The library web3j-deployer and Gradle plugin web3j-deployer-plugin allows you to define your deployment routine for your smart contracts using Web3j. It's an annotation driven approach, with two key annotations used, as shown in the MyDeploymentLogic source file.
To build the wrapper code for the included HelloWorld.sol smart contract, use the usual gradle commands:
./gradlew clean build
Thereafter, to run the deployment logic use the gradle deploy command, specifying the profile and package as needed:
./gradlew -q deploy -Pprofile=rinkeby1 -Ppackage=demo.deploy