Only generate virtual setters where necessary LOXI-76

The previous commit would add virtual Exception-raising setters
globally. This commit adds a method that specifically checks whether
a setter is necessary.

A setter must be generated if the field is writeable in this class
or any super class. Fields can turn non-writeable in a subclass
if a superclass data member is used as a discriminator.

java_gen/java_model.py

@@ -890,6 +890,16 @@ def priv_value(self):
         else:
             return self.java_type.format_value(self.member.value, pub_type=False)
 
+    @property
+    def needs_setter(self):
+        if self.is_writeable:
+            return True
+        super_class = self.msg.super_class
+        if super_class:
+            super_member = super_class.member_by_name(self.name)
+            if super_member:
+                return super_member.needs_setter
+        return False
 
     @property
     def is_writeable(self):

java_gen/templates/_field_accessors.java

@@ -28,7 +28,7 @@
     }
 //:: #endif
 
-//:: if generate_setters:
+//:: if generate_setters and prop.needs_setter:
     //:: setter_template_file_name = "%s/custom/%s_%s.java" % (template_dir, msg.name if not builder else msg.name + '.Builder', prop.setter_name)
     //:: if os.path.exists(setter_template_file_name):
     //:: include(setter_template_file_name, msg=msg, builder=builder, has_parent=has_parent)

java_gen/templates/of_interface.java

@@ -53,7 +53,9 @@ public interface Builder${ "<%s>" % msg.type_annotation if msg.type_annotation e
         ${msg.name}${msg.type_variable} build();
 //:: for prop in msg.members:
         ${prop.java_type.public_type} ${prop.getter_name}()${ "" if prop.is_universal else " throws UnsupportedOperationException"};
+//:: if prop.needs_setter:
         Builder${msg.type_variable} ${prop.setter_name}(${prop.java_type.public_type} ${prop.name})${ "" if prop.is_universal else " throws UnsupportedOperationException"};
+//:: #endif
 //:: #endfor
     }
 }