Add TransactionBuilder#TransactionBuilder(Transaction) constructor.

CHANGELOG.md

@@ -7,6 +7,7 @@ As this project is pre 1.0, breaking changes may happen for minor version bumps.
 ### Update
 * Support resource leeway parameter when simulating Soroban transactions. ([#561](https://github.com/stellar/java-stellar-sdk/pull/561))
 * Support for the new, optional `diagnosticEventsXdr` field on the `SorobanServer.sendTransaction` method. ([#564](https://github.com/stellar/java-stellar-sdk/pull/564))
+* Add `TransactionBuilder#TransactionBuilder(Transaction)` constructor. ([#567](https://github.com/stellar/java-stellar-sdk/pull/567))
 
 ### Breaking changes
 * The types of the following fields have changed. ([#560](https://github.com/stellar/java-stellar-sdk/pull/560))

src/main/java/org/stellar/sdk/TransactionBuilder.java

@@ -2,8 +2,10 @@
 
 import static org.stellar.sdk.TransactionPreconditions.TIMEOUT_INFINITE;
 
+import java.io.IOException;
 import java.math.BigInteger;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.List;
 import java.util.function.Function;
@@ -57,6 +59,41 @@ public TransactionBuilder(TransactionBuilderAccount sourceAccount, Network netwo
     this(AccountConverter.enableMuxed(), sourceAccount, network);
   }
 
+  /**
+   * Construct a new transaction builder from a transaction.
+   *
+   * @param transaction the transaction to rebuild
+   */
+  public TransactionBuilder(Transaction transaction) {
+    AbstractTransaction abstractTransaction;
+    try {
+      // deep copy
+      abstractTransaction =
+          Transaction.fromEnvelopeXdr(
+              transaction.getAccountConverter(),
+              transaction.toEnvelopeXdrBase64(),
+              transaction.getNetwork());
+    } catch (IOException e) {
+      // This should never happen
+      throw new RuntimeException(e);
+    }
+
+    if (!(abstractTransaction instanceof Transaction)) {
+      // This should never happen
+      throw new RuntimeException("TransactionBuilder can only be used to rebuild a Transaction");
+    }
+    Transaction tx = (Transaction) abstractTransaction;
+
+    this.sourceAccount = new Account(tx.getSourceAccount(), tx.getSequenceNumber() - 1);
+    this.accountConverter = tx.accountConverter;
+    this.memo = tx.getMemo();
+    this.operations = Arrays.asList(tx.getOperations());
+    this.baseFee = tx.getFee() / tx.getOperations().length;
+    this.network = tx.getNetwork();
+    this.preconditions = tx.getPreconditions();
+    this.sorobanData = tx.getSorobanData();
+  }
+
   public int getOperationsCount() {
     return operations.size();
   }

src/test/java/org/stellar/sdk/TransactionBuilderTest.java

@@ -4,6 +4,7 @@
 
 import java.io.IOException;
 import java.math.BigInteger;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import org.junit.Test;
@@ -940,6 +941,59 @@ public void testNoNetworkSet() throws FormatException {
     }
   }
 
+  @Test
+  public void testBuilderFromTx() {
+    KeyPair source =
+        KeyPair.fromSecretSeed("SCH27VUZZ6UAKB67BDNF6FA42YMBMQCBKXWGMFD5TZ6S5ZZCZFLRXKHS");
+
+    Account account = new Account(source.getAccountId(), 2908908335136768L);
+    BumpSequenceOperation operation0 = new BumpSequenceOperation.Builder(1L).build();
+    BumpSequenceOperation operation1 = new BumpSequenceOperation.Builder(2L).build();
+    LedgerKey ledgerKey =
+        new LedgerKey.Builder()
+            .discriminant(LedgerEntryType.ACCOUNT)
+            .account(
+                new LedgerKey.LedgerKeyAccount.Builder()
+                    .accountID(
+                        KeyPair.fromAccountId(
+                                "GB7TAYRUZGE6TVT7NHP5SMIZRNQA6PLM423EYISAOAP3MKYIQMVYP2JO")
+                            .getXdrAccountId())
+                    .build())
+            .build();
+    SorobanTransactionData sorobanData =
+        new SorobanTransactionData.Builder()
+            .resources(
+                new SorobanResources.Builder()
+                    .footprint(
+                        new LedgerFootprint.Builder()
+                            .readOnly(new LedgerKey[] {ledgerKey})
+                            .readWrite(new LedgerKey[] {})
+                            .build())
+                    .readBytes(new Uint32(new XdrUnsignedInteger(699)))
+                    .writeBytes(new Uint32(new XdrUnsignedInteger(0)))
+                    .instructions(new Uint32(new XdrUnsignedInteger(34567)))
+                    .build())
+            .resourceFee(new Int64(100L))
+            .ext(new ExtensionPoint.Builder().discriminant(0).build())
+            .build();
+
+    Transaction transaction =
+        new Transaction(
+            AccountConverter.enableMuxed(),
+            account.getAccountId(),
+            980,
+            account.getIncrementedSequenceNumber(),
+            new org.stellar.sdk.Operation[] {operation0, operation1},
+            new MemoText("hello"),
+            new TransactionPreconditions(
+                null, null, BigInteger.ZERO, 0, new ArrayList<>(), new TimeBounds(100, 200)),
+            sorobanData, // For testing purposes, it is impossible to occur in a real environment.
+            Network.PUBLIC);
+
+    TransactionBuilder builder = new TransactionBuilder(transaction);
+    assertEquals(transaction, builder.build());
+  }
+
   @Test
   public void voidBuilderSorobanDataXdrString() {
     KeyPair source =