关于设计模式:设计模式单例模式

//单例饿汉public class Singleton01 {    private static final Singleton01 INSTANCE = new Singleton01();    private Singleton01() {    }    public static Singleton01 getInstance() {        return INSTANCE;    }}

public class Singleton02 {    private volatile static Singleton02 INSTANCE;    //为什么要应用volatile    //执行构造方法和赋值的操作可能会指令重排序,并发状况下可能有两个对象被初始化    //故应用volatile,禁止指令重排序,保障线程平安    private Singleton02() {}    public static Singleton02 getInstance() {        if (INSTANCE == null) {            synchronized (Singleton02.class) {                if (INSTANCE == null) {                    INSTANCE = new Singleton02();                }            }        }        return INSTANCE;    }}

public class Singleton03 {    private Singleton03() {    }    //动态外部类自带懒加载    private static class SingletonHolder {        private static Singleton03 INSTANCE = new Singleton03();    }    public static Singleton03 getInstance() {        return SingletonHolder.INSTANCE;    }}

public enum Singleton04 {    //枚举    INSTANCE;}