Initializable Fields

When deploying a contract, you can specify certain variables to be initialized during the deployment. This allows you to specify things like environment variables during the deploying of a contract, without having to hardcode them into the contract itself.

When a contract is deployed, the network looks for the static{} function. This function is also called a <clinit>. Anything specified within static{} will only ever be run once. For example, if your contract requires some environment variables to be specified at deployment time, you can declare them within static{}. However, if you want to declare them outside of static{}, you can use the @Initializeable annotation. This allows you to declare the variables higher up in your contract, or inside a method within your contract.

All @Initializable fields must be static. The type of an @Initializable field must be a supported AVM ABI type. The data supplied to the data field when deploying your contract must be supplied in the exact same order as the @Initializable field are defined. If not, an ABIException will be thrown. The AVM uses the ABIDecoder or order to parse all @Initializeable annotations.

If you have two variables myInt and myString that you want to declare and initialize them then you would use the @Initializable annotation.

private static int myInt;

private static String myString;

Within the static{} for this contract, you would decode the variables using the ABIDecoder class. Again, any variable you want to initialize need to be decoded in the order that you declared them:

static {
 ABIDecoder decoder = new ABIDecoder(Blockchain.getData());
 myInt = decoder.decodeOneInteger();
 myString = decoder.decodeOneString();


Below is an example of how to use the @Initializable annotation to supply environment variables into a contract in order to interact with an external API. The API call logic has been ignored here to keep this example simple.

// Define environment variables apiKey and Region.
private static String apiKey;

private static int region;

// Set a placeholder for the owner of the contract and API response.
private static Address owner;
private static String initialApiResponse;

// Decode the arguments supplied when deploying the contract.
static {
 apiKey = decoder.decodeOneString();
 region = decoder.decodeOneInt();
 owner = Blockchain.getCaller();
 initialiApiResponse = callApi(apiKey, region);

// Can an external API using the onetime API key and region.
private static String callApi(String suppliedApiKey, int suppliedRegion) {
 // API call logic.
 return apiResponse;

// Have the owner of this contract return the response of the API.
public static String returnMessage() {
 return initialiApiResponse;

// A modifer function that halts a function if it is not called by the owner of this contract.
private static void onlyOwner() {

This contract can be deployed using the Maven CLI and the Aion4J plugin:

mvn aion4j:deploy -Dargs="-T 'ABAF21E45B23C00AA12311C32E' -I 3"