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![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)
Uploaded byConor Svensson
Building Java and Android apps on the blockchain
The document provides an overview of blockchain technology, specifically focusing on Ethereum and its integration with Java through the web3j library. It outlines key concepts such as decentralization, smart contracts, and instructions for setting up and using Ethereum in Java applications. Additionally, it includes details on wallet creation, transaction handling, and reactive programming with web3j.
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Building Java and Android apps on the blockchain
- 1. Java and the Blockchain Blockchainapps with web3j @conors10
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- 6. Ethereum • The worldcomputer • Turing-complete virtual machine • Public blockchain (mainnet & testnet)
- 7. Ether • The fuelof 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 • Buyit • 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 • Computerisedcontract • 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
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- 14. web3j • Complete EthereumJSON-RPC implementation • Sync/async & RX Observable API • Ethereum wallet support • Smart contract wrappers • Command line tools • Android compatible
- 15. web3j artefacts • Maven’sNexus & 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
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- 17. Getting started with Ethereum Freecloud 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
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- 21. Using web3j • Createclient 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 Web3jweb3 = 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
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- 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 newblocks 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 { addressowner; 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 classGreeter 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! Web3jweb3 = 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!
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- 34. Smarter Contracts • Assettokenisation • 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 • Projecthome 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/
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