celo-sdk-java

Introduction

Celo-sdk-java, originally adapted from Ethereum web3j, is a Java library for working with the Celo Blockchain and Celo Core Contracts.

Features

• Connect to a node
• Access web3 object to interact with node's Json RPC API
• Send Transaction with celo's extra fields: (feeCurrency)
• Simple interface to interact with CELO and cUSD
• Simple interface to interact with Celo Core contracts
• Utilities

Getting Started

Prerequisites

Java 11
Gradle 7

Install

Install from repositories:
maven

<dependency>
    <groupId>io.github.gconnect</groupId>
    <artifactId>celo-sdk-java</artifactId>
    <version>0.0.2</version>
</dependency>

build.gradle

implementation 'io.github.gconnect:celo-sdk-java:0.0.2'

Celo SDK Java Maven Central Dependency

Install manually

If you want to generate the jar and import manually.

git clone hhttps://github.com/gconnect/celo-sdk-java.git
./gradlew clean build -xtest
./gradlew publishToMavenLocal

In your settings.gradle file when working on an android project if installing it manually add mavenLocal() inside the dependencyResolutionManagement like this 👇

dependencyResolutionManagement {
    repositoriesMode.set(RepositoriesMode.FAIL_ON_PROJECT_REPOS)
    repositories {
        google()
        mavenCentral()
        mavenLocal()
    }
}

Inside build.gradle file add the below code in the dependencies

implementation 'org.celo:contractkit:0.0.1'

Testing

Most of tests uses Ganache testnet.

To start devchain run ./scripts/start_devchain.sh script.

To reset devchain to the initial state run ./scripts/reset_devchain.sh script. It's recommend to run script before run all tests.

./scripts/reset_devchain.sh
./scripts/start_devchain.sh

./gradlew clean test

Publishing to Sonatype Maven Central

You can read more about it here

1. Create an Account on Sonatype
2. Create an issue and an empty repository with the Sonatype issue ID
3. Follow the steps in 1 above to update your build.gradle file
4. Build and upload library

./gradlew publish

5. Once published go the Nexus Repository Manager. Select Staging repositories. Your published package will be shown here. Then click close. If everything checks out correctly the release button will be clickable for you to release the package to the public.

6. It will take a while maybe 30 minutes for the published package to be available on Maven Central Repository

Initializing the ContractKit

To start working with ContractKit you need a kit instance and a valid net to connect with. In this example will use alfajores (you can read more about it here)

import org.celo.contractkit.ContractKit; 

ContractKit contractKit = ContractKit.build(new HttpService("https://alfajores-forno.celo-testnet.org"));

Initialize the ContractKit with your own node

If you are hosting your own node you can connect our ContractKit to it.

Same as Web3 we support WebSockets, RPC and connecting via IPC. For this last one you will have to initialize the kit with an instance of Web3 that has a valid IPC Provider

import org.celo.contractkit.ContractKit; 
import org.web3j.protocol.Web3j;
import org.web3j.protocol.ipc.UnixIpcService;

Web3j web3j = Web3j.build(new UnixIpcService("/path/to/socketfile"));
ContractKit contractKit = new ContractKit(web3j);

ContractKit Usage

The following are some examples of the capabilities of the ContractKit, assuming it is already connected to a node. If you aren't connected, here is a refresher.

Setting Default Tx Options

kit allows you to set default transaction options:

Web3j web3j = Web3j.build(new HttpService(ContractKit.ALFAJORES_TESTNET));

ContractKitOptions config = new ContractKitOptions.Builder()
    .setFeeCurrency(CeloContract.GoldToken)
    .setGasPrice(BigInteger.valueOf(21_000))
    .build();
ContractKit contractKit = new ContractKit(web3j, config);

Multiple accounts can be added to the kit wallet. The first added account will be used by default.

Credentials credentials = Credentials.create(somePrivateKey);
contractKit.addAccount(credentials);

or

contractKit.addAccount(somePrivateKey);

To change default account to sign transactions

contractKit.setDefaultAccount(publicKey);

Getting the Total Balance

This method from the kit will return the CELO, locked CELO, cUSD and total balance of the address

AccountBalance balance = contractKit.getTotalBalance(myAddress);

Deploy a contract

Deploying a contract with the default account already set.

You can verify the deployment on the Alfajores block explorer here. Wait for the receipt and log it to get the transaction details.

Web3j web3j = Web3j.build(new HttpService(ContractKit.ALFAJORES_TESTNET));

ContractKit contractKit = new ContractKit(web3j);
contractKit.addAccount(PRIVATE_KEY);

GoldToken deployedGoldenToken = contractKit.contracts.getGoldToken().deploy().send();

TransactionReceipt receipt = deployedGoldenToken.getTransactionReceipt().get();
String hash = receipt.getTransactionHash();

Buying all the CELO I can, with the cUSD in my account

ExchangeWrapper exchange = contractKit.contracts.getExchange();
StableTokenWrapper stableToken = contractKit.contracts.getStableToken();
GoldTokenWrapper goldToken = contractKit.contracts.getGoldToken();

BigInteger cUsdBalance = stableToken.balanceOf(myAddress).send();

TransactionReceipt approveTx = stableToken.approve(exchange.getContractAddress(), cUsdBalance).send();
String approveTxHash = approveTx.getTransactionHash();

BigInteger goldAmount = exchange.quoteUsdSell(cUsdBalance).send();
TransactionReceipt sellTx = exchange.sellDollar(cUsdBalance, goldAmount).send();
String sellTxHash = sellTx.getTransactionHash();

Celo Core Contracts. Wrappers / Registry

Interacting with CELO & cUSD

celo-blockchain has two initial coins: CELO and cUSD (stableToken). Both implement the ERC20 standard, and to interact with them is as simple as:

GoldTokenWrapper goldtoken = contractKit.contracts.getGoldToken();
BigInteger goldBalance = goldtoken.balanceOf(someAddress).send();

To send funds:

BigInteger oneGold = Convert.toWei(BigDecimal.ONE, Convert.Unit.ETHER).toBigInteger();
TransactionReceipt tx = goldToken.transfer(someAddress, oneGold).send();
String hash = tx.getTransactionHash();

To interact with cUSD, is the same but with a different contract:

StableTokenWrapper stableToken = contractKit.contracts.getStableToken();

Interacting with Other Celo Contracts

Apart from GoldToken and StableToken, there are many core contracts.

For the moment, we have contract wrappers for:

• Accounts
• Attestations
• BlockchainParameters
• DobleSigningSlasher
• DowntimeSlasher
• Election
• Escrow
• Exchange (Uniswap kind exchange between Gold and Stable tokens)
• GasPriceMinimum
• GoldToken
• Gobernance
• LockedGold
• Reserve
• SortedOracles
• Validators
• StableToken

A Note About Contract Addresses

Celo Core Contracts addresses, can be obtained by looking at the Registry contract. That's actually how kit obtain them.

We expose the registry api, which can be accessed by:

String goldTokenAddress = contractKit.contracts.addressFor(CeloContract.GoldToken);

Sending Custom Transactions

Celo transaction object is not the same as Ethereum's. There are three new fields present:

• feeCurrency (address of the ERC20 contract to use to pay for gas and the gateway fee)
• gatewayFeeRecipient (coinbase address of the full serving the light client's trasactions)
• gatewayFee (value paid to the gateway fee recipient, denominated in the fee currency)

This means that using web3.eth.sendTransaction or myContract.methods.transfer().send() should be avoided.

Instead, kit provides an utility method to send transaction in both scenarios. If you use contract wrappers, there is no need to use this.

For a raw transaction:

CeloRawTransaction tx = CeloRawTransaction.createCeloTransaction(
                BigInteger.ZERO,
                GAS_PRICE,
                GAS_LIMIT,
                someAddress,
                oneGwei,
                contractKit.contracts.addressFor(CeloContract.StableToken),
                null,
                null
        );
EthSendTransaction receipt = contractKit.sendTransaction(tx);
assertNotNull(receipt.getTransactionHash());