spring依賴注入
1. 一個簡單的例子來看看怎麼實例化 ApplicationContext
public interface MessageService {
String getMessage();
}
public class MessageServiceImpl implements MessageService {
public String getMessage() {
return "hello world";
}
}
新建個 application.xml
<?xml version="1.0" encoding="UTF-8" ?>
<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://www.springframework.org/schema/beans"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd" default-autowire="byName">
<bean id="messageService" class="com.javadoop.example.MessageServiceImpl"/>
</beans>
測試類
ApplicationContext applicationContext=new ClassPathXmlApplicationContext("application.xml");
Object bean = applicationContext.getBean("messageService");
2.依賴注入發生的時間
- 用戶第一次通過 getBean 方法向 IOC 容索要 Bean 時， IOC 容器觸發依賴注入
- 當用戶在 Bean 定義資源中爲元素配置了 lazy-init 屬性，即讓容器在解析註冊 Bean 定義
時進行預實例化， 觸發依賴注入
3. getBean()
getBean()方法， 就是用戶向 IOC 容器索取管理的Bean 的方法
@Override
public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
return doGetBean(name, requiredType, null, false);
}
4.doGetBean
真正實現向IOC容器獲取Bean的功能，也是觸發依賴注入功能的地方
//真正實現向IOC容器獲取Bean的功能，也是觸發依賴注入功能的地方
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
//如果指定的是別名，將別名轉換爲規範的Bean名稱
final String beanName = transformedBeanName(name);
Object bean;
//先從緩存中取是否已經有被創建過的單態類型的Bean
//對於單例模式的Bean整個IOC容器中只創建一次，不需要重複創建
Object sharedInstance = getSingleton(beanName);
//IOC容器創建單例模式Bean實例對象
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
//如果指定名稱的Bean在容器中已有單例模式的Bean被創建
//直接返回已經創建的Bean
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
//獲取給定Bean的實例對象，主要是完成FactoryBean的相關處理
//如果是普通 Bean 的話，直接返回 sharedInstance，如果是 FactoryBean 的話，返回它創建的那個實例對象
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
//緩存沒有正在創建的單例模式Bean
//緩存中已經有已經創建的原型模式Bean
//但是由於循環引用的問題導致實例化對象失敗
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
//對IOC容器中是否存在指定名稱的BeanDefinition進行檢查，首先檢查是否
//能在當前的BeanFactory中獲取的所需要的Bean，如果不能則委託當前容器
//的父級容器去查找，如果還是找不到則沿着容器的繼承體系向父級容器查找
BeanFactory parentBeanFactory = getParentBeanFactory();
//當前容器的父級容器存在，且當前容器中不存在指定名稱的Bean
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
//解析指定Bean名稱的原始名稱
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
//委派父級容器根據指定名稱和顯式的參數查找
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
//委派父級容器根據指定名稱和類型查找
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
//創建的Bean是否需要進行類型驗證，一般不需要
if (!typeCheckOnly) {
//向容器標記指定的Bean已經被創建
markBeanAsCreated(beanName);
}
try {
//根據指定Bean名稱獲取其父級的Bean定義
//主要解決Bean繼承時子類合併父類公共屬性問題
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
//獲取當前Bean所有依賴Bean的名稱
String[] dependsOn = mbd.getDependsOn();
//如果當前Bean有依賴Bean,配置的<depend on=''>
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
//遞歸調用getBean方法，獲取當前Bean的依賴Bean
registerDependentBean(dep, beanName);
//把被依賴Bean註冊給當前依賴的Bean
getBean(dep);
}
}
//如果是 singleton scope 的，創建 singleton 的實例
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
//執行創建 Bean
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
//顯式地從容器單例模式Bean緩存中清除實例對象
destroySingleton(beanName);
throw ex;
}
});
//獲取給定Bean的實例對象
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
//IOC容器創建原型模式Bean實例對象
else if (mbd.isPrototype()) {
//原型模式(Prototype)是每次都會創建一個新的對象
Object prototypeInstance = null;
try {
//回調beforePrototypeCreation方法，默認的功能是註冊當前創建的原型對象
beforePrototypeCreation(beanName);
//創建指定Bean對象實例
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
//回調afterPrototypeCreation方法，默認的功能告訴IOC容器指定Bean的原型對象不再創建
afterPrototypeCreation(beanName);
}
//獲取給定Bean的實例對象
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
//要創建的Bean既不是單例模式，也不是原型模式，則根據Bean定義資源中
//配置的生命週期範圍，選擇實例化Bean的合適方法，這種在Web應用程序中
//比較常用，如：request、session、application等生命週期
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
//Bean定義資源中沒有配置生命週期範圍，則Bean定義不合法
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
//這裏又使用了一個匿名內部類，獲取一個指定生命週期範圍的實例
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
//獲取給定Bean的實例對象
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; consider " +
"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
//對創建的Bean實例對象進行類型檢查,，不對的話就拋異常，對的話就返回了
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
上面對向 IOC 容器獲取 Bean 方法的分析， 如果 Bean 定義的單例模式， 則容器在創建之前先從緩存中查找， 整個容器中只存在一個實例對象，如果 Bean 定義的原型模式， 則容器每次都會創建一個新的實例對象
5.createBean
//創建Bean實例對象
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
//判斷需要創建的Bean是否可以實例化，即是否可以通過當前的類加載器加載
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
//校驗和準備Bean中的方法覆蓋
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
//如果Bean配置了初始化前和初始化後的處理器，則試圖返回一個需要創建Bean的代理對象
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
try {
//創建Bean的入口
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException ex) {
throw ex;
}
catch (ImplicitlyAppearedSingletonException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
//真正創建Bean的方法
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
//封裝被創建的Bean對象
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
//獲取實例化對象的類型
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
//調用PostProcessor後置處理器
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
//向容器中緩存單例模式的Bean對象，以防循環引用
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
//這裏是一個匿名內部類，爲了防止循環引用，儘早持有對象的引用
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
//Bean對象的初始化，依賴注入在此觸發
//這個exposedObject在初始化完成之後返回作爲依賴注入完成後的Bean
Object exposedObject = bean;
try {
//將Bean實例對象封裝，並且Bean定義中配置的屬性值賦值給實例對象
populateBean(beanName, mbd, instanceWrapper);
//初始化Bean對象
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
if (earlySingletonExposure) {
//獲取指定名稱的已註冊的單例模式Bean對象
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
//根據名稱獲取的已註冊的Bean和正在實例化的Bean是同一個
if (exposedObject == bean) {
//當前實例化的Bean初始化完成
exposedObject = earlySingletonReference;
}
//當前Bean依賴其他Bean，並且當發生循環引用時不允許新創建實例對象
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
//獲取當前Bean所依賴的其他Bean
for (String dependentBean : dependentBeans) {
//對依賴Bean進行類型檢查
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
//註冊完成依賴注入的Bean
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
上面可以看出是調用了 createBeanInstance 生成的bean的實例，再 populateBean 對bean的屬性進行依賴注入。
6.createBeanInstance
//創建Bean的實例對象
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
//檢查確認Bean是可實例化的
Class<?> beanClass = resolveBeanClass(mbd, beanName);
//使用工廠方法對Bean進行實例化
if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
}
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
if (mbd.getFactoryMethodName() != null) {
//調用工廠方法實例化
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
//使用容器的自動裝配方法進行實例化
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
//配置了自動裝配屬性，使用容器的自動裝配實例化
//容器的自動裝配是根據參數類型匹配Bean的構造方法
return autowireConstructor(beanName, mbd, null, null);
}
else {
//使用默認的無參構造方法實例化
return instantiateBean(beanName, mbd);
}
}
//使用Bean的構造方法進行實例化
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
//使用容器的自動裝配特性，調用匹配的構造方法實例化
return autowireConstructor(beanName, mbd, ctors, args);
}
// No special handling: simply use no-arg constructor.
//使用默認的無參構造方法實例化
return instantiateBean(beanName, mbd);
}
根據指定的初始化策略， 使用靜態工廠、 工廠方法或者容器的自動裝配特性生成 java 實例對象 ， 對於我們最常使用的默認無參構造方法就需要使用相應的初始化策略(JDK 的反射機制或者 CGLIB)來進行初始化了
7.instantiate使用初始化策略實例化Bean對象
//使用初始化策略實例化Bean對象
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
//如果Bean定義中沒有方法覆蓋，則就不需要CGLIB父類類的方法
if (!bd.hasMethodOverrides()) {
Constructor<?> constructorToUse;
synchronized (bd.constructorArgumentLock) {
//獲取對象的構造方法或工廠方法
constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
//如果沒有構造方法且沒有工廠方法
if (constructorToUse == null) {
//使用JDK的反射機制，判斷要實例化的Bean是否是接口
final Class<?> clazz = bd.getBeanClass();
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
if (System.getSecurityManager() != null) {
//這裏是一個匿名內置類，使用反射機制獲取Bean的構造方法
constructorToUse = AccessController.doPrivileged(
(PrivilegedExceptionAction<Constructor<?>>) () -> clazz.getDeclaredConstructor());
}
else {
constructorToUse = clazz.getDeclaredConstructor();
}
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
//使用BeanUtils實例化，通過反射機制調用”構造方法.newInstance(arg)”來進行實例化
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
//使用CGLIB來實例化對象
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
上面的代碼分析， 我們看到了如果 Bean 有方法被覆蓋了， 則使用 JDK 的反射機制進行實例化， 否則， 使用 CGLIB 進行實例化。
8. populateBean 填充對象
//將Bean屬性設置到生成的實例對象上
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
if (bw == null) {
if (mbd.hasPropertyValues()) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
}
else {
// Skip property population phase for null instance.
return;
}
}
// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
// state of the bean before properties are set. This can be used, for example,
// to support styles of field injection.
boolean continueWithPropertyPopulation = true;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
continueWithPropertyPopulation = false;
break;
}
}
}
}
if (!continueWithPropertyPopulation) {
return;
}
//獲取容器在解析Bean定義資源時爲BeanDefiniton中設置的屬性值
PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
//對依賴注入處理，首先處理autowiring自動裝配的依賴注入
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// Add property values based on autowire by name if applicable.
//根據Bean名稱進行autowiring自動裝配處理
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// Add property values based on autowire by type if applicable.
//根據Bean類型進行autowiring自動裝配處理
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
//對非autowiring的屬性進行依賴注入處理
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);
if (hasInstAwareBpps || needsDepCheck) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
if (hasInstAwareBpps) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}
}
if (needsDepCheck) {
checkDependencies(beanName, mbd, filteredPds, pvs);
}
}
if (pvs != null) {
//對屬性進行注入
applyPropertyValues(beanName, mbd, bw, pvs);
}
}
//解析並注入依賴屬性的過程
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
if (pvs.isEmpty()) {
return;
}
//封裝屬性值
MutablePropertyValues mpvs = null;
List<PropertyValue> original;
if (System.getSecurityManager() != null) {
if (bw instanceof BeanWrapperImpl) {
//設置安全上下文，JDK安全機制
((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext());
}
}
if (pvs instanceof MutablePropertyValues) {
mpvs = (MutablePropertyValues) pvs;
//屬性值已經轉換
if (mpvs.isConverted()) {
// Shortcut: use the pre-converted values as-is.
try {
//爲實例化對象設置屬性值
bw.setPropertyValues(mpvs);
return;
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}
//獲取屬性值對象的原始類型值
original = mpvs.getPropertyValueList();
}
else {
original = Arrays.asList(pvs.getPropertyValues());
}
//獲取用戶自定義的類型轉換
TypeConverter converter = getCustomTypeConverter();
if (converter == null) {
converter = bw;
}
//創建一個Bean定義屬性值解析器，將Bean定義中的屬性值解析爲Bean實例對象的實際值
BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);
// Create a deep copy, resolving any references for values.
//爲屬性的解析值創建一個拷貝，將拷貝的數據注入到實例對象中
List<PropertyValue> deepCopy = new ArrayList<>(original.size());
boolean resolveNecessary = false;
for (PropertyValue pv : original) {
//屬性值不需要轉換
if (pv.isConverted()) {
deepCopy.add(pv);
}
//屬性值需要轉換
else {
String propertyName = pv.getName();
//原始的屬性值，即轉換之前的屬性值
Object originalValue = pv.getValue();
//轉換屬性值，例如將引用轉換爲IOC容器中實例化對象引用
Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
//轉換之後的屬性值
Object convertedValue = resolvedValue;
//屬性值是否可以轉換
boolean convertible = bw.isWritableProperty(propertyName) &&
!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
if (convertible) {
//使用用戶自定義的類型轉換器轉換屬性值
convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
}
// Possibly store converted value in merged bean definition,
// in order to avoid re-conversion for every created bean instance.
//存儲轉換後的屬性值，避免每次屬性注入時的轉換工作
if (resolvedValue == originalValue) {
if (convertible) {
//設置屬性轉換之後的值
pv.setConvertedValue(convertedValue);
}
deepCopy.add(pv);
}
//屬性是可轉換的，且屬性原始值是字符串類型，且屬性的原始類型值不是
//動態生成的字符串，且屬性的原始值不是集合或者數組類型
else if (convertible && originalValue instanceof TypedStringValue &&
!((TypedStringValue) originalValue).isDynamic() &&
!(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
pv.setConvertedValue(convertedValue);
//重新封裝屬性的值
deepCopy.add(pv);
}
else {
resolveNecessary = true;
deepCopy.add(new PropertyValue(pv, convertedValue));
}
}
}
if (mpvs != null && !resolveNecessary) {
//標記屬性值已經轉換過
mpvs.setConverted();
}
// Set our (possibly massaged) deep copy.
//進行屬性依賴注入
try {
bw.setPropertyValues(new MutablePropertyValues(deepCopy));
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}
從上面可以看出，(1).屬性值類型不需要轉換時， 不需要解析屬性值， 直接準備進行依賴注入。(2).屬性值需要進行類型轉換時， 如對其他對象的引用等， 首先需要解析屬性值， 然後對解析後的屬性值進行依賴注入。
9.resolveValueIfNecessary 對屬性值的解析
對屬性值的解析是在 BeanDefinitionValueResolver 類中的 resolveValueIfNecessary 方法中進行的，
//解析屬性值，對注入類型進行轉換
@Nullable
public Object resolveValueIfNecessary(Object argName, @Nullable Object value) {
// We must check each value to see whether it requires a runtime reference
// to another bean to be resolved.
//對引用類型的屬性進行解析
if (value instanceof RuntimeBeanReference) {
RuntimeBeanReference ref = (RuntimeBeanReference) value;
//調用引用類型屬性的解析方法
return resolveReference(argName, ref);
}
//對屬性值是引用容器中另一個Bean名稱的解析
else if (value instanceof RuntimeBeanNameReference) {
String refName = ((RuntimeBeanNameReference) value).getBeanName();
refName = String.valueOf(doEvaluate(refName));
//從容器中獲取指定名稱的Bean
if (!this.beanFactory.containsBean(refName)) {
throw new BeanDefinitionStoreException(
"Invalid bean name '" + refName + "' in bean reference for " + argName);
}
return refName;
}
//對Bean類型屬性的解析，主要是Bean中的內部類
else if (value instanceof BeanDefinitionHolder) {
// Resolve BeanDefinitionHolder: contains BeanDefinition with name and aliases.
BeanDefinitionHolder bdHolder = (BeanDefinitionHolder) value;
return resolveInnerBean(argName, bdHolder.getBeanName(), bdHolder.getBeanDefinition());
}
else if (value instanceof BeanDefinition) {
// Resolve plain BeanDefinition, without contained name: use dummy name.
BeanDefinition bd = (BeanDefinition) value;
String innerBeanName = "(inner bean)" + BeanFactoryUtils.GENERATED_BEAN_NAME_SEPARATOR +
ObjectUtils.getIdentityHexString(bd);
return resolveInnerBean(argName, innerBeanName, bd);
}
//對集合數組類型的屬性解析
else if (value instanceof ManagedArray) {
// May need to resolve contained runtime references.
ManagedArray array = (ManagedArray) value;
//獲取數組的類型
Class<?> elementType = array.resolvedElementType;
if (elementType == null) {
//獲取數組元素的類型
String elementTypeName = array.getElementTypeName();
if (StringUtils.hasText(elementTypeName)) {
try {
//使用反射機制創建指定類型的對象
elementType = ClassUtils.forName(elementTypeName, this.beanFactory.getBeanClassLoader());
array.resolvedElementType = elementType;
}
catch (Throwable ex) {
// Improve the message by showing the context.
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Error resolving array type for " + argName, ex);
}
}
//沒有獲取到數組的類型，也沒有獲取到數組元素的類型
//則直接設置數組的類型爲Object
else {
elementType = Object.class;
}
}
//創建指定類型的數組
return resolveManagedArray(argName, (List<?>) value, elementType);
}
//解析list類型的屬性值
else if (value instanceof ManagedList) {
// May need to resolve contained runtime references.
return resolveManagedList(argName, (List<?>) value);
}
//解析set類型的屬性值
else if (value instanceof ManagedSet) {
// May need to resolve contained runtime references.
return resolveManagedSet(argName, (Set<?>) value);
}
//解析map類型的屬性值
else if (value instanceof ManagedMap) {
// May need to resolve contained runtime references.
return resolveManagedMap(argName, (Map<?, ?>) value);
}
//解析props類型的屬性值，props其實就是key和value均爲字符串的map
else if (value instanceof ManagedProperties) {
Properties original = (Properties) value;
//創建一個拷貝，用於作爲解析後的返回值
Properties copy = new Properties();
original.forEach((propKey, propValue) -> {
if (propKey instanceof TypedStringValue) {
propKey = evaluate((TypedStringValue) propKey);
}
if (propValue instanceof TypedStringValue) {
propValue = evaluate((TypedStringValue) propValue);
}
if (propKey == null || propValue == null) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Error converting Properties key/value pair for " + argName + ": resolved to null");
}
copy.put(propKey, propValue);
});
return copy;
}
//解析字符串類型的屬性值
else if (value instanceof TypedStringValue) {
// Convert value to target type here.
TypedStringValue typedStringValue = (TypedStringValue) value;
Object valueObject = evaluate(typedStringValue);
try {
//獲取屬性的目標類型
Class<?> resolvedTargetType = resolveTargetType(typedStringValue);
if (resolvedTargetType != null) {
//對目標類型的屬性進行解析，遞歸調用
return this.typeConverter.convertIfNecessary(valueObject, resolvedTargetType);
}
//沒有獲取到屬性的目標對象，則按Object類型返回
else {
return valueObject;
}
}
catch (Throwable ex) {
// Improve the message by showing the context.
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Error converting typed String value for " + argName, ex);
}
}
else if (value instanceof NullBean) {
return null;
}
else {
return evaluate(value);
}
}
上面的代碼分析， 我們明白了 Spring 是如何將引用類型， 內部類以及集合類型等屬性進行解析的，屬性值解析完成後就可以進行依賴注入了
10 BeanWrapperImpl 對 Bean 屬性的依賴注入：
//實現屬性依賴注入功能
protected void setPropertyValue(PropertyTokenHolder tokens, PropertyValue pv) throws BeansException {
if (tokens.keys != null) {
processKeyedProperty(tokens, pv);
}
else {
processLocalProperty(tokens, pv);
}
}
//實現屬性依賴注入功能
@SuppressWarnings("unchecked")
private void processKeyedProperty(PropertyTokenHolder tokens, PropertyValue pv) {
//調用屬性的getter(readerMethod)方法，獲取屬性的值
Object propValue = getPropertyHoldingValue(tokens);
PropertyHandler ph = getLocalPropertyHandler(tokens.actualName);
if (ph == null) {
throw new InvalidPropertyException(
getRootClass(), this.nestedPath + tokens.actualName, "No property handler found");
}
Assert.state(tokens.keys != null, "No token keys");
String lastKey = tokens.keys[tokens.keys.length - 1];
//注入array類型的屬性值
if (propValue.getClass().isArray()) {
Class<?> requiredType = propValue.getClass().getComponentType();
int arrayIndex = Integer.parseInt(lastKey);
Object oldValue = null;
try {
if (isExtractOldValueForEditor() && arrayIndex < Array.getLength(propValue)) {
oldValue = Array.get(propValue, arrayIndex);
}
Object convertedValue = convertIfNecessary(tokens.canonicalName, oldValue, pv.getValue(),
requiredType, ph.nested(tokens.keys.length));
//獲取集合類型屬性的長度
int length = Array.getLength(propValue);
if (arrayIndex >= length && arrayIndex < this.autoGrowCollectionLimit) {
Class<?> componentType = propValue.getClass().getComponentType();
Object newArray = Array.newInstance(componentType, arrayIndex + 1);
System.arraycopy(propValue, 0, newArray, 0, length);
setPropertyValue(tokens.actualName, newArray);
//調用屬性的getter(readerMethod)方法，獲取屬性的值
propValue = getPropertyValue(tokens.actualName);
}
//將屬性的值賦值給數組中的元素
Array.set(propValue, arrayIndex, convertedValue);
}
catch (IndexOutOfBoundsException ex) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + tokens.canonicalName,
"Invalid array index in property path '" + tokens.canonicalName + "'", ex);
}
}
//注入list類型的屬性值
else if (propValue instanceof List) {
//獲取list集合的類型
Class<?> requiredType = ph.getCollectionType(tokens.keys.length);
List<Object> list = (List<Object>) propValue;
//獲取list集合的size
int index = Integer.parseInt(lastKey);
Object oldValue = null;
if (isExtractOldValueForEditor() && index < list.size()) {
oldValue = list.get(index);
}
//獲取list解析後的屬性值
Object convertedValue = convertIfNecessary(tokens.canonicalName, oldValue, pv.getValue(),
requiredType, ph.nested(tokens.keys.length));
int size = list.size();
//如果list的長度大於屬性值的長度，則多餘的元素賦值爲null
if (index >= size && index < this.autoGrowCollectionLimit) {
for (int i = size; i < index; i++) {
try {
list.add(null);
}
catch (NullPointerException ex) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + tokens.canonicalName,
"Cannot set element with index " + index + " in List of size " +
size + ", accessed using property path '" + tokens.canonicalName +
"': List does not support filling up gaps with null elements");
}
}
list.add(convertedValue);
}
else {
try {
//將值添加到list中
list.set(index, convertedValue);
}
catch (IndexOutOfBoundsException ex) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + tokens.canonicalName,
"Invalid list index in property path '" + tokens.canonicalName + "'", ex);
}
}
}
//注入map類型的屬性值
else if (propValue instanceof Map) {
//獲取map集合key的類型
Class<?> mapKeyType = ph.getMapKeyType(tokens.keys.length);
//獲取map集合value的類型
Class<?> mapValueType = ph.getMapValueType(tokens.keys.length);
Map<Object, Object> map = (Map<Object, Object>) propValue;
// IMPORTANT: Do not pass full property name in here - property editors
// must not kick in for map keys but rather only for map values.
TypeDescriptor typeDescriptor = TypeDescriptor.valueOf(mapKeyType);
//解析map類型屬性key值
Object convertedMapKey = convertIfNecessary(null, null, lastKey, mapKeyType, typeDescriptor);
Object oldValue = null;
if (isExtractOldValueForEditor()) {
oldValue = map.get(convertedMapKey);
}
// Pass full property name and old value in here, since we want full
// conversion ability for map values.
//解析map類型屬性value值
Object convertedMapValue = convertIfNecessary(tokens.canonicalName, oldValue, pv.getValue(),
mapValueType, ph.nested(tokens.keys.length));
//將解析後的key和value值賦值給map集合屬性
map.put(convertedMapKey, convertedMapValue);
}
else {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + tokens.canonicalName,
"Property referenced in indexed property path '" + tokens.canonicalName +
"' is neither an array nor a List nor a Map; returned value was [" + propValue + "]");
}
}
到這類就完成了依賴注入
- 對於集合類型的屬性， 將其屬性值解析爲目標類型的集合後直接賦值給屬性。
- 對於非集合類型的屬性， 大量使用了 JDK 的反射和內省機制， 通過屬性的 getter 方法(readerMethod)獲取指定屬性注入以前的值， 同時調用屬性的 setter 方法(writer Method)爲屬性設置注入後的值。 看到這裏相信很多人都明白了 Spring 的 setter 注入原理。 。