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Signing and Sending a Raw Transaction using BitcoinJ

In this article, we’ll walk through the process of creating a raw transaction on the Ethereum blockchain using the BitcoinJ library. We’ll cover how to sign and send a raw transaction from scratch.

Prerequisites

Before you start, make sure you have:

• BitcoinJ (the official Java implementation of the Bitcoin protocol)

• A Bitcoin client (optional but recommended for debugging purposes)

Creating a Raw Transaction

A raw transaction is a plain text string that represents the contents of a block. To create one, we need to follow these steps:

• Create a new transaction: We’ll use the Transaction class from BitcoinJ to create a new transaction.

• Initialize the unspents list: We’ll store all spent coins in this list.

Code

Transaction transaction = new Transaction(params);

// 遍历未花费列表，组装合适的item

double sum = 0;

String address = null;

List unspents = new ArrayList();

Map spentCoins = new HashMap();


// ...


params.getUnspent().forEach((unspent: Unspent) -> {

    if (unspent.isFunded()) {

        double amount = unspent.getAmount();

        sum += amount;

        spentCoins.put(new Coin(unspent.getHash(), 1), unspent);

    }

});


double totalAmount = sum - spentCoins.values().stream()

        .mapToDouble(Unspent::getAmount).sum();


transaction.setFromAddress(address);

transaction.setToAddress("0x..."); // Replace with your recipient's address

transaction.addTransactionDetails(totalAmount, "raw transaction details");

Key Points

• We iterate over the unspent list and add any coins that are funded to our spentCoins map.

• We calculate the total amount spent in the raw transaction by subtracting the amounts of all coins from the sum.

• We set the sender’s address to a fixed value («0x…») and add details about the raw transaction, including its total amount.

Signing the Raw Transaction

To sign the raw transaction using BitcoinJ, we’ll create a new Signer instance:

Signer signer = new Signer(transaction);

We can then use this signer to generate a signature for the raw transaction:

byte[] signature = signer.signRawTransaction(transaction);

Sending the Raw Transaction

To send the raw transaction, we’ll create a TransactionSender instance:

TransactionSender sender = new TransactionSender(transaction, null); // No recipient address is set

We can then use this sender to broadcast the raw transaction to the Bitcoin network:

sender.broadcast();

Note that in practice, you’d likely want to set a recipient address for the raw transaction when creating it. The example above shows how to do that.

Example Use Case

To send a raw transaction from scratch using BitcoinJ, follow these steps:

• Create a new Transaction instance and initialize the unspent list.

• Iterate over the unspent list and add any funded coins to a map.

• Calculate the total amount spent in the raw transaction.

• Set the sender’s address and add details about the raw transaction.

• Sign the raw transaction using a signer.

• Send the signed raw transaction.

This should give you a good starting point for creating raw transactions on the Ethereum blockchain using BitcoinJ. Remember to replace placeholder addresses with your own recipient’s information!