在上一篇
从feign使用hystrix的全流程分析
中,通过一张图详解了springboot启动的时候工作流程。还没尽兴，今天把bean的实例化过程以及spring如何解决循环依赖，详细分析。略微补充了下和bean实例化相关的图。
本文基于springboot 2.3.12.RELEASE, spring 5.2.15.RELEASE
refresh刷新
-
invokeBeanFactoryPostProcessors 处理beanFactory
-
先完成了configclass的解析(解析了所有的元数据)
-
从resources和Registrar中加载beanDefinition
-
finishBeanFactoryInitialization
public abstract class AbstractApplicationContext extends DefaultResourceLoader implements ConfigurableApplicationContext {
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
......
finishBeanFactoryInitialization(beanFactory);
......
}
}
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
//此处的beanFactory为DefaultListableBeanFactory
beanFactory.preInstantiateSingletons();
}
}
在DefaultListableBeanFactory中，引导到了getBean
public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
@Override
public void preInstantiateSingletons() throws BeansException {
//beanDefinitionNames是在解析以后注册进来的beanName
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
//针对FactoryBean的特殊处理
} else {
//重点关注
getBean(beanName);
}
}
}
}
}
我们看下AbstractBeanFactory，同时看下类关系
public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {
@Override
public Object getBean(String name) throws BeansException {
//从getBean 又转到了doGetBean
return doGetBean(name, null, null, false);
}
protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)throws BeansException {
//①从缓存中获取单例类（重点在于读）
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
} else {
//如果是多例的循环引用，直接抛异常
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
//其他处理
}
if (!typeCheckOnly) {
//标记bean创建
markBeanAsCreated(beanName);
}
try {
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
if (mbd.isSingleton()) {
// ②获取单例bean（重点在于写）
sharedInstance = getSingleton(beanName, () -> {
try {
//创建bean,匿名方法，在getSingleton中被调用
return createBean(beanName, mbd, args);
}
});
//获取bean
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
}catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
return (T) bean;
}
}
在上面的代码里，有两块
-
一个是①从缓存中获取单例类（重点在于读），getSingleton(beanName);
-
单纯只看这里没什么意义，要结合创建流程来看。
-
一个②获取单例bean（重点在于写），最终还是走匿名方法里的createBean逻辑
public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
//① 缓存单例对象，bean name 和 实例的映射
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
//②缓存单例对象的工厂， bean name 和对应工厂的映射
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
//③ 缓存early 单例对象，实例化之后，未初始化的缓存（在spring中是正在创建的状态）
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);
//④ 已经注册单例实例的集合，只有bean name
private final Set<String> registeredSingletons = new LinkedHashSet<>(256);
//⑤正在创建的bean的缓存，set集合
private final Set<String> singletonsCurrentlyInCreation =Collections.newSetFromMap(new ConcurrentHashMap<>(16));
//关注读
@Override
@Nullable
public Object getSingleton(String beanName) {
return getSingleton(beanName, true);
}
//关注读，这里就是广为流传的三级缓存
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// Quick check for existing instance without full singleton lock
//这个时候从①里获取实例，如果有，直接返回，这就是所谓的第一级缓存
Object singletonObject = this.singletonObjects.get(beanName);
// 没有获取到实例，且处于创建中的时候
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
//这个时候从③ earlySingletonObjects 去获取bean的实例
singletonObject = this.earlySingletonObjects.get(beanName);
/还是没有获取到实例，allowEarlyReference传入的是true
if (singletonObject == null && allowEarlyReference) {
//这个时候给①singletonObjects加锁
synchronized (this.singletonObjects) {
// 双重校验，这个时候从①里再次获取
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
// 双重校验，这个时候从③里再次获取
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null) {
//从②里获取 Bean的Factory
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//不为空的时候会放入到③里，同时从②里移除
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
}
}
return singletonObject;
}
//关注创建
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
//给当前对象的singletonObjects 加锁
synchronized (this.singletonObjects) {
//再次校验有无
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
//在这里会放入到⑤里
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
//通过工厂获取object，这个工厂就是匿名方法里的return createBean(beanName, mbd, args);
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
//如果已经创建成功了，继续执行，如果抛异常了，就不往下走了
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
finally {
//在这里会从⑤里移除
afterSingletonCreation(beanName);
}
//创建成功后，会从singletonFactories和earlySingletonObjects移除
if (newSingleton) {
//这里会放入①和④里，从②③里移除
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
protected void beforeSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
this.singletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
最终代码指向了AbstractAutowireCapableBeanFactory中的doCreateBean，这个可以说是spring创建单例的核心，大概分为三步：
- ① createBeanInstance创建实例（内部是反射机制）
- 这里还有一个关键点主动缓存addSingletonFactory，会加入到缓存中
- ② populateBean自动属性填充
- ③ initializeBean初始化bean
三级缓存的操作机制
通过上述可以看到在创建实例以后，spring会把对象加入到缓存中，然后才去做依赖填充和初始化
具体代码如下：
public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory implements AutowireCapableBeanFactory {
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {
RootBeanDefinition mbdToUse = mbd;
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
try {
//在实例化之前BeanPostProcessors 有机会返回一个代理对象
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
try {
//在没有BeanPostProcessors干预的情况下创建bean的地方
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
return beanInstance;
}
}
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//① 创建bean实例
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// 允许 post-processors 修改合并的bean定义
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
mbd.postProcessed = true;
}
}
// 主动缓存单例，以便解析循环引用
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
//这里会写入到DefaultSingletonBeanRegistry的②④，从③里移除，在这里才形成了缓存
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// 初始化bean的实例
Object exposedObject = bean;
try {
//② 属性填充
populateBean(beanName, mbd, instanceWrapper);
//③ 初始化bean
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
}
}
return exposedObject;
}
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
Class<?> beanClass = resolveBeanClass(mbd, beanName);
//省略一堆特殊处理
//当构造函数里有需要注入的对象的时候处理
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
// 没有特殊处理，使用无参构造函数实例化
return instantiateBean(beanName, mbd);
}
protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {
try {
Object beanInstance;
if (System.getSecurityManager() != null) {
beanInstance = AccessController.doPrivileged(
(PrivilegedAction<Object>) () -> getInstantiationStrategy().instantiate(mbd, beanName, this),
getAccessControlContext());
}
else {
//使用CglibSubclassingInstantiationStrategy
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
}
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}
//属性填充
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
//给InstantiationAwareBeanPostProcessors机会在属性设置之前修改bean的状态，特别是字段的注入
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
return;
}
}
}
}
PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
//解析Autowire，这个时候，依赖的对象可能还没有实例化
int resolvedAutowireMode = mbd.getResolvedAutowireMode();
if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// 通过名称注入
if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// 通过类型注入
if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
PropertyDescriptor[] filteredPds = null;
if (hasInstAwareBpps) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
//属性解析
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);
}
//最终完成的是属性注入
if (pvs != null) {
applyPropertyValues(beanName, mbd, bw, pvs);
}
}
protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareMethods(beanName, bean);
return null;
}, getAccessControlContext());
}
else {
//执行实现Aware接口的方法
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
//调用实例化之前的before方法
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
//调用init-method方法
invokeInitMethods(beanName, wrappedBean, mbd);
}
catch (Throwable ex) {
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
}
if (mbd == null || !mbd.isSynthetic()) {
//调用after方法
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
protected void autowireByName(
String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {
String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
for (String propertyName : propertyNames) {
if (containsBean(propertyName)) {
//根据属性名去getBean，这个逻辑很简单，直接就开始了注入的getBean流程
Object bean = getBean(propertyName);
pvs.add(propertyName, bean);
registerDependentBean(propertyName, beanName);
}
}
}
protected void autowireByType(
String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {
TypeConverter converter = getCustomTypeConverter();
if (converter == null) {
converter = bw;
}
Set<String> autowiredBeanNames = new LinkedHashSet<>(4);
String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
for (String propertyName : propertyNames) {
try {
//解析出来属性类型
PropertyDescriptor pd = bw.getPropertyDescriptor(propertyName);
//只能注入非Object类型的对象
if (Object.class != pd.getPropertyType()) {
MethodParameter methodParam = BeanUtils.getWriteMethodParameter(pd);
// Do not allow eager init for type matching in case of a prioritized post-processor.
boolean eager = !(bw.getWrappedInstance() instanceof PriorityOrdered);
DependencyDescriptor desc = new AutowireByTypeDependencyDescriptor(methodParam, eager);
//这里解析依赖的bean，这块比较复杂，resolveDependency为DefaultListableBeanFactory
Object autowiredArgument = resolveDependency(desc, beanName, autowiredBeanNames, converter);
if (autowiredArgument != null) {
pvs.add(propertyName, autowiredArgument);
}
for (String autowiredBeanName : autowiredBeanNames) {
registerDependentBean(autowiredBeanName, beanName);
if (logger.isTraceEnabled()) {
logger.trace("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);
}
}
}
}
spring通过自己的机制把类加载的过程给接手了，通过配置自动加载依赖属性。
ps: 附送一张类加载的流程图：
以下代码为通过反射实例化
public class CglibSubclassingInstantiationStrategy extends SimpleInstantiationStrategy {
}
public class SimpleInstantiationStrategy implements InstantiationStrategy {
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
if (!bd.hasMethodOverrides()) {
Constructor<?> constructorToUse;
//省略获取类构造器
//通过BeanUtils指定构造器实例化类
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
}
public abstract class BeanUtils {
public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
Assert.notNull(ctor, "Constructor must not be null");
try {
ReflectionUtils.makeAccessible(ctor);
if (KotlinDetector.isKotlinReflectPresent() && KotlinDetector.isKotlinType(ctor.getDeclaringClass())) {
return KotlinDelegate.instantiateClass(ctor, args);
}
else {
Class<?>[] parameterTypes = ctor.getParameterTypes();
Assert.isTrue(args.length <= parameterTypes.length, "Can't specify more arguments than constructor parameters");
Object[] argsWithDefaultValues = new Object[args.length];
for (int i = 0 ; i < args.length; i++) {
if (args[i] == null) {
Class<?> parameterType = parameterTypes[i];
argsWithDefaultValues[i] = (parameterType.isPrimitive() ? DEFAULT_TYPE_VALUES.get(parameterType) : null);
}
else {
argsWithDefaultValues[i] = args[i];
}
}
//关注这里，ctor为构造器，通过反射进行了实例化
return ctor.newInstance(argsWithDefaultValues);
}
}
}
属性填充代码跟踪
public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {
//属性解析
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
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;
}
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()) {
for (InjectedElement element : elementsToIterate) {
element.inject(target, beanName, pvs);
}
}
}
private class AutowiredFieldElement extends InjectionMetadata.InjectedElement {
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Field field = (Field) this.member;
Object value;
if (this.cached) {
try {
value = resolvedCachedArgument(beanName, this.cachedFieldValue);
}
catch (NoSuchBeanDefinitionException ex) {
// Unexpected removal of target bean for cached argument -> re-resolve
value = resolveFieldValue(field, bean, beanName);
}
}
else {
value = resolveFieldValue(field, bean, beanName);
}
if (value != null) {
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}
}
@Nullable
private Object resolveFieldValue(Field field, Object bean, @Nullable String beanName) {
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();
Object value;
try {
//又回到了BeanFactory
value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
}
catch (BeansException ex) {
throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
}
synchronized (this) {
if (!this.cached) {
Object cachedFieldValue = null;
if (value != null || this.required) {
cachedFieldValue = desc;
registerDependentBeans(beanName, autowiredBeanNames);
if (autowiredBeanNames.size() == 1) {
String autowiredBeanName = autowiredBeanNames.iterator().next();
if (beanFactory.containsBean(autowiredBeanName) &&
beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
cachedFieldValue = new ShortcutDependencyDescriptor(
desc, autowiredBeanName, field.getType());
}
}
}
this.cachedFieldValue = cachedFieldValue;
this.cached = true;
}
}
return value;
}
}
}
}
public class InjectionMetadata {
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()) {
for (InjectedElement element : elementsToIterate) {
element.inject(target, beanName, pvs);
}
}
}
}
autowireByType 代码跟踪
public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
//解析
result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
return result;
}
@Nullable
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
try {
if (instanceCandidate instanceof Class) {
//type类型解析
instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
}
Object result = instanceCandidate;
return result;
}
finally {
ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
}
}
}
最后在DependencyDescriptor中又回归到了getBean
public class DependencyDescriptor extends InjectionPoint implements Serializable {
public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory)throws BeansException {
//最后又回归到了getBean
return beanFactory.getBean(beanName);
}
}
通过上面得出一个结论，通过Autowire注入的时候，如果不指定beanName，成本还是比较高的。
回归正传，总结一下：
- spring在实例化完成以后，就将实例加入到缓存里；
- 然后采取依赖注入，这个时候未实例化的类在缓存中没有，还会直接走doCreateBean;
- 而已经创建的时候直接在getBean的时候就会返回，并不会执行doCreateBean
- 通过这样的机制解决了循环依赖。
为什么二级缓存不行，非得三级缓存？
- 如果单纯为了解决循环依赖，其实二级缓存①和③就够了；
- 三级缓存是为了保护spring生命周期的正确性，将ObjectFactory对象做成了二级缓存，是为了延迟代理的创建（正常代理对象的生成是在被代理对象初始化后期调用产生的，对应3个步骤）；
- 从addSingleton的调用也可以看到，一旦对象生成就会把②里的remove，添加到①里；
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