我们可能会用各种应用服务部署我们的Java应用，比如Tomcat、WAS、weblogic等。Tomcat和WAS可能会比较少遇到一些奇怪的问题，但是用weblogic部署项目则经常遇到一些比如包冲突问题，路径问题等奇怪但又常见的问题。
今天我就讲讲关于weblogic部署Java项目包冲突的问题。下面我举个例子：
当我在weblogic部署Java项目之后，启动没报任何错，没有异常。但是当我操作某个功能的时候页面就报错了：
后台报了这个错：
Root cause of ServletException. java.lang.LinkageError: loader constraint violation: loader (instance of weblogic/utils/classloaders/ChangeAwareClassLoader)
previously initiated loading for a different type with name "javax/xml/namespace/QName" at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClass(ClassLoader.java:763) at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) at weblogic.utils.classloaders.GenericClassLoader.defineClass(GenericClassLoader.java:343) at weblogic.utils.classloaders.GenericClassLoader.findLocalClass(GenericClassLoader.java:302) at weblogic.utils.classloaders.GenericClassLoader.findClass(GenericClassLoader.java:270) at weblogic.utils.classloaders.ChangeAwareClassLoader.findClass(ChangeAwareClassLoader.java:64) at weblogic.utils.classloaders.ChangeAwareClassLoader.loadClass(ChangeAwareClassLoader.java:49) at java.lang.Class.getDeclaredMethods0(Native Method) at java.lang.Class.privateGetDeclaredMethods(Class.java:2701) at java.lang.Class.privateGetMethodRecursive(Class.java:3048) at java.lang.Class.getMethod0(Class.java:3018) at java.lang.Class.getMethod(Class.java:1784) at org.apache.xmlbeans.XmlBeans.buildMethod(XmlBeans.java:174) at org.apache.xmlbeans.XmlBeans.buildNoArgMethod(XmlBeans.java:190) at org.apache.xmlbeans.XmlBeans.buildGetContextTypeLoaderMethod(XmlBeans.java:200) at org.apache.xmlbeans.XmlBeans.<clinit>(XmlBeans.java:126) at org.openxmlformats.schemas.spreadsheetml.x2006.main.CTWorkbook$Factory.newInstance(Unknown Source) at org.apache.poi.xssf.usermodel.XSSFWorkbook.onWorkbookCreate(XSSFWorkbook.java:290) at org.apache.poi.xssf.usermodel.XSSFWorkbook.<init>(XSSFWorkbook.java:170) at com.kayak.web.base.util.export.ExportExcelXSSF.export(ExportExcelXSSF.java:893) at com.kayak.web.base.action.ExportExcelAction.exportExcel(ExportExcelAction.java:318) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:221) at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:137) at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:110) at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:777) at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:706) at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85) at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:943) at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:877) at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966) at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:868) at javax.servlet.http.HttpServlet.service(HttpServlet.java:727) at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842) at javax.servlet.http.HttpServlet.service(HttpServlet.java:820) at weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(StubSecurityHelper.java:227) at weblogic.servlet.internal.StubSecurityHelper.invokeServlet(StubSecurityHelper.java:125) at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:301) at weblogic.servlet.internal.TailFilter.doFilter(TailFilter.java:26) at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:60) at com.kayak.web.user.filter.LoginCertifyFilter.doFilter(LoginCertifyFilter.java:125) at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:60) at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:60) at com.kayak.web.base.filter.LocalRequestFilter.doFilter(LocalRequestFilter.java:28) at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:60) at weblogic.servlet.internal.RequestEventsFilter.doFilter(RequestEventsFilter.java:27) at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:60) at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.wrapRun(WebAppServletContext.java:3748) at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.run(WebAppServletContext.java:3714) at weblogic.security.acl.internal.AuthenticatedSubject.doAs(AuthenticatedSubject.java:321) at weblogic.security.service.SecurityManager.runAs(SecurityManager.java:120) at weblogic.servlet.internal.WebAppServletContext.securedExecute(WebAppServletContext.java:2283) at weblogic.servlet.internal.WebAppServletContext.execute(WebAppServletContext.java:2182) at weblogic.servlet.internal.ServletRequestImpl.run(ServletRequestImpl.java:1499) at weblogic.work.ExecuteThread.execute(ExecuteThread.java:263) at weblogic.work.ExecuteThread.run(ExecuteThread.java:221)
错误内容很长，但是要学会辨认，我刚开始就被一大堆错误误导了。关键的都是下面的这个：
loader (instance of weblogic/utils/classloaders/ChangeAwareClassLoader) previously initiated loading for a different type with name "javax/xml/namespace/QName"
大概意思就是“加载器之前启动的时候加载了名字叫javax/xml/namespace/QName的其他类型的类”。其实转换一下就是，在启动的时候加载了一个javax/xml/namespace/QName类，我们现在要用的功能需要一个类也叫javax/xml/namespace/QName，但是这个不是我们真正想要的class。
从上面的意思可以知道，这就是有两个相同包包路径和类名称的class，但是有一个想要的却没有，只有一个我们并不想要的class。了解weblogic部署的同学都知道，weblogic加载了一个相同的class就不会在加载其他一样的class了。
当我再操作一次的之后又报另一个错（部分错误内容）：
1 Root cause of ServletException. 2 java.lang.NoClassDefFoundError: Could not initialize class org.apache.xmlbeans.XmlBeans 3 at org.openxmlformats.schemas.spreadsheetml.x2006.main.CTWorkbook$Factory.newInstance(Unknown Source) 4 at org.apache.poi.xssf.usermodel.XSSFWorkbook.onWorkbookCreate(XSSFWorkbook.java:290) 5 at org.apache.poi.xssf.usermodel.XSSFWorkbook.<init>(XSSFWorkbook.java:170) 6 at com.kayak.web.base.util.expor
