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<p style="text-align: center"><span style="font-size: 16px">mybatis与数据库访问相关的配置以及设计
<p style="text-align: left"><span style="font-size: 16px">mybatis不管如何NB，总是要与数据库进行打交道。通过提问的方式，逐步深入
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 
2.对于有连接池的数据源，和无连接池的数据源,我们自己会如何设计?
流程上的区别
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?? 
职责上区别
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现在解开谜底:看实际Mybatis设计如何?
非池化类:
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看下最关键的,获得数据库连接,和我们自己写的没啥区别。简单粗暴
Connection doGetConnection(Properties properties) =
池化类:
池化工作分配? :
??? ? ? ? ? ? ?? 
至此,MYBABTIS对于数据源的创建以及管理结束!看下代码,池化获得连接的代码
Connection getConnection()
<div class="cnblogs_code">
(conn == ) {
</span><span style="color: #008000">//</span><span style="color: #008000">只有有连接还回来,再走这里</span>
<span style="color: #0000ff">if</span> (!<span style="color: #000000">state.idleConnections.isEmpty()) {
</span><span style="color: #008000">//</span><span style="color: #008000"> Pool has available connection</span>
conn = state.idleConnections.remove(0<span style="color: #000000">);
</span><span style="color: #0000ff">if</span><span style="color: #000000"> (log.isDebugEnabled()) {
log.debug(</span>"Checked out connection " + conn.getRealHashCode() + " from pool."<span style="color: #000000">);
}
} </span><span style="color: #0000ff">else</span><span style="color: #000000"> {
</span><span style="color: #008000">//</span><span style="color: #008000">有两种可能:1种,都在使用中,池子没满，再新建
</span><span style="color: #008000">//</span><span style="color: #008000"> Pool does not have available connection</span>
<span style="color: #0000ff">if</span> (state.activeConnections.size() <<span style="color: #000000"> poolMaximumActiveConnections) {
</span><span style="color: #008000">//</span><span style="color: #008000"> Can create new connection
</span><span style="color: #008000">//</span><span style="color: #008000">新建连接</span>
conn = <span style="color: #0000ff">new</span> PooledConnection(dataSource.getConnection(),<span style="color: #0000ff">this</span><span style="color: #000000">);
</span><span style="color: #0000ff">if</span><span style="color: #000000"> (log.isDebugEnabled()) {
log.debug(</span>"Created connection " + conn.getRealHashCode() + "."<span style="color: #000000">);
}
} </span><span style="color: #0000ff">else</span><span style="color: #000000"> {
</span><span style="color: #008000">//</span><span style="color: #008000"> Cannot create new connection
</span><span style="color: #008000">//</span><span style="color: #008000">找一个最老的,用的ArrayList,老的在ArrayList数组的前面</span>
PooledConnection oldestActiveConnection = state.activeConnections.get(0<span style="color: #000000">);
</span><span style="color: #0000ff">long</span> longestCheckoutTime =<span style="color: #000000"> oldestActiveConnection.getCheckoutTime();
</span><span style="color: #008000">//</span><span style="color: #008000">借出超时,不让他做了，直接rollback。。。暴力</span>
<span style="color: #0000ff">if</span> (longestCheckoutTime ><span style="color: #000000"> poolMaximumCheckoutTime) {
</span><span style="color: #008000">//</span><span style="color: #008000"> Can claim overdue connection</span>
state.claimedOverdueConnectionCount++<span style="color: #000000">;
state.accumulatedCheckoutTimeOfOverdueConnections </span>+=<span style="color: #000000"> longestCheckoutTime;
state.accumulatedCheckoutTime </span>+=<span style="color: #000000"> longestCheckoutTime;
state.activeConnections.remove(oldestActiveConnection);
</span><span style="color: #0000ff">if</span> (!<span style="color: #000000">oldestActiveConnection.getRealConnection().getAutoCommit()) {
</span><span style="color: #0000ff">try</span><span style="color: #000000"> {
oldestActiveConnection.getRealConnection().rollback();
} </span><span style="color: #0000ff">catch</span><span style="color: #000000"> (SQLException e) {
</span><span style="color: #008000">/*</span><span style="color: #008000">
Just log a message for debug and continue to execute the following
statement like nothing happend.
Wrap the bad connection with a new PooledConnection,this will help
to not intterupt current executing thread and give current thread a
chance to join the next competion for another valid/good database
connection. At the end of this loop,bad {@link @conn} will be set as null.
</span><span style="color: #008000">*/</span><span style="color: #000000">
log.debug(</span>"Bad connection. Could not roll back"<span style="color: #000000">);
}
}
</span><span style="color: #008000">//</span><span style="color: #008000">拿回来后，不再放到原有的PooledConnection,新建立一个。从新开始.老的REAL connection还被oldestActiveConnection引用,不会内存溢出?</span>
conn = <span style="color: #0000ff">new</span> PooledConnection(oldestActiveConnection.getRealConnection(),<span style="color: #0000ff">this</span><span style="color: #000000">);
conn.setCreatedTimestamp(oldestActiveConnection.getCreatedTimestamp());
conn.setLastUsedTimestamp(oldestActiveConnection.getLastUsedTimestamp());
oldestActiveConnection.invalidate();
</span><span style="color: #0000ff">if</span><span style="color: #000000"> (log.isDebugEnabled()) {
log.debug(</span>"Claimed overdue connection " + conn.getRealHashCode() + "."<span style="color: #000000">);
}
} </span><span style="color: #0000ff">else</span><span style="color: #000000"> {
</span><span style="color: #008000">//</span><span style="color: #008000"> Must wait
</span><span style="color: #008000">//</span><span style="color: #008000">大家都在用着,你只能等着了。</span>
<span style="color: #0000ff">try</span><span style="color: #000000"> {
</span><span style="color: #0000ff">if</span> (!<span style="color: #000000">countedWait) {
state.hadToWaitCount</span>++<span style="color: #000000">;
countedWait </span>= <span style="color: #0000ff">true</span><span style="color: #000000">;
}
</span><span style="color: #0000ff">if</span><span style="color: #000000"> (log.isDebugEnabled()) {
log.debug(</span>"Waiting as long as " + poolTimeToWait + " milliseconds for connection."<span style="color: #000000">);
}
</span><span style="color: #0000ff">long</span> wt =<span style="color: #000000"> System.currentTimeMillis();
state.wait(poolTimeToWait);
state.accumulatedWaitTime </span>+= System.currentTimeMillis() -<span style="color: #000000"> wt;
} </span><span style="color: #0000ff">catch</span><span style="color: #000000"> (InterruptedException e) {
</span><span style="color: #0000ff">break</span><span style="color: #000000">;
}
}
}
}
</span><span style="color: #0000ff">if</span> (conn != <span style="color: #0000ff">null</span><span style="color: #000000">) {
</span><span style="color: #008000">//</span><span style="color: #008000"> ping to server and check the connection is valid or not</span>
<span style="color: #0000ff">if</span><span style="color: #000000"> (conn.isValid()) {
</span><span style="color: #0000ff">if</span> (!<span style="color: #000000">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</span>++<span style="color: #000000">;
state.accumulatedRequestTime </span>+= System.currentTimeMillis() -<span style="color: #000000"> t;
} </span><span style="color: #0000ff">else</span><span style="color: #000000"> {
</span><span style="color: #0000ff">if</span><span style="color: #000000"> (log.isDebugEnabled()) {
log.debug(</span>"A bad connection (" + conn.getRealHashCode() + ") was returned from the pool,getting another connection."<span style="color: #000000">);
}
state.badConnectionCount</span>++<span style="color: #000000">;
localBadConnectionCount</span>++<span style="color: #000000">;
conn </span>= <span style="color: #0000ff">null</span><span style="color: #000000">;
</span><span style="color: #008000">//</span><span style="color: #008000">如果累计有这些个链接失效了,则报个异常.</span>
<span style="color: #0000ff">if</span> (localBadConnectionCount > (poolMaximumIdleConnections +<span style="color: #000000"> poolMaximumLocalBadConnectionTolerance)) {
</span><span style="color: #0000ff">if</span><span style="color: #000000"> (log.isDebugEnabled()) {
log.debug(</span>"PooledDataSource: Could not get a good connection to the database."<span style="color: #000000">);
}
</span><span style="color: #0000ff">throw</span> <span style="color: #0000ff">new</span> SQLException("PooledDataSource: Could not get a good connection to the database."<span style="color: #000000">);
}
}
}
}
}</span></pre>
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（编辑：李大同）
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