When an executable JAR is created by the Project Launcher the compression is controlled with the -compression instruction. However, this über JAR contains bundles and JARs for the run path. Since executable JARs are sometimes used in embedded environments it is important that they are small and the code can loaded quickly. It is difficult to have defaults for this since there are many aspects:
In such a complex trade off there are no singular answers. This
-executable instruction allows therefore to
set the different options and then benchmark the result.
Experience shows that the size can be minimized by rejarring the inner bundles and JARs with the STORE compression but using DEFLATE for the outer/über JAR. This decreases the size because the compression algorithm then works on all class files of all bundles. This is much more efficient for the DEFLATE algorithm then compressing each class file on its own. Gains seen are 20%-30%.
rejar option in the
-executable instruction can take the values
DEFLATE. This will open
the embedded JARs and write them in the given compression in the über JAR.
The default is to not touch the bundle.
strip option can be used to strip resources from JARs embedded in the execetable. Its parameter can define
a list of two GLOBs. The first GLOB must match the file name of the resource and the second GLOB must match the
resource name in the bundle. GLOBs are separated with a colon (‘:’). If no colon is present then the first GLOB
is assumed to be the wildcard GLOB that matches anything.
strip ::= matcher ( ',' matcher )* matcher ::= ( GLOB ':' ) GLOB
*.map– Match all resources ending in
OSGI-OPT/*– Remove the optional resources in bundle
com.example.some.bundle.*:readme.md– Remove the readme’s from bundles withe file name matching
Note hat if you combine multiple GLOBs then you must separate them with a comma (‘,’) and that implies the total must be quoted with single or double quotes. If it is a single GLOB (pair) then quoting is optional. For example:
-executable: rejar=STORE, strip='OSGI-OPT,*.map'
The default is to not strip anything.
Rejarring and stripping should work for unsigned bundles since the signatures should not be affected by the compression algorithms used.