Spring IOC是我们在开发项目中经常使用到的技术,近期接触项目中使用Spring IOC 中的@Autowire
注解的比例非常高,正好借着老师要深入学习Spring IOC
的机会,学习一下 @Autowired
注解的使用过程。
IOC: Inversion of Control控制反转,也叫(Dependency Injection)依赖注入, dao接口的实现不再是业务逻辑层调用工厂类去获取,而是通过容器(spring)来自动的为我们的业务层设置Dao的实现类,这样整个过程就反过来,以前是我们业务层主动去获取dao,而现在是dao主动被设置到业务逻辑层中来了,这个也就是反转控制的由来。通过IOC,我们就可以在不修改任何代码的情况下,无缝地实现数据库的换库迁移。
目前最常见的两种注入方式:1)属性注入;2)构造函数注入。 如下面代码所示:
Spring 团队建议使用的方式是构造函数注入。
构造函数的常见优点是:
构造函数的常见缺点是:
个人感觉,无论优点还是缺点,只要不瞎用就没什么问题。我周围最常用到的是属性注入,下面着重讲解一下属性注入。
@Slf4j
@RunWith(SpringRunner.class)
@SpringBootTest(classes = {AppApiLauncher.class})
public class PkMatchPopUpServiceTest {
@Autowired
PopupService pkMatchPopUpService;
@Test
public void testSendPopUpMessage() {
pkMatchPopUpService.sendRegularPopUpMessage();
}
}
可以看到,就像魔法一样,在加入 @Autowired
注解之后,就可以直接使用他的方法去进行操作了,简直神奇。
点入 @Autowired
注解我们可以看到如下代码:
@Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Autowired {
boolean required() default true;
}
那么它是如何工作的呢,从官方文档 中我们可以看到
Note that actual injection is performed through a BeanPostProcessor which in turn means that you cannot use @Autowired to inject references into BeanPostProcessor or BeanFactoryPostProcessor types. Please consult the javadoc for the AutowiredAnnotationBeanPostProcessor class (which, by default, checks for the presence of this annotation).
大概意思就是说,@Autowire 注解的功能实现都是通过 AutowiredAnnotationBeanPostProcessor
实现的。那就让我们走入AutowiredAnnotationBeanPostProcessor
。它的继承结构如下:
进入 AutowiredAnnotationBeanPostProcessor
我们可以看到在方法postProcessPropertyValues
完成的注入:
public PropertyValues postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
InjectionMetadata metadata = this.findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
metadata.inject(bean, beanName, pvs);
return pvs;
} catch (BeanCreationException var7) {
throw var7;
} catch (Throwable var8) {
throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", var8);
}
}
可以看到 流程如下:
首先注册 Bean
,然后根据获取的 Bean
通过单例模式获取实例,最后生成 Bean
并且进行注册。
registerBeanPostProcessors
方法主要用于处理 BeanPostProcessor 接口。方法如下:
public static void registerBeanPostProcessors(
ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
// 1.找出所有实现BeanPostProcessor接口的类
String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);
// Register BeanPostProcessorChecker that logs an info message when
// a bean is created during BeanPostProcessor instantiation, i.e. when
// a bean is not eligible for getting processed by all BeanPostProcessors.
// BeanPostProcessor的目标计数
int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
// 2.添加BeanPostProcessorChecker(主要用于记录信息)到beanFactory中
beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));
// Separate between BeanPostProcessors that implement PriorityOrdered,
// Ordered, and the rest.
// 3.定义不同的变量用于区分: 实现PriorityOrdered接口的BeanPostProcessor、实现Ordered接口的BeanPostProcessor、普通BeanPostProcessor
// 3.1 priorityOrderedPostProcessors: 用于存放实现PriorityOrdered接口的BeanPostProcessor
List priorityOrderedPostProcessors = new ArrayList();
// 3.2 internalPostProcessors: 用于存放Spring内部的BeanPostProcessor
List internalPostProcessors = new ArrayList();
// 3.3 orderedPostProcessorNames: 用于存放实现Ordered接口的BeanPostProcessor的beanName
List orderedPostProcessorNames = new ArrayList();
// 3.4 nonOrderedPostProcessorNames: 用于存放普通BeanPostProcessor的beanName
List nonOrderedPostProcessorNames = new ArrayList();
// 4.遍历postProcessorNames, 将BeanPostProcessors按3.1 - 3.4定义的变量区分开
for (String ppName : postProcessorNames) {
if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
// 4.1 如果ppName对应的Bean实例实现了PriorityOrdered接口, 则拿到ppName对应的Bean实例并添加到priorityOrderedPostProcessors
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
priorityOrderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
// 4.2 如果ppName对应的Bean实例也实现了MergedBeanDefinitionPostProcessor接口,
// 则将ppName对应的Bean实例添加到internalPostProcessors
internalPostProcessors.add(pp);
}
}
else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
// 4.3 如果ppName对应的Bean实例没有实现PriorityOrdered接口, 但是实现了Ordered接口, 则将ppName添加到orderedPostProcessorNames
orderedPostProcessorNames.add(ppName);
}
else {
// 4.4 否则, 将ppName添加到nonOrderedPostProcessorNames
nonOrderedPostProcessorNames.add(ppName);
}
}
// First, register the BeanPostProcessors that implement PriorityOrdered.
// 5.首先, 注册实现PriorityOrdered接口的BeanPostProcessors
// 5.1 对priorityOrderedPostProcessors进行排序
sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
// 5.2 注册priorityOrderedPostProcessors
registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);
// Next, register the BeanPostProcessors that implement Ordered.
// 6.接下来, 注册实现Ordered接口的BeanPostProcessors
List orderedPostProcessors = new ArrayList();
for (String ppName : orderedPostProcessorNames) {
// 6.1 拿到ppName对应的BeanPostProcessor实例对象
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
// 6.2 将ppName对应的BeanPostProcessor实例对象添加到orderedPostProcessors, 准备执行注册
orderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
// 6.3 如果ppName对应的Bean实例也实现了MergedBeanDefinitionPostProcessor接口,
// 则将ppName对应的Bean实例添加到internalPostProcessors
internalPostProcessors.add(pp);
}
}
// 6.4 对orderedPostProcessors进行排序
sortPostProcessors(orderedPostProcessors, beanFactory);
// 6.5 注册orderedPostProcessors
registerBeanPostProcessors(beanFactory, orderedPostProcessors);
// Now, register all regular BeanPostProcessors.
// 7.注册所有常规的BeanPostProcessors(过程与6类似)
List nonOrderedPostProcessors = new ArrayList();
for (String ppName : nonOrderedPostProcessorNames) {
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
nonOrderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
internalPostProcessors.add(pp);
}
}
registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);
// Finally, re-register all internal BeanPostProcessors.
// 8.最后, 重新注册所有内部BeanPostProcessors(相当于内部的BeanPostProcessor会被移到处理器链的末尾)
// 8.1 对internalPostProcessors进行排序
sortPostProcessors(internalPostProcessors, beanFactory);
// 8.2注册internalPostProcessors
registerBeanPostProcessors(beanFactory, internalPostProcessors);
// Re-register post-processor for detecting inner beans as ApplicationListeners,
// moving it to the end of the processor chain (for picking up proxies etc).
// 9.重新注册ApplicationListenerDetector(跟8类似,主要是为了移动到处理器链的末尾)
beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
}
8.2注册internalPostProcessors 过程
private static void registerBeanPostProcessors(
ConfigurableListableBeanFactory beanFactory, List postProcessors) {
// 1.遍历postProcessors
for (BeanPostProcessor postProcessor : postProcessors) {
// 2.将PostProcessor添加到BeanFactory中的beanPostProcessors缓存
beanFactory.addBeanPostProcessor(postProcessor);
}
}
protected T doGetBean(
final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
// 1.解析beanName,主要是解析别名、去掉FactoryBean的前缀“&”
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 2.尝试从缓存中获取beanName对应的实例
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
// 3.如果beanName的实例存在于缓存中
if (logger.isDebugEnabled()) {
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 + "'");
}
}
// 3.1 返回beanName对应的实例对象(主要用于FactoryBean的特殊处理,普通Bean会直接返回sharedInstance本身)
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
} else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 4.scope为prototype的循环依赖校验:如果beanName已经正在创建Bean实例中,而此时我们又要再一次创建beanName的实例,则代表出现了循环依赖,需要抛出异常。
// 例子:如果存在A中有B的属性,B中有A的属性,那么当依赖注入的时候,就会产生当A还未创建完的时候因为对于B的创建再次返回创建A,造成循环依赖
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 5.获取parentBeanFactory
BeanFactory parentBeanFactory = getParentBeanFactory();
// 5.1 如果parentBeanFactory存在,并且beanName在当前BeanFactory不存在Bean定义,则尝试从parentBeanFactory中获取bean实例
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
// 5.2 将别名解析成真正的beanName
String nameToLookup = originalBeanName(name);
// 5.3 尝试在parentBeanFactory中获取bean对象实例
if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
} else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
// 6.如果不是仅仅做类型检测,而是创建bean实例,这里要将beanName放到alreadyCreated缓存
markBeanAsCreated(beanName);
}
try {
// 7.根据beanName重新获取MergedBeanDefinition(步骤6将MergedBeanDefinition删除了,这边获取一个新的)
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
// 7.1 检查MergedBeanDefinition
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// 8.拿到当前bean依赖的bean名称集合,在实例化自己之前,需要先实例化自己依赖的bean
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
// 8.1 遍历当前bean依赖的bean名称集合
for (String dep : dependsOn) {
// 8.2 检查dep是否依赖于beanName,即检查是否存在循环依赖
if (isDependent(beanName, dep)) {
// 8.3 如果是循环依赖则抛异常
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 8.4 将dep和beanName的依赖关系注册到缓存中
registerDependentBean(dep, beanName);
// 8.5 获取dep对应的bean实例,如果dep还没有创建bean实例,则创建dep的bean实例
getBean(dep);
}
}
// Create bean instance.
// 9.针对不同的scope进行bean的创建
if (mbd.isSingleton()) {
// 9.1 scope为singleton的bean创建(新建了一个ObjectFactory,并且重写了getObject方法)
sharedInstance = getSingleton(beanName, new ObjectFactory
public Object getSingleton(String beanName, ObjectFactory singletonFactory) {
Assert.notNull(beanName, "'beanName' must not be null");
//同步 开始创建单例
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
//判断单例是否被创建 如果已经创建则不在重复创建
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while the singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
//把当前正在创建的bean记录在缓存中,对循环依赖进行检测
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet();
}
try {
//使用回调方法 创建单例bean
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
//移除缓存中对该bean正在加载的状态
afterSingletonCreation(beanName);
}
if (newSingleton) {
//将新创建的bean加入缓存,并且删除加载bean过程中所记录的各种辅助状态
//这些辅助状态主要是在回调方法创建bean时候引入的
addSingleton(beanName, singletonObject);
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
}
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
//根据指定的BeanDefinition信息 解析bean class 并且存储在BeanDefinition中
Class resolvedClass = resolveBeanClass(mbd, beanName);
//对已有的bean definition进行克隆 以防一动态解析的class不能存储在合并的bean definition中
//这里说的动态解析的class是指在bean class的定义中使用EL表达式或者自己定义的beanExpressionResolver
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null){
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
try {
//验证及准备覆盖的方法
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
//给beanPostProcessors一个机会返回代理来替代真正的实例
//这里的beanPostProcessors是指InstantiationAwareBeanPostProcessor类型
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);
}
//创建bean的过程又交给了doCreateBean,spring中以doxxx开头的方法就是真正干活的方法
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
public PropertyValues postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
InjectionMetadata metadata = this.findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
// 注入 bean
metadata.inject(bean, beanName, pvs);
return pvs;
} catch (BeanCreationException var7) {
throw var7;
} catch (Throwable var8) {
throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", var8);
}
}
现在我们进去 inject
,
public void inject(Object target, String beanName, PropertyValues pvs) throws Throwable {
Collection elementsToIterate = this.checkedElements != null ? this.checkedElements : this.injectedElements;
if (!((Collection)elementsToIterate).isEmpty()) {
boolean debug = logger.isDebugEnabled();
InjectionMetadata.InjectedElement element;
for(Iterator var6 = ((Collection)elementsToIterate).iterator(); var6.hasNext(); element.inject(target, beanName, pvs)) {
element = (InjectionMetadata.InjectedElement)var6.next();
if (debug) {
logger.debug("Processing injected element of bean '" + beanName + "': " + element);
}
}
}
}
其逻辑就是遍历,然后调用inject方法,inject方法其实现逻辑如下:
/**
* Either this or {@link #getResourceToInject} needs to be overridden.
*/
protected void inject(Object target, @Nullable String requestingBeanName, @Nullable PropertyValues pvs)
throws Throwable {
if (this.isField) {
Field field = (Field) this.member;
ReflectionUtils.makeAccessible(field);
field.set(target, getResourceToInject(target, requestingBeanName));
}
else {
if (checkPropertySkipping(pvs)) {
return;
}
try {
Method method = (Method) this.member;
ReflectionUtils.makeAccessible(method);
method.invoke(target, getResourceToInject(target, requestingBeanName));
}
catch (InvocationTargetException ex) {
throw ex.getTargetException();
}
}
}
在这里的代码当中我们也可以看到,是inject
也使用了反射技术并且依然是分成字段和方法去处理的。在代码里面也调用了makeAccessible
这样的可以称之为暴力破解的方法,但是反射技术本就是为框架等用途设计的,这也无可厚非。