Function GetJREVersion

src/java/jres/jre.go:231–311

GetJREVersion gets the desired JRE version from environment or uses default Supports BP_JAVA_VERSION (simple version) and JBP_CONFIG_ (complex config)

(ctx *common.Context, jreName string)

// GetJREVersion gets the desired JRE version from environment or uses default
// Supports BP_JAVA_VERSION (simple version) and JBP_CONFIG_<JRE_NAME> (complex config)
func GetJREVersion(ctx *common.Context, jreName string) (libbuildpack.Dependency, error) {
	// Check for simple BP_JAVA_VERSION environment variable first
	// Format: "8", "11", "17", "21", etc. or version patterns like "11.+", "17.*"
	if bpVersion := os.Getenv("BP_JAVA_VERSION"); bpVersion != "" {
		ctx.Log.Debug("Using Java version from BP_JAVA_VERSION: %s", bpVersion)

		// Normalize version to a pattern that FindMatchingVersion understands
		versionPattern := normalizeVersionPattern(bpVersion)

		// Get all available versions for this JRE
		availableVersions := ctx.Manifest.AllDependencyVersions(jreName)
		if len(availableVersions) == 0 {
			return libbuildpack.Dependency{}, fmt.Errorf("no versions found for %s", jreName)
		}

		// Find the highest matching version
		matchedVersion, err := libbuildpack.FindMatchingVersion(versionPattern, availableVersions)
		if err != nil {
			ctx.Log.Warning("Could not find %s matching version %s: %s", jreName, versionPattern, err.Error())
			return libbuildpack.Dependency{}, fmt.Errorf("no version of %s matching %s found", jreName, versionPattern)
		}

		ctx.Log.Debug("Resolved %s version %s from pattern %s", jreName, matchedVersion, versionPattern)
		return libbuildpack.Dependency{Name: jreName, Version: matchedVersion}, nil
	}

	// Check for JBP_CONFIG_<JRE_NAME> environment variable
	// Try both the auto-generated name and the documented name for backward compatibility
	envKey := fmt.Sprintf("JBP_CONFIG_%s", strings.ToUpper(strings.ReplaceAll(jreName, "-", "_")))
	envVal := os.Getenv(envKey)

	// If not found, check for documented environment variable name (e.g., JBP_CONFIG_OPEN_JDK_JRE)
	// This ensures backward compatibility with documented naming conventions
	if envVal == "" {
		if documentedEnvKey, exists := jreNameToDocumentedEnvVar[jreName]; exists {
			envVal = os.Getenv(documentedEnvKey)
			if envVal != "" {
				envKey = documentedEnvKey
			}
		}
	}

	if envVal != "" {
		ctx.Log.Debug("Found %s='%s'", envKey, envVal)

		versionPattern := parseJBPConfigVersion(envVal)
		if versionPattern == "" {
			// No version specified — env var is used for other settings (e.g. memory_calculator).
			// Fall back to manifest default.
			ctx.Log.Debug("%s set but contains no version field, using manifest default", envKey)
		} else {
			ctx.Log.Debug("Parsed version pattern from %s: '%s'", envKey, versionPattern)

			normalizedPattern := normalizeVersionPattern(versionPattern)
			ctx.Log.Debug("Normalized pattern: '%s' -> '%s'", versionPattern, normalizedPattern)

			availableVersions := ctx.Manifest.AllDependencyVersions(jreName)
			if len(availableVersions) == 0 {