《Java多线程编程核心技术》读后感（十四）

单例模式与多线程

立即加载/饿汉模式

立即加载就是使用类的时候已经将对象创建完毕，常见的实现办法就是直接new实例化。

立即加载/饿汉模式实在调用方法前，实例已经被创建了

package Six;

public class MyObject {

    // 立即加载方式==饿汉模式
    private static MyObject myObject = new MyObject();

    private MyObject() {
    }

    public static MyObject getInstance() {
        // 此代码版本为立即加载
        // 此版本代码的缺点是不能有其它实例变量
        // 因为getInstance()方法没有同步
        // 所以有可能出现非线程安全问题
        return myObject;
    }

}

package Six;

public class MyThread extends Thread {

    @Override
    public void run() {
        System.out.println(MyObject.getInstance().hashCode());
    }

}

package Six;

public class Run {

    public static void main(String[] args) {
        MyThread t1 = new MyThread();
        MyThread t2 = new MyThread();
        MyThread t3 = new MyThread();

        t1.start();
        t2.start();
        t3.start();

    }

}

说明对象是同一个，也就实现了立即加载型单例设计模式

延迟加载/"懒汉模式"

延迟加载就是在调用get()方法时实例才被创建，常见的实现办法就是在get()方法中进行new实例化

package Six;

public class MyObject {

    private static MyObject myObject;

    private MyObject() {
    }

    public static MyObject getInstance() {
        // 延迟加载
        if (myObject != null) {
        } else {
            myObject = new MyObject();
        }
        return myObject;
    }

}

package Six;

public class MyThread extends Thread {

    @Override
    public void run() {
        System.out.println(MyObject.getInstance().hashCode());
    }
}

package Six;

public class Run {

    public static void main(String[] args) {
        MyThread t1 = new MyThread();
        t1.start();
    }

}

此实验虽然取得一个对象的实例，但如果实在多线程的环境中，就会出现多个实例的情况

延迟加载/"懒汉模式"缺点

延迟加载在多线程环境中是错误的。

package Six;

public class MyObject {

    private static MyObject myObject;

    private MyObject() {
    }

    public static MyObject getInstance() {
        try {
            if (myObject != null) {
            } else {
                // 模拟在创建对象之前做一些准备性的工作
                Thread.sleep(3000);
                myObject = new MyObject();
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return myObject;
    }
}

package Six;

public class MyThread extends Thread {

    @Override
    public void run() {
        System.out.println(MyObject.getInstance().hashCode());
    }

}

package Six;

public class Run {

    public static void main(String[] args) {
        MyThread t1 = new MyThread();
        MyThread t2 = new MyThread();
        MyThread t3 = new MyThread();

        t1.start();
        t2.start();
        t3.start();

    }

}

打印出了三个对象，并不是单例的，如何解决？见下

延迟加载/“”懒汉模式“”的解决方案

（1）声明synchronized关键字

既然多个线程可以同时进入getInstance()方法，那么只需要对getInstace（）方法声明synchronizaed关键字即可

package Six;

public class MyObject {

    private static MyObject myObject;

    private MyObject() {
    }

    synchronized public static MyObject getInstance() {
        try {
            if (myObject != null) {
            } else {
                // 模拟在创建对象之前做一些准备性的工作
                Thread.sleep(3000);
                myObject = new MyObject();
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return myObject;
    }
}

但此方法的运行效率底下，是同步运行的，下一个线程想要取得对象，则必须等上一个线程释放锁后，才可以继续执行。

（2）尝试同步代码块

同步方法是对方法的整体进行持锁，这对运行效率来讲是不利的。等同于上面的写法

package Six;

public class MyObject {

    private static MyObject myObject;

    private MyObject() {
    }

    public static MyObject getInstance() {
        try {
            synchronized (MyObject.class) {
                if (myObject != null) {
                } else {
                    // 模拟在创建对象之前做一些准备性的工作
                    Thread.sleep(3000);
                    myObject = new MyObject();
                }
            }

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return myObject;
    }
}

（3）针对某些重要的代码进行单独的同步

同步代码块可以针对某些重要的代码进行单独的同步，而其他的代码则不需要同步，这样在运行时，效率完全可以得到大幅度提升

package Six;

public class MyObject {

    private static MyObject myObject;

    private MyObject() {
    }

    public static MyObject getInstance() {
        try {
            if (myObject != null) {
            } else {
                // 模拟在创建对象之前做一些准备性的工作
                Thread.sleep(3000);
                synchronized (MyObject.class) {
                    myObject = new MyObject();
                }

            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return myObject;
    }
}

此方法只对实例化对象的关键代码进行同步，从语句的结构上讲，运行效率得到了提升，但在多线程的环境下还是无法解决得到一个实例对象的结果。

（4）使用DCl双检查锁机制

package Six;

public class MyObject {

    private static MyObject myObject;

    private MyObject() {
    }

    public static MyObject getInstance() {
        try {
            if (myObject != null) {
            } else {
                // 模拟在创建对象之前做一些准备性的工作
                Thread.sleep(3000);
                synchronized (MyObject.class) {
                    if(myObject ==null) {
                        myObject = new MyObject();
                    }

                }

            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return myObject;
    }
}

DCL是大多数多线程结合单例模式使用的解决方案

使用静态内置类实现单例模式

其他代码同上

package Six;

public class MyObject {

    // 内部类方式
        private static class MyObjectHandler {
            private static MyObject myObject = new MyObject();
        }

        private MyObject() {
        }

        public static MyObject getInstance() {
            return MyObjectHandler.myObject;
        }

}

使用staic代码块实现单例模式

package Six;

public class MyObject {

    private static MyObject instance = null;

    private MyObject() {
    }

    static {
        instance = new MyObject();
    }

    public static MyObject getInstance() {
        return instance;
    }

}

package Six;

public class MyThread extends Thread {

    @Override
    public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println(MyObject.getInstance().hashCode());
        }
    }
}

package Six;

public class Run {

    public static void main(String[] args) {
        MyThread t1 = new MyThread();
        MyThread t2 = new MyThread();
        MyThread t3 = new MyThread();

        t1.start();
        t2.start();
        t3.start();

    }

}

使用enum枚举数据类型实现单例模式

package Six;

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

public enum MyObject {
    connectionFactory;

    private Connection connection;

    private MyObject() {
        try {
            System.out.println("调用了MyObject的构造");
            String url = "jdbc:sqlserver://localhost:1079;databaseName=ghydb";
            String username = "sa";
            String password = "";
            String driverName = "com.microsoft.sqlserver.jdbc.SQLServerDriver";
            Class.forName(driverName);
            connection = DriverManager.getConnection(url, username, password);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public Connection getConnection() {
        return connection;
    }
}

package Six;

public class MyThread extends Thread {

    @Override
    public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println(MyObject.connectionFactory.getConnection()
                    .hashCode());
        }
    }
}

package Six;

public class Run {

    public static void main(String[] args) {
        MyThread t1 = new MyThread();
        MyThread t2 = new MyThread();
        MyThread t3 = new MyThread();

        t1.start();
        t2.start();
        t3.start();

    }

}

完善使用enum枚举实现单例模式

前面违反了"职责单一原则”

package Six;

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

public class MyObject {

    public enum MyEnumSingleton {
        connectionFactory;

        private Connection connection;

        private MyEnumSingleton() {
            try {
                System.out.println("创建MyObject对象");
                String url = "jdbc:sqlserver://localhost:1079;databaseName=y2";
                String username = "sa";
                String password = "";
                String driverName = "com.microsoft.sqlserver.jdbc.SQLServerDriver";
                Class.forName(driverName);
                connection = DriverManager.getConnection(url, username,
                        password);
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }

        public Connection getConnection() {
            return connection;
        }
    }

    public static Connection getConnection() {
        return MyEnumSingleton.connectionFactory.getConnection();
    }

}

package Six;

public class MyThread extends Thread {

    @Override
    public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println(MyObject.getConnection().hashCode());
        }
    }
}

package Six;

public class Run {

    public static void main(String[] args) {
        MyThread t1 = new MyThread();
        MyThread t2 = new MyThread();
        MyThread t3 = new MyThread();

        t1.start();
        t2.start();
        t3.start();

    }
}