Spring事务处理原理步骤详解

1、事务处理实现

实现步骤：

* 声明式事务：
*
* 环境搭建：
* 1、导入相关依赖
* 数据源、数据库驱动、Spring-jdbc模块
* 2、配置数据源、JdbcTemplate（Spring提供的简化数据库操作的工具）操作数据
* 3、给方法上标注 @Transactional 表示当前方法是一个事务方法；
* 4、 @EnableTransactionManagement 开启基于注解的事务管理功能；
* @EnableXXX
* 5、配置事务管理器来控制事务;
* @Bean
* public PlatformTransactionManager transactionManager()

代码实现：

@EnableTransactionManagement
@ComponentScan("com.atguigu.tx")
@Configuration
public class TxConfig {

  //数据源
  @Bean
  public DataSource dataSource() throws Exception{
    ComboPooledDataSource dataSource = new ComboPooledDataSource();
    dataSource.setUser("root");
    dataSource.setPassword("123456");
    dataSource.setDriverClass("com.mysql.jdbc.Driver");
    dataSource.setJdbcUrl("jdbc:mysql://localhost:3306/test");
    return dataSource;
  }

  @Bean
  public JdbcTemplate jdbcTemplate() throws Exception{
    //Spring对@Configuration类会特殊处理；给容器中加组件的方法，多次调用都只是从容器中找组件
    JdbcTemplate jdbcTemplate = new JdbcTemplate(dataSource());
    return jdbcTemplate;
  }

  //注册事务管理器在容器中
  @Bean
  public PlatformTransactionManager transactionManager() throws Exception{
    return new DataSourceTransactionManager(dataSource());
  }
}　

2、事务处理原理

原理分析：

* 原理：
* 1）、@EnableTransactionManagement
* 利用TransactionManagementConfigurationSelector给容器中会导入组件
* 导入两个组件
* AutoProxyRegistrar
* ProxyTransactionManagementConfiguration
* 2）、AutoProxyRegistrar：
* 给容器中注册一个 InfrastructureAdvisorAutoProxyCreator 组件；
* InfrastructureAdvisorAutoProxyCreator：？
* 利用后置处理器机制在对象创建以后，包装对象，返回一个代理对象（增强器），代理对象执行方法利用拦截器链进行调用；
*
* 3）、ProxyTransactionManagementConfiguration 做了什么？
* 1、给容器中注册事务增强器；
* 1）、事务增强器要用事务注解的信息，AnnotationTransactionAttributeSource解析事务注解
* 2）、事务拦截器：
* TransactionInterceptor；保存了事务属性信息，事务管理器；
* 他是一个 MethodInterceptor；
* 在目标方法执行的时候；
* 执行拦截器链；
* 事务拦截器：
* 1）、先获取事务相关的属性
* 2）、再获取PlatformTransactionManager，如果事先没有添加指定任何transactionmanger
* 最终会从容器中按照类型获取一个PlatformTransactionManager；
* 3）、执行目标方法
* 如果异常，获取到事务管理器，利用事务管理回滚操作；
* 如果正常，利用事务管理器，提交事务
*
*/

核心代码

1、EnableTransactionManagement注解，注入TransactionManagementConfigurationSelector类

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

2、TransactionManagementConfigurationSelector类，最终会导入AutoProxyRegistrar.class和ProxyTransactionManagementConfiguration.class两个组件。

public class TransactionManagementConfigurationSelector extends AdviceModeImportSelector<EnableTransactionManagement> {

  /**
   * Returns {@link ProxyTransactionManagementConfiguration} or
   * {@code AspectJ(Jta)TransactionManagementConfiguration} for {@code PROXY}
   * and {@code ASPECTJ} values of {@link EnableTransactionManagement#mode()},
   * respectively.
   */
  @Override
  protected String[] selectImports(AdviceMode adviceMode) {
    switch (adviceMode) {
      case PROXY:
        return new String[] {AutoProxyRegistrar.class.getName(),
            ProxyTransactionManagementConfiguration.class.getName()};
      case ASPECTJ:
        return new String[] {determineTransactionAspectClass()};
      default:
        return null;
    }
  }

  private String determineTransactionAspectClass() {
    return (ClassUtils.isPresent("javax.transaction.Transactional", getClass().getClassLoader()) ?
        TransactionManagementConfigUtils.JTA_TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME :
        TransactionManagementConfigUtils.TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME);
  }

}

3、AutoProxyRegistrar类的作用为：

　　给容器中注册一个 InfrastructureAdvisorAutoProxyCreator 组件；

　　最终的目的是：利用后置处理器机制在对象创建以后，包装对象，返回一个代理对象（增强器），代理对象执行方法利用拦截器链进行调用；

@Override
  public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
    boolean candidateFound = false;
    Set<String> annTypes = importingClassMetadata.getAnnotationTypes();
    for (String annType : annTypes) {
      AnnotationAttributes candidate = AnnotationConfigUtils.attributesFor(importingClassMetadata, annType);
      if (candidate == null) {
        continue;
      }
      Object mode = candidate.get("mode");
      Object proxyTargetClass = candidate.get("proxyTargetClass");
      if (mode != null && proxyTargetClass != null && AdviceMode.class == mode.getClass() &&
          Boolean.class == proxyTargetClass.getClass()) {
        candidateFound = true;
        if (mode == AdviceMode.PROXY) {
          AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
          if ((Boolean) proxyTargetClass) {
            AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
            return;
          }
        }
      }
    }
    if (!candidateFound && logger.isInfoEnabled()) {
      String name = getClass().getSimpleName();
      logger.info(String.format("%s was imported but no annotations were found " +
          "having both 'mode' and 'proxyTargetClass' attributes of type " +
          "AdviceMode and boolean respectively. This means that auto proxy " +
          "creator registration and configuration may not have occurred as " +
          "intended, and components may not be proxied as expected. Check to " +
          "ensure that %s has been @Import'ed on the same class where these " +
          "annotations are declared; otherwise remove the import of %s " +
          "altogether.", name, name, name));
    }
  }

InfrastructureAdvisorAutoProxyCreator类的作用与AnnotationAwareAspectJAutoProxyCreator类的作用类似。

@SuppressWarnings("serial")
public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
    implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware {

4、ProxyTransactionManagementConfiguration类

代理事务管理配置类

@Configuration
public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {

  @Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
  @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
  public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() {
    BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
    advisor.setTransactionAttributeSource(transactionAttributeSource());
    advisor.setAdvice(transactionInterceptor());
    if (this.enableTx != null) {
      advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
    }
    return advisor;
  }

  @Bean
  @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
  public TransactionAttributeSource transactionAttributeSource() {
    return new AnnotationTransactionAttributeSource();
  }

  @Bean
  @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
  public TransactionInterceptor transactionInterceptor() {
    TransactionInterceptor interceptor = new TransactionInterceptor();
    interceptor.setTransactionAttributeSource(transactionAttributeSource());
    if (this.txManager != null) {
      interceptor.setTransactionManager(this.txManager);
    }
    return interceptor;
  }

}

TransactionInterceptor类，事务调用：invokeWithinTransaction()方法为最终执行的方法

@Override
  @Nullable
  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);
  }

TransactionAspectSupport类的最终事务方法执行：

@Nullable
  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;
      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;
      }
    }
  }

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