c3p0配置之preferredTestQuery参数默认值探秘

Default: null
Defines the query that will be executed for all connection tests, if the default ConnectionTester (or some other implementation of QueryConnectionTester, or better yet FullQueryConnectionTester) is being used. 
Defining a preferredTestQuery that will execute quickly in your database may dramatically speed up Connection tests. (If no preferredTestQuery is set, the default ConnectionTester executes a getTables() call on the 
Connection's DatabaseMetaData. Depending on your database, this may execute more slowly than a "normal" database query.) NOTE: The table against which your preferredTestQuery will be run must exist in the 
database schema prior to your initialization of your DataSource. If your application defines its own schema, try automaticTestTable instead. [See "Configuring Connection Testing"]

Default: com.mchange.v2.c3p0.impl.DefaultConnectionTester
The fully qualified class-name of an implememtation of the ConnectionTester interface, or QueryConnectionTester if you would like instances to have access to a user-configured preferredTestQuery. This can be
used to customize how c3p0 DataSources test Connections, but with the introduction of automaticTestTable and preferredTestQuery configuration parameters, "rolling your own" should be overkill for most users. 
[See "Configuring Connection Testing"]

    - com.mchange.v2.c3p0.ConnectionTester
        - com.mchange.v2.c3p0.QueryConnectionTester
            - com.mchange.v2.c3p0.FullQueryConnectionTester
                - com.mchange.v2.c3p0.UnifiedConnectionTester
                    - com.mchange.v2.c3p0.AbstractConnectionTester
                        - com.mchange.v2.c3p0.impl.DefaultConnectionTester

private final static ConnectionTester CONNECTION_TESTER = new DefaultConnectionTester();

public int activeCheckConnection(Connection c, String query, Throwable[] rootCauseOutParamHolder)
{
// if (Debug.DEBUG && Debug.TRACE == Debug.TRACE_MAX && logger.isLoggable( MLevel.FINER ) )
// logger.finer("Entering DefaultConnectionTester.activeCheckConnection(Connection c, String query). [query=" + query + "]");

    if (query == null)
        return activeCheckConnectionNoQuery( c, rootCauseOutParamHolder);
    else
    {
    .....
    }
}

private int activeCheckConnectionNoQuery(Connection c,  Throwable[] rootCauseOutParamHolder)
{
// if (Debug.DEBUG && Debug.TRACE == Debug.TRACE_MAX && logger.isLoggable( MLevel.FINER ) )
// logger.finer("Entering DefaultConnectionTester.activeCheckConnection(Connection c). [using default system-table query]");

    ResultSet rs = null;
    try
    {
        rs = c.getMetaData().getTables( null,
                        null,
                        "PROBABLYNOT",
                        new String[] {"TABLE"} );
        return CONNECTION_IS_OKAY;
    }
}

public java.sql.ResultSet getTables(String catalog, String schemaPattern, String tableNamePattern, final String[] types) throws SQLException {
    ...
    try {
　　　　　// 执行sql语句，检测mysql是否可以连通
　　　　　results = stmt.executeQuery((!DatabaseMetaData.this.conn.versionMeetsMinimum(5, 0, 2) ? "SHOW TABLES FROM " : "SHOW FULL TABLES FROM ")
                + StringUtils.quoteIdentifier(catalogStr, DatabaseMetaData.this.quotedId, DatabaseMetaData.this.conn.getPedantic())
                + " LIKE " + StringUtils.quoteIdentifier(tableNamePat, "'", true));
    } catch (SQLException sqlEx) {
         if (SQLError.SQL_STATE_COMMUNICATION_LINK_FAILURE.equals(sqlEx.getSQLState())) { throw sqlEx; }
    }
    ...
}