深海游弋的鱼 – 第 29 页 – 智障儿童欢乐多

Dart Mixin介绍

关于 Dart mixin 的一些理解。理解 mixin 概念的关键在于理解中间类。

Mixins are a way of reusing code in multiple class hierarchies

先来看一个简单例子：

class Piloted {
  int astronauts = 1;
  void describeCrew() {
    print('Number of astronauts: $astronauts');
  }
}

class PilotedCraft extends Spacecraft with Piloted {
  // ···
}

class Piloted {

int astronauts = 1;

void describeCrew() {

print('Number of astronauts: $astronauts');

}

class PilotedCraft extends Spacecraft with Piloted {

// ···

}

PilotedCraft 拥有 astronauts 字段和 describeCrew() 方法。

mixin 是什么？

维基百科中这样定义 mixin：

In object-oriented programming languages, a Mixin is a class that contains methods for use by other classes without having to be the parent class of those other classes.

即，mixin 是另外一个普通类，我们可以在不继承这个类的情况下从这个类”借用”方法和变量。

Support for the mixin keyword was introduced in Dart 2.1. Code in earlier releases usually used abstract class instead.

从这个角度来讲，mixin 不过是 abstract class。

Java tries to make up for this by using Interfaces, but that is not as useful or flexible as mixins.

从这个角度来讲，可以认为 mixin 是带实现的接口。

小节

mixin 有点类似 abstract class
mixin 有点类似 interface
不能继承 mixin
可以使用 mixin, abstract class, class 来作为 mixin

如何使用 mixin?

使用 mixin 的方法很简单：with 关键字后面跟上 mixin 的名字即可。

class Musician extends Performer with Musical {
  // ···
}

class Maestro extends Person
    with Musical, Aggressive, Demented {
  Maestro(String maestroName) {
    name = maestroName;
    canConduct = true;
  }
}

class Musician extends Performer with Musical {

// ···

}

class Maestro extends Person

with Musical, Aggressive, Demented {

Maestro(String maestroName) {

name = maestroName;

canConduct = true;

}

实现 mixin 的方法同样也很简单：创建一个继承自 Object 的类并且不要声明构造方法。如果想让 mixin 作为普通类使用，使用 class 关键字；如果不想让 mixin 作为普通类使用，使用 mixin 关键字代替 class。

mixin Musical {
  bool canPlayPiano = false;
  bool canCompose = false;
  bool canConduct = false;

  void entertainMe() {
    if (canPlayPiano) {
      print('Playing piano');
    } else if (canConduct) {
      print('Waving hands');
    } else {
      print('Humming to self');
    }
  }
}

mixin Musical {

bool canPlayPiano = false;

bool canCompose = false;

bool canConduct = false;

void entertainMe() {

if (canPlayPiano) {

print('Playing piano');

} else if (canConduct) {

print('Waving hands');

} else {

print('Humming to self');

}

on 的用法

The keyword on is used to restrict our mixin’s use to only classes which either extends or implements the class it is declared on. In order to use the on keyword, you must declare your mixin using the mixin keyword

class B {}
mixin Y on B {
  void hi() {
    print('hi');
  }
}
class Q with Y {}

class B {}

mixin Y on B {

void hi() {

print('hi');

}

class Q with Y {}

则有如下错误提示：

Error: 'Object' doesn't implement 'B' so it can't be used with 'Y'.

on 关键字限制了 Y 的使用范围：Y 只能用于继承或实现了 B 的类。修复方式是让 Q 继承自 B：

class Q extends B with Y {}

1	class Q extends B with Y {}

mixin 解决了什么问题？

mixin 解决了多重继承中的 Deadly Diamond of Death(DDD) 问题。

多重继承问题简单描述。各个类的继承关系如下：

class Performer {
    abstract void perform();
}

class Dancer extends Performer {
    void perform() {}
}

class Singer extends Performer {
    void perform() {}
}

class Musician extends Dancer, Singer {
}

class Performer {

abstract void perform();

}

class Dancer extends Performer {

void perform() {}

}

class Singer extends Performer {

void perform() {}

}

class Musician extends Dancer, Singer {

}

问题来了，当调用 Musician.perform() 时，到底会调用哪个 perform() 方法是模糊的。

来看 mixin 如何解决这个问题。见 Dart for Flutter : Mixins in Dart - Flutter Community - Medium

class Performer {
    abstract void perform();
}

mixin Dancer {
    void perform() {}
}

mixin Singer {
    void perform() {}
}

class Musician extends Performer with Dancer, Singer {
}

class Performer {

abstract void perform();

}

mixin Dancer {

void perform() {}

}

mixin Singer {

void perform() {}

}

class Musician extends Performer with Dancer, Singer {

}

现在，当调用 Musician.perform() 时，到底会调用哪个 perform() 方法是确定的。在这里是调用 Singer.perform()。

mixin 有一套明确的机制来选择调用哪个方法。

假设 Musician 类使用多个 mixin (Dancer, Singer)。该类有个方法名为 perform()，Musician 类继承自 Performer 类。

首先，将 Performer 类置于栈顶
其次，后声明的 mixin 优先于先声明的 mixin。按顺序将 mixin 置于栈中，在这里分别是 Dancer, Singer
最后，将 Musician 类自己置于栈中。Musician 类中的 perform() 被优先调用

Dart 使用的是单重继承 (Java 也是单重继承，C++ 是多重继承)。多重继承更为强大，但会引起 Deadly Diamond of Death(DDD) 问题。

Java 使用接口(interface)来部分实现多重继承。多重继承的问题是需要在每个类中实现接口(interface)，所以并不是一个好的方案。(实际上 Java 已经通过默认方法修复了这个问题)

所以 Dart 中就有了 mixin。

理解 mixin

Mixins in Dart work by creating a new class that layers the implementation of the mixin on top of a superclass to create a new class — it is not “on the side” but “on top” of the superclass, so there is no ambiguity in how to resolve lookups.
Mixins is not a way to get multiple inheritance in the classical sense. Mixins is a way to abstract and reuse a family of operations and state. It is similar to the reuse you get from extending a class, but it is compatible with single-inheritance because it is linear.
StackOverflow

class A {
  String getMessage() => 'A';
}

class B {
  String getMessage() => 'B';
}

class P {
  String getMessage() => 'P';
}

class AB extends P with A, B {}

class BA extends P with B, A {}

void main() {
  String result = '';

  AB ab = AB();
  result += ab.getMessage();

  BA ba = BA();
  result += ba.getMessage();

  print(result);
}

class A {

String getMessage() => 'A';

}

class B {

String getMessage() => 'B';

}

class P {

String getMessage() => 'P';

}

class AB extends P with A, B {}

class BA extends P with B, A {}

void main() {

String result = '';

AB ab = AB();

result += ab.getMessage();

BA ba = BA();

result += ba.getMessage();

print(result);

}

以上这段代码输出 BA。

从语义上讲以上这段代码等同于：

class PA = P with A;
class PAB = PA with B;

class AB extends PAB {}

class PB = P with B;
class PBA = PB with A;

class BA extends PBA {}

class PA = P with A;

class PAB = PA with B;

class AB extends PAB {}

class PB = P with B;

class PBA = PB with A;

class BA extends PBA {}

继承结构图是这样的：

Since each mixin application creates a new class, it also creates a new interface (because all Dart classes also define interfaces). As described, the new class extends the superclass and includes copies of the mixin class members, but it also implements the mixin class interface.
In most cases, there is no way to refer to that mixin-application class or its interface; the class for Super with Mixin is just an anonymous superclass of the class declared like class C extends Super with Mixin {}. If you name a mixin application like class CSuper = Super with Mixin {}, then you can refer to the mixin application class and its interface, and it will be a sub-type of both Super and Mixin.

理解 mixin 的关键在于它是线性的。

使用场景

在没有共同父类的各个类中共享代码时
在父类中实现某种方法无意义时

在 Dart 语言中，我们经常可以看到对 mixin 关键字的使用，根据字面理解，就是混合的意思。那么，mixin 如何使用，它的使用场景是什么呢。

继续阅读Dart Mixin介绍

布隆过滤器

什么是 BloomFilter

布隆过滤器（英语：Bloom Filter）是 1970 年由布隆提出的。它实际上是一个很长的二进制向量和一系列随机映射函数。主要用于判断一个元素是否在一个集合中。

通常我们会遇到很多要判断一个元素是否在某个集合中的业务场景，一般想到的是将集合中所有元素保存起来，然后通过比较确定。链表、树、散列表（又叫哈希表，Hash table）等等数据结构都是这种思路。但是随着集合中元素的增加，我们需要的存储空间也会呈现线性增长，最终达到瓶颈。同时检索速度也越来越慢，上述三种结构的检索时间复杂度分别为$O(n)$，$O(logn)$，$O(1)$。

这个时候，布隆过滤器（Bloom Filter）就应运而生。

布隆过滤器原理

了解布隆过滤器原理之前，先回顾下 Hash 函数原理。

哈希函数

哈希函数的概念是：将任意大小的输入数据转换成特定大小的输出数据的函数，转换后的数据称为哈希值或哈希编码，也叫散列值。下面是一幅示意图：

所有散列函数都有如下基本特性：

如果两个散列值是不相同的（根据同一函数），那么这两个散列值的原始输入也是不相同的。这个特性是散列函数具有确定性的结果，具有这种性质的散列函数称为单向散列函数。
散列函数的输入和输出不是唯一对应关系的，如果两个散列值相同，两个输入值很可能是相同的，但也可能不同，这种情况称为“散列碰撞（collision）”。

但是用 hash表存储大数据量时，空间效率还是很低，当只有一个 hash 函数时，还很容易发生哈希碰撞。

布隆过滤器数据结构

BloomFilter 是由一个固定大小的二进制向量或者位图（bitmap）和一系列映射函数组成的。

在初始状态时，对于长度为 m 的位数组，它的所有位都被置为0，如下图所示：

当有变量被加入集合时，通过 K 个映射函数将这个变量映射成位图中的 K 个点，把它们置为 1（假定有两个变量都通过 3 个映射函数）。

查询某个变量的时候我们只要看看这些点是不是都是 1 就可以大概率知道集合中有没有它了

如果这些点有任何一个 0，则被查询变量一定不在；
如果都是 1，则被查询变量很可能存在

为什么说是可能存在，而不是一定存在呢？那是因为映射函数本身就是散列函数，散列函数是会有碰撞的。

误判率

布隆过滤器的误判是指多个输入经过哈希之后在相同的bit位置1了，这样就无法判断究竟是哪个输入产生的，因此误判的根源在于相同的 bit 位被多次映射且置 1。

这种情况也造成了布隆过滤器的删除问题，因为布隆过滤器的每一个 bit 并不是独占的，很有可能多个元素共享了某一位。如果我们直接删除这一位的话，会影响其他的元素。(比如上图中的第 3 位)

特性

一个元素如果判断结果为存在的时候元素不一定存在，但是判断结果为不存在的时候则一定不存在。
布隆过滤器可以添加元素，但是不能删除元素。因为删掉元素会导致误判率增加。

添加与查询元素步骤

添加元素

将要添加的元素给 k 个哈希函数
得到对应于位数组上的 k 个位置
将这k个位置设为 1

查询元素

将要查询的元素给k个哈希函数
得到对应于位数组上的k个位置
如果k个位置有一个为 0，则肯定不在集合中
如果k个位置全部为 1，则可能在集合中

优点

相比于其它的数据结构，布隆过滤器在空间和时间方面都有巨大的优势。布隆过滤器存储空间和插入/查询时间都是常数 $O(K)$，另外，散列函数相互之间没有关系，方便由硬件并行实现。布隆过滤器不需要存储元素本身，在某些对保密要求非常严格的场合有优势。

布隆过滤器可以表示全集，其它任何数据结构都不能；

缺点

但是布隆过滤器的缺点和优点一样明显。误算率是其中之一。随着存入的元素数量增加，误算率随之增加。但是如果元素数量太少，则使用散列表足矣。

另外，一般情况下不能从布隆过滤器中删除元素。我们很容易想到把位数组变成整数数组，每插入一个元素相应的计数器加 1, 这样删除元素时将计数器减掉就可以了。然而要保证安全地删除元素并非如此简单。首先我们必须保证删除的元素的确在布隆过滤器里面。这一点单凭这个过滤器是无法保证的。另外计数器回绕也会造成问题。

在降低误算率方面，有不少工作，使得出现了很多布隆过滤器的变种。

布隆过滤器使用场景和实例

在程序的世界中，布隆过滤器是程序员的一把利器，利用它可以快速地解决项目中一些比较棘手的问题。

如网页 URL 去重、垃圾邮件识别、大集合中重复元素的判断和缓存穿透等问题。

布隆过滤器的典型应用有：

数据库防止穿库。 Google Bigtable，HBase 和 Cassandra 以及 Postgresql 使用BloomFilter来减少不存在的行或列的磁盘查找。避免代价高昂的磁盘查找会大大提高数据库查询操作的性能。
业务场景中判断用户是否阅读过某视频或文章，比如抖音或头条，当然会导致一定的误判，但不会让用户看到重复的内容。
缓存宕机、缓存击穿场景，一般判断用户是否在缓存中，如果在则直接返回结果，不在则查询db，如果来一波冷数据，会导致缓存大量击穿，造成雪崩效应，这时候可以用布隆过滤器当缓存的索引，只有在布隆过滤器中，才去查询缓存，如果没查询到，则穿透到db。如果不在布隆器中，则直接返回。
WEB拦截器，如果相同请求则拦截，防止重复被攻击。用户第一次请求，将请求参数放入布隆过滤器中，当第二次请求时，先判断请求参数是否被布隆过滤器命中。可以提高缓存命中率。Squid 网页代理缓存服务器在 cache digests 中就使用了布隆过滤器。Google Chrome浏览器使用了布隆过滤器加速安全浏览服务
Venti 文档存储系统也采用布隆过滤器来检测先前存储的数据。
SPIN 模型检测器也使用布隆过滤器在大规模验证问题时跟踪可达状态空间。

Coding~

知道了布隆过滤去的原理和使用场景，我们可以自己实现一个简单的布隆过滤器

自定义的 BloomFilter

public class MyBloomFilter {

    /**
     * 一个长度为10 亿的比特位
     */
    private static final int DEFAULT_SIZE = 256 << 22;

    /**
     * 为了降低错误率，使用加法hash算法，所以定义一个8个元素的质数数组
     */
    private static final int[] seeds = {3, 5, 7, 11, 13, 31, 37, 61};

    /**
     * 相当于构建 8 个不同的hash算法
     */
    private static HashFunction[] functions = new HashFunction[seeds.length];

    /**
     * 初始化布隆过滤器的 bitmap
     */
    private static BitSet bitset = new BitSet(DEFAULT_SIZE);

    /**
     * 添加数据
     *
     * @param value 需要加入的值
     */
    public static void add(String value) {
        if (value != null) {
            for (HashFunction f : functions) {
                //计算 hash 值并修改 bitmap 中相应位置为 true
                bitset.set(f.hash(value), true);
            }
        }
    }

    /**
     * 判断相应元素是否存在
     * @param value 需要判断的元素
     * @return 结果
     */
    public static boolean contains(String value) {
        if (value == null) {
            return false;
        }
        boolean ret = true;
        for (HashFunction f : functions) {
            ret = bitset.get(f.hash(value));
            //一个 hash 函数返回 false 则跳出循环
            if (!ret) {
                break;
            }
        }
        return ret;
    }

    /**
     * 模拟用户是不是会员，或用户在不在线。。。
     */
    public static void main(String[] args) {

        for (int i = 0; i < seeds.length; i++) {
            functions[i] = new HashFunction(DEFAULT_SIZE, seeds[i]);
        }

        // 添加1亿数据
        for (int i = 0; i < 100000000; i++) {
            add(String.valueOf(i));
        }
        String id = "123456789";
        add(id);

        System.out.println(contains(id));   // true
        System.out.println("" + contains("234567890"));  //false
    }
}

class HashFunction {

    private int size;
    private int seed;

    public HashFunction(int size, int seed) {
        this.size = size;
        this.seed = seed;
    }

    public int hash(String value) {
        int result = 0;
        int len = value.length();
        for (int i = 0; i < len; i++) {
            result = seed * result + value.charAt(i);
        }
        int r = (size - 1) & result;
        return (size - 1) & result;
    }
}

public class MyBloomFilter {

/**

* 一个长度为10 亿的比特位

private static final int DEFAULT_SIZE = 256 << 22;

/**

* 为了降低错误率，使用加法hash算法，所以定义一个8个元素的质数数组

private static final int[] seeds = {3, 5, 7, 11, 13, 31, 37, 61};

/**

* 相当于构建 8 个不同的hash算法

private static HashFunction[] functions = new HashFunction[seeds.length];

/**

* 初始化布隆过滤器的 bitmap

private static BitSet bitset = new BitSet(DEFAULT_SIZE);

/**

* 添加数据

* @param value 需要加入的值

public static void add(String value) {

if (value != null) {

for (HashFunction f : functions) {

//计算 hash 值并修改 bitmap 中相应位置为 true

bitset.set(f.hash(value), true);

}

/**

* 判断相应元素是否存在

* @param value 需要判断的元素

* @return 结果

public static boolean contains(String value) {

if (value == null) {

return false;

}

boolean ret = true;

for (HashFunction f : functions) {

ret = bitset.get(f.hash(value));

//一个 hash 函数返回 false 则跳出循环

if (!ret) {

break;

}

return ret;

}

/**

* 模拟用户是不是会员，或用户在不在线。。。

public static void main(String[] args) {

for (int i = 0; i < seeds.length; i++) {

functions[i] = new HashFunction(DEFAULT_SIZE, seeds[i]);

}

// 添加1亿数据

for (int i = 0; i < 100000000; i++) {

add(String.valueOf(i));

}

String id = "123456789";

add(id);

System.out.println(contains(id)); // true

System.out.println("" + contains("234567890")); //false

}

class HashFunction {

private int size;

private int seed;

public HashFunction(int size, int seed) {

this.size = size;

this.seed = seed;

}

public int hash(String value) {

int result = 0;

int len = value.length();

for (int i = 0; i < len; i++) {

result = seed * result + value.charAt(i);

}

int r = (size - 1) & result;

return (size - 1) & result;

}

What？我们写的这些早有大牛帮我们实现，还造轮子，真是浪费时间，No，No，No，我们学习过程中是可以造轮子的，造轮子本身就是我们自己对设计和实现的具体落地过程，不仅能提高我们的编程能力，在造轮子的过程中肯定会遇到很多我们没有思考过的问题，成长看的见~~

实际项目使用的时候，领导和我说项目一定要稳定运行，没自信的我放弃了自己的轮子。

Guava 中的 BloomFilter

<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>23.0</version>
</dependency>

<groupId>com.google.guava</groupId>

<artifactId>guava</artifactId>

</dependency>

public class GuavaBloomFilterDemo {

    public static void main(String[] args) {
        //后边两个参数：预计包含的数据量，和允许的误差值
        BloomFilter<Integer> bloomFilter = BloomFilter.create(Funnels.integerFunnel(), 100000, 0.01);
        for (int i = 0; i < 100000; i++) {
            bloomFilter.put(i);
        }
        System.out.println(bloomFilter.mightContain(1));
        System.out.println(bloomFilter.mightContain(2));
        System.out.println(bloomFilter.mightContain(3));
        System.out.println(bloomFilter.mightContain(100001));

        //bloomFilter.writeTo();
    }
}

public class GuavaBloomFilterDemo {

public static void main(String[] args) {

//后边两个参数：预计包含的数据量，和允许的误差值

BloomFilter<Integer> bloomFilter = BloomFilter.create(Funnels.integerFunnel(), 100000, 0.01);

for (int i = 0; i < 100000; i++) {

bloomFilter.put(i);

}

System.out.println(bloomFilter.mightContain(1));

System.out.println(bloomFilter.mightContain(2));

System.out.println(bloomFilter.mightContain(3));

System.out.println(bloomFilter.mightContain(100001));

//bloomFilter.writeTo();

}

分布式环境中，布隆过滤器肯定还需要考虑是可以共享的资源，这时候我们会想到 Redis，是的，Redis 也实现了布隆过滤器。

当然我们也可以把布隆过滤器通过 bloomFilter.writeTo() 写入一个文件，放入OSS、S3这类对象存储中。

Redis 中的 BloomFilter

Redis 提供的 bitMap 可以实现布隆过滤器，但是需要自己设计映射函数和一些细节，这和我们自定义没啥区别。

Redis 官方提供的布隆过滤器到了 Redis 4.0 提供了插件功能之后才正式登场。布隆过滤器作为一个插件加载到 Redis Server 中，给 Redis 提供了强大的布隆去重功能。

在已安装 Redis 的前提下，安装 RedisBloom，有两种方式

直接编译进行安装

git clone https://github.com/RedisBloom/RedisBloom.git

cd RedisBloom

make     #编译 会生成一个rebloom.so文件

redis-server --loadmodule /path/to/rebloom.so   #运行redis时加载布隆过滤器模块

redis-cli    # 启动连接容器中的 redis 客户端验证

git clone https://github.com/RedisBloom/RedisBloom.git

cd RedisBloom

make #编译会生成一个rebloom.so文件

redis-server --loadmodule /path/to/rebloom.so #运行redis时加载布隆过滤器模块

redis-cli # 启动连接容器中的 redis 客户端验证

使用Docker进行安装

docker pull redislabs/rebloom:latest # 拉取镜像

docker run -p 6379:6379 --name redis-redisbloom redislabs/rebloom:latest #运行容器

docker exec -it redis-redisbloom bash

redis-cli

docker pull redislabs/rebloom:latest # 拉取镜像

docker run -p 6379:6379 --name redis-redisbloom redislabs/rebloom:latest #运行容器

docker exec -it redis-redisbloom bash

redis-cli

使用

布隆过滤器基本指令：

bf.add 添加元素到布隆过滤器
bf.exists 判断元素是否在布隆过滤器
bf.madd 添加多个元素到布隆过滤器，bf.add 只能添加一个
bf.mexists 判断多个元素是否在布隆过滤器

127.0.0.1:6379> bf.add user Tom
(integer) 1
127.0.0.1:6379> bf.add user John
(integer) 1
127.0.0.1:6379> bf.exists user Tom
(integer) 1
127.0.0.1:6379> bf.exists user Linda
(integer) 0
127.0.0.1:6379> bf.madd user Barry Jerry Mars
1) (integer) 1
2) (integer) 1
3) (integer) 1
127.0.0.1:6379> bf.mexists user Barry Linda
1) (integer) 1
2) (integer) 0

127.0.0.1:6379> bf.add user Tom

(integer) 1

127.0.0.1:6379> bf.add user John

(integer) 1

127.0.0.1:6379> bf.exists user Tom

(integer) 1

127.0.0.1:6379> bf.exists user Linda

(integer) 0

127.0.0.1:6379> bf.madd user Barry Jerry Mars

1) (integer) 1

2) (integer) 1

3) (integer) 1

127.0.0.1:6379> bf.mexists user Barry Linda

1) (integer) 1

2) (integer) 0

我们只有这几个参数，肯定不会有误判，当元素逐渐增多时，就会有一定的误判了，这里就不做这个实验了。

上面使用的布隆过滤器只是默认参数的布隆过滤器，它在我们第一次 add 的时候自动创建。

Redis 还提供了自定义参数的布隆过滤器，bf.reserve 过滤器名 error_rate initial_size

error_rate：允许布隆过滤器的错误率，这个值越低过滤器的位数组的大小越大，占用空间也就越大
initial_size：布隆过滤器可以储存的元素个数，当实际存储的元素个数超过这个值之后，过滤器的准确率会下降

但是这个操作需要在 add 之前显式创建。如果对应的 key 已经存在，bf.reserve 会报错

127.0.0.1:6379> bf.reserve user 0.01 100
(error) ERR item exists
127.0.0.1:6379> bf.reserve topic 0.01 1000
OK

127.0.0.1:6379> bf.reserve user 0.01 100

(error) ERR item exists

127.0.0.1:6379> bf.reserve topic 0.01 1000

我是一名 Javaer，肯定还要用 Java 来实现的，Java 的 Redis 客户端比较多，有些还没有提供指令扩展机制，笔者已知的 Redisson 和 lettuce 是可以使用布隆过滤器的，我们这里用 Redisson

public class RedissonBloomFilterDemo {

    public static void main(String[] args) {

        Config config = new Config();
        config.useSingleServer().setAddress("redis://127.0.0.1:6379");
        RedissonClient redisson = Redisson.create(config);

        RBloomFilter<String> bloomFilter = redisson.getBloomFilter("user");
        // 初始化布隆过滤器，预计统计元素数量为55000000，期望误差率为0.03
        bloomFilter.tryInit(55000000L, 0.03);
        bloomFilter.add("Tom");
        bloomFilter.add("Jack");
        System.out.println(bloomFilter.count());   //2
        System.out.println(bloomFilter.contains("Tom"));  //true
        System.out.println(bloomFilter.contains("Linda"));  //false
    }
}

public class RedissonBloomFilterDemo {

public static void main(String[] args) {

Config config = new Config();

config.useSingleServer().setAddress("redis://127.0.0.1:6379");

RedissonClient redisson = Redisson.create(config);

RBloomFilter<String> bloomFilter = redisson.getBloomFilter("user");

// 初始化布隆过滤器，预计统计元素数量为55000000，期望误差率为0.03

bloomFilter.tryInit(55000000L, 0.03);

bloomFilter.add("Tom");

bloomFilter.add("Jack");

System.out.println(bloomFilter.count()); //2

System.out.println(bloomFilter.contains("Tom")); //true

System.out.println(bloomFilter.contains("Linda")); //false

}

扩展

为了解决布隆过滤器不能删除元素的问题，布谷鸟过滤器横空出世。论文《Cuckoo Filter：Better Than Bloom》作者将布谷鸟过滤器和布隆过滤器进行了深入的对比。相比布谷鸟过滤器而言布隆过滤器有以下不足：查询性能弱、空间利用效率低、不支持反向操作（删除）以及不支持计数。

由于使用较少，暂不深入。

参考链接

布隆过滤器，这一篇给你讲的明明白白

RocksDB使用入门macOS Big Sur (11.6.6)

这篇文章就简单介绍怎么在macOS下使用RocksDB。

1. RocksDB简要介绍

RocksDB是一个可嵌入的、持久型的Key-Value存储。

不像MySQL，PostgreSQL这样数据库分客户端与服务器端。实际上，可以把它当作一个第三方库，在自己的代码中进行引用，调用相应的接口就可以使用，比如使用C/C++语言，直接在源代码中include相应的头文件就可以。

与嵌入式数据库SQLite的使用比较类似。

目前: RocksDB使用LSM存储引擎，纯C++编写。Java版本RocksJava也已经完美支持。

更多有关RocksDB的介绍参考:

英文官网: https://rocksdb.org/

关于RocksDB的详细使用介绍:

Github Wiki: https://github.com/facebook/rocksdb/wiki

2. RocksDB 安装

macOS上安装RocksDB非常简单，不用单独安装其他任何依赖。直接使用brew工具安装即可。

$ brew install rocksdb

1	$ brew install rocksdb

可以运行一下命令来查看安装信息。

$ brew info rocksdb

1	$ brew info rocksdb

安装结果如下。

$ brew info rocksdb
rocksdb: stable 7.0.3 (bottled), HEAD
Embeddable, persistent key-value store for fast storage
https://rocksdb.org/
/usr/local/Cellar/rocksdb/7.0.3 (138 files, 40.4MB) *
  Poured from bottle on 2022-06-20 at 14:18:52
From: https://mirrors.ustc.edu.cn/homebrew-core.git/Formula/rocksdb.rb
License: GPL-2.0-only or Apache-2.0
==> Dependencies
Build: cmake ✔
Required: gflags ✔, lz4 ✔, snappy ✔, zstd ✔
==> Options
--HEAD
	Install HEAD version
==> Analytics
install: 605 (30 days), 2,584 (90 days), 12,941 (365 days)
install-on-request: 561 (30 days), 2,247 (90 days), 10,385 (365 days)
build-error: 1 (30 days)

$ brew info rocksdb

rocksdb: stable 7.0.3 (bottled), HEAD

Embeddable, persistent key-value store for fast storage

https://rocksdb.org/

/usr/local/Cellar/rocksdb/7.0.3 (138 files, 40.4MB) *

Poured from bottle on 2022-06-20 at 14:18:52

From: https://mirrors.ustc.edu.cn/homebrew-core.git/Formula/rocksdb.rb

License: GPL-2.0-only or Apache-2.0

==> Dependencies

Build: cmake ✔

Required: gflags ✔, lz4 ✔, snappy ✔, zstd ✔

==> Options

--HEAD

Install HEAD version

==> Analytics

install: 605 (30 days), 2,584 (90 days), 12,941 (365 days)

install-on-request: 561 (30 days), 2,247 (90 days), 10,385 (365 days)

build-error: 1 (30 days)

其他操作系统上的安装可以参考：

官方安装教程：https://github.com/facebook/rocksdb/blob/master/INSTALL.md
CentOS 7上安装：https://www.jianshu.com/p/f233528c8303

3. 使用示例

代码目录：

main.cpp
Makefile

main.cpp文件内容：

#include <iostream>
#include <cassert>
#include "rocksdb/db.h"

using namespace std;

int main() {
    rocksdb::DB* db;
    rocksdb::Options options;
    options.create_if_missing = true;
    rocksdb::Status status = rocksdb::DB::Open(options, "/tmp/testdb", &db);
    assert(status.ok());
    cout << "Open rocksdb success." << endl;

    string key1 = "1";
    string value1 = "aaaaa";
    status = db->Put(rocksdb::WriteOptions(), key1, value1);
    assert(status.ok());
    printf("Put[%s,%s] success.\n", key1.c_str(), value1.c_str());
    
    string value;
    status = db->Get(rocksdb::ReadOptions(), key1, &value);
    assert(status.ok());
    printf("Put key[%s] = %s\n", key1.c_str(), value.c_str());

    status = db->Delete(rocksdb::WriteOptions(), key1);
    assert(status.ok());
    printf("Delete key[%s] success.\n", key1.c_str());

    delete db;

    return 0;
}

#include <iostream>

#include <cassert>

#include "rocksdb/db.h"

using namespace std;

int main() {

rocksdb::DB* db;

rocksdb::Options options;

options.create_if_missing = true;

rocksdb::Status status = rocksdb::DB::Open(options, "/tmp/testdb", &db);

assert(status.ok());

cout << "Open rocksdb success." << endl;

string key1 = "1";

string value1 = "aaaaa";

status = db->Put(rocksdb::WriteOptions(), key1, value1);

assert(status.ok());

printf("Put[%s,%s] success.\n", key1.c_str(), value1.c_str());

string value;

status = db->Get(rocksdb::ReadOptions(), key1, &value);

assert(status.ok());

printf("Put key[%s] = %s\n", key1.c_str(), value.c_str());

status = db->Delete(rocksdb::WriteOptions(), key1);

assert(status.ok());

printf("Delete key[%s] success.\n", key1.c_str());

delete db;

return 0;

}

Makefile文件内容：

CXX=g++ -std=c++11
CXXFLAGS=-lrocksdb  # 注意添加链接库

all: main

main: main.cpp
	$(CXX) $(CXXFLAGS) -o $@ $< 

run: main
	./main

clean:
	rm -f main *.o

CXX=g++ -std=c++11

CXXFLAGS=-lrocksdb # 注意添加链接库

all: main

main: main.cpp

$(CXX) $(CXXFLAGS) -o $@ $<

run: main

./main

clean:

rm -f main *.o

运行结果：

make run
g++ -std=c++11 -lrocksdb   -o main main.cpp 
./main
Open rocksdb success.
Put[1,aaaaa] success.
Put key[1] = aaaaa
Delete key[1] success.

make run

g++ -std=c++11 -lrocksdb -o main main.cpp

./main

Open rocksdb success.

Put[1,aaaaa] success.

Put key[1] = aaaaa

Delete key[1] success.

编译时需要注意两点:

支持C11, 添加选项-std=c++11
添加链接库: -lrocksdb

参考链接

RocksDB使用入门 Mac

丰田RAV4 2016款空调滤芯更换以及安装方向

丰田RAV4 2016款空调滤芯更换教程：

1. 不带智能进入和起动系统的车辆：将发动机开关切换至“LOCK”位置。带智能进入和起动系统的车辆：关闭发动机开关。

2. 提起并取出隔板。

3. 拆下手套箱盖。

4. 拆下滤清器盖。

5. 拆下空调滤清器并更换为新滤清器。滤清器上的“↑UP”标记应朝上。

■更换间隔

根据保养计划，检查并更换空调滤清器。在多尘或交通繁忙的地区，更换间隔

可能需要缩短。

■如果通风口的气流明显减弱

滤清器可能堵塞。检查滤清器，如有必要进行更换。

■使用空调系统时

确保滤清器已安装。

使用不带滤清器的空调系统可能会损坏系统。

原厂滤芯的实际安装朝向如下：

博世等标注气流方向的滤芯的实际安装朝向如下：

也就是与原厂的滤芯的箭头方向相反，保证文字都是正的即可。

注意：滤芯是长方形的，原厂的文字是印刻在短边的一侧。买的滤芯文字可能印在了长边一侧，这个时候我们需要转一下方向，不要强行把有文字的一侧露在外面。滤芯是很容易插入的。

参考链接

树莓派3B+/4B报告“low voltage warning”

需求场景

用的好好的树莓派，有一天连接VNC，发现桌面任务栏一直提示

low voltage warning，please check your power supply

1	low voltage warning，please check your power supply

如下图：

看样子是供电不足，可是树莓派的负载并不高啊。

解决方案

搜了一大圈基本都是说电源的问题，现在使用的电源是以前小米2/小米Note手机自带的电源。经过反复测试，发现跟温度有关，冬天的时候问题不大，到了夏天，由于老式电源转换效率低，大量电能被转换成了热量，导致长时间充电后，温度上升很快，引起降压，断电后过一段时间就可以了，但是一旦温度上升到一定程度，问题依旧。

后来买了个努比亚大白20W氮化镓充电器之后，就不会报告这个错误了。氮化镓的充电器转化效率更高，升温更少，并且更耐高温。

由于这个提示只有在连接显示器或VNC连接时才能在GUI桌面任务栏看到，如果是SSH登录就看不到了，非常的不方便。最好是能够通过命令判断是否电压不足。

具体的脚本详见 https://gist.github.com/maxme/d5f000c84a4313aa531288c35c3a8887

通过此脚本可以查看CPU和电压不足的问题。不过每次登录之后需要手动运行，实在不方面。而且脚本中采用了while循环需要手动中断脚本。所以可以把代码中的while循环去掉，并且在SSH登录的时候执行命令病输出到登录信息上。

注意：电压充足的情况下，输出电压为1.2V，而不是 5V，主板的5V是没办法通过脚本直接测量的。我们只能通过脚本测量CPU、内存控制器、内存输入输出、内存本身的电压，主板电压只能用万用表来测量。

效果如下，具体方式参考 ubuntu修改ssh登录提示信息

修改后的raspberry-power-supply-check.sh代码如下:

#!/bin/bash
function throttleCodeMask {
  perl -e "printf \"%s\", $1 & $2 ? \"$3\" : \"$4\""
}
function throttledToText {
  throttledCode=$1
  throttleCodeMask $throttledCode 0x80000 "Soft temperature limit has occurred, " ""
  throttleCodeMask $throttledCode 0x40000 "Throttling has occurred, " ""
  throttleCodeMask $throttledCode 0x20000 "Arm frequency capping has occurred, " ""
  throttleCodeMask $throttledCode 0x10000 "Under-voltage has occurred, " ""
  throttleCodeMask $throttledCode 0x8 "Soft temperature limit active, " ""
  throttleCodeMask $throttledCode 0x4 "Currently throttled, " ""
  throttleCodeMask $throttledCode 0x2 "Arm frequency capped, " ""
  throttleCodeMask $throttledCode 0x1 "Under-voltage detected, " ""
}
temp=$(vcgencmd measure_temp | cut -f2 -d=)
real_clock_speed=$(vcgencmd measure_clock arm | awk -F"=" '{printf ("%0.0f", $2 / 1000000); }' )
sys_clock_speed=$(awk '{printf ("%0.0f",$1/1000); }' </sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq)
voltage=$(vcgencmd measure_volts | cut -f2 -d= | sed 's/000//')
throttled_text=$(throttledToText $(vcgencmd get_throttled | cut -f2 -d=))
echo "$temp $sys_clock_speed / $real_clock_speed MHz $voltage - $throttled_text"

#!/bin/bash

function throttleCodeMask {

perl -e "printf \"%s\", $1 & $2 ? \"$3\" : \"$4\""

}

function throttledToText {

throttledCode=$1

throttleCodeMask $throttledCode 0x80000 "Soft temperature limit has occurred, " ""

throttleCodeMask $throttledCode 0x40000 "Throttling has occurred, " ""

throttleCodeMask $throttledCode 0x20000 "Arm frequency capping has occurred, " ""

throttleCodeMask $throttledCode 0x10000 "Under-voltage has occurred, " ""

throttleCodeMask $throttledCode 0x8 "Soft temperature limit active, " ""

throttleCodeMask $throttledCode 0x4 "Currently throttled, " ""

throttleCodeMask $throttledCode 0x2 "Arm frequency capped, " ""

throttleCodeMask $throttledCode 0x1 "Under-voltage detected, " ""

}

temp=$(vcgencmd measure_temp | cut -f2 -d=)

real_clock_speed=$(vcgencmd measure_clock arm | awk -F"=" '{printf ("%0.0f", $2 / 1000000); }' )

sys_clock_speed=$(awk '{printf ("%0.0f",$1/1000); }' </sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq)

voltage=$(vcgencmd measure_volts | cut -f2 -d= | sed 's/000//')

throttled_text=$(throttledToText $(vcgencmd get_throttled | cut -f2 -d=))

echo "$temp $sys_clock_speed / $real_clock_speed MHz $voltage - $throttled_text"

参考链接

Flutter构建Linux应用

从3.0之后的版本，flutter已经正式支持构建Linux应用

条件

ubuntu 22.04
Android Studio Chipmunk | 2021.2.1
flutter sdk 3.x

方法

1. 打开Linux的平台支持

输入如下指令（如下命令可以使用flutter config查看，2.10以上的版本默认开启）

flutter config --enable-linux-desktop

# 其他平台
flutter config --[no-]enable-macos-desktop

flutter config --[no-]enable-windows-desktop

flutter config --enable-linux-desktop

# 其他平台

flutter config --[no-]enable-macos-desktop

flutter config --[no-]enable-windows-desktop

可以使用flutter doctor查看情况。

安装编译依赖

# 安装Android Studio
$ sudo snap install android-studio --classic

# 安装 Flutter SDK
$ sudo snap install flutter

# 更新 Flutter SDK
$ flutter

$ sudo apt-get install cmake

$ sudo apt-get install ninja-build

$ sudo apt-get install clang

$ sudo apt-get install gtk+-3.0

$ sudo apt-get install libssl-dev

$ sudo apt-get install git

$ sudo apt-get install vim

# 安装Android Studio

$ sudo snap install android-studio --classic

# 安装 Flutter SDK

$ sudo snap install flutter

# 更新 Flutter SDK

$ flutter

$ sudo apt-get install cmake

$ sudo apt-get install ninja-build

$ sudo apt-get install clang

$ sudo apt-get install gtk+-3.0

$ sudo apt-get install libssl-dev

$ sudo apt-get install git

$ sudo apt-get install vim

2. 在旧项目中添加Linux平台支持

在项目地址输入如下指令

flutter create --platforms=linux[,macos,windows] .

1	flutter create --platforms=linux[,macos,windows] .

注意: 项目的名称必须是全小写，如果出现大小写则会报错

"TxxxFxxx" is not a valid Dart package name.

See https://dart.dev/tools/pub/pubspec#name for more information.

"TxxxFxxx" is not a valid Dart package name.

See https://dart.dev/tools/pub/pubspec#name for more information.

3. 构建Linux项目

flutter build linux

1	flutter build linux

4. 报错

Launching lib/main.dart on Linux in debug mode...
Building Linux application...
CMake Error at cmake_install.cmake:66 (file):
  file INSTALL cannot copy file
  "/xxxx/xxxx/build/linux/x64/debug/intermediates_do_not_run/xxx"
  to "/usr/local/xxx": Permission denied.


Exception: Build process failed

Launching lib/main.dart on Linux in debug mode...

Building Linux application...

CMake Error at cmake_install.cmake:66 (file):

file INSTALL cannot copy file

"/xxxx/xxxx/build/linux/x64/debug/intermediates_do_not_run/xxx"

to "/usr/local/xxx": Permission denied.

Exception: Build process failed

出现这个问题

flutter clean //执行这个然后重启 Android Studio

1	flutter clean //执行这个然后重启 Android Studio

参考链接

Flutter构建Windows应用

从2.10之后的版本，flutter已经正式支持构建Windows应用。不过距离实现全平台构建，还需要考虑很多问题，flutter构建的应用更加适合移动端的使用习惯，如果需要构建Windows应用，我的建议是使用一些适合桌面端的widget。

具体方法如下：

条件

需要安装Visual Studio 2019或者Visual Studio 2022，简言之，需要安装Windows 10 SDK。
2.10之前的版本是默认关闭的。Linux和Mac是默认关闭的，可以手动进行打开。

方法

1. 打开Windows的平台支持

PowerShell输入如下指令（如下命令可以使用flutter config查看，2.10以上的版本默认开启）

flutter config --enable-windows-desktop

# 其他平台
flutter config --[no-]enable-macos-desktop

flutter config --[no-]enable-linux-desktop

flutter config --enable-windows-desktop

# 其他平台

flutter config --[no-]enable-macos-desktop

flutter config --[no-]enable-linux-desktop

可以使用flutter doctor查看情况

如果报错：

$ flutter doctor
Flutter assets will be downloaded from https://storage.flutter-io.cn. Make sure you trust this source!
Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, 3.0.1, on Microsoft Windows [版本 10.0.22000.708], locale zh-CN)
[√] Android toolchain - develop for Android devices (Android SDK version 32.0.0)
[√] Chrome - develop for the web
[☠] Visual Studio - develop for Windows (the doctor check crashed)
    X Due to an error, the doctor check did not complete. If the error message below is not helpful, please let us
      know about this issue at https://github.com/flutter/flutter/issues.
    X Exception: Bad UTF-8 encoding (U+FFFD; REPLACEMENT CHARACTER) found while decoding string: [
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          "installationName": "VisualStudio/17.2.3+32526.322",
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          "installationVersion": "17.2.32526.322",
          "productId": "Microsoft.VisualStudio.Product.Community",
          "productPath": "D:\\Program Files\\Microsoft Visual Studio\\2022\\Community\\Common7\\IDE\\devenv.exe",
          "state": 4294967295,
          "isComplete": true,
          "isLaunchable": true,
          "isPrerelease": false,
          "isRebootRequired": false,
          "displayName": "Visual Studio Community 2022",
          "description": "����ǿ��� IDE����ѧ��������Դ��������ߺ͸������ʹ��",
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          "channelUri": "https://aka.ms/vs/17/release/channel",
          "enginePath": "C:\\Program Files (x86)\\Microsoft Visual
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          "thirdPartyNotices": "https://go.microsoft.com/fwlink/?LinkId=661288",
          "updateDate": "2022-06-12T07:42:21.8130931Z",
          "catalog": {
            "buildBranch": "d17.2",
            "buildVersion": "17.2.32526.322",
            "id": "VisualStudio/17.2.3+32526.322",
            "localBuild": "build-lab",
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            "manifestType": "installer",
            "productDisplayVersion": "17.2.3",
            "productLine": "Dev17",
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            "productMilestone": "RTW",
            "productMilestoneIsPreRelease": "False",
            "productName": "Visual Studio",
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          }
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      ]
      . The Flutter team would greatly appreciate if you could file a bug explaining exactly what you were doing
      when this happened:
      https://github.com/flutter/flutter/issues/new/choose
      The source bytes were:
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[√] Android Studio (version 2021.2)
[√] IntelliJ IDEA Ultimate Edition (version 2018.3)
[√] Connected device (4 available)
[√] HTTP Host Availability

! Doctor found issues in 1 category.

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$ flutter doctor

Flutter assets will be downloaded from https://storage.flutter-io.cn. Make sure you trust this source!

Doctor summary (to see all details, run flutter doctor -v):

[√] Flutter (Channel stable, 3.0.1, on Microsoft Windows [版本 10.0.22000.708], locale zh-CN)

[√] Android toolchain - develop for Android devices (Android SDK version 32.0.0)

[√] Chrome - develop for the web

[☠] Visual Studio - develop for Windows (the doctor check crashed)

X Due to an error, the doctor check did not complete. If the error message below is not helpful, please let us

know about this issue at https://github.com/flutter/flutter/issues.

X Exception: Bad UTF-8 encoding (U+FFFD; REPLACEMENT CHARACTER) found while decoding string: [

{

"instanceId": "5cb9d9e1",

"installDate": "2022-06-12T07:42:21Z",

"installationName": "VisualStudio/17.2.3+32526.322",

"installationPath": "D:\\Program Files\\Microsoft Visual Studio\\2022\\Community",

"installationVersion": "17.2.32526.322",

"productId": "Microsoft.VisualStudio.Product.Community",

"productPath": "D:\\Program Files\\Microsoft Visual Studio\\2022\\Community\\Common7\\IDE\\devenv.exe",

"state": 4294967295,

"isComplete": true,

"isLaunchable": true,

"isPrerelease": false,

"isRebootRequired": false,

"displayName": "Visual Studio Community 2022",

"description": "��ǿ�� IDE��ѧ��Դ��ߺ͸��ʹ��",

"channelId": "VisualStudio.17.Release",

"channelUri": "https://aka.ms/vs/17/release/channel",

"enginePath": "C:\\Program Files (x86)\\Microsoft Visual

Studio\\Installer\\resources\\app\\ServiceHub\\Services\\Microsoft.VisualStudio.Setup.Service",

"installedChannelUri": "https://aka.ms/vs/17/release/channel",

"releaseNotes":

"https://docs.microsoft.com/en-us/visualstudio/releases/2022/release-notes-v17.2#17.2.3",

"thirdPartyNotices": "https://go.microsoft.com/fwlink/?LinkId=661288",

"updateDate": "2022-06-12T07:42:21.8130931Z",

"catalog": {

"buildBranch": "d17.2",

"buildVersion": "17.2.32526.322",

"id": "VisualStudio/17.2.3+32526.322",

"localBuild": "build-lab",

"manifestName": "VisualStudio",

"manifestType": "installer",

"productDisplayVersion": "17.2.3",

"productLine": "Dev17",

"productLineVersion": "2022",

"productMilestone": "RTW",

"productMilestoneIsPreRelease": "False",

"productName": "Visual Studio",

"productPatchVersion": "3",

"productPreReleaseMilestoneSuffix": "1.0",

"productSemanticVersion": "17.2.3+32526.322",

"requiredEngineVersion": "3.2.2148.26540"

"properties": {

"campaignId": "2030:fe40db45-8e98-49e0-82b4-afb6aade85b4",

"channelManifestId": "VisualStudio.17.Release/17.2.3+32526.322",

"nickname": "",

"setupEngineFilePath": "C:\\Program Files (x86)\\Microsoft Visual Studio\\Installer\\setup.exe"

}

]

. The Flutter team would greatly appreciate if you could file a bug explaining exactly what you were doing

when this happened:

https://github.com/flutter/flutter/issues/new/choose

The source bytes were:

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184, 246, 200, 203, 195, 226, 183, 209, 202, 185, 211, 195, 34, 44, 13, 10, 32, 32, 32, 32, 34, 99, 104, 97,

110, 110, 101, 108, 73, 100, 34, 58, 32, 34, 86, 105, 115, 117, 97, 108, 83, 116, 117, 100, 105, 111, 46,

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50, 46, 51, 34, 44, 13, 10, 32, 32, 32, 32, 34, 116, 104, 105, 114, 100, 80, 97, 114, 116, 121, 78, 111,

116, 105, 99, 101, 115, 34, 58, 32, 34, 104, 116, 116, 112, 115, 58, 47, 47, 103, 111, 46, 109, 105, 99,

114, 111, 115, 111, 102, 116, 46, 99, 111, 109, 47, 102, 119, 108, 105, 110, 107, 47, 63, 76, 105, 110, 107,

73, 100, 61, 54, 54, 49, 50, 56, 56, 34, 44, 13, 10, 32, 32, 32, 32, 34, 117, 112, 100, 97, 116, 101, 68,

97, 116, 101, 34, 58, 32, 34, 50, 48, 50, 50, 45, 48, 54, 45, 49, 50, 84, 48, 55, 58, 52, 50, 58, 50, 49,

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58, 32, 123, 13, 10, 32, 32, 32, 32, 32, 32, 34, 98, 117, 105, 108, 100, 66, 114, 97, 110, 99, 104, 34, 58,

32, 34, 100, 49, 55, 46, 50, 34, 44, 13, 10, 32, 32, 32, 32, 32, 32, 34, 98, 117, 105, 108, 100, 86, 101,

114, 115, 105, 111, 110, 34, 58, 32, 34, 49, 55, 46, 50, 46, 51, 50, 53, 50, 54, 46, 51, 50, 50, 34, 44, 13,

10, 32, 32, 32, 32, 32, 32, 34, 105, 100, 34, 58, 32, 34, 86, 105, 115, 117, 97, 108, 83, 116, 117, 100,

105, 111, 47, 49, 55, 46, 50, 46, 51, 43, 51, 50, 53, 50, 54, 46, 51, 50, 50, 34, 44, 13, 10, 32, 32, 32,

32, 32, 32, 34, 108, 111, 99, 97, 108, 66, 117, 105, 108, 100, 34, 58, 32, 34, 98, 117, 105, 108, 100, 45,

108, 97, 98, 34, 44, 13, 10, 32, 32, 32, 32, 32, 32, 34, 109, 97, 110, 105, 102, 101, 115, 116, 78, 97, 109,

101, 34, 58, 32, 34, 86, 105, 115, 117, 97, 108, 83, 116, 117, 100, 105, 111, 34, 44, 13, 10, 32, 32, 32,

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32, 70, 105, 108, 101, 115, 32, 40, 120, 56, 54, 41, 92, 92, 77, 105, 99, 114, 111, 115, 111, 102, 116, 32,

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114, 92, 92, 115, 101, 116, 117, 112, 46, 101, 120, 101, 34, 13, 10, 32, 32, 32, 32, 125, 13, 10, 32, 32,

125, 13, 10, 93, 13, 10]

[√] Android Studio (version 2021.2)

[√] IntelliJ IDEA Ultimate Edition (version 2018.3)

[√] Connected device (4 available)

[√] HTTP Host Availability

! Doctor found issues in 1 category.

则需要更新Flutter SDK到3.0.2以上的版本即可，如下：

$ flutter upgrade

1	$ flutter upgrade

具体原因，参考：[windows] Exception: Bad UTF-8 encoding when running flutter doctor or building/running for Windows #102451

2. 在旧项目中添加Windows平台支持

在项目地址输入如下指令

flutter create --platforms=windows[,macos,linux] .

1	flutter create --platforms=windows[,macos,linux] .

注意: 项目的名称必须是全小写，如果出现大小写则会报错

"TxxxFxxx" is not a valid Dart package name.

See https://dart.dev/tools/pub/pubspec#name for more information.

"TxxxFxxx" is not a valid Dart package name.

See https://dart.dev/tools/pub/pubspec#name for more information.

3. 构建Windows项目exe

flutter build windows

1	flutter build windows

参考链接

Android系统中是否开启定位及定位模式的判断

Android系统中包括3中定位模式：

	使用GPS、WLAN和移动网络	使用WLAN和移动网络	仅使用GPS
特点	同时使用GPS、WIFI及基站定位，速度快、精度高，室内定位效果好。缺点：耗流量、耗电量	只使用WIFI和基站定位，需要WIFI或者基站才行，室内效果好。缺点：依赖WIFI或基站，精度一般	不依赖WIFI和基站，室内效果差，户外可靠性好。缺点：室内效果差
代码	LocationManager.GPS_PROVIDER = true； LocationManager.NETWORK_PROVIDER = true;	LocationManager.GPS_PROVIDER = false； LocationManager.NETWORK_PROVIDER = true;	LocationManager.GPS_PROVIDER = true； LocationManager.NETWORK_PROVIDER = false;
是否打开定位服务	Settings.Secure.LOCATION_MODE = 3	Settings.Secure.LOCATION_MODE = 2	Settings.Secure.LOCATION_MODE = 1

使用GPS、WLAN和移动网络

使用WLAN和移动网络

仅使用GPS

特点

同时使用GPS、WIFI及基站定位，速度快、精度高，室内定位效果好。

缺点：耗流量、耗电量

只使用WIFI和基站定位，需要WIFI或者基站才行，室内效果好。

缺点：依赖WIFI或基站，精度一般

不依赖WIFI和基站，室内效果差，户外可靠性好。

缺点：室内效果差

代码

LocationManager.GPS_PROVIDER = true；
LocationManager.NETWORK_PROVIDER = true;

LocationManager.GPS_PROVIDER = false；
LocationManager.NETWORK_PROVIDER = true;

LocationManager.GPS_PROVIDER = true；
LocationManager.NETWORK_PROVIDER = false;

是否打开定位服务

Settings.Secure.LOCATION_MODE = 3

Settings.Secure.LOCATION_MODE = 2

Settings.Secure.LOCATION_MODE = 1

Android系统中检查是否开启了定位服务的代码，参考如下：

    /**
     * 获取设备是否支持GPS定位
     *
     * @param context context
     * @return 如果设备上提供了GPS定位，返回 true ，否则返回false
     */
    public static boolean hasGpsDevice(@NonNull final Context context) {
        final LocationManager lms = (LocationManager) context.getSystemService(android.content.Context.LOCATION_SERVICE);
        if (null != lms) {
            final List<String> providers = lms.getAllProviders();
            if (null != providers) {
                return providers.contains(LocationManager.GPS_PROVIDER);
            }
        }
        return false;
    }

    /**
     * 检查设备是否开启了定位服务
     *
     * @return 定位功能已经开启返回 true,否则返回false
     */
    public static boolean isLocationEnabled(@NonNull final Context context) {
        final LocationManager lms = (LocationManager) context.getSystemService(android.content.Context.LOCATION_SERVICE);
        if (null != lms) {
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
                return lms.isLocationEnabled();
            } else {
                // 目前已知在OPPO Pad Android 11 版本上，设备本身没有GPS设备，此时如果检查GPS定位权限会持续返回 false
                // 导致用户没有办法使用设备，因此我们再没有GPS的设备上，默认直接判断网络定位功能有没有开启
                if (hasGpsDevice(context)) {
                    return lms.isProviderEnabled(LocationManager.GPS_PROVIDER) || lms.isProviderEnabled(LocationManager.NETWORK_PROVIDER);
                }
                return lms.isProviderEnabled(LocationManager.NETWORK_PROVIDER);
            }
        }
        return false;
    }

/**

* 获取设备是否支持GPS定位

* @param context context

* @return 如果设备上提供了GPS定位，返回 true ，否则返回false

public static boolean hasGpsDevice(@NonNull final Context context) {

final LocationManager lms = (LocationManager) context.getSystemService(android.content.Context.LOCATION_SERVICE);

if (null != lms) {

final List<String> providers = lms.getAllProviders();

if (null != providers) {

return providers.contains(LocationManager.GPS_PROVIDER);

}

return false;

}

/**

* 检查设备是否开启了定位服务

* @return 定位功能已经开启返回 true,否则返回false

public static boolean isLocationEnabled(@NonNull final Context context) {

final LocationManager lms = (LocationManager) context.getSystemService(android.content.Context.LOCATION_SERVICE);

if (null != lms) {

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {

return lms.isLocationEnabled();

} else {

// 目前已知在OPPO Pad Android 11 版本上，设备本身没有GPS设备，此时如果检查GPS定位权限会持续返回 false

// 导致用户没有办法使用设备，因此我们再没有GPS的设备上，默认直接判断网络定位功能有没有开启

if (hasGpsDevice(context)) {

return lms.isProviderEnabled(LocationManager.GPS_PROVIDER) || lms.isProviderEnabled(LocationManager.NETWORK_PROVIDER);

}

return lms.isProviderEnabled(LocationManager.NETWORK_PROVIDER);

}

return false;

}

参考链接

苹果ARM SOC M1显微图像

由于PPT需要，特意搜集了一下苹果ARM SOC M1显微图像

继续阅读苹果ARM SOC M1显微图像

进程资源图理解与化简

一了解进程资源图

二化简资源分配图

方法步骤

第一步：先看系统还剩下多少资源没分配，再看有哪些进程是不阻塞（“不阻塞”即：系统有足够的空闲资源分配给它）的
第二步：把不阻塞的进程的所有边都去掉，形成一个孤立的点，再把系统分配给这个进程的资源回收回来
第三步：看剩下的进程有哪些是不阻塞的，然后又把它们逐个变成孤立的点。
第四步：最后，所有的资源和进程都变成孤立的点。这样的图就叫做“可完全简化”。

如果一个图可完全简化，则不会产生死锁；如果一个图不可完全简化（即：图中还有“边”存在），则会产生死锁。这就是“死锁定理”。

实例

解析：

R1有两个资源，一个分配给了P1,一个分配给了P3，此时P2申请R1的资源，因为R1此时没有可用资源，P2堵塞。

R2有三个资源，已经给P1,P2,P3，各自分配了一个资源，而P1此时又再次申请资源R2,P1堵塞

R3有两个资源，已经分配给P2一个，P2申请一个资源，分配给它，所以P3是非阻塞结点

化简的话，看从没有阻塞的结点开始，删去P3周围所有的边，使其成为一个孤立的点，然后看剩下的资源按上述步骤再次进行分配，若到最后只剩下一群孤立的点，则说明该资源图是可以化简的。

参考链接

进程资源图理解与化简

2025 年 9 月
一	二	三	四	五	六	日
1	2	3	4	5	6	7
8	9	10	11	12	13	14
15	16	17	18	19	20	21
22	23	24	25	26	27	28
29	30

mixin 是什么？

小节

如何使用 mixin?

on 的用法

mixin 解决了什么问题？

理解 mixin

使用场景

什么是 BloomFilter

布隆过滤器原理

哈希函数

布隆过滤器数据结构

误判率

特性

添加与查询元素步骤

添加元素

查询元素

优点

缺点

布隆过滤器使用场景和实例

Coding~

自定义的 BloomFilter

Guava 中的 BloomFilter

Redis 中的 BloomFilter

扩展

参考链接

1. RocksDB简要介绍

2. RocksDB 安装

3. 使用示例

编译时需要注意两点:

参考链接

参考链接

需求场景

解决方案

参考链接

条件

方法

1. 打开Linux的平台支持

2. 在旧项目中添加Linux平台支持

3. 构建Linux项目

4. 报错

参考链接

条件

方法

1. 打开Windows的平台支持

2. 在旧项目中添加Windows平台支持

3. 构建Windows项目exe

参考链接

参考链接

一 了解进程资源图

二 化简资源分配图

方法步骤

实例

参考链接

一了解进程资源图

二化简资源分配图