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1. Introduction

This tutorial introduces Web3j, a Java implementation of the popular Web3 abstraction library.

Web3j is used to interact with the Ethereum network by connecting to Ethereum nodes using JSON-RPC or familiar standards like HTTP, WebSockets, IPC.

Ethereum is a whole topic unto itself so let’s first take a quick look at what it is!

2. Ethereum

Ethereum is a (1) cryptocurrency (token symbol ETH), (2) distributed supercomputer, (3) blockchain, and (4) smart contract network written in Solidity.

In other words, Ethereum (the network) is run by a bunch of connected servers called nodes that communicate in a kind of mesh topology (technically, this is not exactly true but close enough to get a more solid understanding of how it all works).

Web3j, and its parent library called Web3, allows web applications to connect to one of those nodes and thereby submit Ethereum transactions, which are, for all intents and purposes, compiled Solidity smart contract functions that have been previously deployed to the Ethereum network. For more information on smart contracts see our article on creating and deploying them with Solidity here.

Each Node broadcasts its changes to every other node so that consensus and verification can be achieved. Thus, each node contains the entire history of the Ethereum blockchain simultaneously thereby creating a redundant backup of all the data, in a tamper-proof way, and via consensus and verification by all the other node in the network.\

For more detailed information on Ethereum, check out the official page.

3. Set Up

To use the full suite of features provided by Web3j, we have to do a little bit more to get set up than usual. First, Web3j is supplied in several, free-standing, modules each of which can be optionally added to the core pom.xml dependency:

<dependency>
    <groupId>org.web3j</groupId>
    <artifactId>core</artifactId>
    <version>3.3.1</version>
</dependency>

Please note that the team at Web3j provides a pre-built Spring Boot Starter with some configuration and limited functionality built right in!

We’ll restrict our focus to the core functionalities in this article (including how to add Web3j to a Spring MVC application, so compatibility with a wider-range of Spring webapps is obtained).

A full list of these modules can be found on Maven Central.

3.1. Compiling Contracts: Truffle or Solc

There are two primary ways to compile and deploy Ethereum smart contracts (.solc files):

  1. The official Solidity compiler.
  2. Truffle (an abstraction suite for testing, deploying, and managing smart contracts).

We’ll stick with Truffle in this article. Truffle simplifies and abstracts the process of compiling smart contracts, migrating them, and deploying them to a network. It also wraps the Solc compiler letting us gain some experience with both.

To set up Truffle:

$ npm install truffle -g
$ truffle version

Four key commands we’ll use to initialize our project respectively, compile our app, deploy our app to the Blockchain, and test it respectively:

$ truffle init
$ truffle compile
$ truffle migrate
$ truffle test

Now, let’s go over a simple example:

pragma solidity ^0.4.17;

contract Example {
  function Example() {
    // constructor
  }
}

Which should yield the following ABI JSON when compiled:

{
  "contractName": "Example",
  "abi": [
    {
      "inputs": [],
      "payable": false,
      "stateMutability": "nonpayable",
      "type": "constructor"
    }
  ],
  "bytecode": "0x60606040523415600e57600080fd5b603580601b6...,
  "deployedBytecode": "0x6060604052600080fd00a165627a7a72305...,
  //...
}

We can then use the supplied bytecode and ABI within our application to interact with the deployed contracts!

3.2. Testing Contracts: Ganache

One of the easiest ways to work with an Ethereum testnet is to launch own Ganache server. We’ll use the pre-built, out-of-the-box, solution since it’s the easiest to set up and configure. It also provides an interface and server shell for Ganache CLI which drives Ganache under-the-hood.

We can connect to our Ganache server on the default supplied URL address: http://localhost:8545 or http://localhost:7545.

There are a couple of other popular approaches to setting up a test network including using Meta-Mask, Infura, or Go-Lang and Geth.

We’ll stick with Ganache in this article since setting up your own GoLang instance (and configuring it as a custom testnet) can be pretty tricky and since the status of Meta-Mask on Chrome is presently uncertain.

We can use Ganache for manual testing scenarios (when debugging or completing our integration testing) or use them for automated testing scenarios (which we have to build our tests around since, in such circumstances, we might not have the available endpoints).

4. Web3 and RPC

Web3 provides a facade and interface for interacting easily with the Ethereum blockchain and Ethereum server nodes. In other words, Web3 facilitates intercommunication between clients and the Ethereum Blockchain by way of JSON-RPC. Web3J is the official Java port of Web3.

We can initialize Web3j for use within our application by passing in a provider (e.g. – the endpoint of a third-party or local Ethereum node):

Web3j web3a = Web3j.build(new HttpService());
Web3j web3b = Web3j.build(new HttpService("YOUR_PROVIDER_HERE"));
Web3j myEtherWallet = Web3j.build(
  new HttpService("https://api.myetherapi.com/eth"));

The third option shows how to add in a third-party provider (thereby connecting with their Ethereum node). But we also have the option to leave our provider option empty. In that case, the default port will be used (8545) on localhost instead.

5. Essential Web3 Methods

Now that we know how to initialize our app to communicate with the Ethereum blockchain, let’s look at a few, core, ways to interact with the Ethereum blockchain.

It’s a good policy to wrap your Web3 methods with a CompleteableFuture to handle the asynchronous nature of JSON-RPC requests made to your configured Ethereum node.

5.1. Current Block Number

We can, for example, return the current block number:

public CompletableFuture<EthBlockNumber> getBlockNumber() {
    EthBlockNumber result = new EthBlockNumber();
    result = this.web3j.ethBlockNumber()
      .sendAsync()
      .get();
    return CompletableFuture.completedFuture(result);
}

5.2. Account

To get the account of a specified address:

public CompletableFuture<EthAccounts> getEthAccounts() {
    EthAccounts result = new EthAccounts();
    result = this.web3j.ethAccounts()
        .sendAsync() 
        .get();
    return CompletableFuture.completedFuture(result);
}

5.3. Number of Account Transactions

To get the number of transactions of a given address:

public CompletableFuture<EthGetTransactionCount> getTransactionCount() {
    EthGetTransactionCount result = new EthGetTransactionCount();
    result = this.web3j.ethGetTransactionCount(DEFAULT_ADDRESS, 
      DefaultBlockParameter.valueOf("latest"))
        .sendAsync() 
        .get();
    return CompletableFuture.completedFuture(result);
}

5.4. Account Balance

And finally, to get the current balance of an address or wallet:

public CompletableFuture<EthGetBalance> getEthBalance() {
    EthGetBalance result = new EthGetBalance();
    this.web3j.ethGetBalance(DEFAULT_ADDRESS, 
      DefaultBlockParameter.valueOf("latest"))
        .sendAsync() 
        .get();
    return CompletableFuture.completedFuture(result);
}

6. Working with Contracts in Web3j

Once we’ve compiled our Solidity contract using Truffle, we can work with our compiled Application Binary Interfaces (ABI) using the standalone Web3j command line tool available here or as a free-standing zip here.

6.1. CLI Magic

We can then automatically generate our Java Smart Contract Wrappers (essentially a POJO exposing the smart contract ABI) using the following command:

$ web3j truffle generate [--javaTypes|--solidityTypes] 
  /path/to/<truffle-smart-contract-output>.json 
  -o /path/to/src/main/java -p com.your.organisation.name

Running the following command in the root of the project:

web3j truffle generate dev_truffle/build/contracts/Example.json 
  -o src/main/java/com/baeldung/web3/contract -p com.baeldung

generated our Example class:

public class Example extends Contract {
    private static final String BINARY = "0x60606040523415600e576...";
    //...
}

6.2. Java POJO’s

Now that we have our Smart Contract Wrapper, we can create a wallet programmatically and then deploy our contract to that address:

WalletUtils.generateNewWalletFile("PASSWORD", new File("/path/to/destination"), true);
Credentials credentials = WalletUtils.loadCredentials("PASSWORD", "/path/to/walletfile");

6.3. Deploy A Contract

We can deploy our contract like so:

Example contract = Example.deploy(this.web3j,
  credentials,
  ManagedTransaction.GAS_PRICE,
  Contract.GAS_LIMIT).send();

And then get the address:

contractAddress = contract.getContractAddress();

6.4. Sending Transactions

To send a Transaction using the Functions of our Contract we  can initialize a Web3j Function with a List of input values and a List of output parameters:

List inputParams = new ArrayList();
List outputParams = new ArrayList();
Function function = new Function("fuctionName", inputParams, outputParams);
String encodedFunction = FunctionEncoder.encode(function);

We can then initialize our Transaction with necessary gas (used to execute of the Transaction) and nonce parameters:

BigInteger nonce = BigInteger.valueOf(100);
BigInteger gasprice = BigInteger.valueOf(100);
BigInteger gaslimit = BigInteger.valueOf(100);

Transaction transaction = Transaction
  .createFunctionCallTransaction("FROM_ADDRESS", 
    nonce, gasprice, gaslimit, "TO_ADDRESS", encodedFunction);

EthSendTransaction transactionResponse = web3j.ethSendTransaction(transaction).sendAsync().get();
transactionHash = transactionResponse.getTransactionHash();

For a full list of smart contract functionalities see the official docs.

7. Conclusion

That’s it! We’ve set up a Java Spring MVC app with Web3j – it’s Blockchain time!

As always, the code examples used in this article are available over on GitHub.

I just announced the new Spring 5 modules in REST With Spring:

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