本系列为SpringBoot深度源码专车系列，第一篇发车！

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专车介绍

该趟专车是开往Spring Boot自动注入原理源码分析的专车

专车问题

• Spring Boot何时注入@Autowired标注的属性？
• 如果注入类型的Bean存在多个Spring Boot是如何处理的？

专车示例

• 定义接口

public interface PersonService {

    String hello(String name);
}

• 定义接口的一个实现

@Service(value = "studentService")
public class StudentServiceImpl implements PersonService {

    @Override
    public String hello(String name) {
        return "[student service] hello " + name;
    }
}

• 定义接口的另一个实现

@Service(value = "teacherService")
public class TeacherServiceImpl implements PersonService {

    @Override
    public String hello(String name) {
        return "[teacher service] hello " + name;
    }
}

• 定义控制器

@RestController
public class TestController {

    @Autowired
    private PersonService studentService;

    @Autowired
    private PersonService teacherService;

    @GetMapping("/hello")
    public String hello(@RequestParam(name = "name") String name) {
        return studentService.hello(name) + "=======>" + teacherService.hello(name);
    }
}

以上示例代码很简单，创建了一个接口，接口有两个实现类，然后在控制器中注入实现类，从而完成业务方法的调用。接下来我们就开始对源码进行分析

专车分析

在分析代码之前我们先回忆一下操作对象的步骤：

• 首先我们会实例化一个对象
• 然后调用对象的set方法来设置对象的属性

有了上面的基础知识，接下来就开始揭秘旅程

寻找入口

在分析源码的时候最关键的一步就是寻找程序的入口，有了入口我们就成功了一半，那么如何寻找程序的入口？针对此处的源码分析，我们可以在TestController类上打一个断点，然后查看调用链

基于调用链路，我们看到有一个doCreateBean方法，该方法就是用来创建bean的，也就是我们上面提到的实例化对象部分

实例化Bean

AbstractAutowireCapableBeanFactory#doCreateBean

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
        throws BeanCreationException {

    // Instantiate the bean.
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        // 创建bean
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    final Object bean = instanceWrapper.getWrappedInstance();
    Class beanType = instanceWrapper.getWrappedClass();
    // ...省略部分代码
    // Initialize the bean instance.
    Object exposedObject = bean;
    try {
        // 填充bean，也就是我们上面提到的调用对象的set方法设置对象属性
        populateBean(beanName, mbd, instanceWrapper);
        exposedObject = initializeBean(beanName, exposedObject, mbd);
    }
    // ...省略部分代码
    return exposedObject;
}

填充bean

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
    // ...省略代码
    PropertyDescriptor[] filteredPds = null;
    if (hasInstAwareBpps) {
        if (pvs == null) {
            pvs = mbd.getPropertyValues();
        }
        // 遍历所有的后置处理器
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof InstantiationAwareBeanPostProcessor) {
                InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                // 通过断点分析我们可以得知此处调用的是AutowiredAnnotationBeanPostProcessor#postProcessProperties
                PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
                if (pvsToUse == null) {
                    if (filteredPds == null) {
                        filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
                    }
                    pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                    if (pvsToUse == null) {
                        return;
                    }
                }
                pvs = pvsToUse;
            }
        }
    }
    if (needsDepCheck) {
        if (filteredPds == null) {
            filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
        }
        checkDependencies(beanName, mbd, filteredPds, pvs);
    }

    if (pvs != null) {
        applyPropertyValues(beanName, mbd, bw, pvs);
    }
}

处理属性

AutowiredAnnotationBeanPostProcessor#postProcessProperties

public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
    // 查找当前bean需要注入的元数据信息，以TestController为例，那么需要注入的就是studentService和teacherService两个属性
    InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
    try {
        // 注入属性
        metadata.inject(bean, beanName, pvs);
    }
    catch (BeanCreationException ex) {
        throw ex;
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
    }
    return pvs;
}

注入属性 AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement#inject

protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
    // 获取属性，此处的属性就是studentService
    Field field = (Field) this.member;
    // 属性对应的value
    Object value;
    if (this.cached) {
        value = resolvedCachedArgument(beanName, this.cachedFieldValue);
    }
    else {
        DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
        desc.setContainingClass(bean.getClass());
        Set autowiredBeanNames = new LinkedHashSet<>(1);
        Assert.state(beanFactory != null, "No BeanFactory available");
        TypeConverter typeConverter = beanFactory.getTypeConverter();
        try {
            // 解析属性依赖
            value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
        }
        catch (BeansException ex) {
            throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
        }
        synchronized (this) {
            if (!this.cached) {
                if (value != null || this.required) {
                    this.cachedFieldValue = desc;
                    registerDependentBeans(beanName, autowiredBeanNames);
                    if (autowiredBeanNames.size() == 1) {
                        String autowiredBeanName = autowiredBeanNames.iterator().next();
                        if (beanFactory.containsBean(autowiredBeanName) &&
                                beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
                            this.cachedFieldValue = new ShortcutDependencyDescriptor(
                                    desc, autowiredBeanName, field.getType());
                        }
                    }
                }
                else {
                    this.cachedFieldValue = null;
                }
                this.cached = true;
            }
        }
    }
    if (value != null) {
        ReflectionUtils.makeAccessible(field);
        // 给属性设置值，完成注入功能
        field.set(bean, value);
    }
}

解析属性依赖 DefaultListableBeanFactory#resolveDependency

public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
        @Nullable Set autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {

    descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
    if (Optional.class == descriptor.getDependencyType()) {
        return createOptionalDependency(descriptor, requestingBeanName);
    }
    else if (ObjectFactory.class == descriptor.getDependencyType() ||
            ObjectProvider.class == descriptor.getDependencyType()) {
        return new DependencyObjectProvider(descriptor, requestingBeanName);
    }
    else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
        return new Jsr330Factory().createDependencyProvider(descriptor, requestingBeanName);
    }
    else {
        Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
                descriptor, requestingBeanName);
        if (result == null) {
            // 解析依赖
            result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
        }
        return result;
    }
}

解析属性依赖 DefaultListableBeanFactory#doResolveDependency

public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
        @Nullable Set autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {

    InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
    try {
        // ...省略代码

        // 解析多个Bean，比如Array、List、Map类型，有兴趣可以自己查看分析
        Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
        if (multipleBeans != null) {
            return multipleBeans;
        }

        // 根据类型获取候选对象，针对studentService而言，该属性的类型为PersonService
        // PersonService有2个实现类，StudentServiceImpl和TeacherServiceImpl
        // 所以此处获取结果为StudentServiceImpl对象和TeacherServiceImpl对象
        Map matchingBeans = findAutowireCandidates(beanName, type, descriptor);
        if (matchingBeans.isEmpty()) {
            if (isRequired(descriptor)) {
                raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
            }
            return null;
        }

        String autowiredBeanName;
        Object instanceCandidate;
        // 重点处理，如果存在多个匹配的bean
        if (matchingBeans.size() > 1) {
            // 从已经匹配的bean中选择一个符合的bean
            autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
            if (autowiredBeanName == null) {
                // 如果bean必须注入或者存在多个匹配的bean，则抛出异常
                if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
                    return descriptor.resolveNotUnique(descriptor.getResolvableType(), matchingBeans);
                }
                else {
                    // In case of an optional Collection/Map, silently ignore a non-unique case:
                    // possibly it was meant to be an empty collection of multiple regular beans
                    // (before 4.3 in particular when we didn't even look for collection beans).
                    return null;
                }
            }
            // 根据bean名称获取对应的示例
            instanceCandidate = matchingBeans.get(autowiredBeanName);
        }
        else {
            // We have exactly one match.
            Map.Entry entry = matchingBeans.entrySet().iterator().next();
            autowiredBeanName = entry.getKey();
            instanceCandidate = entry.getValue();
        }

        if (autowiredBeanNames != null) {
            autowiredBeanNames.add(autowiredBeanName);
        }
        if (instanceCandidate instanceof Class) {
            instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
        }
        Object result = instanceCandidate;
        if (result instanceof NullBean) {
            if (isRequired(descriptor)) {
                raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
            }
            result = null;
        }
        if (!ClassUtils.isAssignableValue(type, result)) {
            throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());
        }
        // 返回对应的示例对象
        return result;
    }
    finally {
        ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
    }
}

此处主要根据类型获取所有匹配的bean，如果匹配的bean有多个，那么最后会选择一个符合条件的bean名称，然后将对应的bena实例返回，调用set方法进行进行注入，到此注入的原理本该结束了。但是还是要分析一下Spring Boot是如何选择出符合条件的bean？

选择符合条件的bean DefaultListableBeanFactory#determineAutowireCandidate

protected String determineAutowireCandidate(Map candidates, DependencyDescriptor descriptor) {
    Class requiredType = descriptor.getDependencyType();
    // 如果bean对应的primary属性为true，则返回bean对应的名称
    String primaryCandidate = determinePrimaryCandidate(candidates, requiredType);
    if (primaryCandidate != null) {
        return primaryCandidate;
    }
    // 如果候选bean使用javax.annotation.Priority标注，返回高优先级bean对应的名称
    String priorityCandidate = determineHighestPriorityCandidate(candidates, requiredType);
    if (priorityCandidate != null) {
        return priorityCandidate;
    }
    // Fallback
    // 如果匹配bean的名称和需要注入的属性名称一致，则返回匹配bean的名称
    for (Map.Entry entry : candidates.entrySet()) {
        String candidateName = entry.getKey();
        Object beanInstance = entry.getValue();
        if ((beanInstance != null && this.resolvableDependencies.containsValue(beanInstance)) ||
                matchesBeanName(candidateName, descriptor.getDependencyName())) {
            return candidateName;
        }
    }
    return null;
}

获取符合条件bean名称总结：

• 依据Bean的primary属性
• 依据javax.annotation.Priority
• 依据注入属性的名称

专车总结

• Bean实例化完成后，填充Bean
• 调用AutowiredAnnotationBeanPostProcessor#postProcessProperties处理属性
• 获取所有需要注入的属性
• 根据注入属性的类型从IOC容器中查找匹配实例
• 如果匹配实例存在多个，根据primary属性--->javax.annotation.Priority注解--->注入属性名称依次过滤，返回符合条件的Bean名称
• 过滤之后，存在一个符合条件的Bean名称，则返回对应的实例，否则抛出异常

专车回顾

回顾一下开头的2个问题：

• Spring Boot何时注入@Autowired标注的属性？
• 如果注入类型的Bean存在多个Spring Boot是如何处理的？

第一个问题：是在Bean实例化后，填充Bean的时候注入@Autowired标注的属性

第二个问题：如果存在多个类型的Bean，会根据primary--->javax.annotation.Priority--->名称依次过滤，得到最终匹配的bean名称