Purpose of buildscript block in Gradle

问题

I am new to Gradle and I am reading the documentation but I don't understand some parts of it. One of these parts is connected with buildscript block. What is its purpose?

If your build script needs to use external libraries, you can add them to the script's classpath in the build script itself. You do this using the buildscript() method, passing in a closure which declares the build script classpath.
buildscript {
  repositories {
    mavenCentral()
  }
  dependencies {
    classpath group: 'commons-codec', name: 'commons-codec', version: '1.2'
  }
}

Ok but what is the difference with:

repositories {
  mavenCentral()
}
dependencies {
  compile group: 'commons-codec', name: 'commons-codec', version: '1.2'
}

For example, why it is necessary to use buildscript?

回答1:

The buildScript block determines which plugins, task classes, and other classes are available for use in the rest of the build script. Without a buildScript block, you can use everything that ships with Gradle out-of-the-box. If you additionally want to use third-party plugins, task classes, or other classes (in the build script!), you have to specify the corresponding dependencies in the buildScript block.

回答2:

The global level dependencies and repositories sections list dependencies that required for building your source and running your source etc.
The buildscript is for the build.gradle file itself. So, this would contain dependencies for say creating RPMs, Dockerfile, and any other dependencies for running the tasks in all the dependent build.gradle

回答3:

I appreciate Peter's answer... but it was not immediately obvious to me what the rest of the build script meant as emphasized in the answer and in the documentation.

Usually bringing in dependent functionality is for use in the Java program or whatever other program you might be writing. Bringing in Spring say, is not to be used in the build script, but in the Java program. Putting it in the buildscript closure ensures that the dependencies are available for use within the gradle build itself. Not the output program.

回答4:

The "buildscript" configuration section is for gradle itself (i.e. changes to how gradle is able to perform the build). So this section will usually include the Android Gradle plugin.

回答5:

It's a bit high level but hope helps.

For me, clear distinction started to shape once I start to understand what is a building block, method, and task. How the syntax looks like, how you can configure them etc. So I suggest you go through all these. After that, you can begin to make sense out of this syntax.

Then it's very important to know what's the type of the object build.gradle (an instance of Project class) so to know what can you have inside a build.gradle file. That would answer where that 'buildScript' and others come from. And to extend your capabilities/features (let's say android), see how plugins can help.

Last but not least, there's a very good tutorial here that talks about closures, delegates which are the concepts essential to understand the script.

回答6:

A bit more explanation by demonstrating Android top-level gradle file.

buildscript {
    // this is where we are going to find the libraries defined in "dependencies block" at below
    repositories {
        google()
        jcenter()
        maven { url 'https://dl.bintray.com/kotlin/kotlin-eap' }
    }

    // everything listed in the dependencies is actually a plugin, which we'll do "apply plugin" in our module level gradle file.
    dependencies {
        classpath 'com.android.tools.build:gradle:3.4.2' // this is android gradle plugin
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version" // kotlin gradle plugin
    }
}

module level gradle file

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'

What is "plugin"? They are just java classes, which implement Plugin interface. Under the interface, it has a method "apply" to add multiple task objects with different names. Task is a class where we can implement the workflow. For instance, the build task consists of the flow of building the app.

So, what does buildscript do? It defines where to find the plugins. What does plugin do? It encompasses multiple tasks. What does task do? It provides us with the build, install, lint, etc.

My understanding might be wrong. Please don't hesitate to correct me if you find anything is misleading.

来源：https://stackoverflow.com/questions/17773817/purpose-of-buildscript-block-in-gradle

标签

classpath

gradle