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Deploy and Interact with Smart Contracts

Working with smart contracts and Java smart contract wrappers

Web3j can auto-generate smart contract wrapper code to deploy and interact with smart contracts without leaving the JVM.

To generate the wrapper code, compile your smart contract:

$ solc <contract>.sol --bin --abi --optimize -o <output-dir>/

Then generate the wrapper code using the Web3j CLI:

web3j generate solidity -b /path/to/<smart-contract>.bin -a /path/to/<smart-contract>.abi -o /path/to/src/main/java -p com.your.organisation.name

Now you can create and deploy your smart contract:

Web3j web3 = Web3j.build(new HttpService());  // defaults to http://localhost:8545/
Credentials credentials = WalletUtils.loadCredentials("password", "/path/to/walletfile");

YourSmartContract contract = YourSmartContract.deploy(
        <web3j>, <credentials>,
        <contractGasProvider>,
        <param1>, ..., <paramN>).send();  // constructor params

Or use an existing contract:

YourSmartContract contract = YourSmartContract.load(
        "0x<address>|<ensName>", <web3j>, <credentials>, <contractGasProvider>);

To transact with a smart contract:

TransactionReceipt transactionReceipt = contract.someMethod(
             <param1>,
             ...).send();

To call a smart contract:

Type result = contract.someMethod(<param1>, ...).send();

For more information refer to Solidity smart contract wrappers.

Smart contract examples

Web3j provides a number of smart contract examples in the project directory codegen/src/test/resources/solidity

It also provides integration tests for demonstrating the deploying and working with those smart contracts in the integration-tests/src/test/java/org/web3j/protocol/scenarios module.

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EIP-20 Ethereum token standard smart contract

There is an Ethereum standard, EIP-20 which started off as an Ethereum Improvement Proposal (EIP), that defines the standard functions that a smart contract providing tokens should implement.

The EIP-20 standard provides function definitions, but does not provide an implementation example. However, there is an implementation provided in codegen/src/test/resources/solidity/contracts, which has been taken from ConsenSys' Tokens project.

Open Zeppelin also provides an example implementation on GitHub.

There are two integration tests that have been written to fully demonstrate the functionality of this token smart contract.

HumanStandardTokenGeneratedIT uses the generated HumanStandardTokenGenerated Solidity smart contract wrappers to demonstrate this.

Alternatively, if you do not wish to use a smart contract wrapper and would like to work directly with the underlying JSON-RPC calls, please refer to HumanStandardTokenIT.