{"id":1442,"date":"2019-08-05T10:03:07","date_gmt":"2019-08-05T10:03:07","guid":{"rendered":"\/blog\/?p=1442"},"modified":"2024-04-25T14:55:59","modified_gmt":"2024-04-25T14:55:59","slug":"coding-smart-contracts-tutorial-part-ii","status":"publish","type":"post","link":"\/blog\/2019\/08\/coding-smart-contracts-tutorial-part-ii\/","title":{"rendered":"Coding Smart Contracts \u2013 Tutorial Part II"},"content":{"rendered":"\n

How to use a smart contract from a java application<\/h2>\n\n\n\n
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Natallia Martchouk, co-founder of trimplement, explains how to develop Ethereum smart contracts.<\/figcaption><\/figure><\/div>\n\n\n\n

In Part I <\/a>of my tutorial I\u2019ve explained how to develop and deploy a simple smart contract. Today we will see how this deployed contract can be used in java applications. 
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We are going to use Parity<\/a> as Ethereum client and Web3j<\/a> java library for interaction with Parity. I\u2019m assuming that you already have installed Web3j<\/em>, solc<\/em> and Parity<\/em> following \u201cPrepare\u201d instructions in Part I<\/a>.<\/p>\n\n\n\n

5. Get Parity Synced<\/strong><\/h3>\n\n\n\n

First of all your Parity<\/em> needs to get synchronized with the Ethereum<\/em> testnet Rinkeby<\/em>, meaning it needs to download the current database status to your local machine. Start your local parity with 
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$parity --chain rinkeby --rpcapi \"eth,net,web3,personal\"
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See also this documentation of Parity about getting synced<\/a>.
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In the meantime, we can prepare everything that we need to call our smart contract from a java application.<\/p>\n\n\n\n\n\n\n\n
6. Create A Smart Contract Wrapper<\/h3>\n\n\n\n
Generate a smart contract wrapper in java, go to the folder where we stored your smart contract and compile the contract using solc<\/em>:<\/p>\n\n\n\n
$ cd myproject\/contracts\/<\/pre>\n\n\n\n
$ mkdir target<\/pre>\n\n\n\n
$ solc Ownable.sol --bin --abi --optimize -o target\/
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Now generate the java wrapper class for your token, replacing `\/yourpath\/to\/src\/main\/java`<\/em> and `com.your.organisation.name`<\/em> with the correct path to the target output dir and java package name.<\/p>\n\n\n\n
$ web3j solidity generate target\/Ownable.bin target\/Ownable.abi -o \/yourpath\/to\/src\/main\/java -p your.package.name 
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Type web3j -h<\/em> to see more options.<\/p>\n\n\n\n
7. Import Your Ethereum Account to Parity<\/h3>\n\n\n\n
Do you remember from our Part I<\/a> that we used MetaMask<\/em> and Remix<\/em> to test our smart contract directly? Now it\u2019s time to test it via the java application. To do so we need to import the private key of the account that was created via MetaMask<\/em> to our local Parity<\/em>. Parity<\/em> stores credentials in so-called keyfiles. To import your private key from MetaMask<\/em> into Parity<\/em> take the following steps (at the time of writing this article, you need to take an indirect way over Geth<\/em> \u2013 probably Parity<\/em> will provide a direct way later):<\/p>\n\n\n\n