Skip to main content

neow3j - Java Smart Contract Quickstart

AxLabs ·  · 5 min read · Source

neow3j v3.19.1

1. Introduction#

Java Smart contracts are made possible through the neow3j devpack, which provides a smart contract library, a compiler and a test framework. Neow3j favors Gradle as the build tool. It offers a Gradle plugin that allows contract compilation via a Gradle task.

2. Setup#



Smart contract development in Java requires a Java SDK of at least version 8. Checkout Adoptium for Java OpenJDK downloads.


For running automated smart contract tests with neow3j you need to have Docker installed.

Development Environment#

You could write smart contracts in any editor, but we recommend using IntelliJ IDEA and Visual Studio Code. IntelliJ offers one of the best Java/Kotlin developer experiences while VS Code offers a powerful extension for development on Neo.

If you use VS Code make sure to install the Neo Blockchain Toolkit extension. It supports contract debugging, easy setup of private blockchains, and an in-editor block explorer. To get familiar with the Blockchain Toolkit checkout the quickstart tutorials here (the tutorials use C# as the smart contract language). For optimal Java support in VS Code we recommend using the Java Extension Pack.


For a quick setup of your smart contract project you can use the template project from our neow3j-boilerplate-contracts repository. It contains a simple "HelloWorld" smart contract.

You can either click on Use this template within GitHub to create your own repository from it, or clone it with the following commands:

git clone neow3j-boilerplate-contracts && git checkout -b devportal-quickstart-tutorial

This tutorial is based on the branch devportal-quickstart-tutorial, it might not be on the same commit as the main branch.

3. Compilation#

Using the Gradle Task#

With the setup from above, we can already compile our HelloWorld contract. Open a terminal in the project's root directory and run

./gradlew neow3jCompile

This compiles the smart contract class HelloWorldSmartContract, and places a NEF file, contract manifest, and debugging information file by default at build/neow3j. NEF file and contract manifest are described here. They are the artifacts that are deployed on-chain. The debugging information file is required by the Neo Debugger (part of the Neo Blockchain Toolkit) to debug your contract.

Using the Compiler in Code#

The neow3j compiler can also be invoked in Java code. This is practical if you are using Java code to deploy your contract and want to compile your contract in the same code.

You will find an example for the compiler usage com.axlabs.helloworld.Deployment in the boilerplate project.

CompilationUnit res = new Compiler().compile(    HelloWorldSmartContract.class.getCanonicalName(),    substitutions);

The name of the contract to compile as well as a map of placeholder substitutions is passed to the compiler. The compilation result is a CompilationUnit which gives access to the NEF file and the contract manifest. They are used to call the ContractManagement contract - a contract that is native to the Neo blockchain.

TransactionBuilder builder = new ContractManagement(neow3j)    .deploy(res.getNefFile(), res.getManifest(), hash160(owner))    .signers(signer);

4. Development#

Understanding Java for the NeoVM

When using neow3j to implement smart contracts you will, at some point, have to understand the differences between normal Java and Java for smart contract. Because smart contract code will run on the NeoVM and not on the JavaVM there are several restrictions. You can read about that here.

Understanding the neow3j Devpack

To get to know the neow3j devpack library you can consult the documentation here. It shows core features of the Neo smart contract API, like storage, events, and native contracts, and explains how they are available in neow3j. Or, checkout one of the other neow3j tutorials on the developer portal to get a better idea of how Java smart contracts look like.

Automated Testing

When writing smart contracts testing is of utmost importance to feel confidence in one's code. Neow3j offers a test framework that is build on top of JUnit 5. It is documented here. The test class HelloWorldSmartContractTest in the boilerplate repository gives a first impression how contract tests with neow3j look.

Manual Testing

Before you deploy your contract on the Neo mainnet, you will probably want to test it on a private network on your machine and then on testnet. For local testing we recommend using the Neo Blockchain Toolkit mentioned in the dev environment section. The boilerplate repository already contains a default.neo-express configuration file that defines a private network with one account called Alice. In VS Code open the command palette and type "Start blockchain". Execute the command. The private net will start. Again in the command palatte type "Transfer assets" and execute the command. Transfer 100 GAS from genesis to Alice. This is will create a transaction on the private net. Now you can run the main method in com.axlabs.helloworld.Deployment (of the boilerplate repo) to deploy the contract on the private net.


The underlying tool used for running private networks is Neo-Express. It can be separately installed and used as a command line tool here.