Spring源码解析——IOC属性填充

正文

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：

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• 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);  
} 

我们来分析下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

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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="dabin.spring01.MyTestBean">
        <property name="name" value="dabin"></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);
    }
}

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()。