While having a J4pClient which sends periodic requests on a server, I restarted that server.
There was a 404 error, probably caused by a request sent during server restart, but then J4p failed with the message "Invalid use of SingleClientConnManager: connection still allocated.
Make sure to release the connection before allocating another one."

20:27:30.062 [main] ERROR c.b.metrics.harvest.HarvestMetrics - Error - cannot harvest metric
org.jolokia.client.exception.J4pException: 404 Not Found
at org.jolokia.client.J4pClient.extractJsonResponse(J4pClient.java:110) ~[jolokia-client-java-0.80.jar:na]
at org.jolokia.client.J4pClient.execute(J4pClient.java:83) ~[jolokia-client-java-0.80.jar:na]
at org.jolokia.client.J4pClient.execute(J4pClient.java:66) ~[jolokia-client-java-0.80.jar:na]
at com.b.metrics.harvest.HarvestMetrics.execute(HarvestMetrics.java:101) [classes/:na]
at com.b.metrics.harvest.HarvestMetrics.main(HarvestMetrics.java:281) [classes/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.6.0_22]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) ~[na:1.6.0_22]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) ~[na:1.6.0_22]
at java.lang.reflect.Method.invoke(Method.java:597) ~[na:1.6.0_22]
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:115) [idea_rt.jar:na]
Harvesting com.b.metrics:type=GroupOfMetrics,name=ApplicationCollector
20:28:30.066 [main] ERROR c.b.metrics.harvest.HarvestMetrics - Error - cannot harvest metric
java.lang.IllegalStateException: Invalid use of SingleClientConnManager: connection still allocated.
Make sure to release the connection before allocating another one.
at org.apache.http.impl.conn.SingleClientConnManager.getConnection(SingleClientConnManager.java:199) ~[httpclient-4.0.1.jar:4.0.1]
at org.apache.http.impl.conn.SingleClientConnManager$1.getConnection(SingleClientConnManager.java:173) ~[httpclient-4.0.1.jar:4.0.1]
at org.apache.http.impl.client.DefaultRequestDirector.execute(DefaultRequestDirector.java:390) ~[httpclient-4.0.1.jar:4.0.1]
at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:641) ~[httpclient-4.0.1.jar:4.0.1]
at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:576) ~[httpclient-4.0.1.jar:4.0.1]
at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:554) ~[httpclient-4.0.1.jar:4.0.1]
at org.jolokia.client.J4pClient.execute(J4pClient.java:82) ~[jolokia-client-java-0.80.jar:na]
at org.jolokia.client.J4pClient.execute(J4pClient.java:66) ~[jolokia-client-java-0.80.jar:na]
at com.b.metrics.harvest.HarvestMetrics.execute(HarvestMetrics.java:101) [classes/:na]