单例模式（下）

public class IdGenerator {
    private AtomicLong id = new AtomicLong(0);
    // 静态方法实现
    public static long getId() {
        return id.incrementAndGet();
    }

}
// 使用示例
long id = IdGenerator.getId();

//  旧使用方式
public  demoFuntion() {
   //...  
   long id = IdGenerator.getInstance().getId(); 
    //...
}

// 2. 新的使用方式：依赖注入
public demofunction(IdGenerator idGenerator) {  
    //... 
    long id = idGenerator.getId();
    //...
}
// 外部调用demofunction()的时候，传入idGeneratorIdGenerator
idGenerator = IdGenerator.getInsance();
demofunction(idGenerator);

public class IdGenerator6 {
    private AtomicLong id = new AtomicLong(0);
    private static final ConcurrentHashMap<Long, IdGenerator6> instances
            = new ConcurrentHashMap<>();

    private IdGenerator6() {}
    public static IdGenerator6 getInstance() {
        Long currentThreadId = Thread.currentThread().getId();
        instances.putIfAbsent(currentThreadId, new IdGenerator6());
        return instances.get(currentThreadId);
    }
    public long getId() {
        return id.incrementAndGet();
    }
}

/**
 * 集群唯一
 *
 * [设计模式之美](https://windcoder.com/go/JKDesignPattern)中仅提供了伪代码，这里根据伪代码尝试做了一个基于Redis的简单实现，代码本身没做测试，请勿直接粘贴复制到生产环境。
 *
 * Redisson 操作redis实现单例对象的序列化存储与反序列化读取。
 * Redisson 实现Redis分布式锁，用于取出前的加锁，与操作后的释放锁。
 * 
 *  关于为何配置自定义序列化与反序列化的问题，可以参考[redisson如何序列化](https://www.php.cn/redis/436670.html)。
 */
public class IdGenerator7 implements Serializable {
    private AtomicLong id = new AtomicLong(0);
    private static IdGenerator7 instance;


    static RedissonClient redisson = getRedissonClient();
    static RLock  lock = redisson.getLock("anyLock");


    private IdGenerator7() {}

    public synchronized static IdGenerator7 getInstance()   {
        if (instance == null) {
            lock.lock();
            RBucket<IdGenerator7> bucket = redisson.getBucket("IdGenerator");
            //如果key存在，就设置key的值为新值value
            //如果key不存在，就设置key的值为value
            // https://www.pianshen.com/article/5465125503/
            bucket.set(new IdGenerator7());
            instance = bucket.get();
        }
        return instance;
    }
    public synchronized void freeInstance() {
        RBucket<IdGenerator7> bucket = redisson.getBucket("IdGenerator");
        bucket.set(new IdGenerator7());
        instance = null; //释放对象
        lock.unlock();

    }
    public long getId() {
        return id.incrementAndGet();
    }

    /**
     * 读取Redisson关于Redis的相关配置，生成Redisson对象。
     * 
     * 这里仅为了方便实现与展示集群唯一的示例。
     * @return
     */
    private static RedissonClient getRedissonClient() {
        Config config = null;
        try {
            config = Config.fromYAML(new File("classpath:redisson.yaml"));
        } catch (IOException e) {
            e.printStackTrace();
        }
        return Redisson.create(config);
    }
}

public class IdGenerator8 {
    private long serverNo;
    private String serverAddress;
    private static final int SERVER_COUNT = 3;

    private static final ConcurrentHashMap<Long, IdGenerator8> instances
            = new ConcurrentHashMap<>();

    static {
        instances.put(1L, new IdGenerator8(1L, "192.134.22.138:8080"));
        instances.put(2L, new IdGenerator8(2L, "192.134.22.139:8080"));
        instances.put(3L, new IdGenerator8(3L, "192.134.22.140:8080"));
    }

    private IdGenerator8(long serverNo, String address) {
        this.serverNo = serverNo;
        this.serverAddress = address;
    }

    public IdGenerator8 getInstance(long serverNo) {
        return instances.get(serverNo);
    }
    public IdGenerator8 getRandomInstance() {
        Random r = new Random();
        int no = r.nextInt(SERVER_COUNT)+1;
        return instances.get(no);
    }

}

public class IdGenerator9 {
    private static final ConcurrentHashMap<String, IdGenerator9> instances
            = new ConcurrentHashMap<>();
    private IdGenerator9() {}

    public static IdGenerator9 getInstance(String name) {
        instances.putIfAbsent(name, new IdGenerator9());
        return instances.get(name);
    }

    public static void main(String[] args) {
        IdGenerator9 i1 =  IdGenerator9.getInstance("User.class");
        IdGenerator9 i2 =  IdGenerator9.getInstance("User.class");
        IdGenerator9 i3 =  IdGenerator9.getInstance("Order.class");
    }
}