No goals have been specified for this build. You must specify a valid lifecycle phase or a goal in t

1. 复现错误

今天在命令框中输入mvn，却报出如下图所示的错误：

No goals have been specified for this build. You must specify a valid lifecycle phase or a goal in the format : or :[:]:. Available lifecycle phases are: validate, initialize, generate-sources, process-sources, generate-resources, process-resources, compile, process-classes, generate-test-sources, process-test-sources, generate-test-resources, process-test-resources, test-compile, process-test-classes, test, prepare-package, package, pre-integrati