--- /dev/null	2018-06-17 23:18:20.806999507 +0800
+++ new/src/share/classes/jdk/jfr/internal/cmd/HelpCommand.java	2019-01-29 10:59:49.296273724 +0800
@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2016, 2019, Oracle and/or its affiliates. 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.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * 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.
+ */
+
+package jdk.jfr.internal.cmd;
+
+import java.util.Deque;
+
+final class HelpCommand extends Command {
+
+    @Override
+    public String getOptionSyntax() {
+        return "[<command>]";
+    }
+
+    @Override
+    public void displayOptionUsage() {
+        println("  <command>   The name of the command to get help for");
+        println();
+        Command.displayAvailableCommands();
+    }
+
+    @Override
+    public String getName() {
+        return "help";
+    }
+
+    @Override
+    public String getDescription() {
+        return "Display help about a command";
+    }
+
+    @Override
+    public void execute(Deque<String> options) {
+        if (options.isEmpty()) {
+            displayUsage();
+        } else {
+            ensureMaxArgumentCount(options, 1);
+            String commandName = options.remove();
+            Command c = Command.valueOf(commandName);
+            if (c == null) {
+                userFailed("Unknown command " + commandName);
+            }
+            c.displayUsage();
+        }
+    }
+}