Java单例的5中写法

Java5种单例写法。

public class Resource {

	/**
	 * 直接初始化
	 * **/
	private static Resource resource_static = new Resource();
	
	public static Resource getResourceInstanceStatic(){
		return resource_static;
	}
	
	/**
	 * 利用synchronized来实现的线程安全
	 * **/
	private static Resource resource_synchronized = null;
	
	public static synchronized Resource getResourceInstanceSynchronized(){
		if(null==resource_synchronized){
			resource_synchronized = new Resource();
		}
		return resource_synchronized;
	}
	
	/**
	 * 利用JVM本身类加载机制实现的lazy初始化
	 * **/
	private static class ResourceHolder{
		public static Resource resource = new Resource();
	}
	public static Resource getResourceInstanceClass(){		
		return ResourceHolder.resource;
	}
	
	/**
	 * DCL double check locking 
	 * 线程不安全 不推荐
	 * **/
	private static Resource resource_dcl = null;
	
	public static Resource getResourceInstanceDCL(){
		if(null==resource_dcl){
			synchronized(Resource.class){
				if(null==resource_dcl){
					resource_dcl = new Resource();
				}
			}
		}
		return resource_dcl;
	}
	
	/**
	 * 线程不安全 不推荐
	 * **/
	private static Resource resource_unsafe = null;
	
	public static Resource getResourceInstanceUnSafe(){
		if(null==resource_unsafe){
			resource_unsafe = new Resource();
		}
		return resource_unsafe;
	}
}