异常场景

从AS2.0升级到2.1，重新编译工程后，抛出了如下异常

Error:A problem occurred configuring root project 'AndroidStudioProjects'.
> Could not resolve all dependencies for configuration ':classpath'.
   > Could not resolve com.android.tools.build:gradle:2.0.0.
     Required by:
         :AndroidStudioProjects:unspecified
      > No cached version of com.android.tools.build:gradle:2.0.0 available for offline mode.

分析

之前为了优化gradle的编译速度，选择了Offline Work模式，取消即可

解决

Settings-Build.Execution,Deployment-Build Tools-Gradle–取消勾选的Offline Work, 重新编译，AS会自动下载.