8185853: Generate readability graph at link time and other startup improvements

[mandy chung]

On 8/4/17 8:48 AM, Alan Bateman wrote:
> This is a patch for jdk10/jdk10 to claw back some of the regression to 
> startup performance in JDK 9 for very short lived applications.
>
> The bulk of the changes are to the "system modules" jlink plugin and 
> the related code in the module system initialization. Specifically, 
> the plugin now generates code to create the readability graph for 
> initial modules known at link time This allows resolution to be 
> skipped at startup for common cases, it also avoids needing to 
> reconstitute the module descriptors for modules that will not be 
> resolved (the EE modules when the initial module is the class path for 
> example). The system module finder has been significantly changed as 
> part of this but all the execution scenarios (images, exploded builds, 
> images patched with exploded build, etc.) that are needed in 
> differetnt JDK development scenarios work as before.
>
> The patch also includes a few misc. changes to reduce the number of 
> class loaded during startup and also some local tuning. There is also 
> a change to the class path initialization to re-fix JDK-6760902 so 
> that lookup of resources on the boot loader append path 
> (-Xbootclasspath/a) is aligned with class loading in the VM in the way 
> that it skips empty path elements. This follows the restoration of 
> tests that Mandy pushed a few days ago with JDK-8185541.
>
> The webrev with the changes is here:
>   http://cr.openjdk.java.net/~alanb/8185853/webrev/index.html

It is good to see more optimization be done at link time that improves 
the startup.

jdk/internal/loader/ClassLoaders.java

79 if (cp.length() == 0) cp = null;

Our launcher and hotspot VM always set "java.class.path" system property 
and so it'll be non-null.  It might help the readers to check if (cp != 
null && cp.length() == 0).

jdk/internal/module/SystemModulesMap.java

128 Constructor<?> ctor = Class.forName(cn).getConstructor();

Class.forName(Module, String) will only search the specified module.  No 
performance difference since the class exists in java.base but it may 
make it clear to the readers.

jdk/tools/jlink/internal/plugins/SystemModulesPlugin.java
    This generates one SystemModules for each module that has a main 
class.  A user may create a launcher at link time but specifies a main 
class which may be different to the one in the module descriptor.   A 
possible future enhancement is to generate a SystemModules for the main 
class if the launcher is created by jlink.

Otherwise, this patch looks good.

Mandy