GH-397: Work around quirk in immutables defaults for unmodifiable col…

…lections

Seems that immutables will not attempt to reconstruct mutable
variants of Lists from the default values, and Maven raises
no exceptions when this fails. Work-around for now is to use
null defaults.

protobuf-maven-plugin/src/main/java/io/github/ascopes/protobufmavenplugin/plugins/JvmPluginResolver.java

@@ -19,6 +19,7 @@
 import static java.nio.charset.StandardCharsets.ISO_8859_1;
 import static java.nio.charset.StandardCharsets.UTF_8;
 import static java.nio.file.StandardOpenOption.CREATE_NEW;
+import static java.util.Objects.requireNonNullElse;
 
 import io.github.ascopes.protobufmavenplugin.dependencies.DependencyResolutionDepth;
 import io.github.ascopes.protobufmavenplugin.dependencies.MavenArtifactPathResolver;
@@ -71,6 +72,12 @@
 public final class JvmPluginResolver {
 
   private static final Set<String> ALLOWED_SCOPES = Set.of("compile", "runtime", "system");
+  private static final List<String> DEFAULT_JVM_ARGS = List.of();
+  private static final List<String> DEFAULT_JVM_CONFIG_ARGS = List.of(
+      "-Xshare:auto",
+      "-XX:+TieredCompilation",
+      "-XX:TieredStopAtLevel=1"
+  );
   private static final Logger log = LoggerFactory.getLogger(JvmPluginResolver.class);
 
   private final HostSystem hostSystem;
@@ -164,13 +171,15 @@ private ArgumentFileBuilder buildArgLine(MavenProtocPlugin plugin)
       args.add(buildJavaPath(modules));
     }
 
-    plugin.getJvmConfigArgs()
+    requireNonNullElse(plugin.getJvmConfigArgs(), DEFAULT_JVM_CONFIG_ARGS)
         .stream()
         .filter(checkValidJvmConfigArg(plugin))
         .forEach(args::add);
 
     args.add(determineMainClass(plugin, dependencies.get(0)));
-    plugin.getJvmArgs().forEach(args::add);
+
+    requireNonNullElse(plugin.getJvmArgs(), DEFAULT_JVM_ARGS)
+        .forEach(args::add);
 
     return args;
   }

protobuf-maven-plugin/src/main/java/io/github/ascopes/protobufmavenplugin/plugins/MavenProtocPlugin.java

@@ -44,9 +44,7 @@ public interface MavenProtocPlugin extends MavenArtifact, ProtocPlugin {
    * @return the list of command line arguments to pass to the JVM.
    * @since 2.6.0
    */
-  default List<String> getJvmArgs() {
-    return List.of();
-  }
+  @Nullable List<String> getJvmArgs();
 
   /**
    * The dependency resolution depth.
@@ -68,20 +66,10 @@ default List<String> getJvmArgs() {
    * <p>Users can use this to control concerns such as heap memory controls,
    * GC and JIT settings, and specifying additional JVM options.
    *
-   * <p>This defaults to allowing shared JIT caches, and optimising the JIT
-   * for short-lived processes. Specifying this attribute overrides the default
-   * values.
-   *
    * @return the list of command line arguments to pass to the JVM.
    * @since 2.6.0
    */
-  default List<String> getJvmConfigArgs() {
-    return List.of(
-        "-Xshare:auto",
-        "-XX:+TieredCompilation",
-        "-XX:TieredStopAtLevel=1"
-    );
-  }
+  @Nullable List<String> getJvmConfigArgs();
 
   /**
    * The main class entrypoint to use if the plugin is not an assembled JAR.