Potential NPE in concurrent access between compress() and removeCached() methods due to unsynchronized Hashtable operations

[NullWagesException]

I've identified a thread-safety issue in the class cache management where concurrent execution of compress() and removeCached() may cause NullPointerException.

1. classes Hashtable is accessed by:

• removeCached() performs classes.remove()
• compress() iterates via classes.elements()

2. No synchronization between these operations

3. Race condition may occur when:

• Thread A starts iteration in compress()
• Thread B removes an element via removeCached()
• Thread A tries to call compress() on removed element → NPE

‌Reproduction:

public class Main {

    public static void main(String[] args) throws Exception {
        ClassPool pool = ClassPool.getDefault();
        int size = 300;
        CountDownLatch countDownLatch = new CountDownLatch(size);
        for (int i = 0; i < size; i++) {
            int finalI = i;
            new Thread(() -> {
                try {
                    ClassFile classFile = new ClassFile(false, "test" + finalI, Object.class.getName());
                    CtClass cc = pool.makeClass(classFile);
                    System.out.println(cc.getSimpleName());
                    Thread.sleep(5);
                    cc.detach();
                } catch (Exception e) {
                    e.printStackTrace();
                    System.exit(0);
                }finally {
                    countDownLatch.countDown();
                }
            }).start();
        }
        countDownLatch.await();
        System.out.println("done");
    }
}

The issue can be easily reproduced during debugging in IntelliJ IDEA when stepping through the nextElement() call.