经过枚举enum实现单例设计

时间 2019-11-29

标签经过枚举 enum 实现设计繁體版

原文原文链接

1、枚举java

经过enum关键字来实现枚举，在枚举中须要注意的有：mysql

1. 枚举中的属性必须放在最前面，通常使用大写字母表示sql

2. 枚举中能够和java类同样定义方法数据库

3. 枚举中的构造方法必须是私有的ide

经过一个java类来模拟枚举的功能：性能

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.能够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：测试

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);
    }
}

2、经过枚举来实现单例ui

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基了基础。下面以数据源中得到Connection链接来举例：this

在开发中常常是经过数据源来得到数据库的Connection,数据源的实现方式有多种，最典型的有两种，一种是C3P0,另一种是DBCP（之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。因为数据源的建立与销毁都是十分消耗性能的，而且系统中有一个数据源通常就能知足开发的要求，所以要将数据源设计为单例。spa

采用分散配置，建立一个jdbc.propertis文件，其内容以下：

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

建立一个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：(不须要properties的后缀)
            /*
             * ResourceBundle rb = PropertyResourceBundle.getBundle("jdbc") ;
             * System.out.println(rb.getString("driverClass"));
             */

            // 方法三：(不须要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() ;
    }
}

结果以下：

-----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 ]

很显然得到了三个Connection链接，可是只调用了一次枚举的构造方法,从而经过枚举实现了单例的设计