VolatileDoubleCheck.java

public class VolatileDoubleCheck {
    private final Object resourceLock = new Object();
    private volatile Resource resource; // the thing we want to make sure we only have 1 of, marked as volatile
    
    public Resource getResource() {
        if (resource == null) {
            synchronized(resourceLock) {
                if (resource == null) {
                    resource = new Resource();
                }
            }
        }
        return resource;
    }
}