Revert "Revert "fix: Issue #1347: ExternalAccountCredentials serializ…

…ation is…" (#1365)

This reverts commit 5a2d943.

oauth2_http/java/com/google/auth/oauth2/ExternalAccountCredentials.java

@@ -42,6 +42,7 @@
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import java.io.IOException;
 import java.io.InputStream;
+import java.io.ObjectInputStream;
 import java.math.BigDecimal;
 import java.net.URI;
 import java.nio.charset.StandardCharsets;
@@ -540,6 +541,13 @@ public CredentialSource getCredentialSource() {
     return credentialSource;
   }
 
+  @SuppressWarnings("unused")
+  private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException {
+    // Properly deserialize the transient transportFactory.
+    input.defaultReadObject();
+    transportFactory = newInstance(transportFactoryClassName);
+  }
+
   @Nullable
   public String getServiceAccountImpersonationUrl() {
     return serviceAccountImpersonationUrl;

oauth2_http/javatests/com/google/auth/oauth2/ExternalAccountCredentialsTest.java

@@ -1117,6 +1117,9 @@ public void serialize() throws IOException, ClassNotFoundException {
         testCredentials.getServiceAccountImpersonationOptions().getLifetime(),
         deserializedCredentials.getServiceAccountImpersonationOptions().getLifetime());
     assertSame(deserializedCredentials.clock, Clock.SYSTEM);
+    assertEquals(
+        MockExternalAccountCredentialsTransportFactory.class,
+        deserializedCredentials.toBuilder().getHttpTransportFactory().getClass());
   }
 
   @Test
@@ -1307,17 +1310,30 @@ protected TestCredentialSource(Map<String, Object> credentialSourceMap) {
       }
     }
 
+    @Override
+    public Builder toBuilder() {
+      return new Builder(this);
+    }
+
     public static Builder newBuilder() {
       return new Builder();
     }
 
     static class Builder extends ExternalAccountCredentials.Builder {
       Builder() {}
 
+      Builder(TestExternalAccountCredentials credentials) {
+        super(credentials);
+      }
+
       @Override
       public TestExternalAccountCredentials build() {
         return new TestExternalAccountCredentials(this);
       }
+
+      public HttpTransportFactory getHttpTransportFactory() {
+        return transportFactory;
+      }
     }
 
     protected TestExternalAccountCredentials(ExternalAccountCredentials.Builder builder) {