code: public boolean isSerializable() {
          try {
              return env.types.isSubtype(type, env.syms.serializableType);
          } catch (CompletionFailure ex) {
              // quietly ignore completion failures
              return false;
          }
      }
docs: Return true if this class implements <code>java.io.Serializable</code>.
  
  Since <code>java.io.Externalizable</code> extends
  <code>java.io.Serializable</code>,
  Externalizable objects are also Serializable.