JAVA 单例模式的几种写法

public class Singleton {

    // 饿汉式 线程安全，类加载时就被初始化了 没有实现懒加载
    /*private Singleton() {}
    private static Singleton  instance = new Singleton();
    public static Singleton getInstance() {
        return  instance;
    }*/


    //  懒汉式一，线程不安全
    /*private Singleton (){}
    private static Singleton instance = null;
    public static Singleton getInstance (){
        if (null == instance) {
            instance = new Singleton();
        }
        return instance;
    }
     */

    // 懒汉式二，线程安全同步锁定方法，每次获取单例都要锁定整个方法
   /* private Singleton(){}
    private static Singleton instance = null;
    public synchronized  static Singleton getInstance () {
        if (null == instance) {
            instance = new Singleton();
        }
        return instance;
    }*/


    //  懒汉式三
    //同步代码块方式，线程安全，只在第一次获取单例时锁定了单例对象，高效
    /*private Singleton(){}
    private static  Singleton instance = null;
    public static Singleton getInstance() {
        if (null == instance) {
            synchronized (Singleton.class) {
                if (null == instance) {
                    instance = new Singleton();
                }
            }
        }
        return  instance;
    }*/

    /**
     * 静态内部类实现，  推荐
     * 线程安全，懒加载，效率高; 静态内部类的凡是在单例类在被JVM加载到内存时并不会立即实例化，而是在第一次调用
     *  getInstance（）方法时才会将SingleTonInner类加载到内存，从而实例化SingleTonInner的静态属性
     * SINGLETON，才开始加载实例化SINGLETON。并且在实例化SINGLETON的时候，JVM帮助我们保证了线程的安全
     */
    private Singleton(){}
    public static Singleton getInstance (){
        return SingletonInner.INSTANCE;
    }
    private static class SingletonInner {
        private static final Singleton INSTANCE = new Singleton();
    }


    /**
     * 枚举类实现
     */
     /*private Singleton(){ }
     public static Singleton getInstance(){
         return  SinglteonEnum.INSTANCE.getInstanceEnum();
     }
     private enum SinglteonEnum {
         INSTANCE;
         private Singleton singlteon;
         SinglteonEnum() {
             singlteon = new Singleton();
         }
          private Singleton getInstanceEnum(){
             return singlteon;
          }
     }*/
}