1、spring事務的簡單概述

由於實現事務功能的方式各不相同，Spring進行了統一的抽象，形成了PlatformTransactionManager事務管理器頂級介面（平臺事務管理器），事務的提交、回滾等操作全部交給它來實現

先來看下三大介面

2、PlatformTransactionManager

相關類方法：

public interface PlatformTransactionManager {
  //獲取事務狀態
   TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException;
   //事務提交
   void commit(TransactionStatus status) throws TransactionException;
   //事務回滾
   void rollback(TransactionStatus status) throws TransactionException;

}複製程式碼
 

繼承關係：

PlatformTransactionManager

AbstractPlatformTransactionManager
　　DataSourceTransactionManager（重點）
　　HibernateTransactionManager
　　JpaTransactionManager複製程式碼

觸發不同的事物管理器

springboot對PlatformTransactionManager的預設配置實現（引用spring-boot-starter-web中jdbc相關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;
      }

   }

}複製程式碼

通過@ConditionalOnMissingBean(PlatformTransactionManager.class)這個註解可以直到如果沒有手動配置

PlatformTransactionManager即使用預設的子類DataSourceTransactionManager來管理實務

3、事務傳播特性和隔離級別

public interface TransactionDefinition {
事務傳播特性：
1、支援事務，如果當前執行緒沒有事務，新建一個事務
  int PROPAGATION_REQUIRED = 0;
2、支援事務，如果當前執行緒沒有事務，則以非事務執行
  int PROPAGATION_SUPPORTS = 1;
3、 當前如果有事務，Spring就會使用該事務；否則會丟擲異常
  int PROPAGATION_MANDATORY = 2;
4、如果當前執行緒存在事務，或者不存在事務，都會新建一個事務，並且新建事務與當前事務是相互隔離的，如果新建事務執行時，會先將當前事務掛起，等新建事務執行完成後，再將放行當前事務，如果新事物出現異常，會正常回滾，但不會影響當前事務
  int PROPAGATION_REQUIRES_NEW = 3;
5、 不支援事務，如果存在事務，則會將當前事務掛起，以非事務執行
  int PROPAGATION_NOT_SUPPORTED = 4;
6、不支援事務，如果當前執行緒存在事務，將會拋異常
  int PROPAGATION_NEVER = 5;
7、如果當前執行緒存在事務，則新建一個回滾點，如果出現異常，則會回滾到上一個回滾點，對於當前事務是不受任何影響的。
  int PROPAGATION_NESTED = 6;
//隔離級別：預設的隔離級別（對mysql資料庫來說就是ISOLATION_ READ_COMMITTED，可以重複讀）
  int ISOLATION_DEFAULT = -1;
//隔離級別：讀未提交
  int ISOLATION_READ_UNCOMMITTED = Connection.TRANSACTION_READ_UNCOMMITTED;
//隔離級別：讀已提交
  int ISOLATION_READ_COMMITTED = Connection.TRANSACTION_READ_COMMITTED;
//隔離級別：可重複讀
  int ISOLATION_REPEATABLE_READ = Connection.TRANSACTION_REPEATABLE_READ;
//隔離級別：序列化  
int ISOLATION_SERIALIZABLE = Connection.TRANSACTION_SERIALIZABLE;
   int TIMEOUT_DEFAULT = -1;
  int getPropagationBehavior();
   int getIsolationLevel();
   int getTimeout();
  boolean isReadOnly();
  @Nullable
  String getName();


}


複製程式碼

4、為什麼說spring事務是aop的一種實現

finishBeanFactoryInitialization(beanFactory);一路向後debug最後可以看到下面的程式碼
複製程式碼

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.
   //這裡就是獲取需要代理的類（如果存在advice）複製程式碼

//獲取bean對應的advice集合。   
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
   if (specificInterceptors != DO_NOT_PROXY) {
      this.advisedBeans.put(cacheKey, Boolean.TRUE);
    //將獲取到的advice集合儲存到代理物件中並返回。
      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;
}複製程式碼

這裡是建立代理的方法
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);
      }
   }
   //將advice封裝到 advisors中
   Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
   proxyFactory.addAdvisors(advisors);
    //這裡的targetSource就是目標物件，在後面呼叫的時候會用到
   proxyFactory.setTargetSource(targetSource);
   customizeProxyFactory(proxyFactory);
   proxyFactory.setFrozen(this.freezeProxy);
   if (advisorsPreFiltered()) {
      proxyFactory.setPreFiltered(true);
   }

   return proxyFactory.getProxy(getProxyClassLoader());
}


specificInterceptors是攔截器（如果是aop則是前置後置這些過濾器，如果是事務，則是事務攔截器）
BeanFactoryCacheOperationSourceAdvisor（儲存了增強的資訊）切面記錄是前置，後置等增強攔截器鏈而事務相關記錄的是事務攔截器private final Map<Object, TransactionAttribute> attributeCache = new ConcurrentHashMap<>(1024);儲存方法對應的事務資訊private final Map<Object, Boolean> advisedBeans = new ConcurrentHashMap<>(256);儲存這個bean是否為增強beanif (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {  return bean;}判斷這個bean是否為代理類，如果不是直接返回單例和多例的區別：
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);  }}將獲取到的bean儲存到singletonObjects中private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);在每次呼叫的時候，直接從singletonObjects這個ConcurrentHashMap中獲取即可。如果是Prototype在從走一遍單例的流程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);}複製程式碼

5、事務方法呼叫

和aop呼叫一樣，最後會呼叫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);
}複製程式碼

然後會呼叫父類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;
      }
   }
}複製程式碼