How to access already deployed Smart Contract using Java?

Question

I would like to get information about token. For example total supply and so on. For this I need the contract. However what is the way to actually load it/access it from Java?

Checking examples in languages such as python, it seems trivial:

contract_address = 'CONTRACT_ADDRESS' 

abi = json.loads(abiJson)

contract = web3.eth.contract(address=contract_address, abi=abi)

totalSupply = contract.functions.totalSupply().call()

However how to actually do this using Java? Do I need to generate .abi file and then compile it to java? This way, wouldn't I need to recompile the project everytime I wanted to find info about another contract?

Thanks for help!

Answer 1

Web3j provides you with the ability to generate Java wrappers from ABI files for Solidity smart contracts. You can do so in different ways, mainly:

• Use web3j-gradle-plugin or web3j-maven-plugin;
• Use web3j-CLI to generate the wrappers.

For your case, you can use the web3j-codegen module to generate the wrappers. Then, load the generated class dynamically.

Since you only need a set of predefined methods that follow ERC20 standard or similar. This might work for you.

Answer 2

• Run task(generateContractWrappers), wrapper contracts will be generated under build/generated/sources/web3j/ERC20.java

• Generate contract instance using above wrapper

    ERC20 contractInstance=ERC20.load(String contractAddress, Web3j web3j, Credentials credentials, ContractGasProvider contractGasProvider)
   //contract gas provider can be created from DefaultGasProvider or StaticGasProvider
   //Credentials credential=Credentials.create(privatekey);

• Call contract functions

    BigInteger totalSupply=contractInstance.totalSupply().send();