android studio新建工程，编译出现
Failed to find Build Tools revision 30.0.3
IOException: https://dl.google.com/android/repository/addons_list-5.xml
java.net.ConnectException: Connection refused: connect
IOException: https://dl.google.com/android/repository/addons_list-4.xml
java.net.ConnectException: Connection refused: connect
IOException: https://dl.google.com/android/repository/addons_list-3.xml
java.net.ConnectException: Connection refused: connect
IOException: https://dl.google.com/android/repository/addons_list-2.xml
java.net.ConnectException: Connection refused: connect
IOException: https://dl.google.com/android/repository/addons_list-1.xml
java.net.ConnectException: Connection refused: connect
Failed to download any source lists!
打开android studio的“SDK Manager”，这里的“SDK Tools”已经勾选下载了，奇怪的是，这里的“Build-Tools 32.1-rc1”，并非报错的“30.0.3”
修改工程build.gradle，指定“buildToolsVersion ‘32.1.0 rc1’”
buildToolsVersion '32.1.0 rc1'
重新sync后解决问题。
另外一种，就是下载对应的版本，选中“SDK Tools”----“Android SDK Build-Tools 32.1-rc1”----勾选“Show Package Details”
会展开对应的版本，勾选下载即可
