如何實現一個單例

1、構造器需要私有化

2、提供一個私有的靜態變量

3、暴露一個公共的獲取單例對象的接口

需要考慮的兩個問題

1、是否支持懶加載

2、是否線程安全

1、餓漢式

public class EagerSingleton {private static final EagerSingleton INSTANCE = new EagerSingleton();private EagerSingleton(){}public EagerSingleton getInstance(){return INSTANCE;}}

不支持懶加載

線程安全

2、懶漢式

public class LazySingleton {private static LazySingleton INSTANCE;private LazySingleton() {}public static LazySingleton getInstance() {if (INSTANCE == null) {INSTANCE = new LazySingleton();}return INSTANCE;}
}

支持懶加載

線程不安全

public class LazySingleton {private static LazySingleton INSTANCE;private LazySingleton() {}public static synchronized LazySingleton getInstance() {if (INSTANCE == null) {INSTANCE = new LazySingleton();}return INSTANCE;}
}

支持懶加載

線程安全

3、雙重檢查鎖

public class DoubleCheckSingleton {private static DoubleCheckSingleton INSTANCE;private DoubleCheckSingleton() {}public DoubleCheckSingleton getInstance() {if (INSTANCE == null) {synchronized (DoubleCheckSingleton.class) {if (INSTANCE == null) {INSTANCE = new DoubleCheckSingleton();}}}return INSTANCE;}
}

支持懶加載

線程安全

4、靜態內部類

public class InnerSingleton {private InnerSingleton() {}public static InnerSingleton getInstance() {return Singleton.INSTANCE;}private static class Singleton {private static final InnerSingleton INSTANCE = new InnerSingleton();}
}

支持懶加載

線程安全

5、枚舉

public enum EnumSingleton {INSTANCE；
}

支持懶加載

線程安全