Springboot源码分析之事务拦截和管理

2019 年 10 月 3 日
筆記

摘要：

在springboot的自动装配事务里面，InfrastructureAdvisorAutoProxyCreator ,TransactionInterceptor,PlatformTransactionManager这三个bean都被装配进来了，InfrastructureAdvisorAutoProxyCreator已经讲过了，就是一个后置处理器，并且优先级不是很高，而是最低，今天的重点是讲解后面两者之间在事务的扮演角色。TransactionInterceptor作为事务的增强子，扮演着增强处理Spring事务的核心角色。

TransactionInterceptor支撑着整个事务功能的架构，逻辑还是相对复杂的，那么现在我们切入正题来分析此拦截器是如何实现事务特性的。

Spring事务三大接口

TransactionDefinition：用于描述隔离级别、超时时间、是否为只读事务和事务传播规则

    public interface TransactionDefinition {          int PROPAGATION_REQUIRED = 0;          int PROPAGATION_SUPPORTS = 1;          int PROPAGATION_MANDATORY = 2;          int PROPAGATION_REQUIRES_NEW = 3;          int PROPAGATION_NOT_SUPPORTED = 4;          int PROPAGATION_NEVER = 5;          int PROPAGATION_NESTED = 6;          int ISOLATION_DEFAULT = -1;          int ISOLATION_READ_UNCOMMITTED = 1;          int ISOLATION_READ_COMMITTED = 2;          int ISOLATION_REPEATABLE_READ = 4;          int ISOLATION_SERIALIZABLE = 8;          int TIMEOUT_DEFAULT = -1;      }

TransactionStatus：代表一个事务的具体运行状态、以及保存点

    public interface TransactionStatus extends SavepointManager, Flushable {         // 判断当前的事务是否是新事务          boolean isNewTransaction();         // 判断该事务里面是否含有保存点          boolean hasSavepoint();           // 这是事务的唯一结果是否进行回滚。因此如果你在外层给try catche住不让事务回滚，就会抛出你可能常见的异常          void setRollbackOnly();            boolean isRollbackOnly();            void flush();         // 不管是commit或者rollback了都算结束了~~~          boolean isCompleted();      }

一般都是使用它的实现类DefaultTransactionStatus,它是Spring默认使用的事务状态。

PlatformTransactionManager：一个高层次的接口，看名字就知道是管理事务的

    public interface PlatformTransactionManager {          TransactionStatus getTransaction(@Nullable TransactionDefinition var1) throws TransactionException;            void commit(TransactionStatus var1) throws TransactionException;            void rollback(TransactionStatus var1) throws TransactionException;      }

事务拦截器

    public class TransactionInterceptor extends TransactionAspectSupport implements MethodInterceptor, Serializable {          public TransactionInterceptor() {          }            public TransactionInterceptor(PlatformTransactionManager ptm, Properties attributes) {              this.setTransactionManager(ptm);              this.setTransactionAttributes(attributes);          }            public TransactionInterceptor(PlatformTransactionManager ptm, TransactionAttributeSource tas) {              this.setTransactionManager(ptm);              this.setTransactionAttributeSource(tas);          }          //最重要的方法，拦截入口          @Nullable          public Object invoke(MethodInvocation invocation) throws Throwable {              Class<?> targetClass = invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null;              Method var10001 = invocation.getMethod();              invocation.getClass();              return this.invokeWithinTransaction(var10001, targetClass, invocation::proceed);          }      //省略无关代码......      }

我们已经知道了，它是个MethodInterceptor，被事务拦截的方法最终都会执行到此增强器身上。
MethodInterceptor是个环绕通知，敲好符合我们的开启、提交、回滚事务等操作，源码分析可以看出，真正做事情的其实还是在父类，它有一个执行事务的模版。

TransactionAspectSupport

    public abstract class TransactionAspectSupport implements BeanFactoryAware, InitializingBean {          private static final Object DEFAULT_TRANSACTION_MANAGER_KEY = new Object();          // currentTransactionStatus() 方法需要使用到它          private static final ThreadLocal<TransactionAspectSupport.TransactionInfo> transactionInfoHolder = new NamedThreadLocal("Current aspect-driven transaction");          protected final Log logger = LogFactory.getLog(this.getClass());          //事务管理器的名称          @Nullable          private String transactionManagerBeanName;          //事务管理器          @Nullable          private PlatformTransactionManager transactionManager;          //事务属性源          @Nullable          private TransactionAttributeSource transactionAttributeSource;          @Nullable          private BeanFactory beanFactory;          // 因为事务管理器可能也会有多个  所以此处做了一个简单的缓存~          private final ConcurrentMap<Object, PlatformTransactionManager> transactionManagerCache = new ConcurrentReferenceHashMap(4);            public TransactionAspectSupport() {          }            @Nullable          protected static TransactionAspectSupport.TransactionInfo currentTransactionInfo() throws NoTransactionException {              return (TransactionAspectSupport.TransactionInfo)transactionInfoHolder.get();          }          //外部调用此Static方法，可议获取到当前事务的状态  从而甚至可议手动来提交、回滚事务          public static TransactionStatus currentTransactionStatus() throws NoTransactionException {              TransactionAspectSupport.TransactionInfo info = currentTransactionInfo();              if (info != null && info.transactionStatus != null) {                  return info.transactionStatus;              } else {                  throw new NoTransactionException("No transaction aspect-managed TransactionStatus in scope");              }          }         //省略无关代码......        // 这里可以发现，若传入的为Properties  内部是实际使用的是NameMatchTransactionAttributeSource 去匹配的，transactionAttributeSource会被覆盖的哟          public void setTransactionAttributes(Properties transactionAttributes) {              NameMatchTransactionAttributeSource tas = new NameMatchTransactionAttributeSource();              tas.setProperties(transactionAttributes);              this.transactionAttributeSource = tas;          }          // 根据方法和目标类来选择          public void setTransactionAttributeSources(TransactionAttributeSource... transactionAttributeSources) {              this.transactionAttributeSource = new CompositeTransactionAttributeSource(transactionAttributeSources);          }         //省略无关代码......        // 接下来就只剩我们最为核心的处理事务的模版方法了         @Nullable          protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,                  final InvocationCallback invocation) throws Throwable {                // If the transaction attribute is null, the method is non-transactional.          // 获取事务属性源~              TransactionAttributeSource tas = getTransactionAttributeSource();          // 获取该方法对应的事务属性（这个特别重要）         // 不同的事务处理方式使用不同的逻辑。对于声明式事务的处理与编程式事务的处理，重要区别在于事务属性上，因为编程式的事务处理是不需要有事务属性的              final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);          // 找到一个合适的事务管理器              final PlatformTransactionManager tm = determineTransactionManager(txAttr);          // 拿到目标方法唯一标识              final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);                if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {                  // Standard transaction demarcation with getTransaction and commit/rollback calls.            // 看是否有必要创建一个事务，根据`事务传播行为`，做出相应的判断                  TransactionInfo txInfo = createTransactionIfNecessary(tm, 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.              //回调方法执行，执行目标方法（原有的业务逻辑）                      retVal = invocation.proceedWithInvocation();                  }                  catch (Throwable ex) {                      // target invocation exception              // 出现异常了，进行回滚（注意：并不是所有异常都会rollback的）                      // 备注：此处若没有事务属性   会commit 兼容编程式事务吧                      completeTransactionAfterThrowing(txInfo, ex);                      throw ex;                  }                  finally {              //清除信息                      cleanupTransactionInfo(txInfo);                  }            // 目标方法完全执行完成后，提交事务~~~                  commitTransactionAfterReturning(txInfo);                  return retVal;              }              else {            //编程式事务处理(CallbackPreferringPlatformTransactionManager) 会走这里              // 原理也差不太多，这里不做详解~~~~                    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;                  }              }          }        // 从容器中找到一个事务管理器          @Nullable          protected PlatformTransactionManager determineTransactionManager(@Nullable TransactionAttribute txAttr) {              if (txAttr != null && this.beanFactory != null) {                // qualifier 就在此处发挥作用了，他就相当于BeanName                  String qualifier = txAttr.getQualifier();                  if (StringUtils.hasText(qualifier)) {                    // 根据此名称 以及PlatformTransactionManager.class 去容器内找                      return this.determineQualifiedTransactionManager(this.beanFactory, qualifier);  // 若没有指定qualifier   那再看看是否指定了 transactionManagerBeanName                  } else if (StringUtils.hasText(this.transactionManagerBeanName)) {                        return this.determineQualifiedTransactionManager(this.beanFactory, this.transactionManagerBeanName);                  } else {                    // 若都没指定，那就不管了。直接根据类型去容器里找 getBean(Class)              // 此处：若容器内有两个PlatformTransactionManager ，那就铁定会报错啦~~~                        PlatformTransactionManager defaultTransactionManager = this.getTransactionManager();                      if (defaultTransactionManager == null) {                          defaultTransactionManager = (PlatformTransactionManager)this.transactionManagerCache.get(DEFAULT_TRANSACTION_MANAGER_KEY);                          if (defaultTransactionManager == null) {                              defaultTransactionManager = (PlatformTransactionManager)this.beanFactory.getBean(PlatformTransactionManager.class);                              this.transactionManagerCache.putIfAbsent(DEFAULT_TRANSACTION_MANAGER_KEY, defaultTransactionManager);                          }                      }                        return defaultTransactionManager;                  }              } else {                // 如果这两个都没配置，所以肯定是手动设置了PlatformTransactionManager的，那就直接返回即可                  return this.getTransactionManager();              }          }            private PlatformTransactionManager determineQualifiedTransactionManager(BeanFactory beanFactory, String qualifier) {              PlatformTransactionManager txManager = (PlatformTransactionManager)this.transactionManagerCache.get(qualifier);              if (txManager == null) {                  txManager = (PlatformTransactionManager)BeanFactoryAnnotationUtils.qualifiedBeanOfType(beanFactory, PlatformTransactionManager.class, qualifier);                  this.transactionManagerCache.putIfAbsent(qualifier, txManager);              }                return txManager;          }            private String methodIdentification(Method method, @Nullable Class<?> targetClass, @Nullable TransactionAttribute txAttr) {              String methodIdentification = this.methodIdentification(method, targetClass);              if (methodIdentification == null) {                  if (txAttr instanceof DefaultTransactionAttribute) {                      methodIdentification = ((DefaultTransactionAttribute)txAttr).getDescriptor();                  }                    if (methodIdentification == null) {                      methodIdentification = ClassUtils.getQualifiedMethodName(method, targetClass);                  }              }                return methodIdentification;          }            @Nullable          protected String methodIdentification(Method method, @Nullable Class<?> targetClass) {              return null;          }      // 若有需要 创建一个TransactionInfo (具体的事务从事务管理器里面getTransaction())          protected TransactionAspectSupport.TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm, @Nullable TransactionAttribute txAttr, final String joinpointIdentification) {            //赋值              if (txAttr != null && ((TransactionAttribute)txAttr).getName() == null) {                  txAttr = new DelegatingTransactionAttribute((TransactionAttribute)txAttr) {                      public String getName() {                          return joinpointIdentification;                      }                  };              }      // 从事务管理器里，通过txAttr拿出来一个TransactionStatus              TransactionStatus status = null;              if (txAttr != null) {                  if (tm != null) {                      status = tm.getTransaction((TransactionDefinition)txAttr);                  } else if (this.logger.isDebugEnabled()) {                      this.logger.debug("Skipping transactional joinpoint [" + joinpointIdentification + "] because no transaction manager has been configured");                  }              }      // 通过TransactionStatus 等，转换成一个通用的TransactionInfo              return this.prepareTransactionInfo(tm, (TransactionAttribute)txAttr, joinpointIdentification, status);          }          protected TransactionAspectSupport.TransactionInfo prepareTransactionInfo(@Nullable PlatformTransactionManager tm, @Nullable TransactionAttribute txAttr, String joinpointIdentification, @Nullable TransactionStatus status) {            //构造一个TransactionInfo              TransactionAspectSupport.TransactionInfo txInfo = new TransactionAspectSupport.TransactionInfo(tm, txAttr, joinpointIdentification);              if (txAttr != null) {                  if (this.logger.isTraceEnabled()) {                      this.logger.trace("Getting transaction for [" + txInfo.getJoinpointIdentification() + "]");                  }                 // 设置事务状态                  txInfo.newTransactionStatus(status);              } else if (this.logger.isTraceEnabled()) {                  this.logger.trace("No need to create transaction for [" + joinpointIdentification + "]: This method is not transactional.");              }      // 这句话是最重要的，把生成的TransactionInfo并绑定到当前线程的ThreadLocal              txInfo.bindToThread();              return txInfo;          }      //比较简单  只用用事务管理器提交事务即可~~~  具体的实现逻辑在事务管理器的commit实现里~~          protected void commitTransactionAfterReturning(@Nullable TransactionAspectSupport.TransactionInfo txInfo) {              if (txInfo != null && txInfo.getTransactionStatus() != null) {                  if (this.logger.isTraceEnabled()) {                      this.logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() + "]");                  }                    txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());              }            }            protected void completeTransactionAfterThrowing(@Nullable TransactionAspectSupport.TransactionInfo txInfo, Throwable ex) {              if (txInfo != null && txInfo.getTransactionStatus() != null) {                  if (this.logger.isTraceEnabled()) {                      this.logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() + "] after exception: " + ex);                  }      // 如果有事务属性了，那就调用rollbackOn看看这个异常需不需要回滚                  if (txInfo.transactionAttribute != null && txInfo.transactionAttribute.rollbackOn(ex)) {                      try {                          txInfo.getTransactionManager().rollback(txInfo.getTransactionStatus());                      } catch (TransactionSystemException var6) {                          this.logger.error("Application exception overridden by rollback exception", ex);                          var6.initApplicationException(ex);                          throw var6;                      } catch (Error | RuntimeException var7) {                          this.logger.error("Application exception overridden by rollback exception", ex);                          throw var7;                      }                  } else {                    // 编程式事务没有事务属性，那就commit吧                      try {                          txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());                      } catch (TransactionSystemException var4) {                          this.logger.error("Application exception overridden by commit exception", ex);                          var4.initApplicationException(ex);                          throw var4;                      } catch (Error | RuntimeException var5) {                          this.logger.error("Application exception overridden by commit exception", ex);                          throw var5;                      }                  }              }            }            protected void cleanupTransactionInfo(@Nullable TransactionAspectSupport.TransactionInfo txInfo) {              if (txInfo != null) {                  txInfo.restoreThreadLocalStatus();              }            }            private static class ThrowableHolderException extends RuntimeException {              public ThrowableHolderException(Throwable throwable) {                  super(throwable);              }                public String toString() {                  return this.getCause().toString();              }          }            private static class ThrowableHolder {              @Nullable              public Throwable throwable;                private ThrowableHolder() {              }          }            @FunctionalInterface          protected interface InvocationCallback {              Object proceedWithInvocation() throws Throwable;          }            protected final class TransactionInfo {            // 当前事务  的事务管理器              @Nullable              private final PlatformTransactionManager transactionManager;            // 当前事务  的事务属性              @Nullable              private final TransactionAttribute transactionAttribute;            //joinpoint标识              private final String joinpointIdentification;            //当前事务    的TransactionStatus              @Nullable              private TransactionStatus transactionStatus;            // 重点就是这个oldTransactionInfo字段                // 这个字段保存了当前事务所在的`父事务`上下文的引用，构成了一个链，准确的说是一个有向无环图                @Nullable              private TransactionAspectSupport.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;              }                @Nullable              public TransactionAttribute getTransactionAttribute() {                  return this.transactionAttribute;              }                public String getJoinpointIdentification() {                  return this.joinpointIdentification;              }              //注意这个方法名，新的一个事务status              public void newTransactionStatus(@Nullable TransactionStatus status) {                  this.transactionStatus = status;              }                @Nullable              public TransactionStatus getTransactionStatus() {                  return this.transactionStatus;              }                public boolean hasTransaction() {                  return this.transactionStatus != null;              }               //绑定当前正在处理的事务的所有信息到ThreadLocal              private void bindToThread() {                // 老的事务  先从线程中拿出来，再把新的（也就是当前）绑定进去~~~~~~                  this.oldTransactionInfo = (TransactionAspectSupport.TransactionInfo)TransactionAspectSupport.transactionInfoHolder.get();                  TransactionAspectSupport.transactionInfoHolder.set(this);              }              //当前事务处理完之后，恢复父事务上下文              private void restoreThreadLocalStatus() {                  TransactionAspectSupport.transactionInfoHolder.set(this.oldTransactionInfo);              }                public String toString() {                  return this.transactionAttribute != null ? this.transactionAttribute.toString() : "No transaction";              }          }      }

事务管理器

file

AbstractPlatformTransactionManager

可见它是对PlatformTransactionManager的一个抽象实现。实现Spring的标准事务工作流
这个基类提供了以下工作流程处理：

确定如果有现有的事务;
应用适当的传播行为;
如果有必要暂停和恢复事务;
提交时检查rollback-only标记;
应用适当的修改当回滚(实际回滚或设置rollback-only);
触发同步回调注册(如果事务同步是激活的)

    public abstract class AbstractPlatformTransactionManager implements PlatformTransactionManager, Serializable {            //始终激活事务同步（请参阅事务的传播属性~）          public static final int SYNCHRONIZATION_ALWAYS = 0;          //仅对实际事务（即，不针对由传播导致的空事务）激活事务同步不支持现有后端事务          public static final int SYNCHRONIZATION_ON_ACTUAL_TRANSACTION = 1;          //永远不激活事务同步          public static final int SYNCHRONIZATION_NEVER = 2;            // 相当于把本类的所有的public static final的变量都收集到此处~~~~          private static final Constants constants = new Constants(AbstractPlatformTransactionManager.class);            // ===========默认值          private int transactionSynchronization = SYNCHRONIZATION_ALWAYS;          // 事务默认的超时时间  为-1表示不超时          private int defaultTimeout = TransactionDefinition.TIMEOUT_DEFAULT;          //Set whether nested transactions are allowed. Default is "false".          private boolean nestedTransactionAllowed = false;          // Set whether existing transactions should be validated before participating（参与、加入）          private boolean validateExistingTransaction = false;            //设置是否仅在参与事务`失败后`将 现有事务`全局`标记为回滚  默认值是true 需要注意~~~          // 表示只要你的事务失败了，就标记此事务为rollback-only 表示它只能给与回滚  而不能再commit或者正常结束了          // 这个调用者经常会犯的一个错误就是：上层事务service抛出异常了，自己把它给try住，并且并且还不throw，那就肯定会报错的：          // 报错信息：Transaction rolled back because it has been marked as rollback-only          // 当然喽，这个属性强制不建议设置为false~~~~~~          private boolean globalRollbackOnParticipationFailure = true;          // 如果事务被全局标记为仅回滚，则设置是否及早失败~~~~          private boolean failEarlyOnGlobalRollbackOnly = false;          // 设置在@code docommit调用失败时是否应执行@code dorollback 通常不需要，因此应避免          private boolean rollbackOnCommitFailure = false;            // 我们发现使用起来有点枚举的意思了，特别是用XML配置的时候  非常像枚举的使用~~~~~~~          // 这也是Constants的重要意义~~~~          public final void setTransactionSynchronizationName(String constantName) {              setTransactionSynchronization(constants.asNumber(constantName).intValue());          }          public final void setTransactionSynchronization(int transactionSynchronization) {              this.transactionSynchronization = transactionSynchronization;          }          //... 省略上面所有字段的一些get/set方法~~~            // 最为重要的一个方法，根据实物定义，获取到一个事务TransactionStatus          @Override          public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {              //doGetTransaction()方法是抽象方法，具体的实现由具体的事务处理器提供（下面会以DataSourceTransactionManager为例子）              Object transaction = doGetTransaction();                //如果没有配置事务属性，则使用默认的事务属性              if (definition == null) {                  definition = new DefaultTransactionDefinition();              }                //检查当前线程是否存在事务  isExistingTransaction此方法默认返回false  但子类都复写了此方法              if (isExistingTransaction(transaction)) {                  // handleExistingTransaction方法为处理已经存在事务的情况                  // 这个方法的实现也很复杂，总之还是对一些传播属性进行解析，各种情况的考虑~~~~~ 如果有新事务产生 doBegin()就会被调用~~~~                  return handleExistingTransaction(definition, transaction, debugEnabled);              }                // 超时时间的简单校验~~~~              if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {                  throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());              }                // 处理事务属性中配置的事务传播特性==============                // PROPAGATION_MANDATORY 如果已经存在一个事务，支持当前事务。如果没有一个活动的事务，则抛出异常              if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {                  throw new IllegalTransactionStateException("No existing transaction found for transaction marked with propagation 'mandatory'");              }                //如果事务传播特性为required、required_new或nested              else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||                      definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||                      definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {                    // 挂起，但是doSuspend()由子类去实现~~~                  // 挂起操作，触发相关的挂起注册的事件，把当前线程事物的所有属性都封装好，放到一个SuspendedResourcesHolder                  // 然后清空清空一下`当前线程事务`                  SuspendedResourcesHolder suspendedResources = suspend(null);                    // 此处，开始创建事务~~~~~                  try {                      boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);                        // //创建一个新的事务状态  就是new DefaultTransactionStatus()  把个属性都赋值上                      DefaultTransactionStatus status = newTransactionStatus(                              definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);                      // 开始事务，抽象方法，由子类去实现~                      doBegin(transaction, definition);                      //初始化和同步事务状态    是TransactionSynchronizationManager这个类  它内部维护了很多的ThreadLocal                      prepareSynchronization(status, definition);                      return status;                  }                  catch (RuntimeException | Error ex) {                      //重新开始 doResume由子类去实现                      resume(null, suspendedResources);                      throw ex;                  }              }              // 走到这里  传播属性就是不需要事务的  那就直接创建一个              else {                  boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);                  // 这个方法相当于先newTransactionStatus,再prepareSynchronization这两步~~~                  // 显然和上面的区别是：中间不回插入调用doBegin()方法，因为没有事务  begin个啥~~                  return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);              }          }              // 再看看commit方法          @Override          public final void commit(TransactionStatus status) throws TransactionException {              //如果是一个已经完成的事物，不可重复提交              if (status.isCompleted()) {                  throw new IllegalTransactionStateException("Transaction is already completed - do not call commit or rollback more than once per transaction");              }                DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;              // 如果已经标记为了需要回滚，那就执行回滚吧              if (defStatus.isLocalRollbackOnly()) {                  processRollback(defStatus, false);                  return;              }                //  shouldCommitOnGlobalRollbackOnly这个默认值是false，目前只有JTA事务复写成true了              // isGlobalRollbackOnly：是否标记为了全局的RollbackOnly              if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {                  processRollback(defStatus, true);                  return;              }              // 提交事务   这里面还是挺复杂的，会考虑到还原点、新事务、事务是否是rollback-only之类的~~              processCommit(defStatus);          }            // rollback方法  里面doRollback方法交给子类去实现~~~          @Override          public final void rollback(TransactionStatus status) throws TransactionException {              DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;              processRollback(defStatus, false);          }      }

从这个抽象类源码分析可以看出，它绝对是一个非常非常典型的模版实现，各个方法实现都是这样。自己先提供实现模版，很多具体的实现方案都开放给子类，比如begin,suspend, resume,commit,rollback等，相当于留好了众多的连接点

DataSourceTransactionManager

    // 它还实现了ResourceTransactionManager接口，提供了getResourceFactory()方法      public class DataSourceTransactionManager extends AbstractPlatformTransactionManager implements ResourceTransactionManager, InitializingBean {          // 显然它管理的就是DataSource  而JTA分布式事务管理可能就是各种各样的数据源了          @Nullable          private DataSource dataSource;          // 不要强制标记为ReadOnly          private boolean enforceReadOnly = false;            // JDBC默认是允许内嵌的事务的          public DataSourceTransactionManager() {              setNestedTransactionAllowed(true);          }          public DataSourceTransactionManager(DataSource dataSource) {              this();              setDataSource(dataSource);              // 它自己的InitializingBean也是做了一个简单的校验而已~~~              afterPropertiesSet();          }            // 手动设置数据源          public void setDataSource(@Nullable DataSource dataSource) {              // 这步处理有必要              // TransactionAwareDataSourceProxy是对dataSource 的包装              if (dataSource instanceof TransactionAwareDataSourceProxy) {                  this.dataSource = ((TransactionAwareDataSourceProxy) dataSource).getTargetDataSource();              } else {                  this.dataSource = dataSource;              }          }            //Return the JDBC DataSource          @Nullable          public DataSource getDataSource() {              return this.dataSource;          }          // @since 5.0 Spring5.0提供的方法   其实还是调用的getDataSource()  判空了而已          protected DataSource obtainDataSource() {              DataSource dataSource = getDataSource();              Assert.state(dataSource != null, "No DataSource set");              return dataSource;          }          // 直接返回的数据源~~~~          @Override          public Object getResourceFactory() {              return obtainDataSource();          }          ...          // 这里返回的是一个`DataSourceTransactionObject`          // 它是一个`JdbcTransactionObjectSupport`，所以它是SavepointManager、实现了SmartTransactionObject接口          @Override          protected Object doGetTransaction() {              DataSourceTransactionObject txObject = new DataSourceTransactionObject();              txObject.setSavepointAllowed(isNestedTransactionAllowed());              // 这个获取有意思~~~~相当于按照线程来的~~~              ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(obtainDataSource());              txObject.setConnectionHolder(conHolder, false);              return txObject;          }            // 检查当前事务是否active          @Override          protected boolean isExistingTransaction(Object transaction) {              DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;              return (txObject.hasConnectionHolder() && txObject.getConnectionHolder().isTransactionActive());          }              // 这是一个核心内容了，里面逻辑需要分析分析~~~          @Override          protected void doBegin(Object transaction, TransactionDefinition definition) {              DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;              Connection con = null;                try {                  if (!txObject.hasConnectionHolder() || txObject.getConnectionHolder().isSynchronizedWithTransaction()) {                      // 从DataSource里获取一个连接（这个DataSource一般是有连接池的~~~）                      Connection newCon = obtainDataSource().getConnection();                      // 把这个链接用ConnectionHolder包装一下~~~                      txObject.setConnectionHolder(new ConnectionHolder(newCon), true);                  }                    txObject.getConnectionHolder().setSynchronizedWithTransaction(true);                  con = txObject.getConnectionHolder().getConnection();                    // 设置isReadOnly、设置隔离界别等~                  Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);                  txObject.setPreviousIsolationLevel(previousIsolationLevel);                    // 这里非常的关键，先看看Connection 是否是自动提交的                  // 如果是 就con.setAutoCommit(false)  要不然数据库默认没执行一条SQL都是一个事务，就没法进行事务的管理了                  if (con.getAutoCommit()) {                      txObject.setMustRestoreAutoCommit(true);                      con.setAutoCommit(false);                  }                  // ====因此从这后面，通过此Connection执行的所有SQL语句只要没有commit就都不会提交给数据库的=====                    // 这个方法特别特别有意思   它自己`Statement stmt = con.createStatement()`拿到一个Statement                  // 然后执行了一句SQL：`stmt.executeUpdate("SET TRANSACTION READ ONLY");`                  // 所以，所以：如果你仅仅只是查询。把事务的属性设置为readonly=true  Spring对帮你对SQl进行优化的                  // 需要注意的是：readonly=true 后，只能读，不能进行dml操作）（只能看到设置事物前数据的变化，看不到设置事物后数据的改变）                  prepareTransactionalConnection(con, definition);                  txObject.getConnectionHolder().setTransactionActive(true);                    int timeout = determineTimeout(definition);                  if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {                      txObject.getConnectionHolder().setTimeoutInSeconds(timeout);                  }                    // Bind the connection holder to the thread.                  // 这一步：就是把当前的链接 和当前的线程进行绑定~~~~                  if (txObject.isNewConnectionHolder()) {                      TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());                  }              } catch (Throwable ex) {                  // 如果是新创建的链接，那就释放~~~~                  if (txObject.isNewConnectionHolder()) {                      DataSourceUtils.releaseConnection(con, obtainDataSource());                      txObject.setConnectionHolder(null, false);                  }                  throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);              }          }            // 真正提交事务          @Override          protected void doCommit(DefaultTransactionStatus status) { DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();              // 拿到链接  然后直接就commit了              Connection con = txObject.getConnectionHolder().getConnection();              try {                  con.commit();              } catch (SQLException ex) {                  throw new TransactionSystemException("Could not commit JDBC transaction", ex);              }          }          //doRollback()方法也类似  这里不再细说      }

小结：

事务属性readonly=true后，只能读操作）（只能看到设置事物前数据的变化，看不到设置事物后数据的改变）但是通过源码我发现，你只设置@Transactional(readOnly = true)这样是不够的，还必须在配置DataSourceTransactionManager的时候，来这么一句dataSourceTransactionManager.setEnforceReadOnly(true)，最终才会对你的只读事务进行优化~~~~
其实如果仅仅只是设置@Transactional(readOnly = true)，最终会把这个Connection设置为只读：con.setReadOnly(true); 它表示将此连接设置为只读模式，作为驱动程序启用数据库优化的提示。将链接设置为只读模式通知数据库后，数据库会对做自己的只读优化。但是，这对数据库而言不一定对于数据库而言这就是readonly事务，这点是非常重要的。（因为毕竟一个事务内可能有多个链接.