web3j Overview

web3j
Web3 Java Ðapp API
@conors10

Ethereum integration
challenges
• Smart contract application binary interface
encoders/decoders
• 256 bit numeric types
• Multiple transaction types
• Wallet management
• …

web3j
• Complete Ethereum JSON-RPC implementation
• Sync/async & RX Observable API
• Ethereum wallet support
• Smart contract wrappers
• Command line tools
• Android compatible

JSON-RPC
• Full implementations:
• Ethereum client API
• Parity & Geth Personal (admin) APIs

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

web3j installation
• 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
• Homebrew:
• brew tap web3j/web3j && brew install web3j

Display client version
Web3ClientVersion clientVersion =
web3.web3ClientVersion()
.sendAsync().get();
System.out.println(“Client version: “ +
clientVersion.getWeb3ClientVersion());
Client version: Geth/v1.6.1-stable-021c3c28/
darwin-amd64/go1.8.1

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

Command line tools
• Send Ether + manage wallet ﬁles
$ web3j wallet send ~/.ethereum/keystore/<walletfile> 0x<destination address>
_ _____ _ _
| | |____ (_) (_)
__ _____| |__ / /_ _ ___
/ / / _ '_ | | | / _
V V / __/ |_) |.___/ / | _ | || (_) |
_/_/ ___|_.__/ ____/| |(_)|_| ___/
_/ |
|__/
Please enter your existing wallet file password:
Wallet for address 0x<source address> loaded
Please confirm address of running Ethereum client you wish to send the transfer request to [http://
localhost:8545/]: https://mainnet.infura.io/<infura token>
Connected successfully to client: Parity//v1.4.4-beta-a68d52c-20161118/x86_64-linux-gnu/rustc1.13.0
What amound would you like to transfer (please enter a numeric value): 10
Please specify the unit (ether, wei, ...) [ether]: ether
Please confim that you wish to transfer 10 ether (10000000000000000000 wei) to address 0x<destination
address>
Please type 'yes' to proceed: yes
Commencing transfer (this may take a few
minutes) ................................................................................................
............................$
Funds have been successfully transferred from 0x<source address> to 0x<destination address>
Transaction hash: 0x<tx hash>
Mined block number: 2673468

Create a wallet
$ 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

Smart contract wrappers
• Generate from Solidity ABI + binary ﬁles
• Java code to:
• Deploy
• Call
• Transact

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;
}
}

Greeter.abi[
{
"constant": true,
"inputs": [
],
"name": "greet",
"outputs": [
{
"name": "",
"type": "string"
}
],
"payable": false,
"type": "function"
},
{
"inputs": [
{
"name": "_greeting",
"type": "string"
}
],
"type": "constructor"
},
...
]

Smart Contract Wrappers
• Compile
$ solc Greeter.sol --bin --abi --
optimize -o build/
• Generate wrappers
$ 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!

web3j + RxJava
• Reactive-functional API
• Observables for all Ethereum client methods
Web3j web3 = Web3j.build(new HttpService()); //
defaults to http://localhost:8545/
web3j.web3ClientVersion().observable().subscribe(
x -> {
System.out.println(x.getWeb3ClientVersion());
});

Event callbacks
• Process events in smart contracts
HumanStandardToken contract = deploy(web3j, ALICE,
GAS_PRICE, GAS_LIMIT,
BigInteger.ZERO,
new Uint256(aliceQty), new Utf8String("web3j tokens"),
new Uint8(BigInteger.TEN), new Utf8String(“w3j$")).get();
contract.transferEventObservable(
<startBlock>, <endBlock>)
.subscribe(event -> {
…
});

Processing all new blocks
Subscription subscription =
web3j.blockObservable(false)
.subscribe(block -> {
System.out.println(
"Sweet, block number " +
block.getBlock().getNumber() +
" has just been created");
}, Throwable::printStackTrace);
TimeUnit.MINUTES.sleep(2);
subscription.unsubscribe();

Replay transactions
web3j.replayTransactionsObservable(
<startBlockNo>, <endBlockNo>)
.subscribe(tx -> {
...
});

Replay all + future
web3j.catchUpToLatestAndSubscribeToNewBlocks
Observable(
<startBlockNo>, <fullTxObjects>)
.subscribe(blk -> {
...
});

Replay Performance
941667 blocks on Ropsten (14th June 2017):
• Blocks excluding transactions in 7m22s.
• Blocks including transactions in 41m16s
(2013 Macbook Pro)

Further Information
• Project home https://web3j.io
• Useful resources http://docs.web3j.io/links.html
• Chat https://gitter.im/web3j/web3j
• Blog http://conorsvensson.com/
• https://blk.io

web3j Overview

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