Warn when default OpenJDK version is being used (#301)

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+* The buildpack now warns when no OpenJDK version is explicitly specified. Users are encouraged to specify a version to ensure future builds use the same OpenJDK version. ([#301](https://github.com/heroku/heroku-buildpack-jvm-common/pull/301))
+
 ### Changed
 
 * Default JDK version for the `heroku-24` stack is now always the latest long-term support version, currently `21`. ([#300](https://github.com/heroku/heroku-buildpack-jvm-common/pull/300))

bin/java

@@ -14,6 +14,41 @@ install_java_with_overlay() {
   local buildDir="${1}"
   local cacheDir="${2:-$(mktemp -d)}"
   if [ ! -f "${buildDir}/.jdk/bin/java" ]; then
+    if [ -z "$(_get_system_property "${buildDir}/system.properties" "java.runtime.version")" ]; then
+      if [ "${STACK}" == "heroku-24" ]; then
+        warning "No OpenJDK version specified
+Your application does not explicitly specify an OpenJDK
+version. The latest long-term support (LTS) version will be
+installed. This currently is OpenJDK ${DEFAULT_JDK_VERSION}.
+
+This default version will change when a new LTS version is
+released. Your application might fail to build with the new
+version. We recommend explicitly setting the required OpenJDK
+version for your application.
+
+To set the OpenJDK version, add or edit the system.properties
+file in the root directory of your application to contain:
+
+java.runtime.version = ${DEFAULT_JDK_VERSION}
+"
+      else
+        warning "No OpenJDK version specified
+Your application does not explicitly specify an OpenJDK
+version. OpenJDK ${DEFAULT_JDK_VERSION} will be installed.
+
+This default version will change at some point. Your
+application might fail to build with the new version. We
+recommend explicitly setting the required OpenJDK version for
+your application.
+
+To set the OpenJDK version, add or edit the system.properties
+file in the root directory of your application to contain:
+
+java.runtime.version = ${DEFAULT_JDK_VERSION}
+"
+      fi
+    fi
+
     local jdkVersion
     jdkVersion=$(get_jdk_version "${buildDir}")
 

lib/jvm.sh

@@ -39,15 +39,12 @@ fi
 
 get_jdk_version() {
   local appDir="${1:?}"
-  if [ -f "${appDir}/system.properties" ]; then
-    detectedVersion="$(_get_system_property "${appDir}/system.properties" "java.runtime.version")"
-    if [ -n "$detectedVersion" ]; then
-      echo "$detectedVersion"
-    else
-      echo "$DEFAULT_JDK_VERSION"
-    fi
+
+  configuredVersion="$(_get_system_property "${appDir}/system.properties" "java.runtime.version")"
+  if [ -n "${configuredVersion}" ]; then
+    echo "${configuredVersion}"
   else
-    echo "$DEFAULT_JDK_VERSION"
+    echo "${DEFAULT_JDK_VERSION}"
   fi
 }
 
@@ -208,7 +205,10 @@ _get_system_property() {
   local escaped_key
   escaped_key="${key//\./\\.}"
 
-  [ -f "$file" ] &&
-    grep -E "^${escaped_key}[[:space:]=]+" "$file" |
-    sed -E -e "s/$escaped_key([\ \t]*=[\ \t]*|[\ \t]+)([_A-Za-z0-9\.-]*).*/\2/g"
+  if [ -f "${file}" ]; then
+    grep -E "^${escaped_key}[[:space:]=]+" "${file}" |
+      sed -E -e "s/${escaped_key}([\ \t]*=[\ \t]*|[\ \t]+)([_A-Za-z0-9\.-]*).*/\2/g"
+  else
+    echo ""
+  fi
 }

test/compile_test.sh

@@ -12,6 +12,7 @@ testCompileWithoutSystemProperties() {
 
   assertCapturedSuccess
 
+  assertCaptured "WARNING: No OpenJDK version specified"
   assertCaptured "Installing OpenJDK 21"
   assertTrue "Java should be present in runtime." "[ -d ${BUILD_DIR}/.jdk ]"
   assertTrue "Java version file should be present." "[ -f ${BUILD_DIR}/.jdk/version ]"

test/spec/java_spec.rb

@@ -19,6 +19,8 @@
             else
               expect(app.output).to include("Installing OpenJDK #{jdk_version}")
             end
+
+            expect(app.output).not_to include("WARNING: No OpenJDK version specified")
             expect(app.output).to include("BUILD SUCCESS")
             expect(successful_body(app)).to eq("Hello from Java!")
           end
@@ -34,6 +36,8 @@
           write_sys_props(Dir.pwd, "maven.version=3.3.9")
         end
         app.deploy do
+          expect(app.output).to include("WARNING: No OpenJDK version specified")
+
           if app.stack == "heroku-24" then
             expect(app.output).to include("Installing OpenJDK 21")
           else
@@ -64,6 +68,8 @@
             else
               expect(app.output).to include("Installing OpenJDK #{jdk_version}")
             end
+
+            expect(app.output).not_to include("WARNING: No OpenJDK version specified")
             expect(app.output).to include("BUILD SUCCESS")
 
             # Workaround (August 2020):