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如何实现一个单例

1、构造器需要私有化

2、提供一个私有的静态变量

3、暴露一个公共的获取单例对象的接口

需要考虑的两个问题

1、是否支持懒加载

2、是否线程安全

1、饿汉式

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

不支持懒加载

线程安全

2、懒汉式

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

支持懒加载

线程不安全

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

支持懒加载

线程安全

3、双重检查锁

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;}
}

支持懒加载

线程安全

4、静态内部类

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

支持懒加载

线程安全

5、枚举

public enum EnumSingleton {INSTANCE；
}

支持懒加载

线程安全