前段时间做了点小东西，随缘记录一下。
在家里日常生活用水是用水塔，通过水泵将水从水井抽到楼顶的水塔用的。这在很常见，通常这些水塔都会有自动液位控制的，一般是一个浮球，机械式的开关。但家里的水塔比较旧没有安装水位控制装置，所以我一直想在水塔添加这样的功能。因为不想用浮球式液位控制，主要是想趁机折腾一下，玩一下技术。最初有些不太实际的脑洞，例如，可不可以通过测量液位变化时水塔的电容反映水位变化；还有根据水压变化计算出液面高度…… 因为不想安装水压测量到水管里，而且还要设法与水泵开关联动，麻烦。大概五年前，上大二的时候，刚学单片机，就想着用 51 做一个东西去根据水塔液位变化自动控制水泵开关。
大概就是这么一个东西，当初用 51 接一个超声波测距模块，还有继电器，联合起来控制水泵。测得距离大于某个值，说明水位低于某个值，就打开水泵抽水，直到距离小于某个值就关掉水泵停止抽水，逻辑很简单的。
不过当时程序可能写得不太好，硬件也没有弄得很好，因为用洞洞板焊接的。那东西在一次暴雨天后就坏了，我弟以为那是遭雷击了，我也不太清楚，反正就坏了。 然后半个月前，我闲着又再次折腾起这东西。这次硬件上使用 Arduino + ESP8266 + 小米路由器 + 小米智能网关 + 加个小米 ZigBee 版智能插座。超声波测距模块由Arduino 驱动，再通过串口发送距离数据到 ESP8266，通过 WiFi TCP 传到小米路由器（跑 OpenWRT)，在小米路由器上的 Linux 写了个程序，基于 ubox (epoll) 的 TCP 和 UDP 客户端。其中 TCP 接收来自 ESP8266 的液位数据，UDP 客户端用于与小米智能网关交互。当水塔液位低于某个水平，这个程序就会向小米智能网关发送命令，去控制米家智能插座打开。这个插座接着水泵，就相当于初版里边的继电器。
之前还弄了个可视化监控的东西，但是后来 PC 机做的服务器崩了，就没有继续玩下去。本来还有个基于 JavaFX 的图形界面，可以查看液位，手动控制开关之类的。 Docker Mosquitto NodeRed Grafana InfluxDB Telegraf adafruit IO

Spring 单例不是 Java 单例。本文讨论 Spring 的单例与单例模式的区别。

前言

单例是 Spring 当中 bean 的默认范围(Scope)。Spring 容器会为某个 bean 定义对象创建唯一的实例，很多时候我们会将这种设计跟《设计模式》(GoF) 书中定义的单例模式作比较。

1. 单例范围 vs 单例模式

Spring 当中的单例范围跟单例模式不是同一个东西。其中的两点差异如下：

• 单例模式确保某个类加载器的某个类只有一个实例
• 而 Spring 单例范围是每个容器的每个bean

  1.1 单例范围的例子

  Spring 的单例实例会被放在缓存中，下次再访问那个命名的 bean 的时候就会从缓存里面取。下边看看例子。

  public class Account {
  
    private String name;
  
    public Account() {
    }
  
    public Account(String name) {
        this.name = name;
    }
  
    public String getName() {
        return name;
    }
  
    @Override
    public String toString() {
        return "Account{" +
                "name='" + name + '\'' +
                '}';
    }
  }
  

  Spring Boot 的 main 方法：

  @SpringBootApplication
  @SpringBootApplication
  public class SpringSingletonApp {
  
    public static void main(String[] args) {
        SpringApplication.run(SpringSingletonApp.class, args);
    }
  
    @Bean(name = "bean1")
    public Account account() {
        return new Account("Test User 1");
    }
  
    @Bean(name = "bean2")
    public Account account1() {
        return new Account("Test User 2");
    }
  }
  

  理解上面的代码：

• 我们创建了同一个类的 2 个实例，并有不同的 bean id。

那么上面代码中 Spring 的 IoC 容器创建了多少个实例？

• 2 个不同的实例，在容器中分别绑定到它们的 id？
• 还是 1 个实例绑定到 2 个 bean id？

1.2 测试用例

我们使用单元测试找出答案。

@SpringBootTest
class SpringSingletonAppTests {

    private static final Logger log = LoggerFactory.getLogger(SpringSingletonAppTests.class);

    @Resource(name = "bean1")
    Account account1;

    @Resource(name = "bean1")
    Account duplicateAccount;

    @Resource(name = "bean2")
    Account account2;

    @Test
    public void testSingletonScope() {
        log.info(account1.getName());
        log.info(account2.getName());

        log.info("Accounts are equal -> {}", account1 == account2);
        log.info("Duplicate account  -> {}", account1 == duplicateAccount);
    }
}

输出为：

20:06:31.165 [main] INFO  i.z.s.SpringSingletonAppTests - Test User 1
20:06:31.165 [main] INFO  i.z.s.SpringSingletonAppTests - Test User 2
20:06:31.165 [main] INFO  i.z.s.SpringSingletonAppTests - Accounts are equal -> false
20:06:31.167 [main] INFO  i.z.s.SpringSingletonAppTests - Duplicate account  -> true

从上面的输出我们发现： Spring 返回了两个不同的实例，单例范围的同一个类可以有多于一个的对象实例。

对于某个 bean id，Spring 容器仅维护唯一的共享单例 bean，在我们上面的例子中，Spring IoC 容器基于同一个类的 bean 定义创建了两个实例，并将它们绑定到对应的 id。 Spring 的 bean 定义就像键值对那样，bean id 就是 key，bean 的实例就是 value。每个 key 引用都会返回同一个 bean 实例（例如 bean1 引用始终返回 id 为 bean1 的 bean）

总结

Spring 单例跟传统的单例模式是不同的。Spring 确保在每个容器对给定 bean id 定义只创建一个 bean 实例。 传统单例模式是保证给定一个类加载器所加载的某个类只有唯一的一个实例。

又再开始读 Spring 源码，做一下笔记，方便自己回顾。

最基础的，关于 BeanFactory 接口，可以认为是容器的根基，其它功能更多的接口都是从这里扩展而来。

@Test
public void testSimpleLoad() {
    BeanFactory bf = new XmlBeanFactory(new ClassPathResource("spring.xml"));
    MyTestBean bean = (MyTestBean) bf.getBean("myTestBean");
    Assertions.assertEquals("testString", bean.getTestStr());
}

上面代码就是最基本的使用。

而对应的 xml 配置文件是这样的:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd">
    <bean id="myTestBean" class="io.zwt.spring001.MyTestBean"/>
</beans>

至于 MyTestBean 只是一个简单的 POJO 就不列出来了。

实际开发中，很少会直接使用 BeanFactory 接口，一般使用 ApplicationContext 等。

从配置文件，到可以使用的 Bean，这个过程里边主要有以下一些步骤：

• 读取配置文件。
• 根据配置文件找到对应的类，并进行实例化。
• 调用实例化后的实例。

15.5 Exercises

Reinforcement

R-15.1 Julia just bought a new computer that uses 64-bit integers to address memory cells. Argue why Julia will never in her life be able to upgrade the main memory of her computer so that it is the maximum-size possible, assuming that you have to have distinct atoms to represent different bits.

R-15.2 Consider an initially empty memory cache consisting of four pages. How many page misses does the LRU algorithm incur on the following page request sequence: (2,3,4,1,2,5,1,3,5,4,1,2,3)?

R-15.3 Consider an initially empty memory cache consisting of four pages. How many page misses does the FIFO algorithm incur on the following page request sequence: (2,3,4,1,2,5,1,3,5,4,1,2,3)?

R-15.4 Consider an initially empty memory cache consisting of four pages. What is the maximum number of page misses that the random algorithm incurs on the following page request sequence: (2,3,4,1,2,5,1,3,5,4,1,2,3)? Show all of the random choices the algorithm made in this case.

R-15.5 Describe, in detail, algorithms for adding an item to, or deleting an item from, an $(a,b)$ tree.

R-15.6 Suppose T is a multiway tree in which each internal node has at least five and at most eight children. For what values of a and b is $T$ a valid $(a,b)$ tree?

R-15.7 For what values of d is the tree $T$ of the previous exercise an order-d B-tree?

R-15.8 Draw the result of inserting, into an initially empty order-7 B-tree, entries with keys (4,40,23,50,11,34,62,78,66,22,90,59,25,72,64,77,39,12), in this order.

Creativity

C-15.9 Describe an efficient external-memory algorithm for removing all the duplicate entries in an array list of size n.

C-15.10 Describe an external-memory data structure to implement the stack ADT so that the total number of disk transfers needed to process a sequence of k push and pop operations is $O(k/B)$.

C-15.11 Describe an external-memory data structure to implement the queue ADT so that the total number of disk transfers needed to process a sequence of k enqueue and dequeue operations is $O(k/B)$.

C-15.12 Describe an external-memory version of the PositionalList ADT (Section 7.3), with block size B, such that an iteration of a list of length n is completed using $O(n/B)$ transfers in the worst case, and all other methods of the ADT require only $O(1)$ transfers.

C-15.13 Change the rules that define red-black trees so that each red-black tree T has a corresponding $(4,8)$ tree, and vice versa.

C-15.14 Describe a modified version of the B-tree insertion algorithm so that each time we create an overflow because of a split of a node w, we redistribute keys among all of w’s siblings, so that each sibling holds roughly the same number of keys (possibly cascading the split up to the parent of w). What is the minimum fraction of each block that will always be filled using this scheme?

C-15.15 Another possible external-memory map implementation is to use a skip list, but to collect consecutive groups of $O(B)$ nodes, in individual blocks, on any level in the skip list. In particular, we define an order-d B-skip list to be such a representation of a skip list structure, where each block contains at least $⌈d/2⌉$ list nodes and at most d list nodes. Let us also choose d in this case to be the maximum number of list nodes from a level of a skip list that can fit into one block. Describe how we should modify the skip-list insertion and removal algorithms for a B-skip list so that the expected height of the structure is $O(logn/logB)$.

C-15.16 Describe how to use a B-tree to implement the Partition ADT (Section 14.7.3) so that the union and find operations each use at most $O(logn/logB)$ disk transfers.

C-15.17 Suppose we are given a sequence S of n elements with integer keys such that some elements in S are colored “blue” and some elements in S are colored “red.” In addition, say that a red element e pairs with a blue element f if they have the same key value. Describe an efficient external-memory algorithm for finding all the red-blue pairs in S. How many disk transfers does your algorithm perform?

C-15.18 Consider the page caching problem where the memory cache can hold m pages, and we are given a sequence P of n requests taken from a pool of $m + 1$ possible pages. Describe the optimal strategy for the offline algorithm and show that it causes at most $m + n/m$ page misses in total, starting from an empty cache.

C-15.19 Describe an efficient external-memory algorithm that determines whether an array of n integers contains a value occurring more than n/2 times.

C-15.20 Consider the page caching strategy based on the least frequently used (LFU) rule, where the page in the cache that has been accessed the least often is the one that is evicted when a new page is requested. If there are ties, LFU evicts the least frequently used page that has been in the cache the longest. Show that there is a sequence P of n requests that causes LFU to miss $Ω(n)$ times for a cache of m pages, whereas the optimal algorithm will miss only $O(m)$ times.

C-15.21 Suppose that instead of having the node-search function $f (d) = 1$ in an order-d B-tree T , we have $f (d) = logd$. What does the asymptotic running time of performing a search in T now become?

Projects

P-15.22 Write a Java class that simulates the best-fit, worst-fit, first-fit, and next-fit algorithms for memory management. Determine experimentally which method is the best under various sequences of memory requests.

P-15.23 Write a Java class that implements all the methods of the sorted map ADT by means of an $(a,b)$ tree, where a and b are integer constants passed as parameters to a constructor.

P-15.24 Implement the B-tree data structure, assuming a block size of 1024 and integer keys. Test the number of “disk transfers” needed to process a sequence of map operations.

1.10 Exercises

Reinforcement

R-1.1 Write a short Java method, inputAllBaseTypes, that inputs a different value of each base type from the standard input device and prints it back to the standard output device.

R-1.2 Suppose that we create an array A of GameEntry objects, which has an integer scores field, and we clone A and store the result in an array B. If we then immediately set A[4].scores equal to 550, what is the score value of the GameEntry object referenced by B[4]?

R-1.3 Write a short Java method, isMultiple, that takes two long values, n and m, and returns true if and only if n is a multiple of m, that is, $n = mi$ for some integer i.

R-1.4 Write a short Java method, isEven, that takes an int i and returns true if and only if i is even. Your method cannot use the multiplication, modulus, or division operators, however.

R-1.5 Write a short Java method that takes an integer n and returns the sum of all positive integers less than or equal to n.

R-1.6 Write a short Java method that takes an integer n and returns the sum of all the odd positive integers less than or equal to n.
R-1.7 Write a short Java method that takes an integer n and returns the sum of the squares of all positive integers less than or equal to n.

R-1.8 Write a short Java method that counts the number of vowel in a given character string.

R-1.9 Write a short Java method that uses a StringBuilder instance to remove all the punctuation from a string s storing a sentence, for example, transforming the string “Let’s try, Mike!” to “Lets try Mike”.

R-1.10 Write a Java class, Flower, that has three instance variables of type String, int, and float, which respectively represent the name of the flower, its number of petals, and price. Your class must include a constructor method that initializes each variable to an appropriate value, and your class should include methods for setting the value of each type, and getting the value of each type.

R-1.11 Modify the CreditCard class from Code Fragment 1.5 to include a method that updates the credit limit.

R-1.12 Modify the CreditCard class from Code Fragment 1.5 so that it ignores any request to process a negative payment amount.

R-1.13 Modify the declaration of the first for loop in the main method in Code Fragment 1.6 so that its charges will cause exactly one of the three credit cards to attempt to go over its credit limit. Which credit card is it?

Creativity

C-1.14 Write a pseudocode description of a method that reverses an array of n integers, so that the numbers are listed in the opposite order than they were before, and compare this method to an equivalent Java method for doing the same thing.

C-1.15 Write a pseudocode description of a method for finding the smallest and largest numbers in an array of integers and compare that to a Java method that would do the same thing.

C-1.16 Write a short program that takes as input three integers, a, b, and c, from the Java console and determines if they can be used in a correct arithmetic formula (in the given order), like “a+b = c,” “a = b - c,” or “a ∗ b = c.”

C-1.17 Write a short Java method that takes an array of int values and determines if there is a pair of distinct elements of the array whose product is even.

C-1.18 The p-norm of a vector v = (v1,v2,…,vn) in n-dimensional space is defined as kvk = qp v1p +v2p +···+vnp. For the special case of p = 2, this results in the traditional Euclidean norm, which represents the length of the vector. For example, the Euclidean norm of a two-dimensional vector with coordinates (4,3) has a Euclidean norm of √42 +32 = √16+9 = √25 = 5. Give an implementation of a method named norm such that norm(v, p) returns the p-norm value of v and norm(v) returns the Euclidean norm of v, where v is represented as an array of coordinates.

C-1.19 Write a Java program that can take a positive integer greater than 2 as input and write out the number of times one must repeatedly divide this number by 2 before getting a value less than 2.

C-1.20 Write a Java method that takes an array of float values and determines if all the numbers are different from each other (that is, they are distinct).

C-1.21 Write a Java method that takes an array containing the set of all integers in the range 1 to 52 and shuffles it into random order. Your method should output each possible order with equal probability.

C-1.22 Write a short Java program that outputs all possible strings formed by using the characters ‘c’, ‘a’, ‘t’, ‘d’, ‘o’, and ‘g’ exactly once.

C-1.23 Write a short Java program that takes two arrays a and b of length n storing int values, and returns the dot product of a and b. That is, it returns an array c of length n such that c[i] = a[i]· b[i], for i = 0,…,n - 1.

C-1.24 Modify the CreditCard class from Code Fragment 1.5 so that printSummary becomes a nonstatic method, and modify the main method from Code Fragment 1.6 accordingly.

C-1.25 Modify the CreditCard class to add a toString() method that returns a String representation of the card (rather than printing it to the console, as done by printSummary). Modify the main method from Code Fragment 1.6 accordingly to use the standard println command.

Projects

P-1.26 Write a short Java program that takes all the lines input to standard input and writes them to standard output in reverse order. That is, each line is output in the correct order, but the ordering of the lines is reversed.

P-1.27 Write a Java program that can simulate a simple calculator, using the Java console as the exclusive input and output device. That is, each input to the calculator, be it a number, like 12.34 or 1034, or an operator, like + or =, can be done on a separate line. After each such input, you should output to the Java console what would be displayed on your calculator.

P-1.28 A common punishment for school children is to write out a sentence multiple times. Write a Java stand-alone program that will write out the following sentence one hundred times: “I will never spam my friends again.” Your program should number each of the sentences and it should make eight different random looking typos.

P-1.29 The birthday paradox says that the probability that two people in a room will have the same birthday is more than half, provided n, the number of people in the room, is more than 23. This property is not really a paradox, but many people find it surprising. Design a Java program that can test this paradox by a series of experiments on randomly generated birthdays, which test this paradox for n = 5,10,15,20,…,100.

P-1.30 (For those who know Java graphical user interface methods:) Define a GraphicalTest class that tests the functionality of the CreditCard class from Code Fragment 1.5 using text fields and buttons.

群

OCA/OCP 考试交流QQ群 157563860

I started with OCA Java SE 8 Programmer I exam last year, it took me 12 days to prepare the OCA exam. It was relatively easy to pass the OCA exam, but it was much harder to pass the OCP 11 exam. It took me roughly 3 months to get fully prepared for the ultimate 1Z0-816, namely Java SE 11 Programmer II exam, for which response to the Oracle Certified Professional: Java SE 11 Developer certification. This is by far the most difficult Java certification exam from Oracle/Sun, not just because it covers topics such as modules, functional programming, concurrent programming, IO. But also it includes some new objectives, like Java Secure Coding Guideline. For those who plan on taking the Oracle Java Certification exam, I strongly recommend you take a look at the official exam objectives before you start your study plan. Buy a good book, I think Selikoff’s book is great, I use that book for my exam preparation. Study the book chapter by chapter, or by topic, or whatever you want. Be sure to do the exercises, it will help you to consolidate your knowledge. It is also helpful to use flashcards to aid the memorization process, for example, some core APIs or some syntax rules. Don’t go directly into the quiz without studying the materials thoroughly, that will just a waste of time and energy. Because that’s very frustrating to see lots of errors. Take your time, start slowly, and gradually level up the difficulty. Below are some useful references.

真题1 某知名互联网下载服务提供商软件工程师笔试题

一、选择题
1. 访问修饰符作用范围由大到小是（ ）。

A.private-protected-default-public
B.public-protected-default-private
C.private-default-protected-public
D.public-default-protected-private

这题没什么好说，当然是选择 B 啦，初学的时候可能有点难记住，不过习惯了就记住了。后来越了解就更容易记住，根本不需要死记硬背。 default 关键字，表示访问权限的时候，其实新的规范（8以上？）改称为 ‘package private’ 可以理解为包内私有访问权限，所以限制程度就是仅次于私有。接着protected和public容易，因为public肯定是范围最宽（大）的。
关于类的访问修饰符的作用范围，Java语言规范的 8.1.1 节有：
The access modifier public pertains only to top level classes and member classes, not to local classes or anonymous classes.
The access modifier protected and private pertain only to member classes within a directly enclosing class declaration.

1. Given

class Booby {
}
class Dooby extends Booby {
}
class Tooby extends Dooby {
}

public class TestClass {
  Booby b = new Booby();
  Tooby t = new Tooby();
  public void do1(List<? super Dooby> dataList) {
    //1 INSERT CODE HERE
  }
  public void do2(List<? extends Dooby> dataList) {
    //2 INSERT CODE HERE
  }
}

and the following four statements:

1. b = dataList.get(0);
2. t = dataList.get(0);
3. dataList.add(b);
4. dataList.add(t);

What can be inserted in the above code?

• Statements 1 and 3 can inserted at //1 and Statements 2 and 4 can be inserted at //2.
• Statement 4 can inserted at //1 and Statement 1 can be inserted at //2.
• Statements 3 and 4 can inserted at //1 and Statements 1 and 2 can be inserted at //2.
• Statements 1 and 2 can inserted at //1 and Statements 3 and 4 can be inserted at //2.
• Statement 1 can inserted at //1 and Statement 4 can be inserted at //2.

Explanation

1. addData1(List<? super Dooby> dataList)
   This means that dataList is a List whose elements are of a class that is either Dooby or a super class of Dooby. We don’t know which super class of Dooby. Thus, if you try to add any object to dataList, it has to be a assignable to Dooby.
   Thus, dataList.add(b); will be invalid because b is not assignable to Dooby.
   Further, if you try to take some object out of dataList, that object will be of a class that is either Dooby or a Superclass of Dooby. Only way you can declare a variable that can be assigned the object retrieved from dataList is Object obj. Thus, t = dataList.get(0); and b = dataList.get(0); are both invalid.

2. addData2(List<? extends Dooby> dataList)
   This means that dataList is a List whose elements are of a class that is either Dooby or a subclass of Dooby. Since we don’t know which subclass of Dooby is the list composed of, there is no way you can add any object to this list.
   If you try to take some object out of dataList, that object will be of a class that is either Dooby or a subclass of Dooby and thus it can be assigned to a variable of class Dooby or its superclass.. Thus, t = dataList.get(0); is invalid.

泛型规则（JLS）
A type argument $T_1$ is said to contain another type argument $T_2$, written $T_2 <= T_1$, is the set of types denoted by $T_2$ is provably a subset of the set of types denoted by $T_1$ under the reflexive and transitive closure of the following rules(where $<:$ denotes subtyping($\S4.10$)):

• $?\space extends\space T<=\space ?\space extends \space S$ if $T <: S$
• $?\space extends \space T<=\space ?$
• $?\space super \space T<=\space ?\space super \space S$ if $T <: S$
• $?\space super \space T<=\space ?$
• $?\space super \space T<=\space ? \space extends \space Object$
• $ T<=\space T$
• $T <= \space ? \space extends \space T$
• $T <= \space ? \space super \space T$

1. Which of the following is an advantage of the Java Platform Module System? B

A. A central repository of all modules
B. Encapsulating packages
C. Encapsulating objects
D. No defined types
E. Platform independence

2. Which statement is true of the following module? D

zoo.staff
|---zoo
|-- staff
|-- Vet.java

A. The directory structure shown is a valid module.
B. The directory structure would be a valid module if module.java were added directly underneath zoo.staff.
C. The directory structure would be a valid module if module.java were added directly underneath zoo.
D. The directory structure would be a valid module if module-info.java were added directly underneath zoo.staff.
E. The directory structure would be a valid module if module-info.java were added directly underneath zoo.
F. None of these changes would make this directory structure a valid module.

解释： Modules are required to have a module-info.java file at the root directory of the module. Option D matches this requirement.

3. B

4. D -> G The -m or –module option is used to specify the module and class name. The -p or -module-path option is used to specify the location of the modules. Option D would be correct if the rest of the command were correct. However, running a program requires specifying the package name with periods (.) instead of slashes. Since the command is incorrect, option G is correct.

5. AF -> AFG Options C and D are incorrect because there is no use keyword. Options A and F are correct because opens is for reflection and uses declares an API that consumes a service. Option G is also correct as the file can be completely empty. This is just something you have to memorize.

6. BDF -> BC Packages inside a module are not exported by default, making option B correct and option A incorrect. Exporting is necessary for other code to use the packages; it is not necessary to call the main() method at the command line, making option C correct and option D incorrect. The module-info.java file has the correct name and compiles, making options E and F incorrect.

7. EF -> DG Options A, B, E, and F are incorrect because they refer to keywords that don’t exist. The requires transitive keyword is used when specifying a module to be used by the requesting module and any other modules that use the requesting module. Therefore, dog needs to specify the transitive relationship, and option G is correct. The module puppy just needs to require dog, and it gets the transitive dependencies, making option D correct.

创建工程

• greetings-service

  在 start.spring.io 选择 2.3.0 M2 版本 Spring Boot，依赖项如下

  Lombok

  RSocket

  Spring Security

• greetings-client

  客户端的依赖项也是

  Lombok

  RSocket

  Spring Security

服务端应用

GreetingsServiceApplication.java