This error appears when attempt to analyze a JVMs heap using the jmap utility.

[root@server1 ~]# jmap -heap <PID>
Attaching to process ID 12345, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.144-b01

using thread-local object allocation.
Mark Sweep Compact GC

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 536870912 (512.0MB)
   NewSize                  = 10485760 (10.0MB)
   MaxNewSize               = 178913280 (170.625MB)
   OldSize                  = 20971520 (20.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
Exception in thread "main" java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at sun.tools.jmap.JMap.runTool(JMap.java:201)
        at sun.tools.jmap.JMap.main(JMap.java:130)
Caused by: java.lang.RuntimeException: unknown CollectedHeap type : class sun.jvm.hotspot.gc_interface.CollectedHeap
        at sun.jvm.hotspot.tools.HeapSummary.run(HeapSummary.java:144)
        at sun.jvm.hotspot.tools.Tool.startInternal(Tool.java:260)
        at sun.jvm.hotspot.tools.Tool.start(Tool.java:223)
        at sun.jvm.hotspot.tools.Tool.execute(Tool.java:118)
        at sun.jvm.hotspot.tools.HeapSummary.main(HeapSummary.java:49)
        ... 6 more

Creating a dump is a workaround for this issue.

~]# jmap -dump:format=b,file=/tmp/testDump <PID>
Dumping heap to /tmp/testDump ...
Heap dump file created

Then, analyze the dump using jhat. This will open a port on the server (7000) in this example. You can then navigate to www.example.com:7000 to view the jhat analysis of the dump.

~]# jhat testDump
Reading from testDump...
Dump file created Sun Oct 22 00:40:01 CDT 2017
Snapshot read, resolving...
Resolving 290870 objects...
Chasing references, expect 58 dots..........................................................
Eliminating duplicate references..........................................................
Snapshot resolved.
Started HTTP server on port 7000
Server is ready.

For example, let's say you are looking into an issue with the objects in a class named myServlet.

public class myServlet {
    example example;
}

The heap dump should have captured the class named myServlet, and the jhat analysis should also show that the class myServlet was captured.