodes with an owner label. We can verify this relationship by running a query written in Neo4j’s powerful Cypher language:

Result result = graphDb.execute(
"MATCH (c:Car) <-[owner]- (p:Person) " +
"WHERE c.make = 'tesla'" +
"RETURN p.firstName, p.lastName");

Here we ask to find a car owner for any car whose make is tesla and give us back his/her firstName and lastName. Unsurprisingly, this returns: {p.firstName=baeldung, p.lastName=baeldung}

3. Cypher Query Language

Neo4j provides a very powerful and pretty intuitive querying language which supports the full range of features one would expect from a database. Let us examine how we can accomplish that standard create, retrieve, update and delete tasks.

3.1. Create Node

Create keyword can be used to create both nodes and relationships.

CREATE (self:Company {name:"Baeldung"})
RETURN self

Here we’ve created a company with a single property name . A node definition is marked by parentheses and its properties are enclosed in curly braces.In this case, self is an alias for the node and Company is a node label.

3.2. Create Relationship

It is possible to create a node and a relationship to that node all in a single query:

Result result = graphDb.execute(
"CREATE (baeldung:Company {name:\"Baeldung\"}) " +
"-[:owns]-> (tesla:Car {make: 'tesla', model: 'modelX'})" +
"RETURN baeldung, tesla");

Here we’ve created nodes baeldung and tesla and established an ownership relationship between them. Creating relationships to pre-existing nodes is, of course, also possible.

3.3. Retrieve Data

MATCH keyword is used to find data in combination with RETURN to control which data points are returned. The WHERE clause can be utilizedto filter out only those nodes that have the properties we desire.

Let usfigure out the name of the company that owns tesla modelX:

Result result = graphDb.execute(
"MATCH (company:Company)-[:owns]-> (car:Car)" +
"WHERE car.make='tesla' and car.model='modelX'" +
"RETURN company.name"); 3.4.Update Nodes

SET keyword can be used for updating node properties or labels. Let us add mileage to our tesla:

Result result = graphDb.execute("MATCH (car:Car)" +
"WHERE car.make='tesla'" +
" SET car.milage=120" +
" SET car :Car:Electro" +
" SET car.model=NULL" +
" RETURN car");

Here we add a new property called milage , modify labels to be both Car and Electro and finally, we delete model property altogether.

3.5.Delete Nodes

DELETE keyword can be used for permanent removal of nodes or relationships from the graph:

graphDb.execute("MATCH (company:Company)" +
" WHERE company.name='Baeldung'" +
" DELETE company");

Here we deleted a company named Baeldung.

3.6.Parameter Binding

In the above examples, we have hard-coded parameter values which isn’t the best practice. Luckily, Neo4j provides a facility for binding variables to a query:

Map<String, Object> params = new HashMap<>();
params.put("name", "baeldung");
params.put("make", "tesla");
params.put("model", "modelS");
Result result = graphDb.execute("CREATE (baeldung:Company {name:$name}) " +
"-[:owns]-> (tesla:Car {make: $make, model: $model})" +
"RETURN baeldung, tesla", params); 4. Java Driver

So far we’ve been looking at interacting with an embedded Neo4j instance, however, in all probability for production, we’d want to run a stand-alone server and connect to it via a provided driver. First, we need to add another dependency in our maven pom.xml :

<dependency>
<groupId>org.neo4j.driver</groupId>
<artifactId>neo4j-java-driver</artifactId>
<version>1.1.1</version>
</dependency>

You can follow this link to check for the latest version of this driver.

Nowwe canestablish a connection:

Driver driver = GraphDatabase.driver(
"bolt://localhost:7687", AuthTokens.basic("neo4j", "12345"));

Then, create a session:

Session session = driver.session();

Finally, we can run some queries:

session.run("CREATE (baeldung:Company {name:\"Baeldung\"}) " +
"-[:owns]-> (tesla:Car {make: 'tesla', model: 'modelX'})" +
"RETURN baeldung, tesla");

Once we are done with all our work we need to close both session and the driver:

session.close();
driver.close(); 5. JDBC Driver

It is also possible to interact with Neo4j via a JDBC driver. Yet another dependency for our pom.xml :

<dependency>
<groupId>org.neo4j</groupId>
<artifactId>neo4j-jdbc-driver</artifactId>
<v