單例模式（Singleton pattern）四種實現

[align=center][size=x-large]PART 1 快速預覽[/size][/align]
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[*][b]單例實現1：經典單例模式（Classic singleton pattern）[/b]
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[*]實現延遲實例化（Lazy instantiaze）；線程不安全（thread-unsafe）
[*]可用來學習單例模式思想，但是因爲線程不安全，所以[color=red]不建議使用[/color]。
[/list]

/**
 * Classic singleton pattern     
 * @author <a href="mailto:[email protected]">futeng</a>
 */
public class Singleton {
      // private static variable
      private static Singleton singleton;

      // private constructor
      private Singleton() {
             // do something useful such as initialized data
      }

      // provides a global point of access to it
      public static Singleton getInstance() {
             if ( singleton == null) {
                   singleton = new Singleton();
            }
             return singleton;
      }
}

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[*][b]單例實現2：對方法同步加鎖式單例（Synchronized method singleton pattern）[/b]
[/list]
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[*]實現延遲實例化（Lazy instantiaze）；線程安全（thread safe）；每次訪問都要等候別的線程離開該方法，性能低。
[*]解決了經典單例模式線程不安全的問題，但是性能低下，所以[color=red]不建議使用[/color]。
[/list]

/**
 * Synchronized method singleton pattern     
 * @author <a href="mailto:[email protected]">futeng</a>
 */
public class Singleton {

      private static Singleton singleton;

      private Singleton() {}

      public static synchronized Singleton getInstance() {
             if ( singleton == null) {
                   singleton = new Singleton();
            }
             return singleton;
      }
}

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[*][b]單例實現3：無延遲實例化式單例（Eagerly instantiaze singleton pattern）[/b]
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[*]非延遲實例化（eagerly instantiaze）；線程安全（thread safe）；
[*]在類加載的第一時間被初始化，[color=red]大部分場景都可勝任[/color]。丟失了延遲實例化特性，這帶來的遺憾是還未被調用就已經實例化了。
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/**
 * Eagerly singleton pattern     
 * @author <a href="mailto:[email protected]">futeng</a>
 */
public class Singleton {

      private static Singleton singleton = new Singleton();

      private Singleton() {}

      public static synchronized Singleton getInstance() {
             return singleton;
      }
}

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[*][b]單例實現4：雙重檢測加鎖式單例（Double-checked locking singleton pattern）[/b]
[/list]
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[*]實現延遲實例化（Lazy instantiaze）；線程安全（thread safe）；
[*]在[color=red]最佳的單例實現[/color]，稍微複雜。
[/list]

/**
 * Double checked singleton pattern     
 * @author <a href="mailto:[email protected]">futeng</a>
 */
public class Singleton {

      private volatile static Singleton singleton = new Singleton();

      private Singleton() {}

      public static Singleton getInstance() {
             if ( singleton == null) {
                   synchronized (Singleton. class) {
                         if ( singleton == null) {
                               singleton = new Singleton();
                        }
                  }
            }
             return singleton;
      }
}

[align=center][size=x-large]PART 2 細細品鑑[/size][/align]