web3j: Web3 Java Ethereum Ðapp API

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web3j is a lightweight, highly modular, reactive, type safe Java and Android library for working with Smart Contracts and 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.

Features

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 (Solidity and Truffle definition formats supported)
Reactive-functional API for working with filters
Ethereum Name Service (ENS) support
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 five runtime dependencies:

RxJava for its reactive-functional API
OKHttp for HTTP connections
Jackson Core for fast JSON serialisation/deserialisation
Bouncy Castle (Spongy Castle on Android) for crypto
Jnr-unixsocket for *nix IPC (not available on Android)

It also uses JavaPoet for generating smart contract wrappers.

Full project documentation is available at docs.web3j.io.

Donate

You can help fund the development of web3j by donating to the following wallet addresses:

Ethereum	0x2dfBf35bb7c3c0A466A6C48BEBf3eF7576d3C420
Bitcoin	1DfUeRWUy4VjekPmmZUNqCjcJBMwsyp61G

Commercial support and training

Commercial support and training is available from blk.io.

Quickstart

A web3j sample project is available that demonstrates a number of core features of Ethereum with web3j, including:

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

Getting started

Add the relevant dependency to your project:

Maven

Java 8:

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

Android:

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

Gradle

Java 8:

compile ('org.web3j:core:3.3.1')

Android:

compile ('org.web3j:core:3.3.1-android')

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 = Web3j.build(new HttpService("https://ropsten.infura.io/your-token"));

For further information refer to Using Infura with web3j

Instructions on obtaining Ether to transact on the network can be found in the testnet section of the docs.

Start sending requests

To send synchronous requests:

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

To send asynchronous requests using a CompletableFuture (Future on Android):

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

To use an RxJava Observable:

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

Note: for Android use:

Web3j web3 = Web3jFactory.build(new HttpService());  // defaults to http://localhost:8545/
...

IPC

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

// OS X/Linux/Unix:
Web3j web3 = Web3j.build(new UnixIpcService("/path/to/socketfile"));
...

// Windows
Web3j web3 = Web3j.build(new WindowsIpcService("/path/to/namedpipefile"));
...

Note: IPC is not currently available on web3j-android.

Working with smart contracts with Java smart contract wrappers

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

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 com.your.organisation.name

Now you can create and deploy your smart contract:

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

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

Alternatively, if you use Truffle, you can make use of its .json output files:

# Inside your Truffle project
$ truffle compile
$ truffle deploy

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

$ cd /path/to/your/web3j/java/project
$ web3j truffle generate /path/to/<truffle-smart-contract-output>.json -o /path/to/src/main/java -p com.your.organisation.name

Whether using Truffle or solc directly, either way you get a ready-to-use Java wrapper for your contract.

So, to use an existing contract:

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

To transact with a smart contract:

TransactionReceipt transactionReceipt = contract.someMethod(
             <param1>,
             ...).send();

To call a smart contract:

Type result = contract.someMethod(<param1>, ...).send();

For more information refer to Smart Contracts.

Filters

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 -> {
    ...
});

Or, if you'd rather replay all blocks to the most current, and be notified of new subsequent blocks being created:

There are a number of other transaction and block replay Observables described in the docs.

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:

subscription.unsubscribe();

Note: filters are not supported on Infura.

For further information refer to Filters and Events and the Web3jRx interface.

Transactions

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

Or if you wish to create your own custom transaction:

Web3j web3 = Web3j.build(new 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).send();
// ...

Although it's far simpler using web3j's Transfer for transacting with Ether.

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

Admin web3j = Admin.build(new HttpService());  // defaults to http://localhost:8545/
PersonalUnlockAccount personalUnlockAccount = web3j.personalUnlockAccount("0x000...", "a password").sendAsync().get();
if (personalUnlockAccount.accountUnlocked()) {
    // send a transaction
}

If you want to make use of Parity's Personal or Trace, or Geth's Personal client APIs, you can use the org.web3j:parity and org.web3j:geth modules respectively.

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.
Asynchronous requests are wrapped in a Java 8 CompletableFutures. web3j provides a wrapper around all async requests to ensure that any exceptions during execution will be captured rather then silently discarded. This is due to the lack of support in CompletableFutures for checked exceptions, which are often rethrown as unchecked exception causing problems with detection. See the Async.run() and its associated test for details.

In both the Java 8 and Android builds:

Quantity payload types are returned as BigIntegers. For simple results, you can obtain the quantity as a String via Response.getResult().
It's also possible to include the raw JSON payload in responses via the includeRawResponse parameter, present in the HttpService and IpcService classes.

Tested clients

Geth
Parity

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

Related projects

For a .NET implementation, check out Nethereum.

For a pure Java implementation of the Ethereum client, check out EthereumJ and Ethereum Harmony.

Projects using web3j

Please submit a pull request if you wish to include your project on the list:

Companies using web3j

Please submit a pull request if you wish to include your company on the list:

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 (excluding integration tests):

$ ./gradlew check

To run the integration tests:

$ ./gradlew  -Pintegration-tests=true :integration-tests:test

Thanks and credits

The Nethereum project for the inspiration
Othera for the great things they are building on the platform
Finhaus guys for putting me onto Nethereum
bitcoinj for the reference Elliptic Curve crypto implementation
Everyone involved in the Ethererum project and its surrounding ecosystem
And of course the users of the library, who've provided valuable input & feedback

t4n6a1ka/web3j