Web3J - Java API

Web3J is lightweight API that connects to and calls a node through JSON-RPC

https://docs.web3j.io/

Getting Started

• Add dependency
• Start a client
• Connect

HttpService service = new HttpService(DemoSettings.NODE_URL);

Web3j connection = Web3j.build(service);

clientVersion = connection.web3ClientVersion().send().getWeb3ClientVersion();

• Unlock account
  • Its still possible, but only over IPC sockets, not over HTTP.
  • Or, unlock on server, and just use the account address

Dealing with account

https://docs.web3j.io/transactions.html#transaction-mechanisms

• Just call with address - unlock account on server
• Use "PersonalUnlockAccount personalUnlockAccount = web3j.personalUnlockAccount("0x000...", "a password").send();" to unlock
  • THIS DOES NOT WORK OVER HTTP!!
• Use wallet
  • wallet file is just the key file under .ethereum...keystore...
  • can use "web3j wallet" utility to create / update (password) wallet file, or create one from private key

Call Smart Contract

Manually https://docs.web3j.io/transactions.html#transacting-with-contract

Through a smart contract wrapper:

• Install web3jfat jar: download the package, unzip, add to path