注：要了解spring事务最好先去了解spring aop，可以参考十一、spring aop之简单使用
简单使用
@Configuration
//1. 开启事务
@EnableTransactionManagement
public class RootConfig {
//2. 定义数据源
@Bean
public DataSource dataSource() {
DruidDataSource dataSource = new DruidDataSource();
return dataSource;
}
//3. 定义事务管理器
@Bean
public PlatformTransactionManager txManager() {
return new DataSourceTransactionManager(dataSource());
}
}
@Service
public class UserService {
@Autowired
private UserMapper userMapper;
//4.声明事务
@Transactional
public void addUser(User user) {
userMapper.addUser(user);
}
}
要使用Spring事务，通常会有以下步骤：
- 在配置文件上声明@EnableTransactionManagement注解开启事务
- 定义数据源
- 定义事务管理器
- @Transactional声明事务
原理解析
@EnableTransactionManagement是开启spring事务，@EnableTransactionManagement源码如下：
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {
boolean proxyTargetClass() default false;
AdviceMode mode() default AdviceMode.PROXY;
int order() default Ordered.LOWEST_PRECEDENCE;
}
又看到熟悉的Import注解，这次使用的是ImportSelector方法，会调用ImportSelector接口的selectImports方法。
@Override
public final String[] selectImports(AnnotationMetadata importingClassMetadata) {
Class<?> annoType = GenericTypeResolver.resolveTypeArgument(getClass(), AdviceModeImportSelector.class);
AnnotationAttributes attributes = AnnotationConfigUtils.attributesFor(importingClassMetadata, annoType);
if (attributes == null) {
throw new IllegalArgumentException(String.format(
"@%s is not present on importing class '%s' as expected",
annoType.getSimpleName(), importingClassMetadata.getClassName()));
}
AdviceMode adviceMode = attributes.getEnum(this.getAdviceModeAttributeName());
//根据AdviceMode返回不同的类型，默认是AdviceMode.PROXY。
String[] imports = selectImports(adviceMode);
if (imports == null) {
throw new IllegalArgumentException(String.format("Unknown AdviceMode: '%s'", adviceMode));
}
return imports;
}
@Override
protected String[] selectImports(AdviceMode adviceMode) {
switch (adviceMode) {
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;
}
}
根据AdviceMode选择创建不同的bean，AdviceMode默认是PROXY，所以返回的是AutoProxyRegistrar和ProxyTransactionManagementConfiguration。我们先来分析AutoProxyRegistrar。
AutoProxyRegistrar
public class AutoProxyRegistrar implements ImportBeanDefinitionRegistrar
由类的声明，AutoProxyRegistrar实现了ImportBeanDefinitionRegistrar，那在创建bean的时候会调用registerBeanDefinitions方法。registerBeanDefinitions方法的实现：
@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
boolean candidateFound = false;
Set<String> annoTypes = importingClassMetadata.getAnnotationTypes();
for (String annoType : annoTypes) {
AnnotationAttributes candidate = AnnotationConfigUtils.attributesFor(importingClassMetadata, annoType);
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;
//只有@EnableTransactionManagement注解才会走到这里
if (mode == AdviceMode.PROXY) {
AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
if ((Boolean) proxyTargetClass) {
AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
return;
}
}
}
}
//...
}
public static BeanDefinition registerAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry, Object source) {
return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);
}
AutoProxyRegistrar这个类主要是注册InfrastructureAdvisorAutoProxyCreator，注册这个bean有什么用呢?先看它的类图
由类图可以看出InfrastructureAdvisorAutoProxyCreator和AnnotationAwareAspectJAutoProxyCreator使用的是相同的父类AbstractAdvisorAutoProxyCreator，那么就会执行和spring aop相同的逻辑。所以AutoProxyRegistrar目的已经很明显了，开启spring aop。接下来再看ProxyTransactionManagementConfiguration的目的
ProxyTransactionManagementConfiguration
@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());
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;
}
}
ProxyTransactionManagementConfiguration是一个配置文件，注册了三个bean，BeanFactoryTransactionAttributeSourceAdvisor、AnnotationTransactionAttributeSource、TransactionInterceptor，而且这三个bean还存在依赖。他们之间的类图如下：
在类图中我们看到几个熟悉的类，Advisor、Advice和Pointcut。ProxyTransactionManagementConfiguration主要的逻辑就是创建一个Advisor对象(BeanFactoryTransactionAttributeSourceAdvisor)，并且注入通知(AnnotationTransactionAttributeSource)和切点(TransactionInterceptor)。
