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在 spring 中，如果一个 bean 的创立过程很简单，咱们能够应用 FactoryBean。
比方像上面的状况，我定义一套 webService 框架。
public interface WebService {void service();
}
public class DefaultWebService implements WebService {
private String serviceName;
public DefaultWebService(String serviceName) {this.serviceName = serviceName;}
@Override
public void service() {System.out.println(serviceName + ": current support service for you....");
}
}
public class WebServiceWrapper implements WebService {
private WebService webService;
public WebServiceWrapper(WebService webService) {this.webService = webService;}
@Override
public void service() {System.out.println("befor service, we need do something....");
webService.service();}
}WebService 接口定义服务的规范，DefaultWebService 是一个默认实现，WebServiceWrapper 的次要作用是在理论服务之前，做一些查看，筹备等工作。
因而当咱们应用 WebService，实际上是心愿应用的是 WebServiceWrapper。
当初 webService 框架实现好了，并打包进去了，当初我须要在我的项目中利用应用 spring 注解个性去应用。
那该怎么办呢？我能够实现 spring 提供的 FactoryBean 接口，就像上面这样：
@Component
public class WebServiceFactoryBean implements FactoryBean {
@Override
public boolean isSingleton() {return true;}
@Override
public Object getObject() throws Exception {return this.createWebService();
}
@Override
public Class<?> getObjectType() {return WebService.class;}
private Object createWebService() {DefaultWebService webService = new DefaultWebService("Backend Service");
// create a proxy
WebServiceWrapper webServiceWrapper = new WebServiceWrapper(webService);
return webServiceWrapper;
}
}紧接着，就能够利用 spring 的性能去应用了
@RestController
public class TestController {
@Autowired
private WebService webService;
@GetMapping(value = "webService")
public void webService() {webService.service();
}
}http://localhost:8080/webService，控制台会输入：
befor service, we need do something….
Backend Service: current support service for you….
通过输入咱们晓得，该 webService，注入的应该是 WebServiceWrapper。
这里的次要疑难是，咱们应用 @Component 托管 spring 的 bean 是 WebServiceFactoryBean。然而最初在 TestController 中，WebServiceWrapper 的相干信息，只有是怎么被注入的呢？也就是 spring 是怎么利用 FactoryBean 来实现特定 bean 工厂的呢？
spring 托管一个 bean 大略的流程是：
bean 信息扫描 -> 实例化 -> 依赖注入 -> 初始化
首先看下 bean 扫描阶段，在所有扫描进去的 BeanDefinition 中，没有发现 WebServiceWrapper 的相干信息，只有 WebServiceFactoryBean 的相干信息。所以 spring 不是在扫描阶段解析出 WebServiceFactoryBean 相干信息的。
再看下 WebServiceFactoryBean 的实例化过程：
public void preInstantiateSingletons() throws BeansException {
// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {if (isFactoryBean(beanName)) {Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
}
else {getBean(beanName);
}
}
}
}这段源码中，有个判断是说如果以后 bean 是 FactoryBean，那么获取该 FactoryBean 自身时，beanName 要加上一个非凡的前缀“&”。因为 WebServiceFactoryBean 是一个 FactoryBean，所以此时调用的是：Object bean = getBean(FACTORY_BEAN_PREFIX + beanName), 入参是：&webServiceFactoryBean
spring 中 getBean 会去调用 doGetBean():
protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
// Create bean instance.
if (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, () -> {
try {return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
});
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
}先创立一个 bean 实例，而后通过 getObjectForBeanInstance()返回一个实例。
这里疑难是，我明明都创立好了一个实例，我间接返回不就好了吗，为什么还要通过 getObjectForBeanInstance()返回呢？上面看看该办法得形容：
/**
获取给定 bean 实例的对象，能够是 bean 实例自身，也能够是 FactoryBean 所创立的对象。*/
protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
// 判断 beanName 是否以 FactoryBean "&" 非凡前缀结尾，如果是以 "&" 结尾
// 阐明以后获取的是 FactoryBean 自身
if (BeanFactoryUtils.isFactoryDereference(name)) {if (beanInstance instanceof NullBean) {return beanInstance;}
if (!(beanInstance instanceof FactoryBean)) {throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
}
if (mbd != null) {mbd.isFactoryBean = true;}
return beanInstance;
}
// 如果该 bean 不是 FactoryBean 那么间接返回
if (!(beanInstance instanceof FactoryBean)) {return beanInstance;}
// 否则就利用 FactoryBean 返回该 bean
Object object = null;
if (mbd != null) {mbd.isFactoryBean = true;}
else {object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}下面的逻辑也是很清晰了，最初看下
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {Object object = doGetObjectFromFactoryBean(factory, beanName);
if (shouldPostProcess) {
try {object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
return object;
}
private Object doGetObjectFromFactoryBean(FactoryBean<?> factory, String beanName) throws BeanCreationException {
Object object;
try {if (System.getSecurityManager() != null) {AccessControlContext acc = getAccessControlContext();
try {object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
}
catch (PrivilegedActionException pae) {throw pae.getException();
}
}
else {**object = factory.getObject();**
}
}
catch (FactoryBeanNotInitializedException ex) {throw new BeanCurrentlyInCreationException(beanName, ex.toString());
}
catch (Throwable ex) {throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
}
// Do not accept a null value for a FactoryBean that's not fully
// initialized yet: Many FactoryBeans just return null then.
if (object == null) {if (isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName, "FactoryBean which is currently in creation returned null from getObject");
}
object = new NullBean();}
return object;
}其实就是调用了 FactoryBean 的 getObject()返回该 bean 的。
通过下面的剖析，WebServiceFactoryBean 是一个 FactoryBean，
getBean(“&webServiceFactoryBean”), 此时 spring 中会创立一个 WebServiceFactoryBean 实例, 而且返回也是 WebServiceFactoryBean 实例自身。因而 spring 容器还利用不到 WebServiceFactoryBean 的 getObject()来返回 WebService 相干实例。
那么 spring 是在什么时候去调用 getBean(“webServiceFactoryBean”)，而后利用 WebServiceFactoryBean 的 getObject()来返回 WebServiceWrapper 的呢？
咱们仅仅晓得的是 TestController 依赖了 WebService，依据后面提到 spring 托管 bean 的流程，TestController 在依赖注入的时候，肯定会找到 WebServiceWrapper！！！上面咱们依据这个思路去 debug 剖析下源码：
上面看下 TestController 的依赖注入阶段：
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreat BeanWrapper instanceWrapper = null;
// Initialize the bean instance.
Object exposedObject = bean;
try {populateBean(beanName, mbd, instanceWrapper);
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
}一个 bean 依赖注入的入口办法就是 populateBean():
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
}
}InstantiationAwareBeanPostProcessor 是一个 bean 的实例化解决接口，
postProcessProperties()次要目标是对实例的属性进行解决。
其中有一个实现类：AutowiredAnnotationBeanPostProcessor 就是用来实现依赖注入的。大略看下该类上的正文和结构器：
/**
主动连贯正文字段、setter 办法和任意配置办法的 BeanPostProcessor 实现。这些要注入的成员是通过正文检测的: 默认状况下，是 Spring 的 @Autowired 和 @Value 正文。还反对 JSR-330 的 @Inject 正文(如果可用的话)，作为 Spring 本人的 @Autowired 的间接代替。*/
public AutowiredAnnotationBeanPostProcessor() {this.autowiredAnnotationTypes.add(Autowired.class);
this.autowiredAnnotationTypes.add(Value.class);
try {this.autowiredAnnotationTypes.add((Class<? extends Annotation>)
ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));
logger.trace("JSR-330'javax.inject.Inject'annotation found and supported for autowiring");
}
catch (ClassNotFoundException ex) {// JSR-330 API not available - simply skip.}
}
其实就是咱们我的项目中应用的 @Autowired，@Value 等注入的实现形式。autowiredAnnotationTypes 指定了反对哪些注解。
上面看下 AutowiredAnnotationBeanPostProcessor 的 postProcessProperties():
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
// 找到须要注入的字段
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
// 对每个字段进行注入
metadata.inject(bean, beanName, pvs);
}
catch (BeanCreationException ex) {throw ex;}
catch (Throwable ex) {throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
}
return pvs;
}
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Collection<InjectedElement> checkedElements = this.checkedElements;
Collection<InjectedElement> elementsToIterate =
(checkedElements != null ? checkedElements : this.injectedElements);
if (!elementsToIterate.isEmpty()) {for (InjectedElement element : elementsToIterate) {
// 对每个依赖字段进行注入
element.inject(target, beanName, pvs);
}
}
}对于 TestController 来说，只有一个 WebService 字段须要注入，因而会只有一个 InjectedElement 要进行 inject。InjectedElement 有两个实现类 AutowiredFieldElement 和 AutowiredMethodElement。因为咱们这里是字段注入，所以看看 AutowiredFieldElement 的 inject();
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
// 以后要注入的字段是什么
Field field = (Field) this.member;
// 给该字段赋值的值到底是什么？Object value;
if (this.cached) {
try {value = resolvedCachedArgument(beanName, this.cachedFieldValue);
}
catch (NoSuchBeanDefinitionException ex) {
// Unexpected removal of target bean for cached argument -> re-resolve
value = resolveFieldValue(field, bean, beanName);
}
}
else {
// 依据该字段信息，解析出 value
value = resolveFieldValue(field, bean, beanName);
}
if (value != null) {
// 如果解析进去的值不为空，那么就进行赋值
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}
}对于 TestController,field 天然是 webService，这里的值，应该是 webService 的一个具体对象，然而显著还没有 cached，因而要进行解析：
private Object resolveFieldValue(Field field, Object bean, @Nullable String beanName) {DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
desc.setContainingClass(bean.getClass());
Set<String> autowiredBeanNames = new LinkedHashSet<>(1);
Assert.state(beanFactory != null, "No BeanFactory available");
TypeConverter typeConverter = beanFactory.getTypeConverter();
Object value;
try {value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
}
return value;
}
public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
if (Optional.class == descriptor.getDependencyType()) {return createOptionalDependency(descriptor, requestingBeanName);
}
else if (ObjectFactory.class == descriptor.getDependencyType() ||
ObjectProvider.class == descriptor.getDependencyType()) {return new DependencyObjectProvider(descriptor, requestingBeanName);
}
else if (javaxInjectProviderClass == descriptor.getDependencyType()) {return new Jsr330Factory().createDependencyProvider(descriptor, requestingBeanName);
}
else {Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(descriptor, requestingBeanName);
if (result == null) {
// 解析 value
result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
}
return result;
}
}
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
// 这里十分要害，实际上就是依据以后 field, 获取它的类型是什么！！！Class<?> type = descriptor.getDependencyType();
// 依据类型去找到对应的候选者
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
}
protected Map<String, Object> findAutowireCandidates(@Nullable String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(this, requiredType, true, descriptor.isEager());
}doResolveDependency()办法中，有个关键点，那就 java 的 Field 提供了以后 Field 的 java 类型是什么。spring 利用了这一点，而后依据类型去找到候选者！！！
此时 type=WebService.class
一路 debug, 在 findAutowireCandidates()办法中，发现 candidateNames 中呈现了，webServiceFactoryBean，也就是说 spring 把 webServiceFactoryBean 当作了一个 WebService. 其实大略测试也晓得是因为 WebServiceFactoryBean 的 getObjectType()返回了 WebService.class 和要依赖注入的字段相匹配。
因而要害的代码是：
public static String[] beanNamesForTypeIncludingAncestors(ListableBeanFactory lbf, Class<?> type, boolean includeNonSingletons, boolean allowEagerInit) {
// 依据 WebService.class 去找到候选者的 bean
String[] result = lbf.getBeanNamesForType(type, includeNonSingletons, allowEagerInit);
}
@Override
public String[] getBeanNamesForType(@Nullable Class<?> type, boolean includeNonSingletons, boolean allowEagerInit) {String[] resolvedBeanNames = cache.get(type);
if (resolvedBeanNames != null) {return resolvedBeanNames;}
resolvedBeanNames = doGetBeanNamesForType(ResolvableType.forRawClass(type), includeNonSingletons, true);
return resolvedBeanNames;
}
private String[] doGetBeanNamesForType(ResolvableType type, boolean includeNonSingletons, boolean allowEagerInit) {List<String> result = new ArrayList<>();
// Check all bean definitions.
for (String beanName : this.beanDefinitionNames) {boolean isFactoryBean = isFactoryBean(beanName, mbd);
BeanDefinitionHolder dbd = mbd.getDecoratedDefinition();
boolean matchFound = false;
boolean allowFactoryBeanInit = (allowEagerInit || containsSingleton(beanName));
boolean isNonLazyDecorated = (dbd != null && !mbd.isLazyInit());
if (!isFactoryBean) {if (includeNonSingletons || isSingleton(beanName, mbd, dbd)) {matchFound = isTypeMatch(beanName, type, allowFactoryBeanInit);
}
}
else {
if (includeNonSingletons || isNonLazyDecorated ||
(allowFactoryBeanInit && isSingleton(beanName, mbd, dbd))) {matchFound = isTypeMatch(beanName, type, allowFactoryBeanInit);
}
if (!matchFound) {
// In case of FactoryBean, try to match FactoryBean instance itself next.
beanName = FACTORY_BEAN_PREFIX + beanName;
if (includeNonSingletons || isSingleton(beanName, mbd, dbd)) {matchFound = isTypeMatch(beanName, type, allowFactoryBeanInit);
}
}
}
if (matchFound) {result.add(beanName);
}
}
}
protected boolean isTypeMatch(String name, ResolvableType typeToMatch, boolean allowFactoryBeanInit)
throws NoSuchBeanDefinitionException {if (beanInstance instanceof FactoryBean) {if (!isFactoryDereference) {Class<?> type = getTypeForFactoryBean((FactoryBean<?>) beanInstance);
return (type != null && typeToMatch.isAssignableFrom(type));
}
else {return typeToMatch.isInstance(beanInstance);
}
}
}
protected Class<?> getTypeForFactoryBean(FactoryBean<?> factoryBean) {
try {if (System.getSecurityManager() != null) {
return AccessController.doPrivileged((PrivilegedAction<Class<?>>) factoryBean::getObjectType, getAccessControlContext());
}
else {return factoryBean.getObjectType();
}
}
catch (Throwable ex) {
// Thrown from the FactoryBean's getObjectType implementation.
logger.info("FactoryBean threw exception from getObjectType, despite the contract saying" +
"that it should return null if the type of its object cannot be determined yet", ex);
return null;
}
}在匹配的过程中，如果一个 bean 是 FactoryBean，那么就去查看它的 getObjectType()返回的类型是否匹配。对于 TestController 来说，它的成员字段 webService 是 WebService 类型，于是 spring 会去容器中找，恰好 WebServiceFactoryBean 的 getObjectType()返回就是 WebService 类型，于是匹配。
在下面 doResolveDependency()中的 findAutowireCandidates()返回了一个 WebServiceFactoryBean 作为匹配的 bean，然而还并没有返回须要注入的 value，上面接着看 doResolveDependency()
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
String autowiredBeanName;
Object instanceCandidate;
String autowiredBeanName;
Object instanceCandidate;
if (matchingBeans.size() > 1) {autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
if (autowiredBeanName == null) {if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {return descriptor.resolveNotUnique(descriptor.getResolvableType(), matchingBeans);
}
else {
// In case of an optional Collection/Map, silently ignore a non-unique case:
// possibly it was meant to be an empty collection of multiple regular beans
// (before 4.3 in particular when we didn't even look for collection beans).
return null;
}
}
instanceCandidate = matchingBeans.get(autowiredBeanName);
}
else {
// We have exactly one match.
Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
autowiredBeanName = entry.getKey();
instanceCandidate = entry.getValue();}
if (autowiredBeanNames != null) {autowiredBeanNames.add(autowiredBeanName);
}
if (instanceCandidate instanceof Class) {instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
}
Object result = instanceCandidate;
return result;
}matchingBeans 目前的只蕴含一个 key-value，其中 key 是 webServiceFactoryBean，value 则是 WebServiceFactoryBean.class。因而真正的 value 是来自于下面的
descriptor.resolveCandidate() 办法：
public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory)
throws BeansException {return beanFactory.getBean(beanName);
}这里留神到到，传入的 beanName 是 webServiceFactoryBean！！！这一下子联想到后面说的，spring 对于获取 FactoryBean 自身必须要对 beanName 加一个非凡的前缀。否则。。。否则就不是返回它本人啦，不必跟踪源码，咱们也可能大略猜得到，对于 FactoryBean 如果 beanName 不加非凡前缀返回的 bean，是调用 FactoryBean 的 getObject()办法返回的！！！联合后面的源码，的确如此。到此 FactoryBean 的实现原理剖析完结。
