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我試圖通過使用REST API在我的neo4j數據庫上運行SPARQL查詢。爲了獲得SPARQL端點，我安裝了Noe4j SPARQL插件（https://github.com/neo4j-contrib/sparql-plugin）。我的系統現在公開這些SPARQL的端點：
Neo4j SPARQL插件REST用法
http://localhost:7474/db/data/ext/SPARQLPlugin/graphdb/insert_quad
http://localhost:7474/db/data/ext/SPARQLPlugin/graphdb/execute_sparql
我試着去使用sencond一個執行查詢。 HTTP POST請求應該與http://neo4j-contrib.github.io/sparql-plugin/中的「示例2」類似，查詢稍微簡單一些。 這是源代碼：
private static final String ENDPOINT = "http://localhost:7474/db/data/ext/SPARQLPlugin/graphdb/execute_sparql";
private static String query = "SELECT ?x WHERE {?x ?y ?z} LIMIT 5";
public static void main(String[] args) {
try {
HttpClient httpclient = new DefaultHttpClient();
HttpPost httppost = new HttpPost(ENDPOINT);
// Request parameters and other properties.
httppost.addHeader(new BasicHeader("Method", "POST"));
httppost.addHeader(new BasicHeader("Accept", "application/json"));
httppost.addHeader(new BasicHeader("Content-Type", "application/json"));
JSONObject holder = new JSONObject();
holder.put("query", query);
System.out.println(holder.toString());
httppost.setEntity(new StringEntity(holder.toString()));
//Execute and get the response.
HttpResponse response = httpclient.execute(httppost);
System.out.println("HEADER:");
System.out.println("Status code : " + response.getStatusLine().getStatusCode());
Header[] hlist = response.getAllHeaders();
for (int i = 0; i < hlist.length; i++) {
System.out.print(hlist[i].getName() + " ");
System.out.println(hlist[i].getValue());
}
System.out.println("-----");
HttpEntity entity = response.getEntity();
if (entity != null) {
BufferedReader rd = new BufferedReader(
new InputStreamReader(entity.getContent()));
String line = "";
while ((line = rd.readLine()) != null) {
System.out.println("x " + line);
}
} else {
System.out.println("Entity is null");
}
} catch (Exception e) {
e.printStackTrace(System.out);
}
}
不幸的是，我沒有得到期望的輸出。輸出是：
{"query":"SELECT ?x WHERE {?x ?y ?z} LIMIT 5"}
HEADER:
Status code : 200
Content-Length 4
Content-Type application/json; charset=UTF-8
Access-Control-Allow-Origin *
Server Jetty(6.1.25)
-----
x null
我很確定這個SPARQL查詢是正確的;我在安裝sparqpl-plugin時也沒有注意到任何問題，所以我不知道爲什麼我得到「null」結果。我試圖通過谷歌尋找解決方案，但我沒有發現任何有用的東西。我怎樣才能解決這個問題 ？ Neo4j的1.9.2的 進出口使用的社區版上運行的Windows 7 安裝SPARQL插件的版本是「0.2快照」
編輯1：如果我運行此查詢：
SELECT ?x WHERE {?x ?y ?z} LIMIT 0
輸出是：
{"query":"SELECT ?x WHERE {?x ?y ?z} LIMIT 0"}
HEADER:
Status code : 200: OK
Content-Length 3
Content-Type application/json; charset=UTF-8
Access-Control-Allow-Origin *
Server Jetty(6.1.25)
-----
x [ ]
也許這有助於在發現錯誤。
從來就還試圖運行的查詢從的Neo4j的HTTP控制檯：
http> POST http://localhost:7474/db/data/ext/SPARQLPlugin/graphdb/execute_sparql {"query":"select ?x where { graph ?g { ?x ?y ?z } }"}
==> 200 OK
==> null
http> POST http://localhost:7474/db/data/ext/SPARQLPlugin/graphdb/execute_sparql {"query":"select ?x where { ?x ?y ?z }"}
==> 200 OK
==> null
編輯2： 從來就試圖重新下載並構建插件。之後我通過在NEO4J_HOME/plugins/sparql中解壓縮來安裝它。然後，在我啓動Neo4j.bat後，我執行了我的java代碼。下面的錯誤在控制檯上來：
x {
x "message" : "com/tinkerpop/blueprints/impls/neo4j/Neo4jGraph",
x "exception" : "NoClassDefFoundError",
x "fullname" : "java.lang.NoClassDefFoundError",
x "stacktrace" : [ "org.neo4j.server.plugin.sparql.SPARQLPlugin.initSail(SPARQLPlugin.java:90)", "org.neo4j.server.plugin.sparql.SPARQLPlugin.executeSPARQL(SPARQLPlugin.java:61)", "java.lang.reflect.Method.invoke(Unknown Source)", "org.neo4j.server.plugins.PluginMethod.invoke(PluginMethod.java:57)", "org.neo4j.server.plugins.PluginManager.invoke(PluginManager.java:168)", "org.neo4j.server.rest.web.ExtensionService.invokeGraphDatabaseExtension(ExtensionService.java:300)", "org.neo4j.server.rest.web.ExtensionService.invokeGraphDatabaseExtension(ExtensionService.java:122)", "java.lang.reflect.Method.invoke(Unknown Source)", "org.neo4j.server.rest.security.SecurityFilter.doFilter(SecurityFilter.java:112)" ],
x "cause" : {
x "message" : "com.tinkerpop.blueprints.impls.neo4j.Neo4jGraph",
x "exception" : "ClassNotFoundException",
x "stacktrace" : [ "java.net.URLClassLoader$1.run(Unknown Source)", "java.net.URLClassLoader$1.run(Unknown Source)", "java.security.AccessController.doPrivileged(Native Method)", "java.net.URLClassLoader.findClass(Unknown Source)", "java.lang.ClassLoader.loadClass(Unknown Source)", "sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)", "java.lang.ClassLoader.loadClass(Unknown Source)", "org.neo4j.server.plugin.sparql.SPARQLPlugin.initSail(SPARQLPlugin.java:90)", "org.neo4j.server.plugin.sparql.SPARQLPlugin.executeSPARQL(SPARQLPlugin.java:61)", "java.lang.reflect.Method.invoke(Unknown Source)", "org.neo4j.server.plugins.PluginMethod.invoke(PluginMethod.java:57)", "org.neo4j.server.plugins.PluginManager.invoke(PluginManager.java:168)", "org.neo4j.server.rest.web.ExtensionService.invokeGraphDatabaseExtension(ExtensionService.java:300)", "org.neo4j.server.rest.web.ExtensionService.invokeGraphDatabaseExtension(ExtensionService.java:122)", "java.lang.reflect.Method.invoke(Unknown Source)", "org.neo4j.server.rest.security.SecurityFilter.doFilter(SecurityFilter.java:112)" ],
x "fullname" : "java.lang.ClassNotFoundException"
x }
x }
從來就發現，「com.tinkerpop.blueprints.impls.neo4j.Neo4jGraph」是一些「藍圖-的Neo4j-graph.jar」的一部分。這個jar文件在我的sparql /文件夾中丟失了，所以我搜索了最近的jar文件並添加了「blueprints-neo4j-graph-2.5.0-20130916.151744-5.jar」。重新啓動服務器後，錯誤不再出現，但我仍然得到「空」結果（在兩個查詢中的任一個）
編輯3：我也嘗試將「BerlinDatasetTest.java」的某些部分從neof4 sparql-plugin的github頁面並手動運行它（在添加所有必需的jar文件之後）。這是源代碼：
public String runQuery(String raw_query) {
String q_result = "";
Sail sail = new GraphSail(new Neo4jGraph("my_db/"));
try {
sail.initialize();
QueryParser parser = new SPARQLParserFactory().getParser();
ParsedQuery query = null;
CloseableIteration<? extends BindingSet, QueryEvaluationException> sparqlResults;
SailConnection conn = sail.getConnection();
query = parser.parseQuery(raw_query, "http://localhost:7474");
sparqlResults = conn.evaluate(query.getTupleExpr(),
query.getDataset(), new EmptyBindingSet(), false);
while (sparqlResults.hasNext()) {
q_result += "; " + sparqlResults.next();
}
conn.close();
sail.shutDown();
} catch (Exception e) {
e.printStackTrace();
}
return q_result;
}
這是結果我得到
java.lang.NullPointerException
at com.tinkerpop.blueprints.oupls.sail.GraphSailConnection.toSesame(GraphSailConnection.java:729)
at com.tinkerpop.blueprints.oupls.sail.GraphSailConnection.fillStatement(GraphSailConnection.java:636)
at com.tinkerpop.blueprints.oupls.sail.GraphSailConnection.access$800(GraphSailConnection.java:36)
at com.tinkerpop.blueprints.oupls.sail.GraphSailConnection$StableStatementIteration.next(GraphSailConnection.java:624)
at com.tinkerpop.blueprints.oupls.sail.GraphSailConnection$StableStatementIteration.next(GraphSailConnection.java:589)
at info.aduna.iteration.IterationWrapper.next(IterationWrapper.java:71)
at net.fortytwo.sesametools.QueryEvaluationIteration.next(QueryEvaluationIteration.java:45)
at net.fortytwo.sesametools.QueryEvaluationIteration.next(QueryEvaluationIteration.java:16)
at info.aduna.iteration.IterationWrapper.next(IterationWrapper.java:71)
at info.aduna.iteration.FilterIteration.findNextElement(FilterIteration.java:69)
at info.aduna.iteration.FilterIteration.hasNext(FilterIteration.java:43)
at info.aduna.iteration.ConvertingIteration.hasNext(ConvertingIteration.java:62)
at info.aduna.iteration.ConvertingIteration.hasNext(ConvertingIteration.java:62)
at info.aduna.iteration.IterationWrapper.hasNext(IterationWrapper.java:57)
at org.openrdf.sail.helpers.SailBaseIteration.hasNext(SailBaseIteration.java:50)
at src.QueryTest.runQuery(QueryTest.java:113)
at src.QueryTest.main(QueryTest.java:140)
「QueryTest.java」是在我運行此功能的Java類的名稱。
編輯4：它的出現，那結果就只有空，如果
query.getDataset()
成爲空（這發生在兩個查詢），如果我添加一個FROM子句或開始使用中的網址WHERE-part，query.Dataset（）不再是null，Im得到一個空結果（這不是null）。這些是兩個「工作」查詢：
select ?x from <localhost:7474> where { graph ?g { ?x ?y ?z } }
select ?x where { graph ?g { ?x <http://localhost:7474#knows> ?z } }
我仍然沒有得到任何結果。
您實際預期會得到多少結果？如果使用查詢'select？x where {graph？g {？x？y？z}}'查詢會發生什麼情況，這樣您就可以搜索所有命名圖表，而不僅僅是默認圖表？ （在某些端點中，默認圖形是指定圖形的聯合，但在其他圖形中並不是這樣）。 –
我已經將一個2gb * .nt文件加載到neo4j中，所以我期望返回很多三元組;因此我添加了「限制5」。不幸的是，如果我使用你的查詢，結果不會改變。 – mxh
您的查詢似乎成功_running_（即，您得到非錯誤結果），所以它看起來像問題在別處。也許不是用2GB數據集運行，你可以設置一個簡單的配置的簡單例子，可能更容易調試？ –