设计模式1-单例模式的8种实现

Posted 2021-08-09 yuec1998

//1、饿汉式（静态变量）
class Singleton1 {
    //构造器私有化
    private Singleton1() {
    }

    private final static Singleton1 instance = new Singleton1();

    public static Singleton1 getInstance() {
        return instance;
    }
}

//2、饿汉式（静态代码块）
class Singleton2 {
    private Singleton2() {
    }

    private static Singleton2 instance;

    //静态代码块
    static {
        instance = new Singleton2();
    }

    public static Singleton2 getInstance() {
        return instance;
    }
}

//3、懒汉式（线程不安全）
class Singleton3 {
    private Singleton3() {
    }

    private static Singleton3 instance;

    //线程不安全
    public static Singleton3 getInstance() {
        if (instance == null) {
            instance = new Singleton3();
        }
        return instance;
    }
}

//4、懒汉式（线程安全）
class Singleton4 {
    private Singleton4() {
    }

    private static Singleton4 instance;

    //加了synchronized，线程安全,效率低
    public static synchronized Singleton4 getInstance() {
        if (instance == null) {
            instance = new Singleton4();
        }
        return instance;
    }
}

//5、懒汉式（线程安全，不良改进版）
class Singleton5 {
    private Singleton5() {
    }

    private static Singleton5 instance;

    //有问题的改进版，高并发时候可能存在多个线程都同时进入到if中，而产生多个实例
    public static Singleton5 getInstance() {
        if (instance == null) {
            synchronized (Singleton5.class) {
                instance = new Singleton5();
            }
        }
        return instance;
    }
}

//6、懒汉式（线程安全，双重检查，推荐版）
class Singleton6 {
    private Singleton6() {
    }

    private static volatile Singleton6 instance;

    public static Singleton6 getInstance() {
        if (instance == null) {
            synchronized (Singleton6.class) {
                if (instance == null) {
                    instance = new Singleton6();
                }
            }
        }
        return instance;
    }
}

//7、懒汉式（静态内部类，推荐使用）
class Singleton7 {
    private Singleton7() {
    }

    private static class SingletonInstance {
        private static final Singleton7 INSTANCE = new Singleton7();
    }

    //利用类加载机制保证初始化实例时只有一个线程，线程安全
    //利用静态内部类特点实现延迟加载，效率高
    public static Singleton7 getInstance() {
        return SingletonInstance.INSTANCE;
    }
}

//8、通过枚举实现,推荐使用
enum SingletonEnum{
    INSTANCE;
    public void helloWord(){
        System.out.println("hello java!");
    }