Most of the Apache OpenWhisk components use the Gradle build tool. As described in more detail in the Gradle Wrapper documentation, Gradle is designed to securely bootstrap itself from a small jar file which is normally located in each OpenWhisk source tree at gradle/wrapper/gradle-wrapper.jar.
However, including the binary gradle-wrapper.jar is not allowed in Apache source releases. Therefore, if after unpacking an OpenWhisk source release you see a build.gradle at the top level of the source tree you will have to restore the missing jar file before you can build anything.
If you have a compatible version of gradle already installed on your machine, this can be done by performing the following steps.
gradle/wrapper/gradle-wrapper.properties to determine the expected version of gradle. Look for the version number embedded in the distributionUrl property.gradle wrapper --gradle-version X.Y.Z replacing X.Y.Z with the version number from gradle-wrapper.properties.If this command succeeds, you should now be able to execute ./gradlew to download the right version of gradle and build OpenWhisk. Consult the README.md for component-specific detailed instructions, such as the build target.
The command may fail because your version of gradle is incompatible with the build.gradle files included in the project. If this happens, you can either (a) install the matching version of gradle or (b) execute the gradle wrapper --gradle-version X.Y.Z command in a scratch directory and then manually copy gradle-wrapper.jar to gradle/wrapper.