Spring 源码（七）Spring 事务源码解析

标签：事务 return Spring Object springframework bean 源码 org 解析

注册后置处理器开启对事务的支持

@EnableTransactionManagement

@EnableTransactionManagement注解的主要作用是开启对事务的支持，源码如下：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {

  boolean proxyTargetClass() default false;

  AdviceMode mode() default AdviceMode.PROXY;

  int order() default Ordered.LOWEST_PRECEDENCE;

}

这里最核心的是TransactionManagementConfigurationSelector类，这个类主要的作用是通过ImportSelector注册了AutoProxyRegistrar和ProxyTransactionManagementConfiguration2个组件，源码如下：

public class TransactionManagementConfigurationSelector extends AdviceModeImportSelector<EnableTransactionManagement> {

  @Override
  protected String[] selectImports(AdviceMode adviceMode) {
    switch (adviceMode) {
      case PROXY:
        // 注册 InfrastructureAdvisorAutoProxyCreator 后置处理器和事务管理器组件
        return new String[] {AutoProxyRegistrar.class.getName(), ProxyTransactionManagementConfiguration.class.getName()};
      case ASPECTJ:
        return new String[] {TransactionManagementConfigUtils.TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME};
      default:
        return null;
    }
  }
}

AutoProxyRegistrar

AutoProxyRegistrar 的主要作用是将InfrastructureAdvisorAutoProxyCreator后置处理器注册到容器，注册这个后置处理器和上一篇Spring AOP注册AnnotationAwareAspectJAutoProxyCreator后置处理器一样，这里就不在重复说明了。InfrastructureAdvisorAutoProxyCreator 他是实现了BeanPostProcessor 接口的后置处理器，所以所有 Bean 的初始化都会调用其 postProcessAfterInitialization方法，这个方法的实现是在其父类AbstractAutoProxyCreator类中。

ProxyTransactionManagementConfiguration

我们通过ProxyTransactionManagementConfiguration来注册事务管理器组件，这个类本身也是一个配置类。在这个配置类中我们将会注册一下三个组件：

BeanFactoryTransactionAttributeSourceAdvisor：事务增强器，包含了切面组件 TransactionInterceptor和标签解析器TransactionAttributeSource
TransactionAttributeSource：@Transaction注解标签解析器
TransactionInterceptor：保存了事务属性信息，事务管理器；它本身也是一个方法拦截器，在invoke方法中进行了事务的处理。

创建代理Bean

上面我们说了所以所有 Bean 的初始化都会调用其 AbstractAutoProxyCreator#postProcessAfterInitialization方法来完成Bean的增强，我们跟进去可以看到这段代码：

@Override
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
  if (bean != null) {
    Object cacheKey = getCacheKey(bean.getClass(), beanName);
    if (!this.earlyProxyReferences.contains(cacheKey)) {
      return wrapIfNecessary(bean, beanName, cacheKey);
    }
  }
  return bean;
}

可以看到生代理对象是在wrapIfNecessary(bean, beanName, cacheKey);方法中完成的，源码如下：

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
  
  // 如果存在建言那么久创建代理类
  // 获取拦截链
  Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
  if (specificInterceptors != DO_NOT_PROXY) {
    this.advisedBeans.put(cacheKey, Boolean.TRUE);
    // 使用拦截链创建代理对象，对原有的Bean进行增强
    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;
}

找到拦截链的的核心方法是 BeanFactoryAdvisorRetrievalHelper#findAdvisorBeans方法

findAdvisorBeans:67, BeanFactoryAdvisorRetrievalHelper (org.springframework.aop.framework.autoproxy)
findCandidateAdvisors:102, AbstractAdvisorAutoProxyCreator (org.springframework.aop.framework.autoproxy)
findEligibleAdvisors:88, AbstractAdvisorAutoProxyCreator (org.springframework.aop.framework.autoproxy)
getAdvicesAndAdvisorsForBean:70, AbstractAdvisorAutoProxyCreator (org.springframework.aop.framework.autoproxy)
wrapIfNecessary:346, AbstractAutoProxyCreator (org.springframework.aop.framework.autoproxy)
postProcessAfterInitialization:298, AbstractAutoProxyCreator (org.springframework.aop.framework.autoproxy)
applyBeanPostProcessorsAfterInitialization:423, AbstractAutowireCapableBeanFactory (org.springframework.beans.factory.support)
initializeBean:1638, AbstractAutowireCapableBeanFactory (org.springframework.beans.factory.support)
doCreateBean:555, AbstractAutowireCapableBeanFactory (org.springframework.beans.factory.support)
createBean:483, AbstractAutowireCapableBeanFactory (org.springframework.beans.factory.support)
getObject:312, AbstractBeanFactory$1 (org.springframework.beans.factory.support)
getSingleton:230, DefaultSingletonBeanRegistry (org.springframework.beans.factory.support)
doGetBean:308, AbstractBeanFactory (org.springframework.beans.factory.support)
getBean:197, AbstractBeanFactory (org.springframework.beans.factory.support)
preInstantiateSingletons:761, DefaultListableBeanFactory (org.springframework.beans.factory.support)
finishBeanFactoryInitialization:867, AbstractApplicationContext (org.springframework.context.support)
refresh:543, AbstractApplicationContext (org.springframework.context.support)
<init>:84, AnnotationConfigApplicationContext (org.springframework.context.annotation)

源码如下：

public List<Advisor> findAdvisorBeans() {
  // Determine list of advisor bean names, if not cached already.
  String[] advisorNames = null;
  synchronized (this) {
    advisorNames = this.cachedAdvisorBeanNames;
    if (advisorNames == null) {
      // 获取所有增强器的名称
      advisorNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
          this.beanFactory, Advisor.class, true, false);
      this.cachedAdvisorBeanNames = advisorNames;
    }
  }
  if (advisorNames.length == 0) {
    return new LinkedList<Advisor>();
  }

  List<Advisor> advisors = new LinkedList<Advisor>();
  for (String name : advisorNames) {
    if (isEligibleBean(name)) {
      if (this.beanFactory.isCurrentlyInCreation(name)) {
        if (logger.isDebugEnabled()) {
          logger.debug("Skipping currently created advisor '" + name + "'");
        }
      }
      else {
        try {
          // 根据名称增强器
          advisors.add(this.beanFactory.getBean(name, Advisor.class));
        }
        ...
      }
    }
  }
  // 返回拦截链
  return advisors;
}

创建代理Bean的核心流程：
单例Bean初始化完成后执行后置处理器 AbstractAutoProxyCreator#postProcessAfterInitialization方法
在容器中找Advisor类型的所有增强器名称，这就会将与事务相关的增强器BeanFactoryTransactionAttributeSourceAdvisor找出来
根据增强器名称获取对应的实例，并生成拦截链
判断代理类型
根据不同的代理类型和拦截链创建代理对象

执行业务方法进行拦截

前面AOP说过不管理是JdkDynamicAopProxy还是CglibAopProxy代理，他们的执行最终都会去调用MethodInterceptor.invoke()方法，而我们事务对应的方法拦截器是TransactionInterceptor类。也就是说我们对事务的增强起始是在TransactionInterceptor的invoke方法中。源码如下：

@Override
public Object invoke(final MethodInvocation invocation) throws Throwable {
  ...
  return invokeWithinTransaction(invocation.getMethod(), targetClass, new InvocationCallback() {
    @Override
    public Object proceedWithInvocation() throws Throwable {
      return invocation.proceed();
    }
  });
}

protected Object invokeWithinTransaction(Method method, Class<?> targetClass, final InvocationCallback invocation)
      throws Throwable {

  // 获取事务属性
  final TransactionAttribute txAttr = getTransactionAttributeSource().getTransactionAttribute(method, targetClass);
  // 获取事务管理器
  final PlatformTransactionManager tm = determineTransactionManager(txAttr);
  // 构造方法唯一标示
  final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

  // 声明式事务
  if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
    // 创建事务
    TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
    Object retVal = null;
    try {
      // 执行被增强的方法
      retVal = invocation.proceedWithInvocation();
    }
    catch (Throwable ex) {
      // 异常回滚
      completeTransactionAfterThrowing(txInfo, ex);
      throw ex;
    }
    finally {
      // 清除信息
      cleanupTransactionInfo(txInfo);
    }
    // 提交事务
    commitTransactionAfterReturning(txInfo);
    return retVal;
  }
  // 编程式事务
  ...
}

从上面我们的源码可以看出，一个事务处理的标准流程：

createTransactionIfNecessary创建一个事务
invocation.proceedWithInvocation();执行业务方法
completeTransactionAfterThrowing(txInfo, ex);如果遇到异常，事务回滚
commitTransactionAfterReturning(txInfo);如果没有异常就提交事务

在创建，回滚和提交事务方法中还有的很多对嵌套事务的逻辑，比如事务的传递性，事务回滚的条件判断等，这里就不说了，有兴趣自己去跟下源码。

标签：事务,return,Spring,Object,springframework,bean,源码,org,解析
From： https://blog.51cto.com/u_15861563/5823700