spring源码深度解析— IOC 之 属性填充

doCreateBean() 主要用于完成 bean 的创建和初始化工作，我们可以将其分为四个过程：

• createBeanInstance() 实例化 bean
• populateBean() 属性填充
• 循环依赖的处理
• initializeBean() 初始化 bean

第一个过程实例化 bean在前面一篇博客中已经分析完了，这篇博客开始分析 属性填充，也就是 populateBean()

protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
    PropertyValues pvs = mbd.getPropertyValues();  

    if (bw == null) {
        if (!pvs.isEmpty()) {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
        }
        else {
            // Skip property population phase for null instance.
            return;
        }
    }  

    // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
    // state of the bean before properties are set. This can be used, for example,
    // to support styles of field injection.
    boolean continueWithPropertyPopulation = true;  

    if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof InstantiationAwareBeanPostProcessor) {
                InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                //返回值为是否继续填充bean
                if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                    continueWithPropertyPopulation = false;
                    break;
                }
            }
        }
    }
    //如果后处理器发出停止填充命令则终止后续的执行
    if (!continueWithPropertyPopulation) {
        return;
    }  

    if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
            mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
        MutablePropertyValues newPvs = new MutablePropertyValues(pvs);  

        // Add property values based on autowire by name if applicable.
        //根据名称自动注入
        if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
            autowireByName(beanName, mbd, bw, newPvs);
        }  

        // Add property values based on autowire by type if applicable.
        //根据类型自动注入
        if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
            autowireByType(beanName, mbd, bw, newPvs);
        }  

        pvs = newPvs;
    }
    //后处理器已经初始化
    boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
    //需要依赖检查
    boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);  

    if (hasInstAwareBpps || needsDepCheck) {
        PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
        if (hasInstAwareBpps) {
            for (BeanPostProcessor bp : getBeanPostProcessors()) {
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                    //对所有需要依赖检查的属性进行后处理
                    pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                    if (pvs == null) {
                        return;
                    }
                }
            }
        }
        if (needsDepCheck) {
            //依赖检查，对应depends-on属性，3.0已经弃用此属性
            checkDependencies(beanName, mbd, filteredPds, pvs);
        }
    }
    //将属性应用到bean中
    //将所有ProtertyValues中的属性填充至BeanWrapper中。
    applyPropertyValues(beanName, mbd, bw, pvs);
} 

我们详细分析下populateBean的流程：
（1）首先进行属性是否为空的判断
（2）通过调用InstantiationAwareBeanPostProcessor的postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)方法来控制程序是否继续进行属性填充
（3）根据注入类型（byName/byType）提取依赖的bean，并统一存入PropertyValues中
（4）应用InstantiationAwareBeanPostProcessor的postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName)方法，对属性获取完毕填充前的再次处理，典型的应用是RequiredAnnotationBeanPostProcesser类中对属性的验证
（5）将所有的PropertyValues中的属性填充至BeanWrapper中
上面步骤中有几个地方是我们比较感兴趣的，它们分别是依赖注入（autowireByName/autowireByType）以及属性填充，接下来进一步分析这几个功能的实现细节

自动注入

Spring 会根据注入类型（ byName / byType ）的不同，调用不同的方法（autowireByName() / autowireByType()）来注入属性值。

autowireByName()

protected void autowireByName(
        String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {

    // 获取 Bean 对象中非简单属性
    String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
    for (String propertyName : propertyNames) {
        // 如果容器中包含指定名称的 bean，则将该 bean 注入到 bean中
        if (containsBean(propertyName)) {
            // 递归初始化相关 bean
            Object bean = getBean(propertyName);
            // 为指定名称的属性赋予属性值
            pvs.add(propertyName, bean);
            // 属性依赖注入
            registerDependentBean(propertyName, beanName);
            if (logger.isDebugEnabled()) {
                logger.debug("Added autowiring by name from bean name '" + beanName +
                        "' via property '" + propertyName + "' to bean named '" + propertyName + "'");
            }
        }
        else {
            if (logger.isTraceEnabled()) {
                logger.trace("Not autowiring property '" + propertyName + "' of bean '" + beanName +
                        "' by name: no matching bean found");
            }
        }
    }
}

该方法逻辑很简单，获取该 bean 的非简单属性，什么叫做非简单属性呢？就是类型为对象类型的属性，但是这里并不是将所有的对象类型都都会找到，比如 8 个原始类型，String 类型 ，Number类型、Date类型、URL类型、URI类型等都会被忽略，如下：

protected String[] unsatisfiedNonSimpleProperties(AbstractBeanDefinition mbd, BeanWrapper bw) {
    Set<String> result = new TreeSet<>();
    PropertyValues pvs = mbd.getPropertyValues();
    PropertyDescriptor[] pds = bw.getPropertyDescriptors();
    for (PropertyDescriptor pd : pds) {
        if (pd.getWriteMethod() != null && !isExcludedFromDependencyCheck(pd) && !pvs.contains(pd.getName()) &&
                !BeanUtils.isSimpleProperty(pd.getPropertyType())) {
            result.add(pd.getName());
        }
    }
    return StringUtils.toStringArray(result);
}

这里获取的就是需要依赖注入的属性。

autowireByName()函数的功能就是根据传入的参数中的pvs中找出已经加载的bean，并递归实例化，然后加入到pvs中

autowireByType

autowireByType与autowireByName对于我们理解与使用来说复杂程度相似，但是实现功能的复杂度却不一样，我们看下方法代码：

protected void autowireByType(
        String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {  

    TypeConverter converter = getCustomTypeConverter();
    if (converter == null) {
        converter = bw;
    }  

    Set<String> autowiredBeanNames = new LinkedHashSet<String>(4);
    //寻找bw中需要依赖注入的属性
    String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
    for (String propertyName : propertyNames) {
        try {
            PropertyDescriptor pd = bw.getPropertyDescriptor(propertyName);
            // Don't try autowiring by type for type Object: never makes sense,
            // even if it technically is a unsatisfied, non-simple property.
            if (!Object.class.equals(pd.getPropertyType())) {
                //探测指定属性的set方法
                MethodParameter methodParam = BeanUtils.getWriteMethodParameter(pd);
                // Do not allow eager init for type matching in case of a prioritized post-processor.
                boolean eager = !PriorityOrdered.class.isAssignableFrom(bw.getWrappedClass());
                DependencyDescriptor desc = new AutowireByTypeDependencyDescriptor(methodParam, eager);
                //解析指定beanName的属性所匹配的值，并把解析到的属性名称存储在autowiredBeanNames中，
                Object autowiredArgument = resolveDependency(desc, beanName, autowiredBeanNames, converter);
                if (autowiredArgument != null) {
                    pvs.add(propertyName, autowiredArgument);
                }
                for (String autowiredBeanName : autowiredBeanNames) {
                    //注册依赖
                    registerDependentBean(autowiredBeanName, beanName);
                    if (logger.isDebugEnabled()) {
                        logger.debug("Autowiring by type from bean name '" + beanName + "' via property '" +
                                propertyName + "' to bean named '" + autowiredBeanName + "'");
                    }
                }
                autowiredBeanNames.clear();
            }
        }
        catch (BeansException ex) {
            throw new UnsatisfiedDependencyException(mbd.getResourceDescription(), beanName, propertyName, ex);
        }
    }
}

根据名称第一步与根据属性第一步都是寻找bw中需要依赖注入的属性，然后遍历这些属性并寻找类型匹配的bean,其中最复杂就是寻找类型匹配的bean。spring中提供了对集合的类型注入支持，如使用如下注解方式：

@Autowired
private List<Test> tests;

这种方式spring会把所有与Test匹配的类型找出来并注入到tests属性中，正是由于这一因素，所以在autowireByType函数，新建了局部遍历autowireBeanNames，用于存储所有依赖的bean，如果只是对非集合类的属性注入来说，此属性并无用处。
对于寻找类型匹配的逻辑实现是封装在了resolveDependency函数中，其实现如下：

public Object resolveDependency(DependencyDescriptor descriptor, String beanName, Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {
    descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
    if (descriptor.getDependencyType().equals(ObjectFactory.class)) {
        //ObjectFactory类注入的特殊处理
        return new DependencyObjectFactory(descriptor, beanName);
    }
    else if (descriptor.getDependencyType().equals(javaxInjectProviderClass)) {
        //javaxInjectProviderClass类注入的特殊处理
        return new DependencyProviderFactory().createDependencyProvider(descriptor, beanName);
    }
    else {
        //通用处理逻辑
        return doResolveDependency(descriptor, descriptor.getDependencyType(), beanName, autowiredBeanNames, typeConverter);
    }
}  

protected Object doResolveDependency(DependencyDescriptor descriptor, Class<?> type, String beanName,
        Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {
    /*
     * 用于支持Spring中新增的注解@Value
     */
    Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
    if (value != null) {
        if (value instanceof String) {
            String strVal = resolveEmbeddedValue((String) value);
            BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
            value = evaluateBeanDefinitionString(strVal, bd);
        }
        TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
        return (descriptor.getField() != null ?
                converter.convertIfNecessary(value, type, descriptor.getField()) :
                converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
    }
    //如果解析器没有成功解析，则需要考虑各种情况
    //属性是数组类型
    if (type.isArray()) {
        Class<?> componentType = type.getComponentType();
        //根据属性类型找到beanFactory中所有类型的匹配bean，
        //返回值的构成为：key=匹配的beanName,value=beanName对应的实例化后的bean（通过getBean(beanName)返回）
        Map<String, Object> matchingBeans = findAutowireCandidates(beanName, componentType, descriptor);
        if (matchingBeans.isEmpty()) {
            //如果autowire的require属性为true而找到的匹配项却为空则只能抛出异常
            if (descriptor.isRequired()) {
                raiseNoSuchBeanDefinitionException(componentType, "array of " + componentType.getName(), descriptor);
            }
            return null;
        }
        if (autowiredBeanNames != null) {
            autowiredBeanNames.addAll(matchingBeans.keySet());
        }
        TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
        //通过转换器将bean的值转换为对应的type类型
        return converter.convertIfNecessary(matchingBeans.values(), type);
    }
    //属性是Collection类型
    else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {
        Class<?> elementType = descriptor.getCollectionType();
        if (elementType == null) {
            if (descriptor.isRequired()) {
                throw new FatalBeanException("No element type declared for collection [" + type.getName() + "]");
            }
            return null;
        }
        Map<String, Object> matchingBeans = findAutowireCandidates(beanName, elementType, descriptor);
        if (matchingBeans.isEmpty()) {
            if (descriptor.isRequired()) {
                raiseNoSuchBeanDefinitionException(elementType, "collection of " + elementType.getName(), descriptor);
            }
            return null;
        }
        if (autowiredBeanNames != null) {
            autowiredBeanNames.addAll(matchingBeans.keySet());
        }
        TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
        return converter.convertIfNecessary(matchingBeans.values(), type);
    }
    //属性是Map类型
    else if (Map.class.isAssignableFrom(type) && type.isInterface()) {
        Class<?> keyType = descriptor.getMapKeyType();
        if (keyType == null || !String.class.isAssignableFrom(keyType)) {
            if (descriptor.isRequired()) {
                throw new FatalBeanException("Key type [" + keyType + "] of map [" + type.getName() +
                        "] must be assignable to [java.lang.String]");
            }
            return null;
        }
        Class<?> valueType = descriptor.getMapValueType();
        if (valueType == null) {
            if (descriptor.isRequired()) {
                throw new FatalBeanException("No value type declared for map [" + type.getName() + "]");
            }
            return null;
        }
        Map<String, Object> matchingBeans = findAutowireCandidates(beanName, valueType, descriptor);
        if (matchingBeans.isEmpty()) {
            if (descriptor.isRequired()) {
                raiseNoSuchBeanDefinitionException(valueType, "map with value type " + valueType.getName(), descriptor);
            }
            return null;
        }
        if (autowiredBeanNames != null) {
            autowiredBeanNames.addAll(matchingBeans.keySet());
        }
        return matchingBeans;
    }
    else {
        Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
        if (matchingBeans.isEmpty()) {
            if (descriptor.isRequired()) {
                raiseNoSuchBeanDefinitionException(type, "", descriptor);
            }
            return null;
        }
        if (matchingBeans.size() > 1) {
            String primaryBeanName = determinePrimaryCandidate(matchingBeans, descriptor);
            if (primaryBeanName == null) {
                throw new NoUniqueBeanDefinitionException(type, matchingBeans.keySet());
            }
            if (autowiredBeanNames != null) {
                autowiredBeanNames.add(primaryBeanName);
            }
            return matchingBeans.get(primaryBeanName);
        }
        // We have exactly one match.
        Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
        if (autowiredBeanNames != null) {
            autowiredBeanNames.add(entry.getKey());
        }
        //已经确定只有一个匹配项
        return entry.getValue();
    }
}

主要就是通过Type从BeanFactory中找到对应的benaName，然后通过getBean获取实例

protected Map<String, Object> findAutowireCandidates(
        @Nullable String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {
    //在BeanFactory找到所有Type类型的beanName
    String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
            this, requiredType, true, descriptor.isEager());
    Map<String, Object> result = new LinkedHashMap<>(candidateNames.length);

    //遍历所有的beanName,通过getBean获取
    for (String candidate : candidateNames) {
        if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, descriptor)) {
            //
            addCandidateEntry(result, candidate, descriptor, requiredType);
        }
    }
    return result;
}

private void addCandidateEntry(Map<String, Object> candidates, String candidateName,
        DependencyDescriptor descriptor, Class<?> requiredType) {

    Object beanInstance = descriptor.resolveCandidate(candidateName, requiredType, this);
    if (!(beanInstance instanceof NullBean)) {
        candidates.put(candidateName, beanInstance);
    }
}

public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory)
        throws BeansException {
    //通过类型找到beanName,然后再找到其实例
    return beanFactory.getBean(beanName);
}

applyPropertyValues

程序运行到这里，已经完成了对所有注入属性的获取，但是获取的属性是以PropertyValues形式存在的，还并没有应用到已经实例化的bean中，这一工作是在applyPropertyValues中。继续跟踪到方法体中：

protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
    if (pvs == null || pvs.isEmpty()) {
        return;
    }  

    MutablePropertyValues mpvs = null;
    List<PropertyValue> original;  

    if (System.getSecurityManager() != null) {
        if (bw instanceof BeanWrapperImpl) {
            ((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext());
        }
    }  

    if (pvs instanceof MutablePropertyValues) {
        mpvs = (MutablePropertyValues) pvs;
        //如果mpvs中的值已经被转换为对应的类型那么可以直接设置到beanwapper中
        if (mpvs.isConverted()) {
            // Shortcut: use the pre-converted values as-is.
            try {
                bw.setPropertyValues(mpvs);
                return;
            }
            catch (BeansException ex) {
                throw new BeanCreationException(
                        mbd.getResourceDescription(), beanName, "Error setting property values", ex);
            }
        }
        original = mpvs.getPropertyValueList();
    }
    else {
        //如果pvs并不是使用MutablePropertyValues封装的类型，那么直接使用原始的属性获取方法
        original = Arrays.asList(pvs.getPropertyValues());
    }  

    TypeConverter converter = getCustomTypeConverter();
    if (converter == null) {
        converter = bw;
    }
    //获取对应的解析器
    BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);  

    // Create a deep copy, resolving any references for values.
    List<PropertyValue> deepCopy = new ArrayList<PropertyValue>(original.size());
    boolean resolveNecessary = false;
    //遍历属性，将属性转换为对应类的对应属性的类型
    for (PropertyValue pv : original) {
        if (pv.isConverted()) {
            deepCopy.add(pv);
        }
        else {
            String propertyName = pv.getName();
            Object originalValue = pv.getValue();
            Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
            Object convertedValue = resolvedValue;
            boolean convertible = bw.isWritableProperty(propertyName) &&
                    !PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
            if (convertible) {
                convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
            }
            // Possibly store converted value in merged bean definition,
            // in order to avoid re-conversion for every created bean instance.
            if (resolvedValue == originalValue) {
                if (convertible) {
                    pv.setConvertedValue(convertedValue);
                }
                deepCopy.add(pv);
            }
            else if (convertible && originalValue instanceof TypedStringValue &&
                    !((TypedStringValue) originalValue).isDynamic() &&
                    !(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
                pv.setConvertedValue(convertedValue);
                deepCopy.add(pv);
            }
            else {
                resolveNecessary = true;
                deepCopy.add(new PropertyValue(pv, convertedValue));
            }
        }
    }
    if (mpvs != null && !resolveNecessary) {
        mpvs.setConverted();
    }  

    // Set our (possibly massaged) deep copy.
    try {
        bw.setPropertyValues(new MutablePropertyValues(deepCopy));
    }
    catch (BeansException ex) {
        throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Error setting property values", ex);
    }
}

我们来看看具体的属性赋值过程

public class MyTestBean {
    private String name ;

    public MyTestBean(String name) {
        this.name = name;
    }

    public MyTestBean() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

<bean id="myTestBean"  class="chenhao.spring01.MyTestBean">
        <property name="name" value="chenhao"></property>
</bean>

如上 bw.setPropertyValues 最终都会走到如下方法

@Override
public void setValue(final @Nullable Object value) throws Exception {
    //获取writeMethod，也就是我们MyTestBean的setName方法
    final Method writeMethod = (this.pd instanceof GenericTypeAwarePropertyDescriptor ?
            ((GenericTypeAwarePropertyDescriptor) this.pd).getWriteMethodForActualAccess() :
            this.pd.getWriteMethod());
    if (System.getSecurityManager() != null) {
        AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
            ReflectionUtils.makeAccessible(writeMethod);
            return null;
        });
        try {
            AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () ->
                    writeMethod.invoke(getWrappedInstance(), value), acc);
        }
        catch (PrivilegedActionException ex) {
            throw ex.getException();
        }
    }
    else {
        ReflectionUtils.makeAccessible(writeMethod);
        //通过反射调用方法进行赋值
        writeMethod.invoke(getWrappedInstance(), value);
    }
}

Debug如下

就是利用反射进行调用对象的set方法赋值

至此，doCreateBean() 第二个过程：属性填充 已经分析完成了，下篇分析第三个过程：循环依赖的处理，其实循环依赖并不仅仅只是在 doCreateBean() 中处理，其实在整个加载 bean 的过程中都有涉及，所以下篇内容并不仅仅只局限于 doCreateBean()。