Improper Synchronization (ARCH4039)

Key java.async.improper-synchronization · Severity warning · Category async

Synchronizing on a mutable/non-final object, on a publicly accessible monitor, or doing double-checked locking without volatile renders the locking ineffective.

Incorrect

class Cache {
    private Object lock = new Object();  // non-final, reassignable
    void put(String k, String v) {
        synchronized (lock) { map.put(k, v); }
    }
}

Correct

class Cache {
    private final Object lock = new Object(); // final, private monitor
    void put(String k, String v) {
        synchronized (lock) { map.put(k, v); }
    }
}

Fix

Synchronize on a private, final lock object; make the field volatile for double-checked locking.