Add support for creating a fat-jar out of a thin-jar

[thomasdarimont]

In scenarios where one has access to an m2 repo or can upload co-located dependencies
it is easy to use the thin jar approach.

However in restricted environments a fatjar is more appropriate. In such cases it would be
very convenient to be able to create a fatjar from a given thin jar.

Perhaps something like this would do the trick:

java -jar thin.jar -Dthin.supersizeme=./fat.jar

[dsyer]

"supersizeme" v. funny

Good idea. Sort of a nuclear option for #15 (and easier to implement).

[dsyer]

Try this out:

    $ java -jar myapp.jar --thin.dryrun --thin.root=target/thin/root
    $ java -jar myapp.jar --thin.archive=maven://org.springframework.boot.experimental:spring-boot-thin-converter:1.0.7.BUILD-SNAPSHOT
    $ java -jar myapp-exec.jar

You can get a usage message by typing

$ java -jar myapp.jar --thin.archive=maven://org.springframework.boot.experimental:spring-boot-thin-converter:1.0.7.BUILD-SNAPSHOT

when there is no target/thin/root to grab the dependencies from. Or just

$ java -jar myapp.jar --thin.archive=maven://org.springframework.boot.experimental:spring-boot-thin-converter:1.0.7.BUILD-SNAPSHOT --help

or

$ java -jar spring-boot-thin-converter-1.0.7.BUILD-SNAPSHOT.jar --help

if you already downloaded it.

I think this is an interesting route to provide additional tooling without needing to add more code (or more dependencies) to the ThinJarLauncher. Essentially you leapfrog from the thin launcher to a tool of your choice (the thin.archive only needs to have a discoverable classpath and main class). The same mechanism could be used, for instance, to provide the features being asked for in #25 and #27.

[dsyer]

I changed it a bit in latest snapshots, so now it is thin.library (not thin.archive) that you need to set to transfer control to a tool. It's more consistent with the way the launcher is the "library" in the default case. E.g. the above is replaced by

$ java -jar myapp.jar --thin.dryrun --thin.root=target/thin/root
$ java -jar myapp.jar --thin.library=org.springframework.boot.experimental:spring-boot-thin-tools-converter:1.0.7.BUILD-SNAPSHOT
$ java -jar myapp-exec.jar

(You don't need the maven:// prefix for a library.)

One advantage of that is that the target jar can be specified with thin.archive if it is different than the main executable. E.g. this works

$ java -jar myapp.jar --thin.dryrun --thin.root=target/thin/root
$ java -jar myotherapp.jar --thin.archive=myapp.jar --thin.library=org.springframework.boot.experimental:spring-boot-thin-tools-converter:1.0.7.BUILD-SNAPSHOT
$ java -jar myapp-exec.jar

Where myotherapp.jar could even be the spring-boot-thin-wrapper itself.