Java and the blockchain - introducing web3j

Java and the
Blockchain
Introducing web3j
@conors10

Blockchain Technologies
2008
2013
2014
2015+

Ethereum
• The world computer
• Turing-complete virtual machine
• Public blockchain (mainnet & testnet)

Ether
• The fuel of the Ethereum blockchain
• Pay miners to process transactions
• Market capitalisation ~$1bn USD (Bitcoin ~$10bn)
• Associated with an address + wallet ﬁle
0x19e03255f667bdfd50a32722df860b1eeaf4d635

Obtaining Ether
• Buy it
• Find someone
• Kraken
• BTC Markets
• Mine it
• mainnet => requires dedicated GPUs
• testnet => quick using your CPU
• Refer to Geth/Parity mining docs

Smart Contracts
• Computerised contract
• Code + data that lives on the blockchain at an
address
• Transactions call functions => state transition

Transactions
• Transfer Ether
• Deploy a smart contract
• Call a smart contract

web3j
• Complete Ethereum JSON-RPC implementation
• Ethereum wallet support
• Smart contract wrappers
• Command line tools
• Android compatible (v1.0.5+)

web3j artefacts
• Maven’s Nexus & Bintray's JFrog repositories
• Java 8: org.web3j:core
• Android: org.web3j:core-android
• web3j releases page:
• Command line tools: web3j-<version>.zip

Getting started with
Ethereum
Free cloud clients @ https://infura.io/
Run a local client (to generate Ether):
$ geth --rpcapi personal,db,eth,net,web3
--rpc --testnet
$ parity --chain testnet

Create a wallet
$ ./web3j-1.0.6/bin/web3j wallet create
_ _____ _ _
| | |____ (_) (_)
__ _____| |__ / /_ _ ___
/ / / _ '_ | | | / _
V V / __/ |_) |.___/ / | _ | || (_) |
_/_/ ___|_.__/ ____/| |(_)|_| ___/
_/ |
|__/
Please enter a wallet file password:
Please re-enter the password:
Please enter a destination directory location [/Users/Conor/
Library/Ethereum/testnet/keystore]: ~/testnet-keystore
Wallet file UTC--2016-11-10T22-52-35.722000000Z--
a929d0fe936c719c4e4d1194ae64e415c7e9e8fe.json successfully
created in: /Users/Conor/testnet-keystore

Wallet ﬁle{
"address":"a929d0fe936c719c4e4d1194ae64e415c7e9e8fe",
"id":"c2fbffdd-f588-43a8-9b0c-facb6fd84dfe",
"version":3,
"crypto":{
"cipher":"aes-128-ctr",
"ciphertext":"27be0c93939fc8262977c4454a6b7c261c931dfd8c030b2d3e60ef76f99bfdc6",
"cipherparams":{
"iv":"5aa4fdc64eef6bd82621c6036a323c41"
},
"kdf":"scrypt",
"kdfparams":{
"dklen":32,
"n":262144,
"p":1,
"r":8,
"salt":"6ebc76f30ee21c9a05f907a1ad1df7cca06dd594cf6c537c5e6c79fa88c9b9d1"
},
"mac":"178eace46da9acbf259e94141fbcb7d3d43041e2ec546cd4fe24958e55a49446"
}
}

Using web3j
• Create client
Web3j web3 = Web3j.build(new HttpService());
// defaults to http://localhost:8545/
• Call method
web3.<method name>([param1, …, paramN).
[send()|sendAsync()]

Display client version
Web3ClientVersion clientVersion =
web3.web3ClientVersion()
.sendAsync().get();
System.out.println(“Client version: “ +
clientVersion.getWeb3ClientVersion());
Client version: Geth/v1.4.18-stable-c72f5459/
darwin/go1.7.3

Sending Ether
Credentials credentials = WalletUtils.loadCredentials(
"password", "/path/to/walletfile");
TransactionReceipt transactionReceipt =
Transfer.sendFundsAsync(
web3,
credentials, “0x<to address>",
BigDecimal.valueOf(0.2),
Convert.Unit.ETHER).get();
System.out.println(“Funds transfer completed…” + …);
Funds transfer completed, transaction hash:
0x16e41aa9d97d1c3374a4cb9599febdb24d4d5648b607c99e01a8
e79e3eab2c34, block number: 1840479

Ethereum Smart Contracts
• Usually written in Solidity
• Statically typed high level language
• Compiled to Ethereum Virtual Machine (EVM) byte
code
• Create Java wrappers with web3j

Greeter.sol
contract mortal {
address owner;
function mortal() { owner = msg.sender; }
function kill() { if (msg.sender == owner) suicide(owner); }
}
contract greeter is mortal {
string greeting;
// constructor
function greeter(string _greeting) public {
greeting = _greeting;
}
// getter
function greet() constant returns (string) {
return greeting;
}
}

Smart Contract Wrappers
• Compile
$ solc Greeter.sol --bin --abi --optimize
-o build/
• Generate wrappers
$ ./web3j-1.0.6/bin/web3j solidity
generate build/greeter.bin build/
greeter.abi -p
org.web3j.example.generated -o src/main/
java/

Greeter.java
public final class Greeter extends Contract { 
private static final String BINARY = “6060604052604...."; 
... 
 
public Future<Utf8String> greet() { 
Function function = new Function<Utf8String>("greet",  
Arrays.<Type>asList(),  
Arrays.<TypeReference<Utf8String>>asList(new
TypeReference<Utf8String>() {})); 
return executeCallSingleValueReturnAsync(function); 
} 
 
public static Future<Greeter> deploy(Web3j web3j, Credentials
credentials, BigInteger gasPrice, BigInteger gasLimit, BigInteger
initialValue, Utf8String _greeting) { 
String encodedConstructor =
FunctionEncoder.encodeConstructor(Arrays.<Type>asList(_greeting)); 
return deployAsync(Greeter.class, web3j, credentials,
gasPrice, gasLimit, BINARY, encodedConstructor, initialValue); 
}
...

Hello Blockchain World!
Credentials credentials =
WalletUtils.loadCredentials(
"my password",
"/path/to/walletfile");
Greeter contract = Greeter.deploy(
web3, credentials, BigInteger.ZERO,
new Utf8String("Hello blockchain world!"))
.get();
Utf8String greeting = contract.greet().get();
System.out.println(greeting.getTypeAsString());
Hello blockchain world!

Smarter Contracts
• Asset tokenisation
• Hold Ether
• EIP-20 smart contract token standard
• See web3j examples

web3j + Ethereum
• web3j simpliﬁes working with Ethereum
• Plenty of documentation
• Smart contract integration tests

Further Information
• Project home http://web3j.io
• Mining http://docs.web3j.io/
transactions.html#obtaining-ether
• Useful resources http://docs.web3j.io/links.html
• Chat https://gitter.im/web3j/web3j
• Blog http://conorsvensson.com/

Java and the blockchain - introducing web3j

Recommended

More Related Content

What's hot (20)

Similar to Java and the blockchain - introducing web3j (20)

More from Conor Svensson (6)

Recently uploaded (20)

Java and the blockchain - introducing web3j