Spring的bean实例化过程

素颜马尾好姑娘i 2021-11-02 04:24 413阅读 0赞

以XmlBeanFactory为例，最简单的取bean方式是：

BeanFactory factory = new XmlBeanFactory(new FileSystemResource("D:\\workspace\\JavaApplication2\\src\\javaapplication2\\spring\\beans.xml"));  
                Car obj = (Car)factory.getBean("car");

Bean的配置文件内容也很简单：

先看起始点，载入先走AbstractBeanFactory

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

doGetBean方法中：

// Create bean instance. 
    if (mbd.isSingleton()) {   
             //传入一个内联类ObjectFactory并实现了getObject方法。 
             sharedInstance = getSingleton(beanName, new ObjectFactory() {   
        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); //有异常则销毁bean 
                throw ex;  
        }  
    }  
    });  
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);  
            //此处开始实例化bean

通过new ObjectFactory()的回调方法，回调当前类继承的createBean方法，该方法在父类AbstractAutowireCapableBeanFactory中：

AbstractAutowireCapableBeanFactory->

protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)  
                throws BeanCreationException {   
      
            // Make sure bean class is actually resolved at this point. 
            resolveBeanClass(mbd, beanName);  //载入该bean的class，并放置到mbd里面,bean的生成不在这里。 
      
            // Prepare method overrides. 
            try {   
                mbd.prepareMethodOverrides();  
            }  
            catch (BeanDefinitionValidationException ex) {   
                throw new BeanDefinitionStoreException(mbd.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, mbd);  //尝试获取一个proxy，普通bean这里一般是空的返回 
                  
                if (bean != null) {   
                    return bean;  
                }  
            }  
            catch (Throwable ex) {   
                throw new BeanCreationException(mbd.getResourceDescription(), beanName,  
                        "BeanPostProcessor before instantiation of bean failed", ex);  
            }  
      
            Object beanInstance = doCreateBean(beanName, mbd, args);  //开始create bean的实例，mbd中包括了需要的class 
            if (logger.isDebugEnabled()) {   
                logger.debug("Finished creating instance of bean '" + beanName + "'");  
            }  
            return beanInstance;  
        }

进入AbstractBeanFactory中的protected Class resolveBeanClass方法：

try {   
        if (mbd.hasBeanClass()) {   
            return mbd.getBeanClass();  
        }  
        if (System.getSecurityManager() != null) {   
            return AccessController.doPrivileged(new PrivilegedExceptionAction<Class>() {   
                public Class run() throws Exception {   
                    return doResolveBeanClass(mbd, typesToMatch);  
                }  
            }, getAccessControlContext());  
        }  
        else {   
            return doResolveBeanClass(mbd, typesToMatch);   <---还要继续进去看生成方法。  
        }  
    }

转入doResolveBeanClass：

private Class doResolveBeanClass(RootBeanDefinition mbd, Class... typesToMatch) throws ClassNotFoundException {   
        if (!ObjectUtils.isEmpty(typesToMatch)) {   
            ClassLoader tempClassLoader = getTempClassLoader();  
            if (tempClassLoader != null) {   
                if (tempClassLoader instanceof DecoratingClassLoader) {   
                    DecoratingClassLoader dcl = (DecoratingClassLoader) tempClassLoader;  
                for (Class<?> typeToMatch : typesToMatch) {   
                    dcl.excludeClass(typeToMatch.getName())  
                }  
            }  
            String className = mbd.getBeanClassName();  
            return (className != null ? ClassUtils.forName(className, tempClassLoader) : null); //通过自己的ClassUtils的forName方法来实例化class 
            }  
        }  
        return mbd.resolveBeanClass(getBeanClassLoader());  <----这里传入了bean的classloader，下面继续看这里  
    }

AbstractBeanDefinition->resolveBeanClass

public Class resolveBeanClass(ClassLoader classLoader) throws ClassNotFoundException {   
            String className = getBeanClassName();  
            if (className == null) {   
                return null;  
            }  
            Class resolvedClass = ClassUtils.forName(className, classLoader);//classloader传入后，仍然是用forName方法加载class 
            this.beanClass = resolvedClass;  
            return resolvedClass;  
        }

再来看forName做了些什么

ClassUtils ->

ClassLoader classLoaderToUse = classLoader;  
    if (classLoaderToUse == null) {   
        classLoaderToUse = getDefaultClassLoader();  
    }  
    try {   
        return classLoaderToUse.loadClass(name); //也比较简单，直接调用loadClass方法加载 
    }

最终将class load进来。

进入

AbstractAutowireCapableBeanFactory->instantiateBean

protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd)  
    ...这里省略没用的        
        else {   
            beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);//下面看这里的实例化 
        }         
                  
        BeanWrapper bw = new BeanWrapperImpl(beanInstance);   //返回一个包装类对象 
        initBeanWrapper(bw);  
        return bw;

SimpleInstantiationStrategy->instantiate

public Object instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner)  
        synchronized (beanDefinition.constructorArgumentLock) {   
                constructorToUse = (Constructor<?>) beanDefinition.resolvedConstructorOrFactoryMethod;  
            ...  
        }  
        return BeanUtils.instantiateClass(constructorToUse);     //BeanUtils来初始化实例 ，给出了实例化需要的构造函数

再来看BeanUtils的实例化方法，比较简单，直接用反射的构造函数来newInstance。

BeanUtils->

public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {   
        try {   
            ReflectionUtils.makeAccessible(ctor);  
            return ctor.newInstance(args);  
        }

AbstractAutowireCapableBeanFactory->

Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)   
      
        // Initialize the bean instance. 
            Object exposedObject = bean;  
            try {   
                populateBean(beanName, mbd, instanceWrapper);  
                if (exposedObject != null) {   
                    exposedObject = initializeBean(beanName, exposedObject, mbd);  
                }  
            }  
        return exposedObject;       //返回给AbstractBeanFactory