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privateprivate 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;}}