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In a multi-threaded situation, un-synchronized lazy initialization of non-volatile fields could mean that a second thread has access to a half-initizliaed object while the first thread is still creating it. Allowing such access could cause serious bugs. Instead. the initizliation block should be synchronized or the variable made volatile.
Similarly, updates of such fields should also be synchronized.
Noncompliant Code Example

protected static Object instance = null;

public static Object getInstance() {
    if (instance != null) {
        return instance;
    }

    instance = new Object();  // Noncompliant
    return instance;
}

Compliant Solution

protected static volatile Object instance = null;

public static Object getInstance() {
    if (instance != null) {
        return instance;
    }

    instance = new Object();
    return instance;
}

or 
protected static Object instance = null;

public static synchronized Object getInstance() {
    if (instance != null) {
        return instance;
    }

    instance = new Object();
    return instance;
}