Neo4j Java开发指南：扩展与嵌入

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"Neo4j Java 开发者参考指南 3.0 版本" 本资源是 Neo4j 的 Java 开发者参考文档，主要针对 Neo4j 3.0 版本，提供了关于如何在 Java 应用中使用 Neo4j、扩展 Neo4j 以及进行高级操作的详细信息。文档涵盖了从嵌入式使用到服务器扩展，再到交易管理等多个方面。 1. **扩展 Neo4j** Neo4j 提供了一个可插拔的扩展架构，允许开发者通过编写 Java 代码来扩展 Cypher 查询语言的功能，或者创建 REST API 的新接口。这需要开发者具备 Java 编程基础，并且能设置好 Java 编译环境。在同一个 JVM 中运行自定义代码和 Neo4j 时，需要注意内存管理和对象生命周期，避免过度创建或保留对象导致内存问题。 2. **在 Java 应用中使用 Neo4j 嵌入式** 从第 2 章开始，文档介绍了如何将 Neo4j 集成到 Java 应用中，包括初始化数据库、执行查询、事务处理等基本操作。这对于希望在应用内部直接访问和管理数据库的开发者非常有用。 3. **遍历框架** 第 3 章详细讲解了 Neo4j 的遍历框架，这是对图数据库进行深度探索的重要工具。遍历框架允许开发者高效地遍历节点和关系网络，实现复杂的查询和数据处理逻辑。 4. **手动索引** 第 4 章介绍了如何为 Neo4j 数据手动创建索引，以提高查询性能。手动索引可以用于特定场景，如快速查找具有特定属性的节点或关系。 5. **交易管理** 在第 5 章中，文档详细阐述了如何在 Neo4j 中管理事务，确保数据的一致性和完整性。这对于需要原子性、一致性、隔离性和持久性的业务操作至关重要。 6. **在线备份** 第 6 章涉及如何从 Java 应用程序进行在线备份。在线备份功能允许在不影响数据库服务的情况下，安全地保存数据库的状态，以备恢复或迁移。此外，文档还可能包含了错误处理、性能调优、安全配置等相关内容。同时，建议读者在阅读过程中配合 Neo4j 的 Java 文档（javadocs/）以获取更全面的信息。该文档适用于希望深入理解和利用 Neo4j 的 Java 开发者，特别是那些需要进行定制化开发和优化性能的项目。

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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