关于spring:Spring-源码学习-16单例-Bean-创建

前言

在 finishBeanFactoryInitialization 中介绍了创立 Bean 的流程大略流程，这里进入单例 Bean 的创立过程。

这里次要分为三个局部创立单例 Bean

1. getSingleton
2. createBean
3. getObjectForBeanInstance

上面进入源码：

getSingleton

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {Assert.notNull(beanName, "Bean name must not be null");
    // 加锁
    synchronized (this.singletonObjects) {
        // 查看 singletonObjects 缓存中是否有
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null) {
            // 查看是否在执行销毁
            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 + "'");
            }
            // 将 Bean 增加到 singletonsCurrentlyInCreation 汇合中, 示意正在创立
            beforeSingletonCreation(beanName);
            boolean newSingleton = false;
            boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
            if (recordSuppressedExceptions) {this.suppressedExceptions = new LinkedHashSet<>();
            }
            try {
                // 调用工厂办法
                // 也就是调用 createBean(beanName, mbd, args)
                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;}
                // 创立胜利, 从 singletonsCurrentlyInCreation 移除
                afterSingletonCreation(beanName);
            }
            if (newSingleton) {
                // 将给定的单例对象增加到该工厂的单例缓存中
                //     this.singletonObjects.put(beanName, singletonObject);
                //     this.singletonFactories.remove(beanName);
                //     this.earlySingletonObjects.remove(beanName);
                //     this.registeredSingletons.add(beanName);
                addSingleton(beanName, singletonObject);
            }
        }
        return singletonObject;
    }
}

返回以给定名称注册的（原始）单例对象，如果尚未注册，则创立并注册一个新对象。

这一块一共能够拆成三局部来了解：

1. 从缓存中获取 singletonObjects

singletonObject 是什么？

/** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

singletonObjects 是一个 ConcurrentHashMap，用来缓存单例对象的实例。

2. 创立 singletonObject

在从缓存中没有获取到 singletonObject，创立新的对象

singletonObject = singletonFactory.getObject();

这一步其实就是调用外边的 createBean(beanName, mbd, args) 办法，这是一个工厂办法。通过 createBean 办法，会创立一个新的 singletonObject。

3. 将创立的 singletonObject 增加到缓存中

protected void addSingleton(String beanName, Object singletonObject) {synchronized (this.singletonObjects) {this.singletonObjects.put(beanName, singletonObject);
        this.singletonFactories.remove(beanName);
        this.earlySingletonObjects.remove(beanName);
        // 曾经胜利创立的单例
        this.registeredSingletons.add(beanName);
    }
}

这一步波及到三个缓存，以及一个胜利创立的单例列表。

/** Cache of singleton objects: bean name to bean instance. */
/** 缓存单例对象，K-V -> BeanName - Bean 实例 */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

/** Cache of singleton factories: bean name to ObjectFactory. */
/** 缓存 Bean 工厂 */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

/** Cache of early singleton objects: bean name to bean instance. */
/** 缓存晚期单例对象 */
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

/** Set of registered singletons, containing the bean names in registration order. */
/** 已注册的单例列表，按注册程序保留 BeanName。*/
private final Set<String> registeredSingletons = new LinkedHashSet<>(256);

将创立的单例对象，增加到 单例缓存 中，同时将 工厂缓存 以及 晚期单例对象缓存 中的对应对象删除。

createBean

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
            throws BeanCreationException {if (logger.isTraceEnabled()) {logger.trace("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.
    // 获取实在的类型
    Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
    if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
        // 创立新的 mbd 避免 其余线程批改
        mbdToUse = new RootBeanDefinition(mbd);
        mbdToUse.setBeanClass(resolvedClass);
    }

    // Prepare method overrides.
    try {
        // 验证并筹备为此 bean 定义的办法代替。查看是否存在具备指定名称的办法。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.
        // 利用实例化之前的后处理器，以解决指定的 bean 是否存在实例化快捷方式。// InstantiationAwareBeanPostProcessor 后置处理器 
        // postProcessBeforeInstantiation 办法可能会曾经实例化 Bean
        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 {
        // 实例化 Bean
        Object beanInstance = doCreateBean(beanName, mbdToUse, args);
        if (logger.isTraceEnabled()) {logger.trace("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);
    }
}

这个办法中波及到：创立 Bean 实例 , 填充 Bean , 利用 PostProcessor。

其中实例化 Bean 是在 doCreateBean 中。当初重点看一下 doCreateBean 办法。

doCreateBean

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

    // Instantiate the bean.
    // Bean 的 对象包装
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        // 从缓存中获取
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        // 缓存中获取不到则间接创立, 这里创立的 BeanInstance !!!
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    // 获取 Bean 实例以及类型
    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 {
                // 如果容许批改 mbd
                // 调用 MergedBeanDefinitionPostProcessor 后置处理器的
                // postProcessMergedBeanDefinition(mbd, beanType, beanName);
                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.
    // mbd 是单例 且 容许循环援用, (默认 true) 且在创立
    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");
        }
        // 先获取 之前的 Bean 的援用, 从 beanPostProcessorCache 中 获取 SmartInstantiationAwareBeanPostProcessor
        // 而后从 SmartInstantiationAwareBeanPostProcessor#getEarlyBeanReference 获取之前的援用
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }

    // Initialize the bean instance.
    Object exposedObject = bean;
    try {
        // 属性赋值
        populateBean(beanName, mbd, instanceWrapper);
        // 执行 init 办法
        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);
        }
    }

    // 这里容许循环依赖
    if (earlySingletonExposure) {
        // 获取晚期的 Bean, 如果没有循环依赖 则获取不到
        Object earlySingletonReference = getSingleton(beanName, false);

        // 有循环依赖
        if (earlySingletonReference != null) {
            // 创立的是不是同一个，可能会有代理对象
            if (exposedObject == bean) {exposedObject = earlySingletonReference;}
            else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                // 获取依赖的 Bean 并 循环放入到 actualDependentBeans
                String[] dependentBeans = getDependentBeans(beanName);
                Set<String> 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.
    try {
        // 注册销毁办法
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }

    return exposedObject;
}

同样是代码很长很长！

分步骤浏览：

如果这个 Bean 是单例 Bean 且容许循环援用且在创立中，则阐明在有循环援用。则调用：

addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));

这一行代码波及到两个办法，别离是 getEarlyBeanReference 和 addSingletonFactory

• getEarlyBeanReference

• addSingletonFactory

这一块能够看到将创立的一个单例对象的 singletonFactory 增加到了 singletonFactories 缓存中。

同时将 earlySingletonObjects 缓存中的单例对象移除。

那什么时候增加到 earlySingletonObjects 缓存中的呢？

这块能够参考 Spring 源码学习 15：finishBeanFactoryInitialization 在 getSingleton 办法中 put 进去的。

为了不便，我把这一小块代码也贴出来：

在这里将缓存从 singletonFactories 移到了 earlySingletonObjects。

Spring 的 Bean 实例化的时候用到的三级缓存其实是：

singletonObjects： 一级缓存，存储单例对象，Bean 曾经实例化，初始化实现。

earlySingletonObjects： 二级缓存，存储 singletonObject，这个 Bean 实例化了，还没有初始化。

singletonFactories： 三级缓存，存储 singletonFactory

上面会初始化 Bean

这里关注重点关注上面一部分：

• populateBean

对 Bean 的属性进行赋值。

这块须要留神的是，在对属性进行赋值时，发现依赖了其余 Bean，就会去先创立其余 Bean。

我这边应用的注解 @Autowired 就会执行上面一部分：

在这里解析属性的时候，就会去创立外部依赖的 Bean。

• initializeBean

getObjectForBeanInstance

获取给定 bean 实例的对象，如果是 FactoryBean，则为 bean 实例自身或其创立的对象。

这一块逻辑绝对比较简单，就是依据后面你创立的 beanInstance，判断其类型，从而创立 Bean 实例。

总结

本文次要介绍了一个 单例 Bean 的创立，当然都是大块大块的源码，须要急躁的啃。

浏览完源码，基本上对循环依赖能有个具体的理解，晓得 Spring 在初始化 Bean 的时候是应用三级缓存来解决循环依赖的额，而前面则会独自筹备一篇文章对循环依赖做介绍。

相干举荐

• Spring 源码学习 15：finishBeanFactoryInitialization（重点）
• Spring 源码学习 14：initApplicationEventMulticaster
• Spring 源码学习 13：initMessageSource