Mybatis源码分析——Mybatis事务如何被Spring管
2021-01-03 本文已影响0人
小波同学
前言
不知道一些同学有没有这种疑问，为什么Mybtis中要配置dataSource，Spring的事务中也要配置dataSource？那么Mybatis和Spring事务中用的Connection是同一个吗？我们常用配置如下
<!--会话工厂 -->
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="dataSource" ref="dataSource" />
</bean>
<!--spring事务管理 -->
<bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
<property name="dataSource" ref="dataSource" />
</bean>
<!--使用注释事务 -->
<tx:annotation-driven transaction-manager="transactionManager" />
看到没，sqlSessionFactory中配置了dataSource，transactionManager也配置了dataSource，我们来回忆一下SqlSessionFactoryBean这个类
public class SqlSessionFactoryBean implements FactoryBean<SqlSessionFactory>, InitializingBean, ApplicationListener<ApplicationEvent> {
protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
// 配置类
Configuration configuration;
// 解析mybatis-Config.xml文件，
// 将相关配置信息保存到configuration
XMLConfigBuilder xmlConfigBuilder = null;
if (this.configuration != null) {
configuration = this.configuration;
if (configuration.getVariables() == null) {
configuration.setVariables(this.configurationProperties);
} else if (this.configurationProperties != null) {
configuration.getVariables().putAll(this.configurationProperties);
}
//资源文件不为空
} else if (this.configLocation != null) {
//根据configLocation创建xmlConfigBuilder，XMLConfigBuilder构造器中会创建Configuration对象
xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties);
//将XMLConfigBuilder构造器中创建的Configuration对象直接赋值给configuration属性
configuration = xmlConfigBuilder.getConfiguration();
} else {
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Property 'configuration' or 'configLocation' not specified, using default MyBatis Configuration");
}
configuration = new Configuration();
if (this.configurationProperties != null) {
configuration.setVariables(this.configurationProperties);
}
}
if (this.objectFactory != null) {
configuration.setObjectFactory(this.objectFactory);
}
if (this.objectWrapperFactory != null) {
configuration.setObjectWrapperFactory(this.objectWrapperFactory);
}
if (this.vfs != null) {
configuration.setVfsImpl(this.vfs);
}
if (hasLength(this.typeAliasesPackage)) {
String[] typeAliasPackageArray = tokenizeToStringArray(this.typeAliasesPackage,
ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
for (String packageToScan : typeAliasPackageArray) {
configuration.getTypeAliasRegistry().registerAliases(packageToScan,
typeAliasesSuperType == null ? Object.class : typeAliasesSuperType);
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Scanned package: '" + packageToScan + "' for aliases");
}
}
}
if (!isEmpty(this.typeAliases)) {
for (Class<?> typeAlias : this.typeAliases) {
configuration.getTypeAliasRegistry().registerAlias(typeAlias);
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Registered type alias: '" + typeAlias + "'");
}
}
}
if (!isEmpty(this.plugins)) {
for (Interceptor plugin : this.plugins) {
configuration.addInterceptor(plugin);
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Registered plugin: '" + plugin + "'");
}
}
}
if (hasLength(this.typeHandlersPackage)) {
String[] typeHandlersPackageArray = tokenizeToStringArray(this.typeHandlersPackage,
ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
for (String packageToScan : typeHandlersPackageArray) {
configuration.getTypeHandlerRegistry().register(packageToScan);
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Scanned package: '" + packageToScan + "' for type handlers");
}
}
}
if (!isEmpty(this.typeHandlers)) {
for (TypeHandler<?> typeHandler : this.typeHandlers) {
configuration.getTypeHandlerRegistry().register(typeHandler);
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Registered type handler: '" + typeHandler + "'");
}
}
}
if (this.databaseIdProvider != null) {//fix #64 set databaseId before parse mapper xmls
try {
configuration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
} catch (SQLException e) {
throw new NestedIOException("Failed getting a databaseId", e);
}
}
if (this.cache != null) {
configuration.addCache(this.cache);
}
if (xmlConfigBuilder != null) {
try {
//解析mybatis-Config.xml文件，并将相关配置信息保存到configuration
xmlConfigBuilder.parse();
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Parsed configuration file: '" + this.configLocation + "'");
}
} catch (Exception ex) {
throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex);
} finally {
ErrorContext.instance().reset();
}
}
if (this.transactionFactory == null) {
//事务默认采用SpringManagedTransaction，这一块非常重要
this.transactionFactory = new SpringManagedTransactionFactory();
}
// 为sqlSessionFactory绑定事务管理器和数据源
// 这样sqlSessionFactory在创建sqlSession的时候可以通过该事务管理器获取jdbc连接，从而执行SQL
configuration.setEnvironment(new Environment(this.environment, this.transactionFactory, this.dataSource));
// 解析mapper.xml
if (!isEmpty(this.mapperLocations)) {
for (Resource mapperLocation : this.mapperLocations) {
if (mapperLocation == null) {
continue;
}
try {
// 解析mapper.xml文件，并注册到configuration对象的mapperRegistry
XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
configuration, mapperLocation.toString(), configuration.getSqlFragments());
xmlMapperBuilder.parse();
} catch (Exception e) {
throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
} finally {
ErrorContext.instance().reset();
}
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Parsed mapper file: '" + mapperLocation + "'");
}
}
} else {
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Property 'mapperLocations' was not specified or no matching resources found");
}
}
// 将Configuration对象实例作为参数，
// 调用sqlSessionFactoryBuilder创建sqlSessionFactory对象实例
return this.sqlSessionFactoryBuilder.build(configuration);
}
}
我们看this.transactionFactory = new SpringManagedTransactionFactory()，Mybatis集成Spring后，默认使用的transactionFactory是SpringManagedTransactionFactory，那我们就来看看其获取Transaction的方法
public class DefaultSqlSessionFactory implements SqlSessionFactory {
private SqlSession openSessionFromConnection(ExecutorType execType, Connection connection) {
try {
boolean autoCommit;
try {
autoCommit = connection.getAutoCommit();
} catch (SQLException e) {
// Failover to true, as most poor drivers
// or databases won't support transactions
autoCommit = true;
}
//从configuration中取出environment对象
final Environment environment = configuration.getEnvironment();
//从environment中取出TransactionFactory
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
//创建Transaction
final Transaction tx = transactionFactory.newTransaction(connection);
//创建包含事务操作的执行器
final Executor executor = configuration.newExecutor(tx, execType);
//构建包含执行器的SqlSession
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) {
if (environment == null || environment.getTransactionFactory() == null) {
return new ManagedTransactionFactory();
}
//这里返回SpringManagedTransactionFactory
return environment.getTransactionFactory();
}
}
public interface TransactionFactory {
Transaction newTransaction(DataSource dataSource, TransactionIsolationLevel level, boolean autoCommit);
}
public class SpringManagedTransactionFactory implements TransactionFactory {
@Override
public Transaction newTransaction(DataSource dataSource, TransactionIsolationLevel level, boolean autoCommit) {
//创建SpringManagedTransaction
return new SpringManagedTransaction(dataSource);
}
}
SpringManagedTransaction
也就是说mybatis的执行事务的事务管理器就切换成了SpringManagedTransaction，下面我们再去看看SpringManagedTransactionFactory类的源码：
public class SpringManagedTransaction implements Transaction {
private static final Log LOGGER = LogFactory.getLog(SpringManagedTransaction.class);
private final DataSource dataSource;
private Connection connection;
private boolean isConnectionTransactional;
private boolean autoCommit;
public SpringManagedTransaction(DataSource dataSource) {
notNull(dataSource, "No DataSource specified");
this.dataSource = dataSource;
}
@Override
public Connection getConnection() throws SQLException {
if (this.connection == null) {
openConnection();
}
return this.connection;
}
private void openConnection() throws SQLException {
//通过DataSourceUtils获取connection，这里和JdbcTransaction不一样
this.connection = DataSourceUtils.getConnection(this.dataSource);
this.autoCommit = this.connection.getAutoCommit();
this.isConnectionTransactional = DataSourceUtils.isConnectionTransactional(this.connection, this.dataSource);
if (LOGGER.isDebugEnabled()) {
LOGGER.debug(
"JDBC Connection ["
+ this.connection
+ "] will"
+ (this.isConnectionTransactional ? " " : " not ")
+ "be managed by Spring");
}
}
@Override
public void commit() throws SQLException {
if (this.connection != null && !this.isConnectionTransactional && !this.autoCommit) {
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Committing JDBC Connection [" + this.connection + "]");
}
//通过connection提交，这里和JdbcTransaction一样
this.connection.commit();
}
}
@Override
public void rollback() throws SQLException {
if (this.connection != null && !this.isConnectionTransactional && !this.autoCommit) {
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Rolling back JDBC Connection [" + this.connection + "]");
}
//通过connection回滚，这里和JdbcTransaction一样
this.connection.rollback();
}
}
@Override
public void close() throws SQLException {
DataSourceUtils.releaseConnection(this.connection, this.dataSource);
}
@Override
public Integer getTimeout() throws SQLException {
ConnectionHolder holder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);
if (holder != null && holder.hasTimeout()) {
return holder.getTimeToLiveInSeconds();
}
return null;
}
}
org.springframework.jdbc.datasource.DataSourceUtils#getConnection
public abstract class DataSourceUtils {
public static Connection getConnection(DataSource dataSource) throws CannotGetJdbcConnectionException {
try {
return doGetConnection(dataSource);
} catch (SQLException var2) {
throw new CannotGetJdbcConnectionException("Failed to obtain JDBC Connection", var2);
} catch (IllegalStateException var3) {
throw new CannotGetJdbcConnectionException("Failed to obtain JDBC Connection: " + var3.getMessage());
}
}
public static Connection doGetConnection(DataSource dataSource) throws SQLException {
Assert.notNull(dataSource, "No DataSource specified");
//TransactionSynchronizationManager重点！！！有没有很熟悉的感觉？？
//还记得我们前面Spring事务源码的分析吗？@Transaction会创建Connection，并放入ThreadLocal中
//这里从ThreadLocal中获取ConnectionHolder
ConnectionHolder conHolder = (ConnectionHolder)TransactionSynchronizationManager.getResource(dataSource);
if (conHolder == null || !conHolder.hasConnection() && !conHolder.isSynchronizedWithTransaction()) {
logger.debug("Fetching JDBC Connection from DataSource");
//如果没有使用@Transaction，那调用Mapper接口方法时，也是通过Spring的方法获取Connection
Connection con = fetchConnection(dataSource);
if (TransactionSynchronizationManager.isSynchronizationActive()) {
try {
ConnectionHolder holderToUse = conHolder;
if (conHolder == null) {
holderToUse = new ConnectionHolder(con);
} else {
conHolder.setConnection(con);
}
holderToUse.requested();
TransactionSynchronizationManager.registerSynchronization(new DataSourceUtils.ConnectionSynchronization(holderToUse, dataSource));
holderToUse.setSynchronizedWithTransaction(true);
if (holderToUse != conHolder) {
//将获取到的ConnectionHolder放入ThreadLocal中，那么当前线程调用下一个接口，下一个接口使用了Spring事务，那Spring事务也可以直接取到Mybatis创建的Connection
//通过ThreadLocal保证了同一线程中Spring事务使用的Connection和Mapper代理类使用的Connection是同一个
TransactionSynchronizationManager.bindResource(dataSource, holderToUse);
}
} catch (RuntimeException var4) {
releaseConnection(con, dataSource);
throw var4;
}
}
return con;
} else {
conHolder.requested();
if (!conHolder.hasConnection()) {
logger.debug("Fetching resumed JDBC Connection from DataSource");
conHolder.setConnection(fetchConnection(dataSource));
}
//所以如果我们业务代码使用了@Transaction注解，在Spring中就已经通过dataSource创建了一个Connection并放入ThreadLocal中
//那么当Mapper代理对象调用方法时，通过SqlSession的SpringManagedTransaction获取连接时，就直接获取到了当前线程中Spring事务创建的Connection并返回
return conHolder.getConnection();
}
}
}
想看怎么获取connHolder
org.springframework.transaction.support.TransactionSynchronizationManager#getResource
public abstract class TransactionSynchronizationManager {
//保存数据库连接的ThreadLocal
private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal("Transactional resources");
@Nullable
public static Object getResource(Object key) {
Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
//获取ConnectionHolder
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;
}
@Nullable
private static Object doGetResource(Object actualKey) {
/**
* 从threadlocal <Map<Object, Object>>中取出来当前线程绑定的map
* map里面存的是<dataSource,ConnectionHolder>
*/
Map<Object, Object> map = (Map)resources.get();
if (map == null) {
return null;
} else {
//map中取出来对应dataSource的ConnectionHolder
Object value = map.get(actualKey);
if (value instanceof ResourceHolder && ((ResourceHolder)value).isVoid()) {
map.remove(actualKey);
if (map.isEmpty()) {
resources.remove();
}
value = null;
}
return value;
}
}
}
我们看到直接从ThreadLocal中取出来的conn，而spring自己的事务也是操作的这个ThreadLocal中的conn来进行事务的开启和回滚，由此我们知道了在同一线程中Spring事务中的Connection和Mybaits中Mapper代理对象中操作数据库的Connection是同一个，当取出来的conn为空时候，调用org.springframework.jdbc.datasource.DataSourceUtils#fetchConnection获取，然后把从数据源取出来的连接返回
public abstract class DataSourceUtils {
private static Connection fetchConnection(DataSource dataSource) throws SQLException {
//从数据源取出来conn
Connection con = dataSource.getConnection();
if (con == null) {
throw new IllegalStateException("DataSource returned null from getConnection(): " + dataSource);
} else {
return con;
}
}
}
我们再来回顾一下上篇文章中的SqlSessionInterceptor
public class SqlSessionTemplate implements SqlSession, DisposableBean {
private final SqlSessionFactory sqlSessionFactory;
private final ExecutorType executorType;
private final SqlSession sqlSessionProxy;
private final PersistenceExceptionTranslator exceptionTranslator;
private class SqlSessionInterceptor implements InvocationHandler {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
// 获取一个sqlSession来执行proxy的method对应的SQL,
// 每次调用都获取创建一个sqlSession线程局部变量，故不同线程相互不影响，在这里实现了SqlSessionTemplate的线程安全性
SqlSession sqlSession = getSqlSession(
SqlSessionTemplate.this.sqlSessionFactory,
SqlSessionTemplate.this.executorType,
SqlSessionTemplate.this.exceptionTranslator);
try {
//直接通过新创建的SqlSession反射调用method
//这也就解释了为什么不需要目标类属性了，这里每次都会创建一个
Object result = method.invoke(sqlSession, args);
// 如果当前操作没有在一个Spring事务中，则手动commit一下
// 如果当前业务没有使用@Transation,那么每次执行了Mapper接口的方法直接commit
// 还记得我们前面讲的Mybatis的一级缓存吗，这里一级缓存不能起作用了，因为每执行一个Mapper的方法，sqlSession都提交了
// sqlSession提交，会清空一级缓存
if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
// force commit even on non-dirty sessions because some databases require
// a commit/rollback before calling close()
sqlSession.commit(true);
}
return result;
} catch (Throwable t) {
Throwable unwrapped = unwrapThrowable(t);
if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
// release the connection to avoid a deadlock if the translator is no loaded. See issue #22
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
sqlSession = null;
Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
if (translated != null) {
unwrapped = translated;
}
}
throw unwrapped;
} finally {
if (sqlSession != null) {
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
}
}
}
}
}
看if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory))和sqlSession.commit(true)行，如果我们没有使用@Transation，Mapper方法执行完后，sqlSession将会提交，也就是说通过org.springframework.jdbc.datasource.DataSourceUtils#fetchConnection获取到的Connection将会commit，相当于Connection是自动提交的，也就是说如果不使用@Transation，Mybatis将没有事务可言。
Mybatis和Spring整合后SpringManagedTransaction和Spring的Transaction的关系：
-
如果开启Spring事务，则先有Spring的Transaction，然后mybatis创建sqlSession时，会创建SpringManagedTransaction并加入sqlSession中，SpringManagedTransaction中的connection会从Spring的Transaction创建的Connection并放入ThreadLocal中获取。
-
如果没有开启Spring事务或者第一个方法没有事务后面的方法有事务，则SpringManagedTransaction创建Connection并放入ThreadLocal中。
spring结合mybatis后mybaits一级缓存失效分为两种情况：
- 如果没有开启事务，每一次sql都是用的新的SqlSession，这时mybatis的一级缓存是失效的。
- 如果有事务，同一个事务中相同的查询使用的相同的SqlSessioon，此时一级缓存是生效的。
如果使用了@Transation呢？那在调用Mapper代理类的方法之前就已经通过Spring的事务生成了Connection并放入ThreadLocal，并且设置事务不自动提交，当前线程多个Mapper代理对象调用数据库操作方法时，将从ThreadLocal获取Spring创建的connection,在所有的Mapper方法调用完后，Spring事务提交或者回滚，到此mybatis的事务是怎么被spring管理的就显而易见了。
还有文章开头的问题，为什么Mybtis中要配置dataSource，Spring的事务中也要配置dataSource？
因为Spring事务在没调用Mapper方法之前就需要开一个Connection，并设置事务不自动提交，那么transactionManager中自然要配置dataSource。那如果我们的Service没有用到Spring事务呢，难道就不需要获取数据库连接了吗？当然不是，此时通过SpringManagedTransaction调用org.springframework.jdbc.datasource.DataSourceUtils#getConnection#fetchConnection方法获取，并将dataSource作为参数传进去，实际上获取的Connection都是通过dataSource来获取的。
