关于spring事务源码的一些小理解

1、spring事务的简单概述

由于实现事务功能的方式各不相同，Spring进行了统一的抽象，形成了PlatformTransactionManager事务管理器顶级接口（平台事务管理器），事务的提交、回滚等操作全部交给它来实现

先来看下三大接口

• PlatformTransactionManager ： 事务管理器

• TransactionDefinition ： 事务的一些基础信息，如超时时间、隔离级别、传播属性等

• TransactionStatus ： 事务的一些状态信息，如是否是一个新的事务、是否已被标记为回滚

2 、 PlatformTransactionManager

相关类方法：

public interface PlatformTransactionManager {
  //获取事务状态
   TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException;
   //事务提交
   void commit(TransactionStatus status) throws TransactionException;
   //事务回滚
   void rollback(TransactionStatus status) throws TransactionException;

}复制代码

继承关系：

PlatformTransactionManager

AbstractPlatformTransactionManager
　　DataSourceTransactionManager（重点）
　　HibernateTransactionManager
　　JpaTransactionManager复制代码

触发不同的事物管理器

springboot对 PlatformTransactionManager的默认配置实现（引用 spring-boot-starter-web中jdbc相关jar ）

public class DataSourceTransactionManagerAutoConfiguration {

   @Configuration
   @ConditionalOnSingleCandidate(DataSource.class)
   static class DataSourceTransactionManagerConfiguration {

      private final DataSource dataSource;

      private final TransactionManagerCustomizers transactionManagerCustomizers;

      DataSourceTransactionManagerConfiguration(DataSource dataSource,
            ObjectProvider<TransactionManagerCustomizers> transactionManagerCustomizers) {
         this.dataSource = dataSource;
         this.transactionManagerCustomizers = transactionManagerCustomizers
               .getIfAvailable();
      }

      @Bean
      @ConditionalOnMissingBean(PlatformTransactionManager.class)
      public DataSourceTransactionManager transactionManager(
            DataSourceProperties properties) {
         DataSourceTransactionManager transactionManager = new DataSourceTransactionManager(
               this.dataSource);
         if (this.transactionManagerCustomizers != null) {
            this.transactionManagerCustomizers.customize(transactionManager);
         }
         return transactionManager;
      }

   }

}复制代码

通过 @ConditionalOnMissingBean(PlatformTransactionManager.class)这个注解可以直到如果没有手动配置

PlatformTransactionManager即使用默认的子类 DataSourceTransactionManager来管理实务

3、事务传播特性和隔离级别

public interface TransactionDefinition {
事务传播特性：
1、支持事务，如果当前线程没有事务，新建一个事务
  int PROPAGATION_REQUIRED = 0;
2、支持事务，如果当前线程没有事务，则以非事务执行
  int PROPAGATION_SUPPORTS = 1;
3、 当前如果有事务，Spring就会使用该事务；否则会抛出异常
  int PROPAGATION_MANDATORY = 2;
4、如果当前线程存在事务，或者不存在事务，都会新建一个事务，并且新建事务与当前事务是相互隔离的，如果新建事务执行时，会先将当前事务挂起，等新建事务执行完成后，再将放行当前事务，如果新事物出现异常，会正常回滚，但不会影响当前事务
  int PROPAGATION_REQUIRES_NEW = 3;
5、 不支持事务，如果存在事务，则会将当前事务挂起，以非事务执行
  int PROPAGATION_NOT_SUPPORTED = 4;
6、不支持事务，如果当前线程存在事务，将会抛异常
  int PROPAGATION_NEVER = 5;
7、如果当前线程存在事务，则新建一个回滚点，如果出现异常，则会回滚到上一个回滚点，对于当前事务是不受任何影响的。
  int PROPAGATION_NESTED = 6;
//隔离级别：默认的隔离级别（对 mysql 数据库来说就是ISOLATION_ READ_COMMITTED，可以重复读）
  int ISOLATION_DEFAULT = -1;
//隔离级别：读未提交
  int ISOLATION_READ_UNCOMMITTED = Connection.TRANSACTION_READ_UNCOMMITTED;
//隔离级别：读已提交
  int ISOLATION_READ_COMMITTED = Connection.TRANSACTION_READ_COMMITTED;
//隔离级别：可重复读
  int ISOLATION_REPEATABLE_READ = Connection.TRANSACTION_REPEATABLE_READ;
//隔离级别：序列化  
int ISOLATION_SERIALIZABLE = Connection.TRANSACTION_SERIALIZABLE;
   int TIMEOUT_DEFAULT = -1;
  int getPropagationBehavior();
   int getIsolationLevel();
   int getTimeout();
  boolean isReadOnly();
  @Nullable
  String getName();


}


复制代码

4、为什么说spring事务是aop的一种实现

finishBeanFactoryInitialization(beanFactory);一路向后debug最后可以看到下面的代码
复制代码

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.
   //这里就是获取需要代理的类（如果存在advice）复制代码

//获取bean对应的advice集合。   
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
   if (specificInterceptors != DO_NOT_PROXY) {
      this.advisedBeans.put(cacheKey, Boolean.TRUE);
    //将获取到的advice集合保存到代理对象中并返回。
      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;
}复制代码

这里是创建代理的方法
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);
      }
   }
   //将advice封装到 advisors中
   Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
   proxyFactory.addAdvisors(advisors);
    //这里的targetSource就是目标对象，在后面调用的时候会用到
   proxyFactory.setTargetSource(targetSource);
   customizeProxyFactory(proxyFactory);
   proxyFactory.setFrozen(this.freezeProxy);
   if (advisorsPreFiltered()) {
      proxyFactory.setPreFiltered(true);
   }

   return proxyFactory.getProxy(getProxyClassLoader());
}


specificInterceptors是拦截器（如果是aop则是前置后置这些过滤器，如果是事务，则是事务拦截器）
BeanFactoryCacheOperationSourceAdvisor（保存了增强的信息）切面记录是前置，后置等增强拦截器链而事务相关记录的是事务拦截器private final Map<Object, TransactionAttribute> attributeCache = new ConcurrentHashMap<>(1024);保存方法对应的事务信息private final Map<Object, Boolean> advisedBeans = new ConcurrentHashMap<>(256);保存这个bean是否为增强beanif (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {  return bean;}判断这个bean是否为代理类，如果不是直接返回单例和多例的区别：
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);  }}将获取到的bean保存到singletonObjects中private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);在每次调用的时候，直接从singletonObjects这个ConcurrentHashMap中获取即可。如果是Prototype在从走一遍单例的流程else if (mbd.isPrototype()) {  // It's a prototype -> create a new instance.  Object prototypeInstance = null;  try {     beforePrototypeCreation(beanName);     prototypeInstance = createBean(beanName, mbd, args);  }  finally {     afterPrototypeCreation(beanName);  }  bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);}复制代码

5、事务方法调用

和aop调用一样，最后会调用 TransactionInterceptor的invock方法

public Object invoke(MethodInvocation invocation) throws Throwable {
   // Work out the target class: may be {@code null}.
   // The TransactionAttributeSource should be passed the target class
   // as well as the method, which may be from an interface.
   Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);

   // Adapt to TransactionAspectSupport's invokeWithinTransaction...
   return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
}复制代码

然后会调用父类 TransactionAspectSupport 的 invokeWithinTransaction方法

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

   // If the transaction attribute is null, the method is non-transactional.
   TransactionAttributeSource tas = getTransactionAttributeSource();
   final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
   final PlatformTransactionManager tm = determineTransactionManager(txAttr);
   final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

   if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
      // Standard transaction demarcation with getTransaction and commit/rollback calls.
      TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
      Object retVal = null;
      try {
         // This is an around advice: Invoke the next interceptor in the chain.
         // This will normally result in a target object being invoked.
         retVal = invocation.proceedWithInvocation();
      }
      catch (Throwable ex) {
         // target invocation exception
         completeTransactionAfterThrowing(txInfo, ex);
         throw ex;
      }
      finally {
         cleanupTransactionInfo(txInfo);
      }
      commitTransactionAfterReturning(txInfo);
      return retVal;
   }

   else {
      final ThrowableHolder throwableHolder = new ThrowableHolder();

      // It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
      try {
         Object result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr, status -> {
            TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
            try {
               return invocation.proceedWithInvocation();
            }
            catch (Throwable ex) {
               if (txAttr.rollbackOn(ex)) {
                  // A RuntimeException: will lead to a rollback.
                  if (ex instanceof RuntimeException) {
                     throw (RuntimeException) ex;
                  }
                  else {
                     throw new ThrowableHolderException(ex);
                  }
               }
               else {
                  // A normal return value: will lead to a commit.
                  throwableHolder.throwable = ex;
                  return null;
               }
            }
            finally {
               cleanupTransactionInfo(txInfo);
            }
         });

         // Check result state: It might indicate a Throwable to rethrow.
         if (throwableHolder.throwable != null) {
            throw throwableHolder.throwable;
         }
         return result;
      }
      catch (ThrowableHolderException ex) {
         throw ex.getCause();
      }
      catch (TransactionSystemException ex2) {
         if (throwableHolder.throwable != null) {
            logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
            ex2.initApplicationException(throwableHolder.throwable);
         }
         throw ex2;
      }
      catch (Throwable ex2) {
         if (throwableHolder.throwable != null) {
            logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
         }
         throw ex2;
      }
   }
}复制代码