作者：等你归去来
cnblogs.com/yougewe/p/10072740.html
项目中常常使用mybatis配合spring进行数据库操作，但是我们知道，数据的操作是要求做到线程安全的，而且按照原来的jdbc的使用方式，每次操作完成之后都要将连接关闭，但是实际使用中我们并没有这么干。
更让人疑惑的点是，spring中默认使用单例形式来加载bean，而往往我们也不会改变这种默认，所以，是所有线程共享数据连接？
让我们来看看真相！
自然是要个栗子的：
我们来看下spring中配置mybatis数据库操作bean(使用 druid 连接池)：
<bean id="dataSource" class="com.alibaba.druid.pool.DruidDataSource">
<property name="url" value="${jdbc.url}" />
<property name="driverClassName" value="${jdbc.driver}" />
<property name="username" value="${jdbc.username}" />
<property name="password" value="${jdbc.password}" />
</bean>
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="dataSource" ref="dataSource" />
<property name="configLocation" value="classpath:mybatis-config.xml" />
</bean>
<!-- scope="prototype" 另说,另讨论，我们先以mapper形式看一下 -->
<bean id="sqlSession" class="org.mybatis.spring.SqlSessionTemplate">
<constructor-arg index="0" ref="sqlSessionFactory" />
</bean>
<!-- 事务 -->
<bean name="transactionManager"
class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
<property name="dataSource" ref="dataSource"></property>
</bean>
而在java代码中使用则是使用依赖注入直接使用 @resource sqlSession, 如下:
@Resource
private SqlSessionTemplate sqlSession;
@Override
public User getUser(Map<String, String> cond) {
// 此句执行db查询
User result = sqlSession.selectOne(NAME_SPACE
+ ".getUser", cond);
return result;
}
这个sqlSession就是直接去操作数据库了看起来是这样，是在bean初始化的时候依赖注入的！
所以，难道每次进入该操作的时候，sqlSession 的实例都会变化吗？答案是否定的。
那么，肯定就是往下使用的时候才发生的变化呗！
再往下走，可以看到，调用了一个代理来进行具体的查询！
// org/mybatis/spring/SqlSessionTemplate.selectOne()
public <T> T selectOne(String statement, Object parameter) {
return this.sqlSessionProxy.<T> selectOne(statement, parameter);
}
为啥要用代理呢？自己直接查不就行了吗？其实，用代理是有好处的，那就可以可以进行另外的包装！
代理是怎么生成的呢？其实只要看一下 SqlSessionTemplate 的构造方法就知道了！
/**
* Constructs a Spring managed {@code SqlSession} with the given
* {@code SqlSessionFactory} and {@code ExecutorType}.
* A custom {@code SQLExceptionTranslator} can be provided as an
* argument so any {@code PersistenceException} thrown by MyBatis
* can be custom translated to a {@code RuntimeException}
* The {@code SQLExceptionTranslator} can also be null and thus no
* exception translation will be done and MyBatis exceptions will be
* thrown
*
* @param sqlSessionFactory
* @param executorType
* @param exceptionTranslator
*/
public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
PersistenceExceptionTranslator exceptionTranslator) {
notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
notNull(executorType, "Property 'executorType' is required");
this.sqlSessionFactory = sqlSessionFactory;
this.executorType = executorType;
this.exceptionTranslator = exceptionTranslator;
// 生成代理 SqlSessionInterceptor 为 InvocationHandler
this.sqlSessionProxy = (SqlSession) newProxyInstance(
SqlSessionFactory.class.getClassLoader(),
new Class[] { SqlSession.class },
new SqlSessionInterceptor());
}
从上面的代码，看不到细节，但是，大致还是知道代理的具体实现了！即使用 SqlSessionInterceptor 去处理具体查询逻辑！
我们来看下 SqlSessionInterceptor 的实现！
/**
* Proxy needed to route MyBatis method calls to the proper SqlSession got
* from Spring's Transaction Manager
* It also unwraps exceptions thrown by {@code Method#invoke(Object, Object...)} to
* pass a {@code PersistenceException} to the {@code PersistenceExceptionTranslator}.
*/
private class SqlSessionInterceptor implements InvocationHandler {
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
SqlSession sqlSession = getSqlSession(
SqlSessionTemplate.this.sqlSessionFactory,
SqlSessionTemplate.this.executorType,
SqlSessionTemplate.this.exceptionTranslator);
try {
Object result = method.invoke(sqlSession, args);
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);
}
}
}
}
SqlSessionInterceptor 是 SqlSessionTemplate 的内部类，目的只有一个，就是处理多个 session 的db操作！
所有请求都被 invoke() 拦截,从而做相应处理：
进入请求，先生成一个新的sqlSession，为本次db操作做准备；
通过反射调用请求进来的方法，将 sqlSession 回调，进行复杂查询及结果映射；
如果需要立即提交事务，do it；
如果出现异常，包装异常信息，重新抛出；
操作完成后，关闭本次session；
到这里，其实我们好像已经明白了，其实外面的 sqlSession 单例，并不会影响具体的db操作控制，所以不用担心session的线程安全问题！
不过，还有个点值得考虑下，如果我一次请求里有多次数据库操作，难道我真的要创建多个sqlSession或者说数据库连接？不会吧！
如果这个问题得不到解决，可能你并不真正了解session的定义了！
所以我们需要继续看一下 session 到底是怎么获取的？
getSqlSession() 方法是在 SqlSessionUtils 中实现的！如下：
/**
* Gets an SqlSession from Spring Transaction Manager or creates a new one if needed.
* Tries to get a SqlSession out of current transaction. If there is not any, it creates a new one.
* Then, it synchronizes the SqlSession with the transaction if Spring TX is active and
* <code>SpringManagedTransactionFactory</code> is configured as a transaction manager.
*
* @param sessionFactory a MyBatis {@code SqlSessionFactory} to create new sessions
* @param executorType The executor type of the SqlSession to create
* @param exceptionTranslator Optional. Translates SqlSession.commit() exceptions to Spring exceptions.
* @throws TransientDataAccessResourceException if a transaction is active and the
* {@code SqlSessionFactory} is not using a {@code SpringManagedTransactionFactory}
* @see SpringManagedTransactionFactory
*/
public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {
notNull(sessionFactory, "No SqlSessionFactory specified");
notNull(executorType, "No ExecutorType specified");
SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
// 如果已经有holder，则直接返回，复用连接
if (holder != null && holder.isSynchronizedWithTransaction()) {
if (holder.getExecutorType() != executorType) {
throw new TransientDataAccessResourceException("Cannot change the ExecutorType when there is an existing transaction");
}
holder.requested();
if (logger.isDebugEnabled()) {
logger.debug("Fetched SqlSession [" + holder.getSqlSession() + "] from current transaction");
}
return holder.getSqlSession();
}
if (logger.isDebugEnabled()) {
logger.debug("Creating a new SqlSession");
}
SqlSession session = sessionFactory.openSession(executorType);
// Register session holder if synchronization is active (i.e. a Spring TX is active)
//
// Note: The DataSource used by the Environment should be synchronized with the
// transaction either through DataSourceTxMgr or another tx synchronization.
// Further assume that if an exception is thrown, whatever started the transaction will
// handle closing / rolling back the Connection associated with the SqlSession.
if (TransactionSynchronizationManager.isSynchronizationActive()) {
Environment environment = sessionFactory.getConfiguration().getEnvironment();
if (environment.getTransactionFactory() instanceof SpringManagedTransactionFactory) {
if (logger.isDebugEnabled()) {
logger.debug("Registering transaction synchronization for SqlSession [" + session + "]");
}
holder = new SqlSessionHolder(session, executorType, exceptionTranslator);
TransactionSynchronizationManager.bindResource(sessionFactory, holder);
TransactionSynchronizationManager.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));
holder.setSynchronizedWithTransaction(true);
holder.requested();
} else {
if (TransactionSynchronizationManager.getResource(environment.getDataSource()) == null) {
if (logger.isDebugEnabled()) {
logger.debug("SqlSession [" + session + "] was not registered for synchronization because DataSource is not transactional");
}
} else {
throw new TransientDataAccessResourceException(
"SqlSessionFactory must be using a SpringManagedTransactionFactory in order to use Spring transaction synchronization");
}
}
} else {
if (logger.isDebugEnabled()) {
logger.debug("SqlSession [" + session + "] was not registered for synchronization because synchronization is not active");
}
}
return session;
}
如上获取 sqlSession 逻辑，主要分两种情况！
如果存在holder，则返回原有的sqlSession，到于这个holder我们稍后再说；
如果没有，则创建一个新连接！
所以，看起来情况还不是太糟，至少有复用的概念了！
那么问题来了，复用？如何做到线程安全？所以我们要看下 SqlSessionHolder 的实现了！
获取holder是通过 TransactionSynchronizationManager.getResource(sessionFactory); 获取的：
public static Object getResource(Object key) {
Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
// 实际获取
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;
}
private static Object doGetResource(Object actualKey) {
Map<Object, Object> map = resources.get();
if (map == null) {
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;
}
咱们忽略对 key 的处理，实际是直接调用 doGetResource() 获取holder。而 doGetResource() 中，则使用了 resources 来保存具体的 kv。 resources 明显是个共享变量，但是看起来这里没有任何的加锁操作！这是为何？
只要看一下 resources 的定义就知道了，其实现为 ThreadLocal, 所以是线程安全了！
private static final ThreadLocal<Map<Object, Object>> resources =
new NamedThreadLocal<Map<Object, Object>>("Transactional resources");
在新的请求进来时，自然是没有值的，所以直接返回null.而后续进入，则获取缓存返回！
而对于没有获取到 holder 的情况，则需要重新创建一个 session 了！
这里获取session由DefaultSqlSessionFactory 进行创建！如下：
// org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSession()
public SqlSession openSession(ExecutorType execType) {
return openSessionFromDataSource(execType, null, false);
}
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
final Environment environment = configuration.getEnvironment();
// SpringManagedTransactionFactory
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
final Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
创建 session 几件事:
根据环境配置，开启一个新事务，该事务管理器会负责后续jdbc连接管理工作；
根据事务创建一个 Executor，备用；
用DefaultSqlSession 将 executor 包装后返回，用于后续真正的db操作；
至此，真正的 sqlSession 已经创建成功！返回后，就可以真正使用了！
等等，创建的session好像并没有保存，那么还是那个问题，每个sql都会创建一个 sqlSession ？ 好吧，是这样的！前面的holder，只是用于存在事务操作的连接！（holder的理解出了偏差哦）
但是有一点，这里虽然创建了多个 sqlSession 实例，但是并不意味着有多个db连接，具体使用db连接时，则一般会会使用连接池来进行优化！如前面提到的 druid 就是个不错的选择！
真实的jdbc连接获取，是在进行真正的 query 时，才进行调用 getConnection() 进行接入！
具体则是在 doQuery() 时，进行st的组装时调用的 ，如下：
// SimpleExecutor.prepareStatement()
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
// 获取 jdbc 连接，返回 java.sql.Connection
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection);
handler.parameterize(stmt);
return stmt;
}
// 调用 BaseExecutor.getConnection()
protected Connection getConnection(Log statementLog) throws SQLException {
// SpringManagedTransaction 管理 connection
Connection connection = transaction.getConnection();
if (statementLog.isDebugEnabled()) {
return ConnectionLogger.newInstance(connection, statementLog, queryStack);
} else {
return connection;
}
}
通过前面通过事务管理工厂创建的 SpringManagedTransaction 进行 connection 获取！一个事务管理器只会存在一次获取数据库连接的操作！
public Connection getConnection() throws SQLException {
if (this.connection == null) {
openConnection();
}
return this.connection;
}
// 而 SpringManagedTransaction 又将connection交由 DataSourceUtils 进行管理！
// org/springframework/jdbc/datasource/DataSourceUtils
public static Connection getConnection(DataSource dataSource) throws CannotGetJdbcConnectionException {
try {
// 真正的连接获取
return doGetConnection(dataSource);
}
catch (SQLException ex) {
throw new CannotGetJdbcConnectionException("Could not get JDBC Connection", ex);
}
}
/**
* Actually obtain a JDBC Connection from the given DataSource.
* Same as {@link #getConnection}, but throwing the original SQLException.
* <p>Is aware of a corresponding Connection bound to the current thread, for example
* when using {@link DataSourceTransactionManager}. Will bind a Connection to the thread
* if transaction synchronization is active (e.g. if in a JTA transaction).
* <p>Directly accessed by {@link TransactionAwareDataSourceProxy}.
* @param dataSource the DataSource to obtain Connections from
* @return a JDBC Connection from the given DataSource
* @throws SQLException if thrown by JDBC methods
* @see #doReleaseConnection
*/
public static Connection doGetConnection(DataSource dataSource) throws SQLException {
Assert.notNull(dataSource, "No DataSource specified");
ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);
if (conHolder != null && (conHolder.hasConnection() || conHolder.isSynchronizedWithTransaction())) {
conHolder.requested();
if (!conHolder.hasConnection()) {
logger.debug("Fetching resumed JDBC Connection from DataSource");
conHolder.setConnection(dataSource.getConnection());
}
return conHolder.getConnection();
}
// Else we either got no holder or an empty thread-bound holder here.
logger.debug("Fetching JDBC Connection from DataSource");
// 通过接入的dataSource进行连接获取，这里将会是最终的jdbc连接
Connection con = dataSource.getConnection();
if (TransactionSynchronizationManager.isSynchronizationActive()) {
logger.debug("Registering transaction synchronization for JDBC Connection");
// Use same Connection for further JDBC actions within the transaction.
// Thread-bound object will get removed by synchronization at transaction completion.
ConnectionHolder holderToUse = conHolder;
if (holderToUse == null) {
holderToUse = new ConnectionHolder(con);
}
else {
holderToUse.setConnection(con);
}
holderToUse.requested();
TransactionSynchronizationManager.registerSynchronization(
new ConnectionSynchronization(holderToUse, dataSource));
holderToUse.setSynchronizedWithTransaction(true);
if (holderToUse != conHolder) {
TransactionSynchronizationManager.bindResource(dataSource, holderToUse);
}
}
return con;
}
上面的实现主要做三件事：
再次确认，是否存在事务处理，holder是否存在，如果有则复用；
如果没有，那再从数据源处获取连接；
获取新连接成功后，检查如果存在事务，则将新获取的连接放入holder中保存起来，以备下次使用；
获取jdbc连接后，就可以真正发起execute()查询了。
数据库连接的疑问算是解答了！我们发现，外部的框架并没有多少为我们节省db连接的动作！而是把最终 getConnection() 交给 datasource 数据源！
而真正解决我们连接复用的问题的，是像 Druid 这样的连接池组件！所以，咱们可以单独来看这些中间件了！
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