這篇文章主要目的是用來介紹plugin，為了對本身非常枯燥的內容進行說明，這裡特意跟前面的文章一樣，準備了一個更加枯燥的例子（如何使用gradle的java plugin來打出一個jar檔案），在枯燥的的gradle的學習的旅程中新增一絲不那麼煩躁的情緒。

gradle的plugin是什麼

回答是什麼的定義往往比較複雜，現在我們知道gradle是什麼了，一個自動構建工具，但是它是如何支援java和groovy的編譯構建呢？這是一個很好的問題，因為答案就是plugin。
不同的plugin提供了不同的功能，從這個角度上來說，gradle就是一個框架，而具體實現則是通過plugin來實現的，從某種意義上來說，maven的設計思路也是如出一轍。
我們不再談論容易引起爭執的設計思路是否一致的問題，至少下面的內容可以使得我們對於gradle的plugin的理解有一個開始：

gradle提供了一個用於構建的框架結構，具體的實際的構建通過不同的plugin來實現。

gradle有哪些plugin

相較於是什麼的問題，有什麼就沒有任何分歧了，在當前穩定的gradle的4.10.2版本，gradle官方提供的plugin主要有如下這些：

試驗示例

這篇文章將通過使用java plugin來對一個簡單的java應用進行jar的打包，來說明在gradle中plugin的使用方式。

事前準備

做一個簡單的HelloWorld的java類，然後將其打成一個jar檔案，具體的檔案結構如下所示

liumiaocn:plugin liumiao$ ls
build.gradle    settings.gradle src
liumiaocn:plugin liumiao$ tree .
.
├── build.gradle
├── settings.gradle
└── src
    └── main
        ├── java
        │   └── com
        │       └── liumiaocn
        │           └── HelloPlugin.java
        └── resources
            └── com
                └── liumiaocn
                    └── resource.xml

8 directories, 4 files
liumiaocn:plugin liumiao$ 

settings.gradle

liumiaocn:plugin liumiao$ cat settings.gradle 
rootProject.name = 'jarPluginSample'
liumiaocn:plugin liumiao$ 

• rootProject.name在建立jar檔案時根據約定會被使用。

build.gradle

liumiaocn:plugin liumiao$ cat build.gradle 
apply plugin: 'java'
version = '1.0'

jar {
    manifest {
        attributes 'Implementation-Title': 'Gradle plugin sample: create jar distribution',
                   'Implementation-Version': version,
                   'Created-By': 'liumiaocn'
    }
}
liumiaocn:plugin liumiao$ 

• apply plugin: ‘java’：是引入java plugin支援唯一需要做的事情
• version：在生成的jar檔案中會按照約定生成在jar檔名中
• jar{}: jar是java plugin所實現的task之一，這裡使用它直接用來生成jar檔案
• manifest{}：用於設定MANIFEST.MF的內容

resource檔案

liumiaocn:plugin liumiao$ cat src/main/resources/com/liumiaocn/resource.xml 
<?xml version="1.0" encoding="UTF-8"?>
liumiaocn:plugin liumiao$ 

• 僅僅是一個示例，用於演示在jar檔案中資原始檔也被打包的過程

HelloWorld類

liumiaocn:plugin liumiao$ cat src/main/java/com/liumiaocn/HelloPlugin.java 
package com.liumiaocn;

public class HelloPlugin {
    private final String pluginName;

    public HelloPlugin(String name) {
        this.pluginName= name;
    }

    public void grettings() {
      System.out.println("Hello, " + this.pluginName);
    } 
}
liumiaocn:plugin liumiao$

確認project資訊

liumiaocn:plugin liumiao$ gradle projects

> Task :projects

------------------------------------------------------------
Root project
------------------------------------------------------------

Root project 'jarPluginSample'
No sub-projects

To see a list of the tasks of a project, run gradle <project-path>:tasks
For example, try running gradle :tasks

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed
liumiaocn:plugin liumiao$ 

打包過程

使用普通方式的jar檔案打包，過程繁瑣的多，而這裡，只需要gradle jar即可

liumiaocn:plugin liumiao$ ls
build.gradle    settings.gradle src
liumiaocn:plugin liumiao$ gradle jar

BUILD SUCCESSFUL in 0s
3 actionable tasks: 3 executed
liumiaocn:plugin liumiao$

• 結果確認
  可以看到jar檔案已經生成在lib下了

liumiaocn:plugin liumiao$ ls
build           build.gradle    settings.gradle src
liumiaocn:plugin liumiao$ tree build
build
├── classes
│   └── java
│       └── main
│           └── com
│               └── liumiaocn
│                   └── HelloPlugin.class
├── libs
│   └── jarPluginSample-1.0.jar
├── resources
│   └── main
│       └── com
│           └── liumiaocn
│               └── resource.xml
└── tmp
    ├── compileJava
    └── jar
        └── MANIFEST.MF

13 directories, 4 files
liumiaocn:plugin liumiao$

• 確認jar檔案內容

liumiaocn:plugin liumiao$ zipinfo build/libs/jarPluginSample-1.0.jar 
Archive:  build/libs/jarPluginSample-1.0.jar
Zip file size: 1292 bytes, number of entries: 6
drwxr-xr-x  2.0 unx        0 b- defN 18-Nov-24 06:49 META-INF/
-rw-r--r--  2.0 unx      146 b- defN 18-Nov-24 06:49 META-INF/MANIFEST.MF
drwxr-xr-x  2.0 unx        0 b- defN 18-Nov-24 06:49 com/
drwxr-xr-x  2.0 unx        0 b- defN 18-Nov-24 06:49 com/liumiaocn/
-rw-r--r--  2.0 unx      758 b- defN 18-Nov-24 06:49 com/liumiaocn/HelloPlugin.class
-rw-rw-r--  2.0 unx       39 b- defN 18-Nov-24 06:49 com/liumiaocn/resource.xml
6 files, 943 bytes uncompressed, 606 bytes compressed:  35.7%
liumiaocn:plugin liumiao$ 

注：jar就是一個壓縮檔案，沒有zipinfo，你可以使用大部分解壓軟體均可看到內部內容

• 確認MANIFEST.MF

liumiaocn:plugin liumiao$ cat build/tmp/jar/MANIFEST.MF 
Manifest-Version: 1.0
Implementation-Title: Gradle plugin sample: create jar distribution
Implementation-Version: 1.0
Created-By: liumiaocn

liumiaocn:plugin liumiao$ 

其他task

除了jar之外，比如gradle clean的task也被實現了，其實就是刪除約定方式申城的構建目錄build等。

liumiaocn:plugin liumiao$ ls
build           build.gradle    settings.gradle src
liumiaocn:plugin liumiao$ gradle clean

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed
liumiaocn:plugin liumiao$ ls
build.gradle    settings.gradle src
liumiaocn:plugin liumiao$

除此之外其他的task以及其關聯關係，這篇文章不再說明，在後續進一步介紹之後再行展開。

liumiaocn:plugin liumiao$ gradle tasks

> Task :tasks

------------------------------------------------------------
All tasks runnable from root project
------------------------------------------------------------

Build tasks
-----------
assemble - Assembles the outputs of this project.
build - Assembles and tests this project.
buildDependents - Assembles and tests this project and all projects that depend on it.
buildNeeded - Assembles and tests this project and all projects it depends on.
classes - Assembles main classes.
clean - Deletes the build directory.
jar - Assembles a jar archive containing the main classes.
testClasses - Assembles test classes.

Build Setup tasks
-----------------
init - Initializes a new Gradle build.
wrapper - Generates Gradle wrapper files.

Documentation tasks
-------------------
javadoc - Generates Javadoc API documentation for the main source code.

Help tasks
----------
buildEnvironment - Displays all buildscript dependencies declared in root project 'jarPluginSample'.
components - Displays the components produced by root project 'jarPluginSample'. [incubating]
dependencies - Displays all dependencies declared in root project 'jarPluginSample'.
dependencyInsight - Displays the insight into a specific dependency in root project 'jarPluginSample'.
dependentComponents - Displays the dependent components of components in root project 'jarPluginSample'. [incubating]
help - Displays a help message.
model - Displays the configuration model of root project 'jarPluginSample'. [incubating]
projects - Displays the sub-projects of root project 'jarPluginSample'.
properties - Displays the properties of root project 'jarPluginSample'.
tasks - Displays the tasks runnable from root project 'jarPluginSample'.

Verification tasks
------------------
check - Runs all checks.
test - Runs the unit tests.

Rules
-----
Pattern: clean<TaskName>: Cleans the output files of a task.
Pattern: build<ConfigurationName>: Assembles the artifacts of a configuration.
Pattern: upload<ConfigurationName>: Assembles and uploads the artifacts belonging to a configuration.

To see all tasks and more detail, run gradle tasks --all

To see more detail about a task, run gradle help --task <task>

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed
liumiaocn:plugin liumiao$ 

總結

plugin在gradle中是一個非常重要的概念，正是使用了這種方式，才使得gradle有了更好的靈活性，這篇文章通過一個簡單的例子介紹了plugin的使用方式，使得我們對其有了一個大體上的認識，詳細的說明，後面會進一步展開。

參考文章

https://docs.gradle.org/current/userguide/plugin_reference.html
https://www.playframework.com/
https://www.antlr.org/
https://guides.gradle.org/migrating-from-maven/
https://docs.gradle.org/current/userguide/java_plugin.html