JAVA_单例模式

一定要掌握并且会白板默写的三种单例模式实现。

饿汉式：

public class Singleton1 {
    // 必须private修饰
    private static final Singleton1 SINGLETON = new Singleton1();
    // 无参构造必须private
    private Singleton1(){}
    // 获取单例的方法，必须public且是静态方法
    public static Singleton1 getInstance(){
        return SINGLETON;
    }
}

懒汉式（双重检查）：

public class Singleton2 {
    // 必须是volatile修饰，保障单例共享变量的有序性和可见性
    private static volatile Singleton2 singleton2 = null;

    private Singleton2(){}

    public static Singleton2 getInstance(){
        if(singleton2 == null){
            synchronized (Singleton2.class){
                if(singleton2 == null){
                    // 这行代码，涉及到的执行操作分成多个步骤，必须保障机器指令的有序性，避免其它线程判断null时出现误判
                    singleton2 = new Singleton2();
                }
            }
        }
        return singleton2;
    }


}

静态内部类：

public class Singleton3 {
    
    private Singleton3(){}
    
    private static class SingletonHolder{
        private static final Singleton3 SINGLETON = new Singleton3();
    }
    
    public static Singleton3 getInstance(){
        return SingletonHolder.SINGLETON;
    }
    
}