系统迁移报This parser does not support specification “null“ version “null“

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部署灰度环境时，因commons-digester不同版本冲突，导致服务启动失败。通过调整jar包加载顺序，删除1.7版本，保留2.1版本，成功解决java.lang.UnsupportedOperationException错误。

今天部署灰度环境，使用以前的程序jar包，启动报错(java.lang.UnsupportedOperationException: This parser does not support specification "null" version "null")，
经查证是由于commons-digester程序包冲突导致（可能是其他jar冲突，仔细检查），删除commons-digester-1.7.jar重启服务，仍然报同样错误，还原删除commons-digester-2.1.jar，然后重启，服务正常无报错。

jar加载顺序：加载jar包和配置文件的时候，排在前面的classpath被加载，后面的将不会被加载，加载顺序和jar的顺序不同的机器，在JVM启动时，加上如下参数：-verbose:class参数可以查看class从哪个jar加载的，ps
原来的进程加载顺序：/home/lib/commons-digester-1.7.jar，/home/lib/commons-digester-2.1.jar
迁移后进程的加载顺序：/home/lib/commons-digester-2.1.jar，/home/lib/commons-digester-1.7.jar

报错冲突的程序：
[root@ea066cec59a1 lib]# ls -lrt|grep -i Digester
-rw-r----- 1 root root 139966 Apr 14 2019 commons-digester-1.7.jar
-rw-r----- 1 root root 196768 Apr 15 2019 commons-digester-2.1.jar

报错堆栈：
[2020-07-30 13:00:11,650] [main] (Digester.java:794) ERROR org.apache.commons.digester.Digester - Digester.getParser:
java.lang.UnsupportedOperationException: This parser does not support specification "null" version "null"
at javax.xml.parsers.SAXParserFactory.setSchema(SAXParserFactory.java:394)
at org.apache.commons.digester.Digester.getFactory(Digester.java:541)
at org.apache.commons.digester.Digester.getParser(Digester.java:791)
at org.apache.commons.digester.Digester.getXMLReader(Digester.java:1068)
at org.apache.commons.digester.Digester.parse(Digester.java:1916)
at com.xx.xxx.complex.xml.XMLHelper.createDefaults(XMLHelper.java:320)
at com.xx.xxx.complex.xml.XMLHelper.getInstance(XMLHelper.java:83)
at com.xx.xxx.complex.datasource.DataSourceFactory.<clinit>(DataSourceFactory.java:23)
at com.xx.xxxx.Startup.contextInitialized(Startup.java:25)
at com.xx.xxxx.Startup.main(Startup.java:67)
07-30 13:00:11.650 org.apache.commons.digester.Digester ERROR - Digester.getParser:
java.lang.UnsupportedOperationException: This parser does not support specification "null" version "null"
at javax.xml.parsers.SAXParserFactory.setSchema(SAXParserFactory.java:394)
at org.apache.commons.digester.Digester.getFactory(Digester.java:541)
at org.apache.commons.digester.Digester.getParser(Digester.java:791)
at org.apache.commons.digester.Digester.getXMLReader(Digester.java:1068)
at org.apache.commons.digester.Digester.parse(Digester.java:1916)
at com.xx.xxx.complex.xml.XMLHelper.createDefaults(XMLHelper.java:320)
at com.xx.xxx.complex.xml.XMLHelper.getInstance(XMLHelper.java:83)
at com.xx.xxx.complex.datasource.DataSourceFactory.<clinit>(DataSourceFactory.java:23)
at com.xx.xxxx.Startup.contextInitialized(Startup.java:25)
at com.xx.xxxx.Startup.main(Startup.java:67)