Cannot locate the chosen ObjectFactory implementation: spring

错误信息：

Caused by: Cannot locate the chosen ObjectFactory implementation: spring - [unknown location]
    at org.apache.struts2.config.AbstractBeanSelectionProvider.alias(AbstractBeanSelectionProvider.java:74)
    at org.apache.struts2.config.AbstractBeanSelectionProvider.alias(AbstractBeanSelectionProvider.java:45)
    at org.apache.struts2.config.DefaultBeanSelectionProvider.register(DefaultBeanSelectionProvider.java:363)
    at com.opensymphony.xwork2.config.impl.DefaultConfiguration.reloadContainer(DefaultConfiguration.java:240)
    at com.opensymphony.xwork2.config.ConfigurationManager.getConfiguration(ConfigurationManager.java:67)
    ... 18 more

背景：

引入Struts基本必需包：
    commons-lang3-3.2.jar
    commons-fileupload-1.3.1.jar
    commons-io-2.2.jar
    freemarker-2.3.22.jar
    javassist-3.11.0.GA.jar
    ognl-3.0.6.jar
    struts2-core-2.3.24.1.jar
    xwork-core-2.3.24.1.jar
    
引入Spring-Web包:
    aopalliance-1.0.jar
    commons-logging-1.1.1.jar
    spring-aop-4.2.4.RELEASE.jar
    spring-beans-4.2.4.RELEASE.jar
    spring-context-4.2.4.RELEASE.jar
    spring-core-4.2.4.RELEASE.jar
    spring-expression-4.2.4.RELEASE.jar
    spring-web-4.2.4.RELEASE.jar

Struts.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE struts PUBLIC
    "-//Apache Software Foundation//DTD Struts Configuration 2.3//EN"
    "http://struts.apache.org/dtds/struts-2.3.dtd">

<struts>
   ......
    <!-- 指定由spring负责action对象的创建 -->
    <constant name="struts.objectFactory" value="spring" />
   ......
</struts>

这个时候报这个错误原因就是还缺少一个jar包：
    struts2-spring-plugin-2.3.15.1.jar   它是整合Struts和Spring的Jar