spring @Transactional原理

2020-01-02 11:43:57 浏览数 (1)

1. 自动导入配置类

和aop自动配置一样,在/META-INF/spring.factories中配置有关transaction的有2个:

代码语言：javascript复制
org.springframework.boot.autoconfigure.EnableAutoConfiguration=
...
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration

这会导致TransactionAutoConfiguration和JtaAutoConfiguration被加载为Bean(若不明白原因, 可见上面的《spring 自动配置》系列). 本文只分析TransactionAutoConfiguration.

2. TransactionAutoConfiguration

该类有两个内部配置类:

  1. TransactionTemplateConfiguration会注入一个TransactionTemplate类型的Bean
  2. EnableTransactionManagementConfiguration会根据spring.aop.proxy-target-class的值决定注入JdkDynamicAutoProxyConfiguration还是CglibAutoProxyConfiguration. 默认情况下,生效的是前者(?).

2.1 按注解依次导入

由于注解的缘故, 会依次导致以下类发生作用:

代码语言：javascript复制
@Configuration
@EnableTransactionManagement(proxyTargetClass = false)
...
public static class JdkDynamicAutoProxyConfiguration {
}
代码语言：javascript复制
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {
    AdviceMode mode() default AdviceMode.PROXY;
    ...
}
代码语言：javascript复制
public class TransactionManagementConfigurationSelector extends AdviceModeImportSelector<EnableTransactionManagement> {
    @Override
    protected String[] selectImports(AdviceMode adviceMode) {
        switch (adviceMode) {
            case PROXY:  // 根据默认值会执行此处
                return new String[] {AutoProxyRegistrar.class.getName(), ProxyTransactionManagementConfiguration.class.getName()};
            case ASPECTJ:
                return new String[] {TransactionManagementConfigUtils.TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME};
            default:
                return null;
        }
    }
}

最终在selectImports方法中, 默认值adviceMode为PROXY, 则AutoProxyRegistrar和ProxyTransactionManagementConfiguration会被导入.

2.2 AutoProxyRegistrar

这里建议debug进行调试, 总之该类生效后, 会回调registerBeanDefinitions函数, 在函数内:

  1. 遍历每个Bean, 直到找到拥有@EnableTransactionManagement且模式为AdviceMode.PROXY的Bean
代码语言：javascript复制
@Override
public void registerBeanDefinitions(..) {
    ...
    if (mode == AdviceMode.PROXY) {
        AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
        if ((Boolean) proxyTargetClass) {
            AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
            return;
        }
    }
    ...
}

debug发现, 注解中proxyTargetClass为真, 则上文两个函数都要执行.

经过debug发现:

  • 前者会注入注册成名为"org.springframework.aop.config.internalAutoProxyCreator", 类型为InfrastructureAdvisorAutoProxyCreator的Bean
  • 后者会取出Bean, 将BeanDefinition的"proxyTargetClass"设置为真

InfrastructureAdvisorAutoProxyCreator 根据注释和观察isEligibleAdvisorBean方法的调用可知,该类负责对BeanDefinition.ROLE_INFRASTRUCTURE基础级别的Bean进行AOP包装. 该类的运作方式在于间接实现了BeanPostProcessor接口，使得从AbstractAutoProxyCreator类中继承的postProcessAfterInitialization方法被调用, 而isEligibleAdvisorBean方法在期间起作用。

2.3 ProxyTransactionManagementConfiguration

可结合注入的组件二：ProxyTransactionManagementConfiguration 的作用是什么?阅读 该类为配置类, 会注册三个Bean:

  1. BeanFactoryTransactionAttributeSourceAdvisor
  2. TransactionAttributeSource
  3. TransactionInterceptor

TransactionAttributeSource 实际创建的是AnnotationTransactionAttributeSource, 用于解析@Transactional注解. 在构造函数内,

代码语言：javascript复制
public AnnotationTransactionAttributeSource(boolean publicMethodsOnly) {
    ...
    this.annotationParsers = new LinkedHashSet<>(2);
    this.annotationParsers.add(new SpringTransactionAnnotationParser());
    ...
}

会创建SpringTransactionAnnotationParser, 并用于解析@Transactional注解

代码语言：javascript复制
@Nullable
protected TransactionAttribute determineTransactionAttribute(AnnotatedElement ae) {
    for (TransactionAnnotationParser annotationParser : this.annotationParsers) {
        TransactionAttribute attr = annotationParser.parseTransactionAnnotation(ae);
        if (attr != null) {
            return attr;
        }
    }
    return null;
}

其中, parseTransactionAnnotation被调用. BeanFactoryTransactionAttributeSourceAdvisor 该类依赖于2和3, 它定义了一个切面(与2相关), 也定义了一个Advisor(与3相关). TransactionInterceptor 最后是最关键的, 用于管理事务的拦截器, 该类的invoke方法会被AOP调用. AOP如何调用请自行了解AOP的原理.

代码语言：javascript复制
@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);
}
代码语言：javascript复制
protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,
            final InvocationCallback invocation) throws Throwable {
    ...
    // If the transaction attribute is null, the method is non-transactional.
    ...
    final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
    final PlatformTransactionManager tm = determineTransactionManager(txAttr);
    ...

    if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
        // Standard transaction demarcation with getTransaction and commit/rollback calls.
        TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);  // 创建事务
        Object retVal = null;
        try {
            retVal = invocation.proceedWithInvocation();  
        }
        catch (Throwable ex) {
            // target invocation exception
            completeTransactionAfterThrowing(txInfo, ex);  // 回滚事务
            throw ex;
        }
        finally {
            cleanupTransactionInfo(txInfo);
        }
        commitTransactionAfterReturning(txInfo);  // 提交事务
        return retVal;
    }

该方法的分析忽略, 请看原文"事务执行的具体步骤"一章.

总之, 在不进行任何指定的情况下, 默认获取PlatformTransactionManager类型的Bean用来管理事务, 一般是DataSourceTransactionManager.

TransactionInterceptor将与DataSourceTransactionManager共同完成整个事务的织入流程和管理. 其中, 管理事务创建/提交/回滚的三个方法值得研究.

3. DataSourceTransactionManager

TransactionInterceptor::createTransactionIfNecessary会一路调用到DataSourceTransactionManager::doBegin:

代码语言：javascript复制
@Override
protected void doBegin(Object transaction, TransactionDefinition definition) {
    DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
    Connection con = null;

    try {
        if (!txObject.hasConnectionHolder() ||
                txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
            Connection newCon = obtainDataSource().getConnection();  // 从连接池中获取连接
            ...
            txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
        }

        txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
        con = txObject.getConnectionHolder().getConnection();

        Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
        txObject.setPreviousIsolationLevel(previousIsolationLevel);

        // 开启事务的关键代码
        if (con.getAutoCommit()) {
            txObject.setMustRestoreAutoCommit(true);
            ...
            con.setAutoCommit(false);
        }

        prepareTransactionalConnection(con, definition);
        txObject.getConnectionHolder().setTransactionActive(true);

        ...
        // 绑定ConnectionHolder
        if (txObject.isNewConnectionHolder()) {
            TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());
        }
    }

    catch (Throwable ex) {
        if (txObject.isNewConnectionHolder()) {
            DataSourceUtils.releaseConnection(con, obtainDataSource());
            txObject.setConnectionHolder(null, false);
        }
        throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
    }
}

在这里我们看到了关键的三处代码:

  1. Connection newCon = obtainDataSource().getConnection();从连接池获取连接
  2. con.setAutoCommit(false);开启事务
  3. TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());绑定该连接, 该连接可以被之后使用
aop 网络安全

0 人点赞