Access Ethereum smart contract in Java application

Question

I have developed smart contract in solidity using truffle and ganache. Now, I want to access that smart contract using some API. So how to do it ? Should I need to use web3j compulsory or is there any other way to do that ?

Answer 1

You could use web3j.

Assuming you deployed you contract to ganache using truffle, in your java code you can use TruffleJsonFunctionWrapperGenerator to generate a wrapper for your contract. Add maven dependencies:

    <dependency>
        <groupId>org.web3j</groupId>
        <artifactId>core</artifactId>
        <version>LATEST</version>
    </dependency>
    <dependency>
        <groupId>org.web3j</groupId>
        <artifactId>codegen</artifactId>
        <version>LATEST</version>
    </dependency>

Generate the wrapper class:

        String outputDir = PROJECT_DIR + \\src\main\java;
        String packageName = "domain.yourCompany.yourApp.somePackage";  //output package
        String[] contractGenerationArgs = new String[7];
        contractGenerationArgs[0] = "generate";
        contractGenerationArgs[1] = "--javaTypes";
        contractGenerationArgs[2] = PATH_TO_TRUFFLE_JSON_OUTPUT_FILE_OF_THE_CONTRACT;
        contractGenerationArgs[3] = "-o";
        contractGenerationArgs[4] = outputDir;
        contractGenerationArgs[5] = "-p";
        contractGenerationArgs[6] = packageName;
        TruffleJsonFunctionWrapperGenerator.run(contractGenerationArgs);

Now use it:

Web3j web3 = Web3j.build(new HttpService("http://localhost:7545/")); //running ganache over port 7545
Web3ClientVersion web3ClientVersion = web3.web3ClientVersion().send();
String account = web3.ethAccounts().send().getAccounts().get(0);
Credentials credentials = Credentials.create(account);
String contractAddress = "0x345ca3e014aaf5dca488057592ee47305d9b3e10"; //The deployed contract address, taken from truffle console or ganache logs
BigInteger gasPrice = new BigInteger(GAS_PRICE);
BigInteger gasLimit = new BigInteger(GAS_LIMIT);
MyContract myContract = MyContract.load(contractAddress, web3, credentials, gasPrice, gasLimit);

//Now you can call methods
myContract.someMethod().send();

If you don't want to use web3j you can also use truffle from the terminal/windows power shell:

1. truffle develop
2. migrate --compile-all --reset (if not deployed already)
3. MyContract.deployed().then(function(instance){app = instance;})
4. app.someMethod()