01_java设计模式之单例模式

java设计模式之单例模式

一.应用场景

在实际开发中,当我们要求一个类只能被实例化一次时,比如各种各样的Manager、各种各样的Factory，可以使用单例模式

二.构造单例的常规步骤

提供一个private static final 的当前类的实例变量INSTANCE
将构造方法设置为private,保证其对外不能通过new关键字实例化
通过getInstance的静态方法返回一个唯一的当前类的实例

三.单例的七种写法

饿汉模式

public class SingleInstance01 {

    private static final SingleInstance01 INSTANCE = new SingleInstance01();

    private SingleInstance01() {}

    public static SingleInstance01 getInstance() {
        return INSTANCE;
    }

}

public class Main {

    public static void main(String[] args) {
        SingleInstance01 s1 = SingleInstance01.getInstance();
        SingleInstance01 s2 = SingleInstance01.getInstance();
        System.out.println(s1 == s2);//输出true
    }

}

类加载到内存后,就会创建一个实例对象
由JVM保证线程安全
JVM保证每一个class只会被load到内存一次,static的变量是在class被load到内存之后,会马上进行实例化,所以也保证了INSTANCE只会被初始化一次
缺点是:不管用到与否,类加载时就会完成实例化

懒汉模式

public class SingleInstance02 {

    private static SingleInstance02 INSTANCE;
    
    private SingleInstance02() {}

    public static SingleInstance02 getInstance() {

        if(INSTANCE == null) {
            doSomeThings();
            INSTANCE = new SingleInstance02();
        }

        return INSTANCE;

    }
    
    private static void doSomeThings() {
        try {
            Thread.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

public class Main {

    public static void main(String[] args) {
        for(int i=0; i<10; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println(SingleInstance02.getInstance().toString());
                }
            }).start();
        }
    }

}
/*************************** 输出结果 *******************************
 com.gale.designpattern.singleinstance.SingleInstance02@3a2cad68
 com.gale.designpattern.singleinstance.SingleInstance02@48ac2be5
 com.gale.designpattern.singleinstance.SingleInstance02@48ac2be5
 com.gale.designpattern.singleinstance.SingleInstance02@4e35792a
 com.gale.designpattern.singleinstance.SingleInstance02@2cb30bd3
 com.gale.designpattern.singleinstance.SingleInstance02@11d32fc4
 com.gale.designpattern.singleinstance.SingleInstance02@d084795
 com.gale.designpattern.singleinstance.SingleInstance02@48ac2be5
 com.gale.designpattern.singleinstance.SingleInstance02@48ac2be5
 com.gale.designpattern.singleinstance.SingleInstance02@48ac2be5
 ******************************************************************/

虽然达到了按需初始化的目的,却带来了线程不安全的问题

在这里插入图片描述

在懒汉模式的基础上,通过synchronized锁定类的class对象解决线程不安全的问题,也叫做线程安全的懒汉模式,但也带来了效率上的下降

public class SingleInstance03 {

    private static SingleInstance03 INSTANCE;

    private SingleInstance03() {}

    public static synchronized SingleInstance03 getInstance() {

        if(INSTANCE == null) {

            doSomeThings();

            INSTANCE = new SingleInstance03();
        }

        return INSTANCE;

    }

    private static void doSomeThings() {
        try {
            Thread.sleep(1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

public class Main {

    public static void main(String[] args) {
        for(int i=0; i<10; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println(SingleInstance03.getInstance().toString());
                }
            }).start();
        }
    }

}
/*************************** 输出结果 *******************************
 com.gale.designpattern.singleinstance.SingleInstance03@1469c295
 com.gale.designpattern.singleinstance.SingleInstance03@1469c295
 com.gale.designpattern.singleinstance.SingleInstance03@1469c295
 com.gale.designpattern.singleinstance.SingleInstance03@1469c295
 com.gale.designpattern.singleinstance.SingleInstance03@1469c295
 com.gale.designpattern.singleinstance.SingleInstance03@1469c295
 com.gale.designpattern.singleinstance.SingleInstance03@1469c295
 com.gale.designpattern.singleinstance.SingleInstance03@1469c295
 com.gale.designpattern.singleinstance.SingleInstance03@1469c295
 com.gale.designpattern.singleinstance.SingleInstance03@1469c295
 ******************************************************************/

试图通过减小同步代码块的方式来提高效率,不可行

public class SingleInstance04 {

    private static SingleInstance04 INSTANCE;

    private SingleInstance04() {}

    public static SingleInstance04 getInstance() {

        if(INSTANCE == null) {
            synchronized (SingleInstance04.class) {
                doSomeThings();
                INSTANCE = new SingleInstance04();
            }
        }

        return INSTANCE;

    }

    private static void doSomeThings() {
        try {
            Thread.sleep(1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

public class Main {

    public static void main(String[] args) {
        for(int i=0; i<10; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println(SingleInstance04.getInstance().toString());
                }
            }).start();
        }
    }

}
/*************************** 输出结果 *******************************
 com.gale.designpattern.singleinstance.SingleInstance04@1ca5fe10
 com.gale.designpattern.singleinstance.SingleInstance04@1ca5fe10
 com.gale.designpattern.singleinstance.SingleInstance04@1ca5fe10
 com.gale.designpattern.singleinstance.SingleInstance04@1ca5fe10
 com.gale.designpattern.singleinstance.SingleInstance04@21b39199
 com.gale.designpattern.singleinstance.SingleInstance04@6fd8b67a
 com.gale.designpattern.singleinstance.SingleInstance04@b6f008f
 com.gale.designpattern.singleinstance.SingleInstance04@18f3035c
 com.gale.designpattern.singleinstance.SingleInstance04@8fddaf1
 com.gale.designpattern.singleinstance.SingleInstance04@1dce2a4
 ******************************************************************/

在这里插入图片描述

双重检查的synchronized单例写法

public class SingleInstance05 {

    private static SingleInstance05 INSTANCE;

    private SingleInstance05() {}

    public static SingleInstance05 getInstance() {

        if(INSTANCE == null) {
            synchronized (SingleInstance05.class) {
                if(INSTANCE == null) {
                    doSomeThings();
                    INSTANCE = new SingleInstance05();
                }
            }
        }

        return INSTANCE;

    }

    private static void doSomeThings() {
        try {
            Thread.sleep(1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

public class Main {

    public static void main(String[] args) {
        for(int i=0; i<10; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println(SingleInstance05.getInstance().toString());
                }
            }).start();
        }
    }

}
/*************************** 输出结果 *******************************
 com.gale.designpattern.singleinstance.SingleInstance05@7dcec037
 com.gale.designpattern.singleinstance.SingleInstance05@7dcec037
 com.gale.designpattern.singleinstance.SingleInstance05@7dcec037
 com.gale.designpattern.singleinstance.SingleInstance05@7dcec037
 com.gale.designpattern.singleinstance.SingleInstance05@7dcec037
 com.gale.designpattern.singleinstance.SingleInstance05@7dcec037
 com.gale.designpattern.singleinstance.SingleInstance05@7dcec037
 com.gale.designpattern.singleinstance.SingleInstance05@7dcec037
 com.gale.designpattern.singleinstance.SingleInstance05@7dcec037
 com.gale.designpattern.singleinstance.SingleInstance05@7dcec037
 ******************************************************************/

静态内部类的单例写法(最完美的写法之一)

public class SingleInstance06 {

    private SingleInstance06() {
    }

    private static class SingleInstance06Holder {
        private static final SingleInstance06 INSTANCE = new SingleInstance06();
    }

    public static SingleInstance06 getInstance() {
        return SingleInstance06Holder. INSTANCE;
    }

}

public class Main {

    public static void main(String[] args) {
        for(int i=0; i<10; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println(SingleInstance06.getInstance().toString());
                }
            }).start();
        }
    }

}
/*************************** 输出结果 *******************************
 com.gale.designpattern.singleinstance.SingleInstance06@418b0eca
 com.gale.designpattern.singleinstance.SingleInstance06@418b0eca
 com.gale.designpattern.singleinstance.SingleInstance06@418b0eca
 com.gale.designpattern.singleinstance.SingleInstance06@418b0eca
 com.gale.designpattern.singleinstance.SingleInstance06@418b0eca
 com.gale.designpattern.singleinstance.SingleInstance06@418b0eca
 com.gale.designpattern.singleinstance.SingleInstance06@418b0eca
 com.gale.designpattern.singleinstance.SingleInstance06@418b0eca
 com.gale.designpattern.singleinstance.SingleInstance06@418b0eca
 com.gale.designpattern.singleinstance.SingleInstance06@418b0eca
 ******************************************************************/

由JVM保证单例和线程安全
加载外部类时不会加载内部类,而是在getInstance方法被调用时,内部类才会被加载,这样就实现了懒加载

枚举单例:不仅可以解决线程同步,还可以防止反序列化(因为枚举类没有构造方法，即使反序列化后，也不能被实例化)(最完美的写法之一)

public enum SingleInstance07 {

    INSTANCE;

    public  static SingleInstance07 getInstance() {
        return INSTANCE;
    }
}

public class Main {

    public static void main(String[] args) {
        for(int i=0; i<10; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println(SingleInstance07.getInstance().hashCode());
                }
            }).start();
        }
    }

}
/*************************** 输出结果 *******************************
 1467106563
 1467106563
 1467106563
 1467106563
 1467106563
 1467106563
 1467106563
 1467106563
 1467106563
 1467106563
 ******************************************************************/

andr_gale

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