关于spring事务源码的一些小理解

时间 2019-12-10

原文原文链接

一、spring事务的简单概述

因为实现事务功能的方式各不相同，Spring进行了统一的抽象，造成了PlatformTransactionManager事务管理器顶级接口（平台事务管理器），事务的提交、回滚等操做所有交给它来实现mysql

先来看下三大接口web

PlatformTransactionManager ：事务管理器spring

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

TransactionStatus ：事务的一些状态信息，如是不是一个新的事务、是否已被标记为回滚数据库

2、PlatformTransactionManager

三、事务传播特性和隔离级别

public interface TransactionDefinition {
事务传播特性：
一、支持事务，若是当前线程没有事务，新建一个事务
  int PROPAGATION_REQUIRED = 0;
二、支持事务，若是当前线程没有事务，则以非事务执行
  int PROPAGATION_SUPPORTS = 1;
三、 当前若是有事务，Spring就会使用该事务；不然会抛出异常
  int PROPAGATION_MANDATORY = 2;
四、若是当前线程存在事务，或者不存在事务，都会新建一个事务，而且新建事务与当前事务是相互隔离的，若是新建事务执行时，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，若是新事物出现异常，会正常回滚，但不会影响当前事务
  int PROPAGATION_REQUIRES_NEW = 3;
五、 不支持事务，若是存在事务，则会将当前事务挂起，以非事务执行
  int PROPAGATION_NOT_SUPPORTED = 4;
六、不支持事务，若是当前线程存在事务，将会抛异常
  int PROPAGATION_NEVER = 5;
七、若是当前线程存在事务，则新建一个回滚点，若是出现异常，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数据库来讲就是ISOLATION_ READ_COMMITTED，能够重复读）
  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();


}


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四、为何说spring事务是aop的一种实现

finishBeanFactoryInitialization(beanFactory);一路向后debug最后能够看到下面的代码
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.
   //这里就是获取须要代理的类（若是存在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);
      }
   }
   //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（若是是aop则是前置后置这些过滤器，若是是事务，则是事务拦截器）
BeanFactoryCacheOperationSourceAdvisor（保存了加强的信息）切面记录是前置，后置等加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是否为加强beanif (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {  return bean;}判断这个bean是否为代理类，若是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中获取便可。若是是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

五、事务方法调用

和aop调用同样，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

而后会调用父类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