我在这里尝试实现的是获取特定节点的关系数量，而其他线程同时向它添加新关系。我使用TestGraphDatabaseFactory().newImpermanentDatabase()图形服务在单元测试中运行我的代码 。
我的代码由大约 50 个线程执行，它看起来像这样：
int numOfRels = 0;
try {
Iterable<Relationship> rels = parentNode.getRelationships(RelTypes.RUNS, Direction.OUTGOING);
while (rels.iterator().hasNext()) {
numOfRels++;
rels.iterator().next();
}
}
catch(Exception e) {
throw e;
}
// Enforce relationship limit
if (numOfRels > 10) {
// do something
}
Transaction tx = graph.beginTx();
try {
Node node = createMyNodeAndConnectToParentNode(...);
tx.success();
return node;
}
catch (Exception e) {
tx.failure();
}
finally {
tx.finish();
}
问题是有时我在上面的 try-catch 块（围绕getRelationships()的块）中得到一个“ArrayIndexOutOfBoundsException：1” 。如果我理解正确 Iterable 不是线程安全的并导致此问题。
我的问题是使用 Neo4j 的 Java API 迭代不断变化的关系和节点的最佳方法是什么？
我收到以下错误：
Exception in thread "Thread-14" org.neo4j.helpers.ThisShouldNotHappenError: Developer: Stefan/Jake claims that: A property key id disappeared under our feet
at org.neo4j.kernel.impl.core.NodeProxy.setProperty(NodeProxy.java:188)
at com.inbiza.connio.neo4j.server.extensions.graph.AppEntity.createMyNodeAndConnectToParentNode(AppEntity.java:546)
at com.inbiza.connio.neo4j.server.extensions.graph.AppEntity.create(AppEntity.java:305)
at com.inbiza.connio.neo4j.server.extensions.TestEmbeddedConnioGraph$appCreatorThread.run(TestEmbeddedConnioGraph.java:61)
at java.lang.Thread.run(Thread.java:722)
Exception in thread "Thread-92" java.lang.ArrayIndexOutOfBoundsException: 1
at org.neo4j.kernel.impl.core.RelationshipIterator.fetchNextOrNull(RelationshipIterator.java:72)
at org.neo4j.kernel.impl.core.RelationshipIterator.fetchNextOrNull(RelationshipIterator.java:36)
at org.neo4j.helpers.collection.PrefetchingIterator.hasNext(PrefetchingIterator.java:55)
at com.inbiza.connio.neo4j.server.extensions.graph.AppEntity.create(AppEntity.java:243)
at com.inbiza.connio.neo4j.server.extensions.TestEmbeddedConnioGraph$appCreatorThread.run(TestEmbeddedConnioGraph.java:61)
at java.lang.Thread.run(Thread.java:722)
Exception in thread "Thread-12" java.lang.ArrayIndexOutOfBoundsException: 1
at org.neo4j.kernel.impl.core.RelationshipIterator.fetchNextOrNull(RelationshipIterator.java:72)
at org.neo4j.kernel.impl.core.RelationshipIterator.fetchNextOrNull(RelationshipIterator.java:36)
at org.neo4j.helpers.collection.PrefetchingIterator.hasNext(PrefetchingIterator.java:55)
at com.inbiza.connio.neo4j.server.extensions.graph.AppEntity.create(AppEntity.java:243)
at com.inbiza.connio.neo4j.server.extensions.TestEmbeddedConnioGraph$appCreatorThread.run(TestEmbeddedConnioGraph.java:61)
at java.lang.Thread.run(Thread.java:722)
Exception in thread "Thread-93" java.lang.ArrayIndexOutOfBoundsException
Exception in thread "Thread-90" java.lang.ArrayIndexOutOfBoundsException
下面是负责创建节点的方法：
static Node createMyNodeAndConnectToParentNode(GraphDatabaseService graph, final Node ownerAccountNode, final String suggestedName, Map properties) {
final String accountId = checkNotNull((String)ownerAccountNode.getProperty("account_id"));
Node appNode = graph.createNode();
appNode.setProperty("urn_name", App.composeUrnName(accountId, suggestedName.toLowerCase().trim()));
int nextId = nodeId.addAndGet(1); // I normally use getOrCreate idiom but to simplify I replaced it with an atomic int - that would do for testing
String urn = App.composeUrnUid(accountId, nextId);
appNode.setProperty("urn_uid", urn);
appNode.setProperty("id", nextId);
appNode.setProperty("name", suggestedName);
Index<Node> indexUid = graph.index().forNodes("EntityUrnUid");
indexUid.add(appNode, "urn_uid", urn);
appNode.addLabel(LabelTypes.App);
appNode.setProperty("version", properties.get("version"));
appNode.setProperty("description", properties.get("description"));
Relationship rel = ownerAccountNode.createRelationshipTo(appNode, RelTypes.RUNS);
rel.setProperty("date_created", fmt.print(new DateTime()));
return appNode;
}
我在看 org.neo4j.kernel.impl.core.RelationshipIterator.fetchNextOrNull()
看起来我的测试生成了一个条件，其中 else if ( (status = fromNode.getMoreRelationships( nodeManager )).loaded() || lastTimeILookedThereWasMoreToLoad ) 没有执行，并且currentTypeIterator状态在两者之间发生了变化。
RelIdIterator currentTypeIterator = rels[currentTypeIndex]; //<-- this is where is crashes
do
{
if ( currentTypeIterator.hasNext() )
...
...
while ( !currentTypeIterator.hasNext() )
{
if ( ++currentTypeIndex < rels.length )
{
currentTypeIterator = rels[currentTypeIndex];
}
else if ( (status = fromNode.getMoreRelationships( nodeManager )).loaded()
// This is here to guard for that someone else might have loaded
// stuff in this relationship chain (and exhausted it) while I
// iterated over my batch of relationships. It will only happen
// for nodes which have more than <grab size> relationships and
// isn't fully loaded when starting iterating.
|| lastTimeILookedThereWasMoreToLoad )
{
....
}
}
} while ( currentTypeIterator.hasNext() );
我还测试了几个锁定场景。下面的一个解决了这个问题。不确定每次基于此迭代关系时是否应该使用锁。
Transaction txRead = graph.beginTx();
try {
txRead.acquireReadLock(parentNode);
long numOfRels = 0L;
Iterable<Relationship> rels = parentNode.getRelationships(RelTypes.RUNS, Direction.OUTGOING);
while (rels.iterator().hasNext()) {
numOfRels++;
rels.iterator().next();
}
txRead.success();
}
finally {
txRead.finish();
}
我对 Neo4j 及其源代码库非常陌生；只是作为我们产品的潜在数据存储进行测试。如果有人从里到外了解 Neo4j 解释这里发生的事情，我将不胜感激。