Fix compatiblity issues.

Currently some public API methods are defined in GenreatedMessage.java
and they have a generric return type:
  class GeneratedMessage {
    class Builder<BuilderType extends Builder<BuilderType>> {
      public BuilderType setField(...);
      public BuilderType setExtension(...);
    }
  }
With these definitions, the compiled byte code of a callsite will have
a direct reference to GeneratedMessage. For example:
  fooBuilder.setField(...);
becomes:
  ##: invokevirtual // Method Builder.setField:(...)LGeneratedMessage.Builder
  ##: checkcast     // class Builder

This will prevent us from updating generated classes to subclass a
different versioned GeneratedMessageV3 class in the future (we can't do
it in a binary compatible way).

This change addresses the problem by overriding these methods directly
in the generated class:
  class Foo {
    class Builder extends GeneratedMessage.Builder<Builder> {
      public Builder setField(...) {
        return super.setField(...);
      }
    }
  }
After this, fooBuilder.setField(...) will be compiled to:
  ##: invokevirtual // Method Builder.setField:(...)LFoo.Builder

The callsites will no longer reference GeneratedMessage directly and we
can change Foo to subclass GeneratedMessageV3 without breaking binary
compatiblity.

The downside of this change is:
  1. It increases generated code size (though it saves some instructions
     on the callsites).
  2. We can never stop generating these overrides because doing that
     will break binary compatibility.

Change-Id: I879afbbc1325a66324a51565e017143489b06e97

1 files changed, 4 insertions, 4 deletions

diff --git a/java/core/src/main/java/com/google/protobuf/GeneratedMessage.java b/java/core/src/main/java/com/google/protobuf/GeneratedMessage.java
index 2c87302b..cea05794 100644
--- a/java/core/src/main/java/com/google/protobuf/GeneratedMessage.java
+++ b/java/core/src/main/java/com/google/protobuf/GeneratedMessage.java
@@ -1396,7 +1396,7 @@ public abstract class GeneratedMessage extends AbstractMessage
       return setExtension((ExtensionLite<MessageType, Type>) extension, value);
     }
     /** Set the value of an extension. */
-    public final <Type> BuilderType setExtension(
+    public <Type> BuilderType setExtension(
         final GeneratedExtension<MessageType, Type> extension, final Type value) {
       return setExtension((ExtensionLite<MessageType, Type>) extension, value);
     }
@@ -1407,7 +1407,7 @@ public abstract class GeneratedMessage extends AbstractMessage
       return setExtension((ExtensionLite<MessageType, List<Type>>) extension, index, value);
     }
     /** Set the value of one element of a repeated extension. */
-    public final <Type> BuilderType setExtension(
+    public <Type> BuilderType setExtension(
         final GeneratedExtension<MessageType, List<Type>> extension,
         final int index, final Type value) {
       return setExtension((ExtensionLite<MessageType, List<Type>>) extension, index, value);
@@ -1418,7 +1418,7 @@ public abstract class GeneratedMessage extends AbstractMessage
       return addExtension((ExtensionLite<MessageType, List<Type>>) extension, value);
     }
     /** Append a value to a repeated extension. */
-    public final <Type> BuilderType addExtension(
+    public <Type> BuilderType addExtension(
         final GeneratedExtension<MessageType, List<Type>> extension, final Type value) {
       return addExtension((ExtensionLite<MessageType, List<Type>>) extension, value);
     }
@@ -1428,7 +1428,7 @@ public abstract class GeneratedMessage extends AbstractMessage
       return clearExtension((ExtensionLite<MessageType, ?>) extension);
     }
     /** Clear an extension. */
-    public final <Type> BuilderType clearExtension(
+    public <Type> BuilderType clearExtension(
         final GeneratedExtension<MessageType, ?> extension) {
       return clearExtension((ExtensionLite<MessageType, ?>) extension);
     }