Web3j – How to Deploy Smart Contracts Using Web3j Java Library

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I am using web3j java library to deploy a smart contract.

// I am encoding the constructor parameters here -- 2 & 3, 10 & 20
String encodedConstructor =
         FunctionEncoder.encodeConstructor(Arrays.asList(new Type(value), ...));

RawTransaction rawTransaction = RawTransaction.createContractTransaction(
    <nonce>,
    <gasPrice>,
    <gasLimit>,
    <value>,
    "0x <compiled smart contract code>" + encodedConstructor);

byte[] signedMessage = TransactionEncoder.signMessage(rawTransaction, <credentials>);

String hexValue = Numeric.toHexString(signedMessage);

I am using

web3.eth.sendRawTransaction(hexValue)

to deploy the contract.

My smart contract has a constructor which accepts some parameters(a, b) and it has a function sum which returns sum of a and b. After deployment if I call the sum function, it always returns me 0.

Need help.

Reference Link: https://docs.web3j.io/transactions.html#creation-of-a-smart-contract

Best Answer

you need to create a Function Object for your smart contract constructor then in data field were you put compiled solidity add the decode result of this function 'compiled smart contract'+'decoded function of the constructor'

Related Solutions

[Ethereum] Web3j test accounts and Credentials object

Here is a more complete program than the one you provided. The only undefined part is Contract_sol_Contract, which you did not define. Notice how privateKey and publicKey are computed.

import org.web3j.crypto.Credentials;
import org.web3j.crypto.ECKeyPair;
import org.web3j.protocol.Web3j;
import org.web3j.protocol.http.HttpService;

import java.io.IOException;
import java.math.BigInteger;

public class SO3 {
    public SO3() throws IOException {
        Web3j web3 = Web3j.build(new HttpService("http://localhost:8545"));
        String account = web3.ethAccounts().send().getAccounts().get(0);
        Credentials credentials = Credentials.create(account);
        ECKeyPair keyPair = credentials.getEcKeyPair();
        BigInteger privateKey = keyPair.getPrivateKey();
        BigInteger publicKey = keyPair.getPublicKey();
        Contract_sol_Contract contract = Contract_sol_Contract.deploy(web3, credentials, new BigInteger("240000"), new BigInteger("2400000"), props).send();
    }
}

I think that the reason that web3j deviated from web3 in requiring just the public or private key is because providing less information is a better security practice. The web3 accounts docs has this warning: The accounts private key. This should never be shared or stored unencrypted in localstorage! Also make sure to null the memory after usage. This means passing around the entire accounts object is discouraged.

Seems the web3 authors knew they created a security issue because earlier on that same page they give this big fat warning: This package has NOT been audited and might potentially be unsafe. Take precautions to clear memory properly, store the private keys safely, and test transaction receiving and sending functionality properly before using in production!.

[Ethereum] how to get latest block number using web3j java APIs

This is the easiest way

Web3j web3j = Web3j.build(new HttpService("https://rinkeby.infura.io/v3/......."));
Block block = web3j.ethGetBlockByNumber(DefaultBlockParameterName.LATEST, false).send().getBlock();
System.out.println(block.getNumber().toString());

Best Answer

Related Solutions

[Ethereum] Web3j test accounts and Credentials object

[Ethereum] how to get latest block number using web3j java APIs

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