Building Java and Android apps on the blockchain
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In this talk, I demonstrate how you can integrate JVM applications with the Ethereum blockchain. I provide an overview of what a 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. You’ll also learn about web3j’s usage of ReactiveX’s Observable, and the great enhancements it was able to bring to the library, both internally and externally for its users working with the blockchain.
![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](https://image.slidesharecdn.com/javaandtheblockchain2-170418045737/85/Building-Java-and-Android-apps-on-the-blockchain-18-320.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()|observable()]](https://image.slidesharecdn.com/javaandtheblockchain2-170418045737/85/Building-Java-and-Android-apps-on-the-blockchain-21-320.jpg)
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Building Java and Android apps on the blockchain
- 1. Java and the Blockchain Blockchain apps with 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 ~$4.5bn USD (Bitcoin ~$20bn) • Associated with an address + wallet file 0x19e03255f667bdfd50a32722df860b1eeaf4d635
- 8. 1 Ether = $42.80 USD
- 9. Obtaining Ether • Buy it • Find someone • Coinbase • BTC Markets • Mine it • mainnet => requires dedicated GPUs • testnet => quick using your CPU • Refer to Geth/Parity mining docs
- 10. Smart Contracts • Computerised contract • Code + data that lives on the blockchain at an address • Transactions call functions => state transition
- 11. Transactions • Transfer Ether • Deploy a smart contract • Call a smart contract
- 12. Transactions
- 14. web3j • Complete Ethereum JSON-RPC implementation • Sync/async & RX Observable API • Ethereum wallet support • Smart contract wrappers • Command line tools • Android compatible
- 15. 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 • Homebrew: • brew tap web3j/web3j && brew install web3j
- 17. 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
- 18. 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
- 21. Using web3j • Create client Web3j web3 = Web3j.build(new HttpService()); // defaults to http://localhost:8545/ • Call method web3.<method name>([param1, …, paramN). [send()|sendAsync()|observable()]
- 22. 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
- 23. 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
- 25. Block #1840479
- 26. 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()); });
- 27. Processing all new blocks Web3j web3 = Web3j.build(new HttpService()); 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();
- 28. 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
- 29. 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; } }
- 30. 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/
- 31. 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); } ...
- 32. 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!
- 34. Smarter Contracts • Asset tokenisation • Hold Ether • EIP-20 smart contract token standard • See web3j examples
- 36. Projects • web3j-core • web3j-android • web3j Spring Boot Starter • web3j-quorum • JP Morgan’s private blockchain technology
- 37. web3j + Ethereum • web3j simplifies working with Ethereum • Plenty of documentation • Smart contract integration tests
- 38. Further Information • Project home https://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/
- 39. Join us