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    <p>Using lazy initialization for the database in a web crawler is probably not worthwhile. Lazy initialization adds complexity and an ongoing speed hit. One case where it is justified is when there is a good chance the data will never be needed. Also, in an interactive application, it can be used to reduce startup time and give the <em>illusion</em> of speed.</p> <p>For a non-interactive application like a web-crawler, which will surely need its database to exist right away, lazy initialization is a poor fit.</p> <p>On the other hand, a web-crawler is easily parallelizable, and will benefit greatly from being multi-threaded. Using it as an exercise to master the <code>java.util.concurrent</code> library would be extremely worthwhile. Specifically, look at <a href="http://java.sun.com/javase/6/docs/api/java/util/concurrent/ConcurrentHashMap.html" rel="noreferrer"><code>ConcurrentHashMap</code></a> and <a href="http://java.sun.com/javase/6/docs/api/java/util/concurrent/ConcurrentSkipListMap.html" rel="noreferrer"><code>ConcurrentSkipListMap</code>,</a> which will allow multiple threads to read and update a shared map.</p> <p>When you get rid of lazy initialization, the simplest Singleton pattern is something like this:</p> <pre><code>class Singleton { static final Singleton INSTANCE = new Singleton(); private Singleton() { } ... } </code></pre> <p>The keyword <code>final</code> is the key here. Even if you provide a <code>static</code> "getter" for the singleton rather than allowing direct field access, making the singleton <code>final</code> helps to ensure correctness and allows more aggressive optimization by the JIT compiler.</p>
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    1. POproper usage of synchronized singleton?
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    1. COI don't think is what he is asking about...
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    2. COI based my answer on the fact that the article cited focuses on the use of double-checked locking to initialize the singleton lazily. This article would be a very bad guide to follow.
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    3. COAh, yes thats correct. Double-checked locking is broken anyway.
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