上一篇博客主要介绍了bean加载的主要流程,本篇博客主要精讲各个步骤的详细实现
1. 缓存中获取单例bean
单例在Spring的同一个容器内只会被创建一次,后续再获取bean直接从单例缓存中获取,当然这里也只是尝试加载,首先尝试从缓存中加载,然后再次尝试从singletonFactorry加载因为在创建单例bean的时候会存在依赖注入的情况,而在创建依赖的时候为了避免循环依赖,Spring创建bean的原则不等bean创建完成就会创建bean的ObjectFactory提早曝光加入到缓存中,一旦下一个bean创建时需要依赖上个bean,则直接使用ObjectFactory,接下来我们看下获取单例bean的方法getSingleton(beanName),进入方法体:
public Object getSingleton(String beanName) {
//参数true是允许早期依赖
return getSingleton(beanName, true);
}
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//检查缓存中是否存在实例
Object singletonObject = this.singletonObjects.get(beanName);
//如果缓存为空且单例bean正在创建中,则锁定全局变量
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
//如果此bean正在加载,则不处理
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
//当某些方法需要提前初始化的时候会直接调用addSingletonFactory把对应的ObjectFactory初始化策略存储在singletonFactory中
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//使用预先设定的getObject方法
singletonObject = singletonFactory.getObject();
记录在缓存中,注意earlySingletonObjects和singletonFactories是互斥的
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
这个方法先尝试从singletonObjects里面获取实例,如果如果获取不到再从earlySingletonObjects里面获取,如果还获取不到,再尝试从singletonFactories里面获取beanName对应的ObjectFactory,然后再调用这个ObjectFactory的getObject方法创建bean,并放到earlySingletonObjects里面去,并且从singletonFactoryes里面remove调这个ObjectFactory,而对于后续所有的内存操作都只为了循环依赖检测时候使用,即allowEarlyReference为true的时候才会使用。
这里涉及到很多个存储bean的不同map,简单解释下:
singletonObjects:用于保存BeanName和创建bean实例之间的关系,beanName–>bean Instance
singletonFactories:用于保存BeanName和创建bean的工厂之间的关系,banName–>ObjectFactory
earlySingletonObjects:也是保存BeanName和创建bean实例之间的关系,与singletonObjects的不同之处在于,当一个单例bean被放到这里面后,那么当bean还在创建过程中,就可以通过getBean方法获取到了,其目的是用来检测循环引用。
registeredSingletons:用来保存当前所有已注册的bean.
2. 从bean的实例中获取对象
获取到bean以后就要获取实例对象了,这里用到的是getObjectForBeanInstance方法。getObjectForBeanInstance是个频繁使用的方法,无论是从缓存中获得bean还是根据不同的scope策略加载bean.总之,我们得到bean的实例后,要做的第一步就是调用这个方法来检测一下正确性,其实就是检测获得Bean是不是FactoryBean类型的bean,如果是,那么需要调用该bean对应的FactoryBean实例中的getObject()作为返回值。
无论是从缓存中获取到的bean还是通过不同的scope策略加载的bean都只是最原始的bean状态,并不一定是我们最终想要的bean.举个例子,加入我们需要对工厂bean进行的处理,那么这里得到的其实是工厂bean的初始状态,但我们真正想要的是工厂bean中定义的factory-method方法中返回的bean,而getObjectForBeanInstance方法就是完成这个。
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
//如果指定的name是工厂相关的(以&开头的)
if (BeanFactoryUtils.isFactoryDereference(name)) {
//如果是NullBean则直接返回此bean
if (beanInstance instanceof NullBean) {
return beanInstance;
}
//如果不是FactoryBean类型,则验证不通过抛出异常
if (!(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
}
// Now we have the bean instance, which may be a normal bean or a FactoryBean.
// If it's a FactoryBean, we use it to create a bean instance, unless the
// caller actually wants a reference to the factory.
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
Object object = null;
if (mbd == null) {
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
我们详细分析下代码会发现此方法中没有重要信息,大多都是一些辅助功能性的检查判断,而真正的核心功能是在getObjectFromFactoryBean(factory, beanName, !synthetic)方法中实现的
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
object = doGetObjectFromFactoryBean(factory, beanName);
// Only post-process and store if not put there already during getObject() call above
// (e.g. because of circular reference processing triggered by custom getBean calls)
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
if (alreadyThere != null) {
object = alreadyThere;
}
else {
if (shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
}
if (containsSingleton(beanName)) {
this.factoryBeanObjectCache.put(beanName, object);
}
}
}
return object;
}
}
else {
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
return object;
}
}
在这段代码中,没有太多东西,基本上做了一件事情,就是如果返回的bean是单例的,就必须保证全局唯一,同时不可重复创建,可以使用缓存来提高性能,也就是说加载过就记录下来以便于下次复用。而在doGetObjectFromFactoryBean我们看到了想要的方法,就是Object=factory.getObejct(),继续看方法
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
object = factory.getObject();
}
}
catch (FactoryBeanNotInitializedException ex) {
throw new BeanCurrentlyInCreationException(beanName, ex.toString());
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
}
// Do not accept a null value for a FactoryBean that's not fully
// initialized yet: Many FactoryBeans just return null then.
if (object == null) {
if (isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
object = new NullBean();
}
return object;
}
如果bean声明为FactoryBean类型,则当提取bean时候提取的不是FactoryBean,而是FactoryBean中对应的getObject方法返回的bean,而doGetObjectFromFactroyBean真是实现这个功能。
3. 获取单例
之前我们说过从缓存中获取单例的过程,但如果缓存中不存在已经加载的单例bean就需要从头开始bean的加载过程了,Spring中使用了getSingleton的重载方法实现bean的加载过程。
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
afterSingletonCreation(beanName);
}
if (newSingleton) {
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
上述代码中其实使用了回调方法,使得程序可以在单例创建的前后做一些准备及处理操作,而真正获取单例bean的方法其实并不是在此方法中实现的,其实现逻辑是在ObjectFactory类型的实例singletonFactory中实现的。而这些准备及处理操作包括如下内容。
(1)检查缓存是否已经加载过
(2)如果没有加载,则记录beanName的正在加载状态
(3)加载单例前记录加载状态。
可能你会觉得beforeSingletonCreation方法是个空实现,里面没有任何逻辑,但其实这个函数中做了一个很重要的操作:记录加载状态,也就是通过this.singletonsCurrentlyInCreation.add(beanName)将当前正要创建的bean记录在缓存中,这样便可以对循环依赖进行检测。
(4)通过调用参数传入的ObjectFactory的个体Object方法实例化bean.
(5)加载单例后的处理方法调用。
同步骤3的记录加载状态相似,当bean加载结束后需要移除缓存中对该bean的正在加载状态的记录。
(6)将结果记录至缓存并删除加载bean过程中所记录的各种辅助状态。
(7)返回处理结果
4. 准备创建bean
createBean是真正创建bean的地方,此方法是定义在AbstractAutowireCapableBeanFactory中,我们看下其源码:
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
try {
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
我们详细分析下代码的流程:
(1)设置class属性或者根据className来解析class
(2)验证MethodOverride属性进行验证及标记
(3)应用初始化前的后处理器,解析指定bean是否存在应用初始化前的短路操作
(4)创建bean