在Android sdk 目录下的samples/android-21/ 文件夹下，任找一个工程，如果在命令行直接编译

可能会报这种错误：

gradle buile.gradle

FAILURE: Build failed with an exception.  
   
* What went wrong:  
Task 'build.gradle' not found in root project 'BorderlessButtons'.  
   
* Try:  
Run gradle tasks to get a list of available tasks. Run with --info or --debug option to get more log output.  
   
* Exception is:  
org.gradle.execution.TaskSelectionException: Task 'build.gradle' not found in root project 'BorderlessButtons'.  
    at org.gradle.execution.TaskSelector.getSelection(TaskSelector.java:101)  
    at org.gradle.execution.TaskSelector.getSelection(TaskSelector.java:76)  
    at org.gradle.execution.commandline.CommandLineTaskParser.parseTasks(CommandLineTaskParser.java:42)  
    at org.gradle.execution.TaskNameResolvingBuildConfigurationAction.configure(TaskNameResolvingBuildConfigurationAction.java:44)

如果是这样，首先回到工程目录下，运行

./gradlew  
./gradlew  build

然后再执行进行编译

gradle build 
gradle tasks //查看android gradle的所有任务
gradle compileReleaseSource  //生成debug apk ，在build/outpus/apk 文件夹下