Web3j 是一个高度模块化、反应式、类型安全的 Java 和 Android 库，用于处理智能合约并与以太坊网络上的客户端（节点）集成

引入依赖

<dependency>
    <groupId>org.web3j</groupId>
    <artifactId>core</artifactId>
    <version>4.3.0</version>
</dependency>

2、工具类

/**
 * @author XiangYu
 * @since 2024/1/7
 */
@Slf4j
public class Web3Client {

    private static final  Web3j web3j = Web3j.build(new HttpService("主网/测试网rpc"));

    private Web3Client(){

    }

    /**
     * 发送交易、只能发送主网默认代币
     * @param privateKey 私钥
     * @param to to
     * @param value 金额
     * @return hash
     */
    public static String sendRawTransaction(String privateKey,String to,String value) {
        try {
            Credentials credentials = getCredentials(privateKey);

            BigDecimal amount = new BigDecimal(value);

            String signed = signed(web3j, to, credentials, amount);

            return web3j.ethSendRawTransaction(signed).send().getTransactionHash();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    /**
     * 发送交易
     *    发送token代币
     * @param privateKey 私钥
     * @param to 接收地址
     * @param amt 转账金额
     * @param contractAddress 合约地址
     * @return hash
     */
    public static String sendRawTransaction(String privateKey,String to,String amt,String contractAddress){
        try {
            Credentials credentials = getCredentials(privateKey);

            Function function = getFunction(to, amt);
            String encodedFunction = FunctionEncoder.encode(function);
            String hexSigned = signed(web3j,contractAddress, credentials, encodedFunction);

            EthSendTransaction ethSendTransaction = web3j.ethSendRawTransaction(hexSigned).send();
            return ethSendTransaction.getTransactionHash();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    /**
     * 签名交易
     * @param web3j web3
     * @param to to
     * @param credentials 私钥凭证
     * @param value 金额
     * @return sign
     */
    private static String signed(String to, Credentials credentials,BigDecimal value) {
        BigInteger nonce = getNonce(web3j, credentials.getAddress());

        RawTransaction rawTransaction = RawTransaction.createTransaction(
                nonce,
                DefaultGasProvider.GAS_PRICE,
                DefaultGasProvider.GAS_LIMIT,
                to,
                Convert.toWei(value, Convert.Unit.ETHER).toBigInteger(),
                ""
        );

        return signed(credentials, rawTransaction);
    }

    /**
     * 签名
     * @param web3j web3
     * @param to to
     * @param credentials 凭证
     * @param encodedFunction 合约函数调用编码
     * @return 签名
     */
    private static String signed(String to, Credentials credentials, String encodedFunction) {
        BigInteger nonce = getNonce(web3j, credentials.getAddress());

        RawTransaction rawTransaction = RawTransaction.createTransaction(nonce,  DefaultGasProvider.GAS_PRICE,
                DefaultGasProvider.GAS_LIMIT, to, encodedFunction);

        return signed(credentials, rawTransaction);
    }

    /**
     * 签名
     * @param credentials 私钥凭证
     * @param rawTransaction 包装交易
     * @return hex sign
     */
    private static String signed(Credentials credentials, RawTransaction rawTransaction) {
        byte[] signedMessage = TransactionEncoder.signMessage(rawTransaction, Web3Manage.INSTANCE.getChainId(), credentials);
        return Numeric.toHexString(signedMessage);
    }

    /**
     * 包装智能合约的transfer函数方法
     * @param to 接收地址
     * @param amt 金额
     * @return transfer function
     */
    private static Function getFunction(String to, String amt) {
        Address address = new Address(to);
        Uint256 value = new Uint256(new BigInteger(Convert.toWei(amt, Convert.Unit.ETHER).toString()));
        List<Type> parametersList = new ArrayList<>();
        parametersList.add(address);
        parametersList.add(value);
        return new Function("transfer", parametersList, new ArrayList<>());
    }

    /**
     * 获取交易账户确认的nonce
     * @param web3j web3j
     * @param address 地址
     * @return nonce
     */
    public static BigInteger getNonce(String address) {
        try {
            EthGetTransactionCount ethGetTransactionCount = web3j.ethGetTransactionCount(
                    address, DefaultBlockParameterName.LATEST).send();
            return ethGetTransactionCount.getTransactionCount();
        } catch (Exception e) {
            e.printStackTrace();
        }
        throw new BscRpcException("nonce too low");
    }

    /**
     * 通过私钥获取账户凭证
     * @param privateKey 私钥
     * @return 凭证
     */
    private static Credentials getCredentials(String privateKey) {
        BigInteger bigInteger = new BigInteger(privateKey, 16);
        ECKeyPair ecKeyPair = ECKeyPair.create(bigInteger);
        return Credentials.create(ecKeyPair);
    }

    /**
     * 钱包生成
     * @param walletFilePath 钱包生成路径
     * @return 凭证
     */
    public static Credentials genWallet(String walletFilePath){
        try {
            // 钱包密码
            String password = "";
            //生成钱包，对应目录下会创建对应的私钥文件。
            String walletFileName = WalletUtils.generateNewWalletFile(password, new File(walletFilePath), false);
            // 加载指定位置的钱包
            return WalletUtils.loadCredentials(password, walletFilePath + "/" + walletFileName);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    /**
     * 校验hash是否符合条件
     * @param hash 金额
     * @return bool
     */
    public static Transaction getTransactionHash(String hash){
        Web3j web3j = Web3Manage.INSTANCE.getWeb3j();
        try {
            // 获取交易信息
            EthTransaction ethTransaction = web3j.ethGetTransactionByHash(hash).send();
            return ethTransaction.getTransaction().orElse(null);
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

}

web3j.js文档
web3j文档
ethers.js文档
参考: Java使用Web3j进行基础和token转账