单例模式

1. 懒汉式:线程是不安全的，若多个线程同时进入if(uniqueInstance==null)

   public class Singleton {
       private static Singleton uniqueInstance;
       private Singleton() {
       }
       public static Singleton getUniqueInstance() {
           if (uniqueInstance == null) {
               uniqueInstance = new Singleton();
           }
           return uniqueInstance;
       }
   }

2. 懒汉式线程安全:一个线程进入方法后，其他试图进入的线程必须等待，性能有损耗

   public static synchronized Singleton getUniqueInstance() {
       if (uniqueInstance == null) {
           uniqueInstance = new Singleton();
       }
       return uniqueInstance;
   }

3. 双重校验锁：先判断是否已经实例化，如果没有才对实例化语句加锁，内部是if(==null)是为了防止两个线程同时进入外层的if语句块内

   public class Singleton {
       private volatile static Singleton uniqueInstance;
       private Singleton() {
       }
       public static Singleton getUniqueInstance() {
           if (uniqueInstance == null) {
               synchronized (Singleton.class) {
                   if (uniqueInstance == null) {
                       uniqueInstance = new Singleton();
                   }
               }
           }
           return uniqueInstance;
       }
   }