Neo4j Java开发指南3.0：扩展与嵌入

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"Neo4j Java Reference 3.0 是一本详细介绍如何在Java环境中使用和扩展Neo4j图数据库的指南。它涵盖了嵌入式使用、遍历框架、手动索引、事务管理、在线备份等内容，并涉及到开发 Neo4j 服务器扩展和Cypher查询语言的能力增强。此参考手册建议读者熟悉Java编程语言并具备Java代码编译环境。书中还提醒了在同一个JVM中运行自定义代码和Neo4j时应注意避免创建过多对象，以防内存管理问题。" Neo4j是一个强大的图数据库系统，尤其适用于处理复杂的数据关系。在Java应用程序中嵌入Neo4j允许开发者充分利用其图形数据模型的优势。以下是 Neo4j Java Reference 3.0 中的一些关键知识点： 1. **嵌入式使用(Using Neo4j embedded in Java applications)**：这部分指导如何将Neo4j数据库集成到Java应用中，使开发者可以直接操作数据库，无需通过网络接口。这通常用于单体应用或测试环境，能提供更高的性能和控制。 2. **遍历框架(The Traversal Framework)**：遍历框架是Neo4j的一种高级特性，允许开发者以图形方式遍历节点和关系，进行复杂的查询。它提供了灵活的策略来控制遍历行为，如深度限制、过滤器和访问策略。 3. **手动索引(Manual Indexing)**：尽管Neo4j支持自动索引，但手动索引允许更精细的控制，特别是在特定性能优化场景下。这部分解释了如何创建和管理索引来加速查询。 4. **事务管理(Transaction Management)**：在图数据库中，事务管理是确保数据一致性和完整性的关键。这里讲述了如何在Java中正确地开始、提交和回滚事务，以及处理并发控制和隔离级别。 5. **在线备份(Online Backup from Java)**：这部分介绍了如何在不影响服务的情况下，从Java应用程序中执行Neo4j的在线备份。这对于数据安全性至关重要，特别是对于需要24/7可用性的系统。 6. **服务器扩展(Extending Neo4j)**：Neo4j的可扩展性使得开发人员可以编写自己的Java插件来扩展服务器功能，比如添加新的API端点或者Cypher函数。这需要对Neo4j服务器架构有深入理解。 7. **许可证(License)**：本参考文献遵循Creative Commons 3.0协议，允许用户在遵循一定条件的情况下自由使用和分享内容。通过这些详细的知识点，开发者可以深入理解Neo4j的内部工作原理，从而更有效地利用这个强大的图数据库系统。无论是构建社交网络分析应用、推荐系统还是其他需要处理复杂关系的数据平台，Neo4j Java Reference 3.0都是一个宝贵的资源。

In our code we’ll interact with the database using GraphDatabaseService which we can get access to by

using the @Context annotation. The following example serves as a template which you can base your

extension on:

Unmanaged extension example

@Path( "/helloworld" )

public class HelloWorldResource

{

Ê private final GraphDatabaseService database;

Ê public HelloWorldResource( @Context GraphDatabaseService database )

Ê {

Ê this.database = database;

Ê }

Ê @GET

Ê @Produces( MediaType.TEXT_PLAIN )

Ê @Path( "/{nodeId}" )

Ê public Response hello( @PathParam( "nodeId" ) long nodeId )

Ê {

Ê // Do stuff with the database

Ê return Response.status( Status.OK ).entity( UTF8.encode( "Hello World, nodeId=" + nodeId ) ).

build();

Ê }

}

The full source code is found here: HelloWorldResource.java

(https://github.com/neo4j/neo4j/blob/3.0/manual/server-

examples/src/main/java/org/neo4j/examples/server/unmanaged/HelloWorldResource.java)

Having built your code, the resulting jar file (and any custom dependencies) should be placed in the

$NEO4J_SERVER_HOME/plugins directory. We also need to tell Neo4j where to look for the extension by

adding some configuration in neo4j.conf:

#Comma separated list of JAXRS packages containing JAXRS Resource, one package name for each mountpoint.

dbms.unmanaged_extension_classes=org.neo4j.examples.server.unmanaged=/examples/unmanaged

Our hello method will now respond to GET requests at the URI:

http://{neo4j_server}:{neo4j_port}/examples/unmanaged/helloworld/{nodeId}. e.g.

curl http://localhost:7474/examples/unmanaged/helloworld/123

which results in

Hello World, nodeId=123

1.2.2. Streaming JSON responses

When writing unmanaged extensions we have greater control over the amount of memory that our

Neo4j queries use. If we keep too much state around it can lead to more frequent full Garbage

Collection and subsequent unresponsiveness by the Neo4j server.

A common way that state can creep in is the creation of JSON objects to represent the result of a

query which we then send back to our application. Neo4j’s Transactional Cypher HTTP endpoint

streams responses back to the client and we should follow in its footsteps.

For example, the following unmanaged extension streams an array of a person’s colleagues:

Unmanaged extension streaming example

@Path("/colleagues")

public class ColleaguesResource

{

Ê private GraphDatabaseService graphDb;

Ê private final ObjectMapper objectMapper;

Ê private static final RelationshipType ACTED_IN = RelationshipType.withName( "ACTED_IN" );

Ê private static final Label PERSON = Label.label( "Person" );

Ê public ColleaguesResource( @Context GraphDatabaseService graphDb )

Ê {

Ê this.graphDb = graphDb;

Ê this.objectMapper = new ObjectMapper();

Ê }

Ê @GET

Ê @Path("/{personName}")

Ê public Response findColleagues( final @PathParam("personName") String personName )

Ê {

Ê StreamingOutput stream = new StreamingOutput()

Ê {

Ê @Override

Ê public void write( OutputStream os ) throws IOException, WebApplicationException

Ê {

Ê JsonGenerator jg = objectMapper.getJsonFactory().createJsonGenerator( os, JsonEncoding

.UTF8 );

Ê jg.writeStartObject();

Ê jg.writeFieldName( "colleagues" );

Ê jg.writeStartArray();

Ê try ( Transaction tx = graphDb.beginTx();

Ê ResourceIterator<Node> persons = graphDb.findNodes( PERSON, "name", personName ) )

Ê {

Ê while ( persons.hasNext() )

Ê {

Ê Node person = persons.next();

Ê for ( Relationship actedIn : person.getRelationships( ACTED_IN, OUTGOING ) )

Ê {

Ê Node endNode = actedIn.getEndNode();

Ê for ( Relationship colleagueActedIn : endNode.getRelationships( ACTED_IN,

INCOMING ) )

Ê {

Ê Node colleague = colleagueActedIn.getStartNode();

Ê if ( !colleague.equals( person ) )

Ê {

Ê jg.writeString( colleague.getProperty( "name" ).toString() );

Ê }

Ê tx.success();

Ê }

Ê jg.writeEndArray();

Ê jg.writeEndObject();

Ê jg.flush();

Ê jg.close();

Ê }

Ê };

Ê return Response.ok().entity( stream ).type( MediaType.APPLICATION_JSON ).build();

Ê }

}

The full source code is found here: ColleaguesResource.java

(https://github.com/neo4j/neo4j/blob/3.0/manual/server-

examples/src/main/java/org/neo4j/examples/server/unmanaged/ColleaguesResource.java)

As well as depending on JAX-RS API this example also uses Jackson — a Java JSON library. You’ll need to

add the following dependency to your Maven POM file (or equivalent):

Unmanaged extension Cypher execution example

@Path("/colleagues-cypher-execution")

public class ColleaguesCypherExecutionResource

{

Ê private final ObjectMapper objectMapper;

Ê private GraphDatabaseService graphDb;

Ê public ColleaguesCypherExecutionResource( @Context GraphDatabaseService graphDb )

Ê {

Ê this.graphDb = graphDb;

Ê this.objectMapper = new ObjectMapper();

Ê }

Ê @GET

Ê @Path("/{personName}")

Ê public Response findColleagues( final @PathParam("personName") String personName )

Ê {

Ê final Map<String, Object> params = MapUtil.map( "personName", personName );

Ê StreamingOutput stream = new StreamingOutput()

Ê {

Ê @Override

Ê public void write( OutputStream os ) throws IOException, WebApplicationException

Ê {

Ê JsonGenerator jg = objectMapper.getJsonFactory().createJsonGenerator( os, JsonEncoding

.UTF8 );

Ê jg.writeStartObject();

Ê jg.writeFieldName( "colleagues" );

Ê jg.writeStartArray();

Ê try ( Transaction tx = graphDb.beginTx();

Ê Result result = graphDb.execute( colleaguesQuery(), params ) )

Ê {

Ê while ( result.hasNext() )

Ê {

Ê Map<String,Object> row = result.next();

Ê jg.writeString( ((Node) row.get( "colleague" )).getProperty( "name" ).toString()

);

Ê }

Ê tx.success();

Ê }

Ê jg.writeEndArray();

Ê jg.writeEndObject();

Ê jg.flush();

Ê jg.close();

Ê }

Ê };

Ê return Response.ok().entity( stream ).type( MediaType.APPLICATION_JSON ).build();

Ê }

Ê private String colleaguesQuery()

Ê {

Ê return "MATCH (p:Person {name: {personName} })-[:ACTED_IN]->()<-[:ACTED_IN]-(colleague) RETURN

colleague";

Ê }

}

The full source code is found here: ColleaguesCypherExecutionResource.java

(https://github.com/neo4j/neo4j/blob/3.0/manual/server-

examples/src/main/java/org/neo4j/examples/server/unmanaged/ColleaguesCypherExecutionResource.java)

Our findColleagues method will now respond to GET requests at the URI:

http://{neo4j_server}:{neo4j_port}/examples/unmanaged/colleagues-cypher-execution/{personName}.

e.g.

curl http://localhost:7474/examples/unmanaged/colleagues-cypher-execution/Keanu%20Reeves

which results in

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