public class A {
private static A a;
private A(){
}

public A getInstance(){
   if(a == null)
        a = new A();
   return a;
}
}

------解决方案--------------------
你在xml配置文件配置了BaseDAO吗？
XML code
    <bean id="dao" class="com.XXX.BaseDAO"
        abstract="false" lazy-init="default" autowire="default"
        dependency-check="default">
        <property name="sessionFactory">
            <ref bean="sessionFactory"></ref>
        </property>
    </bean>

------解决方案--------------------
public class BooksContext {
          //这个地方是关键
	 private static ApplicationContext content=new ClassPathXmlApplicationContext("applicationContext.xml");;

	public static void getnewsTypeById(HttpServletRequest request) {
		SimpleDateFormat  sdf=new SimpleDateFormat("yyyy年MM月dd日hh点mm分");  
		Date date=new Date();  
		String d=sdf.format(date);  
		System.out.println(d);
                //这个地方是关键
		BookService b = (BookService) content.getBean(
				"BookServiceImpl");
		List<BooksLordType> list = b.getAllBooksLordType();
		request.setAttribute("BooksLordTypeList", list);
	}