SpringBoot中不需要再使用PropertyPlaceholderConfigurer来注入属性了，而是用@Value等注解替换了。@Value注解的解析和@Autowired注解用了同一套逻辑。所以，我们一起通过分析@Autowired注解的解析过程来理解@Autowired和@Value的工作原理。
代码分析
@Autowired注解工作原理
@Autowired注解是由AutowiredAnnotationBeanPostProcessor类来解析的，它的类声明如下：
public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {
}
可以看出来，AutowiredAnnotationBeanPostProcessor类是一个BeanPostProcessor接口的实现类。它的类继承关系如下：
需要注意的是，AutowiredAnnotationBeanPostProcessor继承了InstantiationAwareBeanPostProcessorAdapter抽象类，该类对所有的接口方法都给予了默认实现。这样做的好处是，子类可以按需重写方法。代码如下：
public abstract class InstantiationAwareBeanPostProcessorAdapter implements SmartInstantiationAwareBeanPostProcessor {
@Override
@Nullable
public Class<?> predictBeanType(Class<?> beanClass, String beanName) {
return null;
}
@Override
@Nullable
public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, String beanName) throws BeansException {
return null;
}
@Override
public Object getEarlyBeanReference(Object bean, String beanName) throws BeansException {
return bean;
}
@Override
@Nullable
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
return null;
}
@Override
public boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {
return true;
}
@Override
public PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {
return pvs;
}
@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
return bean;
}
@Override
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
return bean;
}
}
这里需要先关注另一个接口MergedBeanDefinitionPostProcessor在AutowiredAnnotationBeanPostProcessor中的实现，MergedBeanDefinitionPostProcessor接口中只定义了一个方法，代码如下：
public interface MergedBeanDefinitionPostProcessor extends BeanPostProcessor {
/**
* Post-process the given merged bean definition for the specified bean.
* @param beanDefinition the merged bean definition for the bean
* @param beanType the actual type of the managed bean instance
* @param beanName the name of the bean
*/
void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName);
}
在AutowiredAnnotationBeanPostProcessor类中的实现代码：
@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
metadata.checkConfigMembers(beanDefinition);
}
findAutowiringMetadata()方法的实现：
private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
// Fall back to class name as cache key, for backwards compatibility with custom callers.
String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
// Quick check on the concurrent map first, with minimal locking.
InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
if (InjectionMetadata.needsRefresh(metadata, clazz)) {
synchronized (this.injectionMetadataCache) {
metadata = this.injectionMetadataCache.get(cacheKey);
if (InjectionMetadata.needsRefresh(metadata, clazz)) {
if (metadata != null) {
metadata.clear(pvs);
}
metadata = buildAutowiringMetadata(clazz);
this.injectionMetadataCache.put(cacheKey, metadata);
}
}
}
return metadata;
}
findAutowiringMetadata()方法是很重要的一个方法。从它的源码我们就能看出来@Autowired为什么优先使用beanName然后才使用类型了。就是这一行代码：String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
然后就是根据cacheKey来找bean对象了，先去injectionMetadataCache缓存找，找不到就构建一个出来，构建的过程在buildAutowiringMetadata()方法里，代码如下：
private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
LinkedList<InjectionMetadata.InjectedElement> elements = new LinkedList<>();
Class<?> targetClass = clazz;
do {
final LinkedList<InjectionMetadata.InjectedElement> currElements = new LinkedList<>();
//逐个遍历类属性，如果该属性上声明了@Autowired注解，且required=true，那么添加到需要自动注入的列表中
ReflectionUtils.doWithLocalFields(targetClass, field -> {
AnnotationAttributes ann = findAutowiredAnnotation(field);
if (ann != null) {
if (Modifier.isStatic(field.getModifiers())) {
if (logger.isWarnEnabled()) {
logger.warn("Autowired annotation is not supported on static fields: " + field);
}
return;
}
boolean required = determineRequiredStatus(ann);
currElements.add(new AutowiredFieldElement(field, required));
}
});
//逐个遍历类方法，如果该方法上声明了@Autowired注解，且required=true，那么添加到需要自动注入的列表中
ReflectionUtils.doWithLocalMethods(targetClass, method -> {
Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
return;
}
AnnotationAttributes ann = findAutowiredAnnotation(bridgedMethod);
if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
if (Modifier.isStatic(method.getModifiers())) {
if (logger.isWarnEnabled()) {
logger.warn("Autowired annotation is not supported on static methods: " + method);
}
return;
}
if (method.getParameterCount() == 0) {
if (logger.isWarnEnabled()) {
logger.warn("Autowired annotation should only be used on methods with parameters: " +
method);
}
}
boolean required = determineRequiredStatus(ann);
PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
currElements.add(new AutowiredMethodElement(method, required, pd));
}
});
elements.addAll(0, currElements);
targetClass = targetClass.getSuperclass();
}
while (targetClass != null && targetClass != Object.class);
return new InjectionMetadata(clazz, elements);
}
buildAutowiringMetadata()方法会遍历类的所有属性和方法，挨个找@Autowired注解，如果找到，就加入到需要自动注入的列表中。findAutowiredAnnotation()方法代码：
@Nullable
private AnnotationAttributes findAutowiredAnnotation(AccessibleObject ao) {
if (ao.getAnnotations().length > 0) {
for (Class<? extends Annotation> type : this.autowiredAnnotationTypes) {
AnnotationAttributes attributes = AnnotatedElementUtils.getMergedAnnotationAttributes(ao, type);
if (attributes != null) {
return attributes;
}
}
}
return null;
}
这里需要注意的是autowiredAnnotationTypes，这是一个类属性，我们看一下它的声明：
private final Set<Class<? extends Annotation>> autowiredAnnotationTypes = new LinkedHashSet<>();
给设置值的地方有三个：
/**
* Create a new AutowiredAnnotationBeanPostProcessor
* for Spring's standard {@link Autowired} annotation.
* <p>Also supports JSR-330's {@link javax.inject.Inject} annotation, if available.
*/
@SuppressWarnings("unchecked")
public AutowiredAnnotationBeanPostProcessor() {
this.autowiredAnnotationTypes.add(Autowired.class);
this.autowiredAnnotationTypes.add(Value.class);
try {
this.autowiredAnnotationTypes.add((Class<? extends Annotation>)
ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));
logger.info("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring");
}
catch (ClassNotFoundException ex) {
// JSR-330 API not available - simply skip.
}
}
/**
* Set the 'autowired' annotation type, to be used on constructors, fields,
* setter methods and arbitrary config methods.
* <p>The default autowired annotation type is the Spring-provided
* {@link Autowired} annotation, as well as {@link Value}.
* <p>This setter property exists so that developers can provide their own
* (non-Spring-specific) annotation type to indicate that a member is
* supposed to be autowired.
*/
public void setAutowiredAnnotationType(Class<? extends Annotation> autowiredAnnotationType) {
Assert.notNull(autowiredAnnotationType, "'autowiredAnnotationType' must not be null");
this.autowiredAnnotationTypes.clear();
this.autowiredAnnotationTypes.add(autowiredAnnotationType);
}
/**
* Set the 'autowired' annotation types, to be used on constructors, fields,
* setter methods and arbitrary config methods.
* <p>The default autowired annotation type is the Spring-provided
* {@link Autowired} annotation, as well as {@link Value}.
* <p>This setter property exists so that developers can provide their own
* (non-Spring-specific) annotation types to indicate that a member is
* supposed to be autowired.
*/
public void setAutowiredAnnotationTypes(Set<Class<? extends Annotation>> autowiredAnnotationTypes) {
Assert.notEmpty(autowiredAnnotationTypes, "'autowiredAnnotationTypes' must not be empty");
this.autowiredAnnotationTypes.clear();
this.autowiredAnnotationTypes.addAll(autowiredAnnotationTypes);
}
需要重点关注的是第一个构造方法，可以看到构造方法中直接将@Autowired注解和@Value注解添加进来了。后面的两个方法是用来修改autowiredAnnotationTypes中的值的。
我们再来看看postProcessMergedBeanDefinition()方法：
@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
metadata.checkConfigMembers(beanDefinition);
}
上面我们梳理了findAutowiringMetadata()方法的代码实现，最后的结果是将@Autowired注解和@Value注解声明的需要自动注入的属性都添加到列表中，然后封装到了InjectionMetadata对象中。
我们再来看看第二行代码checkConfigMembers()方法做了什么：
public void checkConfigMembers(RootBeanDefinition beanDefinition) {
Set<InjectedElement> checkedElements = new LinkedHashSet<>(this.injectedElements.size());
for (InjectedElement element : this.injectedElements) {
Member member = element.getMember();
if (!beanDefinition.isExternallyManagedConfigMember(member)) {
beanDefinition.registerExternallyManagedConfigMember(member);
checkedElements.add(element);
if (logger.isDebugEnabled()) {
logger.debug("Registered injected element on class [" + this.targetClass.getName() + "]: " + element);
}
}
}
this.checkedElements = checkedElements;
}
可以看到，这是InjectionMetadata类提供的方法。方法内部逐个遍历需要自动注入的属性并调用RootBeanDefinition的registerExternallyManagedConfigMember()方法注册到beanDefinition对象上。
如果到这里就完事了。那么，@Value注解的值是什么时候替换的？？？
@Value注解工作原理
接下来，我们需要重新捋一下思路。我们看一下buildAutowiringMetadata()方法都在什么地方用到了：
postProcessMergedBeanDefinition()就不说了，postProcessPropertyValues()和processInjection()是干嘛的呢？
postProcessPropertyValues()方法源码：
@Override
public PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
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;
}
代码比较简单，主要的不同在metadata.inject(bean, beanName, pvs);这行代码，我们看下内部实现：
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Collection<InjectedElement> checkedElements = this.checkedElements;
Collection<InjectedElement> elementsToIterate =
(checkedElements != null ? checkedElements : this.injectedElements);
if (!elementsToIterate.isEmpty()) {
boolean debug = logger.isDebugEnabled();
for (InjectedElement element : elementsToIterate) {
if (debug) {
logger.debug("Processing injected element of bean '" + beanName + "': " + element);
}
element.inject(target, beanName, pvs);
}
}
}
因为这个方法返回值是void的，所以这个方法的核心就是对参数PropertyValues pvs的操作了。发现pvs相关的操作在element.inject(target, beanName, pvs)这行代码里，继续往下看：
protected void inject(Object target, @Nullable String requestingBeanName, @Nullable PropertyValues pvs)
throws Throwable {
if (this.isField) {
Field field = (Field) this.member;
ReflectionUtils.makeAccessible(field);
field.set(target, getResourceToInject(target, requestingBeanName));
}
else {
if (checkPropertySkipping(pvs)) {
return;
}
try {
Method method = (Method) this.member;
ReflectionUtils.makeAccessible(method);
method.invoke(target, getResourceToInject(target, requestingBeanName));
}
catch (InvocationTargetException ex) {
throw ex.getTargetException();
}
}
}
这块就比较有意思了，会判断类型然后执行不同的逻辑，如果是属性就设值；如果是方法就调用。这不是我们关系的点，我们关心的是值是怎么来的，所以需要继续深入getResourceToInject()方法：
@Nullable
protected Object getResourceToInject(Object target, @Nullable String requestingBeanName) {
return null;
}
点进去以后发现是个这。别急，既然是个空方法，肯定有实现方的。往上一翻发现是getResourceToInject()方法是InjectedElement类提供的，而且这哥们还是个抽象类：
public abstract static class InjectedElement {
}
来来来，看看它的实现类：
好了，没跑了，实现肯定在AutowiredFieldElement或AutowiredMethodElement类里，两个类处理差不多，我们看下AutowiredFieldElement类的源码：
private class AutowiredFieldElement extends InjectionMetadata.InjectedElement {
private final boolean required;
private volatile boolean cached = false;
@Nullable
private volatile Object cachedFieldValue;
public AutowiredFieldElement(Field field, boolean required) {
super(field, null);
this.required = required;
}
@Override
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Field field = (Field) this.member;
Object value;
if (this.cached) {
value = resolvedCachedArgument(beanName, this.cachedFieldValue);
}
else {
DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
desc.setContainingClass(bean.getClass());
Set<String> autowiredBeanNames = new LinkedHashSet<>(1);
Assert.state(beanFactory != null, "No BeanFactory available");
TypeConverter typeConverter = beanFactory.getTypeConverter();
try {
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);
field.set(bean, value);
}
}
}
无情啊…直接重写inject()方法什么鬼。算了，不要在意细节。最重要的代码是这句Object arg = beanFactory.resolveDependency(currDesc, beanName, autowiredBeans, typeConverter);我们看下实现：
@Override
@Nullable
public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
if (Optional.class == descriptor.getDependencyType()) {
// 最终它也会走到：doResolveDependency
return createOptionalDependency(descriptor, requestingBeanName);
}
else if (ObjectFactory.class == descriptor.getDependencyType() ||
ObjectProvider.class == descriptor.getDependencyType()) {
return new DependencyObjectProvider(descriptor, requestingBeanName);
}
else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
// 兼容jsr330的javax.inject.Provider
return new Jsr330ProviderFactory().createDependencyProvider(descriptor, requestingBeanName);
}
else {
Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
descriptor, requestingBeanName);
if (result == null) {
result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
}
return result;
}
}
需要关注doResolveDependency()方法，源码：
@Nullable
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
try {
Object shortcut = descriptor.resolveShortcut(this);
if (shortcut != null) {
return shortcut;
}
Class<?> type = descriptor.getDependencyType();
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()));
}
Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
if (multipleBeans != null) {
return multipleBeans;
}
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
if (matchingBeans.isEmpty()) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
return null;
}
String autowiredBeanName;
Object instanceCandidate;
if (matchingBeans.size() > 1) {
autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
if (autowiredBeanName == null) {
if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
return descriptor.resolveNotUnique(type, 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;
}
}
instanceCandidate = matchingBeans.get(autowiredBeanName);
}
else {
// We have exactly one match.
Map.Entry<String, Object> 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());
}
return result;
}
finally {
ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
}
}
doResolveDependency()方法完成了最后的属性解析和注入，仔细看一遍代码即可~~（看不下去的话，参考下这篇博客Spring-Framework 源码阅读之@Autowired和AutowiredAnnotationBeanPostProcessor）