最精简：设计模式之单例模式

懒汉式（线程不安全）

public class SingletonLazy {

    public static void main(String[] args) {
        Order instance = Order.getInstance();
        Order instance2 = Order.getInstance();

        System.out.println(instance == instance2);

    }

}
class Order{
    private Order(){

    }
    private static Order instance = null;
    public static Order getInstance(){
        if (instance == null) {
            return instance = new Order();
        }else {
            return instance;
        }

    }
}

懒汉式（线程安全）双重检测：DCL懒汉式（）

public class SingletonLazy {

    public static void main(String[] args) {
        Order instance = Order.getInstance();
        Order instance2 = Order.getInstance();

        System.out.println(instance == instance2);

    }

}

class Order {
    private Order() {

    }

    private static Order instance = null;

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

        return instance;


    }
}

懒汉式（线程安全）
DCL懒汉式在return instance = new Order();的时候如果有多线程存在的情况下会发生指令重排的情况，所有并不是绝对安全的，解决方法是在private static Order instance = null;上加上Volatile关键字

public class SingletonLazy {

    public static void main(String[] args) {
        Order instance = Order.getInstance();
        Order instance2 = Order.getInstance();

        System.out.println(instance == instance2);

    }

}

class Order {
    private Order() {

    }

    private volatile static Order instance = null;

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

        return instance;


    }
}

饿汉式

public class SingletonHunger {
    public static void main(String[] args) {
        Bank bank = Bank.getInstance();
        Bank bank2 = Bank.getInstance();


        System.out.println(bank == bank2);
    }
}
class Bank{

    private Bank(){

    }

    private static Bank instance = new Bank();

    public static Bank getInstance(){
        return instance;
    }
}