==作者：YB-Chi==

创建一个jdbc.propertis文件，其内容如下：

driverClass = com.mysql.jdbc.Driver
jdbcUrl = jdbc:mysql://localhost:3306/liaokailin
user = root
password = mysqladmin
maxPoolSize = 20
minPoolSize = 5

创建一个MyDataBaseSource的枚举：

import java.sql.Connection;
import java.sql.SQLException;
import java.util.ResourceBundle;

import com.mchange.v2.c3p0.ComboPooledDataSource;

public enum MyDataBaseSource {
	DATASOURCE;
	private ComboPooledDataSource cpds = null;

private MyDataBaseSource() {
	try {
			/*--------获取properties文件内容------------*/

			// 方法一:
			/*
			 * InputStream is =
			 * MyDBSource.class.getClassLoader().getResourceAsStream("jdbc.properties");
			 * Properties p = new Properties(); p.load(is);
			 * System.out.println(p.getProperty("driverClass") );
			 */

			// 方法二：(不需要properties的后缀)
			/*
			 * ResourceBundle rb = PropertyResourceBundle.getBundle("jdbc") ;
			 * System.out.println(rb.getString("driverClass"));
			 */

			// 方法三：(不需要properties的后缀)
			ResourceBundle rs = ResourceBundle.getBundle("jdbc");
			cpds = new ComboPooledDataSource();
			cpds.setDriverClass(rs.getString("driverClass"));
			cpds.setJdbcUrl(rs.getString("jdbcUrl"));
			cpds.setUser(rs.getString("user"));
			cpds.setPassword(rs.getString("password"));
			cpds.setMaxPoolSize(Integer.parseInt(rs.getString("maxPoolSize")));
			cpds.setMinPoolSize(Integer.parseInt(rs.getString("minPoolSize")));    
			 System.out.println("-----调用了构造方法------");

		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	public Connection getConnection() {
		try {
			return cpds.getConnection();
		} catch (SQLException e) {
			return null;
		}
	}

}

测试代码：

public class Test {
	public static void main(String[] args) {
		MyDataBaseSource.DATASOURCE.getConnection() ;
		MyDataBaseSource.DATASOURCE.getConnection() ;
		MyDataBaseSource.DATASOURCE.getConnection() ;
	}
}

结果如下：

-----调用了构造方法------
2013-7-17 17:10:57 com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource getPoolManager
信息: Initializing c3p0 pool... com.mchange.v2.c3p0.ComboPooledDataSource [ acquireIncrement -> 3, acquireRetryAttempts -> 30, acquireRetryDelay -> 1000, autoCommitOnClose -> false, automaticTestTable -> null, breakAfterAcquireFailure -> false, checkoutTimeout -> 0, connectionCustomizerClassName -> null, connectionTesterClassName -> com.mchange.v2.c3p0.impl.DefaultConnectionTester, dataSourceName -> 1hge16d8v1tgb0wppydrzz|2c1e6b, debugUnreturnedConnectionStackTraces -> false, description -> null, driverClass -> com.mysql.jdbc.Driver, factoryClassLocation -> null, forceIgnoreUnresolvedTransactions -> false, identityToken -> 1hge16d8v1tgb0wppydrzz|2c1e6b, idleConnectionTestPeriod -> 0, initialPoolSize -> 3, jdbcUrl -> jdbc:mysql://localhost:3306/kaoqin, maxAdministrativeTaskTime -> 0, maxConnectionAge -> 0, maxIdleTime -> 0, maxIdleTimeExcessConnections -> 0, maxPoolSize -> 20, maxStatements -> 0, maxStatementsPerConnection -> 0, minPoolSize -> 5, numHelperThreads -> 3, preferredTestQuery -> null, properties -> {user=******, password=******}, propertyCycle -> 0, statementCacheNumDeferredCloseThreads -> 0, testConnectionOnCheckin -> false, testConnectionOnCheckout -> false, unreturnedConnectionTimeout -> 0, usesTraditionalReflectiveProxies -> false ]

很显然获得了三个Connection连接，但是只调用了一次枚举的构造方法,从而通过枚举实现了单例的设计