validationQuery验证数据库连接

validationQuery是用来验证数据库连接的查询语句，这个查询语句必须是至少返回一条数据的SELECT语句。每种数据库都有各自的验证语句，下表中收集了几种常见数据库的validationQuery。

如果你想支持多种数据库，你可以根据JDBC驱动来获取validationQuery，这里有个简单的类，根据JDBC驱动名称来获取validationQuery

import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

public class ValidationQuery {

    public String getValidationQuery(String driver) {
        Properties properties = loadProperties();
        return properties.getProperty(driver, "");
    }

    private Properties loadProperties() {
        String propertyFilename = "db.validation.properties";
        try {
            Properties props = new Properties();
            InputStream resourceAsStream = this.getClass().
            getClassLoader().getResourceAsStream(propertyFilename);
            props.load(resourceAsStream);
            resourceAsStream.close();
            return props;
        } catch (IOException e) {
            throw new RuntimeException("Cannot load properties file ‘" + propertyFilename + "‘.", e);
        }
    }

    //Example: Get validationQuery for hsqldb
    public static void main(String[] args) {
        System.out.println(new ValidationQuery().getValidationQuery("org.hsqldb.jdbcDriver"));
    }
}

创建“db.validation.properties”文件，并放置在classpath目录下

#hsqldb
org.hsqldb.jdbcDriver=select 1 from INFORMATION_SCHEMA.SYSTEM_USERS
#Oracle
oracle.jdbc.driver.OracleDriver=select 1 from dual
#DB2
com.ibm.db2.jcc.DB2Driver=select 1 from sysibm.sysdummy1
#mysql
com.mysql.jdbc.Driver=select 1
org.gjt.mm.mysql.Driver=select 1
#microsoft sql
com.microsoft.sqlserver.jdbc.SQLServerDriver=select 1
#postgresql
org.postgresql.Driver=select version();
#ingres
com.ingres.jdbc.IngresDriver=select 1
#derby
org.apache.derby.jdbc.ClientDriver=values 1
#H2
org.h2.Driver=select 1