探究Spring之IOC

spring IOC （控制反转）简单理解就是将对象的控制权由应用程序转交给spring IOC容器。 所有的类都会在spring容器中登记，告诉spring自己是什么，同时又需要什么，然后spring会在系统运行到适当的时候，把你要的东西主动给你，同时也把你交给其他需要你的东西。所有的类的创建、销毁都由 spring来控制，也就是说控制对象生存周期的不再是引用它的对象，而是spring。对于某个具体的对象而言，以前是它控制其他对象，现在是所有对象都被spring控制，所以这叫控制反转。

实现原理：参考资料 https://m.2cto.com/kf/201608/535650.html

这里我先通过分析最简单的 XmlBeanFactory 来看其实现思想。

 

ClassPathResource resource = new ClassPathResource("META-INF/beans.xml");
BeanFactory beanFactory = new XmlBeanFactory(resource);
User user = beanFactory.getBean("user1", User.class);

以上代码代码执行后，XmlBeanFactory执行流程如下。
调用顺序      类名                                    方法名
1       XmlBeanFactory                         构造方法(Resource)
2     DefaultListableBeanFactory               构造方法(BeanFactory)
3     AbstractAutowireCapableBeanFactory       构造方法(BeanFactory)
4     AbstractAutowireCapableBeanFactorys      etParentFactory(BeanFactory)
5     XmlBeanDefinitionReader                  loadBeanDefinitions(Resource)
6     AbstractBeanDefinitionReader             getResourceLoader()

 

1.创建Ioc配置文件的抽象资源，这个抽象资源包含了BeanDefinition的定义信息
2.创建一个BeanFactory，这里使用了DefaultListableBeanFactory
3.创建一个载入BeanDefinition的读取器，这里使用XmlBeanDefinitionReader来载入XML文件形式的BeanDefinition
4.然后将上面定位好的Resource，通过一个回调配置给BeanFactory
5.从定位好的资源位置读入配置信息，具体的解析过程由XmlBeanDefinitionReader完成
6.完成整个载入和注册Bean定义之后，需要的Ioc容器就初步建立起来了

 

 ClassPathXmlApplicationContext的核心过程和XmlBeanFactory的是相同的。注册的bean 都存入了一个ConcurrentHashMap中。

 

DI实现原理 参考资料：http://blog.csdn.net/lisongjia123/article/details/52134396

在ClassPathXmlApplicationContext创建时期就生成对象了

1 AbstractRefreshableApplicationContext   getBeanFactory    返回了DefaultListableBeanFactory 该类继承了 AbstractAutowireCapableBeanFactory

2 AbstractBeanFactory   getBean方法  

@Override
    public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
        return doGetBean(name, requiredType, null, false);
    }

3 AbstractBeanFactory  doGetBean 方法

    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);
        }

        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);
                }
            }

            if (!typeCheckOnly) {
                markBeanAsCreated(beanName);
            }

            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(mbd.getResourceDescription(), beanName,
                                    "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 {
                               //调用此处方法
                                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 name '" + 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);
                    }
                }
            }
            catch (BeansException ex) {
                cleanupAfterBeanCreationFailure(beanName);
                throw ex;
            }
        }

        // Check if required type matches the type of the actual bean instance.
        if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
            try {
                return getTypeConverter().convertIfNecessary(bean, requiredType);
            }
            catch (TypeMismatchException ex) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Failed to convert bean '" + name + "' to required type [" +
                            ClassUtils.getQualifiedName(requiredType) + "]", ex);
                }
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
        }
        return (T) bean;
    }

4 AbstractAutowireCapableBeanFactory   createBean 方法

protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
        if (logger.isDebugEnabled()) {
            logger.debug("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.
        Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
        if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
            mbdToUse = new RootBeanDefinition(mbd);
            mbdToUse.setBeanClass(resolvedClass);
        }

        // Prepare method overrides.
        try {
            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.
            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);
        }
        //调用此处doCreateBean 方法

       Object beanInstance = doCreateBean(beanName, mbdToUse, args); if (logger.isDebugEnabled()) { logger.debug("Finished creating instance of bean '" + beanName + "'"); } return beanInstance; }

 

5 AbstractAutowireCapableBeanFactory  doCreateBean

其余与资料中分析一致，以上流程说明。Spring 在创建 ClassPathXmlApplicationContext 对象时就已经将xml文件中的 bean 创建好了。

Spring当中提供了两种实例化方案： BeanUtils 和 Cglib 方式。BeanUtils实现机制是通过Java的反射机制，Cglib是一个第三方类库采用的是一种字节码加强方式机制。 Spring中采用的默认实例化策略是Cglib。 

接下来我们详细了解下这两种方案。