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我有这样的代码：

import org.neo4j.graphdb.GraphDatabaseService;
import org.neo4j.graphdb.Node;
import org.neo4j.graphdb.Relationship;
import org.neo4j.graphdb.RelationshipType;
import org.neo4j.graphdb.Transaction;
import org.neo4j.graphdb.factory.GraphDatabaseFactory;
import org.neo4j.graphdb.index.Index;

public class Model
{
    private static GraphDatabaseService GraphDb_;
    private static Boolean Init_ = false;
    private static Index<Node> NodeIndex_;

    private static enum RelTypes implements RelationshipType
    {
        Parent, Child
    }

    public Model()
    {

    }

    public static void init(String path)
    {
        if ( Init_ == false )
            GraphDb_ = new GraphDatabaseFactory().newEmbeddedDatabase( path );

        NodeIndex_ = GraphDb_.index().forNodes( "nodes" );
        Init_ = true;
    }

在尝试调用 Model.init("./database/") 后，我得到了字符串“GraphDb_ = new GraphDatabaseFactory().newEmbeddedDatabase(path)”：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at java.util.concurrent.ConcurrentHashMap.put(ConcurrentHashMap.java:1124)
    at java.util.concurrent.ConcurrentHashMap.putAll(ConcurrentHashMap.java:1162)
    at org.neo4j.kernel.configuration.Config.applyChanges(Config.java:130)
    at org.neo4j.kernel.configuration.Config.<init>(Config.java:91)
    at org.neo4j.kernel.InternalAbstractGraphDatabase.<init>(InternalAbstractGraphDatabase.java:218)
    at org.neo4j.kernel.EmbeddedGraphDatabase.<init>(EmbeddedGraphDatabase.java:103)
    at org.neo4j.graphdb.factory.GraphDatabaseFactory$1.newDatabase(GraphDatabaseFactory.java:88)
    at org.neo4j.graphdb.factory.GraphDatabaseBuilder.newGraphDatabase(GraphDatabaseBuilder.java:207)
    at org.neo4j.graphdb.factory.GraphDatabaseFactory.newEmbeddedDatabase(GraphDatabaseFactory.java:69)
    at neosample.Model.init(Model.java:55)

这出现在应用程序启动之前。Neo4J 1.9.2，Java 1.7.0

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