原始碼分析：Spring是如何把Bean註冊到IOC容器中的？
2020-09-28 20:00:16
前言
提到Spring就會想到IOC、DI等概念，這是Spring的核心思想，只要使用過Spring框架的人都知道這些概念，但要問到Spring具體是怎麼實現IOC的，恐怕只能看Spring的原始碼才能找到答案，在我看來只要搞清楚兩個問題，就能對Spring的整體脈絡有個整體的認識
- Bean是如何註冊到IOC註冊中的？
- Bean是如何從IOC容器中get出來的？
一個簡單的例子
我們先從一個簡單的例子開始，基本上在最開始學習Spring的時候都會從xml設定開始，把你需要交給Spring管理的類設定到xml檔案中，你就可以不用管物件的建立了，下面來看一下程式碼
1、首先定義一個User類
@Data
public class User {
private String userName;
private String password;
}
很簡單的一個JavaBean
2、增加一個xml設定
<?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 id="userBean" class="org.kxg.springDemo.User">
<property name="userName" value="jack" />
<property name="password" value="123456" />
</bean>
</beans>
3、讀取設定，並執行
public class XmlBeanTest {
public static void main(String[] args){
ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("bean.xml");
User user = (User) applicationContext.getBean("userBean");
System.out.println(user.getUserName());
}
}
可以看到例子很簡單，堪稱Spring入門的HelloWorld
從上面幾行程式碼可以看出，首先讀取bean.xml中的設定，然後就可以從applicationContext中獲取到User物件，那麼肯定會有User物件註冊到IOC容器中這個步驟
下面我們一起通過原始碼來看一下Bean是如何註冊到Spring IOC容器中的
原始碼解析
從ClassPathXmlApplicationContext開始
public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
this(new String[] {configLocation}, true, null);
}
public ClassPathXmlApplicationContext(String... configLocations) throws BeansException {
this(configLocations, true, null);
}
從ClassPathXmlApplicationContext的構造方法入手，構造方法傳入xml組態檔的路徑，這裡可以傳入單個或多個組態檔
public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
super(parent);
//將傳入的xml設定位置資訊設定到configLocations
setConfigLocations(configLocations);
if (refresh) {
//核心方法
refresh();
}
}
這裡看到，我們傳入的組態檔設定到configLocations，然後呼叫了一個Spring最核心的方法refresh()，這個方法包括了容器啟動的所有內容，是我們學習Spring原始碼的一個入口，可以說你只要把這個方法裡面的內容研究清楚了，對於Spring框架的整個脈絡會有一個全新的認識，下面我們來看看這個方法裡面有些什麼
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// 注意看這個方法
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}
refresh()方法裡面的內容很豐富，從各個方法名稱就大致可以看出來其作用，這裡我們主要看Bean註冊的過程，將目光聚焦到
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
這一行是獲取BeanFactory，裡面進行了Bean的註冊邏輯
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
refreshBeanFactory();
return getBeanFactory();
}
這裡呼叫的是AbstractRefreshableApplicationContext類的refreshBeanFactory()方法，需要注意一下在看Spring原始碼的時候，同一個方法可能會有多個子類都實現了，需要注意區分一下，你當前範例化的是哪個子類
protected final void refreshBeanFactory() throws BeansException {
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
//建立一個BeanFactory
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
customizeBeanFactory(beanFactory);
//這裡進行Bean的載入
loadBeanDefinitions(beanFactory);
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}
因為我們使用的是xml的設定，所以這裡呼叫的是AbstractXmlApplicationContext這個抽象類中的loadBeanDefinitions方法
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
// 構造一個XmlBeanDefinitionReader，用於讀到xml中設定的bean
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
// 設定XmlBeanDefinitionReader
beanDefinitionReader.setEnvironment(this.getEnvironment());
beanDefinitionReader.setResourceLoader(this);
beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
//初始化XmlBeanDefinitionReader
initBeanDefinitionReader(beanDefinitionReader);
//載入Bean
loadBeanDefinitions(beanDefinitionReader);
}
protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
Resource[] configResources = getConfigResources();
if (configResources != null) {
reader.loadBeanDefinitions(configResources);
}
String[] configLocations = getConfigLocations();
if (configLocations != null) {
reader.loadBeanDefinitions(configLocations);
}
}
這裡進行了兩種不同方式的載入，呼叫的是不同的方法，我們傳入的是configLocations
@Override
public int loadBeanDefinitions(String... locations) throws BeanDefinitionStoreException {
Assert.notNull(locations, "Location array must not be null");
int count = 0;
for (String location : locations) {
count += loadBeanDefinitions(location);
}
return count;
}
public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
ResourceLoader resourceLoader = getResourceLoader();
if (resourceLoader == null) {
throw new BeanDefinitionStoreException(
"Cannot load bean definitions from location [" + location + "]: no ResourceLoader available");
}
if (resourceLoader instanceof ResourcePatternResolver) {
// Resource pattern matching available.
try {
Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
int count = loadBeanDefinitions(resources);
if (actualResources != null) {
Collections.addAll(actualResources, resources);
}
if (logger.isTraceEnabled()) {
logger.trace("Loaded " + count + " bean definitions from location pattern [" + location + "]");
}
return count;
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(
"Could not resolve bean definition resource pattern [" + location + "]", ex);
}
}
else {
// Can only load single resources by absolute URL.
Resource resource = resourceLoader.getResource(location);
int count = loadBeanDefinitions(resource);
if (actualResources != null) {
actualResources.add(resource);
}
if (logger.isTraceEnabled()) {
logger.trace("Loaded " + count + " bean definitions from location [" + location + "]");
}
return count;
}
}
這個裡面主要方法是loadBeanDefinitions()，我們繼續往下走
中間省略了一些簡單呼叫
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
Assert.notNull(encodedResource, "EncodedResource must not be null");
if (logger.isTraceEnabled()) {
logger.trace("Loading XML bean definitions from " + encodedResource);
}
Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
if (currentResources == null) {
currentResources = new HashSet<>(4);
this.resourcesCurrentlyBeingLoaded.set(currentResources);
}
if (!currentResources.add(encodedResource)) {
throw new BeanDefinitionStoreException(
"Detected cyclic loading of " + encodedResource + " - check your import definitions!");
}
try {
InputStream inputStream = encodedResource.getResource().getInputStream();
try {
InputSource inputSource = new InputSource(inputStream);
if (encodedResource.getEncoding() != null) {
inputSource.setEncoding(encodedResource.getEncoding());
}
//進行BeanDefinations載入
return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
}
finally {
inputStream.close();
}
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(
"IOException parsing XML document from " + encodedResource.getResource(), ex);
}
finally {
currentResources.remove(encodedResource);
if (currentResources.isEmpty()) {
this.resourcesCurrentlyBeingLoaded.remove();
}
}
}
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
//構造xml的Document結構，解析DOM結構
Document doc = doLoadDocument(inputSource, resource);
//註冊BeanDefinition
int count = registerBeanDefinitions(doc, resource);
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + count + " bean definitions from " + resource);
}
return count;
}
catch (BeanDefinitionStoreException ex) {
throw ex;
}
catch (SAXParseException ex) {
throw new XmlBeanDefinitionStoreException(resource.getDescription(),
"Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
}
catch (SAXException ex) {
throw new XmlBeanDefinitionStoreException(resource.getDescription(),
"XML document from " + resource + " is invalid", ex);
}
catch (ParserConfigurationException ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"Parser configuration exception parsing XML from " + resource, ex);
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"IOException parsing XML document from " + resource, ex);
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"Unexpected exception parsing XML document from " + resource, ex);
}
}
從上面的程式碼可以看出來，Spring是將xml的DOM結構解析後註冊到IOC容器中的
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
int countBefore = getRegistry().getBeanDefinitionCount();
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
return getRegistry().getBeanDefinitionCount() - countBefore;
}
這個方法構造了一個BeanDefinitionDocumentReader，進行註冊BeanDefinition，並且返回了本次註冊Bean的數量
protected void doRegisterBeanDefinitions(Element root) {
// Any nested <beans> elements will cause recursion in this method. In
// order to propagate and preserve <beans> default-* attributes correctly,
// keep track of the current (parent) delegate, which may be null. Create
// the new (child) delegate with a reference to the parent for fallback purposes,
// then ultimately reset this.delegate back to its original (parent) reference.
// this behavior emulates a stack of delegates without actually necessitating one.
BeanDefinitionParserDelegate parent = this.delegate;
this.delegate = createDelegate(getReaderContext(), root, parent);
if (this.delegate.isDefaultNamespace(root)) {
String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
if (StringUtils.hasText(profileSpec)) {
String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
// We cannot use Profiles.of(...) since profile expressions are not supported
// in XML config. See SPR-12458 for details.
if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
if (logger.isDebugEnabled()) {
logger.debug("Skipped XML bean definition file due to specified profiles [" + profileSpec +
"] not matching: " + getReaderContext().getResource());
}
return;
}
}
}
preProcessXml(root);
//進行BeanDefinition轉換，將DOM結構的物件轉換成BeanDefinition
parseBeanDefinitions(root, this.delegate);
postProcessXml(root);
this.delegate = parent;
}
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
if (delegate.isDefaultNamespace(root)) {
NodeList nl = root.getChildNodes();
for (int i = 0; i < nl.getLength(); i++) {
Node node = nl.item(i);
if (node instanceof Element) {
Element ele = (Element) node;
if (delegate.isDefaultNamespace(ele)) {
//Spring預設元素轉換
parseDefaultElement(ele, delegate);
}
else {
//xml中自定義的Element進行解析
delegate.parseCustomElement(ele);
}
}
}
}
else {
delegate.parseCustomElement(root);
}
}
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);
}
}
可以看到xml中預設的設定元素包括import、alias、bean、beans，這些也是最常用的，我們主要看一個bean的轉換
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// Register the final decorated instance.
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to register bean definition with name '" +
bdHolder.getBeanName() + "'", ele, ex);
}
// Send registration event.
getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
}
}
public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {
// Register bean definition under primary name.
String beanName = definitionHolder.getBeanName();
//註冊BeanDefinition
registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
// Register aliases for bean name, if any.
String[] aliases = definitionHolder.getAliases();
if (aliases != null) {
for (String alias : aliases) {
registry.registerAlias(beanName, alias);
}
}
}
// DefaultListableBeanFactory
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
throws BeanDefinitionStoreException {
Assert.hasText(beanName, "Bean name must not be empty");
Assert.notNull(beanDefinition, "BeanDefinition must not be null");
if (beanDefinition instanceof AbstractBeanDefinition) {
try {
((AbstractBeanDefinition) beanDefinition).validate();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Validation of bean definition failed", ex);
}
}
BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
if (existingDefinition != null) {
if (!isAllowBeanDefinitionOverriding()) {
throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
}
else if (existingDefinition.getRole() < beanDefinition.getRole()) {
// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
if (logger.isInfoEnabled()) {
logger.info("Overriding user-defined bean definition for bean '" + beanName +
"' with a framework-generated bean definition: replacing [" +
existingDefinition + "] with [" + beanDefinition + "]");
}
}
else if (!beanDefinition.equals(existingDefinition)) {
if (logger.isDebugEnabled()) {
logger.debug("Overriding bean definition for bean '" + beanName +
"' with a different definition: replacing [" + existingDefinition +
"] with [" + beanDefinition + "]");
}
}
else {
if (logger.isTraceEnabled()) {
logger.trace("Overriding bean definition for bean '" + beanName +
"' with an equivalent definition: replacing [" + existingDefinition +
"] with [" + beanDefinition + "]");
}
}
//將beanDefinition放入beanDefinitionMap中
this.beanDefinitionMap.put(beanName, beanDefinition);
}
else {
if (hasBeanCreationStarted()) {
// Cannot modify startup-time collection elements anymore (for stable iteration)
synchronized (this.beanDefinitionMap) {
//將beanDefinition放入beanDefinitionMap中
this.beanDefinitionMap.put(beanName, beanDefinition);
List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames = updatedDefinitions;
removeManualSingletonName(beanName);
}
}
else {
// Still in startup registration phase
//將beanDefinition放入beanDefinitionMap中
this.beanDefinitionMap.put(beanName, beanDefinition);
this.beanDefinitionNames.add(beanName);
removeManualSingletonName(beanName);
}
this.frozenBeanDefinitionNames = null;
}
if (existingDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}
原始碼跟到這裡，整個流程基本清楚了，最終beanDefinition存到一個beanDefinitionMap中，key為Bean的名稱，value為beanDefinition物件
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);
beanDefinitionMap是一個ConcurrentHashMap，所以本質上Bean最終是被註冊到一個Map中
上面我們說了很多次beanDefinition，最後註冊到容器中的也是這個物件，那它到底是個啥物件?
A BeanDefinition describes a bean instance, which has property values,
constructor argument values, and further information supplied by
concrete implementations.
從註釋中可以看出來BeanDefinition是一個用來描述帶有屬性值、構造方法、還有一些其他進一步資訊的Bean範例（半調子英語，也不知道翻譯的對不對~）
BeanDefinition是對Bean的抽象，因為組態檔中的Bean是多種多樣的，BeanDefinition是對Bean的公共屬性進行抽象，在BeanDefinition中很多屬性是用來描述xml設定中bean的設定屬性的
所以，下面來總結一下整個流程
- 註冊xml組態檔到configLocations
- 呼叫refresh()進行整個Context的重新整理，實際上就是整個Context的啟動
- Bean的載入會讀到組態檔，解析成DOM物件
- 將DOM物件轉換成beanDefinition
- 將beanDefinition存入beanDefinitionMap，完成整個Bean的註冊
沒看明白的同學，可以對照這個流程再回頭去看看，整個流程下來還是挺清晰的
註解方式下Bean的註冊
前面我們講到xml組態檔進行Bean的註冊，xml設定是Spring早期常用的設定方式，現在基本上大部分場景上都推薦使用註解的方式，尤其是SpringBoot時代的來臨，進一步推動了註解方式的全面使用，下面我們來看看註解方式下的Bean註冊，還是從個簡單的例子入手
@Component
public class AnnotionConfig {
@Bean(name = "userBean")
public User getUserBean(){
User user = new User();
user.setUserName("Lucy");
return user;
}
}
public class AnnotionBeanTest {
public static void main(String[] args){
AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext("org.kxg.springDemo");
User user = (User) applicationContext.getBean("userBean");
System.out.println(user.getUserName());
}
}
這裡用到AnnotationConfigApplicationContext，是另外一種容器的實現，傳入一個包名，會自動掃描包下面的Spring註解，然後將其註冊到容器中
public AnnotationConfigApplicationContext(String... basePackages) {
this();
//主要是scan方法完成bean的註冊
scan(basePackages);
//又到了這個方法，有沒有很熟悉~~~
refresh();
}
下面我們重點看一下註解方式的Bean註冊
public int scan(String... basePackages) {
int beanCountAtScanStart = this.registry.getBeanDefinitionCount();
//掃描包，進行Bean註冊
doScan(basePackages);
// Register annotation config processors, if necessary.
if (this.includeAnnotationConfig) {
AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
}
return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
}
//ClassPathBeanDefinitionScanner
protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
Assert.notEmpty(basePackages, "At least one base package must be specified");
Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
for (String basePackage : basePackages) {
//掃描包下打了註解的類，並將其轉換成BeanDefinition
Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
for (BeanDefinition candidate : candidates) {
ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
candidate.setScope(scopeMetadata.getScopeName());
String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
if (candidate instanceof AbstractBeanDefinition) {
postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
}
if (candidate instanceof AnnotatedBeanDefinition) {
AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
}
if (checkCandidate(beanName, candidate)) {
BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
definitionHolder =
AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
beanDefinitions.add(definitionHolder);
//進行BeanDefinition註冊
registerBeanDefinition(definitionHolder, this.registry);
}
}
}
return beanDefinitions;
}
public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {
// Register bean definition under primary name.
String beanName = definitionHolder.getBeanName();
registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
// Register aliases for bean name, if any.
String[] aliases = definitionHolder.getAliases();
if (aliases != null) {
for (String alias : aliases) {
registry.registerAlias(beanName, alias);
}
}
}
看到這個方法，有沒有點眼熟的感覺
上面xml方式進行Bean註冊也呼叫到這個方法了，所以後面的流程都是一樣的，註解方式和xml設定方式從本質上來講，並沒有什麼不同，只是Bean的描述不同而已，最終都會被解析成BeanDefinition，註冊到容器中，至此整個Bean的註冊流程就已經完了。
當然在整個過程中，忽略了很多細節，只看了主線流程。
讀原始碼的時候，很容易陷入細節中，尤其是像Spring這樣通用的框架，它為了通用性和擴充套件性，會把程式碼寫的很「繞」，如果你過於關注細節很容易讓自己陷入實現的細節中，一開始看原始碼的時候，建議不用太關注細節，把主線功能先看完，知道大概的原理後再逐個去擊破~
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