setCharacterEncoding is undefined
When working on a project, after downloading a project from svn to myEclipse , the project reports an error and opens the wrong java file, prompting the following error:
Servlet.service() for servlet jsp threw exception java.lang.Error: Unresolved compilation problem: The method setCharacterEncoding(String) is undefined for the type ServletResponse
Simply put, the setCharacterEncoding method is undefined.
Solution 1
Open the buildpath and find that both servlet.jar and javaEE 5 are imported . All of them have javax.servlet.http.HttpServletRequest , and the default in the project is the class in the imported servlet.jar , and the jar version may be too low, anyway , there is no setCharacterEncoding method in the HttpServletRequest class of the jar package . Just remove servlet.jar .
This method failed.
Solution 2
This method is a method in the interface ServletResponse , but there is no such method in this interface in the referenced servlet.jar . There is also servlet.jar in J2EE , and this method does exist in the servletResponse interface in this jar package . But how to make the project reference this interface in the first place? First delete the reference to servlet.jar , and then add it again.
问题可以解决。但是真的需要每次遇到这个问题都这样吗？难道myEclipse针对兄的jar包，就没有控制引用顺序的方法吗，答案是由。
解决方案3
打开buildpath，切换到Order and Export 选项卡，把J2EE的库移至前边即可，如下图。就这么简单。
总结：之所以出现setCharacterEncoding未定义的错误，是因为引入（或者说使用）的jar包没有这个方法，是真的没有。而有这个方法的jar包又没有被引入。删除错误的jar包固然是个解决办法，但如果有很多个冲突的jar包，或者这些jar包之间不仅仅是版本的关系的时候，使用更改引用顺序将会是一种更好的方法。
之后又遇到了个isWrapperFor 方法未定义的错误。有了上次的经验，去同事那看了下，他那引用的是rt.jar，而我这引用的是class12.jar（我们的jar包是相同的），之索引出现这种问题，可能是引用的顺序造成的。更爱了引用顺序后，然后错误就消失了
方法3 亲测可行。
感谢博主 http://sxyandapp.blog.163.com/blog/static/61488308201282492739161/