private void callChecker(String source, LoadLevel level) {
        manager.registerChecker(new Checker() {
            @Override
            public String source() {
                return source;
            }

            @Override
            public LoadLevel check() {
                return level;
            }

            @Override
            public long intervalMs() {
                return 1;
            }
        });
    }
Should be called at the beginning of each test to initialize the manager.
core/src/test/java/kafka/automq/backpressure/DefaultBackPressureManagerTest.java