maven项目打jar包时报错，Unable to locate Spring NamespaceHandler for XML schema namespace

错误信息

在IDEA里运行正常，一到jar里就报下面这个错误：

org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions(XmlBeanDefinitionReader.java:317) -  Loading XML bean definitions from class path resource [applicationContext.xml]
Exception in thread "main" java.lang.ExceptionInInitializerError
        at com.abc.timetask.MyTimeTask.<init>(MyTimeTask.java:24)
        at com.abc.command.MyCommand.run(MyCommand.java:71)
        at com.abc.command.CliMain.main(CliMain.java:31)
Caused by: org.springframework.beans.factory.parsing.BeanDefinitionParsingException: Configuration problem: Unable to locate Spring NamespaceHandler for XML schema namespace [http://www.springframework.org/schema/context]
Offending resource: class path resource [applicationContext.xml]

        at org.springframework.beans.factory.parsing.FailFastProblemReporter.error(FailFastProblemReporter.java:70)
        at org.springframework.beans.factory.parsing.ReaderContext.error(ReaderContext.java:85)
        at org.springframework.beans.factory.parsing.ReaderContext.error(ReaderContext.java:80)
        at org.springframework.beans.factory.xml.BeanDefinitionParserDelegate.error(BeanDefinitionParserDelegate.java:301)
        at org.springframework.beans.factory.xml.BeanDefinitionParserDelegate.parseCustomElement(BeanDefinitionParserDelegate.java:1408)
        at org.springframework.beans.factory.xml.BeanDefinitionParserDelegate.parseCustomElement(BeanDefinitionParserDelegate.java:1401)
        at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.parseBeanDefinitions(DefaultBeanDefinitionDocumentReader.java:172)
        at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.doRegisterBeanDefinitions(DefaultBeanDefinitionDocumentReader.java:142)
        at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.registerBeanDefinitions(DefaultBeanDefinitionDocumentReader.java:94)
        at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.registerBeanDefinitions(XmlBeanDefinitionReader.java:508)
        at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.doLoadBeanDefinitions(XmlBeanDefinitionReader.java:392)
        at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions(XmlBeanDefinitionReader.java:336)
        at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions(XmlBeanDefinitionReader.java:304)
        at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:181)
        at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:217)
        at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:188)
        at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:252)
        at org.springframework.context.support.AbstractXmlApplicationContext.loadBeanDefinitions(AbstractXmlApplicationContext.java:127)
        at org.springframework.context.support.AbstractXmlApplicationContext.loadBeanDefinitions(AbstractXmlApplicationContext.java:93)
        at org.springframework.context.support.AbstractRefreshableApplicationContext.refreshBeanFactory(AbstractRefreshableApplicationContext.java:129)
        at org.springframework.context.support.AbstractApplicationContext.obtainFreshBeanFactory(AbstractApplicationContext.java:613)
        at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:514)
        at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
        at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
        at com.abc.database.SpringHolder.getContext(SpringHolder.java:21)
        at com.abc.database.SpringHolder.<clinit>(SpringHolder.java:17)
        ... 3 more

原因

本来以为打包时META-INF放错文件夹，放到src/main/resources后打了十多次还是这样。虽然也不知道为啥之前没有问题，但是试了十多遍应该是其他错误导致的。谷歌了一下，Stack Overflow上的答案也不对，直到找到一个博客：
IDEA 打包 maven 项目打包成 jar 文件后，启动错误。Unable to locate Spring NamespaceHandler for XML schema namespace阐述了为什么会导致这个错误的产生。

Spring如何加载XSD文件里面详细描述了spring加载xsd的流程，我遇到的错误就是由于打jar包后META-INF/spring.schemas里面只有spring aop的URL和本地xsd文件映射，没有spring context的映射。按理来说正常的话，应该有spring bean/context/aop所有的映射。

解决方法

在pom.xml里面添加如下的代码：

<?xml version="1.0" encoding="UTF-8"?>
<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/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    
    ...
    
    <build>
        <plugins>
           
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-shade-plugin</artifactId>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>shade</goal>
                        </goals>
                        <configuration>
                            <transformers>
                                <transformer implementation="org.apache.maven.plugins.shade.resource.AppendingTransformer">
                                    <resource>META-INF/spring.handlers</resource>
                                </transformer>
                                 <!-这句话作用是把spring-*.jar所有包里的spring.schemas合并，避免像之前每打包一个spring-*.jar到
                                 可执行jar中，之后的spring.schema就s把之前的覆盖了。-->
                                <transformer implementation="org.apache.maven.plugins.shade.resource.AppendingTransformer">
                                    <resource>META-INF/spring.schemas</resource>
                                </transformer>
                            </transformers>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>                    <!--打jar包的插件-->
                <configuration>
                    <archive>
                        <manifest>
                            <addClasspath>true</addClasspath>
                            <useUniqueVersions>false</useUniqueVersions>
                            <classpathPrefix>lib/</classpathPrefix>
                            <mainClass>com.abc.command.CliMain</mainClass>    <!--jar包入口类-->
                        </manifest>
                    </archive>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

然后使用IDEA中maven package插件打包，不能直接build打包，试了会出错。

成功打包后，在target文件夹生成了jar。