单例模式

确保一个类只有一个实例，而且自行实例化并向整个系统提供这个实例。

特征

• 构造方法一般不外开放

• 通过一个静态方法或枚举返回单例类的对象

• 注意多线程的场景

• 注意单例类对象在反序列化时不会重新创建对象

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单例类的实现

1.懒汉式单例

    // 1.懒汉式单例
    static class Singleton1 {

        // 私有静态变量
        private static Singleton1 mInstance = null;

        // 私有构造方法
        private Singleton1() {
            System.out.println("懒汉式单例");
        }

        // 暴露公有静态方法
        public static Singleton1 getInstance() {
            if (mInstance == null) {
                mInstance = new Singleton1();
            }
            return mInstance;
        }
    }

以上是简单的懒汉式单例实现，如果单例类中有作网络请求、缓存操作或线程池策略等耗时操作，会创建多个不同的实例，以下稍作改动，模拟耗时操作。

main()中添加

        for (int i = 0; i < 100; i++) {
            final int index = i;
            new Thread(){
                @Override
                public void run() {
                    System.out.println("index = " + index + ", instance: " + Singleton1.getInstance());
                }
            }.start();
        }

getInstance()中添加，再运行结果

try {
     Thread.sleep(500);
} catch (InterruptedException e) {
    e.printStackTrace();
}

---------- 节选结果 ----------
index = 1, instance: com.example.designmodedemo.singleton.SingletonMode$Singleton1@6b52350c
懒汉式单例
懒汉式单例
懒汉式单例
index = 7, instance: com.example.designmodedemo.singleton.SingletonMode$Singleton1@4a1a7268
懒汉式单例
懒汉式单例
index = 9, instance: com.example.designmodedemo.singleton.SingletonMode$Singleton1@ce407e7
懒汉式单例
懒汉式单例
懒汉式单例
index = 11, instance: com.example.designmodedemo.singleton.SingletonMode$Singleton1@59ac37b1
index = 8, instance: com.example.designmodedemo.singleton.SingletonMode$Singleton1@1dca18a4
index = 13, instance: com.example.designmodedemo.singleton.SingletonMode$Singleton1@5795dfe4
index = 10, instance: com.example.designmodedemo.singleton.SingletonMode$Singleton1@7898c39
index = 6, instance: com.example.designmodedemo.singleton.SingletonMode$Singleton1@fb49a4f
index = 5, instance: com.example.designmodedemo.singleton.SingletonMode$Singleton1@47c48106
index = 3, instance: com.example.designmodedemo.singleton.SingletonMode$Singleton1@6fada00f

输出的哈希地址不相等说明对象多次创建。想要线程安全，需要加synchronized关键字。

1、直接锁方法。锁方法的缺陷是同步锁粒度太大，耗性能

2、双重校验DCL。同步锁粒度小很多。<u>由于JVM有指令重排特殊性，会导致双重校验失效，静态变量需要添加volatile关键字</u>

private volatile static Singleton1 sInstance = null;

public static Singleton1 getInstance() {
    if (sInstance == null) {// 第一重校验
        try {
            Thread.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        synchronized(Singleton1.class) {
            if (sInstance == null) {// 第二重校验
                sInstance = new Singleton1();
            }
        }
    }
    return sInstance;
}

2.饿汉式单例(线程安全)

    static class Singleton2 {

        private static Singleton2 sInstance = new Singleton2();

        private Singleton2() {}

        public static Singleton2 getInstance() {
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return sInstance;
        }
    }

3.静态内部类单例(线程安全)

static class Singleton3 {

    private Singleton3() {}

    private static class Holder {
        private static final Singleton3 INSTANCE = new Singleton3();
    }

    // 延时加载
    public final static Singleton3 getInstance() {
        return Holder.INSTANCE;
    }
}

4.使用枚举(线程安全)

static class Singleton4 {}

enum EnumDemo {
    INSTANCE;

    private Singleton4 instance;

    EnumDemo() {
        instance = new Singleton4();
    }

    public Singleton4 getInstance() {
        return instance;
    }
}

4.使用容器，如源码中类SystemServiceRegistry

static class Singleton5 {}

static class SingletonManager {
    private static Map<String, Object> map = new HashMap<>();

    public static void putClass(String key, Object instace) {
        if (!map.containsKey(key)) {
            map.put(key, instace);
        }
    }

    public static Object getClass(String key) {
        return map.get(key);
    }
}

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