Inside Spring Framework from Source Code (4)

In the last post, we go through the whole process about how to register a regular <bean>.  Now, let's take a look at how to load <import> tag:

<import resource="services.xml"/>
    <import resource="resources/messageSource.xml"/>
    <import resource="/resources/themeSource.xml"/>

</import>

see the entry code here:

	private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
	if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
	importBeanDefinitionResource(ele);
	}
	else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
	processAliasRegistration(ele);
	}
	else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
	processBeanDefinition(ele, delegate);
	}
	else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
	// recurse
	doRegisterBeanDefinitions(ele);
	}
	}

view raw DefaultBeanDefinitionDocumentReader.java hosted with ❤ by GitHub

importBeanDefinitionResource has all the implementation.

	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;
	}
	// Resolve system properties: e.g. "${user.dir}"
	location = getReaderContext().getEnvironment().resolveRequiredPlaceholders(location);
	Set<Resource> actualResources = new LinkedHashSet<>(4);
	// Discover whether the location is an absolute or relative URI
	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.isTraceEnabled()) {
	logger.trace("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;
	Resource relativeResource = getReaderContext().getResource().createRelative(location);
	if (relativeResource.exists()) {
	importCount = getReaderContext().getReader().loadBeanDefinitions(relativeResource);
	actualResources.add(relativeResource);
	}
	else {
	String baseLocation = getReaderContext().getResource().getURL().toString();
	importCount = getReaderContext().getReader().loadBeanDefinitions(
	StringUtils.applyRelativePath(baseLocation, location), actualResources);
	}
	if (logger.isTraceEnabled()) {
	logger.trace("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[0]);
	getReaderContext().fireImportProcessed(location, actResArray, extractSource(ele));
	}

view raw DefaultBeanDefinitionDocumentReader.java hosted with ❤ by GitHub

the logic is as following:
1. get the value of attribute: resource
2. resolve the location string by replacing system properties: e.g. "${user.dir}"
3. check the location is starting with classpath or regular url.
4. Line 26 will use AbstractBeanDefinitionReader->loadBeanDefinitions to parse the spring beans as mentioned in my previous blog. 

1. get the file resource list (if the location path contains "*" or "?") or return the file resource directly.
2. parse the resource to spring bean one by one.

5. The actualResources is used to store all loaded resources.
6. Line 63 will trig the event of import definition done.