android studio编程时出现的错误：Cannot get property 'XXXX' on extra properties extension as it does not exis

android studio编程时出现的错误：Cannot get property 'XXXX' on extra properties extension as it does not exist

用Android Studio中导入第三方库工程的时候出现的问题：
Error:(28, 0) Cannot get property 'junitVersion' on extra properties extension as it does not exist

出现这种问题原因是第三方库工程 引用了 自定义的 junitVersion 这个名字的ext；

因此在该项目的根目录那个build.gradle里的ext加上junitVersion这个即可，如下
[java] view plain copy ext {
junitVersion = '4.12'
}
4.12这个是自定义的，

其他名称的同理。

************************************************ rootProject 方法介绍 *************************************************************
引用这些自定义ext的值方法如下：
$rootProject.ext.{value}

该方法的好处就是：
用rootProject中的一个值来代替本来的版本号，目前理解成多个module时的用处，即抽取出来统一管理：

例：
[java] view plain copy defaultConfig {
compileSdkVersion rootProject.ext.compileSdkVersion
buildToolsVersion rootProject.ext.buildToolsVersion
minSdkVersion rootProject.ext.minSdkVersion
targetSdkVersion rootProject.ext.targetSdkVersion
}

<pre class="prettyprint" name="code"><code class="hljs cs has-numbering">dependencies {
<span class="hljs-comment">// App's dependencies, including test</span>
compile <span class="hljs-string">"com.android.support:appcompat-v7:$rootProject.supportLibraryVersion"</span>
compile <span class="hljs-string">"com.android.support.test.espresso:espresso-idling-resource:$rootProject.espressoVersion"</span>
compile <span class="hljs-string">"com.google.guava:guava:$rootProject.guavaVersion"</span>

<span class="hljs-comment">// Dependencies for local unit tests</span>
testCompile <span class="hljs-string">"junit:junit:$rootProject.ext.junitVersion"</span>
testCompile <span class="hljs-string">"org.mockito:mockito-all:$rootProject.ext.mockitoVersion"</span>
testCompile <span class="hljs-string">"org.hamcrest:hamcrest-all:$rootProject.ext.hamcrestVersion"</span>
<span class="hljs-string"></span>
}</code><div class="save_code tracking-ad" data-mod="popu_249"><a href="javascript:;"><img src="http://static.blog.csdn.net/images/save_snippets.png"></a></div></pre><br><br>
而project的Gradle（
项目的根目录那个build.gradle
）中是这么写的：
[java] view plain copy // Define versions in a single place
ext {
// Sdk and tools
minSdkVersion = 10
targetSdkVersion = 24
compileSdkVersion = 24
buildToolsVersion = '24.0.2'

// App dependencies
supportLibraryVersion = '24.2.0'
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'
}