When running JShellHeapDumpTest, you sometimes see an EOFException failure. Just before the exception you'll see "null" or "illegal bci". These are being printed out by the following in JMap.java:

    public boolean writeHeapHprofBin(String fileName) {
        try {
            HeapGraphWriter hgw = new HeapHprofBinWriter();
            hgw.write(fileName);
            System.out.println("heap written to " + fileName);
            return true;
        } catch (IOException | RuntimeException exp) {
            System.err.println(exp.getMessage()); <-----------
            return false;
        }
    }

The "null" is because the exception has no message, and this is what String.valueOf(obj) returns in that case. The "illegal bci" is the message from an AssertionFailure exception that is being caught here. I think this code should be changed to the following to there is more reasonable detail in the output:

            exp.printStackTrace(System.err);

Once this is done, you see a better explanation for the failures:

sun.jvm.hotspot.utilities.AssertionFailure: illegal bci
at jdk.hotspot.agent/sun.jvm.hotspot.utilities.Assert.that(Assert.java:32)
at jdk.hotspot.agent/sun.jvm.hotspot.oops.ConstMethod.getLineNumberFromBCI(ConstMethod.java:297)
at jdk.hotspot.agent/sun.jvm.hotspot.oops.Method.getLineNumberFromBCI(Method.java:282)
at jdk.hotspot.agent/sun.jvm.hotspot.utilities.HeapHprofBinWriter.dumpStackFrame(HeapHprofBinWriter.java:751)
at jdk.hotspot.agent/sun.jvm.hotspot.utilities.HeapHprofBinWriter.dumpStackTraces(HeapHprofBinWriter.java:730)
at jdk.hotspot.agent/sun.jvm.hotspot.utilities.HeapHprofBinWriter.write(HeapHprofBinWriter.java:439)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.JMap.writeHeapHprofBin(JMap.java:182)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.JMap.run(JMap.java:97)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.startInternal(Tool.java:260)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.start(Tool.java:223)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.execute(Tool.java:118)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.JMap.main(JMap.java:176)
at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.runJMAP(SALauncher.java:321)
at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.main(SALauncher.java:406)

sun.jvm.hotspot.utilities.AssertionFailure: must be a valid non-zero index
at jdk.hotspot.agent/sun.jvm.hotspot.utilities.Assert.that(Assert.java:32)
at jdk.hotspot.agent/sun.jvm.hotspot.code.NMethod.getMetadataAt(NMethod.java:200)
at jdk.hotspot.agent/sun.jvm.hotspot.code.NMethod.getMethodAt(NMethod.java:206)
at jdk.hotspot.agent/sun.jvm.hotspot.code.DebugInfoReadStream.readMethod(DebugInfoReadStream.java:58)
at jdk.hotspot.agent/sun.jvm.hotspot.code.ScopeDesc.<init>(ScopeDesc.java:64)
at jdk.hotspot.agent/sun.jvm.hotspot.code.ScopeDesc.sender(ScopeDesc.java:121)
at jdk.hotspot.agent/sun.jvm.hotspot.runtime.CompiledVFrame.sender(CompiledVFrame.java:187)
at jdk.hotspot.agent/sun.jvm.hotspot.runtime.VFrame.javaSender(VFrame.java:143)
at jdk.hotspot.agent/sun.jvm.hotspot.runtime.ThreadStackTrace.dumpStack(ThreadStackTrace.java:54)
at jdk.hotspot.agent/sun.jvm.hotspot.utilities.HeapHprofBinWriter.dumpStackTraces(HeapHprofBinWriter.java:718)
at jdk.hotspot.agent/sun.jvm.hotspot.utilities.HeapHprofBinWriter.write(HeapHprofBinWriter.java:439)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.JMap.writeHeapHprofBin(JMap.java:182)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.JMap.run(JMap.java:97)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.startInternal(Tool.java:260)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.start(Tool.java:223)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.execute(Tool.java:118)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.JMap.main(JMap.java:176)
at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.runJMAP(SALauncher.java:321)
at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.main(SALauncher.java:406)

And this is the one for the "null" message:

java.lang.NullPointerException
at jdk.hotspot.agent/sun.jvm.hotspot.utilities.HeapHprofBinWriter.dumpStackTraces(HeapHprofBinWriter.java:727)
at jdk.hotspot.agent/sun.jvm.hotspot.utilities.HeapHprofBinWriter.write(HeapHprofBinWriter.java:439)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.JMap.writeHeapHprofBin(JMap.java:182)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.JMap.run(JMap.java:97)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.startInternal(Tool.java:260)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.start(Tool.java:223)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.execute(Tool.java:118)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.JMap.main(JMap.java:176)
at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.runJMAP(SALauncher.java:321)
at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.main(SALauncher.java:406)