Java 11: What is the difference between javac --release and -source and -target command line parameters?

Question

Does anyone know the difference between using --release and the older -source and -target command line parameters?

In the javac man page: -source Provide source compatibility with specified release -target Generate class files for specific VM version

--release Compile for a specific VM version. Supported targets: 6, 7, 8, 9, 10, 11

Could --release bind both -source and -target to the same version?

Answer 1

I have found the answer in the Java 19 SDK source code for the java compiler:

/**
 * Handles the {@code --release} option.
 *
 * @param additionalOptions a predicate to handle additional options implied by the
 * {@code --release} option. The predicate should return true if all the additional
 * options were processed successfully.
 * @return true if successful, false otherwise
 */
public boolean handleReleaseOptions(Predicate<Iterable<String>> additionalOptions) {
    String platformString = options.get(Option.RELEASE);

    checkOptionAllowed(platformString == null,
            option -> reportDiag(Errors.ReleaseBootclasspathConflict(option)),
            Option.BOOT_CLASS_PATH, Option.XBOOTCLASSPATH, Option.XBOOTCLASSPATH_APPEND,
            Option.XBOOTCLASSPATH_PREPEND,
            Option.ENDORSEDDIRS, Option.DJAVA_ENDORSED_DIRS,
            Option.EXTDIRS, Option.DJAVA_EXT_DIRS,
            Option.SOURCE, Option.TARGET,
            Option.SYSTEM, Option.UPGRADE_MODULE_PATH);

    if (platformString != null) {
        PlatformDescription platformDescription =
                PlatformUtils.lookupPlatformDescription(platformString);

        if (platformDescription == null) {
            reportDiag(Errors.UnsupportedReleaseVersion(platformString));
            return false;
        }

        options.put(Option.SOURCE, platformDescription.getSourceVersion());
        options.put(Option.TARGET, platformDescription.getTargetVersion());

        context.put(PlatformDescription.class, platformDescription);

        if (!additionalOptions.test(platformDescription.getAdditionalOptions()))
            return false;

        JavaFileManager platformFM = platformDescription.getFileManager();
        DelegatingJavaFileManager.installReleaseFileManager(context,
                platformFM,
                getFileManager());
    }

    return true;
}

As the code shows, the --release option will set both source and target to the same value.

In fact, there is a check that forbids using the --release parameter if source or target have already been set.

void checkOptionAllowed(boolean allowed, ErrorReporter r, Option... opts) {
    if (!allowed) {
        Stream.of(opts)
              .filter(options :: isSet)
              .forEach(r :: report);
    }
}