Create an explicit build dependency from JRuby on Java.

Should prevent future flakiness if unreleased JRuby features are dependent on unreleased Java features by populating the local Maven repository with the artifacts needed when running `tests.sh jruby`.

tests.sh

@@ -214,13 +214,19 @@ use_java() {
 # --batch-mode suppresses download progress output that spams the logs.
 MVN="mvn --batch-mode"
 
-build_java() {
+internal_build_java() {
   version=$1
   dir=java_$version
   # Java build needs `protoc`.
   internal_build_cpp
   cp -r java $dir
   cd $dir && $MVN clean
+  # Skip tests here - callers will decide what tests they want to run
+  $MVN install -Dmaven.test.skip=true
+}
+
+build_java() {
+  internal_build_java $1
   # Skip the Kotlin tests on Oracle 7
   if [ "$version" == "oracle7" ]; then
     $MVN test -pl bom,lite,core,util
@@ -447,6 +453,7 @@ build_ruby30() {
 
 build_jruby() {
   internal_build_cpp                # For conformance tests.
+  internal_build_java jdk8 && cd .. # For Maven protobuf jar with local changes
   cd ruby && bash travis-test.sh jruby-9.2.11.1 && cd ..
 }