各位看友，请注意，本文章篇幅很长，得耐心看，希望坚持。
IOC，inversion of control 控制反转，有时候也叫DI，dependency injection 依赖注入，是一种代码解耦的方式。
在一个类中定义一个属性，正常情况下需要在此类中有对此属性赋值的代码，如setter方法，或者在构造函数中赋值，因此类和属性之间的关系已经确定下来了，类和属性间就有了一定的耦合关系。而IOC的精髓就是解耦，类中没有显式的对属性的赋值代码，同时属性的实际类型和值在运行时有系统动态的赋值，Spring对IOC做了很大的扩展，使用者可以很灵活的控制注入的对象类型及值。
Spring内IOC的最直接体现就是@Autowired注解，最常用的方式就是表示在属性上，Spring容器在启动时会将容器内类型是标识了@Autowired的属性类型或者其子类，实现类的Bean通过反射的形式赋值给此属性，或者叫注入的此类中。
Spring中对@Autowired注解的解析是通过一个叫AutowiredAnnotationBeanPostProcessor的BeanPostProcessor(Bean的后处理器)来进行的。BeanPostProcessor负责Bean的一系列处理，实例化前后，初始化前后灯阶段执行相应方法，具体方法的执行顺序如下:
BeanPostPrecessors Invoke Procedure(执行流程)
1.InstantiationAwareBeanPostProcessor.postProcessBeforeInstantiation(Class<?> beanClass, String beanName);
invoke before doCreateBean(),if the method returns not null,the return Object will as the Bean, avoid invoke doCreateBean() ;
2.SmartInstantiationAwareBeanPostProcessor.determineCandidateConstructors(Class<?> beanClass, String beanName);
used to determine to use which Constructor to instance Bean;
3.MergedBeanDefinitionPostProcessor.postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName);
the Bean gets instantiated, modify the RootBeanDefinition ,such as adding PropertyValue etc. before populate Bean;
4.SmartInstantiationAwareBeanPostProcessor.getEarlyBeanReference(Object bean, String beanName);
invoke before populate Bean,used for resolve Circular Reference;
5.InstantiationAwareBeanPostProcessor.postProcessAfterInstantiation(Object bean, String beanName);
invoke in populate Bean, but before fill in values，return false will break the opreation for filling values in Bean;
6.InstantiationAwareBeanPostProcessor.postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName);
invoke in populate Bean, but before fill in values，modify the PropertyValues which will be apply for the Bean,
return null will break the opreation for filling values in Bean;
7.BeanPostProcessor.postProcessBeforeInitialization(Object bean, String beanName);
invoked before invokeInitMethods, modify the finally Bean instance
8.BeanPostProcessor.postProcessAfterInitialization(Object bean, String beanName);
invoked after invokeInitMethods, modify the finally Bean instance
使用者可以自己定义自己的BeanPostProcessor，将其以Bean的形式注册到Spring容器中，Spring容器在启动时会执行其相应方法。
AutowiredAnnotationBeanPostProcessor在Spring容器初始化时会最先执行determineCandidateConstructors方法，先看起源码:
public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {
//此方法的含义是解析Bean类里构造函数上的@Autowired注解，如果有合适的标识了@Autowired的构造函数,
//则在实例化此Bean时会使用此构造函数，@Import和@ComponentScan得到的Bean会如此解析，
//@Bean标识方法生成的Bean不会如此
public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, final String beanName)
throws BeanCreationException {
// Let's check for lookup methods here..
if (!this.lookupMethodsChecked.contains(beanName)) {
try {
ReflectionUtils.doWithMethods(beanClass, new ReflectionUtils.MethodCallback() {
@Override
public void doWith(Method method) throws IllegalArgumentException, IllegalAccessException {
Lookup lookup = method.getAnnotation(Lookup.class);
if (lookup != null) {
LookupOverride override = new LookupOverride(method, lookup.value());
try {
RootBeanDefinition mbd = (RootBeanDefinition) beanFactory.getMergedBeanDefinition(beanName);
mbd.getMethodOverrides().addOverride(override);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(beanName,
"Cannot apply @Lookup to beans without corresponding bean definition");
}
}
}
});
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName, "Lookup method resolution failed", ex);
}
catch (NoClassDefFoundError err) {
throw new BeanCreationException(beanName, "Failed to introspect bean class [" + beanClass.getName() +
"] for lookup method metadata: could not find class that it depends on", err);
}
this.lookupMethodsChecked.add(beanName);
}
// Quick check on the concurrent map first, with minimal locking.
Constructor<?>[] candidateConstructors = this.candidateConstructorsCache.get(beanClass);
if (candidateConstructors == null) {
// Fully synchronized resolution now...
synchronized (this.candidateConstructorsCache) {
//对每个类的构造函数只解析一次，解析完会存储结果，以备下次服用
candidateConstructors = this.candidateConstructorsCache.get(beanClass);
if (candidateConstructors == null) {
Constructor<?>[] rawCandidates;
try {
rawCandidates = beanClass.getDeclaredConstructors();
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Resolution of declared constructors on bean Class [" + beanClass.getName() +
"] from ClassLoader [" + beanClass.getClassLoader() + "] failed", ex);
}
List<Constructor<?>> candidates = new ArrayList<Constructor<?>>(rawCandidates.length);
Constructor<?> requiredConstructor = null;
Constructor<?> defaultConstructor = null;
for (Constructor<?> candidate : rawCandidates) {
//查看构造函数上是否标识@Autowired注解，获取注解的属性
AnnotationAttributes ann = findAutowiredAnnotation(candidate);
if (ann == null) {
Class<?> userClass = ClassUtils.getUserClass(beanClass);
if (userClass != beanClass) {
try {
Constructor<?> superCtor =
userClass.getDeclaredConstructor(candidate.getParameterTypes());
//如果此构造函数是重载了父类的构造函数，则寻找父类的构造函数，查看是否标识@Autowired注解，获取注解的属性
ann = findAutowiredAnnotation(superCtor);
}
catch (NoSuchMethodException ex) {
// Simply proceed, no equivalent superclass constructor found...
}
}
}
//当构造函数上存在@Autowired注解
if (ann != null) {
//@Autowired默认的required = true,当有一个required = true的@Autowired标识的构造函数时，
//不能有其他的构造函数再标识@Autowired，否则会报错
if (requiredConstructor != null) {
throw new BeanCreationException(beanName,
"Invalid autowire-marked constructor: " + candidate +
". Found constructor with 'required' Autowired annotation already: " +
requiredConstructor);
}
//@Autowired的required的属性值，true / false
boolean required = determineRequiredStatus(ann);
if (required) {
if (!candidates.isEmpty()) {
throw new BeanCreationException(beanName,
"Invalid autowire-marked constructors: " + candidates +
". Found constructor with 'required' Autowired annotation: " +
candidate);
}
requiredConstructor = candidate;
}
candidates.add(candidate);
}
else if (candidate.getParameterTypes().length == 0) {
defaultConstructor = candidate;
}
}
if (!candidates.isEmpty()) {
// Add default constructor to list of optional constructors, as fallback.
当没有构造函数标识@Autowired时，设置默认的构造函数作为额外选择
if (requiredConstructor == null) {
if (defaultConstructor != null) {
candidates.add(defaultConstructor);
} //如果没有默认的无参构造函数，且有@Autowired（required = false）的构造函数，则发出警告信
else if (candidates.size() == 1 && logger.isWarnEnabled()) {
logger.warn("Inconsistent constructor declaration on bean with name '" + beanName +
"': single autowire-marked constructor flagged as optional - " +
"this constructor is effectively required since there is no " +
"default constructor to fall back to: " + candidates.get(0));
}
}
candidateConstructors = candidates.toArray(new Constructor<?>[candidates.size()]);
} //若只有一个构造函数，且没标识@Autowired，其参数长度>0，将其作为候选者
else if (rawCandidates.length == 1 && rawCandidates[0].getParameterTypes().length > 0) {
candidateConstructors = new Constructor<?>[] {rawCandidates[0]};
} //没有合适的构造函数
else {
candidateConstructors = new Constructor<?>[0];
}
this.candidateConstructorsCache.put(beanClass, candidateConstructors);
}
}
}
return (candidateConstructors.length > 0 ? candidateConstructors : null);
}
}
AutowiredAnnotationBeanPostProcessor筛选@Autowired标识的构造函数的代码就是这些，筛选出这些构造函数之后，Spring使用ConstructorResolver这个类来择取合适的构造函数，流程如下:
首先对这些构造函数按修饰符优先public排序，修饰符相同再按参数的长短排序，最先解析参数最长的public修饰的构造函数，其优先级最高.
对构造函数的每一个参数解析，如果每一个参数均能从Spring容器中找到合适的Bean，则此将此构造函数作为最优解，如果容器内Bean不能满足所有参数，则解析下一个构造函数。
如果存在两个参数长度相同的构造函数，且容器内Bean均能满足参数解析，则按参数类型和Bean类型的差异性求取参数的差异权重，比如参数是接口，Bean是实现类，则差异加2，参数是集合，Bean是单个类，则转换成集合，差异加2等等，比较两个构造函数的差异权重大小，差异小的那个作为最优解。如果两个差异权重相等，则抛出含有模棱两可的构造函数的BeanCreationException。
当有了最优解的构造函数后，如果下一个构造函数的参数长度等于最优解，则解析此构造函数，如果参数长度小于最优解，则不再解析，直接忽略之后的所有构造函数。
当得到了构造函数最优解之后，将此构造函数存入此Bean的BeanDefinition中，以备下次复用，就是说只对构造函数候选者集合解析一次，下次实例化Bean的时候可以直接得到这个最优解。
以上的情况是基于Spring容器实例化Bean的情况，就是实例化时不会附带Arguments,就是不带参数，如果是使用者自己实例化Bean时，通过BeanFactory的 Object getBean(String name, Object… args) throws BeansException; T getBean(Class requiredType, Object… args) throws BeansException;方法实例化时，按照构造函数的顺序传入实例化参数，则Spring在找合适的构造函数时会忽略之前缓存的最优解，以同样的顺序解析构造函数候选者集合，看看哪些构造函数的参数适合传入的参数，找不到合适的则跑出BeanCreationException。
以上就是@Autowired注解在构造函数上的用法的原理及流程。
重点: BeanFactory的getBean（）方法获取scope = singleton的Bean时，不会生成新的Bean对象，
在scope为request及session的生命周期内，Bean的实例化只会触发一次，也就是说@Autowired的标识的构造函数不是每次调用getBean()均会触发执行。只有scope = prototype的Bean，才会每次均执行。
pring容器在每个Bean实例化之后，调用AutowiredAnnotationBeanPostProcessor的
postProcessMergedBeanDefinition方法:
public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {
@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
if (beanType != null) { //搜索每个Bean内@Autowired注解的信息
InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
metadata.checkConfigMembers(beanDefinition);
}
}
private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, 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);
}
try { //解析@Autowired注解的信息，生成元数据，缓存起来
metadata = buildAutowiringMetadata(clazz);
this.injectionMetadataCache.put(cacheKey, metadata);
}
catch (NoClassDefFoundError err) {
throw new IllegalStateException("Failed to introspect bean class [" + clazz.getName() +
"] for autowiring metadata: could not find class that it depends on", err);
}
}
}
}
return metadata;
}
private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
LinkedList<InjectionMetadata.InjectedElement> elements = new LinkedList<InjectionMetadata.InjectedElement>();
Class<?> targetClass = clazz;
do {
final LinkedList<InjectionMetadata.InjectedElement> currElements =
new LinkedList<InjectionMetadata.InjectedElement>();
ReflectionUtils.doWithLocalFields(targetClass, new ReflectionUtils.FieldCallback() {
@Override
public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException {
//如果类内的属性上有@Autowired注解，则用工具类获取注解信息
AnnotationAttributes ann = findAutowiredAnnotation(field);
if (ann != null) { //@Autowired注解不支持静态方法
if (Modifier.isStatic(field.getModifiers())) {
if (logger.isWarnEnabled()) {
logger.warn("Autowired annotation is not supported on static fields: " + field);
}
return;
} //获取@Autowired注解的required的属性值，如果true，但注入失败会抛出异常，false则不会
boolean required = determineRequiredStatus(ann);
currElements.add(new **AutowiredFieldElement**(field, required));
}
}
});
ReflectionUtils.doWithLocalMethods(targetClass, new ReflectionUtils.MethodCallback() {
@Override
public void doWith(Method method) throws IllegalArgumentException, IllegalAccessException {
Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
return;
} //如果方法上有@Autowired注解，则获取注解信息
AnnotationAttributes ann = findAutowiredAnnotation(bridgedMethod);
if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
//@Autowired不支持静态方法
if (Modifier.isStatic(method.getModifiers())) {
if (logger.isWarnEnabled()) {
logger.warn("Autowired annotation is not supported on static methods: " + method);
}
return;
}
//@Autowired注解标识在方法上的目的就是将容器内的Bean注入到方法的参数中，没有参数就违背了初衷
if (method.getParameterTypes().length == 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);
//在解析完Bean的类型之后，递归的解析其父类，将所有的@Autowired的属性和方法收集起来，
//且类的层级越高其属性会被越优先注入
**targetClass = targetClass.getSuperclass()**;
}
while (targetClass != null && targetClass != Object.class);
return new InjectionMetadata(clazz, elements);
}
}
在将@Autowired注解的信息解析成元数据之后，缓存起来，以备复用，每一个类只解析一次。
在BeanPostProcessor的postProcessMergedBeanDefinition（）方法执行之后，下一步就是执行postProcessPropertyValues（）方法，AutowiredAnnotationBeanPostProcessor的postProcessPropertyValues（）方法就是从Spring IOC容器从找到合适的Bean，注入属性。找到合适的Bean，注入@Autowired标识方法的参数，然后执行此方法。
public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {
@Override
public PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
//从缓存中找到此类的@Autowired元数据，尝试注入。没有@Autowired则会略过
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;
}
}
InjectionMetadata，持有待注入的元数据信息，执行inject（）方法，开始注入属性或方法参数。
public class InjectionMetadata {
private static final Log logger = LogFactory.getLog(InjectionMetadata.class);
private final Class<?> targetClass;
//待注入的属性，是一个InjectedElement集合，按类等层级排列，父类的@Autowired排前
private final Collection<InjectedElement> injectedElements;
private volatile Set<InjectedElement> checkedElements;
public InjectionMetadata(Class<?> targetClass, Collection<InjectedElement> elements) {
this.targetClass = targetClass;
this.injectedElements = elements;
}
public void inject(Object target, String beanName, PropertyValues pvs) throws Throwable {
Collection<InjectedElement> elementsToIterate =
(this.checkedElements != null ? this.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);
}
//解析@Autowired注解生成的元数据类有AutowiredFieldElement，AutowiredMethodElement
//这两个类继承InjectionMetadata .InjectedElement，各自实现了inject方法。
//这两个类是AutowiredAnnotationBeanPostProcessor的私有内部类
element.inject(target, beanName, pvs);
}
}
}
}
下面先看属性的注入:
代码块4
private class AutowiredFieldElement extends InjectionMetadata.InjectedElement {
private final boolean required;
private volatile boolean cached = false;
private volatile Object cachedFieldValue;
public AutowiredFieldElement(Field field, boolean required) {
super(field, null);
this.required = required;
}
@Override
protected void inject(Object bean, String beanName, 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<String>(1);
TypeConverter typeConverter = beanFactory.getTypeConverter();
try {
//如果@Autowired标识的属性有一个合适的待注入对象，则缓存这个Bean的名称，
//如果再次生成这个Bean时，就不需要重新按类型去搜索Spring容器，直接获取这个缓存Bean的名称
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;
//注册Bean依赖
registerDependentBeans(beanName, autowiredBeanNames);
if (autowiredBeanNames.size() == 1) {
String autowiredBeanName = autowiredBeanNames.iterator().next();
if (beanFactory.containsBean(autowiredBeanName)) {
//@Autowired标识属性类型和Bean的类型要匹配，因此Array,Collection,Map类型的属性不支持缓存属性Bean名称
if (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); //通过反射为属性赋值
}
}
}
接下来就需要Spring IOC容器根据DependencyDescriptor依赖描述去寻找容器中合适的Bean:
代码块5
public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
public Object resolveDependency(DependencyDescriptor descriptor, String requestingBeanName,
Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {
descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
if (javaUtilOptionalClass == descriptor.getDependencyType()) {
return new OptionalDependencyFactory().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 Jsr330ProviderFactory().createDependencyProvider(descriptor, requestingBeanName);
}
else {
Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
descriptor, requestingBeanName);
if (result == null) {
//大部分情况下均是执行此步骤，解析依赖
result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
}
return result;
}
}
public Object doResolveDependency(DependencyDescriptor descriptor, String beanName,
Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {
InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
try { //如果是快捷方式的依赖描述，则直接通过候选者的名称来择取Bean，返回
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()));
}
//如果标识@Autowired注解的属性是容器类型，Array,Collection,Map,则在这个方法中解析，
//如果类型是容器类型，且返回了合适的Bean,则解析成功，代码详解请看此段代码的下一段代码
Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
if (multipleBeans != null) { //如果确实时容器类型的属性，则直接返回
return multipleBeans;
}
//非容器类型属性，按类型去IOC容器内择取所有类型匹配的候选者
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
if (matchingBeans.isEmpty()) { //如果没有合适的Bean，但注解的required = true，则抛出异常，若required = false,则注入失败。
if (descriptor.isRequired()) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
return null;
}
String autowiredBeanName;
Object instanceCandidate;
if (matchingBeans.size() > 1) {
//非容器属性，但是有多个候选者，此时需要从中选出最优的那个,代码详解看代码块9
autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
if (autowiredBeanName == null) {
if (descriptor.isRequired() || !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);
}
return (instanceCandidate instanceof Class ?
descriptor.resolveCandidate(autowiredBeanName, type, this) : instanceCandidate);
}
finally {
ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
}
}
}
Spring IOC容器解析Array,Collection,Map类型的属性注入:
代码块6
private Object resolveMultipleBeans(DependencyDescriptor descriptor, String beanName,
Set<String> autowiredBeanNames, TypeConverter typeConverter) {
Class<?> type = descriptor.getDependencyType();
if (type.isArray()) { //如果@Autowired标识的是数组类型的属性
Class<?> componentType = type.getComponentType(); //获取数组的内容类型
ResolvableType resolvableType = descriptor.getResolvableType();
Class<?> resolvedArrayType = resolvableType.resolve();
if (resolvedArrayType != null && resolvedArrayType != type) {
type = resolvedArrayType;
componentType = resolvableType.getComponentType().resolve();
}
if (componentType == null) {
return null;
} //通过类型去IOC容器内择取符合的Bean都是使用这个方法，很重要，此方法请看此段代码的下一段
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, componentType,
new MultiElementDescriptor(descriptor)); //对依赖描述做了封装，区别普通类型的属性注入
if (matchingBeans.isEmpty()) {
return null;
}
if (autowiredBeanNames != null) {
autowiredBeanNames.addAll(matchingBeans.keySet());
}
TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
//将得到的Bean的候选者们转换为属性类型，如从set转换为Array,List等
Object result = converter.convertIfNecessary(matchingBeans.values(), type);
if (getDependencyComparator() != null && result instanceof Object[]) {
Arrays.sort((Object[]) result, adaptDependencyComparator(matchingBeans));
}
return result;
}
else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {
//获取Collection的泛型
Class<?> elementType = descriptor.getResolvableType().asCollection().resolveGeneric();
if (elementType == null) {
return null;
}
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, elementType,
new MultiElementDescriptor(descriptor));
if (matchingBeans.isEmpty()) {
return null;
}
if (autowiredBeanNames != null) {
autowiredBeanNames.addAll(matchingBeans.keySet());
}
TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
Object result = converter.convertIfNecessary(matchingBeans.values(), type);
if (getDependencyComparator() != null && result instanceof List) {
Collections.sort((List<?>) result, adaptDependencyComparator(matchingBeans));
}
return result;
}
else if (Map.class == type) {
ResolvableType mapType = descriptor.getResolvableType().asMap();
Class<?> keyType = mapType.resolveGeneric(0);
if (String.class != keyType) {
return null;
}
Class<?> valueType = mapType.resolveGeneric(1);
if (valueType == null) {
return null;
}
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, valueType,
new MultiElementDescriptor(descriptor));
if (matchingBeans.isEmpty()) {
return null;
}
if (autowiredBeanNames != null) {
autowiredBeanNames.addAll(matchingBeans.keySet());
}
return matchingBeans;
}
else {
return null;
}
}
Spring IOC容器通过类型到容器呢择取符合的Bean的方法:
代码块7
protected Map<String, Object> findAutowireCandidates(
String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {
//Spring容IOC容器中获取所有的符合类型的Bean，包括非Singleton的scope,
//也就是说request,session,prototype的均会被实例化，
//request,session如果此时被实例化会报异常，因为这两个作用域的Bean实际是存储在HttpRequest中的，
//此时还没有Http请求，如果是FactoryBean,则匹配getObject（）放回的类型
String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
this, requiredType, true, descriptor.isEager());
Map<String, Object> result = new LinkedHashMap<String, Object>(candidateNames.length);
//首先从容器自身注册了的依赖解析来匹配，Spring容器自身注册了很多Bean的依赖，
//当使用者想要注入指定类型的Bean时，会优先从已注册的依赖内寻找匹配，这点很重要，代码详解请看此段下一段。
for (Class<?> autowiringType : this.resolvableDependencies.keySet()) {
if (autowiringType.isAssignableFrom(requiredType)) {
Object autowiringValue = this.resolvableDependencies.get(autowiringType);
autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
//如果注册的依赖Bean类型时指定类型的实例或是其父类，接口，则将其作为候选者，注册依赖的类型不会重复
if (requiredType.isInstance(autowiringValue)) {
result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
break;
}
}
}
for (String candidate : candidateNames) {
//@Autowried标识的属性不能和定义其的类的类型相同，就是类不能再注入相同类，会触发无限递归注入
if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate,
addCandidateEntry(result, candidate, descriptor, requiredType);
}
}
if (result.isEmpty() && !indicatesMultipleBeans(requiredType)) {
// Consider fallback matches if the first pass failed to find anything...
DependencyDescriptor fallbackDescriptor = descriptor.forFallbackMatch();
for (String candidate : candidateNames) {
if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, fallbackDescriptor)) {
addCandidateEntry(result, candidate, descriptor, requiredType);
}
}
if (result.isEmpty()) {
// Consider self references as a final pass...
// but in the case of a dependency collection, not the very same bean itself.
for (String candidate : candidateNames) {
if (isSelfReference(beanName, candidate) &&
(!(descriptor instanceof MultiElementDescriptor) || !beanName.equals(candidate)) &&
isAutowireCandidate(candidate, fallbackDescriptor)) {
addCandidateEntry(result, candidate, descriptor, requiredType);
}
}
}
}
return result;
}
/**
* Add an entry to the candidate map: a bean instance if available or just the resolved
* type, preventing early bean initialization ahead of primary candidate selection.
*/
private void addCandidateEntry(Map<String, Object> candidates, String candidateName,
DependencyDescriptor descriptor, Class<?> requiredType) {
//当@Autowired标识的是容器类型的属性，生成的依赖描述类型是MultiElementDescriptor ，
//因此所有的候选者均是合格的，所以会当场实例化他们。而如果属性的类型非容器，那么可能是多个候选者中挑一个，
//此时实例化他们所有就不合适了，最终会把合格的那个实例化，如果没有合格的则不实例化，
//提前实例化对Bean的很多方面有影响，比如AOP，EarlyReference等 */
if (descriptor instanceof MultiElementDescriptor || containsSingleton(candidateName)) {
candidates.put(candidateName, descriptor.resolveCandidate(candidateName, requiredType, this));
}
else {
candidates.put(candidateName, getType(candidateName));
}
}
如下代码所示，Spring在开始实例化Bean之前，已经注册了很多Bean的历依赖关系对，当@Autowired标识的属性类型是已经注册了的依赖类型，则将这些注册的对象作为候选者。这些对象并不一定是容器内的Bean，比如Http请求相关的对象，他们被注入的优先级比容器内Bean高。同时也忽略了一些依赖关系，也就是有些类型的Bean不允许被注入，这个实现方式稍后会看到
代码块8
public abstract class AbstractApplicationContext extends DefaultResourceLoader
implements ConfigurableApplicationContext, DisposableBean {
protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
// Tell the internal bean factory to use the context's class loader etc.
beanFactory.setBeanClassLoader(getClassLoader());
beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));
// Configure the bean factory with context callbacks.
beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);
// BeanFactory interface not registered as resolvable type in a plain factory.
// MessageSource registered (and found for autowiring) as a bean.
beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
beanFactory.registerResolvableDependency(ResourceLoader.class, this);
beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
beanFactory.registerResolvableDependency(ApplicationContext.class, this);
}
}
public abstract class WebApplicationContextUtils {
public static void registerWebApplicationScopes(ConfigurableListableBeanFactory beanFactory, ServletContext sc) {
beanFactory.registerScope(WebApplicationContext.SCOPE_REQUEST, new RequestScope());
beanFactory.registerScope(WebApplicationContext.SCOPE_SESSION, new SessionScope(false));
beanFactory.registerScope(WebApplicationContext.SCOPE_GLOBAL_SESSION, new SessionScope(true));
if (sc != null) {
ServletContextScope appScope = new ServletContextScope(sc);
beanFactory.registerScope(WebApplicationContext.SCOPE_APPLICATION, appScope);
// Register as ServletContext attribute, for ContextCleanupListener to detect it.
sc.setAttribute(ServletContextScope.class.getName(), appScope);
}
beanFactory.registerResolvableDependency(ServletRequest.class, new RequestObjectFactory());
beanFactory.registerResolvableDependency(ServletResponse.class, new ResponseObjectFactory());
beanFactory.registerResolvableDependency(HttpSession.class, new SessionObjectFactory());
beanFactory.registerResolvableDependency(WebRequest.class, new WebRequestObjectFactory());
if (jsfPresent) {
FacesDependencyRegistrar.registerFacesDependencies(beanFactory);
}
}
}
普通属性输入，但是候选者有多个，如何从中挑选出最优解.
代码块9
protected String determineAutowireCandidate(Map<String, Object> candidates, DependencyDescriptor descriptor) {
Class<?> requiredType = descriptor.getDependencyType();
//根据@Primary注解来择取最优解
String primaryCandidate = determinePrimaryCandidate(candidates, requiredType);
if (primaryCandidate != null) {
return primaryCandidate;
}
//根据@Order,@PriorityOrder，及实现Order接口的序号来择取最优解
String priorityCandidate = determineHighestPriorityCandidate(candidates, requiredType);
if (priorityCandidate != null) {
return priorityCandidate;
}
// Fallback
for (Map.Entry<String, Object> entry : candidates.entrySet()) {
String candidateName = entry.getKey();
Object beanInstance = entry.getValue();
//如果通过以上两步都不能选择出最优解，则使用最基本的策略
//首先如果这个类型已经由Spring注册过依赖关系对，则直接使用注册的对象，
//候选者集合是LinkedHashMap,有序Map集合，容器注册的依赖对象位于LinkedHashMap的起始位置
//如果没有注册过此类型的依赖关系，则根据属性的名称来匹配，、
//如果属性名称和某个候选者的Bean名称或别名一致，那么直接将此Bean作为最优解
if ((beanInstance != null && this.resolvableDependencies.containsValue(beanInstance)) ||
matchesBeanName(candidateName, descriptor.getDependencyName())) {
return candidateName;
}
}
return null;
}
protected String determinePrimaryCandidate(Map<String, Object> candidates, Class<?> requiredType) {
String primaryBeanName = null;
for (Map.Entry<String, Object> entry : candidates.entrySet()) {
String candidateBeanName = entry.getKey();
Object beanInstance = entry.getValue();
//候选者可以是父容器内的标识了@Primary的Bean，也可以是当前容器的。SpringMVC容器将Spring容器作为父容器
if (isPrimary(candidateBeanName, beanInstance)) {
if (primaryBeanName != null) {
boolean candidateLocal = containsBeanDefinition(candidateBeanName);
boolean primaryLocal = containsBeanDefinition(primaryBeanName);
//此处确保同一个容器中同一个类型的多个Bean最多只有一个Bean标识了@Primary
if (candidateLocal && primaryLocal) {
throw new NoUniqueBeanDefinitionException(requiredType, candidates.size(),
"more than one 'primary' bean found among candidates: " + candidates.keySet());
}
//如果上一个@Primary的Bean是父容器的，则用当前容器的候选者覆盖之前的@Primary的Bean
else if (candidateLocal) {
primaryBeanName = candidateBeanName;
}
}
else {
primaryBeanName = candidateBeanName;
}
}
}
return primaryBeanName;
}
protected String determineHighestPriorityCandidate(Map<String, Object> candidates, Class<?> requiredType) {
String highestPriorityBeanName = null;
Integer highestPriority = null;
for (Map.Entry<String, Object> entry : candidates.entrySet()) {
String candidateBeanName = entry.getKey();
Object beanInstance = entry.getValue();
Integer candidatePriority = getPriority(beanInstance);
if (candidatePriority != null) {
if (highestPriorityBeanName != null) {
//不能同时存在两个最高优先级的序号
if (candidatePriority.equals(highestPriority)) {
throw new NoUniqueBeanDefinitionException(requiredType, candidates.size(),
"Multiple beans found with the same priority ('" + highestPriority +
"') among candidates: " + candidates.keySet());
} //使用优先级序号最小的Bean作为最优解
else if (candidatePriority < highestPriority) {
highestPriorityBeanName = candidateBeanName;
highestPriority = candidatePriority;
}
}
else {
highestPriorityBeanName = candidateBeanName;
highestPriority = candidatePriority;
}
}
}
return highestPriorityBeanName;
}
@Autowired标识方法的解析过程和标识属性类似，代码详解如下:
private class AutowiredMethodElement extends InjectionMetadata.InjectedElement {
private final boolean required;
private volatile boolean cached = false;
private volatile Object[] cachedMethodArguments;
public AutowiredMethodElement(Method method, boolean required, PropertyDescriptor pd) {
super(method, pd);
this.required = required;
}
@Override
protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {
if (checkPropertySkipping(pvs)) {
return;
}
Method method = (Method) this.member;
Object[] arguments;
if (this.cached) { //如果已经解析过，且缓存了方法的参数注入，则不需要再次解析
// Shortcut for avoiding synchronization...
arguments = resolveCachedArguments(beanName);
}
else {
Class<?>[] paramTypes = method.getParameterTypes();
arguments = new Object[paramTypes.length];
DependencyDescriptor[] descriptors = new DependencyDescriptor[paramTypes.length];
Set<String> autowiredBeans = new LinkedHashSet<String>(paramTypes.length);
TypeConverter typeConverter = beanFactory.getTypeConverter();
for (int i = 0; i < arguments.length; i++) {
MethodParameter methodParam = new MethodParameter(method, i);
DependencyDescriptor currDesc = new DependencyDescriptor(methodParam, this.required);
currDesc.setContainingClass(bean.getClass());
descriptors[i] = currDesc;
try { //根据参数类型从Spring容器中寻找合适的Bean
Object arg = beanFactory.resolveDependency(currDesc, beanName, autowiredBeans, typeConverter);
//如果参数解析失败，但是required = false,则忽略执行此方法
if (arg == null && !this.required) {
arguments = null;
break;
}
arguments[i] = arg;
}
catch (BeansException ex) {
throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(methodParam), ex);
}
}
synchronized (this) {
if (!this.cached) {
if (arguments != null) {
this.cachedMethodArguments = new Object[paramTypes.length];
for (int i = 0; i < arguments.length; i++) {
this.cachedMethodArguments[i] = descriptors[i];
}
registerDependentBeans(beanName, autowiredBeans);
if (autowiredBeans.size() == paramTypes.length) {
Iterator<String> it = autowiredBeans.iterator();
for (int i = 0; i < paramTypes.length; i++) {
String autowiredBeanName = it.next();
if (beanFactory.containsBean(autowiredBeanName)) {
if (beanFactory.isTypeMatch(autowiredBeanName, paramTypes[i])) {
this.cachedMethodArguments[i] = new ShortcutDependencyDescriptor(
descriptors[i], autowiredBeanName, paramTypes[i]);
}
}
}
}
}
else {
this.cachedMethodArguments = null;
}
this.cached = true;
}
}
}
if (arguments != null) {
try {
ReflectionUtils.makeAccessible(method);
//参数解析完毕，执行此方法。@Autowired标识的方法执行的比@PostConstruct早
method.invoke(bean, arguments);
}
catch (InvocationTargetException ex){
throw ex.getTargetException();
}
}
}
private Object[] resolveCachedArguments(String beanName) {
if (this.cachedMethodArguments == null) {
return null;
}
Object[] arguments = new Object[this.cachedMethodArguments.length];
for (int i = 0; i < arguments.length; i++) {
arguments[i] = resolvedCachedArgument(beanName, this.cachedMethodArguments[i]);
}
return arguments;
}
}
