web3j Sample Project
This project provides a very simple starter application for working with web3j and the test Ethereum network (testnet) known as Rinkeby.
The demo deploys a hello world smart contract which contains a greeting message. The current greeting message can be read and modified in the smart contract.
In order to run this demo, three pieces of information are required:
- Ethereum client (or node) endpoint. The simplest thing to do is request a free access token from Infura.
- A wallet file. This can be generated using the Epirus CLI.
- Some Ether. This can be requested from the Rinkeby Faucet.
Once these details have been added to the main Application class, you'll be good to go.
Functionality
This application demonstrates a number of core features of web3j and Ethereum:
- Connecting to a node on the Ethereum network
- Loading an Ethereum wallet file
- Sending Ether from one address to another
- Deploying a smart contract to the network
- Reading a value from the deployed smart contract
- Updating a value in the deployed smart contract
- Viewing an event logged by the smart contract
Background
This application has purposefully been kept as simple as possible with the aim of demonstrating how to deploy and work with a smart contract on the decentralised Ethereum network.
If you require further background information about the above configuration items for the project you can refer to the below.
For a more comprehensive overview, I recommend you read the Java Magazine Article, watch the YouTube talk, and read all of the documentation.
Smart contracts
The contained smart contract is based on the Greeter contract example, with the addition that the value stored in the Greeter can be modified.
The associated Java smart contract wrapper is named Greeter.
For more background on smart contracts, refer to the smart contracts section of the docs.
Infura
If you don't want to sign up to Infura, and would like to run a node yourself, there are instructions in the getting started section of the docs.
Testnets
For more information on the different Ethereum test networks (or testnets), refer to the testnet overview in the docs.
Ether
For background on Ether, refer to the transactions overview section of the docs.
Wallets and transaction signing
For a more technical overview of working with wallet files, refer to the Offline transaction signing section of the docs.
Viewing requests
A logback configuration is included in the project dependencies allowing the logging of protocol messages if required. To enable, simply change the following line in the logback configuration to read:
<logger name="org.web3j.protocol" level="DEBUG"/>