--- a/src/hotspot/share/services/diagnosticCommand.cpp	Wed Dec 12 12:00:02 2018 +0100
+++ b/src/hotspot/share/services/diagnosticCommand.cpp	Wed Dec 12 11:34:08 2018 +0000
@@ -37,6 +37,7 @@
 #include "runtime/fieldDescriptor.inline.hpp"
 #include "runtime/flags/jvmFlag.hpp"
 #include "runtime/handles.inline.hpp"
+#include "runtime/interfaceSupport.inline.hpp"
 #include "runtime/javaCalls.hpp"
 #include "runtime/os.hpp"
 #include "services/diagnosticArgument.hpp"
@@ -123,6 +124,11 @@
   DCmdFactory::register_DCmdFactory(new DCmdFactoryImpl<JMXStopRemoteDCmd>(jmx_agent_export_flags, true,false));
   DCmdFactory::register_DCmdFactory(new DCmdFactoryImpl<JMXStatusDCmd>(jmx_agent_export_flags, true,false));
 
+  // Debug on cmd (only makes sense with JVMTI since the agentlib needs it).
+#if INCLUDE_JVMTI
+  DCmdFactory::register_DCmdFactory(new DCmdFactoryImpl<DebugOnCmdStartDCmd>(full_export, true, false));
+#endif // INCLUDE_JVMTI
+
 }
 
 #ifndef HAVE_EXTRA_DCMD
@@ -1054,3 +1060,43 @@
 int TouchedMethodsDCmd::num_arguments() {
   return 0;
 }
+
+#if INCLUDE_JVMTI
+extern "C" typedef char const* (JNICALL *debugInit_startDebuggingViaCommandPtr)(JNIEnv* env, jthread thread, char const** transport_name,
+                                                                                char const** address, jboolean* first_start);
+static debugInit_startDebuggingViaCommandPtr dvc_start_ptr = NULL;
+
+DebugOnCmdStartDCmd::DebugOnCmdStartDCmd(outputStream* output, bool heap) : DCmdWithParser(output, heap) {
+}
+
+void DebugOnCmdStartDCmd::execute(DCmdSource source, TRAPS) {
+  char const* transport = NULL;
+  char const* addr = NULL;
+  jboolean is_first_start = JNI_FALSE;
+  JavaThread* thread = (JavaThread*) THREAD;
+  jthread jt = JNIHandles::make_local(thread->threadObj());
+  ThreadToNativeFromVM ttn(thread);
+  const char *error = "Could not find jdwp agent.";
+
+  if (!dvc_start_ptr) {
+    for (AgentLibrary* agent = Arguments::agents(); agent != NULL; agent = agent->next()) {
+      if ((strcmp("jdwp", agent->name()) == 0) && (dvc_start_ptr == NULL)) {
+        char const* func = "debugInit_startDebuggingViaCommand";
+        dvc_start_ptr = (debugInit_startDebuggingViaCommandPtr) os::find_agent_function(agent, false, &func, 1);
+      }
+    }
+  }
+
+  if (dvc_start_ptr) {
+    error = dvc_start_ptr(thread->jni_environment(), jt, &transport, &addr, &is_first_start);
+  }
+
+  if (error != NULL) {
+    output()->print_cr("Debugging has not been started: %s", error);
+  } else {
+    output()->print_cr(is_first_start ? "Debugging has been started." : "Debugging is already active.");
+    output()->print_cr("Transport : %s", transport ? transport : "#unknown");
+    output()->print_cr("Address : %s", addr ? addr : "#unknown");
+  }
+}
+#endif // INCLUDE_JVMTI

--- a/src/hotspot/share/services/diagnosticCommand.hpp	Wed Dec 12 12:00:02 2018 +0100
+++ b/src/hotspot/share/services/diagnosticCommand.hpp	Wed Dec 12 11:34:08 2018 +0000
@@ -867,4 +867,26 @@
   virtual void execute(DCmdSource source, TRAPS);
 };
 
+#if INCLUDE_JVMTI
+class DebugOnCmdStartDCmd : public DCmdWithParser {
+public:
+  DebugOnCmdStartDCmd(outputStream* output, bool heap);
+  static const char* name() {
+    return "VM.start_java_debugging";
+  }
+  static const char* description() {
+    return "Starts up the Java debugging if the jdwp agentlib was enabled with the option onjcmd=y.";
+  }
+  static const char* impact() {
+    return "High: Switches the VM into Java debug mode.";
+  }
+  static const JavaPermission permission() {
+    JavaPermission p = { "java.lang.management.ManagementPermission", "monitor", NULL };
+    return p;
+  }
+  static int num_arguments() { return 0; }
+  virtual void execute(DCmdSource source, TRAPS);
+};
+#endif // INCLUDE_JVMTI
+
 #endif // SHARE_VM_SERVICES_DIAGNOSTICCOMMAND_HPP

--- a/src/jdk.jdwp.agent/share/native/libjdwp/debugInit.c	Wed Dec 12 12:00:02 2018 +0100
+++ b/src/jdk.jdwp.agent/share/native/libjdwp/debugInit.c	Wed Dec 12 11:34:08 2018 +0000
@@ -82,6 +82,9 @@
 
 static char *names;                         /* strings derived from OnLoad options */
 
+static jboolean allowStartViaJcmd = JNI_FALSE;  /* if true we allow the debugging to be started via a jcmd */
+static jboolean startedViaJcmd = JNI_FALSE;     /* if false, we have not yet started debugging via a jcmd */
+
 /*
  * Elements of the transports bag
  */
@@ -870,6 +873,7 @@
  "launch=<command line>            run debugger on event             none\n"
  "onthrow=<exception name>         debug on throw                    none\n"
  "onuncaught=y|n                   debug on any uncaught?            n\n"
+ "onjcmd=y|n                       start debug via jcmd?             n\n"
  "timeout=<timeout value>          for listen/attach in milliseconds n\n"
  "mutf8=y|n                        output modified utf-8             n\n"
  "quiet=y|n                        control over terminal messages    n\n"));
@@ -1010,6 +1014,7 @@
     int length;
     char *str;
     char *errmsg;
+    jboolean onJcmd = JNI_FALSE;
 
     /* Set defaults */
     gdata->assertOn     = DEFAULT_ASSERT_ON;
@@ -1229,6 +1234,10 @@
             if ( !get_boolean(&str, &useStandardAlloc) ) {
                 goto syntax_error;
             }
+        } else if (strcmp(buf, "onjcmd") == 0) {
+            if (!get_boolean(&str, &onJcmd)) {
+                goto syntax_error;
+            }
         } else {
             goto syntax_error;
         }
@@ -1254,7 +1263,6 @@
         goto bad_option_with_errmsg;
     }
 
-
     if (!isServer) {
         jboolean specified = bagEnumerateOver(transports, checkAddress, NULL);
         if (!specified) {
@@ -1280,6 +1288,20 @@
         }
     }
 
+    if (onJcmd) {
+        if (launchOnInit != NULL) {
+            errmsg = "Cannot combine onjcmd and launch suboptions";
+            goto bad_option_with_errmsg;
+        }
+        if (!isServer) {
+            errmsg = "Can only use onjcmd with server=y";
+            goto bad_option_with_errmsg;
+        }
+        suspendOnInit = JNI_FALSE;
+        initOnStartup = JNI_FALSE;
+        allowStartViaJcmd = JNI_TRUE;
+    }
+
     return JNI_TRUE;
 
 syntax_error:
@@ -1348,3 +1370,45 @@
     // Last chance to die, this kills the entire process.
     forceExit(EXIT_JVMTI_ERROR);
 }
+
+static jboolean getFirstTransport(void *item, void *arg)
+{
+    TransportSpec** store = arg;
+    *store = item;
+
+    return JNI_FALSE; /* Want the first */
+}
+
+/* Call to start up debugging. */
+JNIEXPORT char const* JNICALL debugInit_startDebuggingViaCommand(JNIEnv* env, jthread thread, char const** transport_name,
+                                                                char const** address, jboolean* first_start) {
+    jboolean is_first_start = JNI_FALSE;
+    TransportSpec* spec = NULL;
+
+    if (!vmInitialized) {
+        return "Not yet initialized. Try again later.";
+    }
+
+    if (!allowStartViaJcmd) {
+        return "Starting debugging via jcmd was not enabled via the onjcmd option of the jdwp agent.";
+    }
+
+    if (!startedViaJcmd) {
+        startedViaJcmd = JNI_TRUE;
+        is_first_start = JNI_TRUE;
+        initialize(env, thread, EI_VM_INIT);
+    }
+
+    bagEnumerateOver(transports, getFirstTransport, &spec);
+
+    if ((spec != NULL) && (transport_name != NULL) && (address != NULL)) {
+        *transport_name = spec->name;
+        *address = spec->address;
+    }
+
+    if (first_start != NULL) {
+        *first_start = is_first_start;
+    }
+
+    return NULL;
+}

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/jdk/com/sun/jdi/OnJcmdTest.java	Wed Dec 12 11:34:08 2018 +0000
@@ -0,0 +1,109 @@
+/*
+ * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, SAP SE. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/**
+ * @test
+ * @bug 8214892
+ * @summary Test that the onjcmd option of the jdwp agent works.
+ *
+ * @author Ralf Schmelter
+ *
+ * @library /test/lib
+ * @run compile --add-exports java.base/jdk.internal.vm=ALL-UNNAMED -g OnJcmdTest.java
+ * @run main/othervm --add-exports java.base/jdk.internal.vm=ALL-UNNAMED -agentlib:jdwp=transport=dt_socket,address=localhost:0,onjcmd=y,server=y OnJcmdTest
+ */
+
+import java.lang.reflect.Method;
+import java.util.Properties;
+
+import jdk.internal.vm.VMSupport;
+import jdk.test.lib.JDKToolFinder;
+import jdk.test.lib.process.OutputAnalyzer;
+import jdk.test.lib.process.ProcessTools;
+
+public class OnJcmdTest {
+
+    private static String getListenerAddress() throws Exception {
+        Properties props = VMSupport.getAgentProperties();
+        return props.getProperty("sun.jdwp.listenerAddress", null);
+    }
+
+    public static void main(String[] args) throws Throwable {
+        // First check if we get the expected errors.
+        OutputAnalyzer output = ProcessTools.executeTestJvm(
+                "-agentlib:jdwp=transport=dt_socket,address=any,onjcmd=y");
+        output.shouldContain("Can only use onjcmd with server=y");
+        output.shouldHaveExitValue(1);
+
+        output = ProcessTools.executeTestJvm(
+                "-agentlib:jdwp=transport=dt_socket,address=any,onjcmd=y,onthrow=a,launch=a");
+        output.shouldContain("Cannot combine onjcmd and launch suboptions");
+        output.shouldHaveExitValue(1);
+
+        // Make sure debugging is not yet started.
+        String prop = getListenerAddress();
+
+        if (prop != null) {
+            throw new RuntimeException("Listener address was set to " + prop);
+        }
+
+        // Now start it (test that it is OK to do this more than once).
+        for (int i = 0; i < 3; ++i) {
+            String jcmd = JDKToolFinder.getJDKTool("jcmd");
+            output = ProcessTools.executeProcess(jcmd,
+                    Long.toString(ProcessTools.getProcessId()),
+                    "VM.start_java_debugging");
+
+            String exp_str = i == 0 ? "Debugging has been started." :
+                                      "Debugging is already active.";
+            output.shouldContain(exp_str);
+            output.shouldContain("Transport : dt_socket");
+            output.shouldHaveExitValue(0);
+        }
+
+        // Now the property should be set, as the jdwp agent waits for a
+        // connection.
+        long t1 = System.currentTimeMillis();
+        long t2 = t1;
+
+        while(t2 - t1 < 4000) {
+            prop = getListenerAddress();
+
+            if (prop != null) {
+                if (prop.equals("localhost:0")) {
+                    throw new RuntimeException("Port was not expanded");
+                } else if (!prop.startsWith("dt_socket:")) {
+                    throw new RuntimeException("Invalid transport prop " + prop);
+                }
+
+                return;
+            }
+
+            Thread.sleep(50);
+            t2 = System.currentTimeMillis();
+        }
+
+        throw new RuntimeException("Debugging backend didn't start");
+    }
+}