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Quickstart

Install Web3j

To start using Web3j you have two options:

Use Web3j CLI

This is the recommended option for full project creation.

Install Web3j CLI

The simplest way to install the Web3j CLI is via the following script:

On Unix:

curl -L get.web3j.io | sh && source ~/.web3j/source.sh

On Windows:

Set-ExecutionPolicy Bypass -Scope Process -Force; iex ((New-Object System.Net.WebClient).DownloadString('https://raw.githubusercontent.com/web3j/web3j-installer/master/installer.ps1'))

For more information on Web3j CLI check the CLI section of the docs.

Create a new project

$ web3j new

This will create a sample project to get you started.

It includes a HelloWorld smart contract and boilerplate code that is easy to follow.

Run your project

Then run the project with the command web3j run <network_url> <wallet_path> <wallet_password>. You can also provide custom parameters to run your project locally.

Use Web3j plugins

To generate Java Wrappers for Solidity Contracts in your project, you need to add one of the Web3j's plugins available for Maven and Gradle.

Gradle plugin

To add the Gradle plugin to your project:

plugins {
    id "org.web3j" version "4.8.4"
}

Once the plugin is downloaded you should see a new set of tasks under the Web3j label:

The simplest way is to create a new folder src/main/solidity and place your smart contracts there. To generate the Java wrappers simply run generateContractWrappers or ./gradlew/generateContractWrappers. The wrappers will generate under build/generated/source/web3j/main/java.

Maven plugin

To add the Maven plugin to your project:

<build>
    <plugins>
        <plugin>
            <groupId>org.web3j</groupId>
            <artifactId>web3j-maven-plugin</artifactId>
            <version>4.8.1</version>
            <configuration>
                <soliditySourceFiles/>
            </configuration>
        </plugin>
    </plugins>
</build>

Use Web3j

Deploying a Smart Contract

To deploy the HelloWorld contract from the previous example:

  Web3j web3j = Web3j.build(new HttpService("<your_node_url>"));
  HelloWorld helloWorld = HelloWorld.deploy(web3j, Credentials.create("your_private_key"), new DefaultGasProvider(), "Hello Blockchain World").send();
  String greeting = helloWorld.greeting().send();
  web3j.shutdown();

There are many ways to create Credentials in Web3j, for more information go to the credentials section.

Loading a Smart Contract

If you have already deployed a contract and would like to interact with it through web3j then the Java Wrappers or your smart contract have a load method.

 Web3j web3j = Web3j.build(new HttpService("<your_node_url>"));
 String greeting;
 HelloWorld helloWorld = HelloWorld.load("your_contract_address", web3j, Credentials.create("your_private_key"), new DefaultGasProvider());
 if (helloWorld.isValid()) {
    greeting = helloWorld.greeting().send();
 }
 web3j.shutdown();

It is important that the loaded contract is checked using the isValid() method. This method will return false if the contract's bytecode does not match with the deployed one.