Copy explicit serialVersionUID from @AutoOneOf classes to their generated subclasses.

Previously the subclasses did not have an explicit `serialVersionUID`, so their implicit ones were likely to change if any details of the code-generation changed.

This change BREAKS SERIAL COMPATIBILITY for `@AutoOneOf` classes that had an explicit `serialVersionUID`.

Also, don't bother generating the shared `Parent_` class in the degenerate case of an `@AutoOneOf` class with no alternatives.

These changes reduce the number of warnings from `CompileWithEclipseTest` from 11 to 3.

RELNOTES=If an `@AutoOneOf` class has a `serialVersionUID` this is now copied to its generated subclasses. THIS BREAKS SERIAL COMPATIBILITY for `@AutoOneOf` classes with explicit `serialVersionUID` declarations, though those were already liable to be broken by arbitrary changes to the generated AutoOneOf code.
PiperOrigin-RevId: 364387942

Author: eamonnmcmanus

value/src/main/java/com/google/auto/value/processor/AutoValueOrOneOfProcessor.java

@@ -433,6 +433,7 @@ final void defineSharedVarsForType(
     vars.equals = methodsToGenerate.containsKey(ObjectMethod.EQUALS);
     vars.hashCode = methodsToGenerate.containsKey(ObjectMethod.HASH_CODE);
     vars.equalsParameterType = equalsParameterType(methodsToGenerate);
+    vars.serialVersionUID = getSerialVersionUID(type);
   }
 
   /** Returns the spelling to be used in the generated code for the given list of annotations. */
@@ -833,8 +834,9 @@ public Boolean visitArray(List<? extends AnnotationValue> list, Void p) {
   }
 
   /**
-   * Returns a string like {@code "1234L"} if {@code type instanceof Serializable} and defines
-   * {@code serialVersionUID = 1234L}; otherwise {@code ""}.
+   * Returns a string like {@code "private static final long serialVersionUID = 1234L"} if {@code
+   * type instanceof Serializable} and defines {@code serialVersionUID = 1234L}; otherwise {@code
+   * ""}.
    */
   final String getSerialVersionUID(TypeElement type) {
     TypeMirror serializable = elementUtils().getTypeElement(Serializable.class.getName()).asType();
@@ -846,7 +848,7 @@ final String getSerialVersionUID(TypeElement type) {
           if (field.getModifiers().containsAll(Arrays.asList(Modifier.STATIC, Modifier.FINAL))
               && field.asType().getKind() == TypeKind.LONG
               && value != null) {
-            return value + "L";
+            return "private static final long serialVersionUID = " + value + "L;";
           } else {
             errorReporter.reportError(
                 field, "serialVersionUID must be a static final long compile-time constant");

value/src/main/java/com/google/auto/value/processor/AutoValueOrOneOfTemplateVars.java

@@ -81,4 +81,11 @@ abstract class AutoValueOrOneOfTemplateVars extends TemplateVars {
    * wildcard, for example {@code <?, ?>}.
    */
   String wildcardTypes;
+
+  /**
+   * The text of the complete serialVersionUID declaration, or empty if there is none. When
+   * non-empty, it will be something like {@code private static final long serialVersionUID =
+   * 123L;}.
+   */
+  String serialVersionUID;
 }

value/src/main/java/com/google/auto/value/processor/AutoValueProcessor.java

@@ -431,7 +431,6 @@ private void defineVarsForType(
         propertyMethodAnnotationMap(type, propertyMethods);
     vars.props =
         propertySet(propertyMethodsAndTypes, annotatedPropertyFields, annotatedPropertyMethods);
-    vars.serialVersionUID = getSerialVersionUID(type);
     // Check for @AutoValue.Builder and add appropriate variables if it is present.
     maybeBuilder.ifPresent(
         builder -> {

value/src/main/java/com/google/auto/value/processor/AutoValueTemplateVars.java

@@ -56,9 +56,6 @@ class AutoValueTemplateVars extends AutoValueOrOneOfTemplateVars {
    */
   String gwtCompatibleAnnotation;
 
-  /** The text of the serialVersionUID constant, or empty if there is none. */
-  String serialVersionUID;
-
   /** The simple name of the generated subclass. */
   String subclass;
   /**

value/src/main/java/com/google/auto/value/processor/autooneof.vm

@@ -94,19 +94,23 @@ final class $generatedClass {
 
   #end
 
+#if (!$props.empty)
   // Parent class that each implementation will inherit from.
   private abstract static class Parent_$formalTypes extends $origClass$actualTypes {
 
-#foreach ($p in $props)
+  $serialVersionUID
+
+  #foreach ($p in $props)
 
     @`java.lang.Override`
     $p.access $p.type ${p.getter}() {
       throw new UnsupportedOperationException(${kindGetter}().toString());
     }
 
-#end
+  #end
 
   }
+#end
 
 #foreach ($p in $props)
 
@@ -120,6 +124,8 @@ final class $generatedClass {
   // Implementation when the contained property is "${p}".
   private static final class Impl_$p$formalTypes extends Parent_$actualTypes {
 
+  $serialVersionUID
+
   #if ($p.type == "void")
 
     // There is only one instance of this class.

value/src/main/java/com/google/auto/value/processor/autovalue.vm

@@ -177,9 +177,7 @@ ${modifiers}class $subclass$formalTypes extends $origClass$actualTypes {
   }
 #end
 
-#if (!$serialVersionUID.empty)
-  private static final long serialVersionUID = $serialVersionUID;
-#end
+  $serialVersionUID
 
 #if ($builderTypeName != "")
 

value/src/test/java/com/google/auto/value/processor/AutoOneOfCompilationTest.java

@@ -43,7 +43,9 @@ public void success() {
             "import java.io.Serializable;",
             "",
             "@AutoOneOf(TaskResult.Kind.class)",
-            "public abstract class TaskResult<V, T extends Throwable> {",
+            "public abstract class TaskResult<V, T extends Throwable> implements Serializable {",
+            "  private static final long serialVersionUID = 1234L;",
+            "",
             "  public enum Kind {VALUE, EXCEPTION, EMPTY}",
             "  public abstract Kind getKind();",
             "",
@@ -92,6 +94,8 @@ public void success() {
             "  // Parent class that each implementation will inherit from.",
             "  private abstract static class Parent_<V, T extends Throwable> "
                 + "extends TaskResult<V, T> {",
+            "    private static final long serialVersionUID = 1234L;",
+            "",
             "    @Override",
             "    public V value() {",
             "      throw new UnsupportedOperationException(getKind().toString());",
@@ -111,6 +115,8 @@ public void success() {
             "  // Implementation when the contained property is \"value\".",
             "  private static final class Impl_value<V, T extends Throwable> "
                 + "extends Parent_<V, T> {",
+            "    private static final long serialVersionUID = 1234L;",
+            "",
             "    private final V value;",
             "",
             "    Impl_value(V value) {",
@@ -152,6 +158,8 @@ public void success() {
             "  // Implementation when the contained property is \"exception\".",
             "  private static final class Impl_exception<V, T extends Throwable> "
                 + "extends Parent_<V, T> {",
+            "    private static final long serialVersionUID = 1234L;",
+            "",
             "    private final Throwable exception;",
             "",
             "    Impl_exception(Throwable exception) {",
@@ -193,13 +201,19 @@ public void success() {
             "  // Implementation when the contained property is \"empty\".",
             "  private static final class Impl_empty<V, T extends Throwable> "
                 + "extends Parent_<V, T> {",
+            "    private static final long serialVersionUID = 1234L;",
+            "",
             "    static final Impl_empty<?, ?> INSTANCE = new Impl_empty<>();",
             "",
             "    private Impl_empty() {}",
             "",
             "    @Override",
             "    public void empty() {}",
             "",
+            "    private Object readResolve() {",
+            "      return INSTANCE;",
+            "    }",
+            "",
             "    @Override",
             "    public String toString() {",
             "      return \"TaskResult{empty}\";",