自学做网站2023第三波疫情已经到来了

【README】

1）声明式事务代码样例：

public interface UserMapper {UserPO qryUserById(@Param("id") String id);@Transactional(propagation = Propagation.REQUIRED, rollbackFor = Exception.class)void insertUser(UserPO userPO);
}public interface UserAccountMapper {@Transactional(propagation = Propagation.REQUIRED, rollbackFor = Exception.class)void insertUserAccount(UserAccountPO userAccountPO);
}

【代码解说】

• 代码调用链路：UserAppService -> userSupport.saveNewUser -> userMapper.insertUser + userAccountMapper.insertUserAccount
• userSupport.saveNewUser 带有 @Transactional 注解； 第1层（最外层）@Transactional标注的方法
  • userMapper.insertUser 带有 @Transactional 注解; 第2层@Transactional标注的方法1
  • userAccountMapper.insertUserAccount 带有 @Transactional 注解; 第2层@Transactional标注的方法2

2）@Transactional：事务注解，用于定义事务元数据，包括事务管理器名称，事务传播行为，超时时间（单位秒），是否只读，回滚的异常类型；

• @Transactional可以标注类与方法，不管是标注类还是方法，@Transactional标注所在的类的bean都会被spring通过aop代理进行增强；

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
@Reflective
public @interface Transactional {@AliasFor("transactionManager")String value() default "";@AliasFor("value")String transactionManager() default "";String[] label() default {};Propagation propagation() default Propagation.REQUIRED;Isolation isolation() default Isolation.DEFAULT;int timeout() default -1;String timeoutString() default "";boolean readOnly() default false;Class<? extends Throwable>[] rollbackFor() default {};String[] rollbackForClassName() default {};Class<? extends Throwable>[] noRollbackFor() default {};String[] noRollbackForClassName() default {};
}

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【步骤1】UserAppService调用userSupport.saveNewUser()

1）因为 userSupport中的saveNewUser方法被@Transaction标注，所以该bean被spring增强为aop代理，所以访问aop代理的入口方法intercept()，即CglibAopProxy#DynamicAdvisedInterceptor静态内部类的 intercept方法；

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【步骤2】获取到TransactionInterceptor

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【步骤3】chain不为空，接着执行CglibMethodInvocation#proceed方法

chain有一个元素： TransactionInterceptor-事务拦截器；

接着执行 467行：retVal = (new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy)).proceed();

【补充】AopContext作用

【补充455行代码】： AopContext实际上是带有ThreadContext的容器对象，用于存储代理对象；

public final class AopContext {private static final ThreadLocal<Object> currentProxy = new NamedThreadLocal("Current AOP proxy");private AopContext() {}public static Object currentProxy() throws IllegalStateException {Object proxy = currentProxy.get();if (proxy == null) {throw new IllegalStateException("Cannot find current proxy: Set 'exposeProxy' property on Advised to 'true' to make it available, and ensure that AopContext.currentProxy() is invoked in the same thread as the AOP invocation context.");} else {return proxy;}}@Nullablestatic Object setCurrentProxy(@Nullable Object proxy) {Object old = currentProxy.get();if (proxy != null) {currentProxy.set(proxy);} else {currentProxy.remove();}return old;}
}

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【步骤4】CglibMethodInvocation#proceed方法

CglibMethodInvocation#proceed方法， 即Cglib方法调用类#proceed方法 （proceed=继续或进行或处理）

【CglibAopProxy#CglibMethodInvocation】

private static class CglibMethodInvocation extends ReflectiveMethodInvocation {public CglibMethodInvocation(Object proxy, @Nullable Object target, Method method, Object[] arguments, @Nullable Class<?> targetClass, List<Object> interceptorsAndDynamicMethodMatchers, MethodProxy methodProxy) {super(proxy, target, method, arguments, targetClass, interceptorsAndDynamicMethodMatchers);}// 调用proceed @Nullablepublic Object proceed() throws Throwable {try {return super.proceed();} catch (RuntimeException var2) {throw var2;} catch (Exception var3) {if (!ReflectionUtils.declaresException(this.getMethod(), var3.getClass()) && !KotlinDetector.isKotlinType(this.getMethod().getDeclaringClass())) {throw new UndeclaredThrowableException(var3);} else {throw var3;}}}
}

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【步骤5】调用ReflectiveMethodInvocation#proceed方法（反射方法调用类）

调用ReflectiveMethodInvocation#proceed方法， ReflectiveMethodInvocation=反射方法调用类

// ReflectiveMethodInvocation=反射方法调用类
// interceptorsAndDynamicMethodMatcher 是一个列表，包含一个元素 TransactionInterceptor 
// currentInterceptorIndex 初始值=-1 
public Object proceed() throws Throwable {if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {return this.invokeJoinpoint();} else {Object interceptorOrInterceptionAdvice = this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
// interceptorOrInterceptionAdvice 就是 TransactionInterceptor ，而不是 InterceptorAndDynamicMethodMatcherif (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {InterceptorAndDynamicMethodMatcher dm = (InterceptorAndDynamicMethodMatcher)interceptorOrInterceptionAdvice;Class<?> targetClass = this.targetClass != null ? this.targetClass : this.method.getDeclaringClass();return dm.matcher().matches(this.method, targetClass, this.arguments) ? dm.interceptor().invoke(this) : this.proceed();} else {return ((MethodInterceptor)interceptorOrInterceptionAdvice).invoke(this); // 调用到这里 }}
}

获取到的interceptorOrInterceptionAdvice的属性如下：

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【步骤6】调用 TransactionInterceptor#invoke（事务拦截器#invoke方法）

调用 TransactionInterceptor#invoke(ReflectiveMethodInvocation)， 传入的入参this是ReflectiveMethodInvocation-反射方法调用对象（包装了 UserSupportImpl#saveNewUser方法的代理对象）

【TransactionInterceptor#invoke】

public Object invoke(MethodInvocation invocation) throws Throwable {Class<?> targetClass = invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null;Method var10001 = invocation.getMethod();Objects.requireNonNull(invocation);return this.invokeWithinTransaction(var10001, targetClass, invocation::proceed);
}

上述 invocation::proceed中的invocation实际是 ReflectiveMethodInvocaion-反射方法调用类，ReflectiveMethodInvocaion封装了代理对象proxy，方法method等属性；

public class ReflectiveMethodInvocation implements ProxyMethodInvocation, Cloneable {protected final Object proxy;@Nullableprotected final Object target;protected final Method method;protected Object[] arguments;@Nullableprivate final Class<?> targetClass;@Nullableprivate Map<String, Object> userAttributes;protected final List<?> interceptorsAndDynamicMethodMatchers;private int currentInterceptorIndex = -1;protected ReflectiveMethodInvocation(Object proxy, @Nullable Object target, Method method, @Nullable Object[] arguments, @Nullable Class<?> targetClass, List<Object> interceptorsAndDynamicMethodMatchers) {this.proxy = proxy;this.target = target;this.targetClass = targetClass;this.method = BridgeMethodResolver.findBridgedMethod(method);this.arguments = AopProxyUtils.adaptArgumentsIfNecessary(method, arguments);this.interceptorsAndDynamicMethodMatchers = interceptorsAndDynamicMethodMatchers;}

【补充】TransactionInterceptor-事务拦截器源码

TransactionInterceptor继承自TransactionAspectSupport；

public class TransactionInterceptor extends TransactionAspectSupport implements MethodInterceptor, Serializable {public TransactionInterceptor() {}public TransactionInterceptor(TransactionManager ptm, TransactionAttributeSource tas) {this.setTransactionManager(ptm);this.setTransactionAttributeSource(tas);}@Nullablepublic Object invoke(MethodInvocation invocation) throws Throwable {Class<?> targetClass = invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null;Method var10001 = invocation.getMethod();Objects.requireNonNull(invocation);// 调用到这里， 注意最后一个入参是 invocation::proceed；return this.invokeWithinTransaction(var10001, targetClass, invocation::proceed);}private void writeObject(ObjectOutputStream oos) throws IOException {
//...}private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {// ...}
}

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【步骤7】调用TransactionAspectSupport#invokeWithinTransaction方法

因为TransactionInterceptor 继承自TransactionAspectSupport； 而invokeWithinTransaction方法的最后一个入参invocation就是封装了目标方法，即UserSupportImpl#saveNewUser方法的包装对象ReflectiveMethodInvocation；

获取到的事务管理器类型是 JdbcTransactionManger, 而事务属性TransactionAttribute保存了注解@Transaction标注的方法的元数据，包括事务传播行为，隔离级别，超时时间，是否只读，事务管理器等属性； 而TransactionAttribute继承自TransactionDefinition-事务定义接口；

【TransactionAttribute】事务属性-TransactionAttribute定义

public interface TransactionAttribute extends TransactionDefinition {@NullableString getQualifier();Collection<String> getLabels();boolean rollbackOn(Throwable ex);
}

【TransactionAspectSupport#invokeWithinTransaction方法】执行到获取PlatformTransactionManager-平台事务管理器；

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【步骤8】根据事务属性txAttr获取事务管理器，tm=JdbcTransactionManager

上述第7步执行到145行，接着判断tm（也就是JdbcTransactionManager）是否是ReactiveTransactionManager类型；

JdbcTransactionManager父类是DataSourceTransactionManager，而DataSourceTransactionManager父类是AbstractPlatformTransactionManager，而AbstractPlatformTransactionManager父类是PlatformTransactionManager；结论：tm(JdbcTransactionManager)不是CallbackPreferringPlatformTransactionManager，所以执行到160行；

又160行得到的ptm还是JdbcTransactionManager，与tm指向同一个对象，162行判断是否为CallbackPreferringPlatformTransactionManager类型不通过，接着执行第220行；

【补充】事务管理器是什么？

public class JdbcTransactionManager extends DataSourceTransactionManager;public class DataSourceTransactionManager extends AbstractPlatformTransactionManager implements ResourceTransactionManager, InitializingBean;public abstract class AbstractPlatformTransactionManager implements PlatformTransactionManager, ConfigurableTransactionManager, Serializable; 

【DataSourceTransactionManager】数据源事务管理器-方法列表

• doBegin-开启事务
• doCommit-提交事务
• doGetTransaction-获取事务
• doResume-恢复事务
• doRollback-回滚事务
• doSuspend-挂起事务；

【DataSourceTransactionManager父类AbstractPlatformTransactionManager】抽象平台事务管理器-部分方法列表

• doBegin 开启事务
• doCommit 提交
• doGetTransation 获取事务
• doResume 恢复事务
• doSuspend 挂起事务
• doRollback 回滚
• setNestedTransactionAllowed -设置允许嵌套事务；

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【步骤9】TransactionAspectSupport#invokeWithinTransaction()-在事务中执行业务逻辑（非常重要）

1）TransactionAspectSupport#invokeWithinTransaction()-在事务中执行业务逻辑，具体步骤如下（跳过了分支判断）：

• 步骤9.1：调用createTransactionIfNecessary方法，创建事务；
• 步骤9.2：调用invocation.proceedWithInvocation()，执行具体业务逻辑（执行目标方法）；
  • 步骤9.2.1（或有） ： 抛出异常，执行completeTransactionAfterThrowing
• 步骤9.3：执行完成（无论是否抛出异常），调用cleanupTransactionInfo()
• 步骤9.4：判断返回值是否不为null 且 事务属性是否不为null
• 步骤9.5：调用commitTransactionAfterReturning()，提交事务；

【TransactionAspectSupport#invokeWithinTransaction()】 在事务中调用；上游是TransactionInterceptor，而TransactionInterceptor是TransactionAspectSupport的子类； 源码如下。

protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass, final InvocationCallback invocation) throws Throwable {TransactionAttributeSource tas = this.getTransactionAttributeSource();TransactionAttribute txAttr = tas != null ? tas.getTransactionAttribute(method, targetClass) : null;TransactionManager tm = this.determineTransactionManager(txAttr);if (this.reactiveAdapterRegistry != null && tm instanceof ReactiveTransactionManager rtm) {// tm类型是JdbcTransactionManager，所以不会执行到这里// ... return txSupport.invokeWithinTransaction(method, targetClass, invocation, txAttr, rtm);} else {PlatformTransactionManager ptm = this.asPlatformTransactionManager(tm);String joinpointIdentification = this.methodIdentification(method, targetClass, txAttr);if (txAttr != null && ptm instanceof CallbackPreferringPlatformTransactionManager cpptm) {// ptm类型是JdbcTransactionManager，所以不会执行到这里// ...} else {// 程序执行到这里 // 1 创建事务，【非常重要】 TransactionInfo txInfo = this.createTransactionIfNecessary(ptm, txAttr, joinpointIdentification);Object retVal;try {// 2 通过反射执行具体业务逻辑， invocation就是包装了业务方法UserSuuportImpl#saveUser()方法的调用对象 retVal = invocation.proceedWithInvocation();} catch (Throwable var22) {// 2.1 抛出异常 （或有）this.completeTransactionAfterThrowing(txInfo, var22);throw var22;} finally {// 3 最后清理事务信息 this.cleanupTransactionInfo(txInfo);}// 4 判断返回值是否不为null 且 事务属性是否不为null if (retVal != null && txAttr != null) {TransactionStatus status = txInfo.getTransactionStatus();if (status != null) {label185: {if (retVal instanceof Future) {// 5 若返回值为 Future类型（异步线程执行结果类）Future<?> future = (Future)retVal;if (future.isDone()) {try {future.get();} catch (ExecutionException var27) {if (txAttr.rollbackOn(var27.getCause())) {status.setRollbackOnly();}} catch (InterruptedException var28) {Thread.currentThread().interrupt();}break label185;}}if (vavrPresent && TransactionAspectSupport.VavrDelegate.isVavrTry(retVal)) {retVal = TransactionAspectSupport.VavrDelegate.evaluateTryFailure(retVal, txAttr, status);}}}}// 5 提交事务 this.commitTransactionAfterReturning(txInfo);return retVal;}}
}

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【步骤9.1】按需创建事务-createTransactionIfNecessary()

TransactionAspectSupport#createTransactionIfNecessary()详情参见 spring声明式事务原理02-调用第1层@Transactional方法-按需创建事务createTransactionIfNecessary
根据事务管理器+事务属性+aop连接点（切点）创建事务，如下：

protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm, @Nullable TransactionAttribute txAttr, final String joinpointIdentification) {if (txAttr != null && ((TransactionAttribute)txAttr).getName() == null) {txAttr = new DelegatingTransactionAttribute((TransactionAttribute)txAttr) {public String getName() {return joinpointIdentification;}};}TransactionStatus status = null;if (txAttr != null) {if (tm != null) {// 通过事务管理器获取事务status = tm.getTransaction((TransactionDefinition)txAttr); } else if (this.logger.isDebugEnabled()) {this.logger.debug("Skipping transactional joinpoint [" + joinpointIdentification + "] because no transaction manager has been configured");}}// 准备事务信息TransactionInfo return this.prepareTransactionInfo(tm, (TransactionAttribute)txAttr, joinpointIdentification, status);
}

【补充】TransactionInfo事务信息定义

1）TransactionInfo是TransactionAspectSupport的 静态内部类，是一个聚合类，封装了事务相关的多个组件：

• transactionManager：事务管理器（封装了事务提交，回滚功能）
• TransactionAttribute：事务属性 （@Transaction注解元数据）
• joinpointIdentification：切点标识（被@Transaction标注的目标方法的全限定名称）
• TransactionStatus：事务状态 （封装获取事务状态的方法，如是否只读，是否回滚，是否保存点，事务是否完成，当前线程是否存在事务等）
• oldTransactionInfo-上一个事务信息对象；

其中 oldTransactionInfo-上一个事务信息对象，是bindToThread()方法与restoreThreadLocalStatus() 需要用到的；

protected static final class TransactionInfo {@Nullableprivate final PlatformTransactionManager transactionManager;@Nullableprivate final TransactionAttribute transactionAttribute;private final String joinpointIdentification;@Nullableprivate TransactionStatus transactionStatus;@Nullableprivate TransactionInfo oldTransactionInfo;public TransactionInfo(@Nullable PlatformTransactionManager transactionManager, @Nullable TransactionAttribute transactionAttribute, String joinpointIdentification) {this.transactionManager = transactionManager;this.transactionAttribute = transactionAttribute;this.joinpointIdentification = joinpointIdentification;}public PlatformTransactionManager getTransactionManager() {Assert.state(this.transactionManager != null, "No PlatformTransactionManager set");return this.transactionManager;}@Nullablepublic TransactionAttribute getTransactionAttribute() {return this.transactionAttribute;}public String getJoinpointIdentification() {return this.joinpointIdentification;}public void newTransactionStatus(@Nullable TransactionStatus status) {this.transactionStatus = status;}@Nullablepublic TransactionStatus getTransactionStatus() {return this.transactionStatus;}public boolean hasTransaction() {return this.transactionStatus != null;}private void bindToThread() {this.oldTransactionInfo = (TransactionInfo)TransactionAspectSupport.transactionInfoHolder.get();TransactionAspectSupport.transactionInfoHolder.set(this);}private void restoreThreadLocalStatus() {TransactionAspectSupport.transactionInfoHolder.set(this.oldTransactionInfo);}    
}// 事务信息持有器(transactionInfoHolder)是一个ThreadLocal对象，
// 用于保存当前事务信息，及上一个事务信息（挂起时），以便恢复上一个事务信息；
private static final ThreadLocal<TransactionInfo> transactionInfoHolder = new NamedThreadLocal("Current aspect-driven transaction");

事务信息持有器(transactionInfoHolder)：是一个ThreadLocal对象，用于保存当前事务信息，及上一个事务信息（挂起时），以便恢复上一个事务信息；

【补充】TransactionStatus事务状态定义

1）TransactionStatus继承自 TransactionExecution, SavepointManager

2）TransactionStatus封装了事务状态方法，包括是否有保存点，当前线程是否存在事务，当前线程是否存在新事务，是否只读，设置回滚状态或判断是否回滚，事务是否完成等状态方法；

3）封装的事务方法包括：

• 保存点相关方法：包括 hasSavepoint ， createSavepoint， rollbackToSavepoint， releaseSavepoint
• 事务相关方法： 包括hasTransaction， isNewTransaction， isReadOnly， setRollbackOnly， isRollbackOnly， isCompleted；
• 嵌套传播模式相关方法：isNested （嵌套模式底层原理是保存点）

public interface TransactionStatus extends TransactionExecution, SavepointManager, Flushable {default boolean hasSavepoint() {return false;}default void flush() {}
}
// 事务执行 
public interface TransactionExecution {default String getTransactionName() {return "";}default boolean hasTransaction() {return true;}default boolean isNewTransaction() {return true;}default boolean isNested() {return false;}default boolean isReadOnly() {return false;}default void setRollbackOnly() {throw new UnsupportedOperationException("setRollbackOnly not supported");}default boolean isRollbackOnly() {return false;}default boolean isCompleted() {return false;}
}
// 保存点管理器
public interface SavepointManager {Object createSavepoint() throws TransactionException;void rollbackToSavepoint(Object savepoint) throws TransactionException;void releaseSavepoint(Object savepoint) throws TransactionException;
}

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【步骤9.2】执行目标方法

1）调用 invocation.proceedWithInvocation() 调用目标方法； invocation的类是 CglibMethodInvocation，即cglib代理方法调用类；

2）invocation.proceedWithInvocation()：调用CglibMethodInvocation#proceed() 方法，又CglibMethodInvocation是封装了目标方法的cglib方法调用代理对象，所以接着会调用目标方法，即UserSupportImpl#saveNewUser()方法；

3）接着调用父类ReflectiveMethodInvocation的proceed()方法；

4）接着执行ReflectiveMethodInvocation#invokeJoinpoint方法，如下。

5）接着执行 AopUtils#invokeJoinpointUsingReflection() 方法

其中 originalMethod.invoke(target, args) 是通过反射调用具体的目标方法；