Spring源码剖析4：其余方式获取Bean的过程分析

2021-10-22 11:40:51

原型Bean加载过程

之前的文章，分析了非懒加载的单例Bean整个加载过程，除了非懒加载的单例Bean之外，Spring中还有一种Bean就是原型（Prototype）的Bean，看一下定义方式：

1 <?xml version="1.0" encoding="UTF-8"?>
2 <beans xmlns="http://www.springframework.org/schema/beans"
3     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
4     xsi:schemaLocation="http://www.springframework.org/schema/beans
5     http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
6 
7     <bean id="prototypeBean" class="org.xrq.action.PrototypeBean" scope="prototype"  />
8     
9 </beans>

原型Bean加载流程总得来说和单例Bean差不多，看一下不同之处，在AbstractBeanFactory的doGetBean的方法的这一步：

 1 else if (mbd.isPrototype()) {
 2     // It's a prototype -> create a new instance.
 3     Object prototypeInstance = null;
 4     try {
 5         beforePrototypeCreation(beanName);
 6         prototypeInstance = createBean(beanName, mbd, args);
 7     }
 8     finally {
 9         afterPrototypeCreation(beanName);
10     }
11     bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
12 }

第6行createBean是一样的，原型Bean实例化的主要区别就在于第6行，它是直接创建bean的，而单例bean我们再对比一下：

 1 if (mbd.isSingleton()) {
 2     sharedInstance = getSingleton(beanName, new ObjectFactory() {
 3         public Object getObject() throws BeansException {
 4             try {
 5                 return createBean(beanName, mbd, args);
 6             }
 7             catch (BeansException ex) {
 8                 // Explicitly remove instance from singleton cache: It might have been put there
 9                 // eagerly by the creation process, to allow for circular reference resolution.
10                 // Also remove any beans that received a temporary reference to the bean.
11                 destroySingleton(beanName);
12                 throw ex;
13             }
14         }
15     });
16     bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
17 }

它优先会尝试getSington，即先尝试从singletonObjects中获取一下bean是否存在，如果存在直接返回singletonObjects中的bean对象。

接着，我们看到原型bean创建和单例bean创建的区别还在于第5行和第9行，先看第5行的代码：

 1 protected void beforePrototypeCreation(String beanName) {
 2     Object curVal = this.prototypesCurrentlyInCreation.get();
 3     if (curVal == null) {
 4         this.prototypesCurrentlyInCreation.set(beanName);
 5     }
 6     else if (curVal instanceof String) {
 7         Set<String> beanNameSet = new HashSet<String>(2);
 8         beanNameSet.add((String) curVal);
 9         beanNameSet.add(beanName);
10         this.prototypesCurrentlyInCreation.set(beanNameSet);
11     }
12     else {
13         Set<String> beanNameSet = (Set<String>) curVal;
14         beanNameSet.add(beanName);
15     }
16 }

这段主要是说bean在创建前要把当前beanName设置到ThreadLocal中去，其目的是保证多线程不会同时创建同一个bean。接着看第9行的代码实现，即bean创建之后做了什么：

 1 protected void afterPrototypeCreation(String beanName) {
 2     Object curVal = this.prototypesCurrentlyInCreation.get();
 3     if (curVal instanceof String) {
 4         this.prototypesCurrentlyInCreation.remove();
 5     }
 6     else if (curVal instanceof Set) {
 7         Set<String> beanNameSet = (Set<String>) curVal;
 8         beanNameSet.remove(beanName);
 9         if (beanNameSet.isEmpty()) {
10             this.prototypesCurrentlyInCreation.remove();
11         }
12     }
13 }

很好理解，就是把当前bean移除一下，这样其它线程就可以创建bean了。第11行的代码不看了，意思是如果bean是FactoryBean的实现类的话，调用getObject()方法获取真正的对象。

byName源码实现

Spring有为开发者提供Autowire（自动装配）的功能，自动装配最常用的就是byName和byType这两种属性。由于自动装配是为了解决对象注入导致的<property>过多的问题，因此很容易找到byName与byType的Spring源码实现应该在属性注入这一块，定位到属性注入的代码AbstractAutowireCapableBeanFactory的populateBean方法，直接截取重点：

 1 if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
 2         mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
 3     MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
 4 
 5     // Add property values based on autowire by name if applicable.
 6     if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
 7         autowireByName(beanName, mbd, bw, newPvs);
 8     }
 9 
10     // Add property values based on autowire by type if applicable.
11     if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
12         autowireByType(beanName, mbd, bw, newPvs);
13     }
14 
15     pvs = newPvs;
16 }

看到第6行~第8行判断是否byName形式，是就执行byName自动装配代码；第11行~第13行判断是否byType形式，是就执行byType自动装配代码。那么首先看一下第7行的byName代码实现：

 1 protected void autowireByName(
 2         String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {
 3 
 4     String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
 5     for (String propertyName : propertyNames) {
 6         if (containsBean(propertyName)) {
 7             Object bean = getBean(propertyName);
 8             pvs.add(propertyName, bean);
 9             registerDependentBean(propertyName, beanName);
10             if (logger.isDebugEnabled()) {
11                 logger.debug("Added autowiring by name from bean name '" + beanName +
12                         "' via property '" + propertyName + "' to bean named '" + propertyName + "'");
13             }
14         }
15         else {
16             if (logger.isTraceEnabled()) {
17                 logger.trace("Not autowiring property '" + propertyName + "' of bean '" + beanName +
18                         "' by name: no matching bean found");
19             }
20         }
21     }
22 }

篇幅问题，代码不一层层跟了，逻辑梳理一下：

第4行，找到Bean中不是简单属性的属性，这句话有点绕，意思就是找到属性是对象类型的属性，但也不是所有的对象类型都会被找到，比如CharSequence类型、Number类型、Date类型、URL类型、URI类型、Locale类型、Class类型就会忽略，具体可见BeanUtils的isSimpleProperty方法
第5行~第7行，遍历所有被找到的属性，如果bean定义中包含了属性名，那么先实例化该属性名对应的bean
第9行registerDependentBean，注册一下当前bean的依赖bean，用于在某个bean被销毁前先将其依赖的bean销毁

其余代码都是一些打日志的，没什么好说的。

byType源码实现

上面说了byName的源码实现，接下来看一下byType源码实现：

 1 protected void autowireByType(
 2         String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {
 3 
 4     TypeConverter converter = getCustomTypeConverter();
 5     if (converter == null) {
 6         converter = bw;
 7     }
 8 
 9     Set<String> autowiredBeanNames = new LinkedHashSet<String>(4);
10     String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
11     for (String propertyName : propertyNames) {
12         try {
13             PropertyDescriptor pd = bw.getPropertyDescriptor(propertyName);
14             // Don't try autowiring by type for type Object: never makes sense,
15             // even if it technically is a unsatisfied, non-simple property.
16             if (!Object.class.equals(pd.getPropertyType())) {
17                 MethodParameter methodParam = BeanUtils.getWriteMethodParameter(pd);
18                 // Do not allow eager init for type matching in case of a prioritized post-processor.
19                 boolean eager = !PriorityOrdered.class.isAssignableFrom(bw.getWrappedClass());
20                 DependencyDescriptor desc = new AutowireByTypeDependencyDescriptor(methodParam, eager);
21                 Object autowiredArgument = resolveDependency(desc, beanName, autowiredBeanNames, converter);
22                 if (autowiredArgument != null) {
23                     pvs.add(propertyName, autowiredArgument);
24                 }
25                 for (String autowiredBeanName : autowiredBeanNames) {
26                     registerDependentBean(autowiredBeanName, beanName);
27                     if (logger.isDebugEnabled()) {
28                         logger.debug("Autowiring by type from bean name '" + beanName + "' via property '" +
29                                 propertyName + "' to bean named '" + autowiredBeanName + "'");
30                     }
31                 }
32                 autowiredBeanNames.clear();
33             }
34         }
35         catch (BeansException ex) {
36             throw new UnsatisfiedDependencyException(mbd.getResourceDescription(), beanName, propertyName, ex);
37         }
38     }
39 }

前面一样，到第10行都是找到Bean中属性是对象类型的属性。

接着就是遍历一下PropertyName，获取PropertyName对应的属性描述，注意一下16行的判断及其对应的注释：不要尝试自动装配Object类型，这没有任何意义，即使从技术角度看它是一个非简单的对象属性。

第18行~第20行跳过（没有太明白是干什么的），byType实现的源码主要在第21行的方法resolveDependency中，这个方法是AbstractAutowireCapableBeanFactory类的实现类DefaultListableBeanFactory中的方法：

 1 public Object resolveDependency(DependencyDescriptor descriptor, String beanName,
 2     Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException  {
 3 
 4     descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
 5     if (descriptor.getDependencyType().equals(ObjectFactory.class)) {
 6         return new DependencyObjectFactory(descriptor, beanName);
 7     }
 8     else if (descriptor.getDependencyType().equals(javaxInjectProviderClass)) {
 9         return new DependencyProviderFactory().createDependencyProvider(descriptor, beanName);
10     }
11     else {
12         return doResolveDependency(descriptor, descriptor.getDependencyType(), beanName, autowiredBeanNames, typeConverter);
13     }
14 }

这里判断一下要自动装配的属性是ObjectFactory.class还是javaxInjectProviderClass还是其他的，我们装配的是其他的，看一下12行的代码实现：

 1 protected Object doResolveDependency(DependencyDescriptor descriptor, Class<?> type, String beanName,
 2     Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException  {
 3 
 4     Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
 5     if (value != null) {
 6         if (value instanceof String) {
 7             String strVal = resolveEmbeddedValue((String) value);
 8             BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
 9             value = evaluateBeanDefinitionString(strVal, bd);
10         }
11         TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
12         return converter.convertIfNecessary(value, type);
13     }
14 
15     if (type.isArray()) {
16         ...
17     }
18     else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {
19         ...
20     }
21     else if (Map.class.isAssignableFrom(type) && type.isInterface()) {
22         ...
23     }
24     else {
25         Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
26         if (matchingBeans.isEmpty()) {
27             if (descriptor.isRequired()) {
28                 raiseNoSuchBeanDefinitionException(type, "", descriptor);
29             }
30             return null;
31         }
32         if (matchingBeans.size() > 1) {
33             String primaryBeanName = determinePrimaryCandidate(matchingBeans, descriptor);
34             if (primaryBeanName == null) {
35                 throw new NoSuchBeanDefinitionException(type, "expected single matching bean but found " +
36                         matchingBeans.size() + ": " + matchingBeans.keySet());
37             }
38             if (autowiredBeanNames != null) {
39                 autowiredBeanNames.add(primaryBeanName);
40             }
41             return matchingBeans.get(primaryBeanName);
42         }
43         // We have exactly one match.
44         Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
45         if (autowiredBeanNames != null) {
46             autowiredBeanNames.add(entry.getKey());
47         }
48         return entry.getValue();
49     }
50 }

第四行结果是null不看了，为了简化代码Array装配、Collection装配、Map装配的代码都略去了，重点看一下普通属性的装配。首先是第25行获取一下自动装配的候选者：

 1 protected Map<String, Object> findAutowireCandidates(
 2     String beanName, Class requiredType, DependencyDescriptor descriptor) {
 3 
 4     String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
 5             this, requiredType, true, descriptor.isEager());
 6     Map<String, Object> result = new LinkedHashMap<String, Object>(candidateNames.length);
 7     for (Class autowiringType : this.resolvableDependencies.keySet()) {
 8         if (autowiringType.isAssignableFrom(requiredType)) {
 9             Object autowiringValue = this.resolvableDependencies.get(autowiringType);
10             autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
11             if (requiredType.isInstance(autowiringValue)) {
12                     result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
13                 break;
14             }
15         }
16     }
17     for (String candidateName : candidateNames) {
18         if (!candidateName.equals(beanName) && isAutowireCandidate(candidateName, descriptor)) {
19             result.put(candidateName, getBean(candidateName));
20         }
21     }
22     return result;
23 }

代码逻辑整理一下：

首先获取候选者bean名称，通过DefaultListableBeanFactory的getBeanNamesForType方法，即找一下所有的Bean定义中指定Type的实现类或者子类
接着第7行~第16行的判断要自动装配的类型是不是要自动装配的纠正类型，这个在【Spring源码分析】非懒加载的单例Bean初始化前后的一些操作一文讲PrepareBeanFactory方法的时候有讲过，如果要自动装配的类型是纠正类型，比如是一个ResourceLoader，那么就会为该类型生成一个代理实例，具体可以看一下第10行的AutowireUtils.resolveAutowiringValue方法的实现
正常来说都是执行的第17行~第21行的代码，逐个判断查找一下beanName对应的BeanDefinition，判断一下是不是自动装配候选者，默认都是的，如果<bean>的autowire-candidate属性设置为false就不是

这样，拿到所有待装配对象的实现类或者子类的候选者，组成一个Map，Key为beanName，Value为具体的Bean。接着回看获取Bean之后的逻辑：

 1 Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
 2     if (matchingBeans.isEmpty()) {
 3         if (descriptor.isRequired()) {
 4             raiseNoSuchBeanDefinitionException(type, "", descriptor);
 5         }
 6         return null;
 7     }
 8     if (matchingBeans.size() > 1) {
 9         String primaryBeanName = determinePrimaryCandidate(matchingBeans, descriptor);
10         if (primaryBeanName == null) {
11             throw new NoSuchBeanDefinitionException(type, "expected single matching bean but found " +
12                     matchingBeans.size() + ": " + matchingBeans.keySet());
13         }
14         if (autowiredBeanNames != null) {
15             autowiredBeanNames.add(primaryBeanName);
16         }
17         return matchingBeans.get(primaryBeanName);
18     }
19     // We have exactly one match.
20     Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
21     if (autowiredBeanNames != null) {
22         autowiredBeanNames.add(entry.getKey());
23     }
24     ...
25 }

整理一下逻辑：

如果拿到的Map是空的且属性必须注入，抛异常
如果拿到的Map中有多个候选对象，判断其中是否有<bean>中属性配置为"primary=true"的，有就拿执行第13行~第15行的代码，没有就第8行的方法返回null，抛异常，这个异常的描述相信Spring用的比较多的应该比较熟悉
如果拿到的Map中只有一个候选对象，直接拿到那个

通过这样一整个流程，实现了byType自动装配，byType自动装配流程比较长，中间细节比较多，还需要多看看才能弄明白。

最后注意一点，即所有待注入的PropertyName-->PropertyValue映射拿到之后都只是放在MutablePropertyValues中，最后由AbstractPropertyAccessor类的setPropertyValues方法遍历并进行逐一注入。

通过FactoryBean获取Bean实例源码实现

我们知道可以通过实现FactoryBean接口，重写getObject()方法实现个性化定制Bean的过程，这部分我们就来看一下Spring源码是如何实现通过FactoryBean获取Bean实例的。代码直接定位到AbstractBeanFactory的doGetBean方法创建单例Bean这部分：

 1 // Create bean instance.
 2 if (mbd.isSingleton()) {
 3     sharedInstance = getSingleton(beanName, new ObjectFactory() {
 4         public Object getObject() throws BeansException {
 5             try {
 6                 return createBean(beanName, mbd, args);
 7             }
 8             catch (BeansException ex) {
 9                 // Explicitly remove instance from singleton cache: It might have been put there
10                 // eagerly by the creation process, to allow for circular reference resolution.
11                 // Also remove any beans that received a temporary reference to the bean.
12                 destroySingleton(beanName);
13                 throw ex;
14             }
15         }
16     });
17     bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
18 }

FactoryBean首先是个Bean且被实例化出来成为一个对象之后才能调用getObject()方法，因此还是会执行第3行~第16行的代码，这段代码之前分析过了就不说了。之后执行第17行的方法：

 1 protected Object getObjectForBeanInstance(
 2         Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
 3 
 4     // Don't let calling code try to dereference the factory if the bean isn't a factory.
 5     if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
 6         throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
 7     }
 8 
 9     // Now we have the bean instance, which may be a normal bean or a FactoryBean.
10     // If it's a FactoryBean, we use it to create a bean instance, unless the
11     // caller actually wants a reference to the factory.
12     if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
13         return beanInstance;
14     }
15 
16     Object object = null;
17     if (mbd == null) {
18         object = getCachedObjectForFactoryBean(beanName);
19     }
20     if (object == null) {
21         // Return bean instance from factory.
22         FactoryBean factory = (FactoryBean) beanInstance;
23         // Caches object obtained from FactoryBean if it is a singleton.
24         if (mbd == null && containsBeanDefinition(beanName)) {
25             mbd = getMergedLocalBeanDefinition(beanName);
26         }
27         boolean synthetic = (mbd != null && mbd.isSynthetic());
28         object = getObjectFromFactoryBean(factory, beanName, !synthetic);
29     }
30     return object;
31 }

首先第5行~第7行判断一下是否beanName以"&"开头并且不是FactoryBean的实现类，不满足则抛异常，因为beanName以"&"开头是FactoryBean的实现类bean定义的一个特征。

接着判断第12行~第14行，如果：

bean不是FactoryBean的实现类
beanName以"&"开头

这两种情况，都直接把生成的bean对象返回出去，不会执行余下的流程。

最后流程走到第16行~第30行，最终调用getObject()方法实现个性化定制bean，先执行第28行的方法：

 1 protected Object getObjectFromFactoryBean(FactoryBean factory, String beanName, boolean shouldPostProcess) {
 2     if (factory.isSingleton() && containsSingleton(beanName)) {
 3         synchronized (getSingletonMutex()) {
 4             Object object = this.factoryBeanObjectCache.get(beanName);
 5             if (object == null) {
 6                 object = doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess);
 7                 this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
 8             }
 9             return (object != NULL_OBJECT ? object : null);
10         }
11     }
12     else {
13         return doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess);
14     }
15 }

第1行~第11行的代码与第12行~第13行的代码最终都是一样的，调用了如下一段：

 1 private Object doGetObjectFromFactoryBean(
 2         final FactoryBean factory, final String beanName, final boolean shouldPostProcess)
 3         throws BeanCreationException {
 4 
 5     Object object;
 6     try {
 7         if (System.getSecurityManager() != null) {
 8             AccessControlContext acc = getAccessControlContext();
 9             try {
10                 object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
11                     public Object run() throws Exception {
12                             return factory.getObject();
13                         }
14                     }, acc);
15             }
16             catch (PrivilegedActionException pae) {
17                 throw pae.getException();
18             }
19         }
20         else {
21             object = factory.getObject();
22         }
23     }
24     catch (FactoryBeanNotInitializedException ex) {
25         throw new BeanCurrentlyInCreationException(beanName, ex.toString());
26     }
27     catch (Throwable ex) {
28         throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
29     }
30         
31     // Do not accept a null value for a FactoryBean that's not fully
32     // initialized yet: Many FactoryBeans just return null then.
33     if (object == null && isSingletonCurrentlyInCreation(beanName)) {
34         throw new BeanCurrentlyInCreationException(
35                 beanName, "FactoryBean which is currently in creation returned null from getObject");
36     }
37 
38     if (object != null && shouldPostProcess) {
39         try {
40             object = postProcessObjectFromFactoryBean(object, beanName);
41         }
42         catch (Throwable ex) {
43             throw new BeanCreationException(beanName, "Post-processing of the FactoryBean's object failed", ex);
44         }
45     }
46 
47     return object;
48 }

第12行和第21行的代码，都一样，最终调用getObject()方法获取对象。回过头去看之前的getObjectFromFactoryBean方法，虽然if...else...逻辑最终都是调用了以上的方法，但是区别在于：

如果FactoryBean接口实现类的isSington方法返回的是true，那么每次调用getObject方法的时候会优先尝试从FactoryBean对象缓存中取目标对象，有就直接拿，没有就创建并放入FactoryBean对象缓存，这样保证了每次单例的FactoryBean调用getObject()方法后最终拿到的目标对象一定是单例的，即在内存中都是同一份
如果FactoryBean接口实现类的isSington方法返回的是false，那么每次调用getObject方法的时候都会新创建一个目标对象

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Spring源码剖析4：其余方式获取Bean的过程分析

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