Spring之事务实现原理及其注解@Transactional底层和传播机制原理

系列文章目录

第一章 Spring之AOP-JDK动态代理源码解析
第二章 Spring之事务实现原理及其注解@Transactional底层和传播机制原理


Spring之事务实现原理及其注解@Transactional底层和传播机制原理

  • 系列文章目录
  • 前言
  • 一、@EnableTransactionManagement入口
  • 二、TransactionManagementConfigurationSelector组件
    • 2.1.selectImports扩展点方法
    • 2.2 组件1-AutoProxyRegistrar
      • 2.2.1 InfrastructureAdvisorAutoProxyCreator
    • 2.3 组件2-ProxyTransactionManagementConfiguration
      • 2.3.1 PointcutAdvisor接口实现类
      • 2.3.2 TransactionAttributeSourcePointcut 切点类
  • 三、TransactionInterceptor事务方法过滤
    • 3.1 invokeWithinTransaction方法
    • 3.2 determineTransactionManager推断事务管理器对象
    • 3.3 methodIdentification获取描述符
    • 3.4 createTransactionIfNecessary是否有必要创建事务
    • 3.5 AbstractPlatformTransactionManager#getTransaction获取事务
      • 3.5.1 第一种情况-事务一开始不存在
      • 3.5.2 第二种情况-事务一开始存在
    • 3.6 事务挂起与恢复
  • 四、事务的传播机制
    • 4.1 PROPAGATION_REQUIRED
    • 4.2 PROPAGATION_SUPPORTS
    • 4.3 PROPAGATION_MANDATORY
    • 4.4 PROPAGATION_REQUIRES_NEW
    • 4.5 PROPAGATION_NOT_SUPPORTED
    • 4.6 PROPAGATION_NEVER
    • 4.7 PROPAGATION_NESTED
  • 总结


前言

本文介绍Spring之事务实现原理,以及注解@Transactional底层实现和7种传播传播机制原理


一、@EnableTransactionManagement入口

Spring支持事务功能的入口注解,通过@Import注解向Spring容器导入TransactionManagementConfigurationSelector组件,该注解实现了ImportSelector接口(扩展点)

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
//@Import注解可以为我们容器中导入组件
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {

	//默认为false
	boolean proxyTargetClass() default false;

	//默认为AdviceMode.PROXY
	AdviceMode mode() default AdviceMode.PROXY;

	int order() default Ordered.LOWEST_PRECEDENCE;

}

注:@EnableTransactionManagement和@EnableAspectJAutoProxy需要一起在配置类中被使用,事务是借组AOP切面来进行实现。

二、TransactionManagementConfigurationSelector组件

2.1.selectImports扩展点方法

导入两个组件:AutoProxyRegistrar和ProxyTransactionManagementConfiguration
当在Spring容器中加载bean定义的时候会回调我们的selectImports方法,方法的返回值是我们需要导入类的全类名路径,然后这个类就会被加载到容器中

@Override
	protected String[] selectImports(AdviceMode adviceMode) {
		switch (adviceMode) {
			/**
			 * 容器中导入二个组件
			 * 一个是AutoProxyRegistrar
			 * 一个是ProxyTransactionManagementConfiguration
			 */
			case PROXY:
				return new String[] {AutoProxyRegistrar.class.getName(),
						ProxyTransactionManagementConfiguration.class.getName()};
			case ASPECTJ:
				return new String[] {
						TransactionManagementConfigUtils.TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME};
			default:
				return null;
		}
	}

2.2 组件1-AutoProxyRegistrar

该组件实现了ImportBeanDefinitionRegistrar接口,也是为一个扩展点,核心是向Spring容器注册一个BeanDefinition

public class AutoProxyRegistrar implements ImportBeanDefinitionRegistrar {

	private final Log logger = LogFactory.getLog(getClass());

	@Override
	public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
		boolean candidateFound = false;
		//获取导入类的注解元数据信息
		Set<String> annTypes = importingClassMetadata.getAnnotationTypes();
		for (String annType : annTypes) {
			AnnotationAttributes candidate = AnnotationConfigUtils.attributesFor(importingClassMetadata, annType);
			if (candidate == null) {
				continue;
			}
			//获取mode属性
			Object mode = candidate.get("mode");
			//获取proxyTargetClass属性
			Object proxyTargetClass = candidate.get("proxyTargetClass");
			if (mode != null && proxyTargetClass != null && AdviceMode.class == mode.getClass() &&
					Boolean.class == proxyTargetClass.getClass()) {
				candidateFound = true;
				if (mode == AdviceMode.PROXY) {
					//注册Bean定义
					AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
					if ((Boolean) proxyTargetClass) {
						//修改bean定义,设置proxyTargetClass为True
						AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
						return;
					}
				}
			}
		}
		if (!candidateFound && logger.isWarnEnabled()) {
			String name = getClass().getSimpleName();
			logger.warn(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));
		}
	}

}

注册的Bean定义为InfrastructureAdvisorAutoProxyCreator

public static BeanDefinition registerAutoProxyCreatorIfNecessary(
			BeanDefinitionRegistry registry, @Nullable Object source) {
		return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);
	}

2.2.1 InfrastructureAdvisorAutoProxyCreator

该类重写了筛选合格的advisor方法,当Spring容器中包含的bean定义的角色类型为BeanDefinition.ROLE_INFRASTRUCTURE时,即为合格的Advisor

public class InfrastructureAdvisorAutoProxyCreator extends AbstractAdvisorAutoProxyCreator {

	@Nullable
	private ConfigurableListableBeanFactory beanFactory;


	@Override
	protected void initBeanFactory(ConfigurableListableBeanFactory beanFactory) {
		super.initBeanFactory(beanFactory);
		this.beanFactory = beanFactory;
	}

	/**
	 * 判断事务注解导入的BeanFactoryTransactionAttributeSourceAdvisor是不是我们想要的Advisor
	 */
	@Override
	protected boolean isEligibleAdvisorBean(String beanName) {
		/**
		 * 1.容器中包含了这个bean定义,
		 * 2.并且bean定义的角色BeanDefinition.ROLE_INFRASTRUCTURE
		 * 这几个条件符合即是我们想要查找的advisor
		 */
		return (this.beanFactory != null && this.beanFactory.containsBeanDefinition(beanName) &&
				this.beanFactory.getBeanDefinition(beanName).getRole() == BeanDefinition.ROLE_INFRASTRUCTURE);
	}

}

2.3 组件2-ProxyTransactionManagementConfiguration

注册启用基于代理的注解驱动的事务管理所需的Spring基础设施 bean。
包含三个bean,一个Advisor增强器,一个Advice通知,一个TransactionAttributeSource事务属性源

@Configuration
public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {

	/**
	 * 导入了Advisor接口的实现类BeanFactoryTransactionAttributeSourceAdvisor-关于事务的切面信息
	 * @return
	 */
	@Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() {
		BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
		advisor.setTransactionAttributeSource(transactionAttributeSource());
		//添加定义的Advice通知
		advisor.setAdvice(transactionInterceptor());
		if (this.enableTx != null) {
			advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
		}
		return advisor;
	}

	/**
	 * 事务属性源对象-用于获取事务属性对象
	 * 初始化事务注解解析器:SpringTransactionAnnotationParser
	 * @return
	 */
	@Bean
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public TransactionAttributeSource transactionAttributeSource() {
		return new AnnotationTransactionAttributeSource();
	}

	/**
	 * 用户拦截事务方法执行的,一个Advice通知
	 * @return
	 */
	@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;
	}

}

2.3.1 PointcutAdvisor接口实现类

BeanFactoryTransactionAttributeSourceAdvisor实现类,当AOP匹配到了当前的Adivisor之后,会去创建动态代理,当动态代理对象执行被代理对象的方法时,如果为JDK动态代理会进入invoke方法执行代理逻辑,在代理逻辑中执行获取匹配的advice通知责任链时,会去调用每个匹配的advisor的getPointcut方法,去进行切点匹配

public class BeanFactoryTransactionAttributeSourceAdvisor extends AbstractBeanFactoryPointcutAdvisor {

	@Nullable
	private TransactionAttributeSource transactionAttributeSource;

	/**
	 * 事务属性源切点
	 */
	private final TransactionAttributeSourcePointcut pointcut = new TransactionAttributeSourcePointcut() {
		@Override
		@Nullable
		protected TransactionAttributeSource getTransactionAttributeSource() {
			return transactionAttributeSource;
		}
	};


	/**
	 * Set the transaction attribute source which is used to find transaction
	 * attributes. This should usually be identical to the source reference
	 * set on the transaction interceptor itself.
	 * @see TransactionInterceptor#setTransactionAttributeSource
	 */
	public void setTransactionAttributeSource(TransactionAttributeSource transactionAttributeSource) {
		this.transactionAttributeSource = transactionAttributeSource;
	}

	/**
	 * Set the {@link ClassFilter} to use for this pointcut.
	 * Default is {@link ClassFilter#TRUE}.
	 */
	public void setClassFilter(ClassFilter classFilter) {
		this.pointcut.setClassFilter(classFilter);
	}

	//重写了getPointCut方法,执行事务代理逻辑的时候会获取切点进行匹配
	@Override
	public Pointcut getPointcut() {
		return this.pointcut;
	}

}

2.3.2 TransactionAttributeSourcePointcut 切点类

主要是一个MethodMatcher接口提供的方法匹配方法

abstract class TransactionAttributeSourcePointcut extends StaticMethodMatcherPointcut implements Serializable {
	
	//放置了ClassFilter,用于过滤当前类或者方法是否有注解@Transactionl
	//最终会进入SpringTransactionAnnotationParser方法
	protected TransactionAttributeSourcePointcut() {
		setClassFilter(new TransactionAttributeSourceClassFilter());
	}

	@Override
	public boolean matches(Method method, @Nullable Class<?> targetClass) {
		if (targetClass != null && TransactionalProxy.class.isAssignableFrom(targetClass)) {
			return false;
		}
		/**
		 * 获取@EnableTransactionManagement注解为Spring容器中导入的ProxyTransactionManagementConfiguration
         * 配置类中的TransactionAttributeSource事务注解属性对象
		 */
		TransactionAttributeSource tas = getTransactionAttributeSource();
		// 通过getTransactionAttribute看方法或者类是否有@Transactional注解,tas为AnnotationTransactionAttributeSource实例,调用getTransactionAttribute会调用父类AbstractFallbackTransactionAttributeSource中的方法
		return (tas == null || tas.getTransactionAttribute(method, targetClass) != null);
	}

	@Override
	public boolean equals(Object other) {
		if (this == other) {
			return true;
		}
		if (!(other instanceof TransactionAttributeSourcePointcut)) {
			return false;
		}
		TransactionAttributeSourcePointcut otherPc = (TransactionAttributeSourcePointcut) other;
		return ObjectUtils.nullSafeEquals(getTransactionAttributeSource(), otherPc.getTransactionAttributeSource());
	}

	@Override
	public int hashCode() {
		return TransactionAttributeSourcePointcut.class.hashCode();
	}

	@Override
	public String toString() {
		return getClass().getName() + ": " + getTransactionAttributeSource();
	}


	/**
	 * Obtain the underlying TransactionAttributeSource (may be {@code null}).
	 * To be implemented by subclasses.
	 */
	@Nullable
	protected abstract TransactionAttributeSource getTransactionAttributeSource();

}

AbstractFallbackTransactionAttributeSource#getTransactionAttribute判断方法或者类是否有@Transactional注解,也就是解析@Transactional注解和方法的描述符,并把TransactionAttribute放入缓存

	public TransactionAttribute getTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
		//判断method所在的class 是不是Object类型
		if (method.getDeclaringClass() == Object.class) {
			return null;
		}

		//构建缓存key
		Object cacheKey = getCacheKey(method, targetClass);
		//先去缓存中获取
		TransactionAttribute cached = this.attributeCache.get(cacheKey);
		//缓存中不为空
		if (cached != null) {
			//判断缓存中的对象是不是空事务属性的对象
			if (cached == NULL_TRANSACTION_ATTRIBUTE) {
				return null;
			}
			//不是的话 就进行返回
			else {
				return cached;
			}
		}
		else {
			//查找事务注解
			TransactionAttribute txAttr = computeTransactionAttribute(method, targetClass);
			// 若解析出来的事务注解属性为空
			if (txAttr == null) {
				//往缓存中存放空事务注解属性
				this.attributeCache.put(cacheKey, NULL_TRANSACTION_ATTRIBUTE);
			}
			else {
				//执行方法的描述符 全类名+方法名
				String methodIdentification = ClassUtils.getQualifiedMethodName(method, targetClass);
				//把方法描述设置到事务属性上去
				if (txAttr instanceof DefaultTransactionAttribute) {
					((DefaultTransactionAttribute) txAttr).setDescriptor(methodIdentification);
				}
				if (logger.isDebugEnabled()) {
					logger.debug("Adding transactional method '" + methodIdentification + "' with attribute: " + txAttr);
				}
				//加入到缓存
				this.attributeCache.put(cacheKey, txAttr);
			}
			return txAttr;
		}
	}

三、TransactionInterceptor事务方法过滤

通过上面的源码分析可知,当开启AOP和事务及方法或者类上面存在@Transactional注解会进入该类的方法执行过滤逻辑,即TransactionInterceptor#invoke方法。(AOP的Advice责任链模式执行)

	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);

		// 执行事务逻辑
		return invokeWithinTransaction(invocation.getMethod(), targetClass, new CoroutinesInvocationCallback() {
			@Override
			@Nullable
			public Object proceedWithInvocation() throws Throwable {
				return invocation.proceed();
			}
			@Override
			public Object getTarget() {
				return invocation.getThis();
			}
			@Override
			public Object[] getArguments() {
				return invocation.getArguments();
			}
		});
	}

3.1 invokeWithinTransaction方法

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

		//获取事务属性源对象,在配置类中添加的
		TransactionAttributeSource tas = getTransactionAttributeSource();
		//txAttr获取到的内容是从解析后的缓存中获取
		// 1.获取解析后的事务属性信息
		final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
		// 获取配置的事务管理器对象
		final PlatformTransactionManager tm = determineTransactionManager(txAttr);
		// 2.从tx属性对象中获取出标注了@Transactionl的方法描述符
		final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

		//处理声明式事务
		if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
			//**************核心,有没有必要创建事务*************
			TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);

			Object retVal;
			try {
				//回调目标方法
				retVal = invocation.proceedWithInvocation();
			}
			catch (Throwable ex) {
				//抛出异常进行回滚处理
				completeTransactionAfterThrowing(txInfo, ex);
				throw ex;
			}
			finally {
				//清空我们的线程变量中transactionInfo的值
				cleanupTransactionInfo(txInfo);
			}
			//提交事务
			commitTransactionAfterReturning(txInfo);
			return retVal;
		}
		// 编程式事务:(回调偏向)
		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;
			}
		}
	}

3.2 determineTransactionManager推断事务管理器对象

@Nullable
	protected PlatformTransactionManager determineTransactionManager(@Nullable TransactionAttribute txAttr) {
		// Do not attempt to lookup tx manager if no tx attributes are set
		if (txAttr == null || this.beanFactory == null) {
			return getTransactionManager();
		}
		//通过@Transactionnal("xxx")指定事务管理器
		String qualifier = txAttr.getQualifier(); 
		if (StringUtils.hasText(qualifier)) {
			return determineQualifiedTransactionManager(this.beanFactory, qualifier);
		}
		else if (StringUtils.hasText(this.transactionManagerBeanName)) {
			return determineQualifiedTransactionManager(this.beanFactory, this.transactionManagerBeanName);
		}
		else {
			//配置类中配置的默认的事务管理器
			PlatformTransactionManager defaultTransactionManager = getTransactionManager();
			if (defaultTransactionManager == null) {
				defaultTransactionManager = this.transactionManagerCache.get(DEFAULT_TRANSACTION_MANAGER_KEY);
				if (defaultTransactionManager == null) {
					// 拿到配置类中配置的事务管理器(使用事务的时候都会配的)
					defaultTransactionManager = this.beanFactory.getBean(PlatformTransactionManager.class);
					this.transactionManagerCache.putIfAbsent(
							DEFAULT_TRANSACTION_MANAGER_KEY, defaultTransactionManager);
				}
			}
			return defaultTransactionManager;
		}
	}

3.3 methodIdentification获取描述符

AbstractFallbackTransactionAttributeSource#getTransactionAttribute方法也调用过,此次会从缓存中获取TransactionAttribute

private String methodIdentification(Method method, @Nullable Class<?> targetClass,
			@Nullable TransactionAttribute txAttr) {

		//全类名+方法名
		String methodIdentification = methodIdentification(method, targetClass);
		if (methodIdentification == null) {
			if (txAttr instanceof DefaultTransactionAttribute) {
				methodIdentification = ((DefaultTransactionAttribute) txAttr).getDescriptor();
			}
			if (methodIdentification == null) {
				methodIdentification = ClassUtils.getQualifiedMethodName(method, targetClass);
			}
		}
		return methodIdentification;
	}

3.4 createTransactionIfNecessary是否有必要创建事务

protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm,
			@Nullable TransactionAttribute txAttr, final String joinpointIdentification) {

		// 如果还没有定义名字,把连接点的ID定义成事务的名称
		if (txAttr != null && txAttr.getName() == null) {
			txAttr = new DelegatingTransactionAttribute(txAttr) {
				@Override
				public String getName() {
					return joinpointIdentification;
				}
			};
		}

		TransactionStatus status = null;
		//当txAttr和tm都不等于空的时候,去通过事务管理器tm获取事务
		if (txAttr != null) {
			if (tm != null) {
				//获取一个事务状态
				status = tm.getTransaction(txAttr);
			}
			else {
				if (logger.isDebugEnabled()) {
					logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
							"] because no transaction manager has been configured");
				}
			}
		}
		//把事物状态和事物属性等信息封装成一个TransactionInfo对象
		return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
	}

3.5 AbstractPlatformTransactionManager#getTransaction获取事务

	@Override
	public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
		//尝试获取一个事务对象
		Object transaction = doGetTransaction();

		// Cache debug flag to avoid repeated checks.
		boolean debugEnabled = logger.isDebugEnabled();

		/**
		 * 判断从上一个方法传递进来的事务属性是不是为空
		 */
		if (definition == null) {

			definition = new DefaultTransactionDefinition();
		}

		/**
		 * 判断是不是已经存在了事务对象(事务嵌套)
		 */
		if (isExistingTransaction(transaction)) {
			//处理存在的事务
			return handleExistingTransaction(definition, transaction, debugEnabled);
		}

		//检查事务设置的超时时间
		if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
			throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
		}

		/**
		 * 由于isExistingTransaction(transaction)跳过了这里,说明当前是不存在事务的
		 * 1.PROPAGATION_MANDATORY传播机制,表示必须运行在事务中,若当前没有事务就抛出异常,那么就会抛出异常
		 */
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
			throw new IllegalTransactionStateException(
					"No existing transaction found for transaction marked with propagation 'mandatory'");
		}
		/**
		 * 2.PROPAGATION_REQUIRED 当前存在事务就加入到当前的事务,没有就新开一个
		 * 3.PROPAGATION_REQUIRES_NEW:新开一个事务,若当前存在事务就挂起当前事务
		 * 4.PROPAGATION_NESTED: 
		    表示如果当前正有一个事务在运行中,则该方法应该运行在 一个嵌套的事务中,
		    被嵌套的事务可以独立于封装事务进行提交或者回滚(保存点),
		    如果封装事务不存在,行为就像 PROPAGATION_REQUIRES NEW
		 */
else if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
				def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
				def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
			/**
			 * 挂起当前事务,在这里为啥传入null?
			 * 因为逻辑走到这里了,经过了上面的isExistingTransaction(transaction) 判断当前是不存在事务的
			 * 所有再这里是挂起当前事务传递一个null进去
			 */
			SuspendedResourcesHolder suspendedResources = suspend(null);
			if (debugEnabled) {
				logger.debug("Creating new transaction with name [" + def.getName() + "]: " + def);
			}
			try {
				//开启事务
				return startTransaction(def, transaction, debugEnabled, suspendedResources);
			}
			catch (RuntimeException | Error ex) {
				resume(null, suspendedResources);
				throw ex;
			}
		}
		else {
			// Create "empty" transaction: no actual transaction, but potentially synchronization.
			if (def.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
				logger.warn("Custom isolation level specified but no actual transaction initiated; " +
						"isolation level will effectively be ignored: " + def);
			}
			boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
			return prepareTransactionStatus(def, null, true, newSynchronization, debugEnabled, null);
		}
	}

3.5.1 第一种情况-事务一开始不存在

在UserService的test方法添加了@Transactional注解

@Component
public class UserService {

	@Autowired
	private JdbcTemplate jdbcTemplate;

	@Autowired
	private UserService userService;

	public UserService() {
		System.out.println("UserService created");
	}

	@Transactional
	public void test(){
		jdbcTemplate.execute("insert into student values(2,'1')");
		userService.b();
	}
}

使用AnnotationConfigApplicationContext去获取Bean并调用test方法

public class MainClass {

    public static void main(String[] args) {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(MainConfig.class);
		UserService userService = (UserService) context.getBean("userService");
		userService.test();

    }
}

以上的这种情况,一开始就是不存在事务,该isExistingTransaction(transaction) 判断不通过
doGetTransaction方法获取事务

protected Object doGetTransaction() {
		//创建一个数据源事务对象
		DataSourceTransactionObject txObject = new DataSourceTransactionObject();
		//是否允许当前事务设置保持点
		txObject.setSavepointAllowed(isNestedTransactionAllowed());
		/**
		 * TransactionSynchronizationManager事务同步管理器对象(该类中都是局部线程变量,ThreadLocal实现)
		 * 用来保存当前事务的信息,我们第一次从这里去线程变量中获取 事务连接持有器对象 通过数据源为key去获取
		 * 第一次进来开始事务 我们的事务同步管理器中没有被存放.所以此时获取出来的conHolder为null
		 */
		ConnectionHolder conHolder =
				(ConnectionHolder) TransactionSynchronizationManager.getResource(obtainDataSource());
		//false代表不是新开
		txObject.setConnectionHolder(conHolder, false);
		//返回事务对象
		return txObject;
	}

startTransaction开启事务方法

private TransactionStatus startTransaction(TransactionDefinition definition, Object transaction,
			boolean debugEnabled, @Nullable SuspendedResourcesHolder suspendedResources) {

		boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
		//这其中的true代表为新开事务
		DefaultTransactionStatus status = newTransactionStatus(
				definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
		//
		doBegin(transaction, definition);
		prepareSynchronization(status, definition);
		return status;
	}

doBegin方法获取数据库连接,创建ConnectionHolder放入事务对象,并设置为新建ConnectionHolder标识true

	protected void doBegin(Object transaction, TransactionDefinition definition) {
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
		Connection con = null;

		try {
			//一开始进入为null
			if (!txObject.hasConnectionHolder() ||
					txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
				//获取连接
				Connection newCon = obtainDataSource().getConnection();
				if (logger.isDebugEnabled()) {
					logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
				}
				//创建ConnectionHolder,设置为新建标识,放入事务对象中
				txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
			}
			//将资源标记为与事务同步
			txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
			//从事务中拿出数据库连接
			con = txObject.getConnectionHolder().getConnection();

			Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
			//设置事务隔离级别
			txObject.setPreviousIsolationLevel(previousIsolationLevel);
			txObject.setReadOnly(definition.isReadOnly());

			// Switch to manual commit if necessary. This is very expensive in some JDBC drivers,
			// so we don't want to do it unnecessarily (for example if we've explicitly
			// configured the connection pool to set it already).
			//设置AutoCommit为false,手动提交
			if (con.getAutoCommit()) {
				txObject.setMustRestoreAutoCommit(true);
				if (logger.isDebugEnabled()) {
					logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
				}
				con.setAutoCommit(false);
			}
			//如果isReadOnly为true,就是执行的stmt.executeUpdate("SET TRANSACTION READ ONLY");
			prepareTransactionalConnection(con, definition);
			//激活事务状态
			txObject.getConnectionHolder().setTransactionActive(true);

			//设置超时
			int timeout = determineTimeout(definition);
			if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
				txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
			}

			// Bind the connection holder to the thread.
			//如果为新建ConnectionHolder,将数据源和ConnectionHolder绑定,并存放到ThreadLocal
			if (txObject.isNewConnectionHolder()) {
				TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());
			}
		}

		catch (Throwable ex) {
			//上面执行异常,并且为新建的ConnectionHolder,归还连接并清空事务中的ConnectionHolder
			if (txObject.isNewConnectionHolder()) {
				DataSourceUtils.releaseConnection(con, obtainDataSource());
				txObject.setConnectionHolder(null, false);
			}
			throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
		}
	}

prepareSynchronization根据需要初始化事务同步,其实就是在ThreadLocal中设置一些属性

protected void prepareSynchronization(DefaultTransactionStatus status, TransactionDefinition definition) {
		if (status.isNewSynchronization()) {
			TransactionSynchronizationManager.setActualTransactionActive(status.hasTransaction());
			TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(
					definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT ?
							definition.getIsolationLevel() : null);
			TransactionSynchronizationManager.setCurrentTransactionReadOnly(definition.isReadOnly());
			TransactionSynchronizationManager.setCurrentTransactionName(definition.getName());
			TransactionSynchronizationManager.initSynchronization();
		}
	}

3.5.2 第二种情况-事务一开始存在

在UserService的test方法和b方法添加了@Transactional注解,并且在test方法还调用了b方法,那么事务是存在的

@Component
public class UserService {

	@Autowired
	private JdbcTemplate jdbcTemplate;

	@Autowired
	private UserService userService;

	@Transactional
	public void test(){
		jdbcTemplate.execute("insert into student values(2,'1')");
		userService.b();
	}

	@Transactional
	public void b(){
		jdbcTemplate.execute("insert into student values(1,'1')");
	}
}

使用AnnotationConfigApplicationContext去获取Bean并调用test方法

public class MainClass {

    public static void main(String[] args) {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(MainConfig.class);
		UserService userService = (UserService) context.getBean("userService");
		userService.test();

    }
}

当在test方法里面调用b方法时,又会进入advice事务过滤逻辑,isExistingTransaction(transaction) 判断会通过
handleExistingTransaction方法处理已存在事务的情况

	private TransactionStatus handleExistingTransaction(
			TransactionDefinition definition, Object transaction, boolean debugEnabled)
			throws TransactionException {
		//如果是PROPAGATION_NEVER传播机制,抛异常
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
			throw new IllegalTransactionStateException(
					"Existing transaction found for transaction marked with propagation 'never'");
		}
		//如果是PROPAGATION_NOT_SUPPORTED传播机制,不支持事务
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
			if (debugEnabled) {
				logger.debug("Suspending current transaction");
			}
			//挂起,得到挂起对象,也就是上一个完整的事务对象相关信息,用于之后的恢复
			//1.清空ConnectionHolder
			//2.解绑数据源和ConnectionHolder的关系,也就是从ThreadLocal中移除绑定关系
			Object suspendedResources = suspend(transaction);
			boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
			//设置TransactionStatus的false代表为不是新开事务
			//返回一个新的事务状态对象Status
			return prepareTransactionStatus(
					definition, null, false, newSynchronization, debugEnabled, suspendedResources);
		}
		//如果是PROPAGATION_REQUIRES_NEW传播机制,开启新的事务
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
			if (debugEnabled) {
				logger.debug("Suspending current transaction, creating new transaction with name [" +
						definition.getName() + "]");
			}
			//挂起,得到挂起对象,也就是上一个完整的事务对象相关信息,用于之后的恢复
			SuspendedResourcesHolder suspendedResources = suspend(transaction);
			try {
				//开启新的事务
				return startTransaction(definition, transaction, debugEnabled, suspendedResources);
			}
			catch (RuntimeException | Error beginEx) {
				//恢复给定的事务
				resumeAfterBeginException(transaction, suspendedResources, beginEx);
				throw beginEx;
			}
		}
		//如果是PROPAGATION_NESTED传播机制,嵌套,创建savepoint
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
			//是否允许嵌套事务
			if (!isNestedTransactionAllowed()) {
				throw new NestedTransactionNotSupportedException(
						"Transaction manager does not allow nested transactions by default - " +
						"specify 'nestedTransactionAllowed' property with value 'true'");
			}
			if (debugEnabled) {
				logger.debug("Creating nested transaction with name [" + definition.getName() + "]");
			}
			//是否为嵌套事务使用保存点
			if (useSavepointForNestedTransaction()) {
				// Create savepoint within existing Spring-managed transaction,
				// through the SavepointManager API implemented by TransactionStatus.
				// Usually uses JDBC 3.0 savepoints. Never activates Spring synchronization.
				//使用已存在的事务去创建savepoint
				DefaultTransactionStatus status =
						prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
				status.createAndHoldSavepoint();
				return status;
			}
			else {
			     //JTA不是使用savepoint来实现嵌套事务
				// Nested transaction through nested begin and commit/rollback calls.
				// Usually only for JTA: Spring synchronization might get activated here
				// in case of a pre-existing JTA transaction.
				return startTransaction(definition, transaction, debugEnabled, null);
			}
		}

		// Assumably PROPAGATION_SUPPORTS or PROPAGATION_REQUIRED.
		if (debugEnabled) {
			logger.debug("Participating in existing transaction");
		}
		if (isValidateExistingTransaction()) {
			if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT) {
				Integer currentIsolationLevel = TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();
				if (currentIsolationLevel == null || currentIsolationLevel != definition.getIsolationLevel()) {
					Constants isoConstants = DefaultTransactionDefinition.constants;
					throw new IllegalTransactionStateException("Participating transaction with definition [" +
							definition + "] specifies isolation level which is incompatible with existing transaction: " +
							(currentIsolationLevel != null ?
									isoConstants.toCode(currentIsolationLevel, DefaultTransactionDefinition.PREFIX_ISOLATION) :
									"(unknown)"));
				}
			}
			if (!definition.isReadOnly()) {
				if (TransactionSynchronizationManager.isCurrentTransactionReadOnly()) {
					throw new IllegalTransactionStateException("Participating transaction with definition [" +
							definition + "] is not marked as read-only but existing transaction is");
				}
			}
		}
		boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
		return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
	}

3.6 事务挂起与恢复

createTransactionIfNecessary方法会返回TransactionInfo对象,该对象存在一个指针指向了旧的TransactionInfo对象,即为链表结构,小编觉得有点像是用到了链表实现的栈结构

protected static final class TransactionInfo {

		@Nullable
		private final PlatformTransactionManager transactionManager;

		@Nullable
		private final TransactionAttribute transactionAttribute;

		private final String joinpointIdentification;

		@Nullable
		private TransactionStatus transactionStatus;

		//旧的事务信息对象
		@Nullable
		private TransactionInfo oldTransactionInfo;
}

在createTransactionIfNecessary方法中最后返回prepareTransactionInfo方法执行的结果,该方法

	protected TransactionInfo prepareTransactionInfo(@Nullable PlatformTransactionManager tm,
			@Nullable TransactionAttribute txAttr, String joinpointIdentification,
			@Nullable TransactionStatus status) {

		TransactionInfo txInfo = new TransactionInfo(tm, txAttr, joinpointIdentification);
		if (txAttr != null) {
			// We need a transaction for this method...
			if (logger.isTraceEnabled()) {
				logger.trace("Getting transaction for [" + txInfo.getJoinpointIdentification() + "]");
			}
			// The transaction manager will flag an error if an incompatible tx already exists.
			txInfo.newTransactionStatus(status);
		}
		else {
			// The TransactionInfo.hasTransaction() method will return false. We created it only
			// to preserve the integrity of the ThreadLocal stack maintained in this class.
			if (logger.isTraceEnabled()) {
				logger.trace("No need to create transaction for [" + joinpointIdentification +
						"]: This method is not transactional.");
			}
		}

		// We always bind the TransactionInfo to the thread, even if we didn't create
		// a new transaction here. This guarantees that the TransactionInfo stack
		// will be managed correctly even if no transaction was created by this aspect.
		//此处为关键地方
		txInfo.bindToThread();
		return txInfo;
	}

bindToThread将TransactionInfo和Thread绑定

private void bindToThread() {
			// Expose current TransactionStatus, preserving any existing TransactionStatus
			// for restoration after this transaction is complete.
			//当前txinfo指向ThreadLocal获取到的,类似栈结果,入栈操作
			this.oldTransactionInfo = transactionInfoHolder.get();
			//重新将当前放入ThreadLocal
			transactionInfoHolder.set(this);
		}

再回过头看一开始的代码

//获取txinfo对象
TransactionInfo txInfo = createTransactionIfNecessary(ptm, 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.
				//执行被代理对象的方法
				retVal = invocation.proceedWithInvocation();
			}
			catch (Throwable ex) {
				// target invocation exception
				completeTransactionAfterThrowing(txInfo, ex);
				throw ex;
			}
			finally {
				//此处是关键
				cleanupTransactionInfo(txInfo);
			}

cleanupTransactionInfo方法,重置ThreadLocal中txinfo对象

protected void cleanupTransactionInfo(@Nullable TransactionInfo txInfo) {
		if (txInfo != null) {
			txInfo.restoreThreadLocalStatus();
		}
	}

restoreThreadLocalStatus该方法有点出栈的意思

private void restoreThreadLocalStatus() {
			// Use stack to restore old transaction TransactionInfo.
			// Will be null if none was set.
			//当前txinfo出栈,this.oldTransactionInfo入栈
			transactionInfoHolder.set(this.oldTransactionInfo);
		}

四、事务的传播机制

4.1 PROPAGATION_REQUIRED

如果不存在事务,则创建一个新的,如果存在,就使用当前的事务;也就是说使用的是同一个事务

4.2 PROPAGATION_SUPPORTS

如果不存在事务,则以非事务方式执行,如果存在,就使用当前的事务

4.3 PROPAGATION_MANDATORY

如果当前不存在事务,则抛出异常,如果存在,就使用当前的事务

4.4 PROPAGATION_REQUIRES_NEW

创建一个新事务,如果存在则挂起当前事务,当前事务执行完之后重新恢复旧事务

4.5 PROPAGATION_NOT_SUPPORTED

不支持事务,始终以非事务方式执行

4.6 PROPAGATION_NEVER

不支持事务,如果当前事务存在,则抛出异常

4.7 PROPAGATION_NESTED

如果当前事务存在,则在嵌套事务中执行,JDBC采用创建SavePoint保存点实现嵌套


总结

以上就是今天要讲的内容,本文介绍了Spring事务的实现原理及其注解@Transactional底层和传播机制原理

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