Taogen's Blog

Random Values

Random Integer

new Random().nextInt(min, max+1) return [min, max]

// Java 8
int size = 1, min = 0, max = 10;
ThreadLocalRandom random = ThreadLocalRandom.current();
int randomNumber = random.ints(size, min, max + 1).findFirst().getAsInt();

int size = 5, min = 0, max = 10;
ThreadLocalRandom random = ThreadLocalRandom.current();
int[] randomNumbers = random.ints(size, min, max + 1).toArray();

int size = 5, min = 0, max = 10;
Random random = new Random();
int[] randomNumbers = random.ints(size, min, max + 1).toArray();

// Java 1.7 or later
int min = 0, max = 10;
ThreadLocalRandom random = ThreadLocalRandom.current();
int randomNum = random.nextInt(min, max + 1);

// Before Java 1.7
int min = 0, max = 10;
Random random = new Random();
int randomNum = random.nextInt((max - min) + 1) + min;

Math.random() return [0, 1)

Random.nextInt(n) is more efficient than Math.random() * n

Math.random() uses Random.nextDouble() internally. Random.nextDouble() uses Random.next() twice to generate a double that has approximately uniformly distributed bits in its mantissa, so it is uniformly distributed in the range 0 to 1-(2^-53).

Random.nextInt(n) uses Random.next() less than twice on average- it uses it once, and if the value obtained is above the highest multiple of n below MAX_INT it tries again, otherwise is returns the value modulo n (this prevents the values above the highest multiple of n below MAX_INT skewing the distribution), so returning a value which is uniformly distributed in the range 0 to n-1.

int randomNum = min + (int) (Math.random() * ((max - min) + 1));

Random float

// TODO

Random String

// TODO

Regex Expression

matches() - Checks if the regexp matches the complete string.

String source = "hello there, I am a Java developer. She is a Web developer.";
Pattern pattern = Pattern.compile("a (.*?) developer");
Matcher matcher = pattern.matcher(source);
System.out.println(matcher.matches()); // false

Pattern pattern2 = Pattern.compile(".*a (.*?) developer.*");
Matcher matcher2 = pattern2.matcher(source);
System.out.println(matcher2.matches()); // true

find() - Get matching substrings

String source = "hello there, I am a Java developer. She is a Web developer.";
Pattern pattern = Pattern.compile("a (.*?) developer", Pattern.CASE_INSENSITIVE);
Matcher matcher = pattern.matcher(source);
// find all match strings
while (matcher.find()) {
    System.out.println(matcher.group()); // a Java developer, a Web developer
    for (int i = 1; i <= matcher.groupCount(); i++) {
        System.out.println(matcher.group(i)); // Java, Web
    }
}
// reset matcher for reuse
matcher.reset();
// only find once
if (matcher.find()) {
    System.out.println(matcher.group()); // a Java developer
    System.out.println(matcher.group(1)); // Java
}

Find the first matching string

// find the fist match strings
if (matcher.find()) {
    System.out.println(matcher.group()); // a Java developer, a Web developer
    for (int i = 1; i <= matcher.groupCount(); i++) {
        System.out.println(matcher.group(i)); // Java, Web
    }
}

Replace group string

String source = "hello there, I am a Java developer. She is a Web developer.";
String regex = "a (.*?) developer";
int groupToReplace = 1;
String replacement = "Good";
Matcher matcher = Pattern.compile(regex).matcher(source);
StringBuilder result = new StringBuilder(source);
int adjust = 0;
while (matcher.find()) {
    int start = matcher.start(groupToReplace);
    int end = matcher.end(groupToReplace);
    result.replace(start + adjust, end + adjust, replacement);
    adjust += replacement.length() - (end - start);
}
System.out.println(result); //hello there, I am a Good developer. She is a Good developer.

Non-greedy regular expression

Followed by the ?. For example .*?

Match special characters

[\\special_character]

For example:

[\\(\\)\\[\\]]

Match multiple line string

// Pattern.DOTALL
Pattern.compile("regexStr", Pattern.DOTALL);

Match case-insensitive string

// Pattern.CASE_INSENSITIVE
Pattern.compile("regexStr", Pattern.CASE_INSENSITIVE)

// (?i) enables case-insensitivity
String regexStr = "(?i).*";
src.matches(regexStr);

// convert string cases
src.toLowerCase();
src.matches(regexStr);

Multiple flags

Pattern.compile("regexStr", Pattern.DOTALL | Pattern.CASE_INSENSITIVE);

Java Code Example articles

• Java Code Examples: Variables and Types
• Java Code Examples: Collection and Stream
• Java Code Examples: Utility classes
• Java Code Examples: Concurrency
• Java Code Examples: Common Logic Processing
• Java Code Examples: IO Streams and Files
• Java Code Examples: DateTime

Create a Multi-Modules Project

1. Create Parent Maven Project

$ mvn -B archetype:generate -DgroupId=com.taogen.demo -DartifactId=parent-project -DarchetypeArtifactId=maven-archetype-quickstart -DarchetypeVersion=1.4
# or 
$ mvn archetype:generate -DgroupId=com.taogen.demo -DartifactId=parent-project -DarchetypeArtifactId=maven-archetype-quickstart -DarchetypeVersion=1.4 -DinteractiveMode=false

2. Create submodules

$ cd parent-project
$ mvn -B archetype:generate -DgroupId=com.taogen.demo -DartifactId=web -DarchetypeArtifactId=maven-archetype-quickstart -DarchetypeVersion=1.4
$ mvn -B archetype:generate -DgroupId=com.taogen.demo -DartifactId=service -DarchetypeArtifactId=maven-archetype-quickstart -DarchetypeVersion=1.4

3. Configuring Parent Project’s pom.xml

3.1. Setting <packaging> to pom

$ cd parent-project
$ vim pom.xml

Add the following <packaging> configuration below the <artifactId> tag In the parent-project‘s pom.xml

<packaging>pom</packaging>

By setting the packaging to pom type, we’re declaring that the project will serve as a parent or an aggregator; it won’t produce further artifacts.

3.2. Configuring Parent Submodules

$ cd parent-project
$ vim pom.xml

Add the following <modules> configuration below the <version> tag In the parent-project‘s pom.xml

<modules>
    <module>web</module>
    <module>service</module>
</modules>

4. Configuring Submodule Project’s pom.xml

In the individual submodules’ pom.xml, add the parent-project in the parent section.

$ vim web/pom.xml
$ vim service/pom.xml

Add the following <parent> configuration below the <moduleVersion> tag In the submodule project’s pom.xml

<parent>
  <artifactId>parent-project</artifactId>
  <groupId>com.taogen.demo</groupId>
  <version>1.0-SNAPSHOT</version>
</parent>

Inherent Parent Dependencies Version

1. Declaring dependencies version in the parent project pom.xml

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-dependencies</artifactId>
            <version>2.6.4</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
        
        <dependency>
            <groupId>org.json</groupId>
            <artifactId>json</artifactId>
            <version>20211205</version>
        </dependency>
    </dependencies>
</dependencyManagement>

2. Inherent dependencies version in Submodules pom.xml

By declaring version in the parent, all submodules using only the groupId and artifactId, and the version will be inherited

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
    </dependency>
    <dependency>
        <groupId>org.json</groupId>
        <artifactId>json</artifactId>
    </dependency>
</dependencies>

if a child module needs to use a different version of a managed dependency, you can override the managed version.

<dependencies>
    ...
    <dependency>
        <groupId>org.json</groupId>
        <artifactId>json</artifactId>
        <version>xxx</version>
    </dependency>
    ...
</dependencies>

3. Verifying Inherent dependencies version is working

$ cd parent-project
$ mvn clean package

Dependency Between Submodules

In this tutorial, the web module depends on the service module. You can add service module dependency in web module project pom.xml.

<dependency>
    <groupId>com.taogen.demo</groupId>
    <artifactId>service</artifactId>
    <version>${project.parent.version}</version>
</dependency>

Notice: To avoid a circular dependency between submodules. A circular dependency occurs when a module A depends on another module B, and the module B depends on module A.

Testing in Spring Boot Multi-Modules Maven Project

Test All Submodules

1. Configuring spring boot

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.6.4</version>
    <relativePath/> <!-- lookup parent from repository -->
</parent>

or

<properties>
    <!-- included spring-boot-starter-parent -->
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
    <!-- custom properties -->
    <spring-boot.version>2.6.4</spring-boot.version>
</properties>

<dependencyManagement>
    <dependencies>
        <!-- included spring-boot-starter-parent -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-dependencies</artifactId>
            <version>${spring-boot.version}</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>

<build>
    <plugins>
        <!-- included spring-boot-starter-parent. For unit testing. -->
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-surefire-plugin</artifactId>
            <version>3.0.0-M5</version>
        </plugin>
    </plugins>
</build>

2. Running all submodule unit tests

$ cd parent-project
$ mvn test

References

• Multi-Module Project with Maven - Baeldung
• Creating a Multi Module Project - Spring Doc
• Spring Boot Maven Plugin Documentation - Spring Doc
• spring-boot-starter-parent
  • Spring Boot Dependency Management
  • 你真的理解 Spring Boot 项目中的 parent 吗？
• Test
  • Testing the Web Layer - Spring Doc

Gotenberg is a great open source project for converting files to PDF.

Starting Up Gotenberg

docker run --rm -p 3000:3000 gotenberg/gotenberg:7

# Run container in background
docker run --rm -d -p 3000:3000 gotenberg/gotenberg:7

View console output of Gotenberg

docker container ls
docker logs -f <containerId>

API Usage

Health Check

http://localhost:3000/health

Chromium

The Chromium module interacts with the Chromium browser to convert HTML documents to PDF.

Converting HTML URLs to PDF

curl \
--request POST 'http://localhost:3000/forms/chromium/convert/url' \
--form 'url="https://my.url"' \
-o my.pdf

Converting local HTML files to PDF

curl \
--request POST 'http://localhost:3000/forms/chromium/convert/html' \
--form 'files=@"/file/path/to/index.html"' \
-o my.pdf

LibreOffice

The LibreOffice module interacts with LibreOffice to convert documents to PDF.

To convert documents to PDF

Files with the following extensions:

.bib` `.doc` `.xml` `.docx` `.fodt` `.html` `.ltx` `.txt` `.odt` `.ott` `.pdb` `.pdf` `.psw` `.rtf` `.sdw` `.stw` `.sxw` `.uot` `.vor` `.wps` `.epub` `.png` `.bmp` `.emf` `.eps` `.fodg` `.gif` `.jpg` `.met` `.odd` `.otg` `.pbm` `.pct` `.pgm` `.ppm` `.ras` `.std` `.svg` `.svm` `.swf` `.sxd` `.sxw` `.tiff` `.xhtml` `.xpm` `.fodp` `.potm` `.pot` `.pptx` `.pps` `.ppt` `.pwp` `.sda` `.sdd` `.sti` `.sxi` `.uop` `.wmf` `.csv` `.dbf` `.dif` `.fods` `.ods` `.ots` `.pxl` `.sdc` `.slk` `.stc` `.sxc` `.uos` `.xls` `.xlt` `.xlsx` `.tif` `.jpeg` `.odp

Converting local document files to PDF

Relative file path

curl \
--request POST 'http://localhost:3000/forms/libreoffice/convert' \
--form 'files=@"test.docx"' \
-o test.pdf

Absolute file path

curl \
--request POST 'http://localhost:3000/forms/libreoffice/convert' \
--form 'files=@"D:\My Workspace\test.docx"' \
-o test.pdf

Multiple files

curl \
--request POST 'http://localhost:3000/forms/libreoffice/convert' \
--form 'files=@"/path/to/file.docx"' \
--form 'files=@"/path/to/file.xlsx"' \
-o my.zip

Request by Java OkHttp

private static final String GOTENBERG_SERVICE_HOST = "http://xxx.xxx.xxx.xxx:3000";

private static final String CONVERT_TO_PDF_URI = "/forms/libreoffice/convert";

private static final OkHttpClient OKHTTP_CLIENT = new OkHttpClient.Builder()
    .connectTimeout(20, TimeUnit.SECONDS)
    .readTimeout(15, TimeUnit.SECONDS)
    .writeTimeout(15, TimeUnit.SECONDS)
    .build();

public static byte[] convertToPdfByGotenberg(InputStream inputStream, String fileName) throws IOException {
    Request.Builder requestBuilder = new Request.Builder()
        .url(HttpUrl.parse(GOTENBERG_SERVICE_HOST + CONVERT_TO_PDF_URI).newBuilder().build());
    String mediaType = URLConnection.guessContentTypeFromName(fileName);
    if (mediaType == null) {
        mediaType = "application/octet-stream";
    }
    MultipartBody.Builder builder = new MultipartBody.Builder()
        .setType(MultipartBody.FORM)
        .addFormDataPart("files", fileName,
                         RequestBody.create(MediaType.parse(mediaType),
                                            IOUtils.toByteArray(inputStream)));
    requestBuilder.post(builder.build());
    try (Response response = OKHTTP_CLIENT.newCall(requestBuilder.build()).execute()) {
        return response.body().bytes();
    }
}

PDF Engines

Merge multiple PDF files to one PDF file.

curl \
--request POST 'http://localhost:3000/forms/pdfengines/merge' \
--form 'files=@"/path/to/pdf1.pdf"' \
--form 'files=@"/path/to/pdf2.pdf"' \
--form 'files=@"/path/to/pdf3.pdf"' \
--form 'files=@"/path/to/pdf4.pdf"' \
-o my.pdf

Webhook

The Webhook module provides a middleware that allows you to upload the output file from multipart/form-data routes to the destination of your choice.

curl \
--request POST 'http://localhost:3000/forms/chromium/convert/url' \
--header 'Gotenberg-Webhook-Extra-Http-Headers: {"MyHeader": "MyValue"}' \
--header 'Gotenberg-Webhook-Url: https://my.webhook.url' \
--header 'Gotenberg-Webhook-Method: PUT' \
--header 'Gotenberg-Webhook-Error-Url: https://my.webhook.error.url' \
--header 'Gotenberg-Webhook-Error-Method: POST' \
--form 'url="https://my.url"'

The middleware reads the following headers:

• Gotenberg-Webhook-Url - the callback to use - required
• Gotenberg-Webhook-Error-Url - the callback to use if error - required
• Gotenberg-Webhook-Method - the HTTP method to use (POST, PATCH, or PUT - default POST).
• Gotenberg-Webhook-Error-Method - the HTTP method to use if error (POST, PATCH, or PUT - default POST).
• Gotenberg-Webhook-Extra-Http-Headers - the extra HTTP headers to send to both URLs (JSON format).

Common Errors

Error: file name is too long

{"level":"error","ts":1649232587.172472,"logger":"api","msg":"create request context: copy to disk: create local file: open /tmp/9e10e36d-c5a9-4623-9fac-92db4a0d0982/xxx.doc: file name too long","trace":"de0b5ce2-5a99-406e-a61d-4abb65ef0294","remote_ip":"xxx.xxx.xxx.xxx","host":"xxx.xxx.xxx.xxx:3000","uri":"/forms/libreoffice/convert","method":"POST","path":"/forms/libreoffice/convert","referer":"","user_agent":"okhttp/4.9.3","status":500,"latency":13161094736,"latency_human":"13.161094736s","bytes_in":6983097,"bytes_out":21}

Solutions

Decreasing your file name length.

Error: file name contains UTF-8 characters

{"level":"error","ts":1649234692.9638329,"logger":"api","msg":"convert to PDF: unoconv PDF: unix process error: wait for unix process: exit status 6","trace":"28d9a196-10e5-4c7d-af6a-178494f49cd1","remote_ip":"xxx.xxx.xxx.xxx","host":"xxx.xxx.xxx.xxx:3000","uri":"/forms/libreoffice/convert","method":"POST","path":"/forms/libreoffice/convert","referer":"","user_agent":"okhttp/4.9.3","status":500,"latency":130617774,"latency_human":"130.617774ms","bytes_in":11559,"bytes_out":21}

Solutions

Encoding filename by URLEncoder.encode(fileName, "UTF-8").

Error: file extension name is not right

{"level":"error","ts":1649234777.9096093,"logger":"api","msg":"validate form data: no form file found for extensions: [.bib .doc .xml .docx .fodt .html .ltx .txt .odt .ott .pdb .pdf .psw .rtf .sdw .stw .sxw .uot .vor .wps .epub .png .bmp .emf .eps .fodg .gif .jpg .jpeg .met .odd .otg .pbm .pct .pgm .ppm .ras .std .svg .svm .swf .sxd .sxw .tif .tiff .xhtml .xpm .odp .fodp .potm .pot .pptx .pps .ppt .pwp .sda .sdd .sti .sxi .uop .wmf .csv .dbf .dif .fods .ods .ots .pxl .sdc .slk .stc .sxc .uos .xls .xlt .xlsx]","trace":"10e6ffd8-2d00-4374-b6fb-0c4ff6af3043","remote_ip":"xxx.xxx.xxx.xxx","host":"xxx.xxx.xxx.xxx:3000","uri":"/forms/libreoffice/convert","method":"POST","path":"/forms/libreoffice/convert","referer":"","user_agent":"okhttp/4.9.3","status":400,"latency":3885166,"latency_human":"3.885166ms","bytes_in":11551,"bytes_out":449}

Solutions

Files in form data should have a correct file extension.

References

Gotenberg Documentation

To get a good job, to become a good developer first.

Keep Learning

• Reading textbooks, documentation.
• Reading open-source code.
• Writing blogs.

Practice Your Craft

• Design yourself systems.
• Build yourself systems.
• Optimizes systems.
• Solving coding problems on LeetCode.

Build Your Reputations

• Contributing to open-source projects on GitHub.
• Answering questions on Stack Overflow.
• Keep writing technical blogs on your website.

What is Empty Cache and Hard Reload for Specific Domain

Clear current domain website cached static files, like HTML, CSS, JS, and images files.

Note: “Hard reload” is only clear current page cached static files not current domain. For current domain we need use “Empty Cache and Hard Reload”

Empty Cache and Hard Reload

Chrome

Method One

1. Press control+shift+I or F12 to open Chrome Developer Tools.
2. Right-click the reload button next to the address bar.
3. Choose: “Empty Cache and Hard Reload”.

Method Two

1. Press control+shift+I or F12 to open Chrome Developer Tools.
2. Under Network (previously General) check Disable cache. (re-enable caching by un-checking this box)
3. Reload the page.

Solution 1: Setting Proxy

First, use a US proxy instead of others, like HK.

Setting HTTP Proxy for git clone by HTTPS URL

setting HTTP proxy for git

git config --global http.proxy socks5h://127.0.0.1:1080
# or just proxy GitHub
git config --global http.https://github.com.proxy socks5h://127.0.0.1:1080

or temporarily setting HTTP proxy for your terminal

// linux
export ALL_PROXY=socks5://127.0.0.1:1080
// windows
set ALL_PROXY=socks5://127.0.0.1:1080

Test Case

Before Setting Proxy: about 200 KiB/s.

git clone https://github.com/mybatis/mybatis-3.git mybatis-clone-test
Cloning into 'mybatis-clone-test'...
remote: Enumerating objects: 397085, done.
remote: Counting objects: 100% (195/195), done.
remote: Compressing objects: 100% (79/79), done.
Receiving objects:   4% (15884/397085), 5.25 MiB | 197.00 KiB/s

After Setting Proxy: about 3 MiB/s.

git clone https://github.com/mybatis/mybatis-3.git mybatis-clone-test
Cloning into 'mybatis-clone-test'...
remote: Enumerating objects: 397085, done.
remote: Counting objects: 100% (195/195), done.
remote: Compressing objects: 100% (79/79), done.
Receiving objects:  14% (55592/397085), 18.63 MiB | 3.16 MiB/s

Proxy types

• Use VPN client: {your_protocol}://127.0.0.1:{your_port}. for example, use client socks proxy socks5h://127.0.0.1:1080.

• Use HTTP proxy server: address:port. For example: http://proxyuser:proxypwd@proxy.server.com:8080

or Setting SSH Proxy for git clone by SSH URL

On Windows, add the following content to your git SSH config file ~/.ssh/config:

ProxyCommand "C:\Program Files\Git\mingw64\bin\connect.exe" -S 127.0.0.1:1080 %h %p

Host github.com
  User git
  Port 22
  Hostname github.com
  IdentityFile "C:\Users\Taogen\.ssh\id_ed25519"
  TCPKeepAlive yes
  IdentitiesOnly yes

Host ssh.github.com
  User git
  Port 443
  Hostname ssh.github.com
  IdentityFile "C:\Users\Taogen\.ssh\id_ed25519"
  TCPKeepAlive yes
  IdentitiesOnly yes

You can put the ProxyCommand ... under a specific Host xxx.xxx.xxx like this if you need to access multiple git hosting sites but set an SSH proxy for only one site.

Host ssh.github.com
	ProxyCommand "C:\Program Files\Git\mingw64\bin\connect.exe" -S 127.0.0.1:1080 %h %p
	...

Git SSH config file path

• Windows:
  • per user SSH configuration file: ~/.ssh/config
  • system SSH configuration file: C:\ProgramData\ssh\ssh_config
• Linux:
  • per user SSH configuration file: ~/.ssh/config
  • system SSH configuration file: /etc/ssh/ssh_config

Git connect program

• Windows: {git_install_path}\Git\mingw64\bin\connect.exe (You can get the file path by running the command line where connect.exe)
• Linux: /usr/bin/nc (You can get the file path by running the command line whereis nc)

Solution 2: Switching from using SSH to HTTPS

Using SSH: about 20 KiB/s

git clone git@github.com:xxx/xxx.git
Receiving objects:   0% (309/396867), 172.01 KiB | 26.00 KiB/s

Using HTTPS: about 400 KiB/s

git clone https://github.com/xxx/xxx.git
Receiving objects: 100% (396867/396867), 113.49 MiB | 402.00 KiB/s, done.

Other Improvements

git config --global http.postbuffer 524288000
git config --global credential.helper "cache --timeout=86400"

References

[1] GitHub 加速终极教程

[2] SSH in git behind proxy on windows 7

[3] ssh_config - OpenBSD

[4] nc - OpenBSD

Running the project

How to configure your local environment to run the project.

How the project was designed

Business Architecture Design

• Business Process
• System Function Structure

Application(System) Architecture Design

Type: Monolithic, Distributed, SOA

Division

• Horizontal division: frontend, middle server, backend server.

• Vertical division: subsystems.

Technical Architecture Design

Technology Stack

Techniques of every layer.

Layers: Persistence layer, data layer, logic layer, application layer, view layer.

Database Design

Interface Design

Implementation of Core Functions

Data Access

Data Source configuration, encapsulation, use

Redis configuration, encapsulation, use

Elasticsearch configuration, encapsulation, use

System Core Functions

User, role, privilege, and menu

Login authentication and authorization

Exception Handler

Logging

File upload and download

Data import and export

Scheduling Tasks

i18n

Code generation

A Module’s CRUD Function Implementation

How to Develop

Git Management

Project File Structure

The process of developing a module

Development Specification

Developing a New Module

Create a database table.

Write module source code.

Running project.

Test module functions.

Java

Constants

java.lang.Math

• E
• PI

java.net.HttpURLConnection

• HTTP_OK
• HTTP_NO_CONTENT
• HTTP_BAD_REQUEST
• HTTP_UNAUTHORIZED
• HTTP_FORBIDDEN
• HTTP_NOT_FOUND
• HTTP_BAD_METHOD
• HTTP_INTERNAL_ERROR

javax.management.timer.Timer

• ONE_DAY
• ONE_HOUR
• ONE_MINUTE
• ONE_SECOND
• ONE_WEEK

Enum

java.nio.charset.StandardCharsets

• ISO_8859_1
• UTF_8.toString()

Utilities

java.util

• Arrays
  • asList(T… a)
  • sort(int[] a)
  • toString(Object[] a)
• Collections
  • emptyList()
  • emptyMap()
  • emptySet()
  • sort(List list)
  • sort(List list, Comparator<? super T> c)
• Objects
  • nonNull()
  • isNull(Object obj)
  • equals(Object a, Object b)
  • deepEquals(Object a, Object b)
  • compare(T a, T b, Comparator<? super T> c)
  • requireNonNull(T obj, String message)
  • toString(Object o, String nullDefault)
• UUID
  • randomUUID()

Others

• Optional
• Random
• StringJoiner

Java Servlet

Constants

jakarta.servlet.http.HttpServletResponse

• SC_OK
• SC_CREATED
• SC_NO_CONTENT
• SC_BAD_REQUEST
• SC_UNAUTHORIZED
• SC_FORBIDDEN
• SC_NOT_FOUND
• SC_INTERNAL_SERVER_ERROR

Spring Framework

Constants

org.springframework.http.HttpHeaders

• ACCEPT_LANGUAGE
• AUTHORIZATION
• CACHE_CONTROL
• CONTENT_DISPOSITION
• CONTENT_TYPE
• USER_AGENT

org.springframework.http.MediaType

• APPLICATION_JSON_VALUE
• APPLICATION_FORM_URLENCODED_VALUE
• MULTIPART_FORM_DATA_VALUE

Enum

org.springframework.http.HttpMethod

• GET.toString()
• POST
• PUT
• DELETE

org.springframework.http.HttpStatus

• OK
• NO_CONTENT
• BAD_REQUEST
• UNAUTHORIZED
• FORBIDDEN
• NOT_FOUND
• METHOD_NOT_ALLOWED
• INTERNAL_SERVER_ERROR

Utilities

org.springframework.util

• StringUtils
  • hasLength(String str)
• CollectionUtils
  • isEmpty(Collection<?> collection)
• ClassUtils

org.springframework.beans

• BeanUtils
  • copyProperties(Object source, Object target)

References

[1] Spring Framework Constant Field Values

[2] Java 8 Constant Field Values

企业微信应用介绍

企业微信应用开发是借助企业微信提供的API，开发针对企业微信平台的应用。企业微信用户可以通过企业微信客户端提供的企业微信应用入口来找到应用。企业微信客户端有 iOS，Android，Windows 等版本，用户可以在多个端同时使用。

企业微信应用按照API接口类型分为：

• 企业内部应用。仅应用开发的企业内部微信用户可以使用的应用。
• 第三方应用。所有企业的微信用户可以使用的应用。该应用在企业微信第三方应用市场可以找到。
• 智慧软硬件应用。面向智能硬件厂商，基于企业微信提供的硬件SDK，升级硬件能力，并提供软硬一体的场景化方案。所有企业的微信用户可以使用的应用。该应用在企业微信第三方应用市场可以找到。

这三种应用的大部分API是相同的，小部分API不同或者仅某种应用可以使用。

按照应用场景分为：

• H5应用。网页类型的应用。
• 小程序应用。
• 群聊机器人。通过机器人，企业应用可以主动向群聊内发送多种类型的消息。
• 管理和辅助类型应用。

企业微信应用的基本使用：

1. 企业微信H5应用（微信用户主动使用该应用）。微信用户在企业微信客户端找到应用入口，进入应用的首页，企业微信用户使用应用页面上的功能。
2. 管理和辅助类型应用（非微信用户使用或被动使用应用）。如通讯录管理、消息推送等。应用的管理后台主动触发相关功能。

企业微信应用开发

企业微信应用涉及的交互：

• 企业微信客户端调用应用服务器的API
• 应用服务器调用企业微信的API
• 企业微信回调应用服务器API。

企业内部应用（自建应用）开发流程

1. 注册企业微信。获取 corpId (Company ID)。
2. 创建应用。登录企业微信管理后台，在应用管理中创建自建应用。上传 App Logo 和输入 App Name。得到 AgentId 和 Secret。
3. 配置应用（非 H5 应用不需要配置）。企业微信管理后台的应用详情页面，1）配置应用主页，2）设置可信域名。
4. 开发应用。通过调用企业微信服务端 API 实现相关业务功能。
5. 将企业微信应用部署到自己的服务器。

Appendixes

• 企业微信官网 （注册创建企业微信账号）
• 企业微信 服务商官网 （服务商后台）
• 企业微信 开发者中心 （开发教程、接口文档、工具资源）
  • 服务端 API 调试工具
• WxJava - GitHub. 微信开发 Java SDK ，支持包括微信支付，开放平台，小程序，企业微信，公众号等的后端开发

HTML pages referred static files (images, js and css etc) can be cached in your browser by setting HTTP response header attributes about cache.

Two main types of cache headers, cache-control and expires, define the caching characteristics for your resources. Typically, cache-control is considered a more modern and flexible approach than expires, but both headers can be used simultaneously.

Cache headers are applied to resources at the server level – for example, in the .htaccess file on an Apache server, used by nearly half of all active websites – to set their caching characteristics. Caching is enabled by identifying a resource or type of resource, such as images or CSS files, and then specifying headers for the resource(s) with the desired caching options.

Setting HTTP cache in Spring framework

Setting HTTP response cache-control header in Spring framework

return ResponseEntity.ok()
    .cacheControl(CacheControl.maxAge(365, TimeUnit.DAYS)
                  .cachePrivate()
                  .mustRevalidate())
    .contentType(MediaType.parseMediaType("application/octet-stream"))
    .header(HttpHeaders.CONTENT_DISPOSITION, "inline; filename=\"" + fileName + "\"")
    .body(resource);

Setting HTTP cache in Nginx Server

Only set HTTP Cache-Control header for HTTP directly response by Nginx, not proxy_pass server HTTP responses. In other words, request static files via the server’s file path. For example:

• expires 30d;
• add_header Cache-Control “public, no-transform”;

conf/nginx.conf

server {
    listen       80;
    server_name  localhost;
    location / {
        autoindex on;
        root   html;
        index  index.html index.htm;
    }
    location ~* \.(js|css|png|jpg|jpeg|gif|svg|ico)$ {
        root /root/upload
        expires 30d;
        add_header Cache-Control "public, no-transform";
    }
}

Cache-Control Directives

Cache

• Cache types: private / public
• Expired time: max-age / s-maxage
• validated-related:
  • no-cache
  • must-revalidate/proxy-revalidate: Indicates that once a resource becomes stale, caches must not use their stale copy without successful validation on the origin server.
  • stale-while-revalidate
  • stale-if-error
  • no-transform
  • immutable: Indicates that the response body will not change over time. The resource, if unexpired, is unchanged on the server and therefore the client should not send a conditional revalidation for it.

Examples:

Cache-Control: public, max-age=604800, must-revalidate

No Cache

• no-store or max-age=0

Note that no-cache does not mean “don’t cache”. no-cache allows caches to store a response but requires them to revalidate it before reuse. If the sense of “don’t cache” that you want is actually “don’t store”, then no-store is the directive to use.

Examples

Cache-Control: no-store

Cache-Control Directive Details

Max Age for Files

• ico/jpg/jpeg/png/gif: max-age=2592000 seconds (30 days) or max-age=31536000 (365 days)
• pdf: max-age=2592000 seconds (30 days)
• css/js: max-age=86400 seconds (1 hours) or max-age=2592000 seconds (30 days)

References

• Caching in HTTP
• HTTP caching - MDN Web Docs
• HTTP Caching - Google Developers
• Guide for Caching and HTTP Cache Headers for Static Content
• Prevent unnecessary network requests with the HTTP Cache
• Cache-Control
  • Cache-Control - MDN Web Docs
  • Cache Headers in Spring MVC