Legacy mode to find modules for tools.jar (or other jars not in the bootclasspath)

[Mandy Chung]

I finally get a chance to revise the legacy mode support (III.B.2 as 
described in [1]) and add the mixed mode legacy application support 
(III.C.a as in [1]).  I wanted to do more extensive testing before 
requesting a formal code review (the testing I have done so far is 
good).  I'm not surprised if I have some strange unexpected issues.

This webrev will give you a better idea how this works as described in [1].
     http://cr.openjdk.java.net/~mchung/jigsaw/legacy-mode/webrev.00/

I'd like to start one discussion in this thread:

With the tradition legacy image, to compile and run with classes from 
tools.jar (or other JDK jars that are not part of the default 
bootclasspath), the -classpath option is required to add it to the 
classpath.

With the new module image, the question is: how should the legacy 
application find the jdk modules corresponding to tools.jar and other 
jars?  See LegacyLauncher.java line 142.

I considered 3 options and propose to use (3a):
1) Default is "jdk.jre" (i.e. entire JRE - see section 0 in [1])
2) Default is "jdk" (i.e. entire JDK including tools.jar and other)
3) Default is "jdk.legacy" that is an aggregator module that optionally 
requires all jdk modules.
   a) install jdk.legacy in jre-module-image and jdk-module-image (i.e. 
legacy mode is only supported in jre-module-image and jdk-module-image.)
   b) install jdk.legacy with the base image (i.e. legacy mode is 
supported in any module image provided that the classes the app depends 
on exist)

1) Default is "jdk.jre"
We could extend this to check if $JAVA_HOME/lib/tools.jar is added in 
the classpath; if so, it will use "jdk" module.

Pros:
- No change is required in the command to compile or run java application
- Legacy applications will run on jre-module-image or jdk-module-image
Cons:
- Require to continue to use -cp tools.jar even if tools.jar doesn't 
exist in the module image
- Legacy applications will not run on jre-base-image (or with some other 
modules installed)

(2) Default is "jdk"

Pros:
- No change in the command to compile or run java application
Cons:
- Legacy app must run on jdk-module-image (not jre-module-image in which 
"jdk" module is not installed)

(3) Default is "jdk.legacy" (or call it "jdk.any")

Pros:
- No change is required in the command to compile or run java application
- Legacy apps will run on jre-module-image or jdk-module-image
Cons:
- One additional aggregator module

Comment, thoughts?

Mandy

[1] http://mail.openjdk.java.net/pipermail/jigsaw-dev/2010-June/001039.html
[2] 
http://mail.openjdk.java.net/pipermail/jigsaw-dev/2010-February/000563.html