前言

项目一般会有多个 module ，这样就容易引起重复引用第三方或者版本不一致，所以需要一个统一的管理。

谷歌官方的方式

/*
app 目录下的 build.gradle
*/
 compileSdkVersion rootProject.ext.compileSdkVersion
 buildToolsVersion rootProject.ext.buildToolsVersion
 defaultConfig {
     minSdkVersion rootProject.ext.minSdkVersion
     targetSdkVersion rootProject.ext.targetSdkVersion
 }
/*
 Dependency versions are defined in the top level build.gradle file. This helps keeping track of
 all versions in a single place. This improves readability and helps managing project complexity.
 依赖版本在顶层的 bulid.gradle 文件中定义。这有助于在一个地方跟踪所有版本。这提高了可读性，并有助  
 于管理项目的复杂性。
 */
dependencies {
    // App's dependencies, including test
    compile "com.android.support:appcompat-v7:$rootProject.supportLibraryVersion"
    compile "com.android.support:cardview-v7:$rootProject.supportLibraryVersion"
    compile "com.android.support:design:$rootProject.supportLibraryVersion"
    compile "com.android.support:recyclerview-v7:$rootProject.supportLibraryVersion"
    compile "com.android.support:support-v4:$rootProject.supportLibraryVersion"
    compile "com.android.support.test.espresso:espresso-idling-resource:$rootProject.espressoVersion"
    compile "com.google.guava:guava:$rootProject.guavaVersion"

    // Dependencies for local unit tests
    testCompile "junit:junit:$rootProject.ext.junitVersion"
    testCompile "org.mockito:mockito-all:$rootProject.ext.mockitoVersion"
    testCompile "org.hamcrest:hamcrest-all:$rootProject.ext.hamcrestVersion"

    // Android Testing Support Library's runner and rules
    androidTestCompile "com.android.support.test:runner:$rootProject.ext.runnerVersion"
    androidTestCompile "com.android.support.test:rules:$rootProject.ext.runnerVersion"

    // Dependencies for Android unit tests
    androidTestCompile "junit:junit:$rootProject.ext.junitVersion"
    androidTestCompile "org.mockito:mockito-core:$rootProject.ext.mockitoVersion"
    androidTestCompile 'com.google.dexmaker:dexmaker:1.2'
    androidTestCompile 'com.google.dexmaker:dexmaker-mockito:1.2'

    // Espresso UI Testing
    androidTestCompile "com.android.support.test.espresso:espresso-core:$rootProject.espressoVersion"
    androidTestCompile "com.android.support.test.espresso:espresso-contrib:$rootProject.espressoVersion"
    androidTestCompile "com.android.support.test.espresso:espresso-intents:$rootProject.espressoVersion"

    // Resolve conflicts between main and test APK:
    androidTestCompile "com.android.support:support-annotations:$rootProject.supportLibraryVersion"
    androidTestCompile "com.android.support:support-v4:$rootProject.supportLibraryVersion"
    androidTestCompile "com.android.support:recyclerview-v7:$rootProject.supportLibraryVersion"
    androidTestCompile "com.android.support:appcompat-v7:$rootProject.supportLibraryVersion"
    androidTestCompile "com.android.support:design:$rootProject.supportLibraryVersion"
}

可以看出官方给的很多示例都写了测试类（gradle中的依赖包大部分都是写测试用的），但我目前接触的代码中机会没有（可能是我太渣了。。。）

project 目录下的 build.gradle
// Define versions in a single place  唯一定义版本的地方  
ext {
    // Sdk and tools
    minSdkVersion = 10
    targetSdkVersion = 25
    compileSdkVersion = 25
    buildToolsVersion = '25.0.0'

    // App dependencies
    supportLibraryVersion = '25.3.1'
    guavaVersion = '18.0'
    junitVersion = '4.12'
    mockitoVersion = '1.10.19'
    powerMockito = '1.6.2'
    hamcrestVersion = '1.3'
    runnerVersion = '0.5'
    rulesVersion = '0.5'
    espressoVersion = '2.2.2'
}

注意：

//两种都可以，不知道有什么区别
rootProject.ext.compileSdkVersion
rootProject.compileSdkVersion

这样看起来是不是很清爽！！！