Lightweight Java library for integration with Ethereum clients
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web3j: Web3 Java Ethereum Ðapp API

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web3j is a lightweight, reactive, type safe Java and Android library for integrating with clients (nodes) on the Ethereum network:

This allows you to work with the Ethereum blockchain, without the additional overhead of having to write your own integration code for the platform.

The Java and the Blockchain talk provides an overview of blockchain, Ethereum and web3j.


  • Complete implementation of Ethereum's JSON-RPC client API over HTTP and IPC
  • Ethereum wallet support
  • Auto-generation of Java smart contract wrappers to create, deploy, transact with and call smart contracts from native Java code
  • Reactive-functional API for working with filters
  • Support for Parity's Personal, and Geth's Personal client APIs
  • Support for Infura, so you don't have to run an Ethereum client yourself
  • Comprehensive integration tests demonstrating a number of the above scenarios
  • Command line tools
  • Android compatible
  • Support for JP Morgan's Quorum via web3j-quorum

It has seven runtime dependencies:

Full project documentation is available at Read the Docs.

Getting started

Add the relevant dependency to your project:


Java 8:





Java 8:

compile ('org.web3j:core:1.1.2')


compile ('org.web3j:core-android:1.1.0')

Start a client

Start up an Ethereum client if you don't already have one running, such as Geth:

$ geth --rpcapi personal,db,eth,net,web3 --rpc --testnet

Or Parity:

$ parity --chain testnet

Or use Infura, which provides free clients running in the cloud:

Web3j web3 = InfuraHttpService(""));

For further information refer to Using Infura with web3j

Start sending requests

To send asynchronous requests using a Future:

Web3j web3 = HttpService());  // defaults to http://localhost:8545/
Web3ClientVersion web3ClientVersion = web3.web3ClientVersion().sendAsync().get();
String clientVersion = web3ClientVersion.getWeb3ClientVersion();

To use an RxJava Observable:

Web3j web3 = HttpService());  // defaults to http://localhost:8545/
web3.web3ClientVersion().observable().subscribe(x -> {
    String clientVersion = x.getWeb3ClientVersion();

To send synchronous requests:

Web3j web3 = HttpService());  // defaults to http://localhost:8545/
Web3ClientVersion web3ClientVersion = web3.web3ClientVersion().send();
String clientVersion = web3ClientVersion.getWeb3ClientVersion();

Note: for Android use:

Web3j web3 = HttpService());  // defaults to http://localhost:8545/


web3j also supports fast inter-process communication (IPC) via file sockets to clients running on the same host as web3j. To connect simply use IpcService instead of HttpService when you create your service:

Web3j web3 = IpcService("/path/to/socketfile"));


web3j functional-reactive nature makes it really simple to setup observers that notify subscribers of events taking place on the blockchain.

To receive all new blocks as they are added to the blockchain:

Subscription subscription = web3j.blockObservable(false).subscribe(block -> {

To receive all new transactions as they are added to the blockchain:

Subscription subscription = web3j.transactionObservable().subscribe(tx -> {

To receive all pending transactions as they are submitted to the network (i.e. before they have been grouped into a block together):

Subscription subscription = web3j.pendingTransactionObservable().subscribe(tx -> {

Topic filters are also supported:

EthFilter filter = new EthFilter(DefaultBlockParameterName.EARLIEST,
        DefaultBlockParameterName.LATEST, <contract-address>)
             .addSingleTopic(...)|.addOptionalTopics(..., ...)|...;
web3j.ethLogObservable(filter).subscribe(log -> {

Subscriptions should always be cancelled when no longer required:


Note: filters are not supported on Infura.

For further information refer to Filters and Events.


web3j provides support for both working with Ethereum wallet files (recommended) and Ethereum client admin commands for sending transactions.

To send Ether to another party using your Ethereum wallet file:

Web3j web3 = HttpService());  // defaults to http://localhost:8545/
Credentials credentials = WalletUtils.loadCredentials("password", "/path/to/walletfile");
TransactionReceipt transactionReceipt = Transfer.sendFunds(
        web3, credentials, "0x...", BigDecimal.valueOf(1.0), Convert.Unit.ETHER);

Or if you wish to create your own custom transaction:

Web3j web3 = HttpService());  // defaults to http://localhost:8545/
Credentials credentials = WalletUtils.loadCredentials("password", "/path/to/walletfile");

// get the next available nonce
EthGetTransactionCount ethGetTransactionCount = web3j.ethGetTransactionCount(
             address, DefaultBlockParameterName.LATEST).sendAsync().get();
BigInteger nonce = ethGetTransactionCount.getTransactionCount();

// create our transaction
RawTransaction rawTransaction  = RawTransaction.createEtherTransaction(
             nonce, <gas price>, <gas limit>, <toAddress>, <value>);

// sign & send our transaction
byte[] signedMessage = TransactionEncoder.signMessage(rawTransaction, credentials);
String hexValue = Hex.toHexString(signedMessage);
EthSendTransaction ethSendTransaction = web3j.ethSendRawTransaction(hexValue).sendAsync().get();
// ...

Although it's far simpler using web3j's Java smart contract wrappers.

Using an Ethereum client's admin commands (make sure you have your wallet in the client's keystore):

Parity parity = HttpService());  // defaults to http://localhost:8545/
PersonalUnlockAccount personalUnlockAccount = parity.personalUnlockAccount("0x000...", "a password").sendAsync().get();
if (personalUnlockAccount.accountUnlocked()) {
    // send a transaction, or use parity.personalSignAndSendTransaction() to do it all in one

Java smart contract wrappers

web3j can auto-generate smart contract wrapper code to deploy and interact with smart contracts without leaving Java.

To generate the wrapper code, compile your smart contract:

$ solc <contract>.sol --bin --abi --optimize -o <output-dir>/

Then generate the wrapper code using web3j's Command line tools:

web3j solidity generate /path/to/<smart-contract>.bin /path/to/<smart-contract>.abi -o /path/to/src/main/java -p

Or in code:

org.web3j.codegen.SolidityFunctionWrapperGenerator /path/to/<smart-contract>.bin /path/to/<smart-contract>.abi -o /path/to/src/main/java -p

Now you can create and deploy your smart contract:

Web3j web3 = HttpService());  // defaults to http://localhost:8545/
Credentials credentials = WalletUtils.loadCredentials("password", "/path/to/walletfile");

YourSmartContract contract = YourSmartContract.deploy(
        <web3j>, <credentials>,
        <param1>, ..., <paramN>).get();  // constructor params

Or use an existing:

YourSmartContract contract = YourSmartContract.load(
        "0x<address>", <web3j>, <credentials>, GAS_PRICE, GAS_LIMIT);

To Transact with a smart contract:

TransactionReceipt transactionReceipt = contract.someMethod(
             new Type(...),

To call a smart contract:

Type result = contract.someMethod(new Type(...), ...).get();

For more information refer to the documentation.

Command line tools

A web3j fat jar is distributed with each release providing command line tools. The command line tools allow you to use some of the functionality of web3j from the command line:

  • Wallet creation
  • Wallet password management
  • Transfer of funds from one wallet to another
  • Generate Solidity smart contract function wrappers

Please refer to the documentation for further information.

Further details

In the Java 8 build:

  • web3j provides type safe access to all responses. Optional or null responses are wrapped in Java 8's Optional type.
  • Async requests are handled using Java 8's CompletableFutures.

In both the Java 8 and Andriod builds:

  • Quantity payload types are returned as BigIntegers. For simple results, you can obtain the quantity as a String via Response.getResult().

Working with filters

See EventFilterIT for an example.

Tested clients

  • Geth
  • Parity

You can run the integration test class CoreIT to verify clients.

Coming soon

  • External key store support
  • IPC interface support
  • WebSocket interface support

Related projects

For a .NET implementation, check out Nethereum.

For a pure Java implementation of the Ethereum client, check out EthereumJ and the work of Ether.Camp.

Build instructions

web3j includes integration tests for running against a live Ethereum client. If you do not have a client running, you can exclude their execution as per the below instructions.

To run a full build including integration tests:

$ ./gradlew check

To run excluding integration tests:

$ ./gradlew -x integrationTest check

Thanks and credits