问Java双重检查锁定代码解释

EN

private static final Object lock = new Object();
private static volatile YourObject instance;

public static YourObject getInstance() {
    YourObject r = instance;    // <---------- This bit, why do we assign it here ? 
    if (r == null) {
        synchronized (lock) {    // What is the benefit of this lock in here
            r = instance;        // assuming instance is null which will pass the first if ( r == null, what do we assign it again ?? I don't get the idea of this step at all. 
            if (r == null) {  
                r = new YourObject();
                instance = r;
            }
        }
    }
    return r;
}

public static ThreadSafeSingleton getInstanceUsingDoubleLocking(){
    if(instance == null){
        synchronized (ThreadSafeSingleton.class) {
            if(instance == null){
                instance = new ThreadSafeSingleton();
            }
        }
    }
    return instance;
}