March 20, 2021

How to connect to Ethereum network using Java / Web3j


We can say that Java is one of the most versatile languages out there, and it continues to be relevant in today's time. Java is so popular because of its massive user base and use cases. In this guide/tutorial, we'll learn how to connect to the Ethereum Blockchain network using Java/ Web3j.

Some Background on Java
Java’s slogan is 'Write once, run anywhere,' which means a Java program should be able to run on any machine. The Java JVM (Java Virtual Machine) enables Java code to run on multiple platforms.

Features of Java:
  • Fully object-oriented.
  • Has one of the most mature developer communities.
  • Backed by tech giant Oracle. 
  • Has tons of frameworks to make reliable apps.
  • Has a massive set of IDEs.

In this guide, we will be connecting to the Ethereum network using Java with the help of a Java library called Web3j.

What is Web3j?

What is Web3j?
Web3j is a Java library that interacts with Ethereum Smart contracts and integrates with Ethereum nodes. It is highly modular, type-safe, and reactive, built for Java and Android development on Ethereum. Web3j eliminates the overhead of writing a custom integration code to connect to the Ethereum blockchain network.

Features of Web3j:
  • Complete Ethereum JSON-RPC client API over HTTP and IPC implementation and support for Ethereum wallet.
  • Auto-generation of Java smart contract wrappers to create, deploy, transact with, and call smart contracts from native Java code (Solidity and Truffle definition formats supported).
  • Reactive-functional API for working with filters.
  • Ethereum Name Service (ENS) support.
  • Support for Hosted Ethereum nodes.
  • Support for ERC20 and ERC721 token standards
  • Command-line tools.
  • Android compatible.

  • Java installed on your system.
  • A Java IDE, we'll use Jet Brains IntelliJ IDEA (community version) here.
  • An Ethereum node.

Installing the web3j java library

To install Web3j and run our code, we'll need Java installed in our systems. To check if you have Java installed or not, type the following in your Terminal/command prompt:

java -version

It must return the installed version of Java on your system. If not installed, download it from here, based on your OS type. 

We can install Web3j using either Maven or Gradle. 

For Maven:

Java 8:


For Gradle:

Java 8:
compile ('org.web3j:core:4.6.3')

compile ('org.web3j:core:4.6.0-android')

We'll use Maven to manage our Java library dependency and install it. First, Let's set up our IntelliJ IDE, create a new project, and select a Maven project, then click next.

Now You'll see a pom.xml file opened up in a window, Take out the code from Line 10-14 and copy-paste the following in your pom.xml file:


Here we're adding web3j dependency in the pom.xml file to interact with the Ethereum blockchain and slf4j dependency to manage the logging error; learn more about it here.

Your pom.xml file must look something like this (Web3j_Demo is the project name):


Then from the right panel, click on Maven and expand your project directory, select Lifecycle, and then double-click on install; this will install Web3j. You’ll see a 'BUILD SUCCESS' message in the IDE terminal (In the image below, see steps 1-5).

Booting our Ethereum node

For our purposes today, we could use pretty much any Ethereum client, such as Geth or OpenEthereum (fka Parity). Since that is a bit too involved for just querying for block height, we'll just grab a free endpoint from QuikNode to make this easy. After you've created your free ethereum endpoint, copy your HTTP Provider endpoint:

You'll need this later, so copy it and save it.

Connecting via Java / Web3j

Now let's write a small piece of Java code to fetch the latest Block number from the Ethereum Blockchain. To accomplish this, Expand your project from the left panel, then click on src, then main, and then right-click on Java, select New, and select a Java class; you can name your Java class anything (ref image below). Then, copy/paste this into your newly created Java class file ( in the following example):

import org.web3j.protocol.Web3j;
import org.web3j.protocol.http.HttpService;
import org.web3j.protocol.core.methods.response.EthBlockNumber;
import java.lang.String;
import java.lang.*;
import java.util.concurrent.ExecutionException;

public class BlockNumber {
    public static void main(String[] args) throws ExecutionException, InterruptedException{
        Web3j web3 = HttpService("ADD_YOUR_ETHEREUM_NODE_URL")); 
        EthBlockNumber result = web3.ethBlockNumber().sendAsync().get(); 
        System.out.println(" The Block Number is: " + result.getBlockNumber().toString());

Make sure to replace `ADD_YOUR_ETHEREUM_NODE_URL` with the http provider from the section above.

A quick explanation of the code above: 

Lines 1-6:
Importing Web3j library, Web3j's HttpService, which will enable us to use the HTTP endpoint, EthBlockNumber method of Web3j, java.lang.String and java.lang.* to convert the block hash into a number, java.util.concurrent.ExecutionException to catch Execution error (line 1 - line 6),

Lines 8-12:
Starting our Java code with public class BlockNumber (line 8), Starting our main class and catching execution and interruption exception (line 9), Supplying our Ethereum node to the web3 variable (line 10), Using ethBlockNumber method to send an async get request to the node (line 11), printing our block number after getting the number using getBlockNumber method and then converting it to a string using toString method (line 12).

Run the code from the left panel by right-clicking on the Java class name and selecting run, or using Ctrl+Shift+F1. A successful output should like this:



Congratulations! You have connected to the Ethereum network using Java / Web3j! Check out Web3j GitHub repository for more information and to examine the code that utilizes web3j. Also, study their documentation to learn more.

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