effective_71慎用延迟初始化

实例域 延迟初始化 可以采用 synchronize方法或者 双重校验锁来完成
类实例域 采用 lazy initialization holder class 模式
正常初始化 优先于 延迟初始化

package book.effectiveJava;

public class _71HadBetterNotLazyInitClass {

	private static class Holder{
		//延迟初始化方式类变量 1
		private static final Target target= new Target();
		public static Target getTarget(){
			return target;
		}
	}

	private static class Target{

		public   InnerTarget instance ;

		private Target(){

		}

		//延迟初始化 字段变量2
		public  InnerTarget getTarget(){
			if(instance==null){
				synchronized (this){
					if(instance==null){
						instance=new InnerTarget();
					}
				}
			}
			return instance;
		}

		//初始化 类变量 字段
		public synchronized InnerTarget getTarget2(){
			if(instance==null){
				instance=new InnerTarget();
			}
			return instance;
		}
		private static class InnerTarget{

		}
	}

	public static void main(String[] args) {
		Object obj1 = Holder.getTarget().getTarget();
		Object obj2 = Holder.getTarget().getTarget();
		System.out.println(obj1==obj2);
	}
}