bitcoin-s/docs/core/txbuilder.md

---
id: txbuilder
title: TxBuilder Example
---

Bitcoin-S features a transaction building API that allows you to construct and sign Bitcoin transactions. Here's an example of how to use it

```scala mdoc:invisible
import scala.concurrent._
import scala.concurrent.duration._

import org.bitcoins.core._
import number._
import config._
import currency._
import org.bitcoins.crypto._
import script.crypto._
import protocol.transaction._
import protocol.script._

import wallet.builder._
import wallet.fee._
import wallet.utxo._

```

```scala mdoc:to-string

implicit val ec: ExecutionContext = ExecutionContext.Implicits.global

// Initialize a transaction builder
val builder = RawTxBuilder()

// generate a fresh private key that we are going to use in the scriptpubkey
val privKey = ECPrivateKey.freshPrivateKey
val pubKey = privKey.publicKey

// this is the script that the TxBuilder is going to create a
// script signature that validly spends this scriptPubKey
val creditingSpk = P2PKHScriptPubKey(pubKey = privKey.publicKey)
val amount = 10000.satoshis

// this is the UTXO we are going to be spending
val utxo =
  TransactionOutput(value = amount, scriptPubKey = creditingSpk)

// the private key that locks the funds for the script we are spending too
val destinationPrivKey = ECPrivateKey.freshPrivateKey

// the amount we are sending -- 5000 satoshis -- to the destinationSPK
val destinationAmount = 5000.satoshis

// the script that corresponds to destination private key, this is what is receiving the money
val destinationSPK =
  P2PKHScriptPubKey(pubKey = destinationPrivKey.publicKey)

// this is where we are sending money too
// we could add more destinations here if we
// wanted to batch transactions
val destinations = {
    val destination0 = TransactionOutput(value = destinationAmount,
                                         scriptPubKey = destinationSPK)

    Vector(destination0)
}

// Add the destinations to the tx builder
builder ++= destinations

// we have to fabricate a transaction that contains the
// UTXO we are trying to spend. If this were a real blockchain
// we would need to reference the UTXO set
val creditingTx = BaseTransaction(version = Int32.one,
                                  inputs = Vector.empty,
                                  outputs = Vector(utxo),
                                  lockTime = UInt32.zero)

// this is the information we need from the crediting TX
// to properly "link" it in the transaction we are creating
val outPoint = TransactionOutPoint(creditingTx.txId, UInt32.zero)
val input = TransactionInput(
    outPoint,
    EmptyScriptSignature,
    sequenceNumber = UInt32.zero)

// Add a new input to our builder
builder += input

// We can now generate a RawTxBuilderResult ready to be finalized
val builderResult = builder.result()

// this contains the information needed to analyze our input during finalization
val inputInfo = P2PKHInputInfo(outPoint, amount, privKey.publicKey)

// this is how much we are going to pay as a fee to the network
// for this example, we are going to pay 1 satoshi per byte
val feeRate = SatoshisPerByte(1.satoshi)

val changePrivKey = ECPrivateKey.freshPrivateKey
val changeSPK = P2PKHScriptPubKey(pubKey = changePrivKey.publicKey)

// We chose a finalizer that adds a change output to our tx based on a fee rate
val finalizer = NonInteractiveWithChangeFinalizer(
    Vector(inputInfo),
    feeRate,
    changeSPK)

// We can now finalize the tx builder result from earlier with this finalizer
val unsignedTxF: Future[Transaction] = finalizer.buildTx(builderResult)

// We now turn to signing the unsigned transaction
// this contains all the information we need to
// validly sign the UTXO above
val utxoInfo = ScriptSignatureParams(inputInfo = inputInfo,
                                     signers = Vector(privKey),
                                     hashType =
                                         HashType.sigHashAll)

// all of the UTXO spending information, since we only have
// one input, this is just one element
val utxoInfos: Vector[ScriptSignatureParams[InputInfo]] = Vector(utxoInfo)

// Yay! Now we use the RawTxSigner object to sign the tx.
// The 'sign' method is going produce a validly signed transaction
// This is going to iterate through each of the UTXOs and use
// the corresponding ScriptSignatureParams to produce a validly
// signed input. This UTXO has:
//   1: one input
//   2: outputs (destination and change outputs)
//   3: a fee rate of 1 satoshi/byte
val signedTx: Transaction = {
  val signedTxF = unsignedTxF.flatMap { unsignedTx =>
    RawTxSigner.sign(
        utx = unsignedTx,
        utxoInfos = utxoInfos,
        expectedFeeRate = feeRate
    )
  }
  Await.result(signedTxF, 30.seconds)
}
```

```scala mdoc:to-string
signedTx.inputs.length

signedTx.outputs.length

//remember, you can call .hex on any bitcoin-s data structure to get the hex representation!
signedTx.hex
```