--- a/langtools/src/share/classes/com/sun/tools/javac/main/Main.java	Fri Mar 26 22:37:04 2010 -0700
+++ b/langtools/src/share/classes/com/sun/tools/javac/main/Main.java	Fri Apr 09 15:39:39 2010 -0700
@@ -28,6 +28,9 @@
 import java.io.File;
 import java.io.IOException;
 import java.io.PrintWriter;
+import java.net.URL;
+import java.security.DigestInputStream;
+import java.security.MessageDigest;
 import java.util.MissingResourceException;
 
 import com.sun.tools.javac.code.Source;
@@ -281,6 +284,15 @@
         if (target.hasInvokedynamic()) {
             options.put("invokedynamic",  "invokedynamic");
         }
+
+        // handle this here so it works even if no other options given
+        String showClass = options.get("showClass");
+        if (showClass != null) {
+            if (showClass.equals("showClass")) // no value given for option
+                showClass = "com.sun.tools.javac.Main";
+            showClass(showClass);
+        }
+
         return filenames.toList();
     }
     // where
@@ -488,6 +500,37 @@
         ex.getCause().printStackTrace();
     }
 
+    /** Display the location and checksum of a class. */
+    void showClass(String className) {
+        out.println("javac: show class: " + className);
+        URL url = getClass().getResource('/' + className.replace('.', '/') + ".class");
+        if (url == null)
+            out.println("  class not found");
+        else {
+            out.println("  " + url);
+            try {
+                final String algorithm = "MD5";
+                byte[] digest;
+                MessageDigest md = MessageDigest.getInstance(algorithm);
+                DigestInputStream in = new DigestInputStream(url.openStream(), md);
+                try {
+                    byte[] buf = new byte[8192];
+                    int n;
+                    do { n = in.read(buf); } while (n > 0);
+                    digest = md.digest();
+                } finally {
+                    in.close();
+                }
+                StringBuilder sb = new StringBuilder();
+                for (byte b: digest)
+                    sb.append(String.format("%02x", b));
+                out.println("  " + algorithm + " checksum: " + sb);
+            } catch (Exception e) {
+                out.println("  cannot compute digest: " + e);
+            }
+        }
+    }
+
     private JavaFileManager fileManager;
 
     /* ************************************************************************

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/langtools/test/tools/javac/T6942649.java	Fri Apr 09 15:39:39 2010 -0700
@@ -0,0 +1,59 @@
+/*
+ * Copyright 2010 Sun Microsystems, Inc.  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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.com if you need additional information or
+ * have any questions.
+ */
+
+
+/*
+ * @test
+ * @bug 6942649
+ * @summary add hidden option to identify location and version of javac classes
+ */
+
+import java.io.*;
+
+public class T6942649 {
+    public static void main(String... args) throws Exception {
+        new T6942649().run();
+    }
+
+    void run() throws Exception {
+        test("-XDshowClass", "com.sun.tools.javac.Main");
+        test("-XDshowClass=com.sun.tools.javac.util.Log", "com.sun.tools.javac.util.Log");
+    }
+
+    void test(String opt, String clazz) throws Exception {
+        System.err.println("test " + opt);
+        StringWriter sw = new StringWriter();
+        PrintWriter pw = new PrintWriter(sw);
+        int rc = com.sun.tools.javac.Main.compile(new String[] { opt }, pw);
+        pw.close();
+        String out = sw.toString();
+        System.err.println("javac rc=" + rc + "\n" + out);
+        if (!out.contains(clazz))
+            throw new Exception("class name not found in output");
+        int lastDot = clazz.lastIndexOf(".");
+        if (!out.contains(clazz.substring(lastDot + 1) + ".class"))
+            throw new Exception("location of class not found in output");
+        if (!out.contains("MD5 checksum: "))
+            throw new Exception("checksum not found in output");
+    }
+}