feat: detect wrong java version in application runtime and give better user feedback

Author: simschla

app/build.gradle

@@ -19,6 +19,44 @@ dependencies {
 	implementation project(':provisioner')
 }
 
+import org.gradle.api.GradleException
+
+def javaExtension = extensions.getByType(JavaPluginExtension)
+def requiredMajor = javaExtension.toolchain.languageVersion.map { it.asInt() }.get()
+
+tasks.named('startScripts', CreateStartScripts).configure {
+	doLast {
+		def unixMarker = 'exec "$JAVACMD" "$@"'
+		def windowsMarker = '"%JAVA_EXE%" %DEFAULT_JVM_OPTS% %JAVA_OPTS% %SPOTLESS_OPTS%  -classpath "%CLASSPATH%" com.diffplug.spotless.cli.SpotlessCLI %*'
+
+		if (!unixScript.text.contains(unixMarker)) {
+			throw new GradleException("Unable to patch Unix launcher: '${unixMarker}' not found.")
+		}
+		if (!windowsScript.text.contains(windowsMarker)) {
+			throw new GradleException("Unable to patch Windows launcher: '${windowsMarker}' not found.")
+		}
+
+		unixScript.text = unixScript.text.replace(unixMarker, """
+spotlessCheckJavaVersion() {
+    local current=\$("\$JAVACMD" -version 2>&1 | awk -F[\\".] 'NR==1 {print \$2}')
+    if [ "\$current" -lt $requiredMajor ]; then
+        echo "Spotless CLI requires Java $requiredMajor or newer but found \$current. Make sure to install Java >= $requiredMajor and add it to the path or set JAVA_HOME env variable." >&2
+        exit 1
+    fi
+}
+spotlessCheckJavaVersion
+$unixMarker""")
+
+		windowsScript.text = windowsScript.text.replace(windowsMarker, """
+for /f "tokens=3 delims=." %%i in ('"%JAVA_EXE%" -version 2^>^&1 ^| findstr /r "[0-9][0-9]*\\.[0-9][0-9]*.*"') do set CURRENT_JAVA_MAJOR=%%i
+if %CURRENT_JAVA_MAJOR% LSS $requiredMajor (
+    echo Spotless CLI requires Java $requiredMajor or newer but found %CURRENT_JAVA_MAJOR%. Make sure to install Java >= $requiredMajor and add it to the path or set JAVA_HOME env variable. >&2
+    exit /b 1
+)
+$windowsMarker""")
+	}
+}
+
 application {
 	mainClass = 'com.diffplug.spotless.cli.SpotlessCLI'
 	applicationName = 'spotless'