spring源码附录（8）import、beans标签的解析

一、import标签的解析

对于过于庞大的项目，分模块应该是解决麻烦的好方法了。spring import标签就可以达到此目的，例如：

<beans>
    <import resource="..."/>
    <import resource="...">
beans>

下面来看源码中的解析：

    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;
        }

        //解析系统属性，例如"${user.dir}"
        // Resolve system properties: e.g. "${user.dir}"
        location = getReaderContext().getEnvironment().resolveRequiredPlaceholders(location);

        Set 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*:"
        }

        //如果是绝对URI则直接根据地址加载对应的配置文件
        // 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 {
            //如果是相对路径则根据相对路径计算出绝对路径
            // 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.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()]);
        getReaderContext().fireImportProcessed(location, actResArray, extractSource(ele));
    }

大体过程如下：

1. 获取resource属性对应的路径。
2. 解析路径中的系统属性
3. 判断location是绝对路径还是相对路径
4. 如果是相对路径则递归调用bean的解析过程，进行另一次的解析
5. 通知监听器，解析完成

二、beans标签的解析

形如：

<bean id="aa" class="test.aa"/>
<beans>
beans>

对于嵌入式beans标签来讲，并没有太多可讲，与单独的配置文件并没有太大差别，无非递归调用beans的解析过程。