默认枚举实例的创建是线程安全的

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

  1. 枚举中可以和java类一样定义方法

  2. 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());

}

}

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:

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);

}

}

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

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

http://www.cnblogs.com/liaokailin/p/3196253.html