Spring加载Bean的流程（源码分析）

Spring加载Bean的流程（源码分析）

2018-08-06 10:23:47 食鱼酱 阅读数 204 文章标签： SpringBean加载Spring源码Spring Bean加载流程 更多

版本

4.3.10

入口

public class Application {

    public static void main(String[] args) {

        ApplicationContext context = new ClassPathXmlApplicationContext("bean.xml");
        Person person = context.getBean("person", Person.class);
        System.out.println(person.toString());
    }
}

对上面的getBean方法进行跟踪，最终到达AbstractBeanFactory类下面的doGetBean()方法 

 

Bean加载源码分析

public abstract class AbstractBeanFactory 
    extends FactoryBeanRegistrySupport 
    implements ConfigurableBeanFactory {
    //以下三个方法是重写父类的getBean方法
    @Override
    public Object getBean(String name) throws BeansException {
        return doGetBean(name, null, null, false);
    }

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

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

    //这个方法是bean加载的关键
    protected <T> T doGetBean(
            final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {
        /*
        1、转换beanName(别名转换)  

        平时开发中传入的参数name可能只是别名，也可能是FactoryBean，所以需要进行解析转换：
        （1）消除修饰符，比如name="&test"，会去除&使name="test"；
        （2）解决spring中alias标签的别名问题
        */      
        final String beanName = transformedBeanName(name);
        Object bean;

        //2、尝试从缓存中加载实例，如果获取不到就从singletonFactories中加载
        Object sharedInstance = getSingleton(beanName);
        //如果缓存中存在对应的bean
        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 + "'");
                }
            }
            //从缓存中获取的bean是原始状态的bean,需要在这里对bean进行bean实例化
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {
            // 如果缓存中没有对应bean
            //4、循环依赖检查
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            // 5、如果缓存中没有数据，就会转到父类工厂去加载
            //获取父工厂
            BeanFactory parentBeanFactory = getParentBeanFactory();
            /*
                !containsBeanDefinition(beanName)就是检测如果当前加载的xml配置文件中不包含beanName所对应的
                配置，就只能到parentBeanFacotory去尝试加载bean。
            */
            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);
            }

            //6、存储XML配置文件的GernericBeanDefinition转换成RootBeanDefinition
            /*
                XML配置文件中读取到的bean信息是存储在GernericBeanDefinition中的，但Bean的后续处理是针
                对于RootBeanDefinition的，所以需要转换后才能进行后续操作。
            */
            try {
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // Guarantee initialization of beans that the current bean depends on.
                //7、初始化依赖的bean
                //bean中可能依赖了其他bean属性，在初始化bean之前会先初始化这个bean所依赖的bean属性。
                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);
                        getBean(dep);
                    }
                }

                //8、创建bean
                //下面这一大段是Spring容器根据不同scope创建bean实例。
                if (mbd.isSingleton()) {
                    sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                        @Override
                        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);
                                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;
    }
}

总结

1、转换beanName(别名转换)

要知道平时开发中传入的参数name可能只是别名，也可能是FactoryBean，所以需要进行解析转换，一般会进行以下解析：

（1）消除修饰符，比如name=”&test”，会去除&使name=”test”；

（2）解决spring中alias标签的别名问题

2、尝试从缓存中加载实例

实例在Spring的同一个容器中只会被创建一次，后面再想获取该bean时，就会尝试从缓存中获取；如果获取不到的话再从singletonFactories中加载，再存入到缓存中。

3、实例化bean（构建实例）

缓存中记录的bean一般只是最原始的bean状态，这时就需要对bean进行实例化。如果得到的是bean的原始状态，但又要对bean进行处理，这时真正需要的是工厂bean中定义的factory-method方法中返回的bean，上面源码中的getObjectForBeanInstance就是来完成这个工作的。

4、循环依赖检查

5、检测parentBeanFactory

从源码可以看出如果缓存中没有数据会转到父类工厂去加载，源码中的!containsBeanDefinition(beanName)就是检测如果当前加载的xml配置文件中不包含beanName所对应的配置，就只能到parentBeanFacotory去尝试加载bean。

6、存储XML配置文件的GernericBeanDefinition转换成RootBeanDefinition；

XML配置文件中读取到的bean信息是存储在GernericBeanDefinition中的，但Bean的后续处理是针对于RootBeanDefinition的，所以需要转换后才能进行后续操作。

7、初始化依赖的bean

这里应该比较好理解，就是bean中可能依赖了其他bean属性，在初始化bean之前会先初始化这个bean所依赖的bean属性。

8、创建bean

Spring容器根据不同scope创建bean实例。