我们知道在gradle中可以用 gradle.properties文件来设置属性。一般.properties文件的位置是定义在和build.gradle和settings.gradle同级的目录下。

1 简单使用

在gradle.properties中定义一个属性是这样的：

PRO_A=12

然后在构建脚本里就可以用到这个属性了：

task A(){
    doLast{
        println(PRO_A)
    }
}

2 和ext属性的关系

首先确定的是，gradle.properties中定义的属性是属定义他的gradle项目的project对象的属性的。

其次，在gradle文档中，没有说明（或者是我没找到）gradle.properties中的属性在是保存在Project对象的哪个位置的。

我个人猜测有两种可能：

1. 通过project.ext属性，保存在这里面。
2. 直接通过扩展project属性的方式，给project拓展属性。

这里倾向于前者，因为第二种给project拓展属性的编程方式，通常用来写gradle plugin。

通过测试验证猜想：

测试方法：定义一个属性，然后打印当前项目的ext属性中的所有的属性。从打印结果中就能看到gradle.preperties文件中的属性是否在ext中了。

定义属性

gradle.properties文件：

PRO_A=I AM PROPERTY A !

build.gradle文件中写一个task测试

task A() {
    doLast {
        println(PRO_A)
        rootProject.extensions.extraProperties.getProperties().forEach { String key, Object value ->
            println("key=$key,value=$value")
        }
    }
}

命令行执行：

▶ gradle A

> Task :sub:A
I AM PROPERTY A !
key=PRO_A,value=I AM PROPERTY A !
key=kotlin.code.style,value=official

将Project的ext属性输出的结果中包含了此前定义的属性，说明gradle.properties中的属性会保存在ext变量中。

3 Project对象的属性的读取顺序和优先级

分割线中内容全部来自gradle文档：Proejct

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A project has 5 property ‘scopes’, which it searches for properties. You can access these properties by name in your build file, or by calling the project’s Project.property(java.lang.String) method. The scopes are:

• The Project object itself. This scope includes any property getters and setters declared by the Project implementation class. For example, Project.getRootProject() is accessible as the rootProject property. The properties of this scope are readable or writable depending on the presence of the corresponding getter or setter method.
• The extra properties of the project. Each project maintains a map of extra properties, which can contain any arbitrary name -> value pair. Once defined, the properties of this scope are readable and writable. See extra properties for more details.
• The extensions added to the project by the plugins. Each extension is available as a read-only property with the same name as the extension.
• The convention properties added to the project by the plugins. A plugin can add properties and methods to a project through the project’s Convention object. The properties of this scope may be readable or writable, depending on the convention objects.
• The tasks of the project. A task is accessible by using its name as a property name. The properties of this scope are read-only. For example, a task called compile is accessible as the compile property.
• The extra properties and convention properties are inherited from the project’s parent, recursively up to the root project. The properties of this scope are read-only.

When reading a property, the project searches the above scopes in order, and returns the value from the first scope it finds the property in. If not found, an exception is thrown. See Project.property(java.lang.String) for more details.

When writing a property, the project searches the above scopes in order, and sets the property in the first scope it finds the property in. If not found, an exception is thrown. See [Project.setProperty(java.lang.String, java.lang.Object)](https://docs.gradle.org/current/dsl/org.gradle.api.Project.html#org.gradle.api.Project:setProperty(java.lang.String, java.lang.Object)) for more details.

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4 总结

1. gradle.properties文件中定义的属性在gradle脚本执行时保存在project.ext属性中。
2. gradle脚本读写属性是有顺序的，他会从5个区域里读写，先读到哪个就先用哪个，读取的优先级为：
   1. project本身的属性。
   2. project的extra（即project.ext）属性中的map中保存的键值对。
   3. 被gradle plugin添加的属性，即project.extensions属性。
   4. 被gradle plugin添加的convention属性。（没用过也没了解过这个属性）
   5. project中的task的名字，也作为project的属性。
   6. project从他的父project继承下来的ext和convention属性，会一直递归到rootProject。