code: public final Jar setAttribute(String name, String value) {
          verifyNotSealed();
          if (jos != null)
              throw new IllegalStateException("Manifest cannot be modified after entries are added.");
          getManifest().getMainAttributes().putValue(name, value);
          return this;
      }
docs: Sets an attribute in the main section of the manifest.
  
  @param name  the attribute's name
  @param value the attribute's value
  @return {@code this}
  @throws IllegalStateException if entries have been added or the JAR has been written prior to calling this methods.