通过枚举enum实现单例设计

一、枚举

  通过enum关键字来实现枚举，在枚举中需要注意的有：

    1. 枚举中的属性必须放在最前面，一般使用大写字母表示

    2\. 枚举中可以和java类一样定义方法

    3\. 枚举中的构造方法必须是私有的

   通过一个java类来模拟枚举的功能：

package org.lkl.imitate_enum; public abstract class WeekDay { public static final WeekDay SUN = new WeekDay() {

    @Override public WeekDay nextDay() { return MON;
    }
}; public static final WeekDay MON = new WeekDay() {

    @Override public WeekDay nextDay() { return SUN;
    }

};

@Override public String toString() { return this == SUN ?"SUN星期天":"MON星期一" ;
} public abstract WeekDay nextDay(); public static void main(String[] args) {
      System.out.println(WeekDay.SUN.nextDay());
      System.out.println(WeekDay.MON.nextDay());
}

}</pre>

     4.可以在枚举属性后面添加()来调用指定参数的构造方法，添加{}来实现其对应的匿名内部类，例如：

<pre style="margin: 0px; padding: 0px; white-space: pre-wrap; word-wrap: break-word; font-family: "Courier New" !important; font-size: 12px !important;">package org.lkl.imitate_enum; public enum TrafficLamp {
RED(30){ public TrafficLamp nextLamp() { return GREEN ;
}
},
GREEN(20){ public TrafficLamp nextLamp() { return YELLOW ;
}
},
YELLOW(10){ public TrafficLamp nextLamp() { return RED ;
}
} ; public abstract TrafficLamp nextLamp() ; private int time ; private TrafficLamp(int time){ this.time = time ;
System.out.println(time);
} public static void main(String[] args) {
System.out.println(TrafficLamp.GREEN);
}
}</pre>

二、通过枚举来实现单例

  实现单例的核心在于private私有化类中的构造方法，在枚举中的构造方法必须是私有的，这就为枚举来实现单例奠定了基础。下面以数据源中获得Connection连接来举例：

  在开发中经常是通过数据源来获得数据库的Connection,数据源的实现方式有多种，最典型的有两种，一种是C3P0,另外一种是DBCP（以后有机会再针对两种数据源进行讨论），这里以C3P0数据源为例。由于数据源的创建与销毁都是十分消耗性能的，并且系统中有一个数据源一般就能满足开发的要求，因此要将数据源设计为单例。

 采用分散配置，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</pre>

创建一个MyDataBaseSource的枚举：

package org.lkl.imitate_enum; 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 = 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：

package org.lkl.imitate_enum; 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