In Travis-CI, test Maven and Gradle separately and concurrently

This should help identify cases where the Gradle build only works if
it runs before or after a Maven build.  It will also help us recognize
any Maven regressions accidentally introduced by our Gradle work.

.travis.yml

@@ -1,3 +1,4 @@
+sudo: false
 language: android
 android:
   components:
@@ -11,22 +12,14 @@ addons:
 before_install:
 - echo $ANDROID_HOME
 - git clone --depth=1 https://github.com/secure-software-engineering/DroidBench.git /tmp/DroidBench
-- export M2_HOME=$HOME/apache-maven-3.5.0
-- if [ ! -d $M2_HOME/bin ]; then curl https://archive.apache.org/dist/maven/maven-3/3.5.0/binaries/apache-maven-3.5.0-bin.tar.gz | tar zxf - -C $HOME; fi
-- export PATH=$M2_HOME/bin:$PATH
-install:
-- mvn clean verify -DskipTests=true -B -q
-- ./gradlew compileJava
-script:
-- xvfb ./gradlew build
-- xvfb-run mvn clean install -B -q
-- bash ./dependent-projects-trigger.sh $AUTH_TOKEN
-- mvn javadoc:aggregate -B -q
-- bash ./upload-javascript.sh $GH_TOKEN
-- ./build-maven-jars.py "install -Dgpg.skip"
-sudo: false
+- source "travis/before-install-$BUILD_SYSTEM"
+install: source "travis/install-$BUILD_SYSTEM"
+script: source "travis/script-$BUILD_SYSTEM"
+before_cache: source "travis/before-cache-$BUILD_SYSTEM"
 cache:
   directories:
+  - "$HOME/.gradle/caches"
+  - "$HOME/.gradle/wrapper"
   - "$HOME/.m2"
   - "$HOME/apache-maven-3.5.0"
   - "$TRAVIS_BUILD_DIR/com.ibm.wala.core.testdata/ocaml/ocamljava-2.0-alpha1/lib"
@@ -34,3 +27,6 @@ env:
   global:
     - secure: KcugjQYnBqeZ7XenZD5QY7jhekVPO0QpQyjDUteLytaokhyRK2g6eNvr/pPerN2uWUvsPwO18P9F+oOupge1cpPZf4cEY8RzLIromyUoRWd6JA0SKciUYdN2kSqnC4uZSJGXeGLoroyEEL4Q2sqimpkbIGxgxYtVniWgJULOyR4=
     - secure: jacvGPYX4ugG/HgwJEEpWnllWsS/ipX+qRJ7qM5mbDYryeqsz0eiaxBxQ6IpDyj0v6O4DUi74lSQs/UxCWcUxvOn/5AweCZfoF1U8vt1xivanR4MbC2kr6rJ6ohICuJ4PHDS4IffncgaP3Y8cfExHq6+d0tbibYkjRIiMUGpSik=
+  matrix:
+    - BUILD_SYSTEM=gradle
+    - BUILD_SYSTEM=maven

travis/before-cache-gradle

@@ -0,0 +1,7 @@
+# -*- mode: sh; sh-shell: sh -*-
+
+caches="$HOME/.gradle/caches"
+
+rm --force --recursive \
+   "$caches/modules-2/modules-2.lock" \
+   "$caches"/*/plugin-resolution

travis/before-install-maven

@@ -0,0 +1,9 @@
+# -*- mode: sh; sh-shell: sh -*-
+
+export M2_HOME=$HOME/apache-maven-3.5.0
+
+if [ ! -d $M2_HOME/bin ]; then
+  curl https://archive.apache.org/dist/maven/maven-3/3.5.0/binaries/apache-maven-3.5.0-bin.tar.gz | tar zxf - -C $HOME
+fi
+
+export PATH=$M2_HOME/bin:$PATH

travis/install-gradle

@@ -0,0 +1,3 @@
+# -*- mode: sh; sh-shell: sh -*-
+
+./gradlew assemble

travis/install-maven

@@ -0,0 +1,3 @@
+# -*- mode: sh; sh-shell: sh -*-
+
+mvn clean verify -DskipTests=true -B -q

travis/script-gradle

@@ -0,0 +1,3 @@
+# -*- mode: sh; sh-shell: sh -*-
+
+xvfb-run ./gradlew check javadoc

travis/script-maven

@@ -0,0 +1,7 @@
+# -*- mode: sh; sh-shell: sh -*-
+
+xvfb-run mvn clean install -B -q
+bash ./dependent-projects-trigger.sh "$AUTH_TOKEN"
+mvn javadoc:aggregate -B -q
+bash ./upload-javascript.sh "$GH_TOKEN"
+./build-maven-jars.py "install -Dgpg.skip"