# Mybatis深入之获取数据库连接
### 简介
主要记录Mybatis何时获取数据库连接以及获取数据库连接的过程。难点在于明白在使用Mybatis数据库连接池情况下的数据库连接的获取过程。
### 何时获取数据库连接
Mybatis只有在真正执行sql操作的时候才会去获取数据库连接。至于如何验证：
### 不深入源码
简单来讲就是有意将数据库配置信息写成、在一个sql执行过程中看哪一步抛数据库连接异常。
~~~
public static void main(String[] args) throws Exception {
String mybatisConfigPath = "config/mybatis/mybatis.xml";
InputStream inputStream = Resources.getResourceAsStream(mybatisConfigPath);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
int count = (Integer)sqlSession.selectOne("org.alien.mybatis.samples.mapper.AuthorMapper.getAllAuthorsCount");
System.out.println(count);
}
~~~
- 上面是一段Mybatis执行代码
- 我们可以将Mybatis连接数据库的信息有意写错
- 再DEBUG模式下一步一步调试看哪一步会抛异常
- 抛异常的那一步就是真正获取数据库连接的一步
异常信息：
### 深入源码
这里简单提一下、具体后面会有。最有迷惑性的是觉得在`openSession()`的时候会获取数据库连接、其实不然：
openSession()最终只是返回一个操作数据库的会话、并不包含数据库连接，DefaultSqlSession（这个是Mybatis初始化的时候返回的一个SqlSession）中的方法：
~~~
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
final Environment environment = configuration.getEnvironment();
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();
}
}
~~~
- 主要装配DefaultSqlSession中执行Sql的Excutor、后面关于一个Sql完整的执行过程会有对其的详细分析
显然真正获取数据库连接的操作是在`sqlSession.selectOne("org.alien.mybatis.samples.mapper.AuthorMapper.getAllAuthorsCount");`进行的。
### 获取数据库连接
在真正的获取数据库连接代码之前、还有许多为sql执行而生的代码、这里暂时忽略或者一些必要的说明、主要重心放在如何获取数据库连接。
书接上回、从前面执行sql代码开始：
~~~
int count = (Integer)sqlSession.selectOne("org.alien.mybatis.samples.mapper.AuthorMapper.getAllAuthorsCount");
~~~
下图是上面代码一系列方法调用过程：
经过一系列调用到SimpleExecutor——》doQuery()：
~~~
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
//RoutingStatementHandler
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
//MappedStatement、这一句是关键
stmt = prepareStatement(handler, ms.getStatementLog());
/*
*参数：
* stmt: PreparedStatementLogger
* resultHandler: null
*/
return handler.<E>query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
~~~
- 上面经过一系列跳转之后各个引用的实例我加了注释、有兴趣的可以自己跟一下、有时候可能要多跟几遍
- 关键代码`stmt=prepareStatement(handler,ms.getStatementLog());`、这一句根据方法名就能猜测、是根据Connection来获取执行Sql的PrepareStatement
- 但是到现在为止我们都没有看到方法的参数中有关于数据库连接的、在此方法中
SimpleExecutor——》prepareStatement();
~~~
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
//获取数据库连接。statementLog:org.apache.ibatis.loggin.slf4j.Slf4Impl
Connection connection = getConnection(statementLog);
//获取执行Sql的Statement——PrepareStatementLogger
//PrepareStatementLogger是PrepareStatement的代理、多了对使用Mybatis执行sql语句时记录sql语句的功能
stmt = handler.prepare(connection);
//将执行Sql需要的参数设置到PrepareStatement中。
handler.parameterize(stmt);
return stmt;
}
~~~
其他的不关注、这里只看数据库连接代码：`Connection connection = getConnection(statementLog);`BaseExecutor——》getConnection():
~~~
protected Connection getConnection(Log statementLog) throws SQLException {
//如果关于数据库连接的日志记录级别是DEBUG级别、则为获取的Connection进行代理、新增日志记录功能、这里不是重点。
if (statementLog.isDebugEnabled()) {
return ConnectionLogger.newInstance(connection, statementLog, queryStack);
} else {
return connection;
}
}
~~~
- Mybatis中关于事务配置项的值是”JDBC”、所以从[Mybatis深入之事务管理 ](http://blog.csdn.net/crave_shy/article/details/46595391)知道这里的transaction其实是：JdbcTransaction
- 最终到JdbcTransaction获取连接的方法中
JdbcTransaction——》openConnection()
~~~
protected void openConnection() throws SQLException {
if (log.isDebugEnabled()) {
log.debug("Opening JDBC Connection");
}
connection = dataSource.getConnection();
if (level != null) {
connection.setTransactionIsolation(level.getLevel());
}
setDesiredAutoCommit(autoCommmit);
}
~~~
- 上面的dataSource从[ Mybatis深入之DataSource实例化过程 ](http://blog.csdn.net/crave_shy/article/details/46584803)知道当使用数据库连接池的时候实例化的是PooledDataSource
PooledDataSource——》getConnection()：
~~~
public Connection getConnection() throws SQLException {
/*
* 为理解方便、将原来代码拆分如下：
*/
PooledConnection pooledConnection = popConnection(dataSource.getUsername(), dataSource.getPassword());
Connection connection = pooledConnection.getProxyConnection();
return connection ;
//return popConnection(dataSource.getUsername(), dataSource.getPassword()).getProxyConnection();
}
~~~
- 从拆分代码看分两步
- 获取数据库真正连接
- 获取真正数据库连接的代理类作为最终返回结果、至于代理是做什么、后面继续
PooledDataSource——》popConnection():
~~~
private PooledConnection popConnection(String username, String password) throws SQLException {
boolean countedWait = false;
PooledConnection conn = null;
long t = System.currentTimeMillis();
int localBadConnectionCount = 0;
while (conn == null) {
synchronized (state) {
if (state.idleConnections.size() > 0) {
// Pool has available connection
conn = state.idleConnections.remove(0);
if (log.isDebugEnabled()) {
log.debug("Checked out connection " + conn.getRealHashCode() + " from pool.");
}
} else {
// Pool does not have available connection
if (state.activeConnections.size() < poolMaximumActiveConnections) {
// Can create new connection
conn = new PooledConnection(dataSource.getConnection(), this);
@SuppressWarnings("unused")
//used in logging, if enabled
Connection realConn = conn.getRealConnection();
if (log.isDebugEnabled()) {
log.debug("Created connection " + conn.getRealHashCode() + ".");
}
} else {
// Cannot create new connection
PooledConnection oldestActiveConnection = state.activeConnections.get(0);
long longestCheckoutTime = oldestActiveConnection.getCheckoutTime();
if (longestCheckoutTime > poolMaximumCheckoutTime) {
// Can claim overdue connection
state.claimedOverdueConnectionCount++;
state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime;
state.accumulatedCheckoutTime += longestCheckoutTime;
state.activeConnections.remove(oldestActiveConnection);
if (!oldestActiveConnection.getRealConnection().getAutoCommit()) {
oldestActiveConnection.getRealConnection().rollback();
}
conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this);
oldestActiveConnection.invalidate();
if (log.isDebugEnabled()) {
log.debug("Claimed overdue connection " + conn.getRealHashCode() + ".");
}
} else {
// Must wait
try {
if (!countedWait) {
state.hadToWaitCount++;
countedWait = true;
}
if (log.isDebugEnabled()) {
log.debug("Waiting as long as " + poolTimeToWait + " milliseconds for connection.");
}
long wt = System.currentTimeMillis();
state.wait(poolTimeToWait);
state.accumulatedWaitTime += System.currentTimeMillis() - wt;
} catch (InterruptedException e) {
break;
}
}
}
}
if (conn != null) {
if (conn.isValid()) {
if (!conn.getRealConnection().getAutoCommit()) {
conn.getRealConnection().rollback();
}
conn.setConnectionTypeCode(assembleConnectionTypeCode(dataSource.getUrl(), username, password));
conn.setCheckoutTimestamp(System.currentTimeMillis());
conn.setLastUsedTimestamp(System.currentTimeMillis());
state.activeConnections.add(conn);
state.requestCount++;
state.accumulatedRequestTime += System.currentTimeMillis() - t;
} else {
if (log.isDebugEnabled()) {
log.debug("A bad connection (" + conn.getRealHashCode() + ") was returned from the pool, getting another connection.");
}
state.badConnectionCount++;
localBadConnectionCount++;
conn = null;
if (localBadConnectionCount > (poolMaximumIdleConnections + 3)) {
if (log.isDebugEnabled()) {
log.debug("PooledDataSource: Could not get a good connection to the database.");
}
throw new SQLException("PooledDataSource: Could not get a good connection to the database.");
}
}
}
}
}
if (conn == null) {
if (log.isDebugEnabled()) {
log.debug("PooledDataSource: Unknown severe error condition. The connection pool returned a null connection.");
}
throw new SQLException("PooledDataSource: Unknown severe error condition. The connection pool returned a null connection.");
}
return conn;
}
~~~
- 先解释一下逻辑、再关注具体的数据库连接方法
1. 先看是否有空闲(idle)状态下的PooledConnection对象，如果有，就直接返回一个可用的PooledConnection对象；否则进行第2步。
2. 查看活动状态的PooledConnection池activeConnections是否已满；如果没有满，则创建一个新的PooledConnection对象，然后放到activeConnections池中，然后返回此PooledConnection对象；否则进行第三步；
3. 看最先进入activeConnections池中的PooledConnection对象是否已经过期：如果已经过期，从activeConnections池中移除此对象，然后创建一个新的PooledConnection对象，添加到activeConnections中，然后将此对象返回；否则进行第4步。
4. 线程等待，循环2步
具体的创建数据库连接代码`conn = new PooledConnection(dataSource.getConnection(), this);`
- 上面代码中的dataSource为UnpooledDataSource、可以从[ Mybatis深入之DataSource实例化过程 ](http://blog.csdn.net/crave_shy/article/details/46584803)了解原因。
所以先要看UnpooledDataSource——getConnection() 经过一系列跳转到同类如下方法：
~~~
private Connection doGetConnection(Properties properties) throws SQLException {
initializeDriver();
Connection connection = DriverManager.getConnection(url, properties);
configureConnection(connection);
return connection;
}
~~~
- 上面我们可以看到很熟悉的加载驱动、获取数据库连接
### 补充
其实关于数据库部分还有很多要写的、比如数据库连接池工作原理、数据库连接何时关闭。这里暂不准备一次将所有的东都放在一起。觉得分开点更容易说清楚理解起来不是那么费劲。
后面会有一篇Mybatis数据库连接池原理来分析它。
更多内容[Mybatis 目录](http://blog.csdn.net/crave_shy/article/details/23932803)
