Spring源码解读4——依赖注入的发生

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<Bean>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。

[code=language-java]public abstract class AbstractBeanFactory{

public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}

protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly){
//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
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
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
......
//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
//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
......
//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

//***这个getMergedLocalBeanDefinition(beanName)肯定是在获取注册过的BeanDefinition
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(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 {
return createBean(beanName, mbd, args);
}
}
});
//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 {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
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);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
}
try {
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);
}

}
return (T) bean;
}

}

上面的源码只是定义了根据Bean定义的模式，采取的不同创建Bean实例对象的策略，具体的Bean实例对象的创建过程由实现了ObejctFactory接口的匿名内部类的createBean方法完成，ObejctFactory使用委派模式，具体的Bean实例创建过程交由其实现类AbstractAutowireCapableBeanFactory完成，我们继续分析AbstractAutowireCapableBeanFactory的createBean方法的源码

[code=language-java]public class AbstractAutowireCapableBeanFactory{
protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args){

Object beanInstance = doCreateBean(beanName, mbd, args);
return beanInstance;
}

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);
//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) {
//在对Bean实例对象生成和依赖注入完成以后，开始对Bean实例对象
//进行初始化，为Bean实例对象应用BeanPostProcessor后置处理器
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
}
......
return exposedObject;

}

//通过对方法源码的分析，我们看到具体的依赖注入实现在以下两个方法中：
//(1).createBeanInstance：生成Bean所包含的java对象实例。
//(2).populateBean ：对Bean属性的依赖注入进行处理。

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
// Make sure bean class is actually resolved at this point.
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) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
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))  {
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
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
Object beanInstance;
//方法getInstantiationStrategy().instantiate调用了SimpleInstantiationStrategy类中的实例化Bean的方法
//方法中使用反射取得构造函数进行实例化
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;

}

方法getInstantiationStrategy().instantiate调用了SimpleInstantiationStrategy类中的实例化Bean的方法

[code=language-java]public class SimpleInstantiationStrategy implements InstantiationStrategy {

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) {

//获取BeanDefinition中的class路径
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.doPrivile(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);
}
}
}

(1).createBeanInstance：生成Bean所包含的java对象实例。已经分析完。这里返回的实例是空的，需要注入属性。

(2).populateBean ：对Bean属性的依赖注入进行处理。开始一分析：

[code=language-java]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();
//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);

// Add property values based on autowire by name if applicable.
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}

// Add property values based on autowire by type if applicable.
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}

pvs = newPvs;
}
... ...

//对属性进行注入
applyPropertyValues(beanName, mbd, bw, pvs);
}

protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
//属性值已经转换
if (pvs instanceof MutablePropertyValues) {
mpvs = (MutablePropertyValues) pvs;
if (mpvs.isConverted()) {
// Shortcut: use the pre-converted values as-is.
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());
}
//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(解析过的属性都放进这个拷贝中，最后再将
//拷贝注入进Bean里)
List<PropertyValue> deepCopy = new ArrayList<PropertyValue>(original.size());
for (PropertyValue pv : original) {
if (pv.isConverted()) {
deepCopy.add(pv);
}
else {
//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);
......
}
......
bw.setPropertyValues(new MutablePropertyValues(deepCopy));
}
public Object resolveValueIfNecessary(Object argName, Object value) {......}

}

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

[code=language-java]//AbstractPropertyAccessor类是BeanWrapperImpl的父类，setPropertyValue采用模板模式，由子类BeanWrapperImpl实现。
public abstract class AbstractPropertyAccessor{
public void setPropertyValues(PropertyValues pvs, boolean ignoreUnknown, boolean ignoreInvalid)
throws BeansException {

List<PropertyAccessException> propertyAccessExceptions = null;
List<PropertyValue> propertyValues = (pvs instanceof MutablePropertyValues ?
((MutablePropertyValues) pvs).getPropertyValueList() : Arrays.asList(pvs.getPropertyValues()));

//pvs里有很多值，故要循环注入
for (PropertyValue pv : propertyValues) {
try {
// This method may throw any BeansException, which won't be caught
// here, if there is a critical failure such as no matching field.
// We can attempt to deal only with less serious exceptions.
setPropertyValue(pv);
}
}
}

[code=language-java]public class BeanWrapperImpl{
public void setPropertyValue(PropertyValue pv){

PropertyDescriptor pd = pv.resolvedDescriptor;
Object originalValue = pv.getValue();
Object 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();
}
}
}
}