MySQL 連接池常常會被用于高并發的 Web 項目中，目的就是為了重復利用連接，避免每次都要獲取和釋放連接，從而提高效率。不過，有時候 MySQL 連接池會出現異常，需要進行重連。

public class MysqlDataSource {
private static final int DEFAULT_RECONNECT_SECONDS = 10;
private long lastConnectTime;
private long lastReconnectedTime;
private String url;
private String username;
private String password;
private int maxPoolSize = 10;
private volatile boolean released;
private volatile boolean paused;
private ListidleConnections = new ArrayList();
private ListactiveConnections = new ArrayList();
public synchronized Connection getConnection() throws SQLException {
if (released) {
throw new SQLException("Connection pool has been released.");
}
if (idleConnections.size() >0) {
Connection conn = idleConnections.remove(0);
if (conn.isValid(1)) {
activeConnections.add(conn);
return conn;
}
}
if (activeConnections.size()< maxPoolSize) {
Connection conn = createConnection();
activeConnections.add(conn);
return conn;
}
throw new SQLException("Connection pool is full.");
}
private Connection createConnection() throws SQLException {
try {
Class.forName("com.mysql.jdbc.Driver");
Connection conn = DriverManager.getConnection(url, username, password);
return conn;
} catch (ClassNotFoundException e) {
throw new SQLException("Cannot find com.mysql.jdbc.Driver");
}
}
public synchronized void releaseConnection(Connection conn) {
activeConnections.remove(conn);
if (idleConnections.size()< maxPoolSize) {
idleConnections.add(conn);
} else {
closeConnection(conn);
}
}
private void closeConnection(Connection conn) {
try {
conn.close();
} catch (SQLException e) {
// ignore
}
}
private void reconnect() throws SQLException {
if (lastReconnectedTime + DEFAULT_RECONNECT_SECONDS * 1000 >System.currentTimeMillis()) {
return;
}
synchronized (this) {
if (lastReconnectedTime + DEFAULT_RECONNECT_SECONDS * 1000 >System.currentTimeMillis()) {
return;
}
lastReconnectedTime = System.currentTimeMillis();
}
Listreconnected = new ArrayList<>();
try {
for (Connection conn : idleConnections) {
if (!conn.isValid(1)) {
Connection newConn = createConnection();
reconnected.add(newConn);
idleConnections.set(idleConnections.indexOf(conn), newConn);
}
}
} finally {
idleConnections.addAll(reconnected);
}
}
}

在上述的代碼中，能夠看到 reconnect() 方法，這個方法主要是用于重連的。當 idle 連接池中的連接發生了失效時，就會通過 createConnection() 方法進行重連。

在高并發的情況下，MySQL 連接池異常重連的問題是比較常見的。如果不及時重連，就會影響服務的正常運行。因此，對于 MySQL 連接池異常重連，我們需要及時定位和解決問題。