【Spring源码这样读】-再次走近容器Spring IOC 二

xlecho 2021-04-16 17:24:19
java spring


继上文的问题,我们本章,继续分析我们的容器到底在做了一些什么事情。

上文我们说到我们的ClassPathResource到底做了什么,但是我们阅读源码之后发现只是做了一些初始化。那我们推测的获取配置文件,然后去配置文件里面获取内容的推测还能成立吗?不需要急,我们一步一步往下看就能看到结果。既然我们看了ClassPathResource,我们在来看看new XmlBeanFactory(classPathResource);到底做了什么。

构造bean工厂类

/**
* Create a new XmlBeanFactory with the given resource,
* which must be parsable using DOM.
* @param resource the XML resource to load bean definitions from
* @throws BeansException in case of loading or parsing errors
*/
public XmlBeanFactory(Resource resource) throws BeansException {
this(resource, null);
}
/**
* Create a new XmlBeanFactory with the given input stream,
* which must be parsable using DOM.
* @param resource the XML resource to load bean definitions from
* @param parentBeanFactory parent bean factory
* @throws BeansException in case of loading or parsing errors
*/
public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException {
super(parentBeanFactory);
this.reader.loadBeanDefinitions(resource);
}

我们使用的就是第一个构造,但是第一个构造调用了第二个,第二个构造只有两步。我们继续往下看

再来看XmlBeanFactory初始化的源码

  • super(parentBeanFactory):用来初始化父类
  • this.reader.loadBeanDefinitions(resource):spring初始化资源加载的真正实现

我们先来看看“super(parentBeanFactory):用来初始化父类”这一步做了什么吧

/**
* 先是调用DefaultListableBeanFactory(@Nullable BeanFactory parentBeanFactory)带参构造方法
*/
public DefaultListableBeanFactory(@Nullable BeanFactory parentBeanFactory) {
super(parentBeanFactory);
}
/**
* 调用父类AbstractAutowireCapableBeanFactory
*/
public AbstractAutowireCapableBeanFactory(@Nullable BeanFactory parentBeanFactory) {
this();
setParentBeanFactory(parentBeanFactory);
}
/**
* 调用AbstractAutowireCapableBeanFactory()无参构造方法
*/
public AbstractAutowireCapableBeanFactory() {
super();
ignoreDependencyInterface(BeanNameAware.class);
ignoreDependencyInterface(BeanFactoryAware.class);
ignoreDependencyInterface(BeanClassLoaderAware.class);
}
/**
* 在初始化XmlBeanFactory容器
*/
public void setParentBeanFactory(@Nullable BeanFactory parentBeanFactory) {
if (this.parentBeanFactory != null && this.parentBeanFactory != parentBeanFactory) {
throw new IllegalStateException("Already associated with parent BeanFactory: " + this.parentBeanFactory);
}
this.parentBeanFactory = parentBeanFactory;
}

到这里是不是比较明了的知道了这一步具体做了什么?其实看完super(parentBeanFactory)的源码发现就做了两件事:获取添加需要忽略指定接口实现类的自动装配和初始化。那我们的第二步具体做了什么呢?

this.reader.loadBeanDefinitions(resource)分析

之前我们说了我们应该是根据配置文件找到对应的类,那源码是这样吗?看看这句代码的实现就知道了。

/**
* 先是调用loadBeanDefinitions(Resource resource)
*/
public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
return loadBeanDefinitions(new EncodedResource(resource));
}
/**
* 这个方法正是我们之前推测的实现
* 从xml配置文件中加载bean
*/
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());
}
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();
}
}
}

当我们贴完代码之后,是不是可以明显的看到我们之前推测的实际证据?

ClassPathResource和InputStream

我们从上面的源码看到,我们定义的类在xml中,源码里面有InputStream。但是传入的只有一个ClassPathResource。那他是怎么封装进入ClassPathResource的呢?

其实这里的关联比较简单。我们不妨来看看这些类的类结构图 在这里插入图片描述

到这里是不是就已经很明了了我们的InputStream是怎么将内容传递进入ClassPathResource

真正获取bean

获取bean才是我们这一段操作的最关键的地方,那bean又是怎么获取出来的呢?这里贴一下主代码吧

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isTraceEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else if (requiredType != null) {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {
return (T) parentBeanFactory.getBean(nameToLookup);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; consider " +
"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {
if (logger.isTraceEnabled()) {
logger.trace("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}

仅从代码量上就能看出来bean的加载经历了一个相当复杂的过程,其中涉及各种各样的考虑。

我们将上面的内容大致总结一下:

  • 转换对应beanName。
  • 尝试从缓存中加载单例。
  • bean的实例化。
  • 原型模式的依赖检查。
  • 检测parentBeanFactory。
  • 将存储XML配置文件的GernericBeanDefinition转换为RootBeanDefinition。
  • 寻找依赖。
  • 针对不同的scope进行bean的创建。
  • 类型转换。(强转)

其实读到这里,我们不难看出,大致跟我们之前的推测出入并不是很大,只是源码的开发人员们对代码的逻辑做的更为严谨,并且做了很多的必要的校验。

版权声明
本文为[xlecho]所创,转载请带上原文链接,感谢
https://my.oschina.net/u/4154037/blog/5022150

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