Can jdeps resolve unnecessary dependencies?

[Waldek Kozaczuk]

Thanks for your responses.

So today I did some experimentation with Java 8 jdeps to analyze how my
app.jar depends at class level on other jars and eventually individual JRE
classes. I would run following command:

jdeps -v -P -R -cp lib/* app.jar # where lib/ contains jars that app.jar
depends on to run and libs did not contain app.jar

It turns our that jdeps would find and present dependent classes in all
jars irregardless if any class in app.jar would directly or indirectly
depend on it per "->" references.

The good example of it was log4j jar where jdeps showed number of classes
that depend directly or indirectly on some java.awt.* or javax.swing.*
classes. And I am sure that this app does not requires AWT nor Swing to
execute. Is it because jdeps finds individual class dependencies by
detecting reflection calls in bytecode of log4j?

Or maybe this "greedy" behavior of jdeps is intende and I would need to
build a tool that would process its generated dependency graph starting
from app.jar and that way find the real subset of JRE it depends on? I am
still thinking that jdeps would not be able to show me classes that were
called through java.lang.reflect.* constructs or Class.forName().

Do my findings make sense?

On Wed, Sep 7, 2016 at 5:03 PM, Mandy Chung <mandy.chung at oracle.com> wrote:

>
> > On Sep 7, 2016, at 8:07 AM, Sander Mak <sander.mak at luminis.eu> wrote:
> >
> >
> >> On 07 Sep 2016, at 16:12, Waldek Kozaczuk <jwkozaczuk at gmail.com> wrote:
> >>
> >> Will I be able to take advantage of jlink's ability to produce
> subset-only
> >> image of the JRE even if I do not use new Java 9 modular jars? In other
> >> words if my app.jar and other dependent jars are old java 8 ones and do
> not
> >> have any module descriptors will jlink be able to identify only needed
> JRE
> >> modules and produce minimal runtime image?
> >
> > Jlink will not be able to do that for you without module descriptors,
> but there's an alternative solution. First, use jdeps to find all the
> platform modules your application needs. Then create an image using jlink
> with an --add-modules <platform_module> argument for each of the identified
> platform modules. You can then run your existing application with all
> application/library JARs on the classpath on top of the resulting image.
>
> As Sander said, jlink creates custom image from packaged modules only. You
> can create a minimal runtime image and your application and other libraries
> can be put on the classpath.
>
> The open issue of adding link-time support for automatic modules is
> JDK-8139947 [1].
>
> Mandy
> [1] https://bugs.openjdk.java.net/browse/JDK-8130047
>
>
>