Question

我尝试运行TraversalExample.java并在课程中创建图表但我得到以下错误

Exception in thread "main" java.lang.NullPointerException
    at setupneo4j.TraversalExample.<init>(TraversalExample.java:23)
    at setupneo4j.TraversalExample.main(TraversalExample.java:33)

这里有什么问题？
我试图在Neo4j的上一篇文章中创建图表但它似乎失败了。

package setupneo4j;

import org.neo4j.graphdb.Direction;
import org.neo4j.graphdb.GraphDatabaseService;
import org.neo4j.graphdb.Node;
import org.neo4j.graphdb.Path;
import org.neo4j.graphdb.Relationship;
import org.neo4j.graphdb.RelationshipType;
import org.neo4j.graphdb.Transaction;
import org.neo4j.graphdb.traversal.Evaluators;
import org.neo4j.graphdb.traversal.TraversalDescription;
import org.neo4j.graphdb.traversal.Uniqueness;

public class TraversalExample
{
    private static GraphDatabaseService db;
    private TraversalDescription friendsTraversal;

    public TraversalExample( GraphDatabaseService db )
    {
        this.db = db;
        // START SNIPPET: basetraverser
        friendsTraversal = db.traversalDescription()
                .depthFirst()
                .relationships( Rels.KNOWS )
                .uniqueness( Uniqueness.RELATIONSHIP_GLOBAL );
        // END SNIPPET: basetraverser
    }

    public static void main( String[] args )
    {

        TraversalExample op = new TraversalExample( db );
        op.shutdownGraph();
    }
    public void shutdownGraph()
    {
        try
        {
            if ( db != null )
            {
                db.shutdown();
            }
        }
        finally
        {
            db = null;
        }
    }

    public Node createTheGraph()
    {
        try ( Transaction tx = db.beginTx() )
        {
            // START SNIPPET: createGraph
            Node A = db.createNode();
            Node B = db.createNode();
            Node C = db.createNode();
            Node D = db.createNode();

            A.createRelationshipTo( C, Rels.KNOWS );
            C.createRelationshipTo( D, Rels.KNOWS );
            A.createRelationshipTo( B, Rels.KNOWS);
            B.createRelationshipTo( C, Rels.KNOWS );
            // END SNIPPET: createGraph
            A.setProperty( "name", "A" );
            B.setProperty( "name", "B" );
            C.setProperty( "name", "C" );
            D.setProperty( "name", "D" );
            tx.success();
            return A;
        }
    }
    public String knowsLikesTraverser( Node node )
    {
        String output = "";
        // START SNIPPET: knowslikestraverser
        for ( Path position : db.traversalDescription()
                .depthFirst()
                .relationships( Rels.KNOWS )
                .relationships( Rels.LIKES, Direction.INCOMING )
                .evaluator( Evaluators.toDepth( 5 ) )
                .traverse( node ) )
        {
            output += position + "\n";
        }
        // END SNIPPET: knowslikestraverser
        return output;
    }

    public String traverseBaseTraverser( Node node )
    {
        String output = "";
        // START SNIPPET: traversebasetraverser
        for ( Path path : friendsTraversal.traverse( node ) )
        {
            output += path + "\n";
        }
        // END SNIPPET: traversebasetraverser
        return output;
    }

    public String depth3( Node node )
    {
        String output = "";
        // START SNIPPET: depth3
        for ( Path path : friendsTraversal
                .evaluator( Evaluators.toDepth( 3 ) )
                .traverse( node ) )
        {
            output += path + "\n";
        }
        // END SNIPPET: depth3
        return output;
    }

    public String depth4( Node node )
    {
        String output = "";
        // START SNIPPET: depth4
        for ( Path path : friendsTraversal
                .evaluator( Evaluators.fromDepth( 2 ) )
                .evaluator( Evaluators.toDepth( 4 ) )
                .traverse( node ) )
        {
            output += path + "\n";
        }
        // END SNIPPET: depth4
        return output;
    }

    public String nodes( Node node )
    {
        String output = "";
        // START SNIPPET: nodes
        for ( Node currentNode : friendsTraversal
                .traverse( node )
                .nodes() )
        {
            output += currentNode.getProperty( "name" ) + "\n";
        }
        // END SNIPPET: nodes
        return output;
    }

    public String relationships( Node node )
    {
        String output = "";
        // START SNIPPET: relationships
        for ( Relationship relationship : friendsTraversal
                .traverse( node )
                .relationships() )
        {
            output += relationship.getType().name() + "\n";
        }
        // END SNIPPET: relationships
        return output;
    }

    // START SNIPPET: sourceRels
    private enum Rels implements RelationshipType
    {
        LIKES, KNOWS
    }
    // END SNIPPET: sourceRels


}

Answer 1

您永远不会实际创建数据库，只需将该字段声明为类属性，将其传递给构造函数，然后将其设置为自身。当你调用它时null，你得到一个NullPointerException。我不知道你从哪里得到代码，但你应该从改变

开始

private static GraphDatabaseService db;

到

private GraphDatabaseService db;

然后在main中初始化db，然后再将其传递给构造函数，类似于

final String DB_PATH = "path/to/your/db"
GraphDatabaseService db = new GraphDatabaseFactory().newEmbeddedDatabase(DB_PATH);

neo4j create graph meet java.lang.NullPointerException

1 个答案: