Hello world

A Neo4j graph consists of:

  • nodes

  • relationships that connect the nodes

  • properties on both nodes and relationships

All relationships have a type. For example, if the graph represents a social network, a relationship type could be KNOWS. If a relationship of the type KNOWS connects two nodes, that is likely to represent two people that know each other. A lot of the semantics of a graph is encoded in the relationship types of the application. Although relationships are directed, they are equally traversed regardless of direction.

For information on project setup, see Embedding Neo4j in your Java application.

The source code of this example isfound at: EmbeddedNeo4j.java

Preparing the database

Relationship types can be created by using an enum. In this example, you only need a single relationship type. This is how to define it:

private enum RelTypes implements RelationshipType

You can also prepare some variables to use:

GraphDatabaseService graphDb;
Node firstNode;
Node secondNode;
Relationship relationship;
private DatabaseManagementService managementService;

The next step is to start the database server. Note that if the directory given for the database does not already exist, it will be created.

managementService = new DatabaseManagementServiceBuilder( databaseDirectory ).build();
graphDb = managementService.database( DEFAULT_DATABASE_NAME );
registerShutdownHook( managementService );

Starting a database server is an expensive operation, so do not start up a new instance every time you need to interact with the database. The instance can be shared by multiple threads, and transactions are thread confined.

As seen, you can register a shutdown hook that makes sure the database shuts down when the JVM exits.

private static void registerShutdownHook( final DatabaseManagementService managementService )
    // Registers a shutdown hook for the Neo4j instance so that it
    // shuts down nicely when the VM exits (even if you "Ctrl-C" the
    // running application).
    Runtime.getRuntime().addShutdownHook( new Thread()
        public void run()
    } );

The next step is to interact with the database.

Wrapping operations in a transaction

All operations have to be performed in a transaction. This is a deliberate design decision since transaction demarcation is an important part of working with a real enterprise database. The example below illustrates transaction handling in Neo4j:

try ( Transaction tx = graphDb.beginTx() )
    // Database operations go here

For more information on transactions, see Transaction management and the Neo4j Javadocs for org.neo4j.graphdb.Transaction.

For brevity, the wrapping of operations in a transaction is not spelled out throughout the manual.

Creating a small graph

You can now create a few nodes. This is how to create a small graph consisting of two nodes, connected with one relationship and some properties:

firstNode = tx.createNode();
firstNode.setProperty( "message", "Hello, " );
secondNode = tx.createNode();
secondNode.setProperty( "message", "World!" );

relationship = firstNode.createRelationshipTo( secondNode, RelTypes.KNOWS );
relationship.setProperty( "message", "brave Neo4j " );

You now have a graph that looks like this:

hello world graph java

Printing the result

After you have created your graph, you can read from it and print the result.

System.out.print( firstNode.getProperty( "message" ) );
System.out.print( relationship.getProperty( "message" ) );
System.out.print( secondNode.getProperty( "message" ) );

Which outputs:

Hello, brave Neo4j World!

For more information on how to view your graph in Neo4j Browser, see Browser Manual → Dedicated web server.

Removing the data

In this case, the data is removed before committing:

// let's remove the data
firstNode = tx.getNodeById( firstNode.getId() );
secondNode = tx.getNodeById( secondNode.getId() );
firstNode.getSingleRelationship( RelTypes.KNOWS, Direction.OUTGOING ).delete();

Deleting a node that still has relationships when the transaction commits will fail. This is to make sure relationships always have a start node and an end node.

Shutting down the database server

Finally, shut down the database server when the application finishes: