Chain3 Java Library

chain3j <> is a lightweight, highly modular, reactive, type safe Java and Android library for working with Smart Contracts and integrating with clients (nodes) on the MOAC network:

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


  • Complete implementation of MOAC’s JSON-RPC client API over HTTP and IPC
  • Auto-generation of Java smart contract wrappers to create, deploy, transact with and call smart contracts from native Java code (Solidity and
  • Reactive-functional API for working with filters
  • Support for MOAC gateway, so you don’t have to run an MOAC client yourself
  • Comprehensive integration tests demonstrating a number of the above scenarios
  • Command line tools
  • Android compatible

It has five runtime dependencies:

It also uses JavaPoet for generating smart contract wrappers.

Commercial support and training

Commercial support and training is available from


A chain3j sample project is available that demonstrates a number of core features of MOAC with chain3j, including:

  • Connecting to a node on the MOAC network
  • Loading an MOAC keystore file
  • Sending MOAC 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

Typically your application should depend on release versions of chain3j, but you may also use snapshot dependencies for early access to features and fixes, refer to the `Snapshot Dependencies`_ section.

Add the relevant dependency to your project:


Java 8:





Java 8:

compile ('io.github.moacchain:chain3j:0.1.0')


compile ('io.github.moacchain:chain3j:0.1.0-android')

Start a client

Start up an MOAC client if you don’t already have one running, check _:

$ ./moac --rpcapi "personal,mc,net,chain3" --rpc --testnet
Chain3j chain3 = HttpService(""));

Start sending requests

To send synchronous requests:

Chain3j chain3 = HttpService());  // defaults to http://localhost:8545/
Chain3ClientVersion chain3ClientVersion = chain3.chain3ClientVersion().send();
String clientVersion = chain3ClientVersion.getChain3ClientVersion();

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

Chain3j chain3 = HttpService());  // defaults to http://localhost:8545/
Chain3ClientVersion chain3ClientVersion = chain3.chain3ClientVersion().sendAsync().get();
String clientVersion = chain3ClientVersion.getChain3ClientVersion();

To use an RxJava Observable:

Chain3j chain3 = HttpService());  // defaults to http://localhost:8545/
chain3.chain3ClientVersion().observable().subscribe(x -> {
    String clientVersion = x.getChain3ClientVersion();

Note: for Android use:

Chain3j chain3 = HttpService());  // defaults to http://localhost:8545/


chain3j also supports fast inter-process communication (IPC) via file sockets to clients running on the same host as chain3j. To connect simply use the relevant IpcService implementation instead of HttpService when you create your service:

// OS X/Linux/Unix:
Chain3j chain3 = UnixIpcService("/path/to/socketfile"));

// Windows
Chain3j chain3 = WindowsIpcService("/path/to/namedpipefile"));

Note: IPC is not currently available on chain3j-android.

Working with smart contracts with Java smart contract wrappers

chain3j 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 chain3j’s Command line tools:

chain3j solidity generate /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:

Chain3j chain3 = HttpService());  // defaults to http://localhost:8545/
Credentials credentials = WalletUtils.loadCredentials("password", "/path/to/walletfile");

YourSmartContract contract = YourSmartContract.deploy(
        <chain3j>, <credentials>,
        <param1>, ..., <paramN>).send();  // constructor params

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

# Open MOAC wallet and start a local MOAC node
# Click CONTRACTS tab and choose the "DEPLOY NEW CONTRACT" button
# Copy the contract codes to the "SOLIDITY CONTRACT SOURCE CODE"
# The codes will be auto compiled.

Then generate the wrapper code using chain3j’s Command line tools:

$ cd /path/to/your/chain3j/java/project
$ chain3j truffle generate /path/to/<truffle-smart-contract-output>.json -o /path/to/src/main/java -p

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>", <chain3j>, <credentials>, GAS_PRICE, GAS_LIMIT);

To transact with a smart contract:

TransactionReceipt transactionReceipt = contract.someMethod(

To call a smart contract:

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

To fine control your gas price:

contract.setGasProvider(new DefaultGasProvider() {

For more information refer to Smart Contracts.


chain3j 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 = chain3j.blockObservable(false).subscribe(block -> {

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

Subscription subscription = chain3j.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 = chain3j.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:

McFilter filter = new McFilter(DefaultBlockParameterName.EARLIEST,
        DefaultBlockParameterName.LATEST, <contract-address>)
             .addSingleTopic(...)|.addOptionalTopics(..., ...)|...;
chain3j.mcLogObservable(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 and the Chain3jRx interface.


chain3j provides support for both working with MOAC wallet files (recommended) and MOAC client admin commands for sending transactions.

To send Mc to another party using your MOAC wallet file:

Chain3j chain3 = HttpService());  // defaults to http://localhost:8545/
Credentials credentials = WalletUtils.loadCredentials("password", "/path/to/walletfile");
TransactionReceipt transactionReceipt = Transfer.sendFunds(
        chain3, credentials, "0x<address>|<ensName>",
        BigDecimal.valueOf(1.0), Convert.Unit.MC)

Or if you wish to create your own custom transaction:

Chain3j chain3 = HttpService());  // defaults to http://localhost:8545/
Credentials credentials = WalletUtils.loadCredentials("password", "/path/to/walletfile");

// get the next available nonce
McGetTransactionCount mcGetTransactionCount = chain3j.mcGetTransactionCount(
             address, DefaultBlockParameterName.LATEST).sendAsync().get();
BigInteger nonce = mcGetTransactionCount.getTransactionCount();

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

// sign & send out transaction with EIP155 signature
byte[] signedMessage = TransactionEncoder.signTxEIP155(rawTransaction, <chainId>, credentials);
String hexValue = Hex.toHexString(signedMessage);
McSendTransaction mcSendTransaction = chain3j.SendRawTransaction(hexValue).send();
// ...

Although it’s far simpler using chain3j’s Transfer for transacting with Mc.

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

Admin chain3j = HttpService());  // defaults to http://localhost:8545/
PersonalUnlockAccount personalUnlockAccount = chain3j.personalUnlockAccount("0x000...", "a password").sendAsync().get();
if (personalUnlockAccount.accountUnlocked()) {
    // send a transaction

Command line tools

A chain3j fat jar is distributed with each release providing command line tools. The command line tools allow you to use some of the functionality of chain3j from the command line:

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

Further details

In the Java 8 build:

  • chain3j 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. chain3j 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 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.

Build instructions

chain3j includes integration tests for running against a live MOAC client. If you do not have a client running, you can exclude their execution as per the below instructions.

To see the compile options:

$ ./gradlew tasks

To run a full build (excluding integration tests):

$ ./gradlew check

Sample maven configuration:

    <name>Sonatype snapshots repo</name>

Thanks and credits

  • The Web3j project for the framework
  • 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