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，只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;

代码地址:https://gitee.com/hong99/design-model/issues/I1IMES

代码实现

懒汉：都是被别人推着才去做的，这样线程不安全(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:
我去吃饭一到饭管菜都已经上好了，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;

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!");
    }
}

public class LazySingleton {
    private static LazySingleton instance;
    private LazySingleton(){}
    public static LazySingleton getInstance() {
        if (instance == null) {
            instance = new LazySingleton();
        }
        return instance;
    }
}

public class LazySingleSafery {
    private static LazySingleSafery lazySingleSafery;

    private LazySingleSafery(){

    }
    
    public static synchronized LazySingleSafery getInstance(){
        if(lazySingleSafery == null){
            lazySingleSafery = new LazySingleSafery();
        }
        return lazySingleSafery;
    }
}

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;
    }
}

public class StaticInnerSingle {
    private static class SingleHolder{
        private static final StaticInnerSingle INSTANCE = new StaticInnerSingle();
    }
    private StaticInnerSingle(){

    }

    public static final StaticInnerSingle getInstance(){
        return SingleHolder.INSTANCE;
    }
}

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 Files\Java\jdk1.8.0_181\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2018.2.3\lib\idea_rt.jar=58612:C:\Program Files\JetBrains\IntelliJ IDEA 2018.2.3\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_181\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\rt.jar;D:\ideaWorkSpace\design-model\target\classes" 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

最后我想说，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。

代码地址:https://gitee.com/hong99/design-model/issues/I1IMES

参考资料：

《大话设计模式》
《研磨设计模式》
《设计模式之禅》
《Java与模式》