實現單例的核心在於private私有化類中的構造方法，在列舉中的構造方法必須是私有的，當一個列舉類中只有一個列舉時只會呼叫一次列舉的構造方法，這就為列舉來實現單例奠定了基礎。下面以資料來源中獲得Connection連線來舉例：

      在開發中經常是通過資料來源來獲得資料庫的Connection,資料來源的實現方式有多種，最典型的有兩種，一種是C3P0,另外一種是DBCP（以後有機會再針對兩種資料來源進行討論），這裡以C3P0資料來源為例。由於資料來源的建立與銷燬都是十分消耗效能的，並且系統中有一個數據源一般就能滿足開發的要求，因此要將資料來源設計為單例。

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

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 {

            /*--------獲取properties檔案內容------------*/
            // 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的字尾)
            /*
             * ResourceBundle rb = PropertyResourceBundle.getBundle("jdbc") ;
             * System.out.println(rb.getString("driverClass"));
             */

            // 方法三：(不需要properties的字尾)
            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("-----呼叫了構造方法------");
            ;
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

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

}
 

package org.lkl.imitate_enum;

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

-----呼叫了構造方法------
2013-7
 
-17 17:10:57 com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource getPoolManager
資訊: 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 ]