Spring事務註解@Transactional的實現原理

@Transactional 註解元數據驅動的聲明式事務

基本原理是：

將對應的方法經過註解元數據，標註在業務方法或者所在的對象上，而後在業務執行期間，經過AOP攔截器反射讀取元數據信息，最終將根據讀取的業務信息構建事務管理支持。

不一樣的方法之間的事務傳播保證在同一個事務內，是經過統一的數據源來實現的，事務開始時將數據源綁定到ThreadLocal中，後續加入的事務從ThreadLocal獲取數據源來保證數據源的統一。

@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface Transactional {

    @AliasFor("transactionManager")
    String value() default "";
    //事務管理器名稱
    @AliasFor("value")
    String transactionManager() default "";
    //事務傳播模式
    Propagation propagation() default Propagation.REQUIRED;
    //事務隔離級別
    Isolation isolation() default Isolation.DEFAULT;
    //超時時間
    int timeout() default TransactionDefinition.TIMEOUT_DEFAULT;
    //是不是隻讀事務
    boolean readOnly() default false;
    //須要回滾的異常類
    Class<? extends Throwable>[] rollbackFor() default {};
    //須要回滾的異常類名稱
    String[] rollbackForClassName() default {};
    //排除回滾的異常類
    Class<? extends Throwable>[] noRollbackFor() default {};
    //排除回滾的異常類名稱
    String[] noRollbackForClassName() default {};
}

這裏經過SpringBoot代碼來分析實現過程，源碼中刪除了部分代碼，只保留了一些重要部分

// 事務自動配置類 注意類裏面使用的@EnableTransactionManagement註解
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration

public class TransactionAutoConfiguration {
    @Configuration
    @ConditionalOnBean(PlatformTransactionManager.class)
    @ConditionalOnMissingBean(AbstractTransactionManagementConfiguration.class)
    public static class EnableTransactionManagementConfiguration {

        //注意這裏使用的@EnableTransactionManagement註解
        @Configuration
        @EnableTransactionManagement(proxyTargetClass = false)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class",
                havingValue = "false", matchIfMissing = false)
        public static class JdkDynamicAutoProxyConfiguration {
        }
        //注意這裏使用的@EnableTransactionManagement註解
        @Configuration
        @EnableTransactionManagement(proxyTargetClass = true)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class",
                havingValue = "true", matchIfMissing = true)
        public static class CglibAutoProxyConfiguration {
        }
    }
}

這裏只分析proxy模式

@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());
        if (this.enableTx != null) {
            advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
        }
        return advisor;
    }

    @Bean
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public TransactionAttributeSource transactionAttributeSource() {
        return new AnnotationTransactionAttributeSource();
    }

    //這裏注入了TransactionInterceptor攔截器bean~~~~
    @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;
    }

}

TransactionInterceptor攔截器經過元數據獲取事務定義信息TransactionDefinition，根據Definition信息獲取PlatformTransactionManager（TM），tm接口抽象了事務的實現流程，默認的tm是DataSourceTransactionManager（經過DataSourceTransactionManagerAutoConfiguration初始化的），tm中的getTransaction根據事務的傳播方式，開啓、加入、掛起事務

@Override
    public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
        Object transaction = doGetTransaction();

        boolean debugEnabled = logger.isDebugEnabled();

        if (definition == null) {
            // 使用默認的Definition
            definition = new DefaultTransactionDefinition();
        }

        if (isExistingTransaction(transaction)) {
            //已經存在事務，進入單獨的方法處理
            return handleExistingTransaction(definition, transaction, debugEnabled);
        }

        // 檢查timeout參數
        if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
            throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
        }

        // 當前必須存在事務，不然拋出異常
        if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
            throw new IllegalTransactionStateException(
                    "No existing transaction found for transaction marked with propagation 'mandatory'");
        }
        else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
                definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
                definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
            //獲取當前的一些事務信息，用於當前事務執行完後恢復
            SuspendedResourcesHolder suspendedResources = suspend(null);
            try {
                boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
                //構造一個新事務的TransactionStatus（包含嵌套事務SavePoint的支持）
                DefaultTransactionStatus status = newTransactionStatus(
                        definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
                //開啓新的事務
                doBegin(transaction, definition);
                prepareSynchronization(status, definition);
                return status;
            }
            catch (RuntimeException | Error ex) {
                //異常，恢復掛起的事務信息
                resume(null, suspendedResources);
                throw ex;
            }
        }
        else {
            //空的事務
            boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
            return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
        }
    }