Spring源码分析之ioc容器是如何创建和获取单实例bean的

首先创造ioc容器 ，这条语句一执行，所有的bean都已经创建好了，并存放在了ioc的容器中。

ApplicationContext ioc = new ClassPathXmlApplicationContext("beans.xml");

ClassPathXmlApplicationContext()方法

点开ClassPathXmlApplicationContext()类，我们发现无论构造器都是调用了this(String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)

public ClassPathXmlApplicationContext(
        String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
            throws BeansException {
    super(parent);
　　 // 解析配置文件的位置
    setConfigLocations(configLocations);
    if (refresh) {
　　　　　//初始化创建所有的bean
        refresh();
    }
}

refresh()方法

接着点进去refresh()方法  代码如下

    @Override
    public void refresh() throws BeansException, IllegalStateException {
　　　　　synchronized (this.startupShutdownMonitor) { // 同步锁，保证多线程下 ioc容器只创建一次
            // Prepare this context for refreshing.
            prepareRefresh();

            // 准备了一个bean工厂  要创建所有的bean 也是ioc最大的接口 ApplicationContext也是其子接口 
　　　　　　　//　同时也解析了xml文件，把xml文件中的所有bean配置信息保存了起来  方便下次使用 
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
          　　
            try {
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);

                // 支持国际化功能
                initMessageSource();

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // 留给子类的方法
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();

                // I初始化所有的单实例bean (接下来看这个方法)
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                finishRefresh();
            }

            catch (BeansException ex) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

                // Reset 'active' flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }

finishBeanFactoryInitialization(beanFactory); 

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));
        }

        // Register a default embedded value resolver if no bean post-processor
        // (such as a PropertyPlaceholderConfigurer bean) registered any before:
        // at this point, primarily for resolution in annotation attribute values.
        if (!beanFactory.hasEmbeddedValueResolver()) {
            beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
        }

        // 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();

        // 初始化所有单实例 初始化单实例工作是这个方法完成的  所以接下来进入这个方法来看
        beanFactory.preInstantiateSingletons();
    }

preInstantiateSingletons()方法

　　这个方法是在 DefaultListableBeanFactory下  这个类下有个beanDefinitionMap属性(ConcurrentHashmap)封装了所有的bean ，以bean的name为键

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

        // 这个beanDefinitionNames封装了所有bean的name
        List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

        // 遍历所有bean的name
        for (String beanName : beanNames) {
            RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
　　　　　　　　// 如果bean是单实例的 且 不是抽象的 且 不是懒加载的 
            if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
　　　　　　　　　　// 第一次创建这肯定为为空  当我们创建完成后 调用getBean()时候  这就不为空了 所以这也是ioc得到bean实例的源码
                if (isFactoryBean(beanName)) {
                    Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
                    if (bean instanceof FactoryBean) {
                        final FactoryBean<?> factory = (FactoryBean<?>) bean;
                        boolean isEagerInit;
                        if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                            isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
                                            ((SmartFactoryBean<?>) factory)::isEagerInit,
                                    getAccessControlContext());
                        }
                        else {
                            isEagerInit = (factory instanceof SmartFactoryBean &&
                                    ((SmartFactoryBean<?>) factory).isEagerInit());
                        }
                        if (isEagerInit) {
                            getBean(beanName);
                        }
                    }
                }
                else {
　　　　　　　　　　　　// 创建bean
                    getBean(beanName);
                }
            }
        }

从上述代码可以看出  无论是第一次创建bean  还是之后我们去ioc容器中取bean实例  都会经过getBean方法 所以下面来看getBean()方法  

getBean()方法 实际执行了了 return doGetBean(name, null, null, false);  

doGetBean(name,null,null,false)  代码有些长 只写关键部分

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

        final String beanName = transformedBeanName(name);
        Object bean;
        Object sharedInstance = getSingleton(beanName);
　　　　　　// 获取单实例 先从缓存中看这个bean有没有被注册  第一次创建 肯定获取为null 第二次获取bean 才会有 所以doGetBean()也是获取bean的源码
        if (sharedInstance != null && args == null) {
            省略.....
        }

        else {
            省略....
           　// 传的参数是false 所以会进来if (!typeCheckOnly) {
　　　　　　　　　　// 标记bean已经创建了 注意：代码执行到这bean并没有被创建 只是先标记一下bean被创建了 
　　　　　　　　　　// 目的是为了在多线程环境下 提前告诉其他线程 这个bean已经创建过了
                markBeanAsCreated(beanName);
            }

            try {
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // 拿到当前bean所依赖的bean 
                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 {
　　　　　　　　　　　　　　　　　// 这是个循环调用 一直向上拿到所依赖的bean 直到没有依赖
                            getBean(dep);
                        }
                        catch (NoSuchBeanDefinitionException ex) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                        }
                    }
                }

                // 创造单实例.
                if (mbd.isSingleton()) {
　　　　　　　　　　　　// 创造bean　　
                    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);
                }
　　　　　　　　　　// 创造原型模式下的实例
                else if (mbd.isPrototype()) {
                    省略....
                }

                else {
                    省略....
                }
            }
            catch (BeansException ex) {
                cleanupAfterBeanCreationFailure(beanName);
                throw ex;
            }
        }

       　省略....
　　　　　
　　　　 //最后返回bean
        return (T) bean;
    }

getSingleton()方法

　　源码不贴了 核心就是两行代码

　　singletonObject = singletonFactory.getObject();  得到单实例bean

　　addSingleton(beanName, singletonObject); 向beanDefinitionMap中添加bean 以便下次可以直接取到

至此Spring就把所有的bean创建完了…………   源码看到吐血！！！

还有好多细节 奈何自己才疏学浅  只能通个大概  剩下的更细节的原理 就请诸君自行学习查看了……  希望能够帮助到所有热爱java的人