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這篇文章主要介紹spring aop底層原理及實現方法，文中介紹的非常詳細，具有一定的參考價值，感興趣的小伙伴們一定要看完！
要分析spring aop的底層原理，首先要會使用，先創建一個普通maven webapp項目，引入spring-context依賴，版本為5.1.1RELEASE
<dependency> <groupId>org.springframework</groupId> <artifactId>spring-context</artifactId> <version>5.1.1.RELEASE</version> </dependency>
然后我使用aspectj作為aop的語法實現，和spring整合起來
<dependency> <groupId>org.aspectj</groupId> <artifactId>aspectjweaver</artifactId> <version>1.9.4</version> </dependency>
接下來我全稱用注解的形式來模擬spring aop的使用，先創建一個配置類去掃描包，開啟aspectJ的自動代理支持
@Configuration
@EnableAspectJAutoProxy
@ComponentScan("com.ww")
public class Wconfig {
}然后新建一個接口和接口的實現類
public interface Dao {
void query();
}
@Component
public class IndexDao implements Dao{
@Override
public void query() {
System.out.println("query......");
}
}創建切面
//代表是一個切面
@Aspect
@Component
public class WAspect {
/**
* execution表達式，可以百度寫法
*/
@Pointcut("execution(* com.ww.dao.*.*(..))")
public void point(){
}
/**
* 在切點上進行前置通知
*/
@Before("point()")
public void beforeAd(){
System.out.println("before-------------");
}
}創建測試方法
public class TestAspect {
public static void main(String[] args) {
AnnotationConfigApplicationContext configApplicationContext = new AnnotationConfigApplicationContext(Wconfig.class);
Dao dao = configApplicationContext.getBean(Dao.class);
dao.query();
}
}執行方法，可以看到在打印query...之前打印了before----------
這個時候我們很想知道為什么這句before會打印在query之前呢，稍微對spring aop有所了解的人應該知道，spring是通過動態代理和字節碼技術來實現aop操作的，也就是經常說的jdk動態代理和cglib動態代理兩種模式，那么，spring究竟是怎么創建的代理對象，又是什么時候產生的代理對象呢，下面我們來一起探究一下源碼，來揭開這個謎底
首先我們透過現象看本質，我先把斷點打在測試方法的最后一行，我們來看這個時候的dao對象
那么接下來我們就要去找到什么時候這個dao對象變成了動態代理對象的，既然在最后一行的時候對象已經變成了代理對象，那么我門自然就猜想是在上一句代碼的位置spring執行了創建代理對象的操作，我們把斷點移到上一行，debug進去
再往下走
這行代碼我看方法名覺得應該是有用的代碼，方法意思應該是spring處理好的bean，跟進去看看
@Nullable
private <T> T resolveBean(ResolvableType requiredType, @Nullable Object[] args, boolean nonUniqueAsNull) {
//這行代碼最有用，處理有beanName的bean，debug進入看一下
NamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, args, nonUniqueAsNull);
if (namedBean != null) {
return namedBean.getBeanInstance();
}
BeanFactory parent = getParentBeanFactory();
if (parent instanceof DefaultListableBeanFactory) {
return ((DefaultListableBeanFactory) parent).resolveBean(requiredType, args, nonUniqueAsNull);
}
else if (parent != null) {
ObjectProvider<T> parentProvider = parent.getBeanProvider(requiredType);
if (args != null) {
return parentProvider.getObject(args);
}
else {
return (nonUniqueAsNull ? parentProvider.getIfUnique() : parentProvider.getIfAvailable());
}
}
return null;
}@SuppressWarnings("unchecked")
@Nullable
private <T> NamedBeanHolder<T> resolveNamedBean(
ResolvableType requiredType, @Nullable Object[] args, boolean nonUniqueAsNull) throws BeansException {
Assert.notNull(requiredType, "Required type must not be null");
Class<?> clazz = requiredType.getRawClass();
Assert.notNull(clazz, "Required type must have a raw Class");
//候選name列表，直覺告訴我這行代碼比較重要
String[] candidateNames = getBeanNamesForType(requiredType);
if (candidateNames.length > 1) {
List<String> autowireCandidates = new ArrayList<>(candidateNames.length);
for (String beanName : candidateNames) {
if (!containsBeanDefinition(beanName) || getBeanDefinition(beanName).isAutowireCandidate()) {
autowireCandidates.add(beanName);
}
}
if (!autowireCandidates.isEmpty()) {
candidateNames = StringUtils.toStringArray(autowireCandidates);
}
}
//因為我的代碼里只有一個bean，所以我覺得應該會執行這里的代碼
if (candidateNames.length == 1) {
String beanName = candidateNames[0];
return new NamedBeanHolder<>(beanName, (T) getBean(beanName, clazz, args));
}
else if (candidateNames.length > 1) {
Map<String, Object> candidates = new LinkedHashMap<>(candidateNames.length);
for (String beanName : candidateNames) {
if (containsSingleton(beanName) && args == null) {
Object beanInstance = getBean(beanName);
candidates.put(beanName, (beanInstance instanceof NullBean ? null : beanInstance));
}
else {
candidates.put(beanName, getType(beanName));
}
}
String candidateName = determinePrimaryCandidate(candidates, clazz);
if (candidateName == null) {
candidateName = determineHighestPriorityCandidate(candidates, clazz);
}
if (candidateName != null) {
Object beanInstance = candidates.get(candidateName);
if (beanInstance == null || beanInstance instanceof Class) {
beanInstance = getBean(candidateName, clazz, args);
}
return new NamedBeanHolder<>(candidateName, (T) beanInstance);
}
if (!nonUniqueAsNull) {
throw new NoUniqueBeanDefinitionException(requiredType, candidates.keySet());
}
}
return null;
}執行完getBeanNamesForType(requiredType)后，我們看idea的變量顯示，果然有一個bean，name是IndexDao
那么接下來自然會進到length==1的那個代碼塊，這時候我再debug進入，這里還是一個getBean方法
在spring容器中還有一些沒有name的其他的bean需要被創建，所以這里我用上了條件斷點，當beanName等于indexDao的時候，才會進入斷點，但是當我F8跑完這行代碼的時候，出乎意料的事情發生了
驚不驚喜，意不意外，getSingleton這行代碼執行結束之后，代理對象就已經被創建了，所以需要debug進入這行代碼去看
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//spring所有的bean被放在ioc容器中的地方，就是這個singletonObjects，這是一個concorrentHashMap。
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}但是我在這里只看到了get方法，那么這些bean是什么時候放到singletonObjects里的呢，我來找找
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
this.singletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}在DefaultSingletonBeanRegistry注冊器中，我找到了singletonObjects.put方法，代表bean是這個時候被放到這個map里去的，接下來我在這行代碼上進行條件斷點，然后我們來看它的調用鏈，找出是什么時候執行的addSingleton方法，其實從這個時候我已經知道，斷點打在測試方法的倒數第二行是不對的，在getBean之前其實代理對象已經產生了
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
//創建bean，核心代碼
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;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}在createBean方法上，我也加上條件斷點，然后debug進入
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
if (logger.isTraceEnabled()) {
logger.trace("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
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);
}
try {
//核心代碼，創建bean實例，需要斷點進入
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}接下來我debug進入doCreateBean方法
debug跟進initializeBean方法，條件斷點在兩個初始化處理器上，我隱約覺得代理對象就是從這兩個方法中產生的，我們拭目以待
protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareMethods(beanName, bean);
return null;
}, getAccessControlContext());
}
else {
invokeAwareMethods(beanName, bean);
}
//包裝的bean
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
//執行前置初始化beanPostProcessor處理器
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
invokeInitMethods(beanName, wrappedBean, mbd);
}
catch (Throwable ex) {
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
}
if (mbd == null || !mbd.isSynthetic()) {
//執行初始化后的beanPostProcessor處理器
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}執行完applyBeanPostProcessorsBeforeInitialization方法，這個時候我們看到warppedBean還是indexDao，并沒有產生代理對象
我猜想在下一個后置處理器中，代理對象將被創建，我debug進去
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
for (BeanPostProcessor processor : getBeanPostProcessors()) {
//經過處理器處理后的bean，我先看看有多少個處理器
Object current = processor.postProcessAfterInitialization(result, beanName);
if (current == null) {
return result;
}
result = current;
}
return result;
}
可以看到我的猜想被證明是正確的，運行完這個后置處理器，代理對象就被創建出來了。 到了這里我們知道了代理對象是從哪里來的了，但是還是沒搞清楚代理對象是怎么創建出來的，這時候我們就需要debug進入到這個處理器內部去瞧瞧了。
@Override
public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
if (bean != null) {
//獲取緩存key,不重要
Object cacheKey = getCacheKey(bean.getClass(), beanName);
if (!this.earlyProxyReferences.contains(cacheKey)) {
//重要方法，需要debug進去看
return wrapIfNecessary(bean, beanName, cacheKey);
}
}
return bean;
}于是乎我又進到了wrapIfNecessary這個方法內部
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
return bean;
}
if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
return bean;
}
if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
// Create proxy if we have advice.
//看到上面這行注釋，可以確定代理類就是從這里產生的，下面這個方法得到的都是一些包括切點信息，通知類型等等的信息
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
if (specificInterceptors != DO_NOT_PROXY) {
this.advisedBeans.put(cacheKey, Boolean.TRUE);
//顧名思義，創建代理，bebug進入看看
Object proxy = createProxy(
bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
this.proxyTypes.put(cacheKey, proxy.getClass());
return proxy;
}
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}protected Object createProxy(Class<?> beanClass, @Nullable String beanName,
@Nullable Object[] specificInterceptors, TargetSource targetSource) {
if (this.beanFactory instanceof ConfigurableListableBeanFactory) {
AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);
}
ProxyFactory proxyFactory = new ProxyFactory();
proxyFactory.copyFrom(this);
if (!proxyFactory.isProxyTargetClass()) {
if (shouldProxyTargetClass(beanClass, beanName)) {
proxyFactory.setProxyTargetClass(true);
}
else {
evaluateProxyInterfaces(beanClass, proxyFactory);
}
}
Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
proxyFactory.addAdvisors(advisors);
proxyFactory.setTargetSource(targetSource);
customizeProxyFactory(proxyFactory);
proxyFactory.setFrozen(this.freezeProxy);
if (advisorsPreFiltered()) {
proxyFactory.setPreFiltered(true);
}
//重要的地方在這里，代理對象是通過這個方法生成的
return proxyFactory.getProxy(getProxyClassLoader());
}
public Object getProxy(@Nullable ClassLoader classLoader) {
//debug進去看看
return createAopProxy().getProxy(classLoader);
}protected final synchronized AopProxy createAopProxy() {
if (!this.active) {
activate();
}
//同樣需要debug進入看看
return getAopProxyFactory().createAopProxy(this);
}我們看到這里有一個if語句，當config中的isOptimize和isProxyTargetClass還有hasNoUserSuppliedProxyInterfaces三個判斷條件只要有一個滿足的話，spring就會選擇cglib的方式進行動態代理，而config中的兩個boolean變量的默認值都是false，而我們的被代理對象又是實現接口的，所以spring會選擇jdk動態代理的實現形式來完成動態代理，當然，我們也可以在這種情況下手動的配置config值來讓spring選擇cglib作為動態代理的實現方式，稍后我會演示
@Override
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {
Class<?> targetClass = config.getTargetClass();
if (targetClass == null) {
throw new AopConfigException("TargetSource cannot determine target class: " +
"Either an interface or a target is required for proxy creation.");
}
if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
return new JdkDynamicAopProxy(config);
}
return new ObjenesisCglibAopProxy(config);
}
else {
//現在知道為什么我們的代理類是動態代理了嗎
return new JdkDynamicAopProxy(config);
}
}
我以spring aop實現的調用鏈圖來結束這次的總結
以上是“spring aop底層原理及實現方法”這篇文章的所有內容，感謝各位的閱讀！希望分享的內容對大家有幫助，更多相關知識，歡迎關注億速云行業資訊頻道！
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