SAXParseException : 文档根元素 "beans" 必须匹配 DOCTYPE 根 "null"

我在将XML配置文件命名为 *application.xml* 时启动会报 *文档根元素 "beans" 必须匹配 DOCTYPE 根 "null"。* 的错误。
此时还没有使用 `@ImportResource` 注解来导入。貌似 *application.xml* 这个名字比较特殊，自动会导入。
*application.xml* 文件仅包含根元素 *beans* 时也还是报错。

完整错误消息如下：

```java
15:47:13.297 [main] ERROR org.springframework.boot.SpringApplication - Application run failed
java.lang.IllegalStateException: Failed to load property source from location 'classpath:/application.xml'
	at org.springframework.boot.context.config.ConfigFileApplicationListener$Loader.load(ConfigFileApplicationListener.java:554)
	at org.springframework.boot.context.config.ConfigFileApplicationListener$Loader.loadForFileExtension(ConfigFileApplicationListener.java:500)
	at org.springframework.boot.context.config.ConfigFileApplicationListener$Loader.load(ConfigFileApplicationListener.java:467)
	at org.springframework.boot.context.config.ConfigFileApplicationListener$Loader.lambda$null$6(ConfigFileApplicationListener.java:448)
	at java.lang.Iterable.forEach(Iterable.java:75)
	at org.springframework.boot.context.config.ConfigFileApplicationListener$Loader.lambda$load$7(ConfigFileApplicationListener.java:447)
	at java.lang.Iterable.forEach(Iterable.java:75)
	at org.springframework.boot.context.config.ConfigFileApplicationListener$Loader.load(ConfigFileApplicationListener.java:444)
	at org.springframework.boot.context.config.ConfigFileApplicationListener$Loader.load(ConfigFileApplicationListener.java:332)
	at org.springframework.boot.context.config.ConfigFileApplicationListener.addPropertySources(ConfigFileApplicationListener.java:207)
	at org.springframework.boot.context.config.ConfigFileApplicationListener.postProcessEnvironment(ConfigFileApplicationListener.java:190)
	at org.springframework.boot.context.config.ConfigFileApplicationListener.onApplicationEnvironmentPreparedEvent(ConfigFileApplicationListener.java:177)
	at org.springframework.boot.context.config.ConfigFileApplicationListener.onApplicationEvent(ConfigFileApplicationListener.java:163)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:172)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:165)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:127)
	at org.springframework.boot.context.event.EventPublishingRunListener.environmentPrepared(EventPublishingRunListener.java:75)
	at org.springframework.boot.SpringApplicationRunListeners.environmentPrepared(SpringApplicationRunListeners.java:54)
	at org.springframework.boot.SpringApplication.prepareEnvironment(SpringApplication.java:347)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:306)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1260)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1248)
	at liujiajia.me.learning.mybatis.Application.main(Application.java:14)
	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 com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)
Caused by: java.util.InvalidPropertiesFormatException: org.xml.sax.SAXParseException; lineNumber: 5; columnNumber: 72; 文档根元素 "beans" 必须匹配 DOCTYPE 根 "null"。
	at sun.util.xml.PlatformXmlPropertiesProvider.load(PlatformXmlPropertiesProvider.java:80)
	at java.util.Properties$XmlSupport.load(Properties.java:1201)
	at java.util.Properties.loadFromXML(Properties.java:881)
	at org.springframework.core.io.support.PropertiesLoaderUtils.fillProperties(PropertiesLoaderUtils.java:137)
	at org.springframework.core.io.support.PropertiesLoaderUtils.loadProperties(PropertiesLoaderUtils.java:122)
	at org.springframework.boot.env.PropertiesPropertySourceLoader.loadProperties(PropertiesPropertySourceLoader.java:59)
	at org.springframework.boot.env.PropertiesPropertySourceLoader.load(PropertiesPropertySourceLoader.java:47)
	at org.springframework.boot.context.config.ConfigFileApplicationListener$Loader.loadDocuments(ConfigFileApplicationListener.java:572)
	at org.springframework.boot.context.config.ConfigFileApplicationListener$Loader.load(ConfigFileApplicationListener.java:526)
	... 28 common frames omitted
Caused by: org.xml.sax.SAXParseException: 文档根元素 "beans" 必须匹配 DOCTYPE 根 "null"。
	at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAXParseException(ErrorHandlerWrapper.java:203)
	at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.error(ErrorHandlerWrapper.java:134)
	at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(XMLErrorReporter.java:396)
	at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(XMLErrorReporter.java:327)
	at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(XMLErrorReporter.java:284)
	at com.sun.org.apache.xerces.internal.impl.dtd.XMLDTDValidator.rootElementSpecified(XMLDTDValidator.java:1599)
	at com.sun.org.apache.xerces.internal.impl.dtd.XMLDTDValidator.handleStartElement(XMLDTDValidator.java:1877)
	at com.sun.org.apache.xerces.internal.impl.dtd.XMLDTDValidator.startElement(XMLDTDValidator.java:742)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanStartElement(XMLDocumentFragmentScannerImpl.java:1358)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$ContentDriver.scanRootElementHook(XMLDocumentScannerImpl.java:1295)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:3129)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$PrologDriver.next(XMLDocumentScannerImpl.java:880)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:606)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:504)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:848)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:777)
	at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:141)
	at com.sun.org.apache.xerces.internal.parsers.DOMParser.parse(DOMParser.java:243)
	at com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderImpl.parse(DocumentBuilderImpl.java:339)
	at sun.util.xml.PlatformXmlPropertiesProvider.getLoadingDoc(PlatformXmlPropertiesProvider.java:106)
	at sun.util.xml.PlatformXmlPropertiesProvider.load(PlatformXmlPropertiesProvider.java:78)
	... 36 common frames omitted
```

*application.xml* 文件的内容：

```xml
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd">
    ...
</beans>
```

将配置文件改名为 *applicationContext.xml* 就不会报错了，但是并没有读入配置。
此时需要在启动类上添加 `@ImportResource("classpath:applicationContext.xml")` 注解。

```java
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.ImportResource;

/**
 * Created by liujiajia on 2019/1/23.
 */
@SpringBootApplication
@ImportResource("classpath:applicationContext.xml")
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}
```

在 [这篇博客](https://blog.csdn.net/dounine/article/details/54344383) 里跟我遇到的问题和解决方法是一样的，只是说了不能起名为 *application.xml*，但并没有说为什么不能起这个名字。

在 [另一片博客](https://blog.csdn.net/weixin_37778823/article/details/82077772) 里说是 *重复配置了同一个bean配置文件* 导致的，但这里只有这么一个配置文件。

在 [另另一片博客](https://blog.csdn.net/lhy66/article/details/6337327) 里说是 *spring1.X使用DOCTYPE，而2.X版本使用schema。* 导致的，但按照文章修改了仍不起作用。

So，如果你知道为什么不能使用 *application.xml* 这个名字，请回复我。

#### 参考

1. [SpringBoot 文档根元素 "beans" 必须匹配 DOCTYPE 根 "null"](https://blog.csdn.net/dounine/article/details/54344383)
2. [文档根元素 "beans" 必须匹配 DOCTYPE 根 "null"](https://blog.csdn.net/weixin_37778823/article/details/82077772)
3. [org.xml.sax.SAXParseException: Document root element "beans", must match DOCTYPE root "null".](https://blog.csdn.net/lhy66/article/details/6337327)

SAXParseException : 文档根元素 "beans" 必须匹配 DOCTYPE 根 "null"

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