mybatis事务源码-mybatis与spring事务管理分析
一丶从jdbc的角度理解什么是事务
从mysql获取一个连接之后, 默认是自动提交, 即执行完sql之后, 就会提交事务. 这种事务的范围是一条sql语句.
将该连接设置非自动提交, 可以执行多条sql语句, 然后由程序决定是提交事务, 还是回滚事务. 这也是我们常说的事务.
Connection connection = dataSource.getConnection();
// connection.setTransactionIsolation(level.getLevel()); //设置事务隔离级别
// 设置是否自动提交, 如果不是自动提交, 则是"开启"事务
connection.setAutoCommit(desiredAutoCommit);
// connection预编译statement, 并执行sql
Statement stmt=connection.preparedStatement();
stmt.execute(sql);
// 提交事务, 或者回滚
connection.commit();
//connection.rollback();
从jdbc使用事务的角度来看, 事务主要是围绕connection展开的, 所以谁可获得connection, 即可控制事务.
二丶mybatis是如何使用事务的
mybatis将jdbc中的事务操作抽象封装成Transaction,用于管理connection的生命周期–创建, 准备, 提交/回滚 和关闭.
/**
* Wraps a database connection.
* Handles the connection lifecycle that comprises: its creation, preparation, commit/rollback and close.
*
* @author Clinton Begin
*/
public interface Transaction {
/**
* Retrieve inner database connection.
* @return DataBase connection
* @throws SQLException
*/
Connection getConnection() throws SQLException;
/**
* Commit inner database connection.
* @throws SQLException
*/
void commit() throws SQLException;
/**
* Rollback inner database connection.
* @throws SQLException
*/
void rollback() throws SQLException;
/**
* Close inner database connection.
* @throws SQLException
*/
void close() throws SQLException;
/**
* Get transaction timeout if set.
* @throws SQLException
*/
Integer getTimeout() throws SQLException;
}
mybatis提供了两种事务实现,一种是完全由jdbc实现的事务JdbcTransaction,包括实现提交和回滚.一种是供容器管理整个生命周期的事务ManagedTransaction,其中将忽略提交和回滚事务的请求, 将提交和回滚事务由容器实现, 但其实这种事务很少用.
jdbcTransaction:
public class JdbcTransaction implements Transaction {
private static final Log log = LogFactory.getLog(JdbcTransaction.class);
protected Connection connection;
protected DataSource dataSource;
protected TransactionIsolationLevel level;
protected boolean autoCommit;
public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) {
dataSource = ds;
level = desiredLevel;
autoCommit = desiredAutoCommit;
}
public JdbcTransaction(Connection connection) {
this.connection = connection;
}
@Override
public Connection getConnection() throws SQLException {
if (connection == null) {
openConnection();
}
return connection;
}
@Override
public void commit() throws SQLException {
if (connection != null && !connection.getAutoCommit()) {
if (log.isDebugEnabled()) {
log.debug("Committing JDBC Connection [" + connection + "]");
}
connection.commit();
}
}
@Override
public void rollback() throws SQLException {
if (connection != null && !connection.getAutoCommit()) {
if (log.isDebugEnabled()) {
log.debug("Rolling back JDBC Connection [" + connection + "]");
}
connection.rollback();
}
}
@Override
public void close() throws SQLException {
if (connection != null) {
resetAutoCommit();
if (log.isDebugEnabled()) {
log.debug("Closing JDBC Connection [" + connection + "]");
}
connection.close();
}
}
protected void setDesiredAutoCommit(boolean desiredAutoCommit) {
try {
if (connection.getAutoCommit() != desiredAutoCommit) {
if (log.isDebugEnabled()) {
log.debug("Setting autocommit to " + desiredAutoCommit + " on JDBC Connection [" + connection + "]");
}
connection.setAutoCommit(desiredAutoCommit);
}
} catch (SQLException e) {
// Only a very poorly implemented driver would fail here,
// and there's not much we can do about that.
throw new TransactionException("Error configuring AutoCommit. "
+ "Your driver may not support getAutoCommit() or setAutoCommit(). "
+ "Requested setting: " + desiredAutoCommit + ". Cause: " + e, e);
}
}
protected void resetAutoCommit() {
try {
if (!connection.getAutoCommit()) {
// MyBatis does not call commit/rollback on a connection if just selects were performed.
// Some databases start transactions with select statements
// and they mandate a commit/rollback before closing the connection.
// A workaround is setting the autocommit to true before closing the connection.
// Sybase throws an exception here.
if (log.isDebugEnabled()) {
log.debug("Resetting autocommit to true on JDBC Connection [" + connection + "]");
}
connection.setAutoCommit(true);
}
} catch (SQLException e) {
if (log.isDebugEnabled()) {
log.debug("Error resetting autocommit to true "
+ "before closing the connection. Cause: " + e);
}
}
}
protected void openConnection() throws SQLException {
if (log.isDebugEnabled()) {
log.debug("Opening JDBC Connection");
}
connection = dataSource.getConnection();
if (level != null) {
connection.setTransactionIsolation(level.getLevel());
}
setDesiredAutoCommit(autoCommit);
}
@Override
public Integer getTimeout() throws SQLException {
return null;
}
}
mybatis事务执行流程:
1. 由于mybatis将事务抽取成一个接口, 便于管理, 所以可以在配置中配置事务管理的实现
2. 解析配置, 将事务管理对象, 保存到Configuration中
3. SqlSessionFactory创建SqlSession时, 将会同时注入tx对象
4. SqlSession执行sql语句时, 会委派给Executor执行, Executor处理主要的逻辑之外, 事务将会委派给事务对象处理, 如从事务对象中获取连接, 使用事务对象提交事务.
BaseExecutor执行器
// 在执行器里获取Connection , 最后是委派给Transaction获取,
// 事务管理, 即是Connection是否设置自动提交, 以及将事务的回滚调用交给事务管理器管理
protected Connection getConnection(Log statementLog) throws SQLException {
Connection connection = transaction.getConnection();
if (statementLog.isDebugEnabled()) {
return ConnectionLogger.newInstance(connection, statementLog, queryStack);
} else {
return connection;
}
}
Transaction封装了connection,然后在transaction内部封装调用connection的操作,如提供不同的Transaction, 来管理connection的生命周期.
三丶spring是如何使用事务的
srpingboot和mybatis整合,测试事务
1) 入口
在配置了DataSourceProperties属性之后, 会创建DataSource, 之后会创建DataSourceTransactionManager作为事务管理器，springboot自动配置类org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration
@Configuration
@ConditionalOnClass({ JdbcTemplate.class, PlatformTransactionManager.class })
@AutoConfigureOrder(Ordered.LOWEST_PRECEDENCE)
@EnableConfigurationProperties(DataSourceProperties.class)
public class DataSourceTransactionManagerAutoConfiguration {
@Configuration
@ConditionalOnSingleCandidate(DataSource.class)
static class DataSourceTransactionManagerConfiguration {
private final DataSource dataSource;
private final TransactionManagerCustomizers transactionManagerCustomizers;
DataSourceTransactionManagerConfiguration(DataSource dataSource,
ObjectProvider<TransactionManagerCustomizers> transactionManagerCustomizers) {
this.dataSource = dataSource;
this.transactionManagerCustomizers = transactionManagerCustomizers.getIfAvailable();
}
@Bean
@ConditionalOnMissingBean(PlatformTransactionManager.class)
public DataSourceTransactionManager transactionManager(DataSourceProperties properties) {
// 默认使用DataSourceTransactionManager, 从使用的角度来说,具有通用性
DataSourceTransactionManager transactionManager = new DataSourceTransactionManager(this.dataSource);
if (this.transactionManagerCustomizers != null) {
this.transactionManagerCustomizers.customize(transactionManager);
}
return transactionManager;
}
}
}
DataSourceTransactionManager继承于AbstractPlatformTransactionManager,而AbstractPlatformTransactionManager位于spring的tx子项目中
2) spring-tx子项目
该项目最主要是用于实现事务管理.
2.1) 最核心接口就是PlatformTransactionManager接口, 定义了事务管理器.
a) #getTransaction(TransactionDefinition): TransactionStatus
Return a currently active transaction or create a new one, according to the specified propagation behavior.
b) #commit(TransactionStatus): void
Commit the given transaction, with regard to its status. If the transaction has been marked rollback-only programmatically, perform a rollback.
c) #rollback(TransactionStatus): void
Perform a rollback of the given transaction.
–更详细的文档则需要看源码或者API文档
2.2) TransactionDefinition 定义了事务的传播行为, 隔离界别, 事务超时时间等
2.3)TransactionStatus 定义了事务的状态, 以便于在提交事务或者回滚事务时决定如何后续行为.
3)使用@Transactional注解声明事务
声明式事务是基于AOP实现的.Spring会对使用@Transactinal注解声明的方法进行动态代理, 生成使用org.springframework.transaction.interceptor.TransactionInterceptor增强对应方法的对象…
3.1) 事务切面方法实现
org.springframework.transaction.interceptor.TransactionAspectSupport
@Override
@Nullable
public Object invoke(MethodInvocation invocation) throws Throwable {
// Work out the target class: may be {@code null}.
// The TransactionAttributeSource should be passed the target class
// as well as the method, which may be from an interface.
Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
// Adapt to TransactionAspectSupport's invokeWithinTransaction...
return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
}
invokeWithinTransaction方法源码：
/**
* General delegate for around-advice-based subclasses, delegating to several other template
* methods on this class. Able to handle {@link CallbackPreferringPlatformTransactionManager}
* as well as regular {@link PlatformTransactionManager} implementations.
* @param method the Method being invoked
* @param targetClass the target class that we're invoking the method on
* @param invocation the callback to use for proceeding with the target invocation
* @return the return value of the method, if any
* @throws Throwable propagated from the target invocation
*/
@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.
TransactionAttributeSource tas = getTransactionAttributeSource();
final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
final PlatformTransactionManager tm = determineTransactionManager(txAttr);
final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
// Standard transaction demarcation with getTransaction and commit/rollback calls.
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.
retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
// target invocation exception
completeTransactionAfterThrowing(txInfo, ex);
throw ex;
}
finally {
cleanupTransactionInfo(txInfo);
}
commitTransactionAfterReturning(txInfo);
return retVal;
}
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;
}
}
}
createTransactionIfNecessary方法源码：
/**
* Create a transaction if necessary based on the given TransactionAttribute.
* Allows callers to perform custom TransactionAttribute lookups through
* the TransactionAttributeSource.
* @param txAttr the TransactionAttribute (may be {@code null})
* @param joinpointIdentification the fully qualified method name
* (used for monitoring and logging purposes)
* @return a TransactionInfo object, whether or not a transaction was created.
* The {@code hasTransaction()} method on TransactionInfo can be used to
* tell if there was a transaction created.
* @see #getTransactionAttributeSource()
*/
@SuppressWarnings("serial")
protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm,
@Nullable TransactionAttribute txAttr, final String joinpointIdentification) {
// If no name specified, apply method identification as transaction name.
if (txAttr != null && txAttr.getName() == null) {
txAttr = new DelegatingTransactionAttribute(txAttr) {
@Override
public String getName() {
return joinpointIdentification;
}
};
}
TransactionStatus status = null;
if (txAttr != null) {
if (tm != null) {
status = tm.getTransaction(txAttr); // 这里使用了配置的PlatformTransactionManager获取事务状态
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
"] because no transaction manager has been configured");
}
}
}
return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}
3.2) 查看springboot自动配置DataSourceTransactionManager实现
@Override
protected Object doGetTransaction() {
DataSourceTransactionObject txObject = new DataSourceTransactionObject();
txObject.setSavepointAllowed(isNestedTransactionAllowed());
ConnectionHolder conHolder =
(ConnectionHolder) TransactionSynchronizationManager.getResource(obtainDataSource());
txObject.setConnectionHolder(conHolder, false);
return txObject; // 刚开始获取事务时, 由于没有开启事务, 所以为null
}
/**
* This implementation sets the isolation level but ignores the timeout.
*/
@Override
protected void doBegin(Object transaction, TransactionDefinition definition) {
DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
Connection con = null;
try {
if (!txObject.hasConnectionHolder() ||
txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
// 从数据源中获取connection
Connection newCon = obtainDataSource().getConnection();
if (logger.isDebugEnabled()) {
logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
}
txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
}
txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
con = txObject.getConnectionHolder().getConnection();
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");
}
con.setAutoCommit(false);
}
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.
if (txObject.isNewConnectionHolder()) {
// 将Connection和datasource关联, 交由事务同步管理器保存管理, 使用ThreadLocal隔离
// TranscationSynchronizationManager也是Spring和mybatis-spring共同合作管理事务的桥梁
// ThreadLocal与当前线程绑定, 即线程隔离, 并且使用了同一个DataSource作为key, 可以获取到同一个ConnectionHolder
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);
}
}
四丶mybatis和spring的事务是如何结合使用的
mybatis-spring项目,用于将Spring和mybatis整合
mybatis源码分析(三) mybatis-spring整合源码分析
mybatis-spring整合,需要配置SqlSessionFactoryBean构建生成SqlSessionFactory
SqlSessionFactoryBean#buildSqlSessionFactory()
//如果为空,则使用默认的SpringManagedTransactionFactory生成SpringManagedTransaction
targetConfiguration.setEnvironment(new Environment(this.environment,
this.transactionFactory == null ? new SpringManagedTransactionFactory() : this.transactionFactory,
this.dataSource));
SpringManagedTransaction#openConnection()
/**
* Gets a connection from Spring transaction manager and discovers if this
* {@code Transaction} should manage connection or let it to Spring.
*
* It also reads autocommit setting because when using Spring Transaction MyBatis
* thinks that autocommit is always false and will always call commit/rollback
* so we need to no-op that calls.
*/
private void openConnection() throws SQLException {
// 从Spring transaction manager获取之前由Spring获取的connection
this.connection = DataSourceUtils.getConnection(this.dataSource);
this.autoCommit = this.connection.getAutoCommit();
this.isConnectionTransactional = DataSourceUtils.isConnectionTransactional(this.connection, this.dataSource);
LOGGER.debug(() ->
"JDBC Connection ["
+ this.connection
+ "] will"
+ (this.isConnectionTransactional ? " " : " not ")
+ "be managed by Spring");
}
有本文第二节可知, mybatis执行事务相关操作, 如获取Connection, 使用connection执行多条sql, 使用connection提交事务或者回滚事务, 都是委派给Transacation执行的,
要想将sql语句的执行由mybatis执行, 事务的提交或者回滚操作由Spring控制, 两者需要关联使用同一个connection, 在不同的方法中调用connection的相关方法操作, (所以, Spring并没有直接使用mybatis sqlSession中提供的提交或者回滚方法) . 如何安全的获取同一个connection?这就需要使用TransactionSynchronizationManager
ThreadLocal
保存connection资源,的 key为DataSource, value为ConnectionHolder
(所以, 事务只支持在一个数据源中)