此文就Spring 事务实现流程进行源码解析，我们可以借此对Spring框架加深理解，下面以xml形式创建一个事务进行分析。

一、基于xml形式开启Transaction

1. 创建数据库user

/* Navicat Premium Data Transfer Source Server         : win-local Source Server Type    : MySQL Source Server Version : 50737 Source Host           : localhost:3306 Source Schema         : db0 Target Server Type    : MySQL Target Server Version : 50737 File Encoding         : 65001 Date: 24/04/2022 20:27:41*/SET NAMES utf8mb4;SET FOREIGN_KEY_CHECKS = 0;-- ------------------------------ Table structure for user-- ----------------------------DROP TABLE IF EXISTS `user`;CREATE TABLE `user`  (  `id` bigint(20) NOT NULL AUTO_INCREMENT,  `name` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL,  `age` int(11) NULL DEFAULT NULL,  PRIMARY KEY (`id`) USING BTREE) ENGINE = InnoDB AUTO_INCREMENT = 1 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;SET FOREIGN_KEY_CHECKS = 1;

2. 创建一个maven 项目

不用Springboot依赖，需要引入mysql驱动依赖、spring-beans、spring-jdbc、Spring-context依赖

    <dependencies>        <dependency>            <groupId>mysql</groupId>            <artifactId>mysql-connector-java</artifactId>            <version>5.1.46</version>        </dependency>        <dependency>            <groupId>org.springframework</groupId>            <artifactId>spring-beans</artifactId>            <version>5.3.18</version>        </dependency>        <!-- spring jdbc 依赖spring-tx -->        <dependency>            <groupId>org.springframework</groupId>            <artifactId>spring-jdbc</artifactId>            <version>5.3.18</version>        </dependency>        <dependency>            <groupId>org.apache.commons</groupId>            <artifactId>commons-dbcp2</artifactId>            <version>2.7.0</version>        </dependency>        <dependency>            <groupId>org.springframework</groupId>            <artifactId>spring-context</artifactId>            <version>5.3.18</version>        </dependency>        <dependency>            <groupId>junit</groupId>            <artifactId>junit</artifactId>            <version>4.13.2</version>        </dependency>    </dependencies>

3. 通过xml形式配置事务

1) 创建Spring 命名空间

首先在resources目录下创建一个spring.xml文件，Spring框架为了声明自己的Xml规范，在<beans>标签里定义了spring 框架指定模块的协议配置, 我们可以通过Index of /schema 访问spring框架的所有模块包，各模块包含了不同版本的xsd文件。

点击进入context目录，查看xsd文件:

比如我要通过xml的形式配置一个bean, 需要在beans标签中声明 xmln的值为：

如果我想用spring的context模块，那么需要声明

xmlns:context=";

同时在xsi: schemeLocation里添加context的url和spring-contxt.xsd的url:

例如我创建一个能在xml中使用spring-beans模块，spring-txt模块，spring-context模块的配置如下:

<?xml version="1.0" encoding="UTF-8" ?><beans xmlns=";       xmlns:xsi=";       xmlns:context=";       xmlns:tx=";       xsi:schemaLocation="                           ;></beans>

如果没有在beans标签里声明协议，那么在配置bean时会出现找不到指定标签的问题。

2) 开启事务配置

在spring.xml文件中添加配置事务配置,使用 annotation-driven 属性开启事务启动,

    <tx:annotation-driven transaction-manager="transactionManager" proxy-target-class="true" mode="proxy"/>

proxy-target-class默认为false, mode默认模式为proxy，可不用配置，待会从源码角度分析不同模式的事务开启。

接着配置transactionManager, 指定class="org.springframework.jdbc.datasource.DataSourceTransactionManager"

    <tx:annotation-driven transaction-manager="transactionManager" proxy-target-class="true" mode="proxy"/>    <bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">        <property name="dataSource" ref="ds"/>    </bean>

DataSourceTransactionManager里包含了DataSource属性配置:

因此我们需要接着配置数据源bean 别名为

    <bean id="ds" class="org.apache.commons.dbcp2.BasicDataSource" destroy-method="close">        <property name="driverClassName" value="com.mysql.jdbc.Driver"/>        <property name="username" value="root"/>        <property name="password" value="root"/>        <property name="url" value="jdbc:mysql://localhost:3306/db0?useSSL=false"/>        <property name="initialSize" value="5"/>        <property name="maxIdle" value="2"/>        <property name="maxTotal" value="100"/>    </bean>

接着给Service配置一个bean, 引用dataSource数据源。

    <!-- 配置bean,指定数据源-->    <bean id="userService" class="service.UserService">        <property name="dataSource" ref="ds"/>    </bean>

3) 创建UserService类

通过dataSouce bean注入JDBCTemplate， 添加一个update(int id,String name)方法, 类上添加@Transactional(propagation = Propagation.REQUIRED)。

package service;import org.springframework.jdbc.core.JdbcTemplate;import org.springframework.transaction.annotation.Propagation;import org.springframework.transaction.annotation.Transactional;import javax.sql.DataSource;/** * @Desc: * @Author: bingbing * @Date: 2022/4/24 0024 21:39 */@Transactional(propagation = Propagation.REQUIRED)public class UserService {    private JdbcTemplate jdbcTemplate;    public void setDataSource(DataSource dataSource) {        this.jdbcTemplate = new JdbcTemplate(dataSource);    }    public String getUserName(int id) {        return jdbcTemplate.query("select * from db0.user where id= ?", rs -> rs.next() ? rs.getString(2) : "", new Object[]{id});    }    public void updateUser(int id, String name) {        jdbcTemplate.update(" update  user set name =? where id= ?", new Object[]{name, id});//        throw new RuntimeException("error!");    }}

4. 测试事务

使用ClassPathXmlApplicationContext类加载spring.xml文件

import org.junit.Test;import org.springframework.context.ApplicationContext;import org.springframework.context.support.ClassPathXmlApplicationContext;import service.UserService;/** * @Desc: * @Author: bingbing * @Date: 2022/4/24 0024 21:42 */public class UserServiceTests {    @Test    public void testTransaction() {        ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");        UserService userService = context.getBean("userService", UserService.class);        String name = userService.getUserName(1);        System.out.println("名字:"+name);        userService.updateUser(1, "bing");        String updateName = userService.getUserName(1);        System.out.println("更新后的名字:" + updateName);    }}

数据库一条记录:

1) 抛出RuntimeException

update方法里放开//throw new RuntimeException("error!"); 注释，执行后

数据库里的记录没有修改，@Tranasctional注解生效。

2) 注释掉RuntimeException

重新执行后，观察结果

数据库也更新过来了。

前面的篇幅从xml的配置形式解释了Transaction集成过程，为什么要从xml形式入手transaction, 是为了后面阅读Spring-tx源码做准备。

二、事务开启入口TxNamespaceHandler

根据spring.xml文件里配置的tx:annitation-driven 关键字在Spring框架里全局搜索，找到目标类TxNamespaceHandler。位于spring-tx模块中的 org.springframework.transaction.config包下。

/* * Copyright 2002-2012 the original author or authors. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *       * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package org.springframework.transaction.config;import org.w3c.dom.Element;import org.springframework.beans.factory.xml.NamespaceHandlerSupport;/** * {@code NamespaceHandler} allowing for the configuration of * declarative transaction management using either XML or using annotations. * * <p>This namespace handler is the central piece of functionality in the * Spring transaction management facilities and offers two approaches * to declaratively manage transactions. * * <p>One approach uses transaction semantics defined in XML using the * {@code <tx:advice>} elements, the other uses annotations * in combination with the {@code <tx:annotation-driven>} element. * Both approached are detailed to great extent in the Spring reference manual. * * @author Rob Harrop * @author Juergen Hoeller * @since 2.0 */public class TxNamespaceHandler extends NamespaceHandlerSupport {	static final String TRANSACTION_MANAGER_ATTRIBUTE = "transaction-manager";	static final String DEFAULT_TRANSACTION_MANAGER_BEAN_NAME = "transactionManager";	static String getTransactionManagerName(Element element) {		return (element.hasAttribute(TRANSACTION_MANAGER_ATTRIBUTE) ?				element.getAttribute(TRANSACTION_MANAGER_ATTRIBUTE) : DEFAULT_TRANSACTION_MANAGER_BEAN_NAME);	}	@Override	public void init() {		registerBeanDefinitionParser("advice", new TxAdviceBeanDefinitionParser());		// 注册事务		registerBeanDefinitionParser("annotation-driven", new AnnotationDrivenBeanDefinitionParser());		registerBeanDefinitionParser("jta-transaction-manager", new JtaTransactionManagerBeanDefinitionParser());	}}

找到了annotation-driven, 这个地方创建了一个AnnotationDrivenBeanDefinitionParser实例。

AnnotationDrivenBeanDefinitionParser

AnnotationDrivenBeanDefinitionParser 类的作用是解析spring.xml里的配置<tx:annotation-driven>标签，并根据配置的mode选择不同的模式取创建Transaction的整个初始化流程，此处也就是整个架Transaction架构的开始地方。

Spring事务注册的模式为动态代理模式，具体实现有2种: aspectj和proxy，可通过配置来选择使用那种形式的事务注册, 如果不配置mode那么使用默认的proxy形式创建，如果我们要使用aspectj模式开启事务，那么就配置mode="aspectj"。

<tx:annotation-driven transaction-manager="transactionManager" mode="aspectj">

我们可以看到Spring事务的开启是默认是以AOP为基础的。

三、AOP驱动事务

AopAutoProxyConfigurer 的configureAutoProxyCreator方法注册了3个Bean， 该3个Bean 是驱动Spring 事务架构的核心支柱，分别是TransactionAttributeSource、TransactionInterceptor、TransactionAttributeSourceAdvisor。

	private static class AopAutoProxyConfigurer {		public static void configureAutoProxyCreator(Element element, ParserContext parserContext) {			AopNamespaceUtils.registerAutoProxyCreatorIfNecessary(parserContext, element);			String txAdvisorBeanName = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME;			if (!parserContext.getRegistry().containsBeanDefinition(txAdvisorBeanName)) {				Object eleSource = parserContext.extractSource(element);				// Create the TransactionAttributeSource definition.				RootBeanDefinition sourceDef = new RootBeanDefinition(						"org.springframework.transaction.annotation.AnnotationTransactionAttributeSource");				sourceDef.setSource(eleSource);				sourceDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);				String sourceName = parserContext.getReaderContext().registerWithGeneratedName(sourceDef);				// Create the TransactionInterceptor definition.				RootBeanDefinition interceptorDef = new RootBeanDefinition(TransactionInterceptor.class);				interceptorDef.setSource(eleSource);				interceptorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);				registerTransactionManager(element, interceptorDef);				interceptorDef.getPropertyValues().add("transactionAttributeSource", new RuntimeBeanReference(sourceName));				String interceptorName = parserContext.getReaderContext().registerWithGeneratedName(interceptorDef);				// Create the TransactionAttributeSourceAdvisor definition.				RootBeanDefinition advisorDef = new RootBeanDefinition(BeanFactoryTransactionAttributeSourceAdvisor.class);				advisorDef.setSource(eleSource);				advisorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);				advisorDef.getPropertyValues().add("transactionAttributeSource", new RuntimeBeanReference(sourceName));				advisorDef.getPropertyValues().add("adviceBeanName", interceptorName);				if (element.hasAttribute("order")) {					advisorDef.getPropertyValues().add("order", element.getAttribute("order"));				}				// 事务通知器 transaction advisor, 基于AOP实现的advisor				parserContext.getRegistry().registerBeanDefinition(txAdvisorBeanName, advisorDef);				CompositeComponentDefinition compositeDef = new CompositeComponentDefinition(element.getTagName(), eleSource);				compositeDef.addNestedComponent(new BeanComponentDefinition(sourceDef, sourceName));				compositeDef.addNestedComponent(new BeanComponentDefinition(interceptorDef, interceptorName));				compositeDef.addNestedComponent(new BeanComponentDefinition(advisorDef, txAdvisorBeanName));				parserContext.registerComponent(compositeDef);			}		}	}

其中TransactionInterceptor是Spring事务的目标方法的增强，通过代理完成Spring 事务的提交、异常处理和回滚。

TransactionInterceptor

TransactionInterceptor是Spring 事务对目标方法的增强器，说简单点就是一层代理，基于Aop实现，实现了spring-aop的Advice接口，同时实现了IntializingBean和BeanFactoryAware接口，只要有事务的执行，那么目标方法的调用类在invoke()方法会生成一个代理对象，通过invoke()方法对目标调用方法进行增强。

@FunctionalInterfacepublic interface MethodInterceptor extends Interceptor {		/**	 * Implement this method to perform extra treatments before and	 * after the invocation. Polite implementations would certainly	 * like to invoke {@link Joinpoint#proceed()}.	 * @param invocation the method invocation joinpoint	 * @return the result of the call to {@link Joinpoint#proceed()};	 * might be intercepted by the interceptor	 * @throws Throwable if the interceptors or the target object	 * throws an exception	 */	Object invoke(MethodInvocation invocation) throws Throwable;}

TransactionInterceptor的invoke()实现:

	@Override	@Nullable	public Object invoke(final 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);	}

创建事务

Spring 创建事务的方式有二种: 声明式事务和编程式事务， 我们可以通过分析一种理解核心流程和原理即可。

进入invokeWithinTransaction()方法直接看声明式事务执行过程的源代码：

	if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {			// Standard transaction demarcation with getTransaction and commit/rollback calls.			// 创建事务			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				// 根据指定异常进行回滚。				completeTransactionAfterThrowing(txInfo, ex);				throw ex;			} finally {				cleanupTransactionInfo(txInfo);			}			// 提交事务			commitTransactionAfterReturning(txInfo);			return retVal;		}

invokeWithinTransaction方法做了哪些事?

1) 通过createTransactionIfNecessary方法创建一个事务，相当于此处开启一个事务。

2) invocation.proceedWithInvocation() 执行目标方法调用。

3) 如果出现异常，那么completeTransactionAfterThrowing处理异常。

4) 在finally 清除掉transaction相关的信息，同时在commitTransactionAfterReturning 提交事务。

回滚事务

如果出现异常，我们可以从上面的源码中发现通过transactionManager执行回滚操作。

        txInfo.getTransactionManager().rollback(txInfo.getTransactionStatus());