Maven, tomcat7, Simple Web App. java.lang.LinkageError: loader constraint violation

Question

我正在按照书本 "Introducing Maven, Apress" 中的说明创建一个简单的 Web 应用程序：HelloWorld!!在 JSP 中。使用 Maven 和 Tomcat7 并使用以下工件：（顺便说一句，我使用的是 win7）

mvn archetype:generate -DarchetypeArtifactId=maven-archetype-webapp

创建的结构如下：

gswm-web
|_pom.xml
|_src
    |_main
          |_resources
          |_webapp
                  |_WEB-INF
                  |        |_web.xml
                  |_index.jsp

和 pom.xml 如下所示，添加了 tomcat 插件：

<project xmlns="http://maven.apache.org/POM/4.0.0"      
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0    
http://maven.apache.org/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.apress.gswmbook</groupId>
<artifactId>gswm-web</artifactId>
<packaging>war</packaging>
<version>1.0-SNAPSHOT</version>
<name>gswm-web Maven Webapp</name>
<url>http://maven.apache.org</url>

<dependencies>
<dependency>
  <groupId>junit</groupId>
  <artifactId>junit</artifactId>
  <version>3.8.1</version>
  <scope>test</scope>
</dependency>
</dependencies>

<build>
<finalName>gswm-web</finalName>
 <plugins>
  <plugin>
    <groupId>org.apache.tomcat.maven</groupId>
    <artifactId>tomcat7-maven-plugin</artifactId>
    <version>2.2</version>
  </plugin>

</plugins>  

</build>
</project>

然后我运行从项目根目录中的命令行执行以下命令：

mvn tomcat7:run

我得到以下信息：

[INFO] Scanning for projects...
[INFO]
[INFO] --------------------------------------------------------------------   
----
[INFO] Building gswm-web Maven Webapp 1.0-SNAPSHOT
[INFO] --------------------------------------------------------------------- 
---
[INFO]
[INFO] >>> tomcat7-maven-plugin:2.2:run (default-cli) @ gswm-web >>>
Downloading: http://repo.maven.apache.org/maven2/javax/servlet/jsp/jsp-   
api/2.1/jsp-api-2.1.pom
Downloaded: http://repo.maven.apache.org/maven2/javax/servlet/jsp/jsp-   
api/2.1/jsp-api-2.1.pom 
Downloading: http://repo.maven.apache.org/maven2/javax/servlet/jsp/jsp-   
api/2.1/jsp-api-2.1.jar
Downloaded: http://repo.maven.apache.org/maven2/javax/servlet/jsp/jsp-   
api/2.1/jsp-api-2.1.jar 
[INFO]
[INFO] --- maven-resources-plugin:2.5:resources (default-resources) @ gswm-   
web ---
[debug] execute contextualize
[WARNING] Using platform encoding (Cp1252 actually) to copy filtered    
resources, i.e. build is 
platform dependent!
[INFO] skip non existing resourceDirectory 
C:\MauricioFiles\Maven\gswm-book-master\gswm-book-   
master\chapter6\final\gswm-web\src\main\resources
[INFO]
[INFO] --- maven-compiler-plugin:2.3.2:compile (default-compile) @ gswm-web    
---
[INFO] No sources to compile
[INFO]
[INFO] <<< tomcat7-maven-plugin:2.2:run (default-cli) @ gswm-web <<<
[INFO]
[INFO] --- tomcat7-maven-plugin:2.2:run (default-cli) @ gswm-web ---
[INFO] Running war on http://localhost:8080/gswm-web
[INFO] Creating Tomcat server configuration at 
C:\MauricioFiles\Maven\gswm-book-master\gswm-book-   
master\chapter6\final\gswm-web\target\tomcat
[INFO] create webapp with contextPath: /gswm-web
Jun 03, 2015 1:50:08 PM org.apache.coyote.AbstractProtocol init
INFO: Initializing ProtocolHandler ["http-bio-8080"]
Jun 03, 2015 1:50:08 PM org.apache.catalina.core.StandardService    
startInternal
INFO: Starting service Tomcat
Jun 03, 2015 1:50:08 PM org.apache.catalina.core.StandardEngine     
startInternal
INFO: Starting Servlet Engine: Apache Tomcat/7.0.47
Jun 03, 2015 1:50:12 PM org.apache.coyote.AbstractProtocol start
INFO: Starting ProtocolHandler ["http-bio-8080"]

但是当我尝试在端口 8080 的浏览器中显示项目时，我收到以下消息：

Jun 03, 2015 1:50:34 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [jsp] in context with path [/gswm-web]     
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"] 
with root cause
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(Class.java:2688)
    at java.lang.Class.getDeclaredMethods(Class.java:1962)
    at      
 org.apache.catalina.util.Introspection.getDeclaredMethods
 (Introspection.java:127)
    at  
 org.apache.catalina.core.DefaultInstanceManager.populateAnnotationsCache
 (DefaultInstanceManager.java:342)
    at org.apache.catalina.core.DefaultInstanceManager.newInstance
(DefaultInstanceManager.java:161)
    at org.apache.catalina.core.DefaultInstanceManager.newInstance
(DefaultInstanceManager.java:149)
    at org.apache.jasper.servlet.JspServletWrapper.getServlet
(JspServletWrapper.java:172)
    at org.apache.jasper.servlet.JspServletWrapper.service
(JspServletWrapper.java:369)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile
(JspServlet.java:390)
    at org.apache.jasper.servlet.JspServlet.service
(JspServlet.java:334)
    at javax.servlet.http.HttpServlet.service
(HttpServlet.java:728)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter
(ApplicationFilterChain.java:305)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter
(ApplicationFilterChain.java:210)
    at    
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter
(ApplicationFilterChain.java:243)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter
(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke
(StandardWrapperValve.java:222)
    at org.apache.catalina.core.StandardContextValve.invoke
(StandardContextValve.java:123)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke
(AuthenticatorBase.java:502)
    at org.apache.catalina.core.StandardHostValve.invoke
(StandardHostValve.java:171)
    at org.apache.catalina.valves.ErrorReportValve.invoke
(ErrorReportValve.java:100)
    at org.apache.catalina.valves.AccessLogValve.invoke
(AccessLogValve.java:953)
    at org.apache.catalina.core.StandardEngineValve.invoke
(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service
(CoyoteAdapter.java:408)
    at org.apache.coyote.http11.AbstractHttp11Processor.process
(AbstractHttp11Processor.java:1041)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process
(AbstractProtocol.java:603)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run
(JIoEndpoint.java:310)
    at java.util.concurrent.ThreadPoolExecutor.runWorker
(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run
(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

在我尝试过的其他事情中，做了一些研究，我在 pom 中添加了以下几行，但没有成功：

<dependency>
    <groupId>javax.servlet.jsp</groupId>
    <artifactId>jsp-api</artifactId>
    <version>2.1</version>
    <scope>provided</scope>
</dependency>

有人知道如何解决这个问题吗？。非常感谢

Answer 1

对 javax.servlet-api 神器执行相同的操作：

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>xxxx</version><!-- Whatever version you are using -->
    <scope>provided</scope>
</dependency>

实际上，违规的 class、HttpServletRequest 就在这个神器中。

Answer 2

pom.xml 中的 servlet-api 依赖项需要 "provided" 范围。这是因为 Tomcat 已经提供（需要并使用自身）servlet-api 依赖项。 Maven 的依赖范围规则定义在这里

http://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#Dependency_Scope

<dependency>
   <groupId>javax.servlet</groupId>
   <artifactId>javax.servlet-api</artifactId>
   <version>xxxx</version><!-- Whatever version you are using -->
   <scope>provided</scope>
</dependency>

Maven, tomcat7, Simple Web App. java.lang.LinkageError: loader constraint violation

Maven, tomcat7, Simple Web App. java.lang.LinkageError: loader constraint violation

maven

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