在本篇文章中，將會介紹如何在spring中進行事務管理，之後對其內部原理進行分析。主要涉及
- @EnableTransactionManagement註解為我們做了什麼？
- 為什麼標註了@Transactional註解的方法就可以具有事務的特性，保持了資料的ACID特性？spring到底是如何具有這樣的偷樑換柱之術的？
- 中間涉獵到一些spring關於註解配置的解析邏輯分析，這一部分比較通用，並不是spring事務管理模組特有的功能。在往後分析spring其他模組程式碼的時候可以借鑑
如何在spring應用中使用事務
我們可以在配置類上標記註解@EnableTransactionManagement，這樣就可以設定spring應用開啟事務管理。 之後我們可以在需要開啟事務的方法上標註@Transactional，spring將利用AOP框架，生成代理類，為方法配置事務增強。下面看一個具體的例子
demo
配置類
@Configuration
@EnableTransactionManagement // 我們這一節的重點研究物件
public class MybatisConfig {
// 各種其他配置，如資料來源配置、mybatis的SqlSessionFactory等
}
複製程式碼需要事務增強的介面類
// 介面類
public interface CountryService {
int createCountry(Country country);
}
// 實現,我們故意讓其丟擲異常
public class CountryServiceImpl implements CountryService {
// ... 注入countryMapper
@Override
@Transactional
public int createCountry(Country country) {
// 使用mybatis mapper來運算元據庫
int result = countryMapper.insert(country);
int i = 1 / 0; // 丟擲RuntimeException，事務回滾
return result;
}
}
複製程式碼使用介面
public class ContextTest {
public static void main(String[] args) {
ApplicationContext context = new AnnotationConfigApplicationContext(MybatisConfig.class);
CountryService countryService = context.getBean(CountryService.class);
Country country = new Country();
country.setCountryName("中國");
country.setCountryCode("CN");
// 由於我們在countryService中，丟擲了異常。因此這裡的資料將發生回滾，不會寫入到資料庫中
countryService.createCountry(country);
}
}
複製程式碼demo說明了什麼？
從以上的demo，我們可以看出來。利用spring的事務管理框架，我們只需要三個步驟即可：
- 通過註解@EnableTransactionManagement，開啟spring的事務管理功能
- 在介面類的需要事務增強的方法上，標註@Transactional
- 在容器中使用增強後的代理類的事務方法，如countryService.createCountry(country)
spring是如何做到的呢？
區區三個步驟，就可以讓我們解耦資料訪問、事務管理這兩個功能模組。神奇的背後，到底隱藏著什麼原理呢？
從@EnableTransactionManagement開始探險
EnableTransactionManagement
@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;
}
複製程式碼從註解的程式碼中，可以看出在其內部通過@Import註解匯入了TransactionManagementConfigurationSelector類
TransactionManagementConfigurationSelector
public class TransactionManagementConfigurationSelector extends AdviceModeImportSelector<EnableTransactionManagement> {
/**
* {@inheritDoc}
* @return {@link ProxyTransactionManagementConfiguration} or
* {@code AspectJTransactionManagementConfiguration} for {@code PROXY} and
* {@code ASPECTJ} values of {@link EnableTransactionManagement#mode()}, respectively
*/
@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;
}
}
}
複製程式碼spring載入配置流程
由於整個解析配置的流程過於複雜，程式碼量繁多。這裡就不一一列出具體程式碼了。下面提供一個主流程的時序圖，有興趣的看官可以跟著流程圖去瀏覽一下相關原始碼。
@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;
if (mode == AdviceMode.PROXY) {
// 該方法內部將註冊一個自動生成代理類（InfrastructureAdvisorAutoProxyCreator）
AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
if ((Boolean) proxyTargetClass) {
AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
return;
}
}
}
}
if (!candidateFound) {
String name = getClass().getSimpleName();
logger.warn(String.format("%s was imported but no annotations were found " +
"having both 'mode' and 'proxyTargetClass' attributes of type " +
"AdviceMode and boolean respectively. This means that auto proxy " +
"creator registration and configuration may not have occurred as " +
"intended, and components may not be proxied as expected. Check to " +
"ensure that %s has been @Import'ed on the same class where these " +
"annotations are declared; otherwise remove the import of %s " +
"altogether.", name, name, name));
}
}
複製程式碼而且，在解析配置的過程中，將處理Import進來的配置類ProxyTransactionManagementConfiguration。 其內部存在三個用@Bean註解標註的方法如下，將向容器註冊其各自返回的bean。
@Configuration
public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {
// 註冊一個切面
@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();
}
// 切面邏輯（Advice）
@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;
}
}
複製程式碼解析配置後的成果
- 註冊了自動代理生成器InfrastructureAdvisorAutoProxyCreator
- 註冊了ProxyTransactionManagementConfiguration，其內部會通過@Bean標註的方法，進而註冊BeanFactoryTransactionAttributeSourceAdvisor、AnnotationTransactionAttributeSource、TransactionInterceptor
spring是如何為我們進行事務增強的
spring通過AOP框架在容器啟動時，自動發現需要事務增強的類或方法（即標註了@Transactional的類或方法），為這些方法嵌入事務切面（即BeanFactoryTransactionAttributeSourceAdvisor）生成代理類，之後我們從容器獲取到的對應的bean就是進行事務增強後的代理類。大致的步驟包括：
- InfrastructureAdvisorAutoProxyCreator作為BeanPostProcessor，在容器啟動期間其postProcessAfterInitialization方法被呼叫，作為建立事務增強代理物件的入口
- 之後從beanfactory中獲取所有Advisor實現類的例項，使用每一個獲取到的Advisor中的Pointcut對當前正在建立的bean進行匹配，在這裡Advisor為BeanFactoryTransactionAttributeSourceAdvisor、Pointcut為TransactionAttributeSourcePointcut
- 匹配過程中會呼叫TransactionAttributeSourcePointcut的matches(Method method, Class targetClass)方法來進行匹配判斷，判斷的工作需要藉助AnnotationTransactionAttributeSource#getTransactionAttribute(Method method, Class targetClass)來解析註解@Transactional
- 如果匹配成功，則證明需要生成事務增強代理。會返回BeanFactoryTransactionAttributeSourceAdvisor例項，作為切面設定到ProxyFactory中，用於生成代理
- 通過ProxyFactory來生成事務增強代理
大致的流程圖如下所示
事務增強代理生成過程的原始碼分析
InfrastructureAdvisorAutoProxyCreator（BeanPostProcessor）
public class InfrastructureAdvisorAutoProxyCreator extends AbstractAdvisorAutoProxyCreator {
// postProcessAfterInitialization介面在其父類中實現
}
複製程式碼public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware {
@Override
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
if (bean != null) {
Object cacheKey = getCacheKey(bean.getClass(), beanName);
if (!this.earlyProxyReferences.contains(cacheKey)) {
// 代理生成邏輯
return wrapIfNecessary(bean, beanName, cacheKey);
}
}
return bean;
}
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
if (beanName != null && this.targetSourcedBeans.contains(beanName)) {
return bean;
}
if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
return bean;
}
if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
// Create proxy if we have advice.
// 在這個方法內部將呼叫TransactionAttributeSourcePointcut#match方法進行匹配，如果匹配成功那麼會返回BeanFactoryTransactionAttributeSourceAdvisor例項
// 這個方法完成了以下幾個工作
// 1. 從beanfactory中獲取所有註冊到beanfactory中的Advisor，將Advisor進行例項化
// 2. 呼叫Advisor中的Pointcut的matches方法，進行匹配。匹配成功則返回當前Advisor
// 3. 在事務管理的框架中，匹配細節由TransactionAttributeSourcePointcut#matches方法負責，其內部會呼叫AnnotationTransactionAttributeSource#getTransactionAttribute方法解析@Transactional註解
// 4. 對於區域性事務來說，解析@Transactional的解析將委託給SpringTransactionAnnotationParser
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
if (specificInterceptors != DO_NOT_PROXY) {
this.advisedBeans.put(cacheKey, Boolean.TRUE);
// 在這個方法內部將使用ProxyFactory來生成代理
Object proxy = createProxy(
bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
this.proxyTypes.put(cacheKey, proxy.getClass());
return proxy;
}
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
}
複製程式碼BeanFactoryTransactionAttributeSourceAdvisor(Advisor)
這裡先看一下BeanFactoryTransactionAttributeSourceAdvisor的類圖，大概瞭解下這個類是在整個事務管理體系中是屬於什麼角色。
public class BeanFactoryTransactionAttributeSourceAdvisor extends AbstractBeanFactoryPointcutAdvisor {
private TransactionAttributeSource transactionAttributeSource;
// 切面內的Pointcut，用於在生成代理的過程中進行匹配方法
private final TransactionAttributeSourcePointcut pointcut = new TransactionAttributeSourcePointcut() {
@Override
protected TransactionAttributeSource getTransactionAttributeSource() {
return transactionAttributeSource;
}
};
/**
* Set the transaction attribute source which is used to find transaction
* attributes. This should usually be identical to the source reference
* set on the transaction interceptor itself.
* @see TransactionInterceptor#setTransactionAttributeSource
*/
public void setTransactionAttributeSource(TransactionAttributeSource transactionAttributeSource) {
this.transactionAttributeSource = transactionAttributeSource;
}
/**
* Set the {@link ClassFilter} to use for this pointcut.
* Default is {@link ClassFilter#TRUE}.
*/
public void setClassFilter(ClassFilter classFilter) {
this.pointcut.setClassFilter(classFilter);
}
@Override
public Pointcut getPointcut() {
return this.pointcut;
}
}
複製程式碼可以看出，這個類間接實現了介面PointcutAdvisor，這是一個切面類（即組合了Pointcut、Advice）。其內部定義的Pointcut為抽象類TransactionAttributeSourcePointcut的匿名實現類。關於AOP的這些概念，可以參考：spring-AOP原理分析一和spring-AOP原理分析二，這裡不再贅述。
TransactionAttributeSourcePointcut(Pointcut)
abstract class TransactionAttributeSourcePointcut extends StaticMethodMatcherPointcut implements Serializable {
// 通過AnnotationTransactionAttributeSource來獲取
@Override
public boolean matches(Method method, Class<?> targetClass) {
if (targetClass != null && TransactionalProxy.class.isAssignableFrom(targetClass)) {
return false;
}
// 獲取到的tas為AnnotationTransactionAttributeSource例項，
// 在ProxyTransactionManagementConfiguration中註冊而來
TransactionAttributeSource tas = getTransactionAttributeSource();
return (tas == null || tas.getTransactionAttribute(method, targetClass) != null);
}
@Override
public boolean equals(Object other) {
if (this == other) {
return true;
}
if (!(other instanceof TransactionAttributeSourcePointcut)) {
return false;
}
TransactionAttributeSourcePointcut otherPc = (TransactionAttributeSourcePointcut) other;
return ObjectUtils.nullSafeEquals(getTransactionAttributeSource(), otherPc.getTransactionAttributeSource());
}
@Override
public int hashCode() {
return TransactionAttributeSourcePointcut.class.hashCode();
}
@Override
public String toString() {
return getClass().getName() + ": " + getTransactionAttributeSource();
}
/**
* Obtain the underlying TransactionAttributeSource (may be {@code null}).
* To be implemented by subclasses.
*/
protected abstract TransactionAttributeSource getTransactionAttributeSource();
}
複製程式碼AnnotationTransactionAttributeSource
public class AnnotationTransactionAttributeSource extends AbstractFallbackTransactionAttributeSource
implements Serializable {
// ... 省略code
}
public abstract class AbstractFallbackTransactionAttributeSource implements TransactionAttributeSource {
@Override
public TransactionAttribute getTransactionAttribute(Method method, Class<?> targetClass) {
if (method.getDeclaringClass() == Object.class) {
return null;
}
// First, see if we have a cached value.
Object cacheKey = getCacheKey(method, targetClass);
Object cached = this.attributeCache.get(cacheKey);
if (cached != null) {
// Value will either be canonical value indicating there is no transaction attribute,
// or an actual transaction attribute.
if (cached == NULL_TRANSACTION_ATTRIBUTE) {
return null;
}
else {
return (TransactionAttribute) cached;
}
}
else {
// We need to work it out.
TransactionAttribute txAttr = computeTransactionAttribute(method, targetClass);
// Put it in the cache.
if (txAttr == null) {
this.attributeCache.put(cacheKey, NULL_TRANSACTION_ATTRIBUTE);
}
else {
String methodIdentification = ClassUtils.getQualifiedMethodName(method, targetClass);
if (txAttr instanceof DefaultTransactionAttribute) {
((DefaultTransactionAttribute) txAttr).setDescriptor(methodIdentification);
}
if (logger.isDebugEnabled()) {
logger.debug("Adding transactional method '" + methodIdentification + "' with attribute: " + txAttr);
}
this.attributeCache.put(cacheKey, txAttr);
}
return txAttr;
}
}
// 解析@Transaction註解的屬性值
protected TransactionAttribute computeTransactionAttribute(Method method, Class<?> targetClass) {
// Don't allow no-public methods as required.
// 1. 只有public方法可以切入事務管理
if (allowPublicMethodsOnly() && !Modifier.isPublic(method.getModifiers())) {
return null;
}
// Ignore CGLIB subclasses - introspect the actual user class.
Class<?> userClass = ClassUtils.getUserClass(targetClass);
// The method may be on an interface, but we need attributes from the target class.
// If the target class is null, the method will be unchanged.
Method specificMethod = ClassUtils.getMostSpecificMethod(method, userClass);
// If we are dealing with method with generic parameters, find the original method.
specificMethod = BridgeMethodResolver.findBridgedMethod(specificMethod);
// First try is the method in the target class.
TransactionAttribute txAttr = findTransactionAttribute(specificMethod);
if (txAttr != null) {
return txAttr;
}
// Second try is the transaction attribute on the target class.
txAttr = findTransactionAttribute(specificMethod.getDeclaringClass());
if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
return txAttr;
}
if (specificMethod != method) {
// Fallback is to look at the original method.
txAttr = findTransactionAttribute(method);
if (txAttr != null) {
return txAttr;
}
// Last fallback is the class of the original method.
txAttr = findTransactionAttribute(method.getDeclaringClass());
if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
return txAttr;
}
}
return null;
}
@Override
protected TransactionAttribute findTransactionAttribute(Method method) {
return determineTransactionAttribute(method);
}
protected TransactionAttribute determineTransactionAttribute(AnnotatedElement ae) {
if (ae.getAnnotations().length > 0) {
// TransactionAnnotationParser的實現類有Ejb3TransactionAnnotationParser、JtaTransactionAnnotationParser、SpringTransactionAnnotationParser
// 對於區域性失誤，我們使用SpringTransactionAnnotationParser來進行解析
for (TransactionAnnotationParser annotationParser : this.annotationParsers) {
TransactionAttribute attr = annotationParser.parseTransactionAnnotation(ae);
if (attr != null) {
return attr;
}
}
}
return null;
}
}
public class SpringTransactionAnnotationParser implements TransactionAnnotationParser, Serializable {
@Override
public TransactionAttribute parseTransactionAnnotation(AnnotatedElement ae) {
AnnotationAttributes attributes = AnnotatedElementUtils.getMergedAnnotationAttributes(ae, Transactional.class);
if (attributes != null) {
return parseTransactionAnnotation(attributes);
}
else {
return null;
}
}
public TransactionAttribute parseTransactionAnnotation(Transactional ann) {
return parseTransactionAnnotation(AnnotationUtils.getAnnotationAttributes(ann, false, false));
}
protected TransactionAttribute parseTransactionAnnotation(AnnotationAttributes attributes) {
RuleBasedTransactionAttribute rbta = new RuleBasedTransactionAttribute();
Propagation propagation = attributes.getEnum("propagation");
rbta.setPropagationBehavior(propagation.value());
Isolation isolation = attributes.getEnum("isolation");
rbta.setIsolationLevel(isolation.value());
rbta.setTimeout(attributes.getNumber("timeout").intValue());
rbta.setReadOnly(attributes.getBoolean("readOnly"));
rbta.setQualifier(attributes.getString("value"));
ArrayList<RollbackRuleAttribute> rollBackRules = new ArrayList<RollbackRuleAttribute>();
Class<?>[] rbf = attributes.getClassArray("rollbackFor");
for (Class<?> rbRule : rbf) {
RollbackRuleAttribute rule = new RollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
String[] rbfc = attributes.getStringArray("rollbackForClassName");
for (String rbRule : rbfc) {
RollbackRuleAttribute rule = new RollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
Class<?>[] nrbf = attributes.getClassArray("noRollbackFor");
for (Class<?> rbRule : nrbf) {
NoRollbackRuleAttribute rule = new NoRollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
String[] nrbfc = attributes.getStringArray("noRollbackForClassName");
for (String rbRule : nrbfc) {
NoRollbackRuleAttribute rule = new NoRollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
rbta.getRollbackRules().addAll(rollBackRules);
return rbta;
}
@Override
public boolean equals(Object other) {
return (this == other || other instanceof SpringTransactionAnnotationParser);
}
@Override
public int hashCode() {
return SpringTransactionAnnotationParser.class.hashCode();
}
}
複製程式碼總結
以上我們從一個demo入手，瞭解瞭如何使用spring來管理事務；之後我們從配置的註解@EnableTransactionManagement切入到spring事務框架的內部原理。期間涉及了幾個主要的類：
- AutoProxyRegistrar其主要職責是註冊InfrastructureAdvisorAutoProxyCreator 1.1 InfrastructureAdvisorAutoProxyCreator，負責在容器啟動期間利用配置資訊生成事務增強的代理類物件
- ProxyTransactionManagementConfiguration其主要職責是註冊AnnotationTransactionAttributeSource、TransactionInterceptor、BeanFactoryTransactionAttributeSourceAdvisor 2.1 AnnotationTransactionAttributeSource負責解析@Transactional註解 2.2 BeanFactoryTransactionAttributeSourceAdvisor作為切面，組合了Pointcut和Advice提供了事務管理的功能
- TransactionAttributeSourcePointcut作為Pointcut，負責匹配方法
- TransactionInterceptor作為Advice，事務管理的邏輯都在這個類中進行實現。
懸念
鑑於篇幅過長了，Ｏ(≧口≦)Ｏ。。。下一節我們再對事務管理的邏輯進行剖析，即對TransactionInterceptor進行分析