设计模式 单例模式（Singleton）

//饿汉式
public class SingletonDemo01 {
    // 类初始化时,会立即加载该对象，线程天生安全,调用效率高
    private static SingletonDemo01 singletonDemo01 = new SingletonDemo01();
    private SingletonDemo01() {
        System.out.println("SingletonDemo01初始化");
    }
    public static SingletonDemo01 getInstance() {
        System.out.println("getInstance");
        return singletonDemo01;
    }
    public static void main(String[] args) {
        SingletonDemo01 s1 = SingletonDemo01.getInstance();
        SingletonDemo01 s2 = SingletonDemo01.getInstance();
        System.out.println(s1 == s2);//结果为ture
    }
 
}

//懒汉式
public class SingletonDemo02 {
    //类初始化时，不会初始化该对象，真正需要使用的时候才会创建该对。
    private static SingletonDemo02 singletonDemo02;
 
    private SingletonDemo02() {
    }
    public synchronized static SingletonDemo02 getInstance() {
        if (singletonDemo02 == null) {
            singletonDemo02 = new SingletonDemo02();
        }
        return singletonDemo02;
    }
 
    public static void main(String[] args) {
        SingletonDemo02 s1 = SingletonDemo02.getInstance();
        SingletonDemo02 s2 = SingletonDemo02.getInstance();
        System.out.println(s1 == s2);//结果为true
    }
}

//静态内部类方式
public class SingletonDemo03 {
    private SingletonDemo03() {
           System.out.println("初始化..");
    }
 
    public static class SingletonClassInstance {
        private static final SingletonDemo03 singletonDemo03 = new SingletonDemo03();
    }
 
    // 方法没有同步
    public static SingletonDemo03 getInstance() {
        System.out.println("getInstance");
        return SingletonClassInstance.singletonDemo03;
    }
 
    public static void main(String[] args) {
        SingletonDemo03 s1 = SingletonDemo03.getInstance();
        SingletonDemo03 s2 = SingletonDemo03.getInstance();
        System.out.println(s1 == s2);//结果为true
    }
}
//优势：兼顾了懒汉模式的内存优化（使用时才初始化）以及饿汉模式的安全性（不会被反射入侵）。
//劣势：需要两个类去做到这一点，虽然不会创建静态内部类的对象，但是其 Class 对象还是会被创建，而且是属于永久带的对象。

//枚举单例式
public class User {
    public static User getInstance() {
        return SingletonDemo04.INSTANCE.getInstance();
    }
 
    private static enum SingletonDemo04 {
        INSTANCE;
        // 枚举元素为单例
        private User user;
 
        private SingletonDemo04() {
            System.out.println("SingletonDemo04");
            user = new User();
        }
 
        public User getInstance() {
            return user;
        }
    }
 
    public static void main(String[] args) {
        User u1 = User.getInstance();
        User u2 = User.getInstance();
        System.out.println(u1 == u2);//结果为true
    }
}

//双重检测方式
public class SingletonDemo04 {
    private SingletonDemo04 singletonDemo04;
 
    private SingletonDemo04() {
 
    }
 
    public SingletonDemo04 getInstance() {
        if (singletonDemo04 == null) {
            synchronized (this) {
                if (singletonDemo04 == null) {
                    singletonDemo04 = new SingletonDemo04();
                }
            }
        }
        return singletonDemo04;
    }
 
}