目錄示例原理DefaultBeanDefinitionDocumentReaderparseDefaultElementimportBeanDefinitionResource總結(jié)示例

我們先來看下配置文件是怎么導(dǎo)入外部配置文件的？先定義一個(gè)spring-import配置文件如下：

<?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'> <import resource='spring-config.xml'/> <bean class='com.john.aop.Person'> </bean></beans>

我們看到里面定義了一個(gè)標(biāo)簽并用屬性resource聲明了導(dǎo)入的資源文件為spring-config.xml。我們?cè)賮砜聪聅pring-config.xml配置文件是怎樣的：

<?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'> <bean class='com.john.aop.Person'> <property name='firstName' value='john'/> <property name='lastName' value='wonder'/> </bean> <bean abstract='true' > <property name='country' value='中國(guó)'/> <property name='gender' value='女'/> </bean></beans>

然后我們可以實(shí)例化一個(gè)BeanFactory來加載spring-import這個(gè)配置文件了。

public static void main(String[] args) { ApplicationContext context=new ClassPathXmlApplicationContext('spring-import.xml'); Person outerPerson=(Person)context.getBean('outerPerson'); System.out.println(outerPerson);}

如果沒問題的話，我們可以獲取到outerPerson這個(gè)bean并打印了。

Person [0, john wonder]原理

我們來通過源碼分析下Spring是如何解析import標(biāo)簽并加載這個(gè)導(dǎo)入的配置文件的。首先我們到DefaultBeanDefinitionDocumentReader類中看下：

DefaultBeanDefinitionDocumentReader

我們可以看到類里面定義了一個(gè)public static final 的IMPORT_ELEMENT變量：

public static final String IMPORT_ELEMENT = 'import';

然后我們可以搜索下哪邊用到了這個(gè)變量，并且定位到parseDefaultElement函數(shù)：

parseDefaultElement

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) { //todo 對(duì) import 標(biāo)簽的解析 2020-11-17 if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) { importBeanDefinitionResource(ele); }}

這里就是我們要找的導(dǎo)入外部配置文件加載Bean定義的源代碼，我們?cè)僦攸c(diǎn)看下importBeanDefinitionResource函數(shù)：

importBeanDefinitionResource

/** * Parse an 'import' element and load the bean definitions * from the given resource into the bean factory. */protected void importBeanDefinitionResource(Element ele) { //// 獲取 resource 的屬性值 String location = ele.getAttribute(RESOURCE_ATTRIBUTE); //// 為空，直接退出 if (!StringUtils.hasText(location)) { getReaderContext().error('Resource location must not be empty', ele); return; } // Resolve system properties: e.g. '${user.dir}' // // 解析系統(tǒng)屬性，格式如 ：'${user.dir}' location = getReaderContext().getEnvironment().resolveRequiredPlaceholders(location); Set<Resource> actualResources = new LinkedHashSet<>(4); // Discover whether the location is an absolute or relative URI //// 判斷 location 是相對(duì)路徑還是絕對(duì)路徑 boolean absoluteLocation = false; try { //以 classpath*: 或者 classpath: 開頭為絕對(duì)路徑 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? //絕對(duì)路徑 也會(huì)調(diào)用loadBeanDefinitions 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 { //如果是相對(duì)路徑，則先計(jì)算出絕對(duì)路徑得到 Resource，然后進(jìn)行解析 // No URL -> considering resource location as relative to the current file. try { int importCount; Resource relativeResource = getReaderContext().getResource().createRelative(location); //如果相對(duì)路徑 這個(gè)資源存在 那么就加載這個(gè)bean 定義 if (relativeResource.exists()) { importCount = getReaderContext().getReader().loadBeanDefinitions(relativeResource); actualResources.add(relativeResource); } else { String baseLocation = getReaderContext().getResource().getURL().toString(); //todo import節(jié)點(diǎn) 內(nèi)部會(huì)調(diào)用loadBeanDefinitions 操作 2020-10-17 importCount = getReaderContext().getReader().loadBeanDefinitions( StringUtils.applyRelativePath(baseLocation, location), actualResources); } if (logger.isTraceEnabled()) { logger.trace('Imported ' + importCount + ' bean definitions from relative location [' + location + ']'); } } }}

我們來解析下這段代碼是怎么一個(gè)流程：

1.首先通過resource標(biāo)簽解析到它的屬性值，并且判讀字符串值是否為空。

2.接下來它還會(huì)通過當(dāng)前上下文環(huán)境去解析字符串路徑里面的占位符，這點(diǎn)我們?cè)谥拔恼轮蟹治鲞^。

2.接下來判斷是否是絕對(duì)路徑，通過調(diào)用ResourcePatternUtils.isUrl(location) || ResourceUtils.toURI(location).isAbsolute();來判斷，劃重點(diǎn)：以 classpath*: 或者 classpath: 開頭為絕對(duì)路徑，或者可以生成一個(gè)URI實(shí)例就是當(dāng)作絕對(duì)路徑，或者也可以URI的isAbsolute來判斷

3.如果是絕對(duì)路徑那么我們通過getReaderContext().getReader()獲取到XmlBeanDefinitionReader然后調(diào)用它的loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources)函數(shù)

4.如果不是絕對(duì)路徑那么我們嘗試生成相對(duì)當(dāng)前資源的路徑(這點(diǎn)很重要)，再通過loadBeanDefinitions方法來加載這個(gè)配置文件中的BeanDefinitions。這里有個(gè)細(xì)節(jié)需要我們注意，就是它為什么要嘗試去判斷資源是否存在？就是如果存在的話那么直接調(diào)用loadBeanDefinitions(Resource resource)方法，也就是說這里肯定是加載單個(gè)資源文件，如方法注釋所說：

/** * Load bean definitions from the specified XML file. * @param resource the resource descriptor for the XML file * @return the number of bean definitions found * @throws BeanDefinitionStoreException in case of loading or parsing errors */@Overridepublic int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException { //load中去注冊(cè)BeanDefinition return loadBeanDefinitions(new EncodedResource(resource));}

從指定的xml文件加載Bean定義。如果不存在，那么就跟絕對(duì)路徑一樣會(huì)調(diào)用loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources)函數(shù)，我們來看看這個(gè)函數(shù)的定義：

/** * Load bean definitions from the specified resource location. * <p>The location can also be a location pattern, provided that the * ResourceLoader of this bean definition reader is a ResourcePatternResolver. * @param location the resource location, to be loaded with the ResourceLoader * (or ResourcePatternResolver) of this bean definition reader * @param actualResources a Set to be filled with the actual Resource objects * that have been resolved during the loading process(要填充加載過程中已解析的實(shí)際資源對(duì)象*的集合). May be {@code null} * to indicate that the caller is not interested in those Resource objects. */public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {

解釋很清楚，這個(gè)location是指從指定資源路徑加載BeanDefinitions。

總結(jié)

從源碼可以看出從外部導(dǎo)入配置文件也就是給了通過一個(gè)總的配置文件來加載各個(gè)單一配置文件擴(kuò)展的機(jī)會(huì)。

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