A Java compiler plugin to prevent invalid thread accesses when writing GUIs.
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GUI Effects Checker

This repository contains a prototype implementation of a Java effect system to ensure the absence of threading access errors from accessing UI objects from threads other than the UI event loop thread.

This is an archival version of the code, corresponding to the ECOOP'13 paper mentioned below.

The current version is distributed with the Checker Framework.

Methods that access GUI objects are assigned the UI effect. Methods that do not access such objects are given the Safe effect, and methods annotated as safe are not permitted to call methods with the UI effect.

Of course, things get a bit more complicated with polymorphism.

ECOOP 2013 Paper

JavaUI is described in detail in an ECOOP 2013 paper titled "JavaUI: Effects for Controlling UI Object Access." The paper and extended technical report are available from the first author's homepage.

The annotated subject programs are available:

The subject programs were checked against Eclipse 3.7 on Ubuntu 12.10, and the scripts to run JavaUI over the subject programs hard-code paths to relevant JARs.


The type system is built on top of the Checker Framework (http://types.cs.washington.edu). It includes a few annotations:

  • @UIEffect: a method annotation indicating the method's effect is UI
  • @SafeEffect: a method annotation indicating the method's effect is Safe (this is the default for unannotated methods, unless the default is overridden by a type or package annotation)
  • @UIType: a type declaration annotation (class, interface) annotation that makes all methods contained within have the UI effect
  • @UIPackage: like @UIType, but for packages

There are also type qualifiers and additional annotations to support polymorphism:

  • @UI: a type qualifier to instantiate effect-generic types with the UI effect
  • @AlwaysSafe: a type qualifier to instantiate effect-generic types with the Safe effect
  • @PolyUIType: a type declaration (class, interface) annotation declaring that the type may have UI and Safe variants, allowing method arguments to be qualified with @PolyUI. This is often used for interfaces like Runnable, which are often used both for tasks run asynchronously on the UI thread and for general asynchronous tasks.
  • @PolyUI: a type qualifier that refers to the particular instantiation of a generic type
  • @PolyUIEffect: a method annotation indicating that the method's effect depends on the instantiation of a generic type.


This code is licensed under Matt Might's aptly-named CRAPL. The license is provided in CRAPL-LICENSE.txt in this repository. For more information on the CRAPL, see Matt Might's article: