-Xbootclasspath/p in the JDK build

[Jonathan Gibbons]

Jonathan Gibbons wrote:
> Aaaargh.
>
> The restrictions on bootclasspath are going to be an issue for 
> building JDK, because -Xbootclasspath/p is used in various places in 
> the build itself.  In particular, this will be a problem when we set 
> SKIP_BOOT_CYCLE=false, because the first time round, the BOOT_DIR will 
> be set to JDK 6, and -Xbootclasspath/p can be used; the second time 
> round, the BOOTDIR will be the newly built JDK 7, and 
> -Xbootclasspath/p might not be available.
>
> Here are places where I know -Xbootclasspath/p is used in the build.
>
> 1. In the langtools build.xml file.
>
> The langtools build uses the BOOT_DIR javac to compile the new javac 
> classes, then immediately uses those classes on the bootclasspath to 
> recompile javac again, for use in the final product. This ensures that 
> the javac in the final JDK is built with the latest sources (and not 
> built with the BOOT_DIR javac.) Both the interim javac and the final 
> javac are made available to the downstream parts of the build.
>
> 2. In the jdk Makefiles
>
> The interim javac referred to in the previous section is used to 
> compile most of the classes in the other repositories. This is done by 
> setting JAVAC_CMD to use the BOOT_DIR javac with the newly compiled 
> javac classes on the bootclasspath.
>
>
> There are 3 ways forward that I can see.
>
> 1. Support -Xbootclasspath/p:  This is arguably simpler than 
> supporting plain -Xbootclasspath.
>
> 2. Leave javac, javadoc, javah to run in JVM legacy mode so that  
> -Xbootclasspath/p: continues to work for these tools. The latest 
> versions of these tools are all used in the build using BOOT_DIR and 
> -Xbootclasspath.
>
> 3. Figure out rules to examine BOOT_DIR and decide if -Xbootclasspath 
> is going to be available, or if we need to build a temporary module 
> library for use with the -L option. Note this will require us to 
> address the issues with -L that I raised in [1].
>
> In any case, if this has not already been implemented, I would 
> recommend that the launcher should reject (give an error) options like 
> -Xbootclasspath if it is not going to accept them. Silently ignoring 
> the options if they are not applicable is almost certainly the wrong 
> thing to do.
>
> -- Jon
>
> [1] 
> http://mail.openjdk.java.net/pipermail/jigsaw-dev/2010-April/000756.html
>


There is probably another way forward, which may be the best way for the 
time being:

4. Examine BOOT_DIR to decide if modules are being used or not.  Then 
use -Xmode:legacy if necessary as well as -Xbootclasspath/p: to run the 
necessary tools in JVM legacy mode.


-- Jon