上一篇跟蹤了IOC容器對配置文件的定位，現在我們繼續跟蹤代碼，看看IOC容器是怎麼加載和註冊配置文件中的信息的。開始之前，首先我們先來了解一下IOC容器所使用的數據結構——-BeanDefinition，它是一個上層接口，有很多實現類，分別對應不同的數據載體。我們平時開發的時候，也會定義很多pojo類，來作為獲取數據的載體。最常見的就是，從數據庫中獲取數據之後，使用一個定義的pojo來裝載，然後我們就可以在程序中使用這個pojo類來編寫各種業務邏輯。同樣，IOC容器首先會讀取配置的XML中各個節點，即各個標籤元素，然後根據不同的標籤元素，使用不同的數據結構來裝載該元素中的各種屬性的值。比如我們最熟悉的<bean>標籤，就是使用AbstractBeanDefinition這個數據結構，接下來的分析中我們可以看到。
先回到上篇資源的定位那裡，代碼如下：
1 public int loadBeanDefinitions(String location, Set<Resource> actualResources) throws BeanDefinitionStoreException {
2 ResourceLoader resourceLoader = getResourceLoader();
3 if (resourceLoader == null) {
4 throw new BeanDefinitionStoreException(
5 "Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
6 }
7
8 if (resourceLoader instanceof ResourcePatternResolver) {
9 // Resource pattern matching available.
10 try {
11 Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
12 int loadCount = loadBeanDefinitions(resources);
13 if (actualResources != null) {
14 for (Resource resource : resources) {
15 actualResources.add(resource);
16 }
17 }
18 if (logger.isDebugEnabled()) {
19 logger.debug("Loaded " + loadCount + " bean definitions from location pattern [" + location + "]");
20 }
21 return loadCount;
22 }
23 catch (IOException ex) {
24 throw new BeanDefinitionStoreException(
25 "Could not resolve bean definition resource pattern [" + location + "]", ex);
26 }
27 }
28 else {
29 // 定位到資源之後，封裝成一個resource對象
30 Resource resource = resourceLoader.getResource(location);
31 int loadCount = loadBeanDefinitions(resource);
32 if (actualResources != null) {
33 actualResources.add(resource);
34 }
35 if (logger.isDebugEnabled()) {
36 logger.debug("Loaded " + loadCount + " bean definitions from location [" + location + "]");
37 }
38 return loadCount;
39 }
40 }
進入loadBeanDefinitions(resource)方法，正式開始加載源碼的跟蹤：
1 @Override
2 public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
3 return loadBeanDefinitions(new EncodedResource(resource));
4 }
1 public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
2 Assert.notNull(encodedResource, "EncodedResource must not be null");
3 if (logger.isInfoEnabled()) {
4 logger.info("Loading XML bean definitions from " + encodedResource);
5 }
6
7 Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
8 if (currentResources == null) {
9 currentResources = new HashSet<EncodedResource>(4);
10 this.resourcesCurrentlyBeingLoaded.set(currentResources);
11 }
12 if (!currentResources.add(encodedResource)) {
13 throw new BeanDefinitionStoreException(
14 "Detected cyclic loading of " + encodedResource + " - check your import definitions!");
15 }
16 try {
17 InputStream inputStream = encodedResource.getResource().getInputStream();
18 try {
19 InputSource inputSource = new InputSource(inputStream);
20 if (encodedResource.getEncoding() != null) {
21 inputSource.setEncoding(encodedResource.getEncoding());
22 }
23 return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
24 }
25 finally {
26 inputStream.close();
27 }
28 }
29 catch (IOException ex) {
30 throw new BeanDefinitionStoreException(
31 "IOException parsing XML document from " + encodedResource.getResource(), ex);
32 }
33 finally {
34 currentResources.remove(encodedResource);
35 if (currentResources.isEmpty()) {
36 this.resourcesCurrentlyBeingLoaded.remove();
37 }
38 }
39 }
進入doLoadBeanDefinitions(inputSource, encodedResource.getResource())方法：
1 protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
2 throws BeanDefinitionStoreException {
3 try {
4 Document doc = doLoadDocument(inputSource, resource);
5 return registerBeanDefinitions(doc, resource);
6 }
7 catch (BeanDefinitionStoreException ex) {
8 throw ex;
9 }
10 catch (SAXParseException ex) {
11 throw new XmlBeanDefinitionStoreException(resource.getDescription(),
12 "Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
13 }
14 catch (SAXException ex) {
15 throw new XmlBeanDefinitionStoreException(resource.getDescription(),
16 "XML document from " + resource + " is invalid", ex);
17 }
18 catch (ParserConfigurationException ex) {
19 throw new BeanDefinitionStoreException(resource.getDescription(),
20 "Parser configuration exception parsing XML from " + resource, ex);
21 }
22 catch (IOException ex) {
23 throw new BeanDefinitionStoreException(resource.getDescription(),
24 "IOException parsing XML document from " + resource, ex);
25 }
26 catch (Throwable ex) {
27 throw new BeanDefinitionStoreException(resource.getDescription(),
28 "Unexpected exception parsing XML document from " + resource, ex);
29 }
30 }
繼續進入registerBeanDefinitions(doc, resource)方法：
1 public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
2 //此時documentReader已經是DefaultBeanDefinitionDocumentReader類了
3 BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
4 int countBefore = getRegistry().getBeanDefinitionCount();
5 documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
6 //返回當前註冊的beanDefinition的個數
7 return getRegistry().getBeanDefinitionCount() - countBefore;
8 }
進入registerBeanDefinitions(doc, createReaderContext(resource))方法：
1 public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
2 this.readerContext = readerContext;
3 logger.debug("Loading bean definitions");
4 Element root = doc.getDocumentElement();
5 doRegisterBeanDefinitions(root);
6 }
進入doRegisterBeanDefinitions(root)方法：
1 protected void doRegisterBeanDefinitions(Element root) {
2 // Any nested <beans> elements will cause recursion in this method. In
3 // order to propagate and preserve <beans> default-* attributes correctly,
4 // keep track of the current (parent) delegate, which may be null. Create
5 // the new (child) delegate with a reference to the parent for fallback purposes,
6 // then ultimately reset this.delegate back to its original (parent) reference.
7 // this behavior emulates a stack of delegates without actually necessitating one.
8 BeanDefinitionParserDelegate parent = this.delegate;
9 this.delegate = createDelegate(getReaderContext(), root, parent);
10
11 if (this.delegate.isDefaultNamespace(root)) {
12 //profile屬性平時使用非常少，該屬性可以用於配置數據庫的切換（常用），使用時，需要在web.xml中配置context-parm
13 //<context-parm>
14 // <parm-name>Spring.profiles.active</parm-name>
15 // <parm-value>dev(在applicationContext.xml中配置的profile屬性的beans的profile屬性值)</parm-name>
16 //</context-parm>
17 //在applicationContext.xml中的配置
18 //<beans profile="dev"> </beans>
19 //<beans profile="produce"> </beans>
20 String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
21 if (StringUtils.hasText(profileSpec)) {
22 String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
23 profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
24 if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
25 if (logger.isInfoEnabled()) {
26 logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec +
27 "] not matching: " + getReaderContext().getResource());
28 }
29 return;
30 }
31 }
32 }
33
34 preProcessXml(root);
35 parseBeanDefinitions(root, this.delegate);
36 postProcessXml(root);
37
38 this.delegate = parent;
39 }
這裏也用到了模板方法，preProcessXml(root)和postProcessXml(root)這兩個方法都是空實現，是留給客戶來實現自己的邏輯的。重點研究一下parseBeanDefinitions(root, this.delegate)方法：
1 protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
2 if (delegate.isDefaultNamespace(root)) {
3 NodeList nl = root.getChildNodes();
4 for (int i = 0; i < nl.getLength(); i++) {
5 Node node = nl.item(i);
6 if (node instanceof Element) {
7 Element ele = (Element) node;
8 if (delegate.isDefaultNamespace(ele)) {
9 parseDefaultElement(ele, delegate);
10 }
11 else {
12 delegate.parseCustomElement(ele);
13 }
14 }
15 }
16 }
17 else {
18 delegate.parseCustomElement(root);
19 }
20 }
parseCustomElement(root)方法不需要怎麼研究，我們平時幾乎不會用到自定義的標籤，所以只跟蹤parseDefaultElement(ele, delegate)裏面的代碼：
private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
//import標籤
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
importBeanDefinitionResource(ele);
}
//alias標籤
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
processAliasRegistration(ele);
}
//bean標籤
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
processBeanDefinition(ele, delegate);
}
//beans標籤
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// recurse
doRegisterBeanDefinitions(ele);
}
}
可以看到，對於不同的標籤，spring採用不同的策略進行處理，重點跟蹤一下處理bean標籤的方法processBeanDefinition(ele, delegate)：
1 protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
2 //委託給delegate去進行各種標籤的解析，parseBeanDefinitionElement方法中包含了各種標籤元素的解析，
3 //並將解析好的內容封裝成BeanDefinitionHolder對象
4 BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
5 if (bdHolder != null) {
6 bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
7 try {
8 // Register the final decorated instance.
9 BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
10 }
11 catch (BeanDefinitionStoreException ex) {
12 getReaderContext().error("Failed to register bean definition with name '" +
13 bdHolder.getBeanName() + "'", ele, ex);
14 }
15 // Send registration event.
16 getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
17 }
18 }
在這個方法中，delegate.parseBeanDefinitionElement(ele)是解析bean元素中各種屬性的方法，registerBeanDefinition(bdHolder, getReaderContext().getRegistry())是將封裝好的數據進行存儲的方法。先看一下解析的方法：
1 public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
2 //獲取bean標籤的id屬性的值
3 String id = ele.getAttribute(ID_ATTRIBUTE);
4 //獲取bean標籤上name屬性的值
5 String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);
6
7 List<String> aliases = new ArrayList<String>();
8 if (StringUtils.hasLength(nameAttr)) {
9 //將name的值進行分割，並將它們當作別名存到aliases中
10 String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
11 aliases.addAll(Arrays.asList(nameArr));
12 }
13
14 String beanName = id;
15 //如果bean標籤的id沒有值，但是name屬性有值，則將name屬性的第一個值當作id的值，並從aliases中將第一個別名移除掉
16 if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
17 beanName = aliases.remove(0);
18 if (logger.isDebugEnabled()) {
19 logger.debug("No XML 'id' specified - using '" + beanName +
20 "' as bean name and " + aliases + " as aliases");
21 }
22 }
23
24 if (containingBean == null) {
25 //檢查bean的唯一性
26 checkNameUniqueness(beanName, aliases, ele);
27 }
28
29 //這裏已經是將XML中bean元素中的所有屬性都封裝到beanDefinition對象中了
30 AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
31 if (beanDefinition != null) {
32 if (!StringUtils.hasText(beanName)) {
33 try {
34 if (containingBean != null) {
35 beanName = BeanDefinitionReaderUtils.generateBeanName(
36 beanDefinition, this.readerContext.getRegistry(), true);
37 }
38 else {
39 beanName = this.readerContext.generateBeanName(beanDefinition);
40 // Register an alias for the plain bean class name, if still possible,
41 // if the generator returned the class name plus a suffix.
42 // This is expected for Spring 1.2/2.0 backwards compatibility.
43 String beanClassName = beanDefinition.getBeanClassName();
44 if (beanClassName != null &&
45 beanName.startsWith(beanClassName) && beanName.length() > beanClassName.length() &&
46 !this.readerContext.getRegistry().isBeanNameInUse(beanClassName)) {
47 aliases.add(beanClassName);
48 }
49 }
50 if (logger.isDebugEnabled()) {
51 logger.debug("Neither XML 'id' nor 'name' specified - " +
52 "using generated bean name [" + beanName + "]");
53 }
54 }
55 catch (Exception ex) {
56 error(ex.getMessage(), ele);
57 return null;
58 }
59 }
60 String[] aliasesArray = StringUtils.toStringArray(aliases);
61 //最後將封裝好的beanDefinition、它的id、以及它的別名一起封裝成BeanDefinitionHolder對象返回
62 return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
63 }
64
65 return null;
66 }
我們可以得到如下信息：
1、獲取bean標籤的id屬性和name屬性的值；
2、name屬性是可以都有多個值的，以逗號或者分號分割；
3、如果id沒有賦值，則取name的第一個值作為id的值。所以，我們一般都會給id賦值，這樣效率高一些；
4、檢查以這個id標識的bean是不是唯一的；
5、進行其他屬性的解析，並最終封裝測AbstractBeanDefinition對象，也就是我們前文中提到的數據結構；
6、最後封裝成BeanDefinitionHolder對象之後返回。
進入parseBeanDefinitionElement(ele, beanName, containingBean)方法，看一下其他元素的解析過程：
1 public AbstractBeanDefinition parseBeanDefinitionElement(
2 Element ele, String beanName, BeanDefinition containingBean) {
3
4 this.parseState.push(new BeanEntry(beanName));
5
6 String className = null;
7 if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
8 className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
9 }
10
11 try {
12 String parent = null;
13 if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
14 parent = ele.getAttribute(PARENT_ATTRIBUTE);
15 }
16 AbstractBeanDefinition bd = createBeanDefinition(className, parent);
17
18 parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
19 bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));
20
21 parseMetaElements(ele, bd);
22 parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
23 parseReplacedMethodSubElements(ele, bd.getMethodOverrides());
24
25 parseConstructorArgElements(ele, bd);
26 parsePropertyElements(ele, bd);
27 parseQualifierElements(ele, bd);
28
29 bd.setResource(this.readerContext.getResource());
30 bd.setSource(extractSource(ele));
31
32 return bd;
33 }
34 catch (ClassNotFoundException ex) {
35 error("Bean class [" + className + "] not found", ele, ex);
36 }
37 catch (NoClassDefFoundError err) {
38 error("Class that bean class [" + className + "] depends on not found", ele, err);
39 }
40 catch (Throwable ex) {
41 error("Unexpected failure during bean definition parsing", ele, ex);
42 }
43 finally {
44 this.parseState.pop();
45 }
46
47 return null;
48 }
解析封裝成BeanDefinitionHolder對象之後，就可以進行註冊了，先回到之前的processBeanDefinition(ele, delegate)：
1 protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
2 //委託給delegate去進行各種標籤的解析，parseBeanDefinitionElement方法中包含了各種標籤元素的解析，
3 //並將解析好的內容封裝成BeanDefinitionHolder對象
4 BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
5 if (bdHolder != null) {
6 bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
7 try {
8 // Register the final decorated instance.
9 BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
10 }
11 catch (BeanDefinitionStoreException ex) {
12 getReaderContext().error("Failed to register bean definition with name '" +
13 bdHolder.getBeanName() + "'", ele, ex);
14 }
15 // Send registration event.
16 getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
17 }
18 }
現在進入BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry())方法進行分析：
1 public static void registerBeanDefinition(
2 BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
3 throws BeanDefinitionStoreException {
4
5 // Register bean definition under primary name.
6 String beanName = definitionHolder.getBeanName();
7 registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
8
9 // Register aliases for bean name, if any.
10 String[] aliases = definitionHolder.getAliases();
11 if (aliases != null) {
12 for (String alias : aliases) {
13 registry.registerAlias(beanName, alias);
14 }
15 }
16 }
這裏的beanName就是之前封裝好的bean的id。這個方法中分別以id和別名作為key來註冊bean，其實就是存儲在map中。
進入registerBeanDefinition(beanName, definitionHolder.getBeanDefinition())，在其子類DefaultListableBeanFactory中有實現：
1 public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
2 throws BeanDefinitionStoreException {
3
4 Assert.hasText(beanName, "Bean name must not be empty");
5 Assert.notNull(beanDefinition, "BeanDefinition must not be null");
6
7 if (beanDefinition instanceof AbstractBeanDefinition) {
8 try {
9 ((AbstractBeanDefinition) beanDefinition).validate();
10 }
11 catch (BeanDefinitionValidationException ex) {
12 throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
13 "Validation of bean definition failed", ex);
14 }
15 }
16
17 BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
18 if (existingDefinition != null) {
19 if (!isAllowBeanDefinitionOverriding()) {
20 throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
21 "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
22 "': There is already [" + existingDefinition + "] bound.");
23 }
24 else if (existingDefinition.getRole() < beanDefinition.getRole()) {
25 // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
26 if (logger.isWarnEnabled()) {
27 logger.warn("Overriding user-defined bean definition for bean '" + beanName +
28 "' with a framework-generated bean definition: replacing [" +
29 existingDefinition + "] with [" + beanDefinition + "]");
30 }
31 }
32 else if (!beanDefinition.equals(existingDefinition)) {
33 if (logger.isInfoEnabled()) {
34 logger.info("Overriding bean definition for bean '" + beanName +
35 "' with a different definition: replacing [" + existingDefinition +
36 "] with [" + beanDefinition + "]");
37 }
38 }
39 else {
40 if (logger.isDebugEnabled()) {
41 logger.debug("Overriding bean definition for bean '" + beanName +
42 "' with an equivalent definition: replacing [" + existingDefinition +
43 "] with [" + beanDefinition + "]");
44 }
45 }
46 this.beanDefinitionMap.put(beanName, beanDefinition);
47 }
48 else {
49 if (hasBeanCreationStarted()) {
50 // Cannot modify startup-time collection elements anymore (for stable iteration)
51 synchronized (this.beanDefinitionMap) {
52 this.beanDefinitionMap.put(beanName, beanDefinition);
53 List<String> updatedDefinitions = new ArrayList<String>(this.beanDefinitionNames.size() + 1);
54 updatedDefinitions.addAll(this.beanDefinitionNames);
55 updatedDefinitions.add(beanName);
56 this.beanDefinitionNames = updatedDefinitions;
57 if (this.manualSingletonNames.contains(beanName)) {
58 Set<String> updatedSingletons = new LinkedHashSet<String>(this.manualSingletonNames);
59 updatedSingletons.remove(beanName);
60 this.manualSingletonNames = updatedSingletons;
61 }
62 }
63 }
64 else {
65 // Still in startup registration phase
66 this.beanDefinitionMap.put(beanName, beanDefinition);
67 this.beanDefinitionNames.add(beanName);
68 this.manualSingletonNames.remove(beanName);
69 }
70 this.frozenBeanDefinitionNames = null;
71 }
72
73 if (existingDefinition != null || containsSingleton(beanName)) {
74 resetBeanDefinition(beanName);
75 }
76 }
我們可以看到：這個beanDefinitionMap就是用來存儲解析好的bean的，以id作為key。至此，就將所有的bean標籤解析好之後封裝成BeanDefinition註冊到了IOC容器中。但是，到目前為止，IOC容器並沒有為我們將這些解析好的數據生成一個一個bean實例，我們仍然不能就這樣直接使用。下一篇接着跟蹤。
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