一线大厂面试必问spring源码系列之「Bean的生命周期」

时间:2021-7-3 作者:qvyue

目录

  • 1. Bean的实例化概述
  • 2. 流程概览
  • 3. 源码分析
  • 4. 演示
  • 4. 总结

一线大厂面试必问spring源码系列之「Bean的生命周期」
image

为源码付出的每一分努力都不会白费。

1. Bean的实例化概述

前一篇分析了BeanDefinition的封装过程,最终将beanName与BeanDefinition以一对一映射关系放到beanDefinitionMap容器中,这一篇重点分析如何利用bean的定义信息BeanDefinition实例化bean。

2. 流程概览

其实bean的实例化过程比较复杂,中间细节很多,为了抓住重点,先将核心流程梳理出来,主要包含以下几个流程:

一线大厂面试必问spring源码系列之「Bean的生命周期」
image

step1: 通过反射创建实例;
step2:给实例属性赋初始值;
step3:如果Bean类实现BeanNameAware接口,则将通过传递Bean的名称来调用setBeanName()方法;如果Bean类实现BeanClassLoaderAware接口,则将通过传递加载此Bean的ClassLoader对象的实例来调用setBeanClassLoader()方法;如果Bean类实现BeanFactoryAware接口,则将通过传递BeanFactory对象的实例来调用setBeanFactory()方法;
step4: 如果有类实现BeanPostProcessors接口,则将在初始化之前调用postProcessBeforeInitialization()方法;
step5:如果Bean类实现了InitializingBean接口,将调用afterPropertiesSet()方法,如果配置文件中的Bean定义包含init-method属性,则该属性的值将解析为Bean类中的方法名称,并将调用该方法;
step6: 如果有类实现BeanPostProcessors接口,则将在初始化之后调用postProcessAfterInitialization()方法;
step7:如果Bean类实现DisposableBean接口,则当Application不再需要Bean引用时,将调用destroy()方法;如果配置文件中的Bean定义包含destroy-method属性,那么将调用Bean类中的相应方法定义。

3. 源码分析

进入AbstractApplicationContext中的fresh()方法,找到finishBeanFactoryInitialization(beanFactory)方法,该类是bean的实例化的入口,具体的实例化由preInstantiateSingletons()方法触发,见如下代码:

 

public void preInstantiateSingletons() throws BeansException 
  if (logger.isTraceEnabled()) {
            logger.trace("Pre-instantiating singletons in " + this);
        }

        // 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.
        //xml解析时,把所有beanName都缓存到beanDefinitionNames了        List beanNames = new ArrayList(this.beanDefinitionNames); 
        // Trigger initialization of all non-lazy singleton beans...        for (String beanName : beanNames) {
            //把父BeanDefinition里面的属性拿到子BeanDefinition中           RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);             //如果不是抽象的,单例的,非懒加载的就实例化
            if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
                //判断bean是否实现了FactoryBean接口              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) ((SmartFactoryBean>) factory)::isEagerInit,
                                    getAccessControlContext());
                        }
                        else {
                            isEagerInit = (factory instanceof SmartFactoryBean &&
                                    ((SmartFactoryBean>) factory).isEagerInit());
                        }
                        if (isEagerInit) {
                            getBean(beanName);                      }
                    }
                }
                else {
                    // 实例化过程
                    getBean(beanName);              }
            }
        }

上述代码主要看getBean方法,随后进入doGetBean方法:

 

 protected  T doGetBean(
            String name, @Nullable Class requiredType, @Nullable Object[] args, boolean typeCheckOnly)
            throws BeansException {

        String beanName = transformedBeanName(name);
        Object bean;

        // 从缓存中获取bean.
        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 {
                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
                                // 创建bean实例
                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);
                }
...
}

由上述代码可知,先从缓存中获取bean,如果没有,则创建bean,最重要的方法就是getSingleton,该方法第二个参数是个函数式接口,进入getSingleton方法,当调用singletonObject = singletonFactory.getObject()时,会触发函数式接口中的createBean方法,随后一路进入doCreateBean,这个方法里面完成了所有实例化所需的步骤:

 

 protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
            throws BeanCreationException {

        // Instantiate the bean.
        // 真正开始创建bean的实例.
        BeanWrapper instanceWrapper = null;
        if (mbd.isSingleton()) {
            instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
        }
        if (instanceWrapper == null) {
            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 {
            // 属性赋值
            populateBean(beanName, mbd, instanceWrapper);
            // 初始化bean
            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);
            }
        }

        if (earlySingletonExposure) {
            Object earlySingletonReference = getSingleton(beanName, false);
            if (earlySingletonReference != null) {
                if (exposedObject == bean) {
                    exposedObject = earlySingletonReference;
                }
                else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                    String[] dependentBeans = getDependentBeans(beanName);
                    Set actualDependentBeans = new LinkedHashSet(dependentBeans.length);
                    for (String dependentBean : dependentBeans) {
                        if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                            actualDependentBeans.add(dependentBean);
                        }
                    }
                    if (!actualDependentBeans.isEmpty()) {
                        throw new BeanCurrentlyInCreationException(beanName,
                                "Bean with name '" + beanName + "' has been injected into other beans [" +
                                StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                                "] in its raw version as part of a circular reference, but has eventually been " +
                                "wrapped. This means that said other beans do not use the final version of the " +
                                "bean. This is often the result of over-eager type matching - consider using " +
                                "'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
                    }
                }
            }
        }

        // Register bean as disposable.
                // 有必要时,注册bean的销毁
        try {
            registerDisposableBeanIfNecessary(beanName, bean, mbd);
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
        }

        return exposedObject;
    }

从上述源码中看出bean的实例化主要分为以下三步:
step1:bean的创建;
step2:给bean的属性赋值;
step3:bean的初始化;
接着得到exposedObject这个已经完全实例化后的bean返回,其中当有必要时,注册bean的销毁,后面再详细看,先抓住主要流程。其中step3也是比较重要的方法,进入该方法:

 

 protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
        if (System.getSecurityManager() != null) {
            AccessController.doPrivileged((PrivilegedAction) () -> {  invokeAwareMethods(beanName, bean);  return null;  }, getAccessControlContext());  }  else {  // 激活aware接口  invokeAwareMethods(beanName, bean);  }    Object wrappedBean = bean;  if (mbd == null || !mbd.isSynthetic()) {  // 初始化前处理的beanPostProcessor  wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);  }    try {  // 激活 init-method方法  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()) {  // 初始化后处理的beanPostProcessor  wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);  }    return wrappedBean;  }

从上面源码可知,梳理出主要的四个步骤:
step1:激活aware接口,完成aware接口的相关操作;
step2:初始化前处理的beanPostProcessor;
step3:完成init-method方法;
step4:初始化后处理的beanPostProcessor;
BeanPostProcessor作用是对初始化后的bean进行增强处理,在该阶段 BeanPostProcessor 会处理当前容器内所有符合条件的实例化后的 bean 对象。它主要是对 Spring 容器提供的 bean 实例对象进行有效的扩展,允许Spring在初始化 bean 阶段对其进行定制化修改,如处理标记接口或者为其提供代理实现。

4. 演示

定义一个MyBeanPostProcessor实现BeanPostProcessor接口

 

public class MyBeanPostProcessor implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("post Process Before Initialization 被调用...");
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("post Process after Initialization 被调用...");
        return bean;
    }
}

定义一个LifeCycleBean类,实现如下接口:

 

public class LifeCycleBean implements BeanNameAware, BeanFactoryAware, BeanClassLoaderAware,
        InitializingBean, DisposableBean {

    private String property;

    public String getProperty() {
        return property;
    }

    public void setProperty(String property) {
        System.out.println("属性注入....");
        this.property = property;
    }

    public LifeCycleBean(){
        System.out.println("构造函数调用...");
    }
    @Override  public void setBeanClassLoader(ClassLoader classLoader) {
        System.out.println("BeanClassLoaderAware 被调用...");
    }

    @Override  public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
        System.out.println("BeanFactoryAware 被调用...");
    }

    @Override  public void setBeanName(String name) {
        System.out.println("BeanNameAware 被调用...");
    }

    @Override  public void destroy() throws Exception {
        System.out.println("DisposableBean destroy 被调用...");
    }

    @Override  public void afterPropertiesSet() throws Exception {
        System.out.println("InitializingBean afterPropertiesSet 被调用...");
    }

    public void initMethod(){
        System.out.println("init-method 被调用...");
    }

    public void destroyMethod(){
        System.out.println("destroy-method 被调用...");
    }

    public void display(){
        System.out.println("方法调用...");
    }

}

指定配置文件spring.xml,配置init-methoddestroy-method方法

 

 

测试类如下:

 

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = {"classpath:spring.xml"})
public class TestSpring {

    @Test
    public void testLifeCycleBean() {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");

    }

执行结果:

 

构造函数调用...
属性注入....
BeanNameAware 被调用...
BeanClassLoaderAware 被调用...
BeanFactoryAware 被调用...
post Process Before Initialization 被调用...
InitializingBean afterPropertiesSet 被调用...
init-method 被调用...
post Process after Initialization 被调用...
DisposableBean destroy 被调用...
destroy-method 被调用...

4. 总结

本篇从一个初学者的角度概览了bean的整个生命周期,并描述了其中的主要流程,阅读源码的初始阶段,优先抓住主要流程,别陷入细节,并通过跑案例、写注解、画流程图等方式加深理解,后续将继续分析bean实例化中的核心流程、设计思想等。

声明:本文内容由互联网用户自发贡献自行上传,本网站不拥有所有权,未作人工编辑处理,也不承担相关法律责任。如果您发现有涉嫌版权的内容,欢迎发送邮件至:qvyue@qq.com 进行举报,并提供相关证据,工作人员会在5个工作日内联系你,一经查实,本站将立刻删除涉嫌侵权内容。