【小家Spring】源码分析Spring的事务拦截器：TransactionInterceptor和事务管理器：PlatformTransactionManager

// 可以看到它继承自SavepointManager，所以它也会处理还原点
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还原点~
	boolean hasSavepoint();
	
	// Set the transaction rollback-only
	// 这是了这个，事务的唯一结果是进行回滚。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：
	// Transaction rolled back because it has been marked as rollback-only
	void setRollbackOnly();
	boolean isRollbackOnly();
	
	//将基础会话刷新到数据存储 for example, all affected Hibernate/JPA sessions
	@Override 
	void flush();
	// Return whether this transaction is completed
	// 不管是commit或者rollback了都算结束了~~~
	boolean isCompleted();
}

// @since 1.2.3 
public abstract class AbstractTransactionStatus implements TransactionStatus {
	// 两个标志位
	private boolean rollbackOnly = false;
	private boolean completed = false;

	// 一个还原点
	@Nullable
	private Object savepoint;

	// 把该属性值保存为true
	@Override
	public void setRollbackOnly() {
		this.rollbackOnly = true;
	}
	// 注意此处并不是直接读取
	@Override
	public boolean isRollbackOnly() {
		return (isLocalRollbackOnly() || isGlobalRollbackOnly());
	}
	public boolean isLocalRollbackOnly() {
		return this.rollbackOnly;
	}
	// Global这里返回false 但是子类DefaultTransactionStatus复写了此方法
	public boolean isGlobalRollbackOnly() {
		return false;
	}
	// 啥都没做  也是由子类去实现
	@Override
	public void flush() {
	}
	...
}

public class SimpleTransactionStatus extends AbstractTransactionStatus {
	private final boolean newTransaction;

	// 构造函数
	public SimpleTransactionStatus() {
		this(true);
	}
	public SimpleTransactionStatus(boolean newTransaction) {
		this.newTransaction = newTransaction;
	}
	@Override
	public boolean isNewTransaction() {
		return this.newTransaction;
	}
}

public class DefaultTransactionStatus extends AbstractTransactionStatus {
	// 它有很多的标志位，成员变量  
	@Nullable
	private final Object transaction;
	// 是否是新事务
	private final boolean newTransaction;
	// 如果为给定事务打开了新的事务同步  该值为true
	private final boolean newSynchronization;
	// 该事务是否标记为了只读
	private final boolean readOnly;
	private final boolean debug;


	@Nullable
	private final Object suspendedResources;

	// 它的唯一构造函数如下：
	public DefaultTransactionStatus(@Nullable Object transaction, boolean newTransaction, boolean newSynchronization,
			boolean readOnly, boolean debug, @Nullable Object suspendedResources) {
		this.transaction = transaction;
		this.newTransaction = newTransaction;
		this.newSynchronization = newSynchronization;
		this.readOnly = readOnly;
		this.debug = debug;
		this.suspendedResources = suspendedResources;
	}
	
	// 直接把底层事务返回
	public Object getTransaction() {
		Assert.state(this.transaction != null, "No transaction active");
		return this.transaction;
	}
	public boolean hasTransaction() {
		return (this.transaction != null);
	}
	
	// 首先
	@Override
	public boolean isNewTransaction() {
		return (hasTransaction() && this.newTransaction);
	}
	...
	// 都由SmartTransactionObject去处理  该接口的实现类有：
	// JdbcTransactionObjectSupport和JtaTransactionObject（分布式事务）
	public boolean isGlobalRollbackOnly() {
		return ((this.transaction instanceof SmartTransactionObject) &&
				((SmartTransactionObject) this.transaction).isRollbackOnly());
	}
	@Override
	public void flush() {
		if (this.transaction instanceof SmartTransactionObject) {
			((SmartTransactionObject) this.transaction).flush();
		}
	}

}

public class TransactionInterceptor extends TransactionAspectSupport implements MethodInterceptor, Serializable {
	// 构造函数：
	// 可议不用特殊的指定PlatformTransactionManager 事务管理器，后面会讲解自定义去获取
	// 可议自己指定Properties 以及 TransactionAttributeSource 
	public TransactionInterceptor() {
	}
	public TransactionInterceptor(PlatformTransactionManager ptm, Properties attributes) {
		setTransactionManager(ptm);
		setTransactionAttributes(attributes);
	}
	public TransactionInterceptor(PlatformTransactionManager ptm, TransactionAttributeSource tas) {
		setTransactionManager(ptm);
		setTransactionAttributeSource(tas);
	}

	// 接下来就是这个invoke方法：就是拦截的入口~
	@Override
	@Nullable
	public Object invoke(MethodInvocation invocation) throws Throwable {
		// 获取目标类
		Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);

		// invokeWithinTransaction：父类TransactionAspectSupport的模板方法
		// invocation::proceed本处执行完成  执行目标方法（当然可能还有其余增强器）
		return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
	}
	...
}

// 通过BeanFactoryAware获取到BeanFactory
// InitializingBean的afterPropertiesSet是对Bean做一些验证（经常会借助它这么来校验Bean~~~）
public abstract class TransactionAspectSupport implements BeanFactoryAware, InitializingBean {

	// 这个类不允许实现Serializable接口，请注意~~
	// NOTE: This class must not implement Serializable because it serves as base
	// class for AspectJ aspects (which are not allowed to implement Serializable)!

	/**
	 * Key to use to store the default transaction manager.
	 */
	private static final Object DEFAULT_TRANSACTION_MANAGER_KEY = new Object();
	// currentTransactionStatus() 方法依托于它
	private static final ThreadLocal<TransactionInfo> transactionInfoHolder = new NamedThreadLocal<>("Current aspect-driven transaction");

	//Subclasses can use this to return the current TransactionInfo.
	// Only subclasses that cannot handle all operations in one method
	// 注意此方法是个静态方法  并且是protected的  说明只有子类能够调用，外部并不可以~~~
	@Nullable
	protected static TransactionInfo currentTransactionInfo() throws NoTransactionException {
		return 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 {
		TransactionInfo info = currentTransactionInfo();
		if (info == null || info.transactionStatus == null) {
			throw new NoTransactionException("No transaction aspect-managed TransactionStatus in scope");
		}
		return info.transactionStatus;
	}

	//==========================================
	// 事务管理器的名称（若设置，会根据此名称去找到事务管理器~~~~）
	@Nullable
	private String transactionManagerBeanName;
	@Nullable
	private PlatformTransactionManager transactionManager;
	@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 void setTransactionAttributeSource(@Nullable TransactionAttributeSource transactionAttributeSource) {
		this.transactionAttributeSource = transactionAttributeSource;
	}
	// 这部操作发现，若传入的为Properties  内部是实际使用的是NameMatchTransactionAttributeSource 去匹配的
	// 备注：若调用了此方法   transactionAttributeSource就会被覆盖的哟
	public void setTransactionAttributes(Properties transactionAttributes) {
		NameMatchTransactionAttributeSource tas = new NameMatchTransactionAttributeSource();
		tas.setProperties(transactionAttributes);
		this.transactionAttributeSource = tas;
	}
	// 若你有多种匹配策略，这也是支持的  可谓非常强大有木有~~~
	public void setTransactionAttributeSources(TransactionAttributeSource... transactionAttributeSources) {
		this.transactionAttributeSource = new CompositeTransactionAttributeSource(transactionAttributeSources);
	}
	...
	// 接下来就只剩我们最为核心的处理事务的模版方法了：
	//protected修饰，不允许其他包和无关类调用
	@Nullable
	protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass, final InvocationCallback invocation) throws Throwable {

		// 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）
		final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);

		// 这个厉害了：就是去找到一个合适的事务管理器（具体策略详见方法~~~)
		final PlatformTransactionManager tm = determineTransactionManager(txAttr);
		// 拿到目标方法唯一标识（类.方法，如service.UserServiceImpl.save）
		final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

		// 如果txAttr为空或者tm 属于非CallbackPreferringPlatformTransactionManager，执行目标增强
		// 在TransactionManager上，CallbackPreferringPlatformTransactionManager实现PlatformTransactionManager接口，暴露出一个方法用于执行事务处理中的回调
		if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
		
			// 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 = null;
			try {
				//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) {
				// 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;
		}

		//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~~~~
		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;
			}
		}
	}

	// 从容器中找到一个事务管理器
	@Nullable
	protected PlatformTransactionManager determineTransactionManager(@Nullable TransactionAttribute txAttr) {
		// 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
		if (txAttr == null || this.beanFactory == null) {
			return getTransactionManager();
		}

		// 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 去容器内招
			return 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 determineQualifiedTransactionManager(this.beanFactory, this.transactionManagerBeanName);
		}
		// 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~~~
		else {
			PlatformTransactionManager defaultTransactionManager = getTransactionManager();
			if (defaultTransactionManager == null) {
				defaultTransactionManager = this.transactionManagerCache.get(DEFAULT_TRANSACTION_MANAGER_KEY);
				if (defaultTransactionManager == null) {
					defaultTransactionManager = this.beanFactory.getBean(PlatformTransactionManager.class);
					this.transactionManagerCache.putIfAbsent(
							DEFAULT_TRANSACTION_MANAGER_KEY, defaultTransactionManager);
				}
			}
			return defaultTransactionManager;
		}
	}
	// ======================================
}

	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 TransactionInfo oldTransactionInfo;
	
		// 唯一的构造函数~~~~
		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;
		}
		
		// 注意这个方法名，新的一个事务status
		public void newTransactionStatus(@Nullable TransactionStatus status) {
			this.transactionStatus = status;
		}
		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() {
			// Expose current TransactionStatus
			// 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 = transactionInfoHolder.get();
			transactionInfoHolder.set(this);
		}

		// 当前事务处理完之后，恢复父事务上下文
		private void restoreThreadLocalStatus() {
			transactionInfoHolder.set(this.oldTransactionInfo);
		}

		@Override
		public String toString() {
			return (this.transactionAttribute != null ? this.transactionAttribute.toString() : "No transaction");
		}
	}

	// 若有需要 创建一个TransactionInfo (具体的事务从事务管理器里面getTransaction()出来~)
	protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm,
			@Nullable TransactionAttribute txAttr, final String joinpointIdentification) {

		// If no name specified, apply method identification as transaction name.
		// 这个简单的说，就是给Name赋值~~~~~
		if (txAttr != null && txAttr.getName() == null) {
			txAttr = new DelegatingTransactionAttribute(txAttr) {
				@Override
				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(txAttr);
			}
			...
		}
		// 通过TransactionStatus 等，转换成一个通用的TransactionInfo
		return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
	}

	protected TransactionInfo prepareTransactionInfo(@Nullable PlatformTransactionManager tm,
			@Nullable TransactionAttribute txAttr, String joinpointIdentification,
			@Nullable TransactionStatus status) {

		// 构造一个TransactionInfo 
		TransactionInfo txInfo = new TransactionInfo(tm, txAttr, joinpointIdentification);
		if (txAttr != null) {
			// 如果已存在不兼容的Tx，事务管理器将标记错误
			txInfo.newTransactionStatus(status);
		}
		,..

		// 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.
		// 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 TransactionInfo txInfo) {
		if (txInfo != null && txInfo.getTransactionStatus() != null) {
			txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
		}
	}

	protected void completeTransactionAfterThrowing(@Nullable TransactionInfo txInfo, Throwable ex) {

		if (txInfo != null && txInfo.getTransactionStatus() != null) {
			// 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());
				}
				...
			}
			// 编程式事务没有事务属性，那就commit吧
			else {
				try {
					txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
				}
				...
			}
		}
	}

	protected void cleanupTransactionInfo(@Nullable TransactionInfo txInfo) {
		if (txInfo != null) {
			txInfo.restoreThreadLocalStatus();
		}
	}

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

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;
	
	// 此处我们直接可以通过属性们来社会，语意思更清晰些了
	// 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);
	}
}

// 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
}

    @Bean
    public PlatformTransactionManager transactionManager(DataSource dataSource) {
        DataSourceTransactionManager dataSourceTransactionManager = new DataSourceTransactionManager(dataSource);
        dataSourceTransactionManager.setEnforceReadOnly(true); // 让事务管理器进行只读事务层面上的优化  建议开启
        return dataSourceTransactionManager;
    }