【spring源码】bean的实例化（转载）

首先来看一段代码，看过上一节的朋友肯定对这段代码并不陌生。这一段代码诠释了Spring加载bean的完整过程，包括读取配置文件，扫描包，加载类，实例化bean，注入bean属性依赖。

上一节介绍了Spring是如何加载class文件的，本节主要围绕finishBeanFactoryInitialization(beanFactory)方法，聊聊Spring是如何实例化bean的，从上面代码片段中的注解不难看出，此方法主要的任务就是实例化非懒加载的单例bean。闲话少叙，看代码。

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    // Initialize conversion service for this context.
    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));
    }  

    // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
    String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
    for (String weaverAwareName : weaverAwareNames) {
        getBean(weaverAwareName);
    }  

    // Stop using the temporary ClassLoader for type matching.
    beanFactory.setTempClassLoader(null);  

    // Allow for caching all bean definition metadata, not expecting further changes.
    beanFactory.freezeConfiguration();  

    // Instantiate all remaining (non-lazy-init) singletons.
    beanFactory.preInstantiateSingletons();
}
 

上面代码主要看最后一句beanFactory.preInstantiateSingletons()。

public void preInstantiateSingletons() throws BeansException {
    if (this.logger.isDebugEnabled()) {
        this.logger.debug("Pre-instantiating singletons in " + this);
    }
    List<String> beanNames;
    synchronized (this.beanDefinitionMap) {
        // Iterate over a copy to allow for init methods which in turn register new bean definitions.
        // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
        beanNames = new ArrayList<String>(this.beanDefinitionNames);
    }
    for (String beanName : beanNames) {
        RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
        if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
            if (isFactoryBean(beanName)) {
                final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
                boolean isEagerInit;
                if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                    isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
                        @Override
                        public Boolean run() {
                            return ((SmartFactoryBean<?>) factory).isEagerInit();
                        }
                    }, getAccessControlContext());
                }
                else {
                    isEagerInit = (factory instanceof SmartFactoryBean &&
                            ((SmartFactoryBean<?>) factory).isEagerInit());
                }
                if (isEagerInit) {
                    getBean(beanName);
                }
            }
            else {
                getBean(beanName);
            }
        }
    }
}
 

此方法首先将加载进来的beanDefinitionNames循环分析，如果是我们自己配置的bean就会走else中的getBean(beanName)，接着看。

@Override
public Object getBean(String name) throws BeansException {
    return doGetBean(name, null, null, false);
}
 

doGetBean方法内容太多，一段一段看。

protected <T> T doGetBean(
        final String name, final Class<T> requiredType, 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.isDebugEnabled()) {
            if (isSingletonCurrentlyInCreation(beanName)) {
                logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
                        "' that is not fully initialized yet - a consequence of a circular reference");
            }
            else {
                logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
            }
        }
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }  

这里主要看Object sharedInstance = getSingleton(beanName)。

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        synchronized (this.singletonObjects) {
            singletonObject = this.earlySingletonObjects.get(beanName);
            if (singletonObject == null && allowEarlyReference) {
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    singletonObject = singletonFactory.getObject();
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
 

这里能看到，Spring会把实例化好的bean存入singletonObjects，这是一个ConcurrentHashMap，

private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(64);  

当然这里我们bean并未实例化过，所以这里应该也不能get出什么东西来，也就是返回null了。if子句也就不会执行了。那么接着看else子句的内容。

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 (args != null) {
            // Delegation to parent with explicit args.
            return (T) parentBeanFactory.getBean(nameToLookup, args);
        }
        else {
            // No args -> delegate to standard getBean method.
            return parentBeanFactory.getBean(nameToLookup, requiredType);
        }
    }
 

这两条验证也都不会实现，接写来就是重点了。

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 dependsOnBean : dependsOn) {
            if (isDependent(beanName, dependsOnBean)) {
                throw new BeanCreationException("Circular depends-on relationship between '" +
                        beanName + "' and '" + dependsOnBean + "'");
            }
            registerDependentBean(dependsOnBean, beanName);
            getBean(dependsOnBean);
        }
    }  

    // Create bean instance.
    if (mbd.isSingleton()) {
        sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
            @Override
            public Object getObject() throws BeansException {
                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 '" + scopeName + "'");
        }
        try {
            Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
                @Override
                public Object getObject() throws BeansException {
                    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);
        }
    }
}
 

在这里拿到RootBeanDefinition并check，并获得bean的依赖，并循环迭代实例化bean。例如class A依赖于class B，就会先实例化B。下面的if ... else ...就是真正实例化bean的地方。其实真正实例化bean的方法是createBean(beanName, mbd, args)，只是区分了isSingleton或isPrototype，两者的区别在于，单例的(Singleton)被缓存起来，而Prototype是不用缓存的。首先看一下createBean(beanName, mbd, args)。createBean方法中除了做了一些实例化bean前的检查准备工作外，最核心的方法就是

Object beanInstance = doCreateBean(beanName, mbd, args);
 

由于这个过程涉及到的代码都是一大坨，就不贴出所有代码了。

首先就是创建一个bean的实例且封装到BeanWrapper中，在这里bean已经实例化了。具体的实现方法是在org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner)中。

在这里不难看出实例化分两种情况，如果没有无参构造器是就生成CGLIB子类，否则就直接反射成实例。

既然已经有了实例对象了，那么，Spring是如何将bean的属性注入到bean的呢？返回到上面的doCreateBean方法中。往下看找到populateBean(beanName, mbd, instanceWrapper);，内幕就在这里。只贴部分代码：

这里是调用InstantiationAwareBeanPostProcessor的具体子类的ibp.postProcessPropertyValues方法注入属性。当我们使用@Resource注解的时候，具体的子类是CommonAnnotationBeanPostProcessor；如果使用的是@Autowired注解，则具体的子类是AutowiredAnnotationBeanPostProcessor。此方法内是委托InjectionMetadata对象来完成属性注入。

findAutowiringMetadata方法能拿到使用了特定注解的属性(Field)、方法(Method)及依赖的关系保存到checkedElements集合<Set>里，然后再执行自己的inject方法。

真正干事的还是InjectedElement的inject方法。

其实别看代码这么多，最关键的部分就是：

在这里也就真相大白了，就是通过JDK反射特性，直接set值的。