会话Bean引用实体Bean的时候出现了NullPointer异常，在线等这里的高手指点，谢谢拉！

Question

这是写在Session Bean中：
InitialContext ctx=new InitialContext();
BookLocalHome bookHome=(BookLocalHome)ctx.lookup("RefBookBean");
BookLocal book=bookHome.create();
if(book==null){
System.out.println("the book object is null!");
                        }
ejb-jar.xml文件:
 <enterprise-beans>
          <session >
         <description><![CDATA[Description for BookSession]]></description>
         <display-name>Name for BookSession</display-name>         <ejb-name>BookSession</ejb-name>         <home>com.xiaoliang.interfaces.BookSessionHome</home>
         <remote>com.xiaoliang.interfaces.BookSession</remote>
         <ejb-class>com.xiaoliang.ejb.BookSessionBean</ejb-class>
         <session-type>Stateless</session-type>
         <transaction-type>Container</transaction-type>
         <ejb-local-ref>
         <ejb-ref-name>RefBookBean</ejb-ref-name>
         <ejb-ref-type>Entity</ejb-ref-type>
         <local-home>com.xiaoliang.interfaces.BookLocalHome</local-home>
         <local>com.xiaoliang.interfaces.BookLocal</local>
         <ejb-link>BookLocal</ejb-link>
         </ejb-local-ref>
      </session>
      <entity >
         <description><![CDATA[Description for Borrow]]></description>
         <display-name>Name for Borrow</display-name>         <ejb-name>BorrowLocal</ejb-name>         <local-home>com.xiaoliang.interfaces.BorrowLocalHome</local-home>
         <local>com.xiaoliang.interfaces.BorrowLocal</local>         <ejb-class>com.xiaoliang.ejb.BorrowBean</ejb-class>
         <persistence-type>Container</persistence-type>
         <prim-key-class>java.lang.String</prim-key-class>
         <reentrant>False</reentrant>
         <cmp-version>2.x</cmp-version>
         <abstract-schema-name>BorrowLocal</abstract-schema-name>
         <cmp-field>
           <field-name>BookId</field-name>
         </cmp-field>
          <cmp-field>
           <field-name>BookName</field-name>
         </cmp-field>
         <cmp-field>
           <field-name>BorrowUser</field-name>
         </cmp-field>
          <cmp-field>
           <field-name>BorrowDate</field-name>
         </cmp-field>
         <cmp-field>
           <field-name>ReturnDate</field-name>
         </cmp-field>
         <primkey-field>BorrowUser</primkey-field>
               </entity>      <entity >
         <description><![CDATA[Description for Book]]></description>
         <display-name>Name for Book</display-name>         <ejb-name>BookLocal</ejb-name>         <local-home>com.xiaoliang.interfaces.BookLocalHome</local-home>
         <local>com.xiaoliang.interfaces.BookLocal</local>         <ejb-class>com.xiaoliang.ejb.BookBean</ejb-class>
         <persistence-type>Bean</persistence-type>
         <prim-key-class>java.lang.String</prim-key-class>
         <reentrant>False</reentrant>
         <abstract-schema-name>BookLocal</abstract-schema-name>
         </entity>
jboss.xml文件：
<jboss>
  <enterprise-beans>
    <session>
       <ejb-name>BookSession</ejb-name>
       <local-jndi-name>BookSession</local-jndi-name>
    </session>
    <entity>
      <ejb-name>BookLocal</ejb-name>
      <local-jndi-name>BookLocal</local-jndi-name>
    </entity>
     <entity>
      <ejb-name>BorrowLocal</ejb-name>
      <local-jndi-name>BorrowLocal</local-jndi-name>
    </entity>
  </enterprise-beans>
</jboss>
运行之后出现java.lang.NullPointer异常，打印出the book object is null
也不知哪出了问题，跪等高人指点！！！