springboot static属性 注入

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beanif (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