java.lang.LinkageError: loader constraint violation

Maven项目在加入servlet.api和jsp.api后报如下的错误

严重: Servlet.service() for servlet jsp threw exception
java.lang.LinkageError: loader constraint violation: loader (instance of org/apache/jasper/servlet/JasperLoader) previously initiated loading for a different type with name “javax/servlet/http/HttpServletRequest”
at java.lang.Class.getDeclaredMethods0(Native Method)
at java.lang.Class.privateGetDeclaredMethods(Unknown Source)
at java.lang.Class.getDeclaredMethods(Unknown Source)
at org.apache.catalina.util.DefaultAnnotationProcessor.processAnnotations(DefaultAnnotationProcessor.java:226)
at org.apache.jasper.servlet.JspServletWrapper.getServlet(JspServletWrapper.java:148)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:329)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)atorg.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
at java.lang.Thread.run(Unknown Source)

错误原因:eclipse和tomcat自带servlet.api和jsp.api。当在Maven项目中eclipse自带的api并不会自动导入，这时需要在pom.xml手动导入，而这个时候就会和Tomcat自带的包冲突导致错误发生。

解决办法：为手动导入的jar包设置scope范围provided，如下：

javax.servlet
javax.servlet-api
3.1.0
provided

javax.servlet
jsp-api
2.0
provided

保存运行后错误得到解决。

注：provided代表只在编译和测试时运行，当你把项目部署到Tomcat后就不会再启用可以避开Tomcat自带的jar包。