五、IOC容器依赖注入
1.依赖注入的时间
当Spring IoC容器完成了Bean定义资源的定位、载入和解析注册以后，IoC容器中已经管理类Bean定义的相关数据，但是此时IoC容器还没有对所管理的Bean进行依赖注入，依赖注入在以下两种情况发生：
(1).用户第一次通过getBean方法向IoC容索要Bean时，IoC容器触发依赖注入。
(2).当用户在Bean定义资源中为元素配置了lazy-init属性，即让容器在解析注册Bean定义时进行预实例化，触发依赖注入。
2.依赖注入的入口AbstractBeanFactory的getBean()
BeanFactory接口定义了Spring IoC容器的基本功能规范，是Spring IoC容器所应遵守的最底层和最基本的编程规范。BeanFactory接口中定义了几个getBean方法，就是用户向IoC容器索取管理的Bean的方法，我们通过分析其子类的具体实现，理解Spring IoC容器在用户索取Bean时如何完成依赖注入。
在BeanFactory中我们看到getBean（String…）函数，它的具体实现在AbstractBeanFactory中
//获取IoC容器中指定名称的Bean
public Object getBean(String name) throws BeansException {
//doGetBean才是真正向IoC容器获取被管理Bean的过程
return doGetBean(name, null, null, false);
}
//获取IoC容器中指定名称和类型的Bean
public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
//doGetBean才是真正向IoC容器获取被管理Bean的过程
return doGetBean(name, requiredType, null, false);
}
//获取IoC容器中指定名称和参数的Bean
public Object getBean(String name, Object... args) throws BeansException {
//doGetBean才是真正向IoC容器获取被管理Bean的过程
return doGetBean(name, null, args, false);
}
//获取IoC容器中指定名称、类型和参数的Bean
public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {
//doGetBean才是真正向IoC容器获取被管理Bean的过程
return doGetBean(name, requiredType, args, false);
}
//真正实现向IoC容器获取Bean的功能，也是触发依赖注入功能的地方
@SuppressWarnings("unchecked")
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
//根据指定的名称获取被管理Bean的名称，剥离指定名称中对容器的相关依赖
//如果指定的是别名，将别名转换为规范的Bean名称
final String beanName = transformedBeanName(name);
Object bean;
//先从缓存中取是否已经有被创建过的单态类型的Bean，对于单态模式的Bean整
//个IoC容器中只创建一次，不需要重复创建
Object sharedInstance = getSingleton(beanName);
//IoC容器创建单态模式Bean实例对象
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
//如果指定名称的Bean在容器中已有单态模式的Bean被创建，直接返回
//已经创建的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 + "'");
}
}
//获取给定Bean的实例对象，主要是完成FactoryBean的相关处理
//注意：BeanFactory是管理容器中Bean的工厂，而FactoryBean是
//创建创建对象的工厂Bean，两者之间有区别
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {//缓存没有正在创建的单态模式Bean
//缓存中已经有已经创建的原型模式Bean，但是由于循环引用的问题导致实
//例化对象失败
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
//对IoC容器中是否存在指定名称的BeanDefinition进行检查，首先检查是否
//能在当前的BeanFactory中获取的所需要的Bean，如果不能则委托当前容器
//的父级容器去查找，如果还是找不到则沿着容器的继承体系向父级容器查找
BeanFactory parentBeanFactory = getParentBeanFactory();
//当前容器的父级容器存在，且当前容器中不存在指定名称的Bean
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
//解析指定Bean名称的原始名称
String nameToLookup = originalBeanName(name);
if (args != null) {
//委派父级容器根据指定名称和显式的参数查找
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
//委派父级容器根据指定名称和类型查找
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
//创建的Bean是否需要进行类型验证，一般不需要
if (!typeCheckOnly) {
//向容器标记指定的Bean已经被创建
markBeanAsCreated(beanName);
}
//根据指定Bean名称获取其父级的Bean定义，主要解决Bean继承时子类
//合并父类公共属性问题
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
//获取当前Bean所有依赖Bean的名称
String[] dependsOn = mbd.getDependsOn();
//如果当前Bean有依赖Bean
if (dependsOn != null) {
for (String dependsOnBean : dependsOn) {
//递归调用getBean方法，获取当前Bean的依赖Bean
getBean(dependsOnBean);
//把被依赖Bean注册给当前依赖的Bean
registerDependentBean(dependsOnBean, beanName);
}
}
//创建单态模式Bean的实例对象
if (mbd.isSingleton()) {
//这里使用了一个匿名内部类，创建Bean实例对象，并且注册给所依赖的对象
sharedInstance = getSingleton(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
try {
//创建一个指定Bean实例对象，如果有父级继承，则合并子//类和父类的定义
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
//显式地从容器单态模式Bean缓存中清除实例对象
destroySingleton(beanName);
throw ex;
}
}
});
//获取给定Bean的实例对象
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
//IoC容器创建原型模式Bean实例对象
else if (mbd.isPrototype()) {
//原型模式(Prototype)是每次都会创建一个新的对象
Object prototypeInstance = null;
try {
//回调beforePrototypeCreation方法，默认的功能是注册当前创//建的原型对象
beforePrototypeCreation(beanName);
//创建指定Bean对象实例
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
//回调afterPrototypeCreation方法，默认的功能告诉IoC容器指//定Bean的原型对象不再创建了
afterPrototypeCreation(beanName);
}
//获取给定Bean的实例对象
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
//要创建的Bean既不是单态模式，也不是原型模式，则根据Bean定义资源中
//配置的生命周期范围，选择实例化Bean的合适方法，这种在Web应用程序中
//比较常用，如：request、session、application等生命周期
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
//Bean定义资源中没有配置生命周期范围，则Bean定义不合法
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
}
try {
//这里又使用了一个匿名内部类，获取一个指定生命周期范围的实例
Object scopedInstance = scope.get(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
}
});
//获取给定Bean的实例对象
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);
}
}
}
//对创建的Bean实例对象进行类型检查
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return (T) bean;
}从上述代码中我们可以看出，getBean()是依赖注入的起点，之后会通过调用createBean()来真正获得Bean实例对象。在这个过程中，Bean对象会根据BeanDefinition定义的要求生成。在AbstractAutowireCapableBeanFactory中实现了这个createBean,createBean不但生成了需要的Bean，还对Bean初始进行了处理，比如实现了在BeanDefinition中的init-method属性的定义，Bean后置处理器等。
3.Bean实例对象的创建：AbstractAutowireCapableBeanFactory的createBean()
//创建Bean实例对象
protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
//判断需要创建的Bean是否可以实例化，即是否可以通过当前的类加载器加载
resolveBeanClass(mbd, beanName);
//校验和准备Bean中的方法覆盖
try {
mbd.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
//如果Bean配置了初始化前和初始化后的处理器，则试图返回一个需要创建//Bean的代理对象
Object bean = resolveBeforeInstantiation(beanName, mbd);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
//创建Bean的入口
Object beanInstance = doCreateBean(beanName, mbd, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
//真正创建Bean的方法
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
//封装被创建的Bean对象
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()){//单态模式的Bean，先从容器中缓存中获取同名Bean
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//创建实例对象
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
//获取实例化对象的类型
Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
//调用PostProcessor后置处理器
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
//向容器中缓存单态模式的Bean对象，以防循环引用
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");
}
//这里是一个匿名内部类，为了防止循环引用，尽早持有对象的引用
addSingletonFactory(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
//Bean对象的初始化，依赖注入在此触发
//这个exposedObject在初始化完成之后返回作为依赖注入完成后的Bean
Object exposedObject = bean;
try {
//将Bean实例对象封装，并且Bean定义中配置的属性值赋值给实例对象
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
//初始化Bean对象
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) {
//获取指定名称的已注册的单态模式Bean对象
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
//根据名称获取的以注册的Bean和正在实例化的Bean是同一个
if (exposedObject == bean) {
//当前实例化的Bean初始化完成
exposedObject = earlySingletonReference;
}
//当前Bean依赖其他Bean，并且当发生循环引用时不允许新创建实例对象
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actu alDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
//获取当前Bean所依赖的其他Bean
for (String dependentBean : dependentBeans) {
//对依赖Bean进行类型检查
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.");
}
}
}
}
//注册完成依赖注入的Bean
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}在这里我们可以看到，与依赖注入密切相关的方法有createBeanInstance和populateBean.其中createBeanInstance是生成Bean所包含的java对象，populateBean是对Bean属性的依赖注入进行处理，接下来分别解析这两个方法。
4. java对象实例的创建 ：AbstractAutowireCapableBeanFactory的createBeanInstance()
Bean所包含的java对象实例生成方式有很多种，可以通过工厂方法生成也可以通过容器的autowire特性生成，这些生成方式都是由相关的BeanDefinition来指定。如下代码所示:
//创建Bean的实例对象
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
//检查确认Bean是可实例化的
Class beanClass = resolveBeanClass(mbd, beanName);
//使用工厂方法对Bean进行实例化
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());
}
if (mbd.getFactoryMethodName() != null) {
//调用工厂方法实例化
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
//使用容器的自动装配方法进行实例化
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) {
//配置了自动装配属性，使用容器的自动装配实例化
//容器的自动装配是根据参数类型匹配Bean的构造方法
return autowireConstructor(beanName, mbd, null, null);
}
else {
//使用默认的无参构造方法实例化
return instantiateBean(beanName, mbd);
}
}
//使用Bean的构造方法进行实例化
Constructor[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
//使用容器的自动装配特性，调用匹配的构造方法实例化
return autowireConstructor(beanName, mbd, ctors, args);
}
//使用默认的无参构造方法实例化
return instantiateBean(beanName, mbd);
}
//使用默认的无参构造方法实例化Bean对象
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
try {
Object beanInstance;
final BeanFactory parent = this;
//获取系统的安全管理接口，JDK标准的安全管理API
if (System.getSecurityManager() != null) {
//这里是一个匿名内置类，根据实例化策略创建实例对象
//默认的实例化策略是CglibSubclassingInstantiationStrategy，也就是使用CGLIB来对Bena进行实例化
beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
public Object run() {
return getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
}, getAccessControlContext());
}
else {
//将实例化的对象封装起来
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}这里通过CGLIB来对Bean进行实例化。一般使用SimpleInstanitiationStrategy类，它提供了两种实例化Java对象的方法，一种是通过BeanUtils,它使用JVM的反射功能。另一种是CGLIB来生成。代码如下:
//使用初始化策略实例化Bean对象
public Object instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner) {
//如果Bean定义中没有方法覆盖，则就不需要CGLIB父类类的方法
if (beanDefinition.getMethodOverrides().isEmpty()) {
Constructor<?> constructorToUse;
synchronized (beanDefinition.constructorArgumentLock) {
//获取对象的构造方法或工厂方法
constructorToUse = (Constructor<?>) beanDefinition.resolvedConstructorOrFactoryMethod;
//如果没有构造方法且没有工厂方法
if (constructorToUse == null) {
//使用JDK的反射机制，判断要实例化的Bean是否是接口
final Class clazz = beanDefinition.getBeanClass();
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
if (System.getSecurityManager() != null) {
//这里是一个匿名内置类，使用反射机制获取Bean的构造方法
constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor>() {
public Constructor run() throws Exception {
return clazz.getDeclaredConstructor((Class[]) null);
}
});
}
else {
constructorToUse = clazz.getDeclaredConstructor((Class[]) null);
}
beanDefinition.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Exception ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
//使用BeanUtils实例化，通过反射机制调用”构造方法.newInstance(arg)”来进行实例化
return BeanUtils.instantiateClass(constructorToUse);
}
else {
//使用CGLIB来实例化对象
return instantiateWithMethodInjection(beanDefinition, beanName, owner);
}
}
通过上面的代码分析，我们看到了如果Bean有方法被覆盖了，则使用JDK的反射机制进行实例化，否则，使用CGLIB进行实例化。
instantiateWithMethodInjection方法调用SimpleInstantiationStrategy的子类CglibSubclassingInstantiationStrategy使用CGLIB来进行初始化，其源码如下
//使用CGLIB进行Bean对象实例化
public Object instantiate(Constructor ctor, Object[] args) {
//CGLIB中的类
Enhancer enhancer = new Enhancer();
//将Bean本身作为其基类
enhancer.setSuperclass(this.beanDefinition.getBeanClass());
enhancer.setCallbackFilter(new CallbackFilterImpl());
enhancer.setCallbacks(new Callback[] {
NoOp.INSTANCE,
new LookupOverrideMethodInterceptor(),
new ReplaceOverrideMethodInterceptor()
});
//使用CGLIB的create方法生成实例对象
return (ctor == null) ?
enhancer.create() :
enhancer.create(ctor.getParameterTypes(), args);
}
5.依赖关系处理：AbstractAutowireCapableBeanFactory.populateBean()
//将Bean属性设置到生成的实例对象上
protected void populateBean(String beanName, AbstractBeanDefinition mbd, BeanWrapper bw) {
//获取容器在解析Bean定义资源时为BeanDefiniton中设置的属性值
PropertyValues pvs = mbd.getPropertyValues();
//实例对象为null
if (bw == null) {
//属性值不为空
if (!pvs.isEmpty()) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
}
else {
//实例对象为null，属性值也为空，不需要设置属性值，直接返回
return;
}
}
//在设置属性之前调用Bean的PostProcessor后置处理器
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;
}
//依赖注入开始，首先处理autowire自动装配的注入
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
//对autowire自动装配的处理，根据Bean名称自动装配注入
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
//根据Bean类型自动装配注入
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
//检查容器是否持有用于处理单态模式Bean关闭时的后置处理器
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
//Bean实例对象没有依赖，即没有继承基类
boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);
if (hasInstAwareBpps || needsDepCheck) {
//从实例对象中提取属性描述符
PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw);
if (hasInstAwareBpps) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
//使用BeanPostProcessor处理器处理属性值
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}
}
if (needsDepCheck) {
//为要设置的属性进行依赖检查
checkDependencies(beanName, mbd, filteredPds, pvs);
}
}
//对属性进行注入
applyPropertyValues(beanName, mbd, bw, pvs);
}
//解析并注入依赖属性的过程
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
if (pvs == null || pvs.isEmpty()) {
return;
}
//封装属性值
MutablePropertyValues mpvs = null;
List<PropertyValue> original;
if (System.getSecurityManager()!= null) {
if (bw instanceof BeanWrapperImpl) {
//设置安全上下文，JDK安全机制
((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext());
}
}
if (pvs instanceof MutablePropertyValues) {
mpvs = (MutablePropertyValues) pvs;
//属性值已经转换
if (mpvs.isConverted()) {
try {
//为实例化对象设置属性值
bw.setPropertyValues(mpvs);
return;
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}
//获取属性值对象的原始类型值
original = mpvs.getPropertyValueList();
}
else {
original = Arrays.asList(pvs.getPropertyValues());
}
//获取用户自定义的类型转换
TypeConverter converter = getCustomTypeConverter();
if (converter == null) {
converter = bw;
}
//创建一个Bean定义属性值解析器，将Bean定义中的属性值解析为Bean实例对象
//的实际值
BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);
//为属性的解析值创建一个拷贝，将拷贝的数据注入到实例对象中
List<PropertyValue> deepCopy = new ArrayList<PropertyValue>(original.size());
boolean resolveNecessary = false;
for (PropertyValue pv : original) {
//属性值不需要转换
if (pv.isConverted()) {
deepCopy.add(pv);
}
//属性值需要转换
else {
String propertyName = pv.getName();
//原始的属性值，即转换之前的属性值
Object originalValue = pv.getValue();
//转换属性值，例如将引用转换为IoC容器中实例化对象引用
Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
//转换之后的属性值
Object convertedValue = resolvedValue;
//属性值是否可以转换
boolean convertible = bw.isWritableProperty(propertyName) &&
!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
if (convertible) {
//使用用户自定义的类型转换器转换属性值
convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
}
//存储转换后的属性值，避免每次属性注入时的转换工作
if (resolvedValue == originalValue) {
if (convertible) {
//设置属性转换之后的值
pv.setConvertedValue(convertedValue);
}
deepCopy.add(pv);
}
//属性是可转换的，且属性原始值是字符串类型，且属性的原始类型值不是
//动态生成的字符串，且属性的原始值不是集合或者数组类型
else if (convertible && originalValue instanceof TypedStringValue &&
!((TypedStringValue) originalValue).isDynamic() &&
!(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
pv.setConvertedValue(convertedValue);
deepCopy.add(pv);
}
else {
resolveNecessary = true;
//重新封装属性的值
deepCopy.add(new PropertyValue(pv, convertedValue));
}
}
}
if (mpvs != null && !resolveNecessary) {
//标记属性值已经转换过
mpvs.setConverted();
}
//进行属性依赖注入
//这里是依赖注入发生的地方，会在BeanWrapperImpl中完成
try {
bw.setPropertyValues(new MutablePropertyValues(deepCopy));
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}分析上述代码，我们可以看出，对属性的注入过程分以下两种情况：
(1).属性值类型不需要转换时，不需要解析属性值，直接准备进行依赖注入。
(2).属性值需要进行类型转换时，如对其他对象的引用等，首先需要解析属性值，然后对解析后的属性值进行依赖注入。
对属性值的解析是在BeanDefinitionValueResolver类中的resolveValueIfNecessary方法中进行的，对属性值的依赖注入是通过bw.setPropertyValues方法实现的，在分析属性值的依赖注入之前，我们先分析一下对属性值的解析过程。
6.依赖属性类型转换: BeanDefinitionValueResolver.resolveValueIfNecessary
//解析属性值，对注入类型进行转换
public Object resolveValueIfNecessary(Object argName, Object value) {
//对引用类型的属性进行解析
if (value instanceof RuntimeBeanReference) {
RuntimeBeanReference ref = (RuntimeBeanReference) value;
//调用引用类型属性的解析方法
return resolveReference(argName, ref);
}
//对属性值是引用容器中另一个Bean名称的解析
else if (value instanceof RuntimeBeanNameReference) {
String refName = ((RuntimeBeanNameReference) value).getBeanName();
refName = String.valueOf(evaluate(refName));
//从容器中获取指定名称的Bean
if (!this.beanFactory.containsBean(refName)) {
throw new BeanDefinitionStoreException(
"Invalid bean name '" + refName + "' in bean reference for " + argName);
}
return refName;
}
//对Bean类型属性的解析，主要是Bean中的内部类
else if (value instanceof BeanDefinitionHolder) {
BeanDefinitionHolder bdHolder = (BeanDefinitionHolder) value;
return resolveInnerBean(argName, bdHolder.getBeanName(), bdHolder.getBeanDefinition());
}
else if (value instanceof BeanDefinition) {
BeanDefinition bd = (BeanDefinition) value;
return resolveInnerBean(argName, "(inner bean)", bd);
}
//对集合数组类型的属性解析
else if (value instanceof ManagedArray) {
ManagedArray array = (ManagedArray) value;
//获取数组的类型
Class elementType = array.resolvedElementType;
if (elementType == null) {
//获取数组元素的类型
String elementTypeName = array.getElementTypeName();
if (StringUtils.hasText(elementTypeName)) {
try {
//使用反射机制创建指定类型的对象
elementType = ClassUtils.forName(elementTypeName, this.beanFactory.getBeanClassLoader());
array.resolvedElementType = elementType;
}
catch (Throwable ex) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Error resolving array type for " + argName, ex);
}
}
//没有获取到数组的类型，也没有获取到数组元素的类型，则直接设置数
//组的类型为Object
else {
elementType = Object.class;
}
}
//创建指定类型的数组
return resolveManagedArray(argName, (List<?>) value, elementType);
}
//解析list类型的属性值
else if (value instanceof ManagedList) {
return resolveManagedList(argName, (List<?>) value);
}
//解析set类型的属性值
else if (value instanceof ManagedSet) {
return resolveManagedSet(argName, (Set<?>) value);
}
//解析map类型的属性值
else if (value instanceof ManagedMap) {
return resolveManagedMap(argName, (Map<?, ?>) value);
}
//解析props类型的属性值，props其实就是key和value均为字符串的map
else if (value instanceof ManagedProperties) {
Properties original = (Properties) value;
//创建一个拷贝，用于作为解析后的返回值
Properties copy = new Properties();
for (Map.Entry propEntry : original.entrySet()) {
Object propKey = propEntry.getKey();
Object propValue = propEntry.getValue();
if (propKey instanceof TypedStringValue) {
propKey = evaluate((TypedStringValue) propKey);
}
if (propValue instanceof TypedStringValue) {
propValue = evaluate((TypedStringValue) propValue);
}
copy.put(propKey, propValue);
}
return copy;
}
//解析字符串类型的属性值
else if (value instanceof TypedStringValue) {
TypedStringValue typedStringValue = (TypedStringValue) value;
Object valueObject = evaluate(typedStringValue);
try {
//获取属性的目标类型
Class<?> resolvedTargetType = resolveTargetType(typedStringValue);
if (resolvedTargetType != null) {
//对目标类型的属性进行解析，递归调用
return this.typeConverter.convertIfNecessary(valueObject, resolvedTargetType);
}
//没有获取到属性的目标对象，则按Object类型返回
else {
return valueObject;
}
}
catch (Throwable ex) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Error converting typed String value for " + argName, ex);
}
}
else {
return evaluate(value);
}
}
//解析引用类型的属性值
private Object resolveReference(Object argName, RuntimeBeanReference ref) {
try {
//获取引用的Bean名称
String refName = ref.getBeanName();
refName = String.valueOf(evaluate(refName));
//如果引用的对象在父类容器中，则从父类容器中获取指定的引用对象
if (ref.isToParent()) {
if (this.beanFactory.getParentBeanFactory() == null) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Can't resolve reference to bean '" + refName +
"' in parent factory: no parent factory available");
}
return this.beanFactory.getParentBeanFactory().getBean(refName);
}
//从当前的容器中获取指定的引用Bean对象，如果指定的Bean没有被实例化
//则会递归触发引用Bean的初始化和依赖注入
else {
Object bean = this.beanFactory.getBean(refName);
//将当前实例化对象的依赖引用对象
this.beanFactory.registerDependentBean(refName, this.beanName);
return bean;
}
}
catch (BeansException ex) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
}
}
//解析array类型的属性
private Object resolveManagedArray(Object argName, List<?> ml, Class elementType) {
//创建一个指定类型的数组，用于存放和返回解析后的数组
Object resolved = Array.newInstance(elementType, ml.size());
for (int i = 0; i < ml.size(); i++) {
//递归解析array的每一个元素，并将解析后的值设置到resolved数组中，索引为i
Array.set(resolved, i,
resolveValueIfNecessary(new KeyedArgName(argName, i), ml.get(i)));
}
return resolved;
}
//解析list类型的属性
private List resolveManagedList(Object argName, List<?> ml) {
List<Object> resolved = new ArrayList<Object>(ml.size());
for (int i = 0; i < ml.size(); i++) {
//递归解析list的每一个元素
resolved.add(
resolveValueIfNecessary(new KeyedArgName(argName, i), ml.get(i)));
}
return resolved;
}
//解析set类型的属性
private Set resolveManagedSet(Object argName, Set<?> ms) {
Set<Object> resolved = new LinkedHashSet<Object>(ms.size());
int i = 0;
//递归解析set的每一个元素
for (Object m : ms) {
resolved.add(resolveValueIfNecessary(new KeyedArgName(argName, i), m));
i++;
}
return resolved;
}
//解析map类型的属性
private Map resolveManagedMap(Object argName, Map<?, ?> mm) {
Map<Object, Object> resolved = new LinkedHashMap<Object, Object>(mm.size());
//递归解析map中每一个元素的key和value
for (Map.Entry entry : mm.entrySet()) {
Object resolvedKey = resolveValueIfNecessary(argName, entry.getKey());
Object resolvedValue = resolveValueIfNecessary(
new KeyedArgName(argName, entry.getKey()), entry.getValue());
resolved.put(resolvedKey, resolvedValue);
}
return resolved;
}通过上面的代码分析，我们明白了Spring是如何将引用类型，内部类以及集合类型等属性进行解析的，属性值解析完成后就可以进行依赖注入了，依赖注入的过程就是Bean对象实例设置到它所依赖的Bean对象属性上去，在第7步中我们已经说过，依赖注入是通过bw.setPropertyValues方法实现的，该方法也使用了委托模式，在BeanWrapper接口中至少定义了方法声明，依赖注入的具体实现交由其实现类BeanWrapperImpl来完成，下面我们就分析依BeanWrapperImpl中赖注入相关的源码。
7.属性值依赖注入： BeanWrapperImpl.setPropertyValue
BeanWrapperImpl类主要是对容器中完成初始化的Bean实例对象进行属性的依赖注入，即把Bean对象设置到它所依赖的另一个Bean的属性中去，依赖注入的相关源码如下：
//实现属性依赖注入功能
private void setPropertyValue(PropertyTokenHolder tokens, PropertyValue pv) throws BeansException {
//PropertyTokenHolder主要保存属性的名称、路径，以及集合的size等信息
String propertyName = tokens.canonicalName;
String actualName = tokens.actualName;
//keys是用来保存集合类型属性的size
if (tokens.keys != null) {
//将属性信息拷贝
PropertyTokenHolder getterTokens = new PropertyTokenHolder();
getterTokens.canonicalName = tokens.canonicalName;
getterTokens.actualName = tokens.actualName;
getterTokens.keys = new String[tokens.keys.length - 1];
System.arraycopy(tokens.keys, 0, getterTokens.keys, 0, tokens.keys.length - 1);
Object propValue;
try {
//获取属性值，该方法内部使用JDK的内省( Introspector)机制，调用属性//的getter(readerMethod)方法，获取属性的值
propValue = getPropertyValue(getterTokens);
}
catch (NotReadablePropertyException ex) {
throw new NotWritablePropertyException(getRootClass(), this.nestedPath + propertyName,
"Cannot access indexed value in property referenced " +
"in indexed property path '" + propertyName + "'", ex);
}
//获取集合类型属性的长度
String key = tokens.keys[tokens.keys.length - 1];
if (propValue == null) {
throw new NullValueInNestedPathException(getRootClass(), this.nestedPath + propertyName,
"Cannot access indexed value in property referenced " +
"in indexed property path '" + propertyName + "': returned null");
}
//注入array类型的属性值
else if (propValue.getClass().isArray()) {
//获取属性的描述符
PropertyDescriptor pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
//获取数组的类型
Class requiredType = propValue.getClass().getComponentType();
//获取数组的长度
int arrayIndex = Integer.parseInt(key);
Object oldValue = null;
try {
//获取数组以前初始化的值
if (isExtractOldValueForEditor()) {
oldValue = Array.get(propValue, arrayIndex);
}
//将属性的值赋值给数组中的元素
Object convertedValue = convertIfNecessary(propertyName, oldValue, pv.getValue(), requiredType,
new PropertyTypeDescriptor(pd, new MethodParameter(pd.getReadMethod(), -1), requiredType));
Array.set(propValue, arrayIndex, convertedValue);
}
catch (IndexOutOfBoundsException ex) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Invalid array index in property path '" + propertyName + "'", ex);
}
}
//注入list类型的属性值
else if (propValue instanceof List) {
PropertyDescriptor pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
//获取list集合的类型
Class requiredType = GenericCollectionTypeResolver.getCollectionReturnType(
pd.getReadMethod(), tokens.keys.length);
List list = (List) propValue;
//获取list集合的size
int index = Integer.parseInt(key);
Object oldValue = null;
if (isExtractOldValueForEditor() && index < list.size()) {
oldValue = list.get(index);
}
//获取list解析后的属性值
Object convertedValue = convertIfNecessary(propertyName, oldValue, pv.getValue(), requiredType,
new PropertyTypeDescriptor(pd, new MethodParameter(pd.getReadMethod(), -1), requiredType));
if (index < list.size()) {
//为list属性赋值
list.set(index, convertedValue);
}
//如果list的长度大于属性值的长度，则多余的元素赋值为null
else if (index >= list.size()) {
for (int i = list.size(); i < index; i++) {
try {
list.add(null);
}
catch (NullPointerException ex) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Cannot set element with index " + index + " in List of size " +
list.size() + ", accessed using property path '" + propertyName +
"': List does not support filling up gaps with null elements");
}
}
list.add(convertedValue);
}
}
//注入map类型的属性值
else if (propValue instanceof Map) {
PropertyDescriptor pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
//获取map集合key的类型
Class mapKeyType = GenericCollectionTypeResolver.getMapKeyReturnType(
pd.getReadMethod(), tokens.keys.length);
//获取map集合value的类型
Class mapValueType = GenericCollectionTypeResolver.getMapValueReturnType(
pd.getReadMethod(), tokens.keys.length);
Map map = (Map) propValue;
//解析map类型属性key值
Object convertedMapKey = convertIfNecessary(null, null, key, mapKeyType,
new PropertyTypeDescriptor(pd, new MethodParameter(pd.getReadMethod(), -1), mapKeyType));
Object oldValue = null;
if (isExtractOldValueForEditor()) {
oldValue = map.get(convertedMapKey);
}
//解析map类型属性value值
Object convertedMapValue = convertIfNecessary(
propertyName, oldValue, pv.getValue(), mapValueType,
new TypeDescriptor(new MethodParameter(pd.getReadMethod(), -1, tokens.keys.length + 1)));
//将解析后的key和value值赋值给map集合属性
map.put(convertedMapKey, convertedMapValue);
}
else {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Property referenced in indexed property path '" + propertyName +
"' is neither an array nor a List nor a Map; returned value was [" + pv.getValue() + "]");
}
}
//对非集合类型的属性注入
else {
PropertyDescriptor pd = pv.resolvedDescriptor;
if (pd == null || !pd.getWriteMethod().getDeclaringClass().isInstance(this.object)) {
pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
//无法获取到属性名或者属性没有提供setter(写方法)方法
if (pd == null || pd.getWriteMethod() == null) {
//如果属性值是可选的，即不是必须的，则忽略该属性值
if (pv.isOptional()) {
logger.debug("Ignoring optional value for property '" + actualName +
"' - property not found on bean class [" + getRootClass().getName() + "]");
return;
}
//如果属性值是必须的，则抛出无法给属性赋值，因为每天提供setter方法异常
else {
PropertyMatches matches = PropertyMatches.forProperty(propertyName, getRootClass());
throw new NotWritablePropertyException(
getRootClass(), this.nestedPath + propertyName,
matches.buildErrorMessage(), matches.getPossibleMatches());
}
}
pv.getOriginalPropertyValue().resolvedDescriptor = pd;
}
Object oldValue = null;
try {
Object originalValue = pv.getValue();
Object valueToApply = originalValue;
if (!Boolean.FALSE.equals(pv.conversionNecessary)) {
if (pv.isConverted()) {
valueToApply = pv.getConvertedValue();
}
else {
if (isExtractOldValueForEditor() && pd.getReadMethod() != null) {
//获取属性的getter方法(读方法)，JDK内省机制
final Method readMethod = pd.getReadMethod();
//如果属性的getter方法不是public访问控制权限的，即访问控制权限比较严格，
//则使用JDK的反射机制强行访问非public的方法(暴力读取属性值)
if (!Modifier.isPublic(readMethod.getDeclaringClass().getModifiers()) &&
!readMethod.isAccessible()) {
if (System.getSecurityManager()!= null) {
//匿名内部类，根据权限修改属性的读取控制限制
AccessController.doPrivileged(new PrivilegedAction<Object>() {
public Object run() {
readMethod.setAccessible(true);
return null;
}
});
}
else {
readMethod.setAccessible(true);
}
}
try {
//属性没有提供getter方法时，调用潜在的读取属性值//的方法，获取属性值
if (System.getSecurityManager() != null) {
oldValue = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
public Object run() throws Exception {
return readMethod.invoke(object);
}
}, acc);
}
else {
oldValue = readMethod.invoke(object);
}
}
catch (Exception ex) {
if (ex instanceof PrivilegedActionException) {
ex = ((PrivilegedActionException) ex).getException();
}
if (logger.isDebugEnabled()) {
logger.debug("Could not read previous value of property '" +
this.nestedPath + propertyName + "'", ex);
}
}
}
//设置属性的注入值
valueToApply = convertForProperty(propertyName, oldValue, originalValue, pd);
}
pv.getOriginalPropertyValue().conversionNecessary = (valueToApply != originalValue);
}
//根据JDK的内省机制，获取属性的setter(写方法)方法
final Method writeMethod = (pd instanceof GenericTypeAwarePropertyDescriptor ?
((GenericTypeAwarePropertyDescriptor) pd).getWriteMethodForActualAccess() :
pd.getWriteMethod());
//如果属性的setter方法是非public，即访问控制权限比较严格，则使用JDK的反射机制，
//强行设置setter方法可访问(暴力为属性赋值)
if (!Modifier.isPublic(writeMethod.getDeclaringClass().getModifiers()) && !writeMethod.isAccessible()) {
//如果使用了JDK的安全机制，则需要权限验证
if (System.getSecurityManager()!= null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() {
public Object run() {
writeMethod.setAccessible(true);
return null;
}
});
}
else {
writeMethod.setAccessible(true);
}
}
final Object value = valueToApply;
if (System.getSecurityManager() != null) {
try {
//将属性值设置到属性上去
AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
public Object run() throws Exception {
writeMethod.invoke(object, value);
return null;
}
}, acc);
}
catch (PrivilegedActionException ex) {
throw ex.getException();
}
}
else {
writeMethod.invoke(this.object, value);
}
}
catch (TypeMismatchException ex) {
throw ex;
}
catch (InvocationTargetException ex) {
PropertyChangeEvent propertyChangeEvent =
new PropertyChangeEvent(this.rootObject, this.nestedPath + propertyName, oldValue, pv.getValue());
if (ex.getTargetException() instanceof ClassCastException) {
throw new TypeMismatchException(propertyChangeEvent, pd.getPropertyType(), ex.getTargetException());
}
else {
throw new MethodInvocationException(propertyChangeEvent, ex.getTargetException());
}
}
catch (Exception ex) {
PropertyChangeEvent pce =
new PropertyChangeEvent(this.rootObject, this.nestedPath + propertyName, oldValue, pv.getValue());
throw new MethodInvocationException(pce, ex);
}
}
}通过对上面注入依赖代码的分析，我们已经明白了Spring IoC容器是如何将属性的值注入到Bean实例对象中去的：
(1).对于集合类型的属性，将其属性值解析为目标类型的集合后直接赋值给属性。
(2).对于非集合类型的属性，大量使用了JDK的反射和内省机制，通过属性的getter方法(reader method)获取指定属性注入以前的值，同时调用属性的setter方法(writer method)为属性设置注入后的值。看到这里相信很多人都明白了Spring的setter注入。
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