引言
spring加载xml的对象信息解析实例化成各个bean的过程我在这里就不细讲了,毕竟从头开始看很容易绕晕大家,反而让大家觉得这并不需要写。我们姑且认为spring已经加载好了各类对象信息封装成BeanDefinition,并已经实例化存储在了某个地方。不管是懒汉还是饿汉,都要经历反射出对象实例,然后初始化,我们先从spring中比较好理解的地方来入手IOC,那就是IOC中的bean在实例化之后的初始化操作。主要涉及到BeanPostProcessor,InitializingBean这两个的应用。
IOC的bean初始化
protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
invokeInitMethods(beanName, wrappedBean, mbd);
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
为了方便阅读,我删除了部分不相干的代码。
AbstractAutowireCapableBeanFactory#initializeBean就是初始化IOC容器中的Bean的主要方法,从这个方法入手,来看看IOC加载Bean到底做了什么?
将实例化好的bean传入该方法:
--> 调用BeanPostProcessor的postProcessBeforeInitialization方法
--> 调用bean实例的初始化方法
--> 调用BeanPostProcessor的postProcessAfterInitialization方法
以下三段代码就是这三个过程:
@Override
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
result = beanProcessor.postProcessBeforeInitialization(result, beanName);
if (result == null) {
return result;
}
}
return result;
}
循环所有实现了BeanPostProcessor类的bean,并执行相应的对象初始化之前的方法postProcessBeforeInitialization
protected void invokeInitMethods(String beanName, final Object bean, RootBeanDefinition mbd)
throws Throwable {
boolean isInitializingBean = (bean instanceof InitializingBean);
if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
if (logger.isDebugEnabled()) {
logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
}
if (System.getSecurityManager() != null) {
try {
AccessController.doPrivileged(new PrivilegedExceptionAction
该段代码给一些实现了InitializingBean的bean进行初始化操作。
注意:
1:spring为bean提供了两种初始化的方式,需要实现InitializingBean接口,重写afterPropertiesSet方法,或者在配置文件中同过init-method指定,两种方式可以同时使用,但是会先执行afterPropertiesSet再执行init-method。下文还有demo证明。
2:实现InitializingBean接口是直接调用afterPropertiesSet方法,比通过反射调用init-method指定的方法效率相对来说要高点。但是init-method方式消除了对spring的依赖
3:如果调用afterPropertiesSet方法时出错,则不调用init-method指定的方法。
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
result = beanProcessor.postProcessAfterInitialization(result, beanName);
if (result == null) {
return result;
}
}
return result;
}
再次循环所有实现了BeanPostProcessor类的bean,并执行相应的对象初始化之前的方法postProcessAfterInitialization。顺便提一下,以后要写的Spring AOP的底层处理也是通过实现BeanPostProcessor来执行代理包装逻辑的。
IOC的bean注入
我们的ioc对象初始化好了,接下来就要看看最关键的依赖注入了。先思考一个问题,spring在什么时候把对象注入进去的?这里先不解释,我们看这段源码:
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
if (pvs instanceof MutablePropertyValues) {
...
}
else {
original = Arrays.asList(pvs.getPropertyValues());
}
TypeConverter converter = getCustomTypeConverter();
if (converter == null) {
converter = bw;
}
BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);
// Create a deep copy, resolving any references for values.
List deepCopy = new ArrayList(original.size());
boolean resolveNecessary = false;
for (PropertyValue pv : original) {
if (pv.isConverted()) {
deepCopy.add(pv);
}
else {
String propertyName = pv.getName();
Object originalValue = pv.getValue();
//在这里解析并塞入了注入的对象
Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
Object convertedValue = resolvedValue;
boolean convertible = bw.isWritableProperty(propertyName) &&
!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
if (convertible) {
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.
if (resolvedValue == originalValue) {
if (convertible) {
pv.setConvertedValue(convertedValue);
}
deepCopy.add(pv);
}
else if (convertible && originalValue instanceof TypedStringValue &&
!((TypedStringValue) originalValue).isDynamic() &&
!(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
pv.setConvertedValue(convertedValue);
deepCopy.add(pv);
}
else {
resolveNecessary = true;
deepCopy.add(new PropertyValue(pv, convertedValue));
}
}
}
}
我还是删了很多其他逻辑,这里主要的一句话就是Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
在这里分析并塞入了注入对象的关联。具体怎么操作还得看自己去翻阅。接下来我用最简化的代码方式来展示我的寻找过程:
AbstractBeanFactory类
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
我们肯定找到拿到每个bean的如果,如果不存在就会创建。
AbstractBeanFactory类
@SuppressWarnings("unchecked")
protected T doGetBean(
final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory
然后再进入createBean(),它的实现是在 AbstractAutowireCapableBeanFactory 当中:
AbstractAutowireCapableBeanFactory类
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
return beanInstance;
}
AbstractAutowireCapableBeanFactory类
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
}
}
重点关注 createBeanInstance() 和 populateBean() 这两个方法。其中,createBeanInstance方法生成了Bean所包含的Java对象:
AbstractAutowireCapableBeanFactory类
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
if (resolved) {
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
}
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
Object beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
重点关注 getInstantiationStrategy()这个方法,可以看到instantiateBean方法的功能实现是通过调用getInstantiationStrategy().instantiate方法实现的。 getInstantiationStrategy方法的作用是获得实例化的策略对象,也就是指通过哪种方案进行实例化的过程。继续跟踪下去我们可以发现,Spring当中提供了两种实例化方案: BeanUtils和Cglib方式。BeanUtils实现机制是通过Java的反射机制,Cglib是一个第三方类库采用的是一种字节码加强方式机制。Spring中采用的默认实例化策略是Cglib。
接下来就是重头戏建立bean的依赖关系了。我们回到doCreateBean方法中的populateBean(beanName, mbd, instanceWrapper);
protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
applyPropertyValues(beanName, mbd, bw, pvs);
}
有没有发现,我们已经来到我开始讲的applyPropertyValues方法了?
下面我们就不看源码了,我们用反射来把value注入进去,这样更容易理解。
try {
Method declaredMethod = bean.getClass().getDeclaredMethod(
"set" + propertyValue.getName().substring(0, 1).toUpperCase()
+ propertyValue.getName().substring(1), value.getClass());
declaredMethod.setAccessible(true);
declaredMethod.invoke(bean, value);
} catch (NoSuchMethodException e) {
Field declaredField = bean.getClass().getDeclaredField(propertyValue.getName());
declaredField.setAccessible(true);
declaredField.set(bean, value);
}
拿到set参数的方法,如果出现异常,表示并没有写set方法,则就粗暴的方法塞入field。(此处并不是源码,是我根据源码理解写的比较通俗易懂的方式)
这是我觉得在繁多的spring源码中找出一段目标代码先看到,更容易让人能够跟着思路走下去。
题外话:
synchronized (this.dependenciesForBeanMap) {
Set dependenciesForBean =
this.dependenciesForBeanMap.computeIfAbsent(dependentBeanName, k -> new LinkedHashSet<>(8));
dependenciesForBean.add(canonicalName);
}
在看IOC源码的时候看到Map的computeIfAbsent让我恍然大悟,我来用以前怎么写这段代码的来解释这段代码什么意思如下:
Set dependenciesForBean = this.dependenciesForBeanMap.get(dependentBeanName);
if (dependenciesForBean != null){
dependenciesForBean = new LinkedHashSet<>(8);
}
dependenciesForBean.add(dependentBeanName);
dependenciesForBean.add(canonicalName);
这是jdk1.8才支持的lambda写法,是不是代码更简洁?
再来分享一种简洁的方法,计算每个学生的总分记录到map中:
List students = new ArrayList<>();
students.add(new Student("张三", "语文", 18));
students.add(new Student("张三", "数学", 20));
Map resultMap = new HashMap<>();
for (Student student : students) {
resultMap.merge(student.getName(), student.getScore(), (a, b) -> b + a);
}