Java and the blockchain - introducing web3j
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An overview of what blockchain is, how the Ethereum blockchain works, and how you can deploy and interact with smart contracts on Ethereum, all from within the JVM, thanks to web3j.
![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](https://image.slidesharecdn.com/javaandtheblockchain-161123104954/85/Java-and-the-blockchain-introducing-web3j-17-320.jpg)
![Using web3j
• Create client
Web3j web3 = Web3j.build(new HttpService());
// defaults to http://localhost:8545/
• Call method
web3.<method name>([param1, …, paramN).
[send()|sendAsync()]](https://image.slidesharecdn.com/javaandtheblockchain-161123104954/85/Java-and-the-blockchain-introducing-web3j-20-320.jpg)
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Java and the blockchain - introducing web3j
- 1. Java and the Blockchain Introducing web3j @conors10
- 6. Ethereum • The world computer • Turing-complete virtual machine • Public blockchain (mainnet & testnet)
- 7. 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
- 8. 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
- 9. Smart Contracts • Computerised contract • Code + data that lives on the blockchain at an address • Transactions call functions => state transition
- 10. Transactions • Transfer Ether • Deploy a smart contract • Call a smart contract
- 11. Transactions
- 13. web3j • Complete Ethereum JSON-RPC implementation • Ethereum wallet support • Smart contract wrappers • Command line tools • Android compatible (v1.0.5+)
- 14. 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
- 16. 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
- 17. 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
- 20. Using web3j • Create client Web3j web3 = Web3j.build(new HttpService()); // defaults to http://localhost:8545/ • Call method web3.<method name>([param1, …, paramN). [send()|sendAsync()]
- 21. Display client version Web3j web3 = Web3j.build(new HttpService()); 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
- 22. Sending Ether Web3j web3 = Web3j.build(new HttpService()); 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
- 24. Block #1840479
- 25. 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
- 26. 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; } }
- 27. 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/
- 28. 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); } ...
- 29. Hello Blockchain World! Web3j web3 = Web3j.build(new HttpService()); 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!
- 31. Smarter Contracts • Asset tokenisation • Hold Ether • EIP-20 smart contract token standard • See web3j examples
- 32. web3j + Ethereum • web3j simplifies working with Ethereum • Plenty of documentation • Smart contract integration tests
- 33. 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/