org.hibernate.MappingException

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
%%%% Error Creating SessionFactory %%%%
org.hibernate.MappingException: An association from the table author_book refers to an unmapped class: Author
	at org.hibernate.cfg.Configuration.secondPassCompileForeignKeys(Configuration.java:1252)
	at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1170)
	at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1286)
	at po.HibernateSessionFactory.<clinit>(HibernateSessionFactory.java:31)
	at po.Test.select(Test.java:17)
	at po.Test.main(Test.java:12)
%%%% Error Creating SessionFactory %%%%
org.hibernate.InvalidMappingException: Could not parse mapping document from resource po/Author.hbm.xml
	at org.hibernate.cfg.Configuration.addResource(Configuration.java:569)
	at org.hibernate.cfg.Configuration.parseMappingElement(Configuration.java:1587)
	at org.hibernate.cfg.Configuration.parseSessionFactory(Configuration.java:1555)
	at org.hibernate.cfg.Configuration.doConfigure(Configuration.java:1534)
	at org.hibernate.cfg.Configuration.doConfigure(Configuration.java:1508)
	at org.hibernate.cfg.Configuration.configure(Configuration.java:1428)
	at po.HibernateSessionFactory.rebuildSessionFactory(HibernateSessionFactory.java:69)
	at po.HibernateSessionFactory.getSession(HibernateSessionFactory.java:53)
	at po.Test.select(Test.java:17)
	at po.Test.main(Test.java:12)
Caused by: org.hibernate.DuplicateMappingException: Duplicate collection role mapping po.Author.bookSet
	at org.hibernate.cfg.Mappings.addCollection(Mappings.java:124)
	at org.hibernate.cfg.HbmBinder.createClassProperties(HbmBinder.java:2066)
	at org.hibernate.cfg.HbmBinder.createClassProperties(HbmBinder.java:2041)
	at org.hibernate.cfg.HbmBinder.bindRootPersistentClassCommonValues(HbmBinder.java:359)
	at org.hibernate.cfg.HbmBinder.bindRootClass(HbmBinder.java:273)
	at org.hibernate.cfg.HbmBinder.bindRoot(HbmBinder.java:144)
	at org.hibernate.cfg.Configuration.add(Configuration.java:669)
	at org.hibernate.cfg.Configuration.addInputStream(Configuration.java:504)
	at org.hibernate.cfg.Configuration.addResource(Configuration.java:566)
	... 9 more
Exception in thread "main" java.lang.NullPointerException
	at po.Test.select(Test.java:18)
	at po.Test.main(Test.java:12)

   修改的办法：
         在多对多的关系中使用myeclipse容易出现这种错误：
            
      比如：
 

     <set name="authorSet" table="author_book" inverse="true" cascade="save-update"  >
            <key column="bid"/>
            <many-to-many class="Author" column="aid"/>
        </set>

   

   在many-to-many 中class没有加上包名导致的以上错误：
   修改如下：

     <set name="authorSet" table="author_book" inverse="true" cascade="save-update"  >
            <key column="bid"/>
            <many-to-many class="com.ch.army.Author" column="aid"/>
        </set>