Exchanger.java

public class Exchanger<T> {

    private T current;
    private final ConditionVariable takeCond = new ConditionVariable();
    private final ConditionVariable putCond = new ConditionVariable(true);

    public T take() {
        while(true) {
            takeCond.block();
            synchronized (this) {
                if(current == null)
                    continue;

                T res = current;
                current = null;
                takeCond.close();
                putCond.open();
                return res;
            }
        }
    }

    public void put(T val) {
        while(true) {
            putCond.block();
            synchronized (this) {
                if(current != null) {
                    continue;
                }

                current = val;
                putCond.close();
                takeCond.open();
                return;
            }
        }
    }
}