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  1. JDK
  2. JDK-8227229

Deprecate the launcher -Xdebug/-debug flags that have not done anything since Java 6

    Details

    • Type: Enhancement
    • Status: New
    • Priority: P4
    • Resolution: Unresolved
    • Affects Version/s: 14
    • Fix Version/s: None
    • Component/s: tools
    • Labels:
      None
    • Subcomponent:
    • Introduced In Version:
      6

      Description

      The Xdebug (and its alias -debug) launcher flag does nothing and is documented as existing "for backwards compatibility". The history is rather complex but fortunately it is summarized in JDK-6272174:

      -------------------------------------------------------------------------------------
      History, to the best of my memory --

      (1) Long ago and far away

       -Xdebug used to mean use JVMDI
       -Djava.compiler=whatever was how you could specify a JIT compiler to run in the Classic VM
       -Djava.compiler=NONE was the Classic VM way of forcing interpreted execution and had to be used with -Xdebug because JVMDI ran in the interpreter

      (2) HotSpot came along

       -Djava.compiler=whatever was meaningless and ignored, but,
       -Djava.compiler=NONE presumably so that the "-Xdebug -Djava.compiler=NONE" command line worked, was made the same as -Xint

      (3) Then full-speed debugging came along

       Everybody's command line was still launching the debugee with "-Xdebug -Djava.compiler=NONE", so a horrible hack was added out of cautiousness:

       -Djava.compiler=NONE was left equal the same as -Xint, but was ignored if combined with -Xdebug *gag*

       Worse, I think this was me

      (4) Then JVM TI came along (JDK5)

       -Xdebug now meant set up the internal JVMDI emulating JVM TI environment enabling all the JVMDI events, like breakpoint.
       There wasn't a JVMDI flag anymore, so the full-speed hack (3) was changed to check for breakpoints being enabled

      (5) Then JVMDI was removed (JDK6)

       -Xdebug is now just ignored

       So, -Xdebug won't enable breakpoints.

      ---

      So Xdebug has done nothing since JDK 6, but unfortunately continues to be referred to in relation to JDWP! This occurs in our own JDWP usage information:

       > java -Xrunjdwp:help
                     Java Debugger JDWP Agent Library
                     --------------------------------
      ...
      Warnings
      --------
        - The older -Xrunjdwp interface can still be used, but will be removed in
          a future release, for example:
              java -Xdebug -Xrunjdwp:[help]|[<option>=<value>, ...]

      ---

      This then makes its way into other web documents e.g.

      ---
      To run a regular serverless Java class Test with debugging enabled in the Oracle HotSpot JVM, you need to use the following command:

      java -Xdebug -Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=y Test

      As you can see, you basically need only two JVM options: -Xdebug and -Xrunjdwp. Note that these are X-arguments, which means that they are not standardized and may not work as expected in other JVM implementations.

      The -Xdebug argument enables the debugging itself, and the -Xrunjdwp argument configures the JDWP protocol with several important parameters. [1]
      ---
      [1] https://stackify.com/java-remote-debugging/ (December 2017)

      So I propose that we officially deprecate -Xdebug/-debug in 14 so that we can remove it in 15.

      There are obviously multiple pieces to this and we need to update the JDWP usage information independent of this.

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              • Assignee:
                Unassigned
                Reporter:
                dholmes David Holmes
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                • Created:
                  Updated: