Setup asciidoc-to-html build system

Our markdown documentation is published to GitHub pages via its
automatic markdown files publishing support[1]. However, this mechanism
does not support asciidoc files.

We are in the process of converting all of our documentation to
asciidoc, so we'll need to setup our own GitHub pages build and
deployment system.

Let's reuse the asciidoc-to-html build system in addressbook-level4[2]
by copying the relevant code/files.

[1] https://github.com/blog/2289-publishing-with-github-pages-now-as-easy-as-1-2-3
[2] https://github.com/se-edu/addressbook-level4

.gitignore

@@ -24,3 +24,4 @@ test/data/
 
 # Gradle files
 /.gradle/
+/build/

.travis.yml

@@ -0,0 +1,28 @@
+language: java
+matrix:
+  include:
+    - jdk: oraclejdk9
+
+script: >-
+    ./gradlew
+
+deploy:
+  skip_cleanup: true
+  provider: script
+  script: ./config/travis/deploy_github_pages.sh
+  on:
+    branch: master
+
+addons:
+  apt:
+    packages:
+      - oracle-java9-installer
+
+before_cache:
+  - rm -f  $HOME/.gradle/caches/modules-2/modules-2.lock
+  - rm -fr $HOME/.gradle/caches/*/plugin-resolution/
+
+cache:
+  directories:
+    - $HOME/.gradle/caches/
+    - $HOME/.gradle/wrapper/

build.gradle

@@ -0,0 +1,35 @@
+// Gradle Configuration File
+// For more details take a look at the Java Quickstart chapter in the Gradle
+// user guide available at http://gradle.org/docs/4.8.1/userguide/tutorial_java_projects.html
+
+plugins {
+    id 'org.asciidoctor.convert' version '1.5.6'
+}
+
+asciidoctor {
+    backends 'html5'
+    sourceDir 'docs'
+    outputDir "${buildDir}/docs"
+
+    attributes = [
+        linkcss: true,
+        stylesheet: 'gh-pages.css',
+        'source-highlighter': 'coderay',
+        icons: 'font',
+        experimental: true,
+        sectlinks: true,
+        idprefix: '',  // for compatibility with GitHub preview
+        idseparator: '-',
+    ]
+}
+
+// Copies stylesheets into the directory containing generated HTML files as
+// Asciidoctor does not copy linked CSS files to the output directory when rendering.
+// This is needed for linked stylesheets and embedded stylesheets which import other files.
+task copyStylesheets(type: Copy) {
+    from "${asciidoctor.sourceDir}/stylesheets"
+    into "${asciidoctor.outputDir}/html5/stylesheets"
+}
+asciidoctor.dependsOn copyStylesheets
+
+defaultTasks 'clean', 'asciidoctor'

config/travis/deploy_github_pages.sh

@@ -0,0 +1,42 @@
+#!/bin/sh
+# Pushes files generated by Asciidoctor and associated files to gh-pages branch for commits to master branch.
+
+set -o errexit # exit with nonzero exit code if any line fails
+
+if [ -z "$GITHUB_TOKEN" ]; then
+  echo 'GITHUB_TOKEN is not set up in Travis. Skipping deploy.'
+  exit 0
+fi;
+
+set -o nounset # exit if variable is unset
+
+cd build/docs/html5
+
+git init
+git config user.name 'Deployment Bot (Travis)'
+git config user.email '[email protected]'
+
+git config credential.helper 'store --file=.git/credentials'
+echo "https://${GITHUB_TOKEN}:@github.com" > .git/credentials
+
+git remote add upstream "https://github.com/${TRAVIS_REPO_SLUG}.git"
+
+# Reset to gh-pages branch, or create orphan branch if gh-pages does not exist in remote.
+if git ls-remote --exit-code --heads upstream gh-pages; then
+    git fetch --depth=1 upstream gh-pages
+    git reset upstream/gh-pages
+elif [ $? -eq 2 ]; then # exit code of git ls-remote is 2 if branch does not exist
+    git checkout --orphan gh-pages
+else # error occurred
+    exit $?
+fi
+
+# Exit if there are no changes to gh-pages files.
+if changes=$(git status --porcelain) && [ -z "$changes" ]; then
+    echo 'No changes to GitHub Pages files; exiting.'
+    exit 0
+fi
+
+git add -A .
+git commit -m "Rebuild pages at ${TRAVIS_COMMIT}"
+git push --quiet upstream HEAD:gh-pages