Tact
Tact is a Java program and package that inserts runtime checks into Java bytecode. The goal of these checks is to aid the development of multi-threaded programs by ensuring that only the intended threads access any object, either by sole ownership or holding appropriate locks.
Example
The most convenient way to run tact is on a jar:
java -jar path/to/tact.jar files.jar
This will inject runtime checks into the bytecode of all classes in files.jar.
It's also possible to inject individual class files:
java -jar path/to/tact.jar Hello.class
Like with jars, this will inject the class's bytecode.
What's Going On?
The above examples will guard access to all non-final objects used by files.jar and the Hello class with the following strategy:
An object is owned by the Thread that creates it. No other Thread can access it, and any that try will throw an IllegalAccessError.
This is strict, but the goal is to catch unintended object accesses. This is also impractical for every multi-threaded program, because they all have to create Thread objects before starting them. So, other strategies can be specified in the program's source with the unh.edu.cs.tact pacakge, after calling Checker.init(). The first aid is the Checker.releaseAndStart() method, which atomically relinquishes the current Thread's ownership of a Thread or Runnable object, then starts the new Thread.
Runnable r = new MyTask();
Checker.releaseAndStart(r);
The next aid is the GuardedBy annotation. This can be applied to individual fields of a class to specify that they can only be accessed when the current Thread holds a certain lock.
@GuardedBy("this") public int sharedData;
…
synchronized(this){ sharedData = 7; } // OK
…
sharedData = 13; // throws IllegalAccessError
Only "this", static and non-static members of the form "full.package.and.Class.field", and class objects of the form "full.package.and.Class.class" are accepted as guard locks. Oftentimes, these compile-time guards are insufficient. A runtime method exists to fill this role:
Checker.guardBy(obj, guard);
synchronized(guard){ obj.field = 7; } // OK
obj.field = 13; // throws IllegalAccessError
Remember, Checker.init() must be called before any of these methods. See the JavaDoc for more methods and details.
Java 7
The bytecode generated by tact currently fails to pass the Java 7 VM's
"Stack Map Frame" check. Therefore, injected programs must be run with
java's -XX:-UseSplitVerifier flag.
TODO
Tact is still incomplete, and the correctness of the current implementation is still not fully vetted. Use at your own risk!