Spring容器式

优点：线程安全，原子性、可见性、有序性得到保证
缺点：性能不好，消耗的资源较多

public class ContainerSingleton {
    private ContainerSingleton() {
    }
    private static Map<String, Object> ioc = new ConcurrentHashMap<>();
    public static Object getBean(String className) {
        synchronized (ioc) {
            if (!ioc.containsKey(className)) {
                Object obj = null;
                try {
                    obj = Class.forName(className).newInstance();
                    ioc.put(className, obj);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                return obj;
            }
            return ioc.get(className);
        }
    }
}