@@ -34,10 +34,15 @@
 import com.google.api.client.http.javanet.NetHttpTransport;
 import com.google.api.core.InternalApi;
 import com.google.auth.http.HttpTransportFactory;
+import java.io.Serializable;
 import java.security.GeneralSecurityException;
 import java.security.KeyStore;
+import java.security.KeyStoreException;
+import java.security.cert.Certificate;
+import java.util.Enumeration;
 import java.util.Objects;
 import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 /**
  * An HttpTransportFactory that creates {@link NetHttpTransport} instances configured for mTLS
@@ -49,8 +54,25 @@
  */
 @NullMarked
 @InternalApi
-public class MtlsHttpTransportFactory implements HttpTransportFactory {
-  private final KeyStore mtlsKeyStore;
+public class MtlsHttpTransportFactory implements HttpTransportFactory, Serializable {
+  private static final long serialVersionUID = 1L;
+  private final transient @Nullable KeyStore mtlsKeyStore;
+  private final transient boolean hasKeyStore;
+
+  /**
+   * Default no-arg constructor creating an instance without a KeyStore ({@code hasKeyStore() ==
+   * false}). This constructor is invoked reflectively by {@code
+   * ExternalAccountCredentials.readObject()} during deserialization (since {@code transportFactory}
+   * is transient on {@code ExternalAccountCredentials}), after which {@code
+   * IdentityPoolCredentials.readObject()} reconstructs {@code X509Provider} from the serialized
+   * certificate configuration and replaces the transport factory with {@link
+   * #MtlsHttpTransportFactory(KeyStore)}. Not intended for direct use; callers configuring mTLS
+   * should use {@link #MtlsHttpTransportFactory(KeyStore)}.
+   */
+  public MtlsHttpTransportFactory() {
+    this.mtlsKeyStore = null;
+    this.hasKeyStore = false;
+  }
 
   /**
    * Constructs a factory for mTLS transports.
@@ -61,6 +83,41 @@ public class MtlsHttpTransportFactory implements HttpTransportFactory {
    */
   public MtlsHttpTransportFactory(KeyStore mtlsKeyStore) {
     this.mtlsKeyStore = Objects.requireNonNull(mtlsKeyStore, "mtlsKeyStore cannot be null");
+    this.hasKeyStore = checkHasKeyStore(this.mtlsKeyStore);
+  }
+
+  /**
+   * Returns whether this factory was constructed with a non-null {@link KeyStore} containing client
+   * certificates for mTLS. A factory created via the no-arg constructor (e.g. during
+   * deserialization), with an empty KeyStore, or with a KeyStore containing only trusted CA
+   * certificates (without a private key entry and certificate chain) will return {@code false}.
+   */
+  public boolean hasKeyStore() {
+    return this.hasKeyStore;
+  }
+
+  private static boolean checkHasKeyStore(@Nullable KeyStore keyStore) {
+    if (keyStore == null) {
+      return false;
+    }
+    try {
+      Enumeration<String> aliases = keyStore.aliases();
+      if (aliases == null) {
+        return false;
+      }
+      while (aliases.hasMoreElements()) {
+        String alias = aliases.nextElement();
+        if (keyStore.isKeyEntry(alias)) {
+          Certificate[] chain = keyStore.getCertificateChain(alias);
+          if (chain != null && chain.length > 0) {
+            return true;
+          }
+        }
+      }
+      return false;
+    } catch (KeyStoreException e) {
+      return false;
+    }
   }
 
   @Override
 
@@ -432,6 +432,7 @@ static ExternalAccountCredentials fromJson(
       Map<String, Object> json, HttpTransportFactory transportFactory) {
     String audience = (String) json.get("audience");
     String subjectTokenType = (String) json.get("subject_token_type");
+    String actorTokenType = (String) json.get("actor_token_type");
     String tokenUrl = (String) json.get("token_url");
 
     Map<String, Object> credentialSourceMap = (Map<String, Object>) json.get("credential_source");
@@ -488,6 +489,7 @@ static ExternalAccountCredentials fromJson(
         .setHttpTransportFactory(transportFactory)
         .setAudience(audience)
         .setSubjectTokenType(subjectTokenType)
+        .setActorTokenType(actorTokenType)
         .setTokenUrl(tokenUrl)
         .setTokenInfoUrl(tokenInfoUrl)
         .setCredentialSource(new IdentityPoolCredentialSource(credentialSourceMap))
@@ -533,6 +535,22 @@ private boolean shouldBuildImpersonatedCredential() {
    */
   protected AccessToken exchangeExternalCredentialForAccessToken(
       StsTokenExchangeRequest stsTokenExchangeRequest) throws IOException {
+    return exchangeExternalCredentialForAccessToken(stsTokenExchangeRequest, this.transportFactory);
+  }
+
+  /**
+   * Exchanges the external credential for a Google Cloud access token using the specified transport
+   * factory. This overload allows callers to provide a per-cycle transport factory, for example one
+   * pinned to a specific mTLS certificate.
+   *
+   * @param stsTokenExchangeRequest the Security Token Service token exchange request
+   * @param cycleTransportFactory the HTTP transport factory to use for this exchange
+   * @return the access token returned by the Security Token Service
+   * @throws OAuthException if the call to the Security Token Service fails
+   */
+  protected AccessToken exchangeExternalCredentialForAccessToken(
+      StsTokenExchangeRequest stsTokenExchangeRequest, HttpTransportFactory cycleTransportFactory)
+      throws IOException {
     // Handle service account impersonation if necessary.
     if (this.shouldBuildImpersonatedCredential()) {
       this.impersonatedCredentials = this.buildImpersonatedCredentials();
@@ -543,7 +561,9 @@ protected AccessToken exchangeExternalCredentialForAccessToken(
 
     StsRequestHandler.Builder requestHandler =
         StsRequestHandler.newBuilder(
-            tokenUrl, stsTokenExchangeRequest, transportFactory.create().createRequestFactory());
+            tokenUrl,
+            stsTokenExchangeRequest,
+            cycleTransportFactory.create().createRequestFactory());
 
     // If this credential was initialized with a Workforce configuration then the
     // workforcePoolUserProject must be passed to the Security Token Service via the internal
 
@@ -31,73 +31,194 @@
 
 package com.google.auth.oauth2;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import com.google.api.client.json.GenericJson;
 import com.google.api.client.json.JsonObjectParser;
+import com.google.api.client.util.Data;
 import com.google.auth.oauth2.IdentityPoolCredentialSource.CredentialFormatType;
 import com.google.common.io.CharStreams;
 import java.io.BufferedReader;
-import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.InputStreamReader;
+import java.io.Reader;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.LinkOption;
 import java.nio.file.Paths;
 import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 /**
- * Internal provider for retrieving the subject tokens for {@link IdentityPoolCredentials} to
- * exchange for GCP access tokens via a local file.
+ * Internal provider for retrieving the subject and actor tokens for {@link IdentityPoolCredentials}
+ * to exchange for GCP access tokens via a local file.
+ *
+ * <p>Note: Despite the name, this class handles both subject <em>and</em> actor tokens. The class
+ * name retains "Subject" for serialization backward compatibility; renaming it would break
+ * deserialization of previously serialized credentials.
  */
 @NullMarked
-class FileIdentityPoolSubjectTokenSupplier implements IdentityPoolSubjectTokenSupplier {
+class FileIdentityPoolSubjectTokenSupplier
+    implements IdentityPoolSubjectTokenSupplier, IdentityPoolActorTokenSupplier {
 
-  private final long serialVersionUID = 2475549052347431992L;
+  private static final long serialVersionUID = 7152208690659890358L;
 
   private final IdentityPoolCredentialSource credentialSource;
 
-  /**
-   * Constructor for FileIdentitySubjectTokenProvider
-   *
-   * @param credentialSource the credential source to use.
-   */
   FileIdentityPoolSubjectTokenSupplier(IdentityPoolCredentialSource credentialSource) {
-    this.credentialSource = credentialSource;
+    this.credentialSource = checkNotNull(credentialSource, "credentialSource cannot be null");
   }
 
   @Override
   public String getSubjectToken(ExternalAccountSupplierContext context) throws IOException {
-    String credentialFilePath = this.credentialSource.getCredentialLocation();
+    return getToken(credentialSource.subjectTokenFieldName);
+  }
+
+  @Override
+  public String getActorToken(ExternalAccountSupplierContext context) throws IOException {
+    if (credentialSource.credentialFormatType == CredentialFormatType.TEXT) {
+      throw new IllegalArgumentException(
+          "Actor tokens are only supported for JSON-formatted credential files with distinct field"
+              + " names.");
+    }
+    return getToken(credentialSource.actorTokenFieldName);
+  }
+
+  /**
+   * Reads the credential file once and returns both the subject and actor tokens atomically.
+   *
+   * <p>This method ensures that both tokens are extracted from the same file read, avoiding
+   * potential race conditions when the file is being updated between reads.
+   *
+   * @param context the supplier context
+   * @return a {@link TokenPair} containing both the subject and actor tokens
+   * @throws IOException if the file cannot be read or the required fields are missing
+   */
+  TokenPair readTokens(ExternalAccountSupplierContext context) throws IOException {
+    if (credentialSource.credentialFormatType != CredentialFormatType.JSON) {
+      throw new IOException(
+          "readTokens() is only supported for JSON-formatted credential sources.");
+    }
+
+    String subjectFieldName = credentialSource.subjectTokenFieldName;
+    if (subjectFieldName == null) {
+      throw new IOException("Subject token field name must be specified for JSON credentials.");
+    }
+
+    String credentialFilePath = credentialSource.getCredentialLocation();
     if (!Files.exists(Paths.get(credentialFilePath), LinkOption.NOFOLLOW_LINKS)) {
       throw new IOException(
           String.format(
               "Invalid credential location. The file at %s does not exist.", credentialFilePath));
     }
-    try {
-      return parseToken(
-          Files.newInputStream(new File(credentialFilePath).toPath()), this.credentialSource);
-    } catch (IOException e) {
-      throw new IOException(
-          "Error when attempting to read the subject token from the credential file.", e);
+
+    GenericJson parsedJson = readAndParseJsonFile(credentialFilePath);
+    String subject = extractField(parsedJson, subjectFieldName);
+
+    String actor = null;
+    if (credentialSource.actorTokenFieldName != null) {
+      actor = extractField(parsedJson, credentialSource.actorTokenFieldName);
     }
+
+    return new TokenPair(subject, actor);
   }
 
-  static String parseToken(InputStream inputStream, IdentityPoolCredentialSource credentialSource)
-      throws IOException {
-    if (credentialSource.credentialFormatType == CredentialFormatType.TEXT) {
+  private String getToken(@Nullable String targetFieldName) throws IOException {
+    if (credentialSource.credentialFormatType == CredentialFormatType.JSON
+        && targetFieldName == null) {
+      throw new IOException("Target field name must be specified for JSON credentials.");
+    }
+
+    String credentialFilePath = credentialSource.getCredentialLocation();
+    if (!Files.exists(Paths.get(credentialFilePath), LinkOption.NOFOLLOW_LINKS)) {
+      throw new IOException(
+          String.format(
+              "Invalid credential location. The file at %s does not exist.", credentialFilePath));
+    }
+
+    if (credentialSource.credentialFormatType == CredentialFormatType.JSON) {
+      GenericJson parsedJson = readAndParseJsonFile(credentialFilePath);
+      return extractField(parsedJson, targetFieldName);
+    }
+
+    try (InputStream inputStream = Files.newInputStream(Paths.get(credentialFilePath))) {
       BufferedReader reader =
           new BufferedReader(new InputStreamReader(inputStream, StandardCharsets.UTF_8));
       return CharStreams.toString(reader);
+    } catch (IOException e) {
+      throw new IOException("Error when attempting to read the token from the credential file.", e);
+    }
+  }
+
+  private static GenericJson readAndParseJsonFile(String credentialFilePath) throws IOException {
+    try (InputStream inputStream = Files.newInputStream(Paths.get(credentialFilePath))) {
+      JsonObjectParser parser = new JsonObjectParser(OAuth2Utils.JSON_FACTORY);
+      return parser.parseAndClose(inputStream, StandardCharsets.UTF_8, GenericJson.class);
+    } catch (Exception e) {
+      throw new IOException("Error when attempting to read the token from the credential file.", e);
     }
+  }
+
+  private static String extractField(GenericJson json, String fieldName) throws IOException {
+    Object value = json.get(fieldName);
+    if (value == null || Data.isNull(value)) {
+      throw new IOException("Invalid token field name. No token was found for field: " + fieldName);
+    }
+    if (!(value instanceof String)) {
+      throw new IOException(
+          "Token field value for "
+              + fieldName
+              + " must be a String but was: "
+              + value.getClass().getName());
+    }
+    return (String) value;
+  }
+
+  /** Used primarily for UrlIdentityPoolSubjectTokenSupplier */
+  static String parseToken(
+      InputStream inputStream,
+      IdentityPoolCredentialSource credentialSource,
+      @Nullable String targetFieldName)
+      throws IOException {
+    try (InputStream in = inputStream;
+        Reader reader = new InputStreamReader(in, StandardCharsets.UTF_8)) {
+      if (credentialSource.credentialFormatType == CredentialFormatType.TEXT) {
+        return CharStreams.toString(new BufferedReader(reader));
+      }
+
+      if (targetFieldName == null) {
+        throw new IOException("Target field name must be specified for JSON credentials.");
+      }
+
+      JsonObjectParser parser = new JsonObjectParser(OAuth2Utils.JSON_FACTORY);
+      GenericJson fileContents =
+          parser.parseAndClose(in, StandardCharsets.UTF_8, GenericJson.class);
+
+      Object value = fileContents.get(targetFieldName);
+      if (value == null || Data.isNull(value)) {
+        throw new IOException(
+            "Invalid token field name. No token was found for field: " + targetFieldName);
+      }
+      if (!(value instanceof String)) {
+        throw new IOException(
+            "Token field value for "
+                + targetFieldName
+                + " must be a String but was: "
+                + value.getClass().getName());
+      }
+      return (String) value;
+    }
+  }
 
-    JsonObjectParser parser = new JsonObjectParser(OAuth2Utils.JSON_FACTORY);
-    GenericJson fileContents =
-        parser.parseAndClose(inputStream, StandardCharsets.UTF_8, GenericJson.class);
+  /** Holds a pair of subject and actor tokens read atomically from the same file. */
+  static class TokenPair {
+    final String subject;
+    final @Nullable String actor;
 
-    if (!fileContents.containsKey(credentialSource.subjectTokenFieldName)) {
-      throw new IOException("Invalid subject token field name. No subject token was found.");
+    TokenPair(String subject, @Nullable String actor) {
+      this.subject = subject;
+      this.actor = actor;
     }
-    return (String) fileContents.get(credentialSource.subjectTokenFieldName);
   }
 }