详解Spring中getBean()方法

我们日常会经常使用getBean()方法从spring容器中获取我们需要的bean。那么，getBean()方法是怎么一步步完成从spring容器中获取bean的呢？

下面我们就通过源码分析一下。

一、主要代码分析

基于：spring5.1

入口：AbstractBeanFactory类的doGetBean()：

 public Object getBean(String name) throws BeansException {return doGetBean(name, null, null, false);}protected <T> T doGetBean(final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)  throws BeansException {//beanName转换，这里主要有两方面的考虑：//1:如果beanName是以&开头的，则表明是要返回FactoryBean本身//而不是其生产出来的对象，而FactoryBean存储的时候，跟普通的bean一样//2:如果传入的是别名，则是需要转换为实际的beanName的           final String beanName = transformedBeanName(name);Object bean;// 检查bean是否已经存在了缓存中Object sharedInstance = getSingleton(beanName);//不为空则表明之前被创建过if (sharedInstance != null && args == null) {if (logger.isDebugEnabled()) {.............}//这里对于普通的bean，则会直接的返回，//如果是FactoryBean类型的则会创建对应的实例返回bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);}else {//如果是正在创建的Prototype类型的bean,无法处理该类型循环依赖的问题，则直接抛出异常信息      if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}// 查看父类中是否有相关的bean的定义信息BeanFactory parentBeanFactory = getParentBeanFactory();if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {// 如果name是以&开头的  则返回&+beanName的格式String nameToLookup = originalBeanName(name);if (args != null) {//递归去父类中查找return (T) parentBeanFactory.getBean(nameToLookup, args);}else {//递归去父类中查找return parentBeanFactory.getBean(nameToLookup, requiredType);}}if (!typeCheckOnly) {markBeanAsCreated(beanName);}try {//将父类的定义的BeanDefinition与子类的BeanDefinition进行合并覆盖final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);checkMergedBeanDefinition(mbd, beanName, args);//判断bean是否有dependsOn 类的依赖//如果没有循环依赖，则先创建所dependsOn依赖的beanString[] dependsOn = mbd.getDependsOn();if (dependsOn != null) {for (String dep : dependsOn) {//这里主要是判断是否有以下这种类型的依赖：//<bean id="beanA" class="BeanA" depends-on="beanB">//<bean id="beanB" class="BeanB" depends-on="beanA">//如果有，则直接抛出异常if (isDependent(beanName, dep)) {.......}registerDependentBean(dep, beanName);getBean(dep);}}// 处理完bean的依赖关系，则开始来创建beanif (mbd.isSingleton()) {//创建单例的bean，这里的createBean下文将详细讲解sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {@Overridepublic Object getObject() throws BeansException {try {return createBean(beanName, mbd, args);}catch (BeansException ex) {destroySingleton(beanName);throw ex;}}});bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}else if (mbd.isPrototype()) {//创建原型的beanObject prototypeInstance = null;try {beforePrototypeCreation(beanName);prototypeInstance = createBean(beanName, mbd, args);}finally {afterPrototypeCreation(beanName);}bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);}else {//创建其他生命周期的beanString scopeName = mbd.getScope();final Scope scope = this.scopes.get(scopeName);if (scope == null) {.........}try {Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {@Overridepublic Object getObject() throws BeansException {beforePrototypeCreation(beanName);try {return createBean(beanName, mbd, args);}finally {afterPrototypeCreation(beanName);}}});bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);}catch (IllegalStateException ex) {........}}}catch (BeansException ex) {cleanupAfterBeanCreationFailure(beanName);throw ex;}}// bean的类型转换if (requiredType != null &&
bean != null && !requiredType.isAssignableFrom(bean.getClass())) {try {return getTypeConverter().convertIfNecessary(bean, requiredType);}catch (TypeMismatchException ex) {........}}return (T) bean;}

二、主要方法分析

整个getBean大的流程如上所示，下面我们分析一下上面源码中几个比较重要的方法：

1、transformedBeanName()方法：

 protected String transformedBeanName(String name) {//如果beanName是以&开头的  则截取掉开头的&return canonicalName(BeanFactoryUtils.transformedBeanName(name));}public String canonicalName(String name) {String canonicalName = name;// 循环的去获取别名，直到获取到真实的beanName//aliasA->aliasB->beanNameString resolvedName;do {resolvedName = this.aliasMap.get(canonicalName);if (resolvedName != null) {canonicalName = resolvedName;}}while (resolvedName != null);return canonicalName;}

2、getSingleton方法

 public Object getSingleton(String beanName) {return getSingleton(beanName, true);}//判断是否有缓存起来的beanprotected Object getSingleton(String beanName, boolean allowEarlyReference) {//singletonObjects保存的为实例化并赋值过的bean,可以直接使用Object singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {synchronized (this.singletonObjects) {//earlySingletonObjects主要是保存允许提前暴露出来的bean//主要是为了解决循环依赖的问题singletonObject = this.earlySingletonObjects.get(beanName);if (singletonObject == null && allowEarlyReference) {ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);if (singletonFactory != null) {singletonObject = singletonFactory.getObject();this.earlySingletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);}}}}return (singletonObject != NULL_OBJECT ? singletonObject : null);}

3、getObjectForBeanInstance方法

 protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {//如果name是以&开头的 但是不是FactoryBean，则直接抛出异常if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());}// 则beanInstance可能是一个普通的bean,也可能是一个FactoryBean// 如果是一个普通的bean，则直接返回if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {return beanInstance;}//FactoryBean创建出bean实例返回Object object = null;if (mbd == null) {object = getCachedObjectForFactoryBean(beanName);}if (object == null) {FactoryBean<?> factory = (FactoryBean<?>) beanInstance;if (mbd == null && containsBeanDefinition(beanName)) {mbd = getMergedLocalBeanDefinition(beanName);}boolean synthetic = (mbd != null && mbd.isSynthetic());object = getObjectFromFactoryBean(factory, beanName, !synthetic);}return object;}

三、流程图

通过流程图我们可以更加直观的看出，getBean()方法的主要流程。

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详解Spring中getBean()方法

一、主要代码分析

二、主要方法分析

三、流程图

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