mybatis与数据库访问相关的配置以及设计
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
- 我们常用的MyBatis配置中哪些是与数据库相关？
- 数据源配置:
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${driver}"/>
<property name="url" value="${url}"/>
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>
</dataSource>
</environment>
看到这个配置文件,第一个直觉会想到由谁读取配置文件,谁有读取了配置信息？先忽略这个疑问，跳过。直接看下面的问题
1.配置数据源信息后,由谁来创建、管理数据源?
根据JDBC驱动中约束的接口,Connection需要DataSource中获取
如果自己设计,是否直接可以由工厂返回Connection?有什么好处,有什么坏处? //TODO
没看代码前:
实际Mybatis设计，没有直接返回Connection,而返回了dataSource
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
现在解开谜底:看实际Mybatis设计如何?
非池化类:
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
private Connection doGetConnection(Properties properties) throws SQLException {
initializeDriver();
Connection connection = DriverManager.getConnection(url, properties);
configureConnection(connection);
return connection;
}
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
@Override
public Connection getConnection() throws SQLException {
return popConnection(dataSource.getUsername(), dataSource.getPassword()).getProxyConnection();
}
while (conn == null) { //够用就行,拿到一个就返回
synchronized (state) {
//只有有连接还回来,再走这里
if (!state.idleConnections.isEmpty()) {
// Pool has available connection
conn = state.idleConnections.remove(0);
if (log.isDebugEnabled()) {
log.debug("Checked out connection " + conn.getRealHashCode() + " from pool.");
}
} else {
//有两种可能:1种,都在使用中,池子没满，再新建
// Pool does not have available connection
if (state.activeConnections.size() < poolMaximumActiveConnections) {
// Can create new connection
//新建连接
conn = new PooledConnection(dataSource.getConnection(), this);
if (log.isDebugEnabled()) {
log.debug("Created connection " + conn.getRealHashCode() + ".");
}
} else {
// Cannot create new connection
//找一个最老的,用的ArrayList,老的在ArrayList数组的前面
PooledConnection oldestActiveConnection = state.activeConnections.get(0);
long longestCheckoutTime = oldestActiveConnection.getCheckoutTime();
//借出超时,不让他做了，直接rollback。。。暴力
if (longestCheckoutTime > poolMaximumCheckoutTime) {
// Can claim overdue connection
state.claimedOverdueConnectionCount++;
state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime;
state.accumulatedCheckoutTime += longestCheckoutTime;
state.activeConnections.remove(oldestActiveConnection);
if (!oldestActiveConnection.getRealConnection().getAutoCommit()) {
try {
oldestActiveConnection.getRealConnection().rollback();
} catch (SQLException e) {
/*
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.
*/
log.debug("Bad connection. Could not roll back");
}
}
//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?
conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this);
conn.setCreatedTimestamp(oldestActiveConnection.getCreatedTimestamp());
conn.setLastUsedTimestamp(oldestActiveConnection.getLastUsedTimestamp());
oldestActiveConnection.invalidate();
if (log.isDebugEnabled()) {
log.debug("Claimed overdue connection " + conn.getRealHashCode() + ".");
}
} else {
// Must wait
//大家都在用着,你只能等着了。
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) {
// ping to server and check the connection is valid or not
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;
//如果累计有这些个链接失效了,则报个异常.
if (localBadConnectionCount > (poolMaximumIdleConnections + poolMaximumLocalBadConnectionTolerance)) {
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.");
}
}
}
}
}