Fix freezing when fetching IBSN and no results are found

CHANGELOG.md

@@ -96,12 +96,13 @@ Note that this project **does not** adhere to [Semantic Versioning](http://semve
 - We fixed an issue in the preferences where custom columns could be added to the entry table with no qualifier. [#9913](https://github.com/JabRef/jabref/issues/9913)
 - We fixed an issue where the encoding header in a bib file was not respected when the file contained a BOM (Byte Order Mark). [#9926](https://github.com/JabRef/jabref/issues/9926)
 - We fixed an issue where cli help output for import and export format was inconsistent. [koppor#429](https://github.com/koppor/jabref/issues/429)
-- We fixed an issue where no preview could be generated for some entry types and led to an exception [#9947](https://github.com/JabRef/jabref/issues/9947)
+- We fixed an issue where no preview could be generated for some entry types and led to an exception. [#9947](https://github.com/JabRef/jabref/issues/9947)
 - We fixed an issue where the Linux terminal working directory argument was malformed and therefore ignored upon opening a terminal [#9953](https://github.com/JabRef/jabref/issues/9953)
-- We fixen an issue under Linux where under some systems the file instead of the folder was opened [#9607](https://github.com/JabRef/jabref/issues/9607)
+- We fixen an issue under Linux where under some systems the file instead of the folder was opened. [#9607](https://github.com/JabRef/jabref/issues/9607)
 - We fixed an issue where an Automatic Keyword Group could not be deleted in the UI. [#9778](https://github.com/JabRef/jabref/issues/9778)
 - We fixed an issue where the citation key pattern `[edtrN_M]` returned the wrong editor. [#9946](https://github.com/JabRef/jabref/pull/9946)
-- We fixed an issue where empty grey containers would remain in the groups panel, if displaying of group item count is turned off [#9972](https://github.com/JabRef/jabref/issues/9972)
+- We fixed an issue where empty grey containers would remain in the groups panel, if displaying of group item count is turned off. [#9972](https://github.com/JabRef/jabref/issues/9972)
+- We fixed an issue where fetching an ISBN could lead to application freezing when the fetcher did not return any results. [#9979](https://github.com/JabRef/jabref/issues/9979)
 
 ### Removed
 

src/main/java/org/jabref/logic/importer/fetcher/isbntobibtex/IsbnFetcher.java

@@ -2,7 +2,9 @@
 
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.Iterator;
 import java.util.List;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.regex.Pattern;
 
@@ -62,22 +64,23 @@ public Optional<BibEntry> performSearchById(String identifier) throws FetcherExc
             identifier = removeNewlinesAndSpacesFromIdentifier(identifier);
             bibEntry = openLibraryIsbnFetcher.performSearchById(identifier);
         } catch (FetcherException ex) {
+            LOGGER.debug("Got a fetcher exception for IBSN search", ex);
             if (retryIsbnFetcher.isEmpty()) {
                 throw ex;
-            } else {
-                LOGGER.debug("Got a fetcher exception for IBSN search", ex);
-                LOGGER.debug("Try using the alternate ISBN fetchers to find an entry.");
             }
         } finally {
-            while (bibEntry.isEmpty() && retryIsbnFetcher.iterator().hasNext()) {
-                AbstractIsbnFetcher fetcher = retryIsbnFetcher.iterator().next();
-                LOGGER.debug("No entry found for ISBN=" + identifier + "; trying " + fetcher.getName() + " next.");
+            LOGGER.debug("Trying using the alternate ISBN fetchers to find an entry.");
+            // do not move the iterator in the loop as this would always return a new one and thus create and endless loop
+            Iterator<AbstractIsbnFetcher> iterator = retryIsbnFetcher.iterator();
+            while (bibEntry.isEmpty() && iterator.hasNext()) {
+                AbstractIsbnFetcher fetcher = iterator.next();
+                LOGGER.debug("No entry found for ISBN {}; trying {} next.", identifier, fetcher.getName());
                 bibEntry = fetcher.performSearchById(identifier);
             }
         }
 
         if (bibEntry.isEmpty()) {
-            LOGGER.debug("Could not found a entry for ISBN=" + identifier);
+            LOGGER.debug("Could not found a entry for ISBN {}", identifier);
         }
 
         return bibEntry;
@@ -94,9 +97,7 @@ public List<BibEntry> performSearch(BibEntry entry) throws FetcherException {
     }
 
     public IsbnFetcher addRetryFetcher(AbstractIsbnFetcher retryFetcher) {
-        if (retryFetcher == null) {
-            throw new IllegalArgumentException("Please provide a valid isbn fetcher.");
-        }
+        Objects.requireNonNull(retryFetcher, "Please provide a valid isbn fetcher.");
         retryIsbnFetcher.add(retryFetcher);
         return this;
     }