由浅入深之层层升级探索单例模式

前言:设计模式源于生活

单例的基本概念

单例模式确保某各类只有一个实例，而且自行实例化并向整个系统提供这个实例。选择单例模式就是为了避免不一致状态

单例模式特点

1、单例类只能有一个实例。
2、单例类必须自己创建自己唯一的实例。
3、单例类必须给所有其它对象提供这一实例。

单例模式的优缺点

1、单例类只有一个实例
2、共享资源，全局使用
3、节省创建时间，提高性能

单例模式应用场景

项目定义的配置文件都是单例
SpringIOC容器默认就是单例模式
线程池和数据库连接池都是单例

饿汉模式

public class SingletonV1 {

    /**
     * 当类初始化,就会创建该对象
     */
    private static SingletonV1 singletonV1 = new SingletonV1();

    /**
     * 构造方法需要私有化,禁止初始化
     */
    private SingletonV1() {
    }

    public static SingletonV1 getInstance() {
        return singletonV1;
    }

    /**
     * 测试单例
     *
     * @param args
     */
    public static void main(String[] args) {
        SingletonV1 instance1 = SingletonV1.getInstance();
        SingletonV1 instance2 = SingletonV1.getInstance();

        System.out.println(instance1);
        System.out.println(instance2);
    }
}

饿汉模式-静态代码块

public class SingletonV5 {
    private static SingletonV5 singletonV5 = null;

    //类加载读取
    static {
        singletonV5 = new SingletonV5();
    }

    public static SingletonV5 getInstance(){
        return singletonV5;
    }

    public static void main(String[] args) {
        SingletonV5 instance1 = SingletonV5.getInstance();
        SingletonV5 instance2 = SingletonV5.getInstance();
        System.out.println(instance1 == instance2);
    }
}

懒汉模式-线程不安全

public class SingletonV2 {

    private static SingletonV2 singleton001;

    /**
     * 构造方法需要私有化,禁止初始化
     */
    private SingletonV2() {
    }

    /**
     * 多线程下,可能会造成线程不安全,违背单例模式
     * @return
     */
    public static SingletonV2 getInstance() {
        if (null == singleton001) {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            singleton001 = new SingletonV2();
        }
        return singleton001;
    }


    /**
     * 测试单例
     * @param args
     */
    public static void main(String[] args) {
        for (int i = 0; i < 15; i++) {
            new Thread(new Runnable() {
                public void run() {
                    SingletonV2 instance = SingletonV2.getInstance();
                    System.out.println(instance);
                }
            }).start();
        }
    }
}

懒汉模式-线程安全

public class SingletonV3 {
    private static SingletonV3 singletonV2;

    private SingletonV3() {
    }

    /**
     * 效率低,每次获取单例都需要上锁
     * @return
     */
    public static synchronized SingletonV3 getInstance() {
        if (null == singletonV2) {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            singletonV2 = new SingletonV3();
        }
        return singletonV2;
    }

    /**
     * 测试单例
     * @param args
     */
    public static void main(String[] args) {
        for (int i = 0; i < 15; i++) {
            new Thread(new Runnable() {
                public void run() {
                    SingletonV3 instance = SingletonV3.getInstance();
                    System.out.println(instance);
                }
            }).start();
        }
    }
}

双重检验锁

public class SingletonV4 {

    private static SingletonV4 singletonV4;

    private SingletonV4() {
        System.out.println("--初始化--");
    }

    /**
     * 双重检验锁
     * 能够保证线程安全,且效率高
     * @return
     */
    public static SingletonV4 getInstance() {
        if (null == singletonV4) {
            synchronized (SingletonV4.class) {
                if (null == singletonV4) {
                    singletonV4 = new SingletonV4();
                }
            }
        }
        return singletonV4;
    }

    public static void main(String[] args) {
        SingletonV4 instance1 = SingletonV4.getInstance();
        SingletonV4 instance2 = SingletonV4.getInstance();
        System.out.println(instance1);
        System.out.println(instance2);
    }
}

破解单例模式-基于双重检验锁

public class SingletonV7 {

    private static SingletonV7 singletonV7;

    private SingletonV7() throws Exception {
        System.out.println("--初始化--");
    }

    /**
     * 双重检验锁
     * 能够保证线程安全,且效率高
     * @return
     */
    public static SingletonV7 getInstance() throws Exception {
        if (null == singletonV7) {
            synchronized (SingletonV7.class) {
                if (null == singletonV7) {
                    singletonV7 = new SingletonV7();
                }
            }
        }
        return singletonV7;
    }

    public static void main(String[] args) throws Exception {
        //获取单例对象
        SingletonV7 instance1 = SingletonV7.getInstance();

        //通过反射创建对象,暴力破解单例模式
        Class aClass = Class.forName("com.dream.sunny.SingletonV7");
        Constructor declaredConstructor = aClass.getDeclaredConstructor();
        declaredConstructor.setAccessible(true);
        SingletonV7 instance2 = (SingletonV7) declaredConstructor.newInstance();

        System.out.println(instance1);
        System.out.println(instance2);
        System.out.println(instance1 == instance2);
    }
}

可以发现,我第一次获取完对象后,第二次通过反射的形式,来创建对象,所以变相的破解了单例模式

双重检验锁-三级校验

public class SingletonV8 {

    private static SingletonV8 singletonV8;

    private SingletonV8() throws Exception {
        synchronized (SingletonV8.class){
            if(singletonV8 != null){
                throw new  RuntimeException("不要试图使用反射破坏异常");
            }
        }
        System.out.println("--初始化--");
    }

    /**
     * 双重检验锁
     * 能够保证线程安全,且效率高
     * @return
     */
    public static SingletonV8 getInstance() throws Exception {
        if (null == singletonV8) {
            synchronized (SingletonV8.class) {
                if (null == singletonV8) {
                    singletonV8 = new SingletonV8();
                }
            }
        }
        return singletonV8;
    }

    public static void main(String[] args) throws Exception {
        //获取单例对象
        SingletonV8 instance1 = SingletonV8.getInstance();

//        //通过反射创建对象,暴力破解单例模式
        Class aClass = Class.forName("com.dream.sunny.SingletonV8");
        Constructor declaredConstructor = aClass.getDeclaredConstructor();
        declaredConstructor.setAccessible(true);
        SingletonV8 instance2 = (SingletonV8) declaredConstructor.newInstance();
    }
}

因为反射是需要通过无参构造方法进行创建对象,所以可以在无参构造方法,多加一层校验,判断单例对象是否存在

双重检验锁-三级校验-再次破坏

public class SingletonV9 {
    private static SingletonV9 singletonV9;

    private SingletonV9() throws Exception {
        synchronized (SingletonV9.class){
            if(singletonV9 != null){
                throw new  RuntimeException("不要试图使用反射破坏异常");
            }
        }
        System.out.println("--初始化--");
    }

    /**
     * 双重检验锁
     * 能够保证线程安全,且效率高
     * @return
     */
    public static SingletonV9 getInstance() throws Exception {
        if (null == singletonV9) {
            synchronized (SingletonV9.class) {
                if (null == singletonV9) {
                    singletonV9 = new SingletonV9();
                }
            }
        }
        return singletonV9;
    }

    public static void main(String[] args) throws Exception {


        //通过反射创建对象,暴力破解单例模式
        Class aClass = Class.forName("com.dream.sunny.SingletonV9");
        Constructor declaredConstructor = aClass.getDeclaredConstructor();
        declaredConstructor.setAccessible(true);
        SingletonV9 instance2 = (SingletonV9) declaredConstructor.newInstance();

        //获取单例对象
        SingletonV9 instance1 = SingletonV9.getInstance();

        System.out.println(instance1);
        System.out.println(instance2);
    }
}

双重检验锁-三级校验升级版

public class SingletonV10 {
    private static SingletonV10 singletonV10;

    private SingletonV10() throws Exception {
        synchronized (SingletonV10.class){
            if(singletonV10 != null){
                throw new  RuntimeException("不要试图使用反射破坏异常");
            }else{
                singletonV10 = this;
            }
        }
        System.out.println("--初始化--");
    }

    /**
     * 双重检验锁
     * 能够保证线程安全,且效率高
     * @return
     */
    public static SingletonV10 getInstance() throws Exception {
        if (null == singletonV10) {
            synchronized (SingletonV10.class) {
                if (null == singletonV10) {
                    singletonV10 = new SingletonV10();
                }
            }
        }
        return singletonV10;
    }

    public static void main(String[] args) throws Exception {


        //通过反射创建对象,暴力破解单例模式
        Class aClass = Class.forName("com.dream.sunny.SingletonV10");
        Constructor declaredConstructor = aClass.getDeclaredConstructor();
        declaredConstructor.setAccessible(true);
        SingletonV10 instance2 = (SingletonV10) declaredConstructor.newInstance();

        //获取单例对象
        SingletonV10 instance1 = SingletonV10.getInstance();

        System.out.println(instance1);
        System.out.println(instance2);
    }
}

枚举类,实现单例模式（枚举实现单例模式是无法被破解的,因为枚举没有空的构造方法）

public enum SingletonV11 {

    INSTANCE;

    SingletonV11(){
        System.out.println("初始化方法");
    }

    public static void main(String[] args) {
        SingletonV11 instance1 = SingletonV11.INSTANCE;
        SingletonV11 instance2 = SingletonV11.INSTANCE;

        System.out.println(instance1);
        System.out.println(instance2);
    }
}

总结

到这里几种写法都介绍完了，至于选择用哪种形式的单例模式，取决于你的项目本身，是否是有复杂的并发环境，还是需要控制单例对象的资源消耗。