单例模式

单线程环境

饿汉式：通过静态常量、静态代码块实现，线程同步

懒汉式：判断null之后再new对象，线程不同步

静态内部类

线程安全，延迟加载，效率高

仅当调用getInstance()方法时，静态内部类才进行加载，加载遵循双亲委派模式，加载过程线程安全


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public class Singleton {
    private Singleton() {
    }

    private static class SingletonInstance {
        private static final Singleton INSTANCE = new Singleton();
    }

    public static Singleton getInstance() {
        return SingletonInstance.INSTANCE;
    }
}

双重检查

线程安全，延迟加载，效率高


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public class Singleton {
    private static volatile Singleton singleton;

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (singleton == null) {
            synchronized (Singleton.class) {
                if (singleton == null) {

                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }
}