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源码分析

org.springframework.boot.web.servlet.server.DocumentRoot

/**	 * Returns the absolute document root when it points to a valid directory, logging a	 * warning and returning {@code null} otherwise.	 * @return the valid document root	 */	final File getValidDirectory() {		File file = this.directory;        // If document root not explicitly set see if we are running from a war archive		file = (file != null) ? file : getWarFileDocumentRoot();        // If not a war archive maybe it is an exploded war		file = (file != null) ? file : getExplodedWarFileDocumentRoot();        // Or maybe there is a document root in a well-known location		file = (file != null) ? file : getCommonDocumentRoot();		if (file == null && this.logger.isDebugEnabled()) {			logNoDocumentRoots();		}		else if (this.logger.isDebugEnabled()) {			this.logger.debug("Document root: " + file);		}		return file;	}

发现有三种取路径方式： 

war包 getWarFileDocumentRoot

导出包 getExplodedWarFileDocumentRoot

文档 getCommonDocumentRoot

内置tomcat启动应该属于第三种。

private static final String[] COMMON_DOC_ROOTS = { "src/main/webapp", "public","static" };

private File getCommonDocumentRoot() {		for (String commonDocRoot : COMMON_DOC_ROOTS) {			File root = new File(commonDocRoot);			if (root.exists() && root.isDirectory()) {				return root.getAbsoluteFile();			}		}		return null;	}

百度得知 取的是

System.getProperty("user.dir")

相当于 

File root = new File(System.getProperty("user.dir")+"src/main/webapp")；

输出 System.getProperty("user.dir") 发现是项目层目录，而不是模块层目录

解决方案

方法一：启动项增加配置参数

方法二：Spring Boot 插件启动

参考文章

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