Java 单例的线程安全

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双检查double-check

public static JCoDestination getJcoConnectionPool() {
        if (null != jcoDestination) {
            return jcoDestination;
        }
        try {
        	synchronized (JcoConnectionPoolFactory.class) {
        		if(null == jcoDestination){
	        		connect();
	                //得到连接
	                jcoDestination = JCoDestinationManager.getDestination(ABAP_AS_POOLED);
        		}
			}
        } catch (JCoException e) {
            e.printStackTrace();
            logger.info("SAP JCO 连接失败！");
        } catch (Exception e) {
            e.printStackTrace();
            logger.info("SAP JCO 连接异常！");
        }
        return jcoDestination;
    }


 

 更好的处理方式

 Initialization on Demand Holder

 依赖内部类机制，JLS来保证线程安全，不需要自己同步

public class Singleton{
    private static class SingletonHolder{
        private final static Singleton INSTANCE = new Singleton();
    }

    public static Singleton getInstance(){
         return SingletonHolder.INSTANCE;
    }
}