If JNI is used to lock an object and then all references to the object are dropped without the thread unlocking the object, then we have an in-use inflated ObjectMonitor whose associated object can be null due to it being GC'd.

Prior to the fix for JDK-8320515, if we run the test for that fix we get:

# Internal Error (/home/stefank/git/jdk/open/src/hotspot/share/services/management.cpp:1274), pid=1546709, tid=1546754
# assert(object != nullptr) failed: must be a Java object
...
V [libjvm.so+0x1330ce8] jmm_DumpThreads+0x1a48 (management.cpp:1274)
j sun.management.ThreadImpl.dumpThreads0([JZZI)[Ljava/lang/management/ThreadInfo;+0 java.management@22-internal
j sun.management.ThreadImpl.dumpAllThreads(ZZI)[Ljava/lang/management/ThreadInfo;+28 java.management@22-internal
j sun.management.ThreadImpl.dumpAllThreads(ZZ)[Ljava/lang/management/ThreadInfo;+5 java.management@22-internal
j IterateMonitorWithDeadObjectTest.dumpThreadsWithLockedMonitors()V+7
j IterateMonitorWithDeadObjectTest.main([Ljava/lang/String;)V+11

If we remove that assert we hit an NPE in the Java layer:

java.lang.NullPointerException: Cannot invoke "Object.getClass()" because "lock" is null
at java.management/java.lang.management.ThreadInfo.<init>(ThreadInfo.java:172)
at java.management/sun.management.ThreadImpl.dumpThreads0(Native Method)
at java.management/sun.management.ThreadImpl.dumpAllThreads(ThreadImpl.java:518)
at java.management/sun.management.ThreadImpl.dumpAllThreads(ThreadImpl.java:506)
at IterateMonitorWithDeadObjectTest.dumpThreadsWithLockedMonitors(IterateMonitorWithDeadObjectTest.java:44)
at IterateMonitorWithDeadObjectTest.main(IterateMonitorWithDeadObjectTest.java:66)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
at java.base/java.lang.reflect.Method.invoke(Method.java:580)
at com.sun.javatest.regtest.agent.MainActionHelper$AgentVMRunnable.run(MainActionHelper.java:333)
at java.base/java.lang.Thread.run(Thread.java:1570)

After the fix for JDK-8320515 such ObjectMonitors are elided from the set of monitors gathered by ObjectMonitorsDump::do_monitors

We need to decide whether such ObjectMonitors should be included in the results of jmm_DumpThreads and if so re-enable them and fix the code accordingly to tolerate the possibility of a null object.