第一章 Spring之AOP-JDK动态代理源码解析
第二章 Spring之事务实现原理及其注解@Transactional底层和传播机制原理
本文介绍Spring之事务实现原理，以及注解@Transactional底层实现和7种传播传播机制原理
Spring支持事务功能的入口注解，通过@Import注解向Spring容器导入TransactionManagementConfigurationSelector组件，该注解实现了ImportSelector接口（扩展点）
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
//@Import注解可以为我们容器中导入组件
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {
//默认为false
boolean proxyTargetClass() default false;
//默认为AdviceMode.PROXY
AdviceMode mode() default AdviceMode.PROXY;
int order() default Ordered.LOWEST_PRECEDENCE;
}
注：@EnableTransactionManagement和@EnableAspectJAutoProxy需要一起在配置类中被使用，事务是借组AOP切面来进行实现。
导入两个组件：AutoProxyRegistrar和ProxyTransactionManagementConfiguration
当在Spring容器中加载bean定义的时候会回调我们的selectImports方法，方法的返回值是我们需要导入类的全类名路径，然后这个类就会被加载到容器中
@Override
protected String[] selectImports(AdviceMode adviceMode) {
switch (adviceMode) {
/**
* 容器中导入二个组件
* 一个是AutoProxyRegistrar
* 一个是ProxyTransactionManagementConfiguration
*/
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;
}
}
该组件实现了ImportBeanDefinitionRegistrar接口，也是为一个扩展点，核心是向Spring容器注册一个BeanDefinition
public class AutoProxyRegistrar implements ImportBeanDefinitionRegistrar {
private final Log logger = LogFactory.getLog(getClass());
@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
boolean candidateFound = false;
//获取导入类的注解元数据信息
Set<String> annTypes = importingClassMetadata.getAnnotationTypes();
for (String annType : annTypes) {
AnnotationAttributes candidate = AnnotationConfigUtils.attributesFor(importingClassMetadata, annType);
if (candidate == null) {
continue;
}
//获取mode属性
Object mode = candidate.get("mode");
//获取proxyTargetClass属性
Object proxyTargetClass = candidate.get("proxyTargetClass");
if (mode != null && proxyTargetClass != null && AdviceMode.class == mode.getClass() &&
Boolean.class == proxyTargetClass.getClass()) {
candidateFound = true;
if (mode == AdviceMode.PROXY) {
//注册Bean定义
AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
if ((Boolean) proxyTargetClass) {
//修改bean定义，设置proxyTargetClass为True
AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
return;
}
}
}
}
if (!candidateFound && logger.isWarnEnabled()) {
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));
}
}
}
注册的Bean定义为InfrastructureAdvisorAutoProxyCreator
public static BeanDefinition registerAutoProxyCreatorIfNecessary(
BeanDefinitionRegistry registry, @Nullable Object source) {
return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);
}
该类重写了筛选合格的advisor方法，当Spring容器中包含的bean定义的角色类型为BeanDefinition.ROLE_INFRASTRUCTURE时，即为合格的Advisor
public class InfrastructureAdvisorAutoProxyCreator extends AbstractAdvisorAutoProxyCreator {
@Nullable
private ConfigurableListableBeanFactory beanFactory;
@Override
protected void initBeanFactory(ConfigurableListableBeanFactory beanFactory) {
super.initBeanFactory(beanFactory);
this.beanFactory = beanFactory;
}
/**
* 判断事务注解导入的BeanFactoryTransactionAttributeSourceAdvisor是不是我们想要的Advisor
*/
@Override
protected boolean isEligibleAdvisorBean(String beanName) {
/**
* 1.容器中包含了这个bean定义，
* 2.并且bean定义的角色BeanDefinition.ROLE_INFRASTRUCTURE
* 这几个条件符合即是我们想要查找的advisor
*/
return (this.beanFactory != null && this.beanFactory.containsBeanDefinition(beanName) &&
this.beanFactory.getBeanDefinition(beanName).getRole() == BeanDefinition.ROLE_INFRASTRUCTURE);
}
}
注册启用基于代理的注解驱动的事务管理所需的Spring基础设施 bean。
包含三个bean，一个Advisor增强器，一个Advice通知，一个TransactionAttributeSource事务属性源
@Configuration
public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {
/**
* 导入了Advisor接口的实现类BeanFactoryTransactionAttributeSourceAdvisor-关于事务的切面信息
* @return
*/
@Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() {
BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
advisor.setTransactionAttributeSource(transactionAttributeSource());
//添加定义的Advice通知
advisor.setAdvice(transactionInterceptor());
if (this.enableTx != null) {
advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
}
return advisor;
}
/**
* 事务属性源对象-用于获取事务属性对象
* 初始化事务注解解析器：SpringTransactionAnnotationParser
* @return
*/
@Bean
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public TransactionAttributeSource transactionAttributeSource() {
return new AnnotationTransactionAttributeSource();
}
/**
* 用户拦截事务方法执行的，一个Advice通知
* @return
*/
@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;
}
}
BeanFactoryTransactionAttributeSourceAdvisor实现类，当AOP匹配到了当前的Adivisor之后，会去创建动态代理，当动态代理对象执行被代理对象的方法时，如果为JDK动态代理会进入invoke方法执行代理逻辑，在代理逻辑中执行获取匹配的advice通知责任链时，会去调用每个匹配的advisor的getPointcut方法，去进行切点匹配
public class BeanFactoryTransactionAttributeSourceAdvisor extends AbstractBeanFactoryPointcutAdvisor {
@Nullable
private TransactionAttributeSource transactionAttributeSource;
/**
* 事务属性源切点
*/
private final TransactionAttributeSourcePointcut pointcut = new TransactionAttributeSourcePointcut() {
@Override
@Nullable
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);
}
//重写了getPointCut方法，执行事务代理逻辑的时候会获取切点进行匹配
@Override
public Pointcut getPointcut() {
return this.pointcut;
}
}
主要是一个MethodMatcher接口提供的方法匹配方法
abstract class TransactionAttributeSourcePointcut extends StaticMethodMatcherPointcut implements Serializable {
//放置了ClassFilter，用于过滤当前类或者方法是否有注解@Transactionl
//最终会进入SpringTransactionAnnotationParser方法
protected TransactionAttributeSourcePointcut() {
setClassFilter(new TransactionAttributeSourceClassFilter());
}
@Override
public boolean matches(Method method, @Nullable Class<?> targetClass) {
if (targetClass != null && TransactionalProxy.class.isAssignableFrom(targetClass)) {
return false;
}
/**
* 获取@EnableTransactionManagement注解为Spring容器中导入的ProxyTransactionManagementConfiguration
* 配置类中的TransactionAttributeSource事务注解属性对象
*/
TransactionAttributeSource tas = getTransactionAttributeSource();
// 通过getTransactionAttribute看方法或者类是否有@Transactional注解，tas为AnnotationTransactionAttributeSource实例，调用getTransactionAttribute会调用父类AbstractFallbackTransactionAttributeSource中的方法
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.
*/
@Nullable
protected abstract TransactionAttributeSource getTransactionAttributeSource();
}
AbstractFallbackTransactionAttributeSource#getTransactionAttribute判断方法或者类是否有@Transactional注解，也就是解析@Transactional注解和方法的描述符，并把TransactionAttribute放入缓存
public TransactionAttribute getTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
//判断method所在的class 是不是Object类型
if (method.getDeclaringClass() == Object.class) {
return null;
}
//构建缓存key
Object cacheKey = getCacheKey(method, targetClass);
//先去缓存中获取
TransactionAttribute cached = this.attributeCache.get(cacheKey);
//缓存中不为空
if (cached != null) {
//判断缓存中的对象是不是空事务属性的对象
if (cached == NULL_TRANSACTION_ATTRIBUTE) {
return null;
}
//不是的话 就进行返回
else {
return cached;
}
}
else {
//查找事务注解
TransactionAttribute txAttr = computeTransactionAttribute(method, targetClass);
// 若解析出来的事务注解属性为空
if (txAttr == null) {
//往缓存中存放空事务注解属性
this.attributeCache.put(cacheKey, NULL_TRANSACTION_ATTRIBUTE);
}
else {
//执行方法的描述符 全类名+方法名
String methodIdentification = ClassUtils.getQualifiedMethodName(method, targetClass);
//把方法描述设置到事务属性上去
if (txAttr instanceof DefaultTransactionAttribute) {
((DefaultTransactionAttribute) txAttr).setDescriptor(methodIdentification);
}
if (logger.isDebugEnabled()) {
logger.debug("Adding transactional method '" + methodIdentification + "' with attribute: " + txAttr);
}
//加入到缓存
this.attributeCache.put(cacheKey, txAttr);
}
return txAttr;
}
}
通过上面的源码分析可知，当开启AOP和事务及方法或者类上面存在@Transactional注解会进入该类的方法执行过滤逻辑,即TransactionInterceptor#invoke方法。（AOP的Advice责任链模式执行）
public Object invoke(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);
// 执行事务逻辑
return invokeWithinTransaction(invocation.getMethod(), targetClass, new CoroutinesInvocationCallback() {
@Override
@Nullable
public Object proceedWithInvocation() throws Throwable {
return invocation.proceed();
}
@Override
public Object getTarget() {
return invocation.getThis();
}
@Override
public Object[] getArguments() {
return invocation.getArguments();
}
});
}
protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,
final InvocationCallback invocation) throws Throwable {
//获取事务属性源对象，在配置类中添加的
TransactionAttributeSource tas = getTransactionAttributeSource();
//txAttr获取到的内容是从解析后的缓存中获取
// 1.获取解析后的事务属性信息
final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
// 获取配置的事务管理器对象
final PlatformTransactionManager tm = determineTransactionManager(txAttr);
// 2.从tx属性对象中获取出标注了@Transactionl的方法描述符
final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
//处理声明式事务
if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
//**************核心，有没有必要创建事务*************
TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
Object retVal;
try {
//回调目标方法
retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
//抛出异常进行回滚处理
completeTransactionAfterThrowing(txInfo, ex);
throw ex;
}
finally {
//清空我们的线程变量中transactionInfo的值
cleanupTransactionInfo(txInfo);
}
//提交事务
commitTransactionAfterReturning(txInfo);
return retVal;
}
// 编程式事务：（回调偏向）
else {
final ThrowableHolder throwableHolder = new ThrowableHolder();
// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
try {
Object result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr, status -> {
TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
try {
return invocation.proceedWithInvocation();
}
catch (Throwable ex) {
if (txAttr.rollbackOn(ex)) {
// A RuntimeException: will lead to a rollback.
if (ex instanceof RuntimeException) {
throw (RuntimeException) ex;
}
else {
throw new ThrowableHolderException(ex);
}
}
else {
// A normal return value: will lead to a commit.
throwableHolder.throwable = ex;
return null;
}
}
finally {
cleanupTransactionInfo(txInfo);
}
});
// Check result state: It might indicate a Throwable to rethrow.
if (throwableHolder.throwable != null) {
throw throwableHolder.throwable;
}
return result;
}
catch (ThrowableHolderException ex) {
throw ex.getCause();
}
catch (TransactionSystemException ex2) {
if (throwableHolder.throwable != null) {
logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
ex2.initApplicationException(throwableHolder.throwable);
}
throw ex2;
}
catch (Throwable ex2) {
if (throwableHolder.throwable != null) {
logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
}
throw ex2;
}
}
}
@Nullable
protected PlatformTransactionManager determineTransactionManager(@Nullable TransactionAttribute txAttr) {
// Do not attempt to lookup tx manager if no tx attributes are set
if (txAttr == null || this.beanFactory == null) {
return getTransactionManager();
}
//通过@Transactionnal("xxx")指定事务管理器
String qualifier = txAttr.getQualifier();
if (StringUtils.hasText(qualifier)) {
return determineQualifiedTransactionManager(this.beanFactory, qualifier);
}
else if (StringUtils.hasText(this.transactionManagerBeanName)) {
return determineQualifiedTransactionManager(this.beanFactory, this.transactionManagerBeanName);
}
else {
//配置类中配置的默认的事务管理器
PlatformTransactionManager defaultTransactionManager = getTransactionManager();
if (defaultTransactionManager == null) {
defaultTransactionManager = this.transactionManagerCache.get(DEFAULT_TRANSACTION_MANAGER_KEY);
if (defaultTransactionManager == null) {
// 拿到配置类中配置的事务管理器（使用事务的时候都会配的）
defaultTransactionManager = this.beanFactory.getBean(PlatformTransactionManager.class);
this.transactionManagerCache.putIfAbsent(
DEFAULT_TRANSACTION_MANAGER_KEY, defaultTransactionManager);
}
}
return defaultTransactionManager;
}
}
AbstractFallbackTransactionAttributeSource#getTransactionAttribute方法也调用过，此次会从缓存中获取TransactionAttribute
private String methodIdentification(Method method, @Nullable Class<?> targetClass,
@Nullable TransactionAttribute txAttr) {
//全类名+方法名
String methodIdentification = methodIdentification(method, targetClass);
if (methodIdentification == null) {
if (txAttr instanceof DefaultTransactionAttribute) {
methodIdentification = ((DefaultTransactionAttribute) txAttr).getDescriptor();
}
if (methodIdentification == null) {
methodIdentification = ClassUtils.getQualifiedMethodName(method, targetClass);
}
}
return methodIdentification;
}
protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm,
@Nullable TransactionAttribute txAttr, final String joinpointIdentification) {
// 如果还没有定义名字，把连接点的ID定义成事务的名称
if (txAttr != null && txAttr.getName() == null) {
txAttr = new DelegatingTransactionAttribute(txAttr) {
@Override
public String getName() {
return joinpointIdentification;
}
};
}
TransactionStatus status = null;
//当txAttr和tm都不等于空的时候，去通过事务管理器tm获取事务
if (txAttr != null) {
if (tm != null) {
//获取一个事务状态
status = tm.getTransaction(txAttr);
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
"] because no transaction manager has been configured");
}
}
}
//把事物状态和事物属性等信息封装成一个TransactionInfo对象
return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}
@Override
public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
//尝试获取一个事务对象
Object transaction = doGetTransaction();
// Cache debug flag to avoid repeated checks.
boolean debugEnabled = logger.isDebugEnabled();
/**
* 判断从上一个方法传递进来的事务属性是不是为空
*/
if (definition == null) {
definition = new DefaultTransactionDefinition();
}
/**
* 判断是不是已经存在了事务对象（事务嵌套）
*/
if (isExistingTransaction(transaction)) {
//处理存在的事务
return handleExistingTransaction(definition, transaction, debugEnabled);
}
//检查事务设置的超时时间
if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
}
/**
* 由于isExistingTransaction(transaction)跳过了这里，说明当前是不存在事务的
* 1.PROPAGATION_MANDATORY传播机制，表示必须运行在事务中，若当前没有事务就抛出异常，那么就会抛出异常
*/
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
throw new IllegalTransactionStateException(
"No existing transaction found for transaction marked with propagation 'mandatory'");
}
/**
* 2.PROPAGATION_REQUIRED 当前存在事务就加入到当前的事务,没有就新开一个
* 3.PROPAGATION_REQUIRES_NEW:新开一个事务,若当前存在事务就挂起当前事务
* 4.PROPAGATION_NESTED:
表示如果当前正有一个事务在运行中，则该方法应该运行在 一个嵌套的事务中，
被嵌套的事务可以独立于封装事务进行提交或者回滚(保存点)，
如果封装事务不存在,行为就像 PROPAGATION_REQUIRES NEW
*/
else if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
/**
* 挂起当前事务，在这里为啥传入null?
* 因为逻辑走到这里了,经过了上面的isExistingTransaction(transaction) 判断当前是不存在事务的
* 所有再这里是挂起当前事务传递一个null进去
*/
SuspendedResourcesHolder suspendedResources = suspend(null);
if (debugEnabled) {
logger.debug("Creating new transaction with name [" + def.getName() + "]: " + def);
}
try {
//开启事务
return startTransaction(def, transaction, debugEnabled, suspendedResources);
}
catch (RuntimeException | Error ex) {
resume(null, suspendedResources);
throw ex;
}
}
else {
// Create "empty" transaction: no actual transaction, but potentially synchronization.
if (def.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
logger.warn("Custom isolation level specified but no actual transaction initiated; " +
"isolation level will effectively be ignored: " + def);
}
boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
return prepareTransactionStatus(def, null, true, newSynchronization, debugEnabled, null);
}
}
在UserService的test方法添加了@Transactional注解
@Component
public class UserService {
@Autowired
private JdbcTemplate jdbcTemplate;
@Autowired
private UserService userService;
public UserService() {
System.out.println("UserService created");
}
@Transactional
public void test(){
jdbcTemplate.execute("insert into student values(2,'1')");
userService.b();
}
}
使用AnnotationConfigApplicationContext去获取Bean并调用test方法
public class MainClass {
public static void main(String[] args) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(MainConfig.class);
UserService userService = (UserService) context.getBean("userService");
userService.test();
}
}
以上的这种情况，一开始就是不存在事务，该isExistingTransaction(transaction) 判断不通过
doGetTransaction方法获取事务
protected Object doGetTransaction() {
//创建一个数据源事务对象
DataSourceTransactionObject txObject = new DataSourceTransactionObject();
//是否允许当前事务设置保持点
txObject.setSavepointAllowed(isNestedTransactionAllowed());
/**
* TransactionSynchronizationManager事务同步管理器对象(该类中都是局部线程变量，ThreadLocal实现)
* 用来保存当前事务的信息,我们第一次从这里去线程变量中获取 事务连接持有器对象 通过数据源为key去获取
* 第一次进来开始事务 我们的事务同步管理器中没有被存放.所以此时获取出来的conHolder为null
*/
ConnectionHolder conHolder =
(ConnectionHolder) TransactionSynchronizationManager.getResource(obtainDataSource());
//false代表不是新开
txObject.setConnectionHolder(conHolder, false);
//返回事务对象
return txObject;
}
startTransaction开启事务方法
private TransactionStatus startTransaction(TransactionDefinition definition, Object transaction,
boolean debugEnabled, @Nullable SuspendedResourcesHolder suspendedResources) {
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
//这其中的true代表为新开事务
DefaultTransactionStatus status = newTransactionStatus(
definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
//
doBegin(transaction, definition);
prepareSynchronization(status, definition);
return status;
}
doBegin方法获取数据库连接，创建ConnectionHolder放入事务对象，并设置为新建ConnectionHolder标识true
protected void doBegin(Object transaction, TransactionDefinition definition) {
DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
Connection con = null;
try {
//一开始进入为null
if (!txObject.hasConnectionHolder() ||
txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
//获取连接
Connection newCon = obtainDataSource().getConnection();
if (logger.isDebugEnabled()) {
logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
}
//创建ConnectionHolder，设置为新建标识，放入事务对象中
txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
}
//将资源标记为与事务同步
txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
//从事务中拿出数据库连接
con = txObject.getConnectionHolder().getConnection();
Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
//设置事务隔离级别
txObject.setPreviousIsolationLevel(previousIsolationLevel);
txObject.setReadOnly(definition.isReadOnly());
// Switch to manual commit if necessary. This is very expensive in some JDBC drivers,
// so we don't want to do it unnecessarily (for example if we've explicitly
// configured the connection pool to set it already).
//设置AutoCommit为false，手动提交
if (con.getAutoCommit()) {
txObject.setMustRestoreAutoCommit(true);
if (logger.isDebugEnabled()) {
logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
}
con.setAutoCommit(false);
}
//如果isReadOnly为true，就是执行的stmt.executeUpdate("SET TRANSACTION READ ONLY");
prepareTransactionalConnection(con, definition);
//激活事务状态
txObject.getConnectionHolder().setTransactionActive(true);
//设置超时
int timeout = determineTimeout(definition);
if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
}
// Bind the connection holder to the thread.
//如果为新建ConnectionHolder，将数据源和ConnectionHolder绑定，并存放到ThreadLocal
if (txObject.isNewConnectionHolder()) {
TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());
}
}
catch (Throwable ex) {
//上面执行异常，并且为新建的ConnectionHolder，归还连接并清空事务中的ConnectionHolder
if (txObject.isNewConnectionHolder()) {
DataSourceUtils.releaseConnection(con, obtainDataSource());
txObject.setConnectionHolder(null, false);
}
throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
}
}
prepareSynchronization根据需要初始化事务同步，其实就是在ThreadLocal中设置一些属性
protected void prepareSynchronization(DefaultTransactionStatus status, TransactionDefinition definition) {
if (status.isNewSynchronization()) {
TransactionSynchronizationManager.setActualTransactionActive(status.hasTransaction());
TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(
definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT ?
definition.getIsolationLevel() : null);
TransactionSynchronizationManager.setCurrentTransactionReadOnly(definition.isReadOnly());
TransactionSynchronizationManager.setCurrentTransactionName(definition.getName());
TransactionSynchronizationManager.initSynchronization();
}
}
在UserService的test方法和b方法添加了@Transactional注解，并且在test方法还调用了b方法，那么事务是存在的
@Component
public class UserService {
@Autowired
private JdbcTemplate jdbcTemplate;
@Autowired
private UserService userService;
@Transactional
public void test(){
jdbcTemplate.execute("insert into student values(2,'1')");
userService.b();
}
@Transactional
public void b(){
jdbcTemplate.execute("insert into student values(1,'1')");
}
}
使用AnnotationConfigApplicationContext去获取Bean并调用test方法
public class MainClass {
public static void main(String[] args) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(MainConfig.class);
UserService userService = (UserService) context.getBean("userService");
userService.test();
}
}
当在test方法里面调用b方法时，又会进入advice事务过滤逻辑，isExistingTransaction(transaction) 判断会通过
handleExistingTransaction方法处理已存在事务的情况
private TransactionStatus handleExistingTransaction(
TransactionDefinition definition, Object transaction, boolean debugEnabled)
throws TransactionException {
//如果是PROPAGATION_NEVER传播机制，抛异常
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
throw new IllegalTransactionStateException(
"Existing transaction found for transaction marked with propagation 'never'");
}
//如果是PROPAGATION_NOT_SUPPORTED传播机制，不支持事务
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
if (debugEnabled) {
logger.debug("Suspending current transaction");
}
//挂起，得到挂起对象，也就是上一个完整的事务对象相关信息，用于之后的恢复
//1.清空ConnectionHolder
//2.解绑数据源和ConnectionHolder的关系，也就是从ThreadLocal中移除绑定关系
Object suspendedResources = suspend(transaction);
boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
//设置TransactionStatus的false代表为不是新开事务
//返回一个新的事务状态对象Status
return prepareTransactionStatus(
definition, null, false, newSynchronization, debugEnabled, suspendedResources);
}
//如果是PROPAGATION_REQUIRES_NEW传播机制，开启新的事务
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
if (debugEnabled) {
logger.debug("Suspending current transaction, creating new transaction with name [" +
definition.getName() + "]");
}
//挂起，得到挂起对象，也就是上一个完整的事务对象相关信息，用于之后的恢复
SuspendedResourcesHolder suspendedResources = suspend(transaction);
try {
//开启新的事务
return startTransaction(definition, transaction, debugEnabled, suspendedResources);
}
catch (RuntimeException | Error beginEx) {
//恢复给定的事务
resumeAfterBeginException(transaction, suspendedResources, beginEx);
throw beginEx;
}
}
//如果是PROPAGATION_NESTED传播机制，嵌套，创建savepoint
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
//是否允许嵌套事务
if (!isNestedTransactionAllowed()) {
throw new NestedTransactionNotSupportedException(
"Transaction manager does not allow nested transactions by default - " +
"specify 'nestedTransactionAllowed' property with value 'true'");
}
if (debugEnabled) {
logger.debug("Creating nested transaction with name [" + definition.getName() + "]");
}
//是否为嵌套事务使用保存点
if (useSavepointForNestedTransaction()) {
// Create savepoint within existing Spring-managed transaction,
// through the SavepointManager API implemented by TransactionStatus.
// Usually uses JDBC 3.0 savepoints. Never activates Spring synchronization.
//使用已存在的事务去创建savepoint
DefaultTransactionStatus status =
prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
status.createAndHoldSavepoint();
return status;
}
else {
//JTA不是使用savepoint来实现嵌套事务
// Nested transaction through nested begin and commit/rollback calls.
// Usually only for JTA: Spring synchronization might get activated here
// in case of a pre-existing JTA transaction.
return startTransaction(definition, transaction, debugEnabled, null);
}
}
// Assumably PROPAGATION_SUPPORTS or PROPAGATION_REQUIRED.
if (debugEnabled) {
logger.debug("Participating in existing transaction");
}
if (isValidateExistingTransaction()) {
if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT) {
Integer currentIsolationLevel = TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();
if (currentIsolationLevel == null || currentIsolationLevel != definition.getIsolationLevel()) {
Constants isoConstants = DefaultTransactionDefinition.constants;
throw new IllegalTransactionStateException("Participating transaction with definition [" +
definition + "] specifies isolation level which is incompatible with existing transaction: " +
(currentIsolationLevel != null ?
isoConstants.toCode(currentIsolationLevel, DefaultTransactionDefinition.PREFIX_ISOLATION) :
"(unknown)"));
}
}
if (!definition.isReadOnly()) {
if (TransactionSynchronizationManager.isCurrentTransactionReadOnly()) {
throw new IllegalTransactionStateException("Participating transaction with definition [" +
definition + "] is not marked as read-only but existing transaction is");
}
}
}
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
}
createTransactionIfNecessary方法会返回TransactionInfo对象，该对象存在一个指针指向了旧的TransactionInfo对象，即为链表结构，小编觉得有点像是用到了链表实现的栈结构
protected static final class TransactionInfo {
@Nullable
private final PlatformTransactionManager transactionManager;
@Nullable
private final TransactionAttribute transactionAttribute;
private final String joinpointIdentification;
@Nullable
private TransactionStatus transactionStatus;
//旧的事务信息对象
@Nullable
private TransactionInfo oldTransactionInfo;
}
在createTransactionIfNecessary方法中最后返回prepareTransactionInfo方法执行的结果，该方法
protected TransactionInfo prepareTransactionInfo(@Nullable PlatformTransactionManager tm,
@Nullable TransactionAttribute txAttr, String joinpointIdentification,
@Nullable TransactionStatus status) {
TransactionInfo txInfo = new TransactionInfo(tm, txAttr, joinpointIdentification);
if (txAttr != null) {
// We need a transaction for this method...
if (logger.isTraceEnabled()) {
logger.trace("Getting transaction for [" + txInfo.getJoinpointIdentification() + "]");
}
// The transaction manager will flag an error if an incompatible tx already exists.
txInfo.newTransactionStatus(status);
}
else {
// The TransactionInfo.hasTransaction() method will return false. We created it only
// to preserve the integrity of the ThreadLocal stack maintained in this class.
if (logger.isTraceEnabled()) {
logger.trace("No need to create transaction for [" + joinpointIdentification +
"]: This method is not transactional.");
}
}
// We always bind the TransactionInfo to the thread, even if we didn't create
// a new transaction here. This guarantees that the TransactionInfo stack
// will be managed correctly even if no transaction was created by this aspect.
//此处为关键地方
txInfo.bindToThread();
return txInfo;
}
bindToThread将TransactionInfo和Thread绑定
private void bindToThread() {
// Expose current TransactionStatus, preserving any existing TransactionStatus
// for restoration after this transaction is complete.
//当前txinfo指向ThreadLocal获取到的，类似栈结果，入栈操作
this.oldTransactionInfo = transactionInfoHolder.get();
//重新将当前放入ThreadLocal
transactionInfoHolder.set(this);
}
再回过头看一开始的代码
//获取txinfo对象
TransactionInfo txInfo = createTransactionIfNecessary(ptm, txAttr, joinpointIdentification);
Object retVal;
try {
// This is an around advice: Invoke the next interceptor in the chain.
// This will normally result in a target object being invoked.
//执行被代理对象的方法
retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
// target invocation exception
completeTransactionAfterThrowing(txInfo, ex);
throw ex;
}
finally {
//此处是关键
cleanupTransactionInfo(txInfo);
}
cleanupTransactionInfo方法，重置ThreadLocal中txinfo对象
protected void cleanupTransactionInfo(@Nullable TransactionInfo txInfo) {
if (txInfo != null) {
txInfo.restoreThreadLocalStatus();
}
}
restoreThreadLocalStatus该方法有点出栈的意思
private void restoreThreadLocalStatus() {
// Use stack to restore old transaction TransactionInfo.
// Will be null if none was set.
//当前txinfo出栈，this.oldTransactionInfo入栈
transactionInfoHolder.set(this.oldTransactionInfo);
}
如果不存在事务，则创建一个新的，如果存在，就使用当前的事务；也就是说使用的是同一个事务
如果不存在事务，则以非事务方式执行，如果存在，就使用当前的事务
如果当前不存在事务，则抛出异常，如果存在，就使用当前的事务
创建一个新事务，如果存在则挂起当前事务，当前事务执行完之后重新恢复旧事务
不支持事务，始终以非事务方式执行
不支持事务，如果当前事务存在，则抛出异常
如果当前事务存在，则在嵌套事务中执行，JDBC采用创建SavePoint保存点实现嵌套
以上就是今天要讲的内容，本文介绍了Spring事务的实现原理及其注解@Transactional底层和传播机制原理