每日一发设计模式 - 单例模式(singleton)

singleton

单例模式,是一种常用的软件设计模式。在它的核心结构中只包含一个被称为单例的特殊类。通过单例模式可以保证系统中,应用该模式的类一个类只有一个实例。即一个类只有一个对象实例。

为什么要用单例模式

在实际的项目中,有一些东西是大家共用的,比如:系统配置、redis实例、数据库实例等,我们不希望每个用户在使用这些资源的时候,自己去新建一份实例,这样会造成资源的浪费。
目的:保证一个类只有一个实例
使用场景:当你想控制实例个数,节省系统资源的时候
解决方案:判断该类是否已经有实例,如果有则返回,没有则创建
关键代码:私有化构造函数,让使用方不能随意的去实例
实现方案:懒汉、饿汉、静态内部类、枚举

  1. 懒汉 - 线程不安全
package com.my.test.singleton;

//懒汉 - 线程不安全
public class SingletonExample01 {

    private SingletonExample01(){

    }

    private static SingletonExample01 instance = null;

    public static SingletonExample01 getInstance(){
        if(instance == null){
            instance = new SingletonExample01();
        }
        return instance;
    }
}
  1. 懒汉 - 线程安全
package com.my.test.singleton;

//懒汉 - 线程安全
public class SingletonExample02 {

    private SingletonExample02(){

    }

    private static SingletonExample02 instance = null;

    public static synchronized SingletonExample02 getInstance(){
        if(instance == null){
            instance = new SingletonExample02();
        }
        return instance;
    }
}
  1. 双重校验锁
package com.my.test.singleton;

//懒汉 - 双重校验锁
public class SingletonExample03 {

    private SingletonExample03(){

    }

    private static volatile SingletonExample03 instance = null;

    public static SingletonExample03 getInstance(){
        if(instance == null){
            synchronized (SingletonExample03.class){
                if(instance == null){
                    instance = new SingletonExample03();
                }
            }
        }
        return instance;
    }
}
  1. 饿汉模式
package com.my.test.singleton;

//饿汉模式
public class SingletonExample04 {

    private SingletonExample04(){

    }

    // 方法一
//    private static SingletonExample04 instance = new SingletonExample04();


    // 方法二
    private static SingletonExample04 instance = null;

    static {
        instance = new SingletonExample04();
    }

    public static SingletonExample04 getInstance(){
        return instance;
    }
}
  1. 静态内部类
package com.my.test.singleton;

// 静态内部类
public class SingletonExample05 {

    private SingletonExample05(){

    }

    private static class InstanceSingletonExample05{
        private static SingletonExample05 instance = new SingletonExample05();
    }

    public static SingletonExample05 getInstance(){
        return InstanceSingletonExample05.instance;
    }
}
  1. 枚举
package com.my.test.singleton;


// 枚举
public class SingletonExample06 {

    private SingletonExample06(){
    }

    public static SingletonExample06 getInstance(){
        return InstanceSingletonExample06.INSTANCE.getSingletonExample06();
    }

    private enum InstanceSingletonExample06{
        INSTANCE;

        private SingletonExample06 singletonExample06 = new SingletonExample06();

        public SingletonExample06 getSingletonExample06(){
            return singletonExample06;
        }
    }
}
  1. 枚举2.0
package com.my.test.singleton;

public enum SingletonEnumExample{
    INSTANCE;

    public void test(){
        // do everything
    }
}

实例场景之 - 获取jdbc连接

我们新增两个类,第一个是非单例的获取jdbc连接

package com.mk.designDemo.singleton;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class MDataSourceNoSingle {

    private static final Logger logger = LoggerFactory.getLogger(MDataSourceNoSingle.class);

    private static Connection connection = null;

    public Connection getConnection() {
        try {
            Class.forName("com.mysql.jdbc.Driver");
            connection = DriverManager.getConnection("jdbc:mysql://******:3306/*****?characterEncoding=UTF-8", "root", "root");
            logger.info("线程{}实例化connection", Thread.currentThread().getName());
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        }
        return connection;
    }

}

第二个使用了单例模式

package com.mk.designDemo.singleton;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class MDataSourceSingle {

    private static final Logger logger = LoggerFactory.getLogger(MDataSourceSingle.class);

    private static Connection connection = null;

    static {
        try {
            Class.forName("com.mysql.jdbc.Driver");
            connection = DriverManager.getConnection("jdbc:mysql://******:3306/*****?characterEncoding=UTF-8", "root", "root");
            logger.info("线程{}实例化connection", Thread.currentThread().getName());
        } catch (ClassNotFoundException | SQLException e) {
            logger.error(e.getMessage(), e);
        }
    }
    
    private MDataSourceSingle() {
    }

    public static Connection getConnection() {
        return connection;
    }
}

然后我们写了一个简单的controller,里面有两个接口,用来验证结果

package com.mk.designDemo.controller;

import com.mk.designDemo.singleton.MDataSourceNoSingle;
import com.mk.designDemo.singleton.MDataSourceSingle;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import java.sql.Connection;

@RestController
public class SingletonController {

    private static final Logger logger = LoggerFactory.getLogger(SingletonController.class);

    private final static String sql = "select * from t_trade_date limit 10";

    @RequestMapping(path = "/noSingleton")
    public String noSingleton() {
        MDataSourceNoSingle mDataSourceNoSingle = new MDataSourceNoSingle();
        Connection connection = mDataSourceNoSingle.getConnection();
        doExecute(connection, sql);
        return "OK";
    }

    @RequestMapping(path = "/singleton")
    public String singleton() {
        doExecute(MDataSourceSingle.getConnection(), sql);
        return "OK";
    }


    private void doExecute(Connection connection, String sql) {
        logger.info("do execute sql:{}", sql);
    }

}

当我们调用三次非单例的接口时,查看日志输出如下,connection被实例化了多次:

2019-07-09 15:47:31.966 [http-nio-80-exec-1] DEBUG o.s.web.servlet.DispatcherServlet - GET "/web/noSingleton", parameters={}
2019-07-09 15:47:31.970 [http-nio-80-exec-1] DEBUG o.s.w.s.m.m.a.RequestMappingHandlerMapping - Mapped to public java.lang.String com.mk.designDemo.controller.SingletonController.noSingleton()
2019-07-09 15:47:32.633 [http-nio-80-exec-1] INFO  c.m.d.singleton.MDataSourceNoSingle - 线程http-nio-80-exec-1实例化connection
2019-07-09 15:47:32.633 [http-nio-80-exec-1] INFO  c.m.d.controller.SingletonController - do execute sql:select * from t_trade_date limit 10
2019-07-09 15:47:32.671 [http-nio-80-exec-1] DEBUG o.s.w.s.m.m.a.RequestResponseBodyMethodProcessor - Using 'text/html', given [text/html, application/xhtml+xml, image/webp, image/apng, application/xml;q=0.9, */*;q=0.8] and supported [text/plain, */*, text/plain, */*, application/json, application/*+json, application/json, application/*+json]
2019-07-09 15:47:32.672 [http-nio-80-exec-1] DEBUG o.s.w.s.m.m.a.RequestResponseBodyMethodProcessor - Writing ["OK"]
2019-07-09 15:47:32.693 [http-nio-80-exec-1] DEBUG o.s.web.servlet.DispatcherServlet - Completed 200 OK
2019-07-09 15:47:40.005 [http-nio-80-exec-4] DEBUG o.s.web.servlet.DispatcherServlet - GET "/web/noSingleton", parameters={}
2019-07-09 15:47:40.007 [http-nio-80-exec-4] DEBUG o.s.w.s.m.m.a.RequestMappingHandlerMapping - Mapped to public java.lang.String com.mk.designDemo.controller.SingletonController.noSingleton()
2019-07-09 15:47:40.361 [http-nio-80-exec-4] INFO  c.m.d.singleton.MDataSourceNoSingle - 线程http-nio-80-exec-4实例化connection
2019-07-09 15:47:40.361 [http-nio-80-exec-4] INFO  c.m.d.controller.SingletonController - do execute sql:select * from t_trade_date limit 10
2019-07-09 15:47:40.362 [http-nio-80-exec-4] DEBUG o.s.w.s.m.m.a.RequestResponseBodyMethodProcessor - Using 'text/html', given [text/html, application/xhtml+xml, image/webp, image/apng, application/xml;q=0.9, */*;q=0.8] and supported [text/plain, */*, text/plain, */*, application/json, application/*+json, application/json, application/*+json]
2019-07-09 15:47:40.362 [http-nio-80-exec-4] DEBUG o.s.w.s.m.m.a.RequestResponseBodyMethodProcessor - Writing ["OK"]
2019-07-09 15:47:40.363 [http-nio-80-exec-4] DEBUG o.s.web.servlet.DispatcherServlet - Completed 200 OK
2019-07-09 15:47:48.459 [http-nio-80-exec-7] DEBUG o.s.web.servlet.DispatcherServlet - GET "/web/noSingleton", parameters={}
2019-07-09 15:47:48.461 [http-nio-80-exec-7] DEBUG o.s.w.s.m.m.a.RequestMappingHandlerMapping - Mapped to public java.lang.String com.mk.designDemo.controller.SingletonController.noSingleton()
2019-07-09 15:47:48.801 [http-nio-80-exec-7] INFO  c.m.d.singleton.MDataSourceNoSingle - 线程http-nio-80-exec-7实例化connection
2019-07-09 15:47:48.801 [http-nio-80-exec-7] INFO  c.m.d.controller.SingletonController - do execute sql:select * from t_trade_date limit 10
2019-07-09 15:47:48.802 [http-nio-80-exec-7] DEBUG o.s.w.s.m.m.a.RequestResponseBodyMethodProcessor - Using 'text/html', given [text/html, application/xhtml+xml, image/webp, image/apng, application/xml;q=0.9, */*;q=0.8] and supported [text/plain, */*, text/plain, */*, application/json, application/*+json, application/json, application/*+json]
2019-07-09 15:47:48.802 [http-nio-80-exec-7] DEBUG o.s.w.s.m.m.a.RequestResponseBodyMethodProcessor - Writing ["OK"]
2019-07-09 15:47:48.803 [http-nio-80-exec-7] DEBUG o.s.web.servlet.DispatcherServlet - Completed 200 OK

然后我们调用单例的接口三次,日志输出如下,我们可以看到,只有第一次进行了实例化,后面再也没有继续实例化

2019-07-09 15:49:22.868 [http-nio-80-exec-10] DEBUG o.s.web.servlet.DispatcherServlet - GET "/web/singleton", parameters={}
2019-07-09 15:49:22.871 [http-nio-80-exec-10] DEBUG o.s.w.s.m.m.a.RequestMappingHandlerMapping - Mapped to public java.lang.String com.mk.designDemo.controller.SingletonController.singleton()
2019-07-09 15:49:23.234 [http-nio-80-exec-10] INFO  c.m.d.singleton.MDataSourceSingle - 线程http-nio-80-exec-10实例化connection
2019-07-09 15:49:23.235 [http-nio-80-exec-10] INFO  c.m.d.controller.SingletonController - do execute sql:select * from t_trade_date limit 10
2019-07-09 15:49:23.237 [http-nio-80-exec-10] DEBUG o.s.w.s.m.m.a.RequestResponseBodyMethodProcessor - Using 'text/html', given [text/html, application/xhtml+xml, image/webp, image/apng, application/xml;q=0.9, */*;q=0.8] and supported [text/plain, */*, text/plain, */*, application/json, application/*+json, application/json, application/*+json]
2019-07-09 15:49:23.237 [http-nio-80-exec-10] DEBUG o.s.w.s.m.m.a.RequestResponseBodyMethodProcessor - Writing ["OK"]
2019-07-09 15:49:23.239 [http-nio-80-exec-10] DEBUG o.s.web.servlet.DispatcherServlet - Completed 200 OK
2019-07-09 15:49:25.032 [http-nio-80-exec-1] DEBUG o.s.web.servlet.DispatcherServlet - GET "/web/singleton", parameters={}
2019-07-09 15:49:25.034 [http-nio-80-exec-1] DEBUG o.s.w.s.m.m.a.RequestMappingHandlerMapping - Mapped to public java.lang.String com.mk.designDemo.controller.SingletonController.singleton()
2019-07-09 15:49:25.034 [http-nio-80-exec-1] INFO  c.m.d.controller.SingletonController - do execute sql:select * from t_trade_date limit 10
2019-07-09 15:49:25.035 [http-nio-80-exec-1] DEBUG o.s.w.s.m.m.a.RequestResponseBodyMethodProcessor - Using 'text/html', given [text/html, application/xhtml+xml, image/webp, image/apng, application/xml;q=0.9, */*;q=0.8] and supported [text/plain, */*, text/plain, */*, application/json, application/*+json, application/json, application/*+json]
2019-07-09 15:49:25.036 [http-nio-80-exec-1] DEBUG o.s.w.s.m.m.a.RequestResponseBodyMethodProcessor - Writing ["OK"]
2019-07-09 15:49:25.038 [http-nio-80-exec-1] DEBUG o.s.web.servlet.DispatcherServlet - Completed 200 OK
2019-07-09 15:49:25.926 [http-nio-80-exec-2] DEBUG o.s.web.servlet.DispatcherServlet - GET "/web/singleton", parameters={}
2019-07-09 15:49:25.926 [http-nio-80-exec-2] DEBUG o.s.w.s.m.m.a.RequestMappingHandlerMapping - Mapped to public java.lang.String com.mk.designDemo.controller.SingletonController.singleton()
2019-07-09 15:49:25.927 [http-nio-80-exec-2] INFO  c.m.d.controller.SingletonController - do execute sql:select * from t_trade_date limit 10
2019-07-09 15:49:25.928 [http-nio-80-exec-2] DEBUG o.s.w.s.m.m.a.RequestResponseBodyMethodProcessor - Using 'text/html', given [text/html, application/xhtml+xml, image/webp, image/apng, application/xml;q=0.9, */*;q=0.8] and supported [text/plain, */*, text/plain, */*, application/json, application/*+json, application/json, application/*+json]
2019-07-09 15:49:25.928 [http-nio-80-exec-2] DEBUG o.s.w.s.m.m.a.RequestResponseBodyMethodProcessor - Writing ["OK"]
2019-07-09 15:49:25.929 [http-nio-80-exec-2] DEBUG o.s.web.servlet.DispatcherServlet - Completed 200 OK

验证通过,OVER!

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