设计模式-单例模式

时间:2022-07-25
本文章向大家介绍设计模式-单例模式，主要内容包括其使用实例、应用技巧、基本知识点总结和需要注意事项，具有一定的参考价值，需要的朋友可以参考一下。

单列模式是什么？

单列模式是创建型模式的一种，同样一个对象(实例)在内存中只会有一份，并只有一个全局的访问点。单列分为好多种实现，其中有线程安全和线程不安全，大类有懒汉和饿汉；

单列模式可以干嘛？

可以解决一个对象重复的创建和销毁，减少内存的开销，保存只在内存只有一份。就比如:一个学校一般只有一个正校长;

代码实现

懒汉：都是被别人推着才去做的，线程不安全(也有安全)

饿汉: 先创建对象，每次直接返回实例化后单例结果。

比如:

我去吃饭一到饭馆菜都已经上好了，就直接开吃了，这个是饿汉;

我去吃饭一到那里是等了半天，一边上菜一边催，才好不容易把菜上全了，这样的叫懒汉，要别人催才给;

/**
 *
 * 功能描述:单例(俄汉)
 *
 * @param:
 * @return:
 * @auther: csh
 * @date: 2020/5/17 10:49
 */
public class HungrySingle {
    //创建 HungrySingle 的一个对象    、
    private static HungrySingle instance = new HungrySingle();

    //让构造函数为 private，这样该类就不会被实例化
    private HungrySingle() {
    }      //获取唯一可用的对象

    public static HungrySingle getInstance() {
        return instance;
    }

    public void showMessage() {
        System.out.println("Hello World!");
    }
}
/**
 *
 * 功能描述:懒汉（线程不安全）
 *
 * @param:
 * @return:
 * @auther: csh
 * @date: 2020/5/17 10:55
 */
public class LazySingleton {
    private static LazySingleton instance;
    private LazySingleton(){}
    public static LazySingleton getInstance() {
        if (instance == null) {
            instance = new LazySingleton();
        }
        return instance;
    }
}
/**
 * @Auther: csh
 * @Date: 2020/5/17 11:27
 * @Description:懒汉 （线程安全） 缺点：性能较低
 */
public class LazySingleSafery {
    private static LazySingleSafery lazySingleSafery;

    private LazySingleSafery(){

    }
    /**
     *
     * 功能描述:使用了同步类锁导致性能低下
     *
     * @param:
     * @return:
     * @auther: csh
     * @date: 2020/5/17 11:32
     */
    public static synchronized LazySingleSafery getInstance(){
        if(lazySingleSafery == null){
            lazySingleSafery = new LazySingleSafery();
        }
        return lazySingleSafery;
    }
}
/**
 * @Auther: csh
 * @Date: 2020/5/17 11:33
 * @Description:双检锁/双重校验锁（DCL，即 double-checked locking）
 * 性能较高
 */
public class DoubleCheckLazySingleSafery {
    private static DoubleCheckLazySingleSafery instance;
    private DoubleCheckLazySingleSafery(){

    }

    public static DoubleCheckLazySingleSafery getInstance(){
        if(instance == null){
            synchronized (DoubleCheckLazySingleSafery.class){
                if(instance == null){
                    instance = new DoubleCheckLazySingleSafery();
                }
            }
        }
        return instance;
    }
}
/**
 * @Auther: csh
 * @Date: 2020/5/17 11:39
 * @Description:登记式/静态内部类  线程安全
 */
public class StaticInnerSingle {
    private static class SingleHolder{
        private static final StaticInnerSingle INSTANCE = new StaticInnerSingle();
    }
    private StaticInnerSingle(){

    }

    public static final StaticInnerSingle getInstance(){
        return SingleHolder.INSTANCE;
    }
}
/**
 * @Auther: csh
 * @Date: 2020/5/17 10:43
 * @Description:实现
 */
public class SingletonPatternDemo {
    public static void main(String[] args) {
        //不合法的构造函数
        //编译时错误：构造函数 HungrySingle() 是不可见的
        //HungrySingle object = new HungrySingle();

        //获取唯一可用的对象 饿汉
        ExecutorService lazyThreadPool = Executors.newFixedThreadPool(10);
        for (int i =0;i<10;i++){
            lazyThreadPool.execute(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(10);
                    }catch (Exception e){
                        e.printStackTrace();
                    }
                    System.out.println("饿汉（安全）"+ HungrySingle.getInstance());
                }
            });
        }
        //懒汉 线程不安全
        ExecutorService hungryThreadPool = Executors.newFixedThreadPool(10);
        for (int i =0;i<10;i++){
            hungryThreadPool.execute(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(10);
                    }catch (Exception e){
                        e.printStackTrace();
                    }
                    System.out.println("懒汉（不安全）"+LazySingleton.getInstance());
                }
            });
        }

        //懒汉 线程安全 性能低
        ExecutorService hungrySaferyThreadPool = Executors.newFixedThreadPool(10);
        for (int i =0;i<10;i++){
            hungrySaferyThreadPool.execute(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(10);
                    }catch (Exception e){
                        e.printStackTrace();
                    }
                    System.out.println("懒汉（安全）"+LazySingleSafery.getInstance());
                }
            });
        }

        //懒汉 线程安全 性能高
        ExecutorService DoubleCheckThreadPool = Executors.newFixedThreadPool(10);
        for (int i =0;i<10;i++){
            DoubleCheckThreadPool.execute(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(10);
                    }catch (Exception e){
                        e.printStackTrace();
                    }
                    System.out.println("懒汉双重检查（安全）"+DoubleCheckLazySingleSafery.getInstance());
                }
            });
        }


        //懒汉 线程安全 性能高
        ExecutorService staticInnerSingleThreadPool = Executors.newFixedThreadPool(10);
        for (int i =0;i<10;i++){
            staticInnerSingleThreadPool.execute(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(10);
                    }catch (Exception e){
                        e.printStackTrace();
                    }
                    System.out.println("登记式/静态内部类（安全）"+StaticInnerSingle.getInstance());
                }
            });
        }


    }
}

结果

"C:Program FilesJavajdk1.8.0_181binjava.exe" "-javaagent:C:Program FilesJetBrainsIntelliJ IDEA 2018.2.3libidea_rt.jar=58612:C:Program FilesJetBrainsIntelliJ IDEA 2018.2.3bin" -Dfile.encoding=UTF-8 -classpath "C:Program FilesJavajdk1.8.0_181jrelibcharsets.jar;C:Program FilesJavajdk1.8.0_181jrelibdeploy.jar;C:Program FilesJavajdk1.8.0_181jrelibextaccess-bridge-64.jar;C:Program FilesJavajdk1.8.0_181jrelibextcldrdata.jar;C:Program FilesJavajdk1.8.0_181jrelibextdnsns.jar;C:Program FilesJavajdk1.8.0_181jrelibextjaccess.jar;C:Program FilesJavajdk1.8.0_181jrelibextjfxrt.jar;C:Program FilesJavajdk1.8.0_181jrelibextlocaledata.jar;C:Program FilesJavajdk1.8.0_181jrelibextnashorn.jar;C:Program FilesJavajdk1.8.0_181jrelibextsunec.jar;C:Program FilesJavajdk1.8.0_181jrelibextsunjce_provider.jar;C:Program FilesJavajdk1.8.0_181jrelibextsunmscapi.jar;C:Program FilesJavajdk1.8.0_181jrelibextsunpkcs11.jar;C:Program FilesJavajdk1.8.0_181jrelibextzipfs.jar;C:Program FilesJavajdk1.8.0_181jrelibjavaws.jar;C:Program FilesJavajdk1.8.0_181jrelibjce.jar;C:Program FilesJavajdk1.8.0_181jrelibjfr.jar;C:Program FilesJavajdk1.8.0_181jrelibjfxswt.jar;C:Program FilesJavajdk1.8.0_181jrelibjsse.jar;C:Program FilesJavajdk1.8.0_181jrelibmanagement-agent.jar;C:Program FilesJavajdk1.8.0_181jrelibplugin.jar;C:Program FilesJavajdk1.8.0_181jrelibresources.jar;C:Program FilesJavajdk1.8.0_181jrelibrt.jar;D:ideaWorkSpacedesign-modeltargetclasses" com.hong.base.singlemodel.SingletonPatternDemo
饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7
饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7
饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7
饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7
饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7
饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7
饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7
饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7
饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7
饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7
懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752
懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752
懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752
懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752
懒汉（不安全）com.hong.base.singlemodel.LazySingleton@710f2fc4
懒汉（不安全）com.hong.base.singlemodel.LazySingleton@1bd08742
懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752
懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752
懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752
懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752
懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412
懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412
懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412
懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412
懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412
懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412
懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412
懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412
懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412
懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412
懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d
懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d
懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d
懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d
懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d
懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d
懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d
懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d
懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d
懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d
登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c
登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c
登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c
登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c
登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c
登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c
登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c
登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c
登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c
登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c

最后，判断一个对象是不是单列只需要看对象里面有没有private 类名 (){} 如果有肯定是单例。

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