In a multi-threaded situation, un-synchronized lazy initialization of static fields could mean that a second thread has access to a half-initialized object while the first thread is still creating it. Allowing such access could cause serious bugs. Instead. the initialization block should be synchronized.

Similarly, updates of such fields should also be synchronized.

This rule raises an issue whenever a lazy static initialization is done on a class with at least one synchronized method, indicating intended usage in multi-threaded applications.

Noncompliant Code Example

private static Properties fPreferences = null;

private static Properties getPreferences() {
        if (fPreferences == null) {
            fPreferences = new Properties(); // Noncompliant
            fPreferences.put("loading", "true");
            fPreferences.put("filterstack", "true");
            readPreferences();
        }
        return fPreferences;
    }
}

Compliant Solution

private static Properties fPreferences = null;

private static synchronized Properties getPreferences() {
        if (fPreferences == null) {
            fPreferences = new Properties();
            fPreferences.put("loading", "true");
            fPreferences.put("filterstack", "true");
            readPreferences();
        }
        return fPreferences;
    }
}