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Java

Introduction​

The CKB-SDK-Java is a versatile and reliable toolkit for integrating the Nervos CKB blockchain into Java applications. Java's platform independence, robustness, and extensive library support make it a suitable choice for enterprise-level blockchain solutions. This SDK is particularly useful for developing applications that require strong consistency, reliability, and integration with existing enterprise systems.

Requirements​

ComponentsVersionDescription
Java8Java Development Kit (JDK)
Gradle≥ 5.0Build automation tool

To verify that you have Java and Gradle installed, you can run this in your terminal:

java -version
gradle -v

Setup Project​

  1. Create a new project: Open a terminal and run the following commands to create a folder for your Java project:
mkdir ckb-java-example
cd ckb-java-example
gradle init
  1. Edit build.gradle: Open the build.gradle file and add the ckb-sdk-java dependency:
build.gradle
dependencies {
    implementation 'org.nervos.ckb:ckb:{version}'
}

Replace {version} with the latest version of the CKB-SDK-Java

  1. Build the project: Run the following command in the project root directory:
gradle build

Setup Client​

CKB-SDK-Java provides a convenient client that enables you to easily interact with CKB nodes. You can connect to different networks (Testnet, Devnet, and Mainnet) by specifying the appropriate URL.

import org.nervos.ckb.service.Api;
import org.nervos.ckb.service.CkbRpcApi;

String testnetUrl = "https://testnet.ckb.dev"; // Testnet
String devnetUrl = "http://127.0.0.1:8114"; // Devnet
String mainnetUrl = "https://mainnet.ckb.dev/rpc"; // Mainnet

// Connect to Testnet
CkbRpcApi ckbApi = new Api(testnetUrl);

Examples​

Get Block Info​

You can leverage the above client to call any RPC APIs provided by CKB in Java. Here is a simple example to get block info using a block hash.

byte[] blockHash = Numeric.hexStringToByteArray("0x77fdd22f6ae8a717de9ae2b128834e9b2a1424378b5fc95606ba017aab5fed75");
Block block = ckbApi.getBlock(blockHash);
System.out.println("Block: " + block);

For more details about JSON-RPC APIs, please refer to CKB RPC

Build a Transaction​

ckb-sdk-java encapsulates the common logic into a user-friendly transaction builder. It could greatly free you from getting into Script details and from tedious manual work of building transaction including adding celldeps, transaction fee calculation, change output set and so on.

Here is an example to build a CKB transfer transaction with the help of transaction builder and CKB node.

note

The address and key are for demo purposes only and should not be used in a production environment.

String sender = "ckt1qzda0cr08m85hc8jlnfp3zer7xulejywt49kt2rr0vthywaa50xwsq2qf8keemy2p5uu0g0gn8cd4ju23s5269qk8rg4r";
String receiver = "ckt1qzda0cr08m85hc8jlnfp3zer7xulejywt49kt2rr0vthywaa50xwsqg958atl2zdh8jn3ch8lc72nt0cf864ecqdxm9zf";
Iterator<TransactionInput> iterator = new InputIterator(sender);
TransactionBuilderConfiguration configuration = new TransactionBuilderConfiguration(Network.TESTNET);
configuration.setFeeRate(1000);
TransactionWithScriptGroups txWithGroups = new CkbTransactionBuilder(configuration, iterator)
    .addOutput(receiver, 50100000000L)
    .setChangeOutput(sender)
    .build();

For more use cases of building transaction with a CKB node, please refer to:

Sign & Send Transaction​

Once the TransactionWithScriptGroups is prepared, you can follow these steps to sign and send transaction to CKB network:

  1. Sign transaction with your private key.
  2. Send signed transaction to CKB node, and wait for it to be confirmed.
TransactionWithScriptGroups txWithScriptGroups = ...; // Your built transaction
// 0. Set your private key
String privateKey = "0x6c9ed03816e31...";
// 1. Sign transaction with your private key
TransactionSigner.getInstance(Network.TESTNET).signTransaction(txWithScriptGroups, privateKey);
// 2. Send transaction to CKB node
byte[] txHash = ckbApi.sendTransaction(txWithScriptGroups.txView);
System.out.println(Numeric.toHexString(txHash));

Generate a New Address​

In CKB world, a Lock Script

A Script that enforces access and ownership of a Cell.
can be represented as an address. The secp256k1_blake160 is the most commonly used address. Here's how to generate it.

// Generate a random address
ECKeyPair keyPair = Keys.createEcKeyPair();
Script script = Script.generateSecp256K1Blake160SignhashAllScript(keyPair));
Address address = new Address(script, Network.TESTNET);
System.out.println(address.encode());

Convert Public Key to Address​

Convert elliptic curve public key to an address (secp256k1_blake160):

// The public key sent is an elliptic curve public key of compressed format - a 65-length hex (not include hex prefix 0x).
byte[] publicKey = Numeric.hexStringToByteArray("0x24a501efd328e062c8675f2365970728c859c592beeefd6be8ead3d901330bc01");
Script script = Script.generateSecp256K1Blake160SignhashAllScript(publicKey);
Address address = new Address(script, Network.TESTNET);
System.out.println(address.encode());

Parse Address​

Parse an address from an encoded string and get its network, Script, and encoded string in other formats.

Address address = Address.decode("ckt1qyqxgp7za7dajm5wzjkye52asc8fxvvqy9eqlhp82g");
Script script = address.getScript();
Network network = address.getNetwork();
System.out.println(address.encode());
note

Short address and full bech32 address are deprecated. For more details about addresses, check out CKB Address and RFC-0021.

Additional Resources