package pattern;

public class SingleInstance{
      private static SingleInstance sInstance = new SingleInstance();
      private SingleInstance(){}
      public static SingleInstance getInstance(){
         return sInstance;
      }
      public static void main(String[] arg) {
          System.out.println(SingleInstance.getInstance());
          System.out.println(SingleInstance.getInstance());
          System.out.println(SingleInstance2.getInstance());
          System.out.println(SingleInstance2.getInstance());
          System.out.println(SingleInstance3.getInstance());
          System.out.println(SingleInstance3.getInstance());
          System.out.println(SingleInstance4.getInstance());
          System.out.println(SingleInstance4.getInstance());
          System.out.println(SingleInstance5.getInstance());
          System.out.println(SingleInstance5.getInstance());
      }
}

class SingleInstance2 {
      private static SingleInstance2 sInstance;
      private SingleInstance2(){}
      public static SingleInstance2 getInstance(){
        if( null == sInstance){
            sInstance = new SingleInstance2();
        }
        return sInstance;
      }
      /* 上述的代码在多个线程密集调用getInstance时，存在创建多个实例的可能。
       * 比如线程A进入null == sInstance这段代码块，而在A线程未创建完成实例时，
       * 如果线程B也进入了该代码块，必然会造成两个实例的产生。
       * 
       */
}

class SingleInstance3 {
    private static SingleInstance3 sInstance;
    private SingleInstance3() {}
    public static synchronized SingleInstance3 getInstance(){
      if (null == sInstance){
         sInstance = new SingleInstance3();
      }
    return sInstance;
    }
}
/* Volatile是轻量级的synchronized，它在多处理器开发中保证了共享变量的“可见性”。
 * 可见性的意思是当一个线程修改一个共享变量时，另外一个线程能读到这个修改的值。
 * 使用volatile修饰sInstance变量之后，可以确保多个线程之间正确处理sInstance变量。
 * 
 */
class SingleInstance4 {
       private static volatile SingleInstance4 sInstance;
       private SingleInstance4(){}
       public static SingleInstance4 getInstance(){
          if(null==sInstance){
              synchronized(SingleInstance4.class){
                if(null == sInstance){
                   sInstance = new SingleInstance4();
                }
              }
          }
       return sInstance;
       }
    }

/* 在Java中，类的静态初始化会在类被加载时触发，我们利用这个原理，可以实现利用这一特性，
 * 结合内部类，可以实现如下的代码，进行懒汉式创建实例。
 * 
 */
class SingleInstance5 {
      private SingleInstance5(){}
      public static SingleInstance5 getInstance() {
        return SingleInstanceHolder.sInstance;
      }
      private static class SingleInstanceHolder {
        private static SingleInstance5 sInstance = new SingleInstance5();
      }
    }