eclipse不支持sun.*包的问题处理

在项目中使用BASE64Decoder，eclipse的编辑器莫名报错，

Multiple markers at this line 
- Access restriction: The type BASE64Decoder is not accessible due to restriction on 
required library C:\Program Files\Java\jre6\lib\rt.jar 
- Access restriction: The method decodeBuffer(String) from the type CharacterDecoder is 
not accessible due to restriction on required library C:\Program Files\Java\jre6\lib\rt.jar 
- Access restriction: The constructor BASE64Decoder() is not accessible due to 
restriction on required library C:\Program Files\Java\jre6\lib\rt.jar

可见eclipse对rt.jar中的sun.*没有权限可以访问，在项目上右键点击Properties-Java Build Path,选择右侧的Libraries标签，展开JRE System Library，选中Access rules，点击Edit,Add一条rule，其中Resolution选择Accessible，Rule Pattern填入sun/**，保存后错误即消失。