我想你们的努力一定会有回报的 —— 《龙与虎》
由一个简单的例子引出本文
代码如下：
user.java
@Data
public class User {
private String username;
private int id;
}
userbean.xml
<?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 name="user" class="com.mryan.springcore.demo.User">
<property name="username" value="MRyan"></property>
<property name="id" value="666"></property>
</bean>
</beans>
启动类，并实现了ApplicationRunner方法
@SpringBootApplication
public class DemoApplication implements CommandLineRunner {
public static void main(String[] args) {
SpringApplication.run(DemoApplication.class, args);
}
@Override
public void run(String... args) throws Exception {
ApplicationContext context = new ClassPathXmlApplicationContext("userbean.xml");
User user = (User) context.getBean("user");
System.out.println(user.toString());
}
}
xml配置文件向spring ioc容器中注册一个bean，并运行程序打印bean信息。
成功打印结果。
我们会发现貌似我们什么都不用知道，spring框架都替我们想到了，我们几乎不用做什么，spring框架替我们做了，那么问题来了，它是怎么做到的呢？对象到底是怎么创建的呢？
在阅读源码之前我们先补一下知识
知识小课堂
1. 我们可以用XML，配置类，注解等方式来定义bean信息，容器会读取bean信息，那容器如何能兼容读取这些不同方式定义的bean信息？
spring为我们定义了一个BeanDefinitionReader的统一抽象层，专门读取bean信息
2. 获得了bean信息我们可以直接用反射的方式获取对象这是我们的线性思维,显然是不合理的。
spring中引用了工厂模式为我们创建bean的实例，工厂类名为BeanFactory
3. 对象的创建过程分为实例化分配内存，和初始化。
spring中对象的创建也不例外，除了bean的实例化还需要对属性进行填充值，当然还需要执行自定义初始方法init-method（源码中见）
4. 在不同的阶段如果要做不同的操作怎么办？
spring中为我们创建了观察者模式：包括监听器，事件发布，当发布不同的事件之后，监听器接受到之后开始做相应的处理工作
源码分析
启动类ApplicationContext 处打断点一步一步调试
@SpringBootApplication
public class DemoApplication implements CommandLineRunner {
public static void main(String[] args) {
SpringApplication.run(DemoApplication.class, args);
}
@Override
public void run(String... args) throws Exception {
ApplicationContext context = new ClassPathXmlApplicationContext("userbean.xml");//断点处
User user = (User) context.getBean("user");
System.out.println(user.toString());
}
}
进入ClassPathXmlApplicationContext中
public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
this(new String[] {configLocation}, true, null);
}
public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
//使用给定的父上下文创建一个新的AbstractXmlApplicationContext
super(parent);
//设置此应用程序上下文的配置位置
setConfigLocations(configLocations);
if (refresh) {
//核心
refresh();
}
}
继续debug进入refresh方法，下面对于一些非核心的方法选择性跳过，只分析核心代码，对于refresh方法也可以看之前我写的文章《SpringBoot源码分析》 Context初始化流程
@Override
public void refresh() throws BeansException, IllegalStateException {
//1 加锁，确保了同一时间startupShutdownMonitor的状态不发生改变。
synchronized (this.startupShutdownMonitor) {
//2 为刷新做准备 设置启动日期和活跃状态等
prepareRefresh();
//3 ****核心****
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
//4 准备BeanFactory 各种设置 各种register这里不做叙述
prepareBeanFactory(beanFactory);
try {
//5 ****核心****
postProcessBeanFactory(beanFactory);
//6 执行BeanFactoryPostProcessors 实例化并且执行已经注册了的BeanFactoryPostProcessors
invokeBeanFactoryPostProcessors(beanFactory);
//7 实例化并且注册所有BeanPostProcessor的Beans
registerBeanPostProcessors(beanFactory);
//8 国际化配置
initMessageSource();
//9 初始化应用事件广播器。事件广播器用来向 ApplicationListener 通知各种应用产生的事件，是一个标准的观察者模式。
initApplicationEventMulticaster();
//10 是留给子类的扩展步骤，用来让特定的 Context 子类初始化其他的 Bean。
onRefresh();
//11 把实现了 ApplicationListener 的 Bean 注册到事件广播器，并对广播器中的早期未广播事件进行通知。观察者模式：包括监听器，事件发布，当发布不同的事件之后，监听器接受到之后开始做相应的处理工作
registerListeners();
// 12 ****核心****
finishBeanFactoryInitialization(beanFactory);
// 13 完成上下文的刷新工作，调用 LifecycleProcessor 的 onFresh() 方法以及发布 ContextRefreshedEvent 事件。
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
destroyBeans();
cancelRefresh(ex);
throw ex;
}
finally {
//14
resetCommonCaches();
}
}
}
在3处
根据代码命名我们很容易猜到obtainFreshBeanFactory方法作用是获取BeanFactory，并加载bean定义信息
进入方法
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
//判断当前容器中是否由BeanFactory，如果由就销毁，没有则创建
refreshBeanFactory();
//返回BeanFactory
return getBeanFactory();
}
进入refreshBeanFactory方法
@Override
protected final void refreshBeanFactory() throws BeansException {
//判断当前的beanFactory是否为空，不为空则证明当前容器中存在BeanFactory进行销毁，否则创建BeanFactory
if (hasBeanFactory()) {// return (this.beanFactory != null);
destroyBeans();
closeBeanFactory();
}
try {
//创建BeanFactory
DefaultListableBeanFactory beanFactory = createBeanFactory();
//设置序列号ID
beanFactory.setSerializationId(getId());
//自定义BeanFactory
customizeBeanFactory(beanFactory);
loadBeanDefinitions(beanFactory);
this.beanFactory = beanFactory;
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}
可以发现此时beanDefiinitionMap中就存在了之前定义的bean信息
在5处：
进入postProcessBeanFactory方法
protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
}
空的方法，这其实是Spring为我们提供让我们自行扩展的方法，在上面我们获取了BeanDefinition，在放入BeanFactory之前我们可以对bean信息进行修改等操作，我们就可以实现BeanFactoryPostProcessor并重写postProcessBeanFactory方法在其中添加逻辑代码。
在第12处开始
就进入最最最核心的地方。
进入finishBeanFactoryInitialization内部
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// 为此上下文初始化转换服务。
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
}
// 如果之前没有任何bean后处理器（例如，PropertyPlaceholderConfigurer Bean）进行注册，请注册默认的嵌入式值解析器：此时，主要用于解析注释属性值。
if (!beanFactory.hasEmbeddedValueResolver()) {
beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
}
// 尽早初始化LoadTimeWeaverAware Bean，以便尽早注册其转换器。
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
// 停止使用临时ClassLoader进行类型匹配。
beanFactory.setTempClassLoader(null);
// 允许缓存所有bean定义元数据，而不期望进一步的更改。
beanFactory.freezeConfiguration();
// ****核心**** 实例化所有剩余的（非延迟初始化）单例。
beanFactory.preInstantiateSingletons();
}
我们可以看到最后一行标注核心的代码，我们继续进入方法
@Override
public void preInstantiateSingletons() throws BeansException {
****省略之前
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// 触发所有非惰性单例bean的初始化...
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
//如果它不是抽象的并且是单例是懒加载的才进入此判断语句
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//如果FactoryBean中存在该beanName走此判断语句
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged(
(PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
//以上不符合条件 显然跳到了这里
else {
getBean(beanName);
}
}
}
***省略之后
首先核心代码处for循环，从beanDefinitionNames中获取BeanName，显而易见本例中只有一个beanName=user
进入getBean方法
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
在进入doGetBean方法
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
***省略以上代码
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
//断点
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
destroySingleton(beanName);
throw ex;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
***省略以下代码
如果是单例的话进入判断，进入createBean方法 此时完成对bean的创建
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
***省略以上代码
try {
//doCreateBean 实际创建指定的bean
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
进入doCreateBean方法 此时才进行Bean的实例化
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
// 实例化bean
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//*****核心***** 使用适当的实例化策略为指定的bean创建一个新实例
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isTraceEnabled()) {
logger.trace("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
来看看createBeanInstance做了什么吧
使用适当的实例化策略为指定的bean创建一个新实例，
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
Class<?> beanClass = resolveBeanClass(mbd, beanName);
if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
}
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
// Candidate constructors for autowiring?
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
//默认构造的首选构造函数 案例中没有给定构造器
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
//无需特殊处理：只需使用no-arg构造函数
return instantiateBean(beanName, mbd);
}
上述流程进入到instantiateBean方法
此时才会使用其默认构造函数实例化给定的bean
protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {
try {
Object beanInstance;
if (System.getSecurityManager() != null) {
beanInstance = AccessController.doPrivileged(
//核心 在此工厂中以给定名称返回Bean的实例
(PrivilegedAction<Object>) () ->
getInstantiationStrategy().instantiate(mbd, beanName, this),
getAccessControlContext());
}
else {
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
}
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}
进入instantiate方法中
此时在此工厂中以给定名称返回Bean的实例，并且过程中调用ctor.newInstance(argsWithDefaultValues); 完成了实例化
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
if (!bd.hasMethodOverrides()) {
Constructor<?> constructorToUse;
synchronized (bd.constructorArgumentLock) {
constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
if (constructorToUse == null) {
final Class<?> clazz = bd.getBeanClass();
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
if (System.getSecurityManager() != null) {
constructorToUse = AccessController.doPrivileged(
(PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
}
else {
constructorToUse = clazz.getDeclaredConstructor();
}
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
//此处执行ctor.newInstance(argsWithDefaultValues); 真正的完成对象的实例化
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
继续step over结果逐层返回直到到AbstractAutowireCapableBeanFactory类的doCreateBean方法
可以看到当前bean对象已经被实例化出来了，但是还没有设置属性值。
下方有段核心代码
//填充bean 其实就是填充属性值
populateBean(beanName, mbd, instanceWrapper);
结果逐层返回，我们发现属性值填充成功
接着流程走到initializeBean方法
protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareMethods(beanName, bean);
return null;
}, getAccessControlContext());
}
else {
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
//BeanPostProcessor(前置方法）
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
//bean初始化用户自定义初始方法init-method
invokeInitMethods(beanName, wrappedBean, mbd);
}
catch (Throwable ex) {
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
}
if (mbd == null || !mbd.isSynthetic()) {
//BeanPostProcessor（后置方法）
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
到此为止完成了bean对象的读取，创建，实例化，执行用户自定义初始方法init-method。
可能源码不好理解，没关系，我画了个图，方便过下流程
作者：MRyan
本文采用 知识共享署名-相同方式共享 4.0 国际许可协议 进行许可。
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