J2EE常见bug

java.lang.IncompatibleClassChangeError: class org.hibernate.cfg.ExtendedMappings has interface org.hibernate.cfg.Mappings as super class

hibernate3.5.5版本后的hibernate3.jar中已经包括annotation，所以不用引用hibernate-annotations.jar和hibernate-commons-annotations.jar。如果引用这2个jar包则会报java.lang.IncompatibleClassChangeError: class org.hibernate.cfg.ExtendedMappings has interface org.hibernate.cfg.Mappings as super class 错误 如果之后出现java.lang.NoClassDefFoundError: javax/persistence/Cacheable这个错误，则需要引用hibernate-jpa-2.0-api-1.0.0.Final.jar这个jar包。

Caused by: java.lang.NoClassDefFoundError: javax/persistence/Entity

hibernate-jpa-2.0-api-1.0.0.Final.jar与 ejb3-persistence.jar有很多重复的类，只用hibernate-jpa-2.0-api-1.0.0.Final.jar就可以 

InvalidPropertyException: Invalid property 'packagesToScan' of bean class [org.springframework.orm.hibernate3.annotation.AnnotationSessionFactoryBean]: No property 'packagesToScan' found at

把spring.jar换成2.5.6及以上的即可