[Ethereum] How to get contract object using ABI json and address with web3j

abijson-rpcweb3j

I worked with web3js(JavaScript) and used web3.eth.Contract with 2 params (json, address) and successfully got contract object.
Now I started to work with web3j(Java) and don`t understand how I have to work now.
How can I do this ?

Best Answer

I found a solution

I installed epirus wrapper, then I generated java package with command epirus solidity generate

-a=/root/my_json.abi -o=/root/ -p=my_java

(before I changed extension of my file from json to abi). After getting this package, I could use smart contract object with his methods from json file.

Also, after installation of epirus and before generating package I used this commands

JAVA_HOME=$(dirname $( readlink -f $(which java) ))
JAVA_HOME=$(realpath "$JAVA_HOME"/../)
export JAVA_HOME

sudo update-alternatives --config java

Don`t know exactly what they do, but without them I could not use epirus.

To check if epirus ready to generate I used command

epirus -V

It returns version of wrapper. If not - use two commands above.

Ismael, thank you for the direction)

Related Solutions

Web3j – Using Web3j to Load a Smart Contract and Specify Credentials

To generate the Credentials if you have a plain public key and private key, you need to convert them in to the hex representation and then pass them in to the constructor for Credentials

String hexPrivateKey = String.format("%040x", new BigInteger(1, privateKey.getBytes()));
String hexPublicKey = String.format("%040x", new BigInteger(1, publicKey.getBytes()));  
Credentials creds = Credentials.create(hexPrivateKey, hexPublicKey);

Where publicKey and privateKey are Strings containing your plain pub and priv keys

[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!.

Best Answer

Related Solutions

Web3j – Using Web3j to Load a Smart Contract and Specify Credentials

[Ethereum] Web3j test accounts and Credentials object

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