Swaps ERC20 tokens with 0x.org. Shows how to set target allowance.

ZeroExERC20Swap.java

package org.example.crypto;

import lombok.extern.slf4j.Slf4j;
import org.apache.logging.log4j.core.config.Configurator;
import org.web3j.contracts.eip20.generated.ERC20;
import org.web3j.crypto.Credentials;
import org.web3j.crypto.WalletUtils;
import org.web3j.protocol.Web3j;
import org.web3j.protocol.core.DefaultBlockParameterName;
import org.web3j.protocol.core.methods.response.TransactionReceipt;
import org.web3j.protocol.http.HttpService;
import org.web3j.tx.RawTransactionManager;
import org.web3j.tx.TransactionManager;
import org.web3j.tx.gas.DefaultGasProvider;
import org.web3j.tx.response.PollingTransactionReceiptProcessor;
import org.web3j.tx.response.TransactionReceiptProcessor;

import java.math.BigInteger;
import java.util.Optional;

@Slf4j
public class ZeroExERC20Swap {
    // for URL addresses see https://docs.0x.org/introduction/0x-cheat-sheet#0x-api-endpoints
    public static final String BSC_BASE_URL = "https://bsc.api.0x.org";
    public static final String POLYGON_BASE_URL = "https://polygon.api.0x.org";
    public static final String MUMBAI_BASE_URL = "https://mumbai.api.0x.org";

    public static final String BSC_KISS_ME = "0x3ad2ea050c3a103961475e10c2332e043e170840";
    public static final String POLYGON_MATIC = "0xDef1C0ded9bec7F1a1670819833240f027b25EfF";
    public static final String POLYGON_USDC = "0x2791Bca1f2de4661ED88A30C99A7a9449Aa84174";
    public static final String POLYGON_VHC = "0x51b5619f5180e333d18b6310c8d540aea43a0371";
    public static final String MUMBAI_KMC = "0x87acc29a0c619cC99921A1d3DE71f389B5e93C12";
    public static final String MUMBAI_TUSDC = "0x8B9b8543516f7Eef3f5B54929B133b841a50a55A";

    private static final String MNEMONIC = "...";

    private static Web3j buildMumbaiWeb3j() {
        var url = "https://polygon-mumbai.infura.io/v3/...";
        return Web3j.build(new HttpService(url));
    }

    private static Credentials getCredentials() {
        return WalletUtils.loadBip39Credentials("", MNEMONIC);
    }

    private static void runInMumbai() throws Exception {
        var web3j = buildMumbaiWeb3j();
        var creds = getCredentials();
        var walletAddress = creds.getAddress();
        log.info("Wallet address: {}", walletAddress);
        log.info("Balance: {}", web3j.ethGetBalance(walletAddress, DefaultBlockParameterName.LATEST).send().getBalance());

        var txManager = new RawTransactionManager(web3j, creds, web3j.ethChainId().send().getChainId().longValue());

        var tusdcContract = ERC20.load(MUMBAI_TUSDC, web3j, txManager, new DefaultGasProvider());
        var tusdcBalance = tusdcContract.balanceOf(walletAddress).send();
        log.info("TUSDC balance: {}", tusdcBalance);

        var kmcContract = ERC20.load(MUMBAI_KMC, web3j, txManager, new DefaultGasProvider());
        var kmcBalance = kmcContract.balanceOf(walletAddress).send();
        log.info("$KMC balance: {}", kmcBalance);

        var client = new ZeroExClient(MUMBAI_BASE_URL);
        var sellAmount = BigInteger.valueOf(100000L);
        var priceResponse = client.getPrice(GetPriceRequest.builder()
                .sellToken(MUMBAI_TUSDC)
                .buyToken(MUMBAI_KMC)
                .sellAmount(Optional.of(sellAmount))
                .build());
        log.info("Price: {}; allowanceTarget: {}", priceResponse.price, priceResponse.allowanceTarget);

        log.info("Setting allowance...");
        var txReceipt = tusdcContract.approve(priceResponse.allowanceTarget, sellAmount).send();
        TransactionReceiptProcessor receiptProcessor = new PollingTransactionReceiptProcessor(
                web3j,
                TransactionManager.DEFAULT_POLLING_FREQUENCY,
                TransactionManager.DEFAULT_POLLING_ATTEMPTS_PER_TX_HASH);
        TransactionReceipt receipt = receiptProcessor
                .waitForTransactionReceipt(txReceipt.getTransactionHash());
        log.info("Done. Receipt: {}; Status: {}", receipt.getTransactionHash(), receipt.getStatus());

        var quote = client.getQuote(GetQuoteRequest.builder()
                .sellToken(MUMBAI_TUSDC)
                .buyToken(MUMBAI_KMC)
                .sellAmount(Optional.of(sellAmount))
                .takerAddress(Optional.of(walletAddress))
                .build());

        log.info("Quote: {}", quote.price);

        log.info("Swapping tokens...");
        var txHash = txManager
                .sendTransaction(quote.gasPrice, quote.gas, quote.to, quote.data, quote.value)
                .getTransactionHash();
        var swapTxReceipt = receiptProcessor.waitForTransactionReceipt(txHash);
        log.info("Done. Receipt: {}; Status: {}", swapTxReceipt.getTransactionHash(), swapTxReceipt.getStatus());

        log.info("TUSDC balance: {}", tusdcContract.balanceOf(walletAddress).send());
        log.info("$KMC balance: {}", kmcContract.balanceOf(walletAddress).send());
    }

    public static void main(String[] args) throws Exception {
        Configurator.initialize(null, "classpath:log4j2-local.xml");
        runInMumbai();
    }
}