Mybatis数据源与数据库连接池

现在让我们看一下popConnection()方法到底做了什么：

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

[java] view plain copy

1. /* 
2.  * 传递一个用户名和密码，从连接池中返回可用的PooledConnection 
3.  */  
4. private PooledConnection popConnection(String username, String password) throws SQLException  
5. {  
6.     boolean countedWait = false;  
7.     PooledConnection conn = null;  
8.     long t = System.currentTimeMillis();  
9.     int localBadConnectionCount = 0;  
10.   
11.     while (conn == null)  
12.     {  
13.         synchronized (state)  
14.         {  
15.             if (state.idleConnections.size() > 0)  
16.             {  
17.                 // 连接池中有空闲连接，取出第一个  
18.                 conn = state.idleConnections.remove(0);  
19.                 if (log.isDebugEnabled())  
20.                 {  
21.                     log.debug("Checked out connection " + conn.getRealHashCode() + " from pool.");  
22.                 }  
23.             }  
24.             else  
25.             {  
26.                 // 连接池中没有空闲连接，则取当前正在使用的连接数小于最大限定值，  
27.                 if (state.activeConnections.size() < poolMaximumActiveConnections)  
28.                 {  
29.                     // 创建一个新的connection对象  
30.                     conn = new PooledConnection(dataSource.getConnection(), this);  
31.                     @SuppressWarnings("unused")  
32.                     //used in logging, if enabled  
33.                     Connection realConn = conn.getRealConnection();  
34.                     if (log.isDebugEnabled())  
35.                     {  
36.                         log.debug("Created connection " + conn.getRealHashCode() + ".");  
37.                     }  
38.                 }  
39.                 else  
40.                 {  
41.                     // Cannot create new connection 当活动连接池已满，不能创建时，取出活动连接池的第一个，即最先进入连接池的PooledConnection对象  
42.                     // 计算它的校验时间，如果校验时间大于连接池规定的最大校验时间，则认为它已经过期了，利用这个PoolConnection内部的realConnection重新生成一个PooledConnection  
43.                     //  
44.                     PooledConnection oldestActiveConnection = state.activeConnections.get(0);  
45.                     long longestCheckoutTime = oldestActiveConnection.getCheckoutTime();  
46.                     if (longestCheckoutTime > poolMaximumCheckoutTime)  
47.                     {  
48.                         // Can claim overdue connection  
49.                         state.claimedOverdueConnectionCount++;  
50.                         state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime;  
51.                         state.accumulatedCheckoutTime += longestCheckoutTime;  
52.                         state.activeConnections.remove(oldestActiveConnection);  
53.                         if (!oldestActiveConnection.getRealConnection().getAutoCommit())  
54.                         {  
55.                             oldestActiveConnection.getRealConnection().rollback();  
56.                         }  
57.                         conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this);  
58.                         oldestActiveConnection.invalidate();  
59.                         if (log.isDebugEnabled())  
60.                         {  
61.                             log.debug("Claimed overdue connection " + conn.getRealHashCode() + ".");  
62.                         }  
63.                     }  
64.                     else  
65.                     {  
66.   
67.                         //如果不能释放，则必须等待有  
68.                         // Must wait  
69.                         try  
70.                         {  
71.                             if (!countedWait)  
72.                             {  
73.                                 state.hadToWaitCount++;  
74.                                 countedWait = true;  
75.                             }  
76.                             if (log.isDebugEnabled())  
77.                             {  
78.                                 log.debug("Waiting as long as " + poolTimeToWait + " milliseconds for connection.");  
79.                             }  
80.                             long wt = System.currentTimeMillis();  
81.                             state.wait(poolTimeToWait);  
82.                             state.accumulatedWaitTime += System.currentTimeMillis() - wt;  
83.                         }  
84.                         catch (InterruptedException e)  
85.                         {  
86.                             break;  
87.                         }  
88.                     }  
89.                 }  
90.             }  
91.   
92.             //如果获取PooledConnection成功，则更新其信息  
93.   
94.             if (conn != null)  
95.             {  
96.                 if (conn.isValid())  
97.                 {  
98.                     if (!conn.getRealConnection().getAutoCommit())  
99.                     {  
100.                         conn.getRealConnection().rollback();  
101.                     }  
102.                     conn.setConnectionTypeCode(assembleConnectionTypeCode(dataSource.getUrl(), username, password));  
103.                     conn.setCheckoutTimestamp(System.currentTimeMillis());  
104.                     conn.setLastUsedTimestamp(System.currentTimeMillis());  
105.                     state.activeConnections.add(conn);  
106.                     state.requestCount++;  
107.                     state.accumulatedRequestTime += System.currentTimeMillis() - t;  
108.                 }  
109.                 else  
110.                 {  
111.                     if (log.isDebugEnabled())  
112.                     {  
113.                         log.debug("A bad connection (" + conn.getRealHashCode() + ") was returned from the pool, getting another connection.");  
114.                     }  
115.                     state.badConnectionCount++;  
116.                     localBadConnectionCount++;  
117.                     conn = null;  
118.                     if (localBadConnectionCount > (poolMaximumIdleConnections + 3))  
119.                     {  
120.                         if (log.isDebugEnabled())  
121.                         {  
122.                             log.debug("PooledDataSource: Could not get a good connection to the database.");  
123.                         }  
124.                         throw new SQLException("PooledDataSource: Could not get a good connection to the database.");  
125.                     }  
126.                 }  
127.             }  
128.         }  
129.   
130.     }  
131.   
132.     if (conn == null)  
133.     {  
134.         if (log.isDebugEnabled())  
135.         {  
136.             log.debug("PooledDataSource: Unknown severe error condition.  The connection pool returned a null connection.");  
137.         }  
138.         throw new SQLException("PooledDataSource: Unknown severe error condition.  The connection pool returned a null connection.");  
139.     }  
140.   
141.     return conn;  
142. }  

对应的处理流程图如下所示：

如上所示,对于PooledDataSource的getConnection()方法内，先是调用类PooledDataSource的popConnection()方法返回了一个PooledConnection对象，然后调用了PooledConnection的getProxyConnection()来返回Connection对象。