• 【源码阅读】事务源码

    拜神

    看源码先拜神，事务管理的顶级接口 PlatformTransactionManager 是 Rod Johnson 和 Juergen Hoeller 两位大神写的。嗯，我看的大多源码都是拜 Juergen Hoeller 这位大神
    在这里插入图片描述
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    编程式事务

    接口介绍

    1. Spring 事务的三大接口：PlatformTransactionManager、TransactionDefinition、TransactionStatus
    2. TransactionTemplate 是对上述三个接口的封装，采用了模板设计模式

    PlatformTransactionManager

    事务管理的顶级接口。该接口其实极其简单，只有三个方法，（1）获取或创建事务；（2）提交事务；（3）回滚事务

    同时为了简化事务管理器的实现，该接口提供了一个抽象类 AbstractPlatformTransactionManager，规定了事务管理器的基本框架，仅将依赖于具体平台的特性作为抽象方法留给子类实现，默认实现有 JtaTransactionManager 和 DataSourceTransactionManager。例如使用 JPA，AbstractPlatformTransactionManager 就有多一个 JpaTransactionManager 的实现

    该接口也在最上边的注释上说明了，该接口是供应用直接使用的，不建议作为 API 的首选，建议使用 TransactionTemplate 或者 基于 AOP 的切面

    /**
 * This is the central interface in Spring's transaction infrastructure.
 * Applications can use this directly, but it is not primarily meant as API:
 * Typically, applications will work with either TransactionTemplate or
 * declarative transaction demarcation through AOP.
 *
 * <p>For implementors, it is recommended to derive from the provided
 * {@link org.springframework.transaction.support.AbstractPlatformTransactionManager}
 * class, which pre-implements the defined propagation behavior and takes care
 * of transaction synchronization handling. Subclasses have to implement
 * template methods for specific states of the underlying transaction,
 * for example: begin, suspend, resume, commit.
 *
 * <p>The default implementations of this strategy interface are
 * {@link org.springframework.transaction.jta.JtaTransactionManager} and
 * {@link org.springframework.jdbc.datasource.DataSourceTransactionManager},
 * which can serve as an implementation guide for other transaction strategies.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @since 16.05.2003
 * @see org.springframework.transaction.support.TransactionTemplate
 * @see org.springframework.transaction.interceptor.TransactionInterceptor
 */
public interface PlatformTransactionManager {

	/**
	 * Return a currently active transaction or create a new one, according to
	 * the specified propagation behavior.
	 * <p>Note that parameters like isolation level or timeout will only be applied
	 * to new transactions, and thus be ignored when participating in active ones.
	 * <p>Furthermore, not all transaction definition settings will be supported
	 * by every transaction manager: A proper transaction manager implementation
	 * should throw an exception when unsupported settings are encountered.
	 * <p>An exception to the above rule is the read-only flag, which should be
	 * ignored if no explicit read-only mode is supported. Essentially, the
	 * read-only flag is just a hint for potential optimization.
	 * @param definition the TransactionDefinition instance (can be {@code null} for defaults),
	 * describing propagation behavior, isolation level, timeout etc.
	 * @return transaction status object representing the new or current transaction
	 * @throws TransactionException in case of lookup, creation, or system errors
	 * @throws IllegalTransactionStateException if the given transaction definition
	 * cannot be executed (for example, if a currently active transaction is in
	 * conflict with the specified propagation behavior)
	 * @see TransactionDefinition#getPropagationBehavior
	 * @see TransactionDefinition#getIsolationLevel
	 * @see TransactionDefinition#getTimeout
	 * @see TransactionDefinition#isReadOnly
	 */
	TransactionStatus getTransaction(@Nullable TransactionDefinition definition)
			throws TransactionException;

	/**
	 * Commit the given transaction, with regard to its status. If the transaction
	 * has been marked rollback-only programmatically, perform a rollback.
	 * <p>If the transaction wasn't a new one, omit the commit for proper
	 * participation in the surrounding transaction. If a previous transaction
	 * has been suspended to be able to create a new one, resume the previous
	 * transaction after committing the new one.
	 * <p>Note that when the commit call completes, no matter if normally or
	 * throwing an exception, the transaction must be fully completed and
	 * cleaned up. No rollback call should be expected in such a case.
	 * <p>If this method throws an exception other than a TransactionException,
	 * then some before-commit error caused the commit attempt to fail. For
	 * example, an O/R Mapping tool might have tried to flush changes to the
	 * database right before commit, with the resulting DataAccessException
	 * causing the transaction to fail. The original exception will be
	 * propagated to the caller of this commit method in such a case.
	 * @param status object returned by the {@code getTransaction} method
	 * @throws UnexpectedRollbackException in case of an unexpected rollback
	 * that the transaction coordinator initiated
	 * @throws HeuristicCompletionException in case of a transaction failure
	 * caused by a heuristic decision on the side of the transaction coordinator
	 * @throws TransactionSystemException in case of commit or system errors
	 * (typically caused by fundamental resource failures)
	 * @throws IllegalTransactionStateException if the given transaction
	 * is already completed (that is, committed or rolled back)
	 * @see TransactionStatus#setRollbackOnly
	 */
	void commit(TransactionStatus status) throws TransactionException;

	/**
	 * Perform a rollback of the given transaction.
	 * <p>If the transaction wasn't a new one, just set it rollback-only for proper
	 * participation in the surrounding transaction. If a previous transaction
	 * has been suspended to be able to create a new one, resume the previous
	 * transaction after rolling back the new one.
	 * <p><b>Do not call rollback on a transaction if commit threw an exception.</b>
	 * The transaction will already have been completed and cleaned up when commit
	 * returns, even in case of a commit exception. Consequently, a rollback call
	 * after commit failure will lead to an IllegalTransactionStateException.
	 * @param status object returned by the {@code getTransaction} method
	 * @throws TransactionSystemException in case of rollback or system errors
	 * (typically caused by fundamental resource failures)
	 * @throws IllegalTransactionStateException if the given transaction
	 * is already completed (that is, committed or rolled back)
	 */
	void rollback(TransactionStatus status) throws TransactionException;

}


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    TransactionDefinition

    接口注释解释该接口定义 Spring 的事务属性的接口，也就是说一个事务的配置都在这里做，这个接口包含了事务的传播等级、隔离级别、事务是否只读、事务超时时间

    /**
 * Interface that defines Spring-compliant transaction properties.
 * Based on the propagation behavior definitions analogous to EJB CMT attributes.
 *
 * <p>Note that isolation level and timeout settings will not get applied unless
 * an actual new transaction gets started. As only {@link #PROPAGATION_REQUIRED},
 * {@link #PROPAGATION_REQUIRES_NEW} and {@link #PROPAGATION_NESTED} can cause
 * that, it usually doesn't make sense to specify those settings in other cases.
 * Furthermore, be aware that not all transaction managers will support those
 * advanced features and thus might throw corresponding exceptions when given
 * non-default values.
 *
 * <p>The {@link #isReadOnly() read-only flag} applies to any transaction context,
 * whether backed by an actual resource transaction or operating non-transactionally
 * at the resource level. In the latter case, the flag will only apply to managed
 * resources within the application, such as a Hibernate {@code Session}.
 *
 * @author Juergen Hoeller
 * @since 08.05.2003
 * @see PlatformTransactionManager#getTransaction(TransactionDefinition)
 * @see org.springframework.transaction.support.DefaultTransactionDefinition
 * @see org.springframework.transaction.interceptor.TransactionAttribute
 */
public interface TransactionDefinition {

	/**
	 * Support a current transaction; create a new one if none exists.
	 * Analogous to the EJB transaction attribute of the same name.
	 * <p>This is typically the default setting of a transaction definition,
	 * and typically defines a transaction synchronization scope.
	 */
	int PROPAGATION_REQUIRED = 0;

	/**
	 * Support a current transaction; execute non-transactionally if none exists.
	 * Analogous to the EJB transaction attribute of the same name.
	 * <p><b>NOTE:</b> For transaction managers with transaction synchronization,
	 * {@code PROPAGATION_SUPPORTS} is slightly different from no transaction
	 * at all, as it defines a transaction scope that synchronization might apply to.
	 * As a consequence, the same resources (a JDBC {@code Connection}, a
	 * Hibernate {@code Session}, etc) will be shared for the entire specified
	 * scope. Note that the exact behavior depends on the actual synchronization
	 * configuration of the transaction manager!
	 * <p>In general, use {@code PROPAGATION_SUPPORTS} with care! In particular, do
	 * not rely on {@code PROPAGATION_REQUIRED} or {@code PROPAGATION_REQUIRES_NEW}
	 * <i>within</i> a {@code PROPAGATION_SUPPORTS} scope (which may lead to
	 * synchronization conflicts at runtime). If such nesting is unavoidable, make sure
	 * to configure your transaction manager appropriately (typically switching to
	 * "synchronization on actual transaction").
	 * @see org.springframework.transaction.support.AbstractPlatformTransactionManager#setTransactionSynchronization
	 * @see org.springframework.transaction.support.AbstractPlatformTransactionManager#SYNCHRONIZATION_ON_ACTUAL_TRANSACTION
	 */
	int PROPAGATION_SUPPORTS = 1;

	/**
	 * Support a current transaction; throw an exception if no current transaction
	 * exists. Analogous to the EJB transaction attribute of the same name.
	 * <p>Note that transaction synchronization within a {@code PROPAGATION_MANDATORY}
	 * scope will always be driven by the surrounding transaction.
	 */
	int PROPAGATION_MANDATORY = 2;

	/**
	 * Create a new transaction, suspending the current transaction if one exists.
	 * Analogous to the EJB transaction attribute of the same name.
	 * <p><b>NOTE:</b> Actual transaction suspension will not work out-of-the-box
	 * on all transaction managers. This in particular applies to
	 * {@link org.springframework.transaction.jta.JtaTransactionManager},
	 * which requires the {@code javax.transaction.TransactionManager} to be
	 * made available it to it (which is server-specific in standard Java EE).
	 * <p>A {@code PROPAGATION_REQUIRES_NEW} scope always defines its own
	 * transaction synchronizations. Existing synchronizations will be suspended
	 * and resumed appropriately.
	 * @see org.springframework.transaction.jta.JtaTransactionManager#setTransactionManager
	 */
	int PROPAGATION_REQUIRES_NEW = 3;

	/**
	 * Do not support a current transaction; rather always execute non-transactionally.
	 * Analogous to the EJB transaction attribute of the same name.
	 * <p><b>NOTE:</b> Actual transaction suspension will not work out-of-the-box
	 * on all transaction managers. This in particular applies to
	 * {@link org.springframework.transaction.jta.JtaTransactionManager},
	 * which requires the {@code javax.transaction.TransactionManager} to be
	 * made available it to it (which is server-specific in standard Java EE).
	 * <p>Note that transaction synchronization is <i>not</i> available within a
	 * {@code PROPAGATION_NOT_SUPPORTED} scope. Existing synchronizations
	 * will be suspended and resumed appropriately.
	 * @see org.springframework.transaction.jta.JtaTransactionManager#setTransactionManager
	 */
	int PROPAGATION_NOT_SUPPORTED = 4;

	/**
	 * Do not support a current transaction; throw an exception if a current transaction
	 * exists. Analogous to the EJB transaction attribute of the same name.
	 * <p>Note that transaction synchronization is <i>not</i> available within a
	 * {@code PROPAGATION_NEVER} scope.
	 */
	int PROPAGATION_NEVER = 5;

	/**
	 * Execute within a nested transaction if a current transaction exists,
	 * behave like {@link #PROPAGATION_REQUIRED} otherwise. There is no
	 * analogous feature in EJB.
	 * <p><b>NOTE:</b> Actual creation of a nested transaction will only work on
	 * specific transaction managers. Out of the box, this only applies to the JDBC
	 * {@link org.springframework.jdbc.datasource.DataSourceTransactionManager}
	 * when working on a JDBC 3.0 driver. Some JTA providers might support
	 * nested transactions as well.
	 * @see org.springframework.jdbc.datasource.DataSourceTransactionManager
	 */
	int PROPAGATION_NESTED = 6;


	/**
	 * Use the default isolation level of the underlying datastore.
	 * All other levels correspond to the JDBC isolation levels.
	 * @see java.sql.Connection
	 */
	int ISOLATION_DEFAULT = -1;

	/**
	 * Indicates that dirty reads, non-repeatable reads and phantom reads
	 * can occur.
	 * <p>This level allows a row changed by one transaction to be read by another
	 * transaction before any changes in that row have been committed (a "dirty read").
	 * If any of the changes are rolled back, the second transaction will have
	 * retrieved an invalid row.
	 * @see java.sql.Connection#TRANSACTION_READ_UNCOMMITTED
	 */
	int ISOLATION_READ_UNCOMMITTED = Connection.TRANSACTION_READ_UNCOMMITTED;

	/**
	 * Indicates that dirty reads are prevented; non-repeatable reads and
	 * phantom reads can occur.
	 * <p>This level only prohibits a transaction from reading a row
	 * with uncommitted changes in it.
	 * @see java.sql.Connection#TRANSACTION_READ_COMMITTED
	 */
	int ISOLATION_READ_COMMITTED = Connection.TRANSACTION_READ_COMMITTED;

	/**
	 * Indicates that dirty reads and non-repeatable reads are prevented;
	 * phantom reads can occur.
	 * <p>This level prohibits a transaction from reading a row with uncommitted changes
	 * in it, and it also prohibits the situation where one transaction reads a row,
	 * a second transaction alters the row, and the first transaction re-reads the row,
	 * getting different values the second time (a "non-repeatable read").
	 * @see java.sql.Connection#TRANSACTION_REPEATABLE_READ
	 */
	int ISOLATION_REPEATABLE_READ = Connection.TRANSACTION_REPEATABLE_READ;

	/**
	 * Indicates that dirty reads, non-repeatable reads and phantom reads
	 * are prevented.
	 * <p>This level includes the prohibitions in {@link #ISOLATION_REPEATABLE_READ}
	 * and further prohibits the situation where one transaction reads all rows that
	 * satisfy a {@code WHERE} condition, a second transaction inserts a row
	 * that satisfies that {@code WHERE} condition, and the first transaction
	 * re-reads for the same condition, retrieving the additional "phantom" row
	 * in the second read.
	 * @see java.sql.Connection#TRANSACTION_SERIALIZABLE
	 */
	int ISOLATION_SERIALIZABLE = Connection.TRANSACTION_SERIALIZABLE;


	/**
	 * Use the default timeout of the underlying transaction system,
	 * or none if timeouts are not supported.
	 */
	int TIMEOUT_DEFAULT = -1;


	/**
	 * Return the propagation behavior.
	 * <p>Must return one of the {@code PROPAGATION_XXX} constants
	 * defined on {@link TransactionDefinition this interface}.
	 * @return the propagation behavior
	 * @see #PROPAGATION_REQUIRED
	 * @see org.springframework.transaction.support.TransactionSynchronizationManager#isActualTransactionActive()
	 */
	int getPropagationBehavior();

	/**
	 * Return the isolation level.
	 * <p>Must return one of the {@code ISOLATION_XXX} constants defined on
	 * {@link TransactionDefinition this interface}. Those constants are designed
	 * to match the values of the same constants on {@link java.sql.Connection}.
	 * <p>Exclusively designed for use with {@link #PROPAGATION_REQUIRED} or
	 * {@link #PROPAGATION_REQUIRES_NEW} since it only applies to newly started
	 * transactions. Consider switching the "validateExistingTransactions" flag to
	 * "true" on your transaction manager if you'd like isolation level declarations
	 * to get rejected when participating in an existing transaction with a different
	 * isolation level.
	 * <p>Note that a transaction manager that does not support custom isolation levels
	 * will throw an exception when given any other level than {@link #ISOLATION_DEFAULT}.
	 * @return the isolation level
	 * @see #ISOLATION_DEFAULT
	 * @see org.springframework.transaction.support.AbstractPlatformTransactionManager#setValidateExistingTransaction
	 */
	int getIsolationLevel();

	/**
	 * Return the transaction timeout.
	 * <p>Must return a number of seconds, or {@link #TIMEOUT_DEFAULT}.
	 * <p>Exclusively designed for use with {@link #PROPAGATION_REQUIRED} or
	 * {@link #PROPAGATION_REQUIRES_NEW} since it only applies to newly started
	 * transactions.
	 * <p>Note that a transaction manager that does not support timeouts will throw
	 * an exception when given any other timeout than {@link #TIMEOUT_DEFAULT}.
	 * @return the transaction timeout
	 */
	int getTimeout();

	/**
	 * Return whether to optimize as a read-only transaction.
	 * <p>The read-only flag applies to any transaction context, whether backed
	 * by an actual resource transaction ({@link #PROPAGATION_REQUIRED}/
	 * {@link #PROPAGATION_REQUIRES_NEW}) or operating non-transactionally at
	 * the resource level ({@link #PROPAGATION_SUPPORTS}). In the latter case,
	 * the flag will only apply to managed resources within the application,
	 * such as a Hibernate {@code Session}.
	 * <p>This just serves as a hint for the actual transaction subsystem;
	 * it will <i>not necessarily</i> cause failure of write access attempts.
	 * A transaction manager which cannot interpret the read-only hint will
	 * <i>not</i> throw an exception when asked for a read-only transaction.
	 * @return {@code true} if the transaction is to be optimized as read-only
	 * @see org.springframework.transaction.support.TransactionSynchronization#beforeCommit(boolean)
	 * @see org.springframework.transaction.support.TransactionSynchronizationManager#isCurrentTransactionReadOnly()
	 */
	boolean isReadOnly();

	/**
	 * Return the name of this transaction. Can be {@code null}.
	 * <p>This will be used as the transaction name to be shown in a
	 * transaction monitor, if applicable (for example, WebLogic's).
	 * <p>In case of Spring's declarative transactions, the exposed name will be
	 * the {@code fully-qualified class name + "." + method name} (by default).
	 * @return the name of this transaction
	 * @see org.springframework.transaction.interceptor.TransactionAspectSupport
	 * @see org.springframework.transaction.support.TransactionSynchronizationManager#getCurrentTransactionName()
	 */
	@Nullable
	String getName();

}


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    这玩意怎么用的呢。我们回头看看 @Transactional 注解

    @Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface Transactional {

    //...
	Propagation propagation() default Propagation.REQUIRED;

	Isolation isolation() default Isolation.DEFAULT;

    int timeout() default TransactionDefinition.TIMEOUT_DEFAULT;
    //...
}


public enum Propagation {

	REQUIRED(TransactionDefinition.PROPAGATION_REQUIRED),

	SUPPORTS(TransactionDefinition.PROPAGATION_SUPPORTS),

	MANDATORY(TransactionDefinition.PROPAGATION_MANDATORY),

	REQUIRES_NEW(TransactionDefinition.PROPAGATION_REQUIRES_NEW),

	NOT_SUPPORTED(TransactionDefinition.PROPAGATION_NOT_SUPPORTED),

	NEVER(TransactionDefinition.PROPAGATION_NEVER),

	NESTED(TransactionDefinition.PROPAGATION_NESTED);


	private final int value;


	Propagation(int value) {
		this.value = value;
	}

	public int value() {
		return this.value;
	}

}

public enum Isolation {

	DEFAULT(TransactionDefinition.ISOLATION_DEFAULT),

	READ_UNCOMMITTED(TransactionDefinition.ISOLATION_READ_UNCOMMITTED),

	READ_COMMITTED(TransactionDefinition.ISOLATION_READ_COMMITTED),

	REPEATABLE_READ(TransactionDefinition.ISOLATION_REPEATABLE_READ),

	SERIALIZABLE(TransactionDefinition.ISOLATION_SERIALIZABLE);

	private final int value;

	Isolation(int value) {
		this.value = value;
	}

	public int value() {
		return this.value;
	}

}


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    所以就是注解引用了枚举对象，然后枚举对象里边的值其实取自 TransactionDefinition

    TransactionStatus

    记录事务的执行状态，用于flush 或者 回滚

    /**
 * Representation of the status of a transaction.
 *
 * <p>Transactional code can use this to retrieve status information,
 * and to programmatically request a rollback (instead of throwing
 * an exception that causes an implicit rollback).
 *
 * <p>Includes the {@link SavepointManager} interface to provide access
 * to savepoint management facilities. Note that savepoint management
 * is only available if supported by the underlying transaction manager.
 *
 * @author Juergen Hoeller
 * @since 27.03.2003
 * @see #setRollbackOnly()
 * @see PlatformTransactionManager#getTransaction
 * @see org.springframework.transaction.support.TransactionCallback#doInTransaction
 * @see org.springframework.transaction.interceptor.TransactionInterceptor#currentTransactionStatus()
 */
public interface TransactionStatus extends SavepointManager, Flushable {

	/**
	 * Return whether the present transaction is new; otherwise participating
	 * in an existing transaction, or potentially not running in an actual
	 * transaction in the first place.
	 */
	boolean isNewTransaction();

	/**
	 * Return whether this transaction internally carries a savepoint,
	 * that is, has been created as nested transaction based on a savepoint.
	 * <p>This method is mainly here for diagnostic purposes, alongside
	 * {@link #isNewTransaction()}. For programmatic handling of custom
	 * savepoints, use the operations provided by {@link SavepointManager}.
	 * @see #isNewTransaction()
	 * @see #createSavepoint()
	 * @see #rollbackToSavepoint(Object)
	 * @see #releaseSavepoint(Object)
	 */
	boolean hasSavepoint();

	/**
	 * Set the transaction rollback-only. This instructs the transaction manager
	 * that the only possible outcome of the transaction may be a rollback, as
	 * alternative to throwing an exception which would in turn trigger a rollback.
	 * <p>This is mainly intended for transactions managed by
	 * {@link org.springframework.transaction.support.TransactionTemplate} or
	 * {@link org.springframework.transaction.interceptor.TransactionInterceptor},
	 * where the actual commit/rollback decision is made by the container.
	 * @see org.springframework.transaction.support.TransactionCallback#doInTransaction
	 * @see org.springframework.transaction.interceptor.TransactionAttribute#rollbackOn
	 */
	void setRollbackOnly();

	/**
	 * Return whether the transaction has been marked as rollback-only
	 * (either by the application or by the transaction infrastructure).
	 */
	boolean isRollbackOnly();

	/**
	 * Flush the underlying session to the datastore, if applicable:
	 * for example, all affected Hibernate/JPA sessions.
	 * <p>This is effectively just a hint and may be a no-op if the underlying
	 * transaction manager does not have a flush concept. A flush signal may
	 * get applied to the primary resource or to transaction synchronizations,
	 * depending on the underlying resource.
	 */
	@Override
	void flush();

	/**
	 * Return whether this transaction is completed, that is,
	 * whether it has already been committed or rolled back.
	 * @see PlatformTransactionManager#commit
	 * @see PlatformTransactionManager#rollback
	 */
	boolean isCompleted();

}

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    TransactionTemplate

    TransactionTemplate 注释的接是就是简化程序化事务划分和事务异常处理的模板类
    比如它继承了 DefaultTransactionDefinition， DefaultTransactionDefinition 定义了事务默认的 TransactionDefinition
    它里边核心的方法是：execute(TransactionCallback action)

    /**
 * Template class that simplifies programmatic transaction demarcation and
 * transaction exception handling.
 *
 * <p>The central method is {@link #execute}, supporting transactional code that
 * implements the {@link TransactionCallback} interface. This template handles
 * the transaction lifecycle and possible exceptions such that neither the
 * TransactionCallback implementation nor the calling code needs to explicitly
 * handle transactions.
 *
 * <p>Typical usage: Allows for writing low-level data access objects that use
 * resources such as JDBC DataSources but are not transaction-aware themselves.
 * Instead, they can implicitly participate in transactions handled by higher-level
 * application services utilizing this class, making calls to the low-level
 * services via an inner-class callback object.
 *
 * <p>Can be used within a service implementation via direct instantiation with
 * a transaction manager reference, or get prepared in an application context
 * and passed to services as bean reference. Note: The transaction manager should
 * always be configured as bean in the application context: in the first case given
 * to the service directly, in the second case given to the prepared template.
 *
 * <p>Supports setting the propagation behavior and the isolation level by name,
 * for convenient configuration in context definitions.
 *
 * @author Juergen Hoeller
 * @since 17.03.2003
 * @see #execute
 * @see #setTransactionManager
 * @see org.springframework.transaction.PlatformTransactionManager
 */
@SuppressWarnings("serial")
public class TransactionTemplate extends DefaultTransactionDefinition implements TransactionOperations, InitializingBean {

	/** Logger available to subclasses. */
	protected final Log logger = LogFactory.getLog(getClass());

	@Nullable
	private PlatformTransactionManager transactionManager;


	/**
	 * Construct a new TransactionTemplate for bean usage.
	 * <p>Note: The PlatformTransactionManager needs to be set before
	 * any {@code execute} calls.
	 * @see #setTransactionManager
	 */
	public TransactionTemplate() {
	}

	/**
	 * Construct a new TransactionTemplate using the given transaction manager.
	 * @param transactionManager the transaction management strategy to be used
	 */
	public TransactionTemplate(PlatformTransactionManager transactionManager) {
		this.transactionManager = transactionManager;
	}

	/**
	 * Construct a new TransactionTemplate using the given transaction manager,
	 * taking its default settings from the given transaction definition.
	 * @param transactionManager the transaction management strategy to be used
	 * @param transactionDefinition the transaction definition to copy the
	 * default settings from. Local properties can still be set to change values.
	 */
	public TransactionTemplate(PlatformTransactionManager transactionManager, TransactionDefinition transactionDefinition) {
		super(transactionDefinition);
		this.transactionManager = transactionManager;
	}


	/**
	 * Set the transaction management strategy to be used.
	 */
	public void setTransactionManager(@Nullable PlatformTransactionManager transactionManager) {
		this.transactionManager = transactionManager;
	}

	/**
	 * Return the transaction management strategy to be used.
	 */
	@Nullable
	public PlatformTransactionManager getTransactionManager() {
		return this.transactionManager;
	}

	@Override
	public void afterPropertiesSet() {
		if (this.transactionManager == null) {
			throw new IllegalArgumentException("Property 'transactionManager' is required");
		}
	}


	@Override
	@Nullable
	public <T> T execute(TransactionCallback<T> action) throws TransactionException {
        // 判断 PlatformTransactionManager 不为空
		Assert.state(this.transactionManager != null, "No PlatformTransactionManager set");

        //
		if (this.transactionManager instanceof CallbackPreferringPlatformTransactionManager) {
			return ((CallbackPreferringPlatformTransactionManager) this.transactionManager).execute(this, action);
		}
		else {

            // 调用 platformTransactionManager.getTransaction(this)，上边讲过 PlatformTransactionManager 的方法在 AbstractPlatformTransactionManager 都有实现
            // 遗留一：下边会讲 AbstractPlatformTransactionManager 这里不展开 只需要知道这里是创建了一个事务
			TransactionStatus status = this.transactionManager.getTransaction(this);
			T result;
			try {
                // 就是执行业务代码了
                // 遗留二：下边会讲 TransactionCallback
				result = action.doInTransaction(status);
			}
			catch (RuntimeException | Error ex) {
				// Transactional code threw application exception -> rollback
                // 事务回滚，方法就在下边，其实就是调了 platformTransactionManager.rollback 
                // 遗留一
				rollbackOnException(status, ex);
				throw ex;
			}
			catch (Throwable ex) {
				// Transactional code threw unexpected exception -> rollback
                // 事务回滚 对未知异常做了封装
				rollbackOnException(status, ex);
				throw new UndeclaredThrowableException(ex, "TransactionCallback threw undeclared checked exception");
			}
            // 事务提交
            // 遗留一
			this.transactionManager.commit(status);
			return result;
		}
	}

	/**
	 * Perform a rollback, handling rollback exceptions properly.
	 * @param status object representing the transaction
	 * @param ex the thrown application exception or error
	 * @throws TransactionException in case of a rollback error
	 */
	private void rollbackOnException(TransactionStatus status, Throwable ex) throws TransactionException {
		Assert.state(this.transactionManager != null, "No PlatformTransactionManager set");

		logger.debug("Initiating transaction rollback on application exception", ex);
		try {
            // 遗留一
			this.transactionManager.rollback(status);
		}
		catch (TransactionSystemException ex2) {
			logger.error("Application exception overridden by rollback exception", ex);
			ex2.initApplicationException(ex);
			throw ex2;
		}
		catch (RuntimeException | Error ex2) {
			logger.error("Application exception overridden by rollback exception", ex);
			throw ex2;
		}
	}


	@Override
	public boolean equals(Object other) {
		return (this == other || (super.equals(other) && (!(other instanceof TransactionTemplate) ||
				getTransactionManager() == ((TransactionTemplate) other).getTransactionManager())));
	}

}


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    举一个我们项目使用 TransactionTemplate 的例子
    这里为什么要用 TransactionTemplate 是因为 @PostConstruct 注解上边加事务注解是不生效的。具体原因等讲完遗留一就明白了

        @PostConstruct
    public void init() {
        TransactionTemplate tmpl = new TransactionTemplate(jpaTransactionManager);
        tmpl.execute(new TransactionCallbackWithoutResult() {
            @Override
            protected void doInTransactionWithoutResult(TransactionStatus status) {
                FwUserTokenInfo userTokenInfo = serviceAccountService.getServiceAccount();
                AccessControlContext.setAcUser(userTokenInfo.getUser());
                AccessControlDefaultPolicyUploader.super.init();
            }
        });

    }

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    TransactionCallback

    这里先看遗留二吧，比较简单
    注释写的很清楚，事务代码的回调接口，与 TransactionTemplate 一起使用，通常作为方法实现中的匿名类

    /**
 * Callback interface for transactional code. Used with {@link TransactionTemplate}'s
 * {@code execute} method, often as anonymous class within a method implementation.
 *
 * <p>Typically used to assemble various calls to transaction-unaware data access
 * services into a higher-level service method with transaction demarcation. As an
 * alternative, consider the use of declarative transaction demarcation (e.g. through
 * Spring's {@link org.springframework.transaction.annotation.Transactional} annotation).
 *
 * @author Juergen Hoeller
 * @since 17.03.2003
 * @see TransactionTemplate
 * @see CallbackPreferringPlatformTransactionManager
 * @param <T> the result type
 */
@FunctionalInterface
public interface TransactionCallback<T> {

	/**
	 * Gets called by {@link TransactionTemplate#execute} within a transactional context.
	 * Does not need to care about transactions itself, although it can retrieve and
	 * influence the status of the current transaction via the given status object,
	 * e.g. setting rollback-only.
	 * <p>Allows for returning a result object created within the transaction, i.e. a
	 * domain object or a collection of domain objects. A RuntimeException thrown by the
	 * callback is treated as application exception that enforces a rollback. Any such
	 * exception will be propagated to the caller of the template, unless there is a
	 * problem rolling back, in which case a TransactionException will be thrown.
	 * @param status associated transaction status
	 * @return a result object, or {@code null}
	 * @see TransactionTemplate#execute
	 * @see CallbackPreferringPlatformTransactionManager#execute
	 */
	@Nullable
	T doInTransaction(TransactionStatus status);

}

// 抽象方法实现了 TransactionCallback，我上边的例子其实就是使用了它
/**
 * Simple convenience class for TransactionCallback implementation.
 * Allows for implementing a doInTransaction version without result,
 * i.e. without the need for a return statement.
 *
 * @author Juergen Hoeller
 * @since 28.03.2003
 * @see TransactionTemplate
 */
public abstract class TransactionCallbackWithoutResult implements TransactionCallback<Object> {

	@Override
	@Nullable
	public final Object doInTransaction(TransactionStatus status) {
		doInTransactionWithoutResult(status);
		return null;
	}

	/**
	 * Gets called by {@code TransactionTemplate.execute} within a transactional
	 * context. Does not need to care about transactions itself, although it can retrieve
	 * and influence the status of the current transaction via the given status object,
	 * e.g. setting rollback-only.
	 * <p>A RuntimeException thrown by the callback is treated as application
	 * exception that enforces a rollback. An exception gets propagated to the
	 * caller of the template.
	 * <p>Note when using JTA: JTA transactions only work with transactional
	 * JNDI resources, so implementations need to use such resources if they
	 * want transaction support.
	 * @param status associated transaction status
	 * @see TransactionTemplate#execute
	 */
	protected abstract void doInTransactionWithoutResult(TransactionStatus status);

}


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    AbstractPlatformTransactionManager

    遗留一， 这里比较重要了，注释也写得很清楚，是 Spring 处理事务的抽象基类，是具体平台事务的基础（也就是你想自定义一套事务管理最好以它为基础）
    直接看源码吧
    源码太长，分开看吧

    /**
 * Abstract base class that implements Spring's standard transaction workflow,
 * serving as basis for concrete platform transaction managers like
 * {@link org.springframework.transaction.jta.JtaTransactionManager}.
 *
 * <p>This base class provides the following workflow handling:
 * <ul>
 * <li>determines if there is an existing transaction;
 * <li>applies the appropriate propagation behavior;
 * <li>suspends and resumes transactions if necessary;
 * <li>checks the rollback-only flag on commit;
 * <li>applies the appropriate modification on rollback
 * (actual rollback or setting rollback-only);
 * <li>triggers registered synchronization callbacks
 * (if transaction synchronization is active).
 * </ul>
 *
 * <p>Subclasses have to implement specific template methods for specific
 * states of a transaction, e.g.: begin, suspend, resume, commit, rollback.
 * The most important of them are abstract and must be provided by a concrete
 * implementation; for the rest, defaults are provided, so overriding is optional.
 *
 * <p>Transaction synchronization is a generic mechanism for registering callbacks
 * that get invoked at transaction completion time. This is mainly used internally
 * by the data access support classes for JDBC, Hibernate, JPA, etc when running
 * within a JTA transaction: They register resources that are opened within the
 * transaction for closing at transaction completion time, allowing e.g. for reuse
 * of the same Hibernate Session within the transaction. The same mechanism can
 * also be leveraged for custom synchronization needs in an application.
 *
 * <p>The state of this class is serializable, to allow for serializing the
 * transaction strategy along with proxies that carry a transaction interceptor.
 * It is up to subclasses if they wish to make their state to be serializable too.
 * They should implement the {@code java.io.Serializable} marker interface in
 * that case, and potentially a private {@code readObject()} method (according
 * to Java serialization rules) if they need to restore any transient state.
 *
 * @author Juergen Hoeller
 * @since 28.03.2003
 * @see #setTransactionSynchronization
 * @see TransactionSynchronizationManager
 * @see org.springframework.transaction.jta.JtaTransactionManager
 */
@SuppressWarnings("serial")
public abstract class AbstractPlatformTransactionManager implements PlatformTransactionManager, Serializable {
    // 对 这里就是给你看看注释，然后拜拜大神 Juergen Hoeller
}


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    AbstractPlatformTransactionManager

    /*
 * @author Juergen Hoeller
 */
@SuppressWarnings("serial")
public abstract class AbstractPlatformTransactionManager implements PlatformTransactionManager, Serializable {

	//---------------------------------------------------------------------
	// Implementation of PlatformTransactionManager
	//---------------------------------------------------------------------

	/**
	 * This implementation handles propagation behavior. Delegates to
	 * {@code doGetTransaction}, {@code isExistingTransaction}
	 * and {@code doBegin}.
	 * @see #doGetTransaction
	 * @see #isExistingTransaction
	 * @see #doBegin
	 */
	@Override
	public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
        // do 开头。spring 中 do 开头的方法都是干实事的
		Object transaction = doGetTransaction();

		// Cache debug flag to avoid repeated checks.
		boolean debugEnabled = logger.isDebugEnabled();

		if (definition == null) {
			// Use defaults if no transaction definition given.
			definition = new DefaultTransactionDefinition();
		}

        // 判断事务是否存在且活跃
		if (isExistingTransaction(transaction)) {
			// Existing transaction found -> check propagation behavior to find out how to behave.
			return handleExistingTransaction(definition, transaction, debugEnabled);
		}

		// Check definition settings for new transaction.
		if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
			throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
		}

		// No existing transaction found -> check propagation behavior to find out how to proceed.
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
			throw new IllegalTransactionStateException(
					"No existing transaction found for transaction marked with propagation 'mandatory'");
		}
		else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
				definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
				definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
			SuspendedResourcesHolder suspendedResources = suspend(null);
			if (debugEnabled) {
				logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);
			}
			try {
				boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
                // 创建事务状态
				DefaultTransactionStatus status = newTransactionStatus(definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
                // 开始事务
				doBegin(transaction, definition);
                // 事务状态同步
				prepareSynchronization(status, definition);
				return status;
			}
			catch (RuntimeException | Error ex) {
				resume(null, suspendedResources);
				throw ex;
			}
		}
		else {
			// Create "empty" transaction: no actual transaction, but potentially synchronization.
			if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
				logger.warn("Custom isolation level specified but no actual transaction initiated; " +
						"isolation level will effectively be ignored: " + definition);
			}
			boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
			return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
		}
	}


    /**
	 * This implementation of rollback handles participating in existing
	 * transactions. Delegates to {@code doRollback} and
	 * {@code doSetRollbackOnly}.
	 * @see #doRollback
	 * @see #doSetRollbackOnly
	 */
	@Override
	public final void rollback(TransactionStatus status) throws TransactionException {
		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;
		processRollback(defStatus, false);
	}

	/**
	 * Process an actual rollback.
	 * The completed flag has already been checked.
	 * @param status object representing the transaction
	 * @throws TransactionException in case of rollback failure
	 */
	private void processRollback(DefaultTransactionStatus status, boolean unexpected) {
		try {
			boolean unexpectedRollback = unexpected;

			try {
				triggerBeforeCompletion(status);

				if (status.hasSavepoint()) {
					if (status.isDebug()) {
						logger.debug("Rolling back transaction to savepoint");
					}
					status.rollbackToHeldSavepoint();
				}
				else if (status.isNewTransaction()) {
					if (status.isDebug()) {
						logger.debug("Initiating transaction rollback");
					}
					doRollback(status);
				}
				else {
					// Participating in larger transaction
					if (status.hasTransaction()) {
						if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {
							if (status.isDebug()) {
								logger.debug("Participating transaction failed - marking existing transaction as rollback-only");
							}
							doSetRollbackOnly(status);
						}
						else {
							if (status.isDebug()) {
								logger.debug("Participating transaction failed - letting transaction originator decide on rollback");
							}
						}
					}
					else {
						logger.debug("Should roll back transaction but cannot - no transaction available");
					}
					// Unexpected rollback only matters here if we're asked to fail early
					if (!isFailEarlyOnGlobalRollbackOnly()) {
						unexpectedRollback = false;
					}
				}
			}
			catch (RuntimeException | Error ex) {
				triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
				throw ex;
			}

			triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);

			// Raise UnexpectedRollbackException if we had a global rollback-only marker
			if (unexpectedRollback) {
				throw new UnexpectedRollbackException(
						"Transaction rolled back because it has been marked as rollback-only");
			}
		}
		finally {
			cleanupAfterCompletion(status);
		}
	}


	/**
	 * This implementation of commit handles participating in existing
	 * transactions and programmatic rollback requests.
	 * Delegates to {@code isRollbackOnly}, {@code doCommit}
	 * and {@code rollback}.
	 * @see org.springframework.transaction.TransactionStatus#isRollbackOnly()
	 * @see #doCommit
	 * @see #rollback
	 */
	@Override
	public final void commit(TransactionStatus status) throws TransactionException {
        // 判断当前事务状态是否已经结束 
		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;
        // 判断当前事务是否有回滚标识
		if (defStatus.isLocalRollbackOnly()) {
			if (defStatus.isDebug()) {
				logger.debug("Transactional code has requested rollback");
			}
            // 事务回滚
			processRollback(defStatus, false);
			return;
		}

        // 全局事务 且被标记为需要回滚
		if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {
			if (defStatus.isDebug()) {
				logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
			}
            // 事务回滚
			processRollback(defStatus, true);
			return;
		}

        // 事务提交
		processCommit(defStatus);
	}

    /**
	 * Process an actual commit.
	 * Rollback-only flags have already been checked and applied.
	 * @param status object representing the transaction
	 * @throws TransactionException in case of commit failure
	 */
	private void processCommit(DefaultTransactionStatus status) throws TransactionException {
		try {
			boolean beforeCompletionInvoked = false;

			try {
				boolean unexpectedRollback = false;
                // 空方法，没找到实现
				prepareForCommit(status);
                // 事务提交前，触发回调
				triggerBeforeCommit(status);
                // 事务完成前，触发回调
				triggerBeforeCompletion(status);
				beforeCompletionInvoked = true;

				if (status.hasSavepoint()) {
					if (status.isDebug()) {
						logger.debug("Releasing transaction savepoint");
					}
					unexpectedRollback = status.isGlobalRollbackOnly();
					status.releaseHeldSavepoint();
				}
                // 是否是新事务，如果是新事务，将执行提交操作，比如传播行为是 REQUIRED 中嵌套了一个 REQUIRED，那么内部的事务就不是新的事务，外部的事务是新事务
				else if (status.isNewTransaction()) {
					if (status.isDebug()) {
						logger.debug("Initiating transaction commit");
					}
					unexpectedRollback = status.isGlobalRollbackOnly();
                    // 提交事务
                    // 扩展一
					doCommit(status);
				}
				else if (isFailEarlyOnGlobalRollbackOnly()) {
					unexpectedRollback = status.isGlobalRollbackOnly();
				}

				// Throw UnexpectedRollbackException if we have a global rollback-only
				// marker but still didn't get a corresponding exception from commit.
				if (unexpectedRollback) {
					throw new UnexpectedRollbackException("Transaction silently rolled back because it has been marked as rollback-only");
				}
			}
			catch (UnexpectedRollbackException ex) {
				// can only be caused by doCommit
                // 事务完成之后，执行一些回调
				triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
				throw ex;
			}
			catch (TransactionException ex) {
				// can only be caused by doCommit
                // 提交过程中有异常，执行回滚操作
				if (isRollbackOnCommitFailure()) {
					doRollbackOnCommitException(status, ex);
				}
				else {
                    // 事务完成之后，执行一些回调
					triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
				}
				throw ex;
			}
			catch (RuntimeException | Error ex) {
				if (!beforeCompletionInvoked) {
					triggerBeforeCompletion(status);
				}
                // 提交过程中有异常，执行回滚操作
				doRollbackOnCommitException(status, ex);
				throw ex;
			}

			// Trigger afterCommit callbacks, with an exception thrown there
			// propagated to callers but the transaction still considered as committed.
			try {
                // 事务 commit 之后，执行一些回调
				triggerAfterCommit(status);
			}
			finally {
                // 事务完成之后，执行一些回调
				triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
			}

		}
		finally {
            // 事务执行完毕之后，执行一些清理操作
			cleanupAfterCompletion(status);
		}
	}

	private void cleanupAfterCompletion(DefaultTransactionStatus status) {
        // 将事务状态置为已完成
		status.setCompleted();
        // 是否是新的事务同步
		if (status.isNewSynchronization()) {
            // 扩展二 清空当前线程 ThreadLocal 里边的所有东西
			TransactionSynchronizationManager.clear();
		}
        // 是否是新的事务
		if (status.isNewTransaction()) {
            // 提交事务并清理
			doCleanupAfterCompletion(status.getTransaction());
		}
        // 当前事务状态是否被挂起
		if (status.getSuspendedResources() != null) {
			if (status.isDebug()) {
				logger.debug("Resuming suspended transaction after completion of inner transaction");
			}
			Object transaction = (status.hasTransaction() ? status.getTransaction() : null);
            // 恢复被挂起的事务
			resume(transaction, (SuspendedResourcesHolder) status.getSuspendedResources());
		}
	}

    // 抽象方法，所以还要往下走
    // 该方法作用：返回当前事务的事务状态
    // 它的实现有很多，但里边的逻辑都差不多，毕竟只是拿到当前事务状态：CciLocalTransactionManager DataSourceTransactionManager HibernateTransactionManager JpaTransactionManager JtaTransactionManager...
    // 扩展一：找一个实现看一下
    protected abstract Object doGetTransaction() throws TransactionException;

    // 扩展一
    protected abstract void doBegin(Object transaction, TransactionDefinition definition) throws TransactionException;

    // 扩展一
	protected abstract void doCommit(DefaultTransactionStatus status) throws TransactionException;

    // 扩展一
    protected void doCleanupAfterCompletion(Object transaction) {};

    // 扩展一
    protected abstract void doRollback(DefaultTransactionStatus status) throws TransactionException;

}

// 扩展一
public class DataSourceTransactionManager extends AbstractPlatformTransactionManager implements ResourceTransactionManager, InitializingBean {

    protected Object doGetTransaction() {
        // 构造了个 DataSourceTransactionObject 对象，对象里边没啥就俩字段：private boolean newConnectionHolder;和 private boolean mustRestoreAutoCommit;
        DataSourceTransactionManager.DataSourceTransactionObject txObject = new DataSourceTransactionManager.DataSourceTransactionObject();
        // 默认是false：private boolean nestedTransactionAllowed = false; 表示是否允许嵌套事务
        txObject.setSavepointAllowed(this.isNestedTransactionAllowed());
        // 这一个是核心，无论哪种实现都需要调这个方法，区别只是结果做了强转
        // 这里需要注意一点就是入参传入的是 DataSource 对象，然后返回当前线程资源的持有者 ConnectionHolder，后期连接归 ConnectionHolder 管 
        // 扩展二
        ConnectionHolder conHolder = (ConnectionHolder)TransactionSynchronizationManager.getResource(this.obtainDataSource());
        // 将 conHolder 丢到 DataSourceTransactionObject 中，第二个参数表示是否是一个新的连接，这里肯定为 false，因为通过 datasource 获取的连接才是新的连接
        txObject.setConnectionHolder(conHolder, false);
        return txObject;
    }

    protected DataSource obtainDataSource() {
        DataSource dataSource = this.getDataSource();
        Assert.state(dataSource != null, "No DataSource set");
        return dataSource;
    }

    @Nullable
    public DataSource getDataSource() {
        return this.dataSource;
    }

    /**
	 * This implementation sets the isolation level but ignores the timeout.
	 */
	@Override
	protected void doBegin(Object transaction, TransactionDefinition definition) {
        // 强转成 DataSource 事务对象
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
		Connection con = null;

		try {
			if (!txObject.hasConnectionHolder() || txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
                // 获取一个事务的连接
				Connection newCon = obtainDataSource().getConnection();
				if (logger.isDebugEnabled()) {
					logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
				}
                // 将事务的连接放到 DataSourceTransactionObject 对象中，并标记为新的连接
				txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
			}

            // 标记事务连接为 true
			txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
            // 获取事务的连接
			con = txObject.getConnectionHolder().getConnection();

            // 获取事务的隔离级别，并赋值。实现比较简单不展开了
            // 这里边其实做了两件事：（1）// Set read-only flag; (2)// Apply specific isolation level, if any.
			Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
			txObject.setPreviousIsolationLevel(previousIsolationLevel);

			// 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).
			if (con.getAutoCommit()) {
				txObject.setMustRestoreAutoCommit(true);
				if (logger.isDebugEnabled()) {
					logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
				}
                // 改为不自动提交
				con.setAutoCommit(false);
			}

            // 设置 TransactionSynchronizationManager 中的一些变量值
			prepareTransactionalConnection(con, definition);
            // 设置来连接对象可用
			txObject.getConnectionHolder().setTransactionActive(true);

			int timeout = determineTimeout(definition);
			if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
                // 设置连接对象的超时时间
				txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
			}

			// Bind the connection holder to the thread.
            // 核心方法！！！ 绑定线程！！！
			if (txObject.isNewConnectionHolder()) {
                // 扩展二
				TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());
			}
		}

		catch (Throwable ex) {
			if (txObject.isNewConnectionHolder()) {
				DataSourceUtils.releaseConnection(con, obtainDataSource());
				txObject.setConnectionHolder(null, false);
			}
			throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
		}
	}

	@Override
	protected void doCommit(DefaultTransactionStatus status) {
        // 获取当前线程对象
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
		// 获取当前连接
        Connection con = txObject.getConnectionHolder().getConnection();
		if (status.isDebug()) {
			logger.debug("Committing JDBC transaction on Connection [" + con + "]");
		}
		try {
            // 提交事务 具体不展开了 
			con.commit();
		}
		catch (SQLException ex) {
			throw new TransactionSystemException("Could not commit JDBC transaction", ex);
		}
	}

	@Override
	protected void doCleanupAfterCompletion(Object transaction) {
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;

		// Remove the connection holder from the thread, if exposed.
		if (txObject.isNewConnectionHolder()) {
            // 解绑资源，对应前边的 bindResource，就是在 ThreadLocal 里边删掉资源
			TransactionSynchronizationManager.unbindResource(obtainDataSource());
		}

		// Reset connection.
        // 重置连接
		Connection con = txObject.getConnectionHolder().getConnection();
		try {
			if (txObject.isMustRestoreAutoCommit()) {
				con.setAutoCommit(true);
			}
			DataSourceUtils.resetConnectionAfterTransaction(con, txObject.getPreviousIsolationLevel());
		}
		catch (Throwable ex) {
			logger.debug("Could not reset JDBC Connection after transaction", ex);
		}

		if (txObject.isNewConnectionHolder()) {
			if (logger.isDebugEnabled()) {
				logger.debug("Releasing JDBC Connection [" + con + "] after transaction");
			}
			DataSourceUtils.releaseConnection(con, this.dataSource);
		}

        // 清理事务
		txObject.getConnectionHolder().clear();
	}

	@Override
	protected void doRollback(DefaultTransactionStatus status) {
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
		Connection con = txObject.getConnectionHolder().getConnection();
		if (status.isDebug()) {
			logger.debug("Rolling back JDBC transaction on Connection [" + con + "]");
		}
		try {
            // 回滚 具体不展开了 
			con.rollback();
		}
		catch (SQLException ex) {
			throw new TransactionSystemException("Could not roll back JDBC transaction", ex);
		}
	}

}

// 扩展二
// 这也是一个比较核心的抽象类
public abstract class TransactionSynchronizationManager {

    // 存储事务资源信息
    private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal<>("Transactional resources");

    // 存储事务过程中的一些回调接口(TransactionSynchronization接口，这个可以在事务的过程中给开发者提供一些回调用的)
	private static final ThreadLocal<Set<TransactionSynchronization>> synchronizations = new NamedThreadLocal<>("Transaction synchronizations");

    // 存储当前正在运行的事务的名称
	private static final ThreadLocal<String> currentTransactionName = new NamedThreadLocal<>("Current transaction name");

    // 存储当前正在运行的事务是否是只读的
 	private static final ThreadLocal<Boolean> currentTransactionReadOnly = new NamedThreadLocal<>("Current transaction read-only status");

    // 存储当前正在运行的事务的隔离级别
	private static final ThreadLocal<Integer> currentTransactionIsolationLevel = new NamedThreadLocal<>("Current transaction isolation level");

    // 存储当前正在运行的事务是否是活动状态，事务启动的时候会被激活
	private static final ThreadLocal<Boolean> actualTransactionActive = new NamedThreadLocal<>("Actual transaction active");

	/**
	 * Retrieve a resource for the given key that is bound to the current thread.
	 * @param key the key to check (usually the resource factory)
	 * @return a value bound to the current thread (usually the active
	 * resource object), or {@code null} if none
	 * @see ResourceTransactionManager#getResourceFactory()
	 */
    // 返回当前线程的资源持有者
	@Nullable
	public static Object getResource(Object key) {
        // 扩展三 最后都是调用了扩展三的代码，获取 DataSource 的 bean
		Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
        // 获取连接对象 
		Object value = doGetResource(actualKey);
		if (value != null && logger.isTraceEnabled()) {
			logger.trace("Retrieved value [" + value + "] for key [" + actualKey + "] bound to thread [" + Thread.currentThread().getName() + "]");
		}
		return value;
	}

	/**
	 * Actually check the value of the resource that is bound for the given key.
	 */
	@Nullable
	private static Object doGetResource(Object actualKey) {
		Map<Object, Object> map = resources.get();
		if (map == null) {
            // 最开始必然走这里，因为没 put 进 resources
			return null;
		}
		Object value = map.get(actualKey);
		// Transparently remove ResourceHolder that was marked as void...
		if (value instanceof ResourceHolder && ((ResourceHolder) value).isVoid()) {
			map.remove(actualKey);
			// Remove entire ThreadLocal if empty...
			if (map.isEmpty()) {
				resources.remove();
			}
			value = null;
		}
		return value;
	}

	/**
	 * Bind the given resource for the given key to the current thread.
	 * @param key the key to bind the value to (usually the resource factory)
	 * @param value the value to bind (usually the active resource object)
	 * @throws IllegalStateException if there is already a value bound to the thread
	 * @see ResourceTransactionManager#getResourceFactory()
	 */
	public static void bindResource(Object key, Object value) throws IllegalStateException {
        // 获取当前线程
		Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
		Assert.notNull(value, "Value must not be null");
		Map<Object, Object> map = resources.get();
		// set ThreadLocal Map if none found
		if (map == null) {
			map = new HashMap<>();
            // 更新到 ThreadLocal 中
			resources.set(map);
		}
		Object oldValue = map.put(actualKey, value);
		// Transparently suppress a ResourceHolder that was marked as void...
		if (oldValue instanceof ResourceHolder && ((ResourceHolder) oldValue).isVoid()) {
			oldValue = null;
		}
		if (oldValue != null) {
			throw new IllegalStateException("Already value [" + oldValue + "] for key [" + actualKey + "] bound to thread [" + Thread.currentThread().getName() + "]");
		}
		if (logger.isTraceEnabled()) {
			logger.trace("Bound value [" + value + "] for key [" + actualKey + "] to thread [" + Thread.currentThread().getName() + "]");
		}
	}

	/**
	 * Clear the entire transaction synchronization state for the current thread:
	 * registered synchronizations as well as the various transaction characteristics.
	 * @see #clearSynchronization()
	 * @see #setCurrentTransactionName
	 * @see #setCurrentTransactionReadOnly
	 * @see #setCurrentTransactionIsolationLevel
	 * @see #setActualTransactionActive
	 */
    public static void clear() {
		synchronizations.remove();
		currentTransactionName.remove();
		currentTransactionReadOnly.remove();
		currentTransactionIsolationLevel.remove();
		actualTransactionActive.remove();
	}

}

// 扩展三
public class DefaultScopedObject implements ScopedObject, Serializable {

	private final ConfigurableBeanFactory beanFactory;

	private final String targetBeanName;


	/**
	 * Creates a new instance of the {@link DefaultScopedObject} class.
	 * @param beanFactory the {@link ConfigurableBeanFactory} that holds the scoped target object
	 * @param targetBeanName the name of the target bean
	 */
	public DefaultScopedObject(ConfigurableBeanFactory beanFactory, String targetBeanName) {
		Assert.notNull(beanFactory, "BeanFactory must not be null");
		Assert.hasText(targetBeanName, "'targetBeanName' must not be empty");
		this.beanFactory = beanFactory;
		this.targetBeanName = targetBeanName;
	}


	@Override
	public Object getTargetObject() {
		return this.beanFactory.getBean(this.targetBeanName);
	}

	@Override
	public void removeFromScope() {
		this.beanFactory.destroyScopedBean(this.targetBeanName);
	}

}


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    响应式事务

    未完待续。。。

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  • 原文地址：https://blog.csdn.net/lbh_paopao/article/details/125545642