最近在研究图计算相关技术，前一阶段学了Neo4j的知识，在学习的过程也对比一些其它的开源图数据库，如OrientDB，还有一些通用框架TinkerPop。
在OrientDB的对比学习过程中，希望首先把已有的数据导入到新的数据库中，还好OrientDB官方提供了Neo4j数据导入工具orientdb-neo4j-importer。然而一切皆有坑...
按照官方文档进行数据导入，执行命令：
sh orientdb-neo4j-importer-2.2.31/bin/orientdb-neo4j-importer.sh -neo4jdbdir ../graph.db/ -neo4jlibdir ../neo4jlib/出现以下报错：
Exception in thread "main" java.lang.NoClassDefFoundError: com/orientechnologies/orient/core/OConstants
at com.orientechnologies.orient.neo4jimporter.ONeo4jImporterMain.main(ONeo4jImporterMain.java:15)
Caused by: java.lang.ClassNotFoundException: com.orientechnologies.orient.core.OConstants
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 1 more尝试把importer/lib/orientdb-neo4j-importer-2.2.31.jar拷贝到orientdb安装目录的lib文件夹下，再次执行还是同样的报错。
查看源代码也没发现此处有问题，再通过sh打印详细信息发现环境变量ORIENTDB_HOME设置有问题，执行以下命令定义环境变量：
export ORIENTDB_HOME=/root/orientdb-community-importers-2.2.31再次执行命令，顺利通过！
[root@datanode orientdb-community-importers-2.2.31]# sh -x orientdb-neo4j-importer-2.2.31/bin/orientdb-neo4j-importer.sh -neo4jdbdir ../graph.db/ -neo4jlibdir ../neo4jlib/
+ PRG=orientdb-neo4j-importer-2.2.31/bin/orientdb-neo4j-importer.sh
+ '[' -h orientdb-neo4j-importer-2.2.31/bin/orientdb-neo4j-importer.sh ']'
++ dirname orientdb-neo4j-importer-2.2.31/bin/orientdb-neo4j-importer.sh
+ PRGDIR=orientdb-neo4j-importer-2.2.31/bin
+ '[' -f /root/orientdb-community-importers-2.2.31/lib/orientdb-neo4j-importer-2.2.31.jar ']'
+ export ORIENTDB_HOME
+ '[' -f /usr/java/jdk1.8.0_91/bin/java ']'
+ JAVA=/usr/java/jdk1.8.0_91/bin/java
+ export JAVA
+ LOG_FILE=/root/orientdb-community-importers-2.2.31/config/orientdb-neo4j-importer-log.properties
+ JAVA_MAX_DIRECT=-XX:MaxDirectMemorySize=512g
+ JAVA_OPTS_SCRIPT='-XX:MaxDirectMemorySize=512g -Djava.util.logging.config.file=/root/orientdb-community-importers-2.2.31/config/orientdb-neo4j-importer-log.properties'
+ ARGS=
+ for var in '"$@"'
+ ARGS=' -neo4jdbdir'
+ for var in '"$@"'
+ ARGS=' -neo4jdbdir ../graph.db/'
+ for var in '"$@"'
+ ARGS=' -neo4jdbdir ../graph.db/ -neo4jlibdir'
+ for var in '"$@"'
+ ARGS=' -neo4jdbdir ../graph.db/ -neo4jlibdir ../neo4jlib/'
+ [[ 4 -gt 1 ]]
+ key=-neo4jdbdir
+ case $key in
+ shift
+ [[ 3 -gt 1 ]]
+ key=../graph.db/
+ case $key in
+ shift
+ [[ 2 -gt 1 ]]
+ key=-neo4jlibdir
+ case $key in
+ NEO4JLIB=../neo4jlib/
+ shift
+ shift
+ [[ 0 -gt 1 ]]
+ exec /usr/java/jdk1.8.0_91/bin/java -client -cp '../neo4jlib//*:/root/orientdb-community-importers-2.2.31/lib/*' -XX:MaxDirectMemorySize=512g -Djava.util.logging.config.file=/root/orientdb-community-importers-2.2.31/config/orientdb-neo4j-importer-log.properties com.orientechnologies.orient.neo4jimporter.ONeo4jImporterMain -neo4jdbdir ../graph.db/ -neo4jlibdir ../neo4jlib/
Neo4j to OrientDB Importer v.2.2.31 (build 285537d2767275f460df32c6a3be01bfff6a517c, branch 2.2.x) - Copyrights (c) 2016 OrientDB LTD
WARNING: 'o' option not found. Defaulting to 'false'.
WARNING: 'i' option not found. Defaulting to 'false'.
WARNING: 'odbdir' option not found. Defaulting to '/root/databases/neo4j_import'.
Please make sure that there are no running servers on:
'../graph.db' (Neo4j)
and:
'/root/databases/neo4j_import' (OrientDB)
Initializing Neo4j...Done
Initializing OrientDB...Done
Importing Neo4j database:
'../graph.db'
into OrientDB database:
'/root/databases/neo4j_import'
Getting all Nodes from Neo4j and creating corresponding Vertices in OrientDB...
4 OrientDB Vertices have been created (100% done)
Done
Creating internal Indices on properties 'Neo4jNodeID' & 'Neo4jLabelList' on all OrientDB Vertices Classes...
4 OrientDB Indices have been created (100% done)
Done
Getting all Relationships from Neo4j and creating corresponding Edges in OrientDB...
2 OrientDB Edges have been created (100% done)
Done
Getting Constraints from Neo4j and creating corresponding ones in OrientDB...
0 OrientDB UNIQUE Indices have been created
Done
Getting Indices from Neo4j and creating corresponding ones in OrientDB...
0 OrientDB Indices have been created
Done
Import completed!
Shutting down OrientDB...Done
Shutting down Neo4j...Done
===============
Import Summary:
===============
- Found Neo4j Nodes : 4
-- With at least one Label : 4
--- With multiple Labels : 0
-- Without Labels : 0
- Imported OrientDB Vertices : 4 (100%)
- Found Neo4j Relationships : 2
- Imported OrientDB Edges : 2 (100%)
- Found Neo4j Constraints : 0
- Imported OrientDB Constraints (UNIQUE Indices created) : 0
- NOT UNIQUE Indices created due to failure in creating UNIQUE Indices : 0
- Found Neo4j (non-constraint) Indices : 0
- Imported OrientDB Indices : 0
- Additional internal Indices created : 4
- Total Import time: : 6 seconds
-- Initialization time : 5 seconds
-- Time to Import Nodes : 0 seconds (35.4 nodes/sec)
-- Time to Import Relationships : 0 seconds (22.47 rels/sec)
-- Time to Import Constraints and Indices : 0 seconds (0 indices/sec)
-- Time to Create Internal Indices (on vertex properties 'Neo4jNodeID' & 'Neo4jLabelList') : 1 seconds (7.01 indices/sec)以为一切搞定，启动OrientDB数据库，并没有看到新导入的数据库... 再仔细查看发现没有添加选项-odbdir（但是默认值应该是DB_HOME/databases/neo4j_import，不知道为何没有生效...），数据库导入到其它路径下，拷贝过来后可以正常使用了。
（如图，neo4j_import数据库）