spring-mvc源码-bean定义加载-默认标签解析(import)

import标签的解析是调用的org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader#importBeanDefinitionResource方法，源码如下：

	protected void importBeanDefinitionResource(Element ele) {
		String location = ele.getAttribute(RESOURCE_ATTRIBUTE);
		if (!StringUtils.hasText(location)) {
			getReaderContext().error("Resource location must not be empty", ele);
			return;
		}

		//从上下文获取系统属性，解析${}占位符并使用相应的属性值来代替
		location = getReaderContext().getEnvironment().resolveRequiredPlaceholders(location);

		Set actualResources = new LinkedHashSet(4);

		//这里是对import的位置做出判断,是相对还是绝对路径,默认是相对
		boolean absoluteLocation = false;
		try {
			absoluteLocation = ResourcePatternUtils.isUrl(location) || ResourceUtils.toURI(location).isAbsolute();
		}
		catch (URISyntaxException ex) {
			// cannot convert to an URI, considering the location relative
			// unless it is the well-known Spring prefix "classpath*:"
		}

		// Absolute or relative?
		if (absoluteLocation) {
			try {
				int importCount = getReaderContext().getReader().loadBeanDefinitions(location, actualResources);
				if (logger.isDebugEnabled()) {
					logger.debug("Imported " + importCount + " bean definitions from URL location [" + location + "]");
				}
			}
			catch (BeanDefinitionStoreException ex) {
				getReaderContext().error(
						"Failed to import bean definitions from URL location [" + location + "]", ele, ex);
			}
		}
		else {
			// No URL -> considering resource location as relative to the current file.
			try {
				int importCount;
				//将路径配置转为资源
				Resource relativeResource = getReaderContext().getResource().createRelative(location);
				//如果可以获取到资源，则回调到AbstractBeanDefinitionReader的loadBeanDefinitions方法进行加载
				if (relativeResource.exists()) {
					importCount = getReaderContext().getReader().loadBeanDefinitions(relativeResource);
					actualResources.add(relativeResource);
				}
				else {
					//获取不到资源，则加上基础路径，再进行加载操作
					String baseLocation = getReaderContext().getResource().getURL().toString();
					importCount = getReaderContext().getReader().loadBeanDefinitions(
							StringUtils.applyRelativePath(baseLocation, location), actualResources);
				}
				if (logger.isDebugEnabled()) {
					logger.debug("Imported " + importCount + " bean definitions from relative location [" + location + "]");
				}
			}
			catch (IOException ex) {
				getReaderContext().error("Failed to resolve current resource location", ele, ex);
			}
			catch (BeanDefinitionStoreException ex) {
				getReaderContext().error("Failed to import bean definitions from relative location [" + location + "]",
						ele, ex);
			}
		}
		Resource[] actResArray = actualResources.toArray(new Resource[actualResources.size()]);
		//调用事件监听器ReaderEventListener#importProcessed()方法 通知处理的import的结果
		getReaderContext().fireImportProcessed(location, actResArray, extractSource(ele));
	}

其中有一步获取系统属性，解析${}占位符并使用相应的属性值来代替，这个的主要场景是资源文件可能是按环境分离的，如：

这里可以从系统属性和环境变量中获取，类似于：System.getProperties()，System.getenv()

这些属性，在上下文中已经存在，

如系统属性：

os.name

如环境变量：

JAVA_HOME

最后一步，都会调到org.springframework.beans.factory.xml.XmlBeanDefinitionReader#loadBeanDefinitions(org.springframework.core.io.Resource)这个方法解析xml文件，具体解析又回到了bean的定义加载，因为本身也在解析xml文件流程中，只是里面又嵌套了xml文件，递归调用，可看这篇文章：https://blog.csdn.net/matt8/article/details/106352083