一Spring事务开启原理

Posted 2021-09-25 柠檬时间

Spring事务简单实现步骤

1. 在事务配置类上声明@EnableTransactionManagement注解开启事务
2. 在事务配置类上定义数据源
3. 在事务配置类上定义事务管理器
4. 在相关类或者方法上使用@Transactional声明事务

代码例子如下：

@Configuration
@EnableTransactionManagement
public class RootConfig{
    
    @Bean
    public DataSource dataSource(){
        DruidDataSource dataSource = new DruidDataSource();
        dataSource.setXXX();
        ...
        return dataSource;
    }
    
    @Bean
    public PlatfromTransactionManager txManager(){
        return new DataSourceTransactionManager(dataSource());
    }
}

@Service
public class UserService{

    @Autowired
    private UserRepository userRepository;
    
    @Transactional
    public void addUser(User user){
        userRepository.save(user);
    }
}

@EnableTransactionManagement开启事务原理

@EnableTransactionManagement接口类是通过Import了TransactionManagementConfigurationSelector来实现其注入，代码如下：

@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;
}

而TransactionManagementConfigurationSelector又使用selectImport方法来实现其注入，代码如下：

@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;
    }
}

其中主要功能点为根据AdviceMode选择创建不同的bean，AdviceMode的默认代理方式是proxy，即jdk代理。所以返回的是AutoProxyRegistrar和ProxyTransactionManagementConfiguration。

AutoProxyRegistrar

我们先看AutoProxyRegistrar，AutoProxyRegistrar通过实现ImportBeanDefinitionRegistrar，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);
}

可以看到，它通过注册InfrastructureAdvisorAutoProxyCreator来启动Spring Aop。

ProxyTransactionManagementConfiguration

接下来再看ProxyTransactionManagementConfiguration的作用，代码如下：

@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，而这三个类分别继承Advisor、Advice和Pointcut。即AOP切面所需组成。