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Code Howtos

This page provides some development support in the form of howtos. See also High Level Documentation.

Generic code how tos

We really recommend reading the book Java by Comparison.

Please read

  • try not to abbreviate names of variables, classes or methods
  • use lowerCamelCase instead of snake_case
  • name enums in singular, e.g. Weekday instead of Weekdays (except if they represent flags)

Error Handling in JabRef

Throwing and Catching Exceptions


  • All exceptions we throw should be or extend JabRefException; This is especially important if the message stored in the Exception should be shown to the user. JabRefException has already implemented the getLocalizedMessage() method which should be used for such cases (see details below!).
  • Catch and wrap all API exceptions (such as IOExceptions) and rethrow them
    • Example:

         try {
             // ...
         } catch (IOException ioe) {
             throw new JabRefException("Something went wrong...",
                 Localization.lang("Something went wrong...", ioe);
  • Never, ever throw and catch Exception or Throwable
  • Errors should only be logged when they are finally caught (i.e., logged only once). See Logging for details.
  • If the Exception message is intended to be shown to the User in the UI (see below) provide also a localizedMessage (see JabRefException).

(Rationale and further reading:

Outputting Errors in the UI

Principle: Error messages shown to the User should not contain technical details (e.g., underlying exceptions, or even stack traces). Instead, the message should be concise, understandable for non-programmers and localized. The technical reasons (and stack traces) for a failure should only be logged.

To show error message two different ways are usually used in JabRef:

  • showing an error dialog
  • updating the status bar at the bottom of the main window
TODO: Usage of status bar and Swing Dialogs

Using the EventSystem

What the EventSystem is used for

Many times there is a need to provide an object on many locations simultaneously. This design pattern is quite similar to Java’s Observer, but it is much simpler and readable while having the same functional sense.

Main principle

EventBus represents a communication line between multiple components. Objects can be passed through the bus and reach the listening method of another object which is registered on that EventBus instance. Hence, the passed object is available as a parameter in the listening method.

Register to the EventBus

Any listening method has to be annotated with @Subscribe keyword and must have only one accepting parameter. Furthermore, the object which contains such listening method(s) has to be registered using the register(Object) method provided by EventBus. The listening methods can be overloaded by using different parameter types.

Posting an object

post(object) posts an object through the EventBus which has been used to register the listening/subscribing methods.

Short example

/* */


public class Listener {

   private int value = 0;

   public void listen(int value) {
      this.value = value;

   public int getValue() {
      return this.value;
/* */


public class Main {
   private static EventBus eventBus = new EventBus();

   public static void main(String[] args) {
      Main main = new Main();
      Listener listener = new Listener();
      eventBus.register(listener);; // 1 represents the passed event

      // Output should be 1

Event handling in JabRef

The event package contains some specific events which occur in JabRef.

For example: Every time an entry was added to the database a new EntryAddedEvent is sent through the eventBus which is located in BibDatabase.

If you want to catch the event you’ll have to register your listener class with the registerListener(Object listener) method in BibDatabase. EntryAddedEvent provides also methods to get the inserted BibEntry.


JabRef uses the logging facade SLF4j. All log messages are passed internally to tinylog which handles any filtering, formatting and writing of log messages.

  • Obtaining a logger for a class:

    private static final Logger LOGGER = LoggerFactory.getLogger(<ClassName>.class);
  • If the logging event is caused by an exception, please add the exception to the log message as:

      catch (SomeException e) {
         LOGGER.warn("Warning text.", e);
  • SLF4J also support parameterized logging, e.g. if you want to print out multiple arguments in a log statement use a pair of curly braces. Examples
  • When running tests, is used. It is located under src/test/resources. As default, only info is logged. When developing, it makes sense to use debug as log level. One can change the log level per class using the pattern level@class=debug is set to debug. In the .properties file, this is done for org.jabref.model.entry.BibEntry.

Using Localization correctly

More information about this topic from the translator side is provided at Translating JabRef Interface.

All labeled UI elements, descriptions and messages shown to the user should be localized, i.e., should be displayed in the chosen language.

JabRef uses ResourceBundles (see Oracle Tutorial) to store key=value pairs for each String to be localized.

To show an localized String the following org.jabref.logic.l10n.Localization has to be used. The Class currently provides three methods to obtain translated strings:

    public static String lang(String key);

    public static String lang(String key, String... params);

    public static String menuTitle(String key, String... params);

The actual usage might look like:

    Localization.lang("Get me a translated String");
    Localization.lang("Using %0 or more %1 is also possible", "one", "parameter");
    Localization.menuTitle("Used for Menus only");

General hints:

  • Use the String you want to localize directly, do not use members or local variables: Localization.lang("Translate me"); instead of Localization.lang(someVariable) (possibly in the form someVariable = Localization.lang("Translate me")
  • Use %x-variables where appropriate: Localization.lang("Exported %0 entries.", number) instead of Localization.lang("Exported ") + number + Localization.lang(" entries.");
  • Use a full stop/period (“.”) to end full sentences

The tests check whether translation strings appear correctly in the resource bundles.

  1. Add new Localization.lang("KEY") to Java file. Run the LocalizationConsistencyTestunder (src/test/org.jabref.logic.


  2. Tests fail. In the test output a snippet is generated which must be added to the English translation file.
  3. Add snippet to English translation file located at src/main/resources/l10n/
  4. Please do not add translations for other languages directly in the properties. They will be overwritten by Crowdin

Adding a new Language

  1. Add the new Language to the Language enum in
  2. Create an empty <locale code>.properties file
  3. Configure the new language in Crowdin

If the language is a variant of a language zh_CN or pt_BR it is necessary to add a language mapping for Crowdin to the crowdin.yml file in the root. Of course the properties file also has to be named according to the language code and locale.

Cleanup and Formatters

We try to build a cleanup mechanism based on formatters. The idea is that we can register these actions in arbitrary places, e.g., onSave, onImport, onExport, cleanup, etc. and apply them to different fields. The formatters themselves are independent of any logic and therefore easy to test.

Example: NormalizePagesFormatter

Drag and Drop

Drag and Drop makes usage of the Dragboard. For JavaFX the following tutorial is helpful. Note that the data has to be serializable which is put on the dragboard. For drag and drop of Bib-entries between the maintable and the groups panel, a custom Dragboard is used, CustomLocalDragboard which is a generic alternative to the system one.

For accessing or putting data into the Clipboard use the ClipboardManager.

Get the JabRef frame panel

JabRefFrame and BasePanel are the two main classes. You should never directly call them, instead pass them as parameters to the class.

Get Absolute Filename or Path for file in File directory

Optional<Path> file = FileHelper.expandFilename(database, fileText, preferences.getFilePreferences());

String path Can be the files name or a relative path to it. The Preferences should only be directly accessed in the GUI. For the usage in logic pass them as parameter

Setting a Database Directory for a .bib File

  • @comment{jabref-meta: fileDirectory:<directory>
  • “fileDirectory” is determined by Globals.pref.get(“userFileDir”) (which defaults to “fileDirectory”
  • There is also “fileDirectory-<username>”, which is determined by Globals.prefs.get(“userFileDirIndividual”)
  • Used at DatabasePropertiesDialog

How to work with Preferences

model and logic must not know JabRefPreferences. See ProxyPreferences for encapsulated preferences and for a detailed discussion.

See (via for the current way how to deal with preferences.

Defaults should go into the model package. See Comments in this Commit

Test Cases

General hints on tests

Imagine you want to test the method format(String value) in the class BracesFormatter which removes double braces in a given string.

  • Placing: all tests should be placed in a class named classTest, e.g. BracesFormatterTest.
  • Naming: the name should be descriptive enough to describe the whole test. Use the format methodUnderTest_ expectedBehavior_context (without the dashes). So for example formatRemovesDoubleBracesAtBeginning. Try to avoid naming the tests with a test prefix since this information is already contained in the class name. Moreover, starting the name with test leads often to inferior test names (see also the Stackoverflow discussion about naming).
  • Test only one thing per test: tests should be short and test only one small part of the method. So instead of

    testFormat() {
     assertEqual("test", format("test"));
     assertEqual("{test", format("{test"));
     assertEqual("test", format("test}}"));

    we would have five tests containing a single assert statement and named accordingly (formatDoesNotChangeStringWithoutBraces, formatDoesNotRemoveSingleBrace, , etc.). See JUnit AntiPattern for background.

  • Do not just test happy paths, but also wrong/weird input.
  • It is recommend to write tests before you actually implement the functionality (test driven development).
  • Bug fixing: write a test case covering the bug and then fix it, leaving the test as a security that the bug will never reappear.
  • Do not catch exceptions in tests, instead use the assertThrows(Exception.class, ()->doSomethingThrowsEx()) feature of junit-jupiter to the test method.

Lists in tests

  • Use assertEquals(Collections.emptyList(), actualList); instead of assertEquals(0, actualList.size()); to test whether a list is empty.
  • Similarly, use assertEquals(Arrays.asList("a", "b"), actualList); to compare lists instead of

           assertEquals(2, actualList.size());
           assertEquals("a", actualList.get(0));
           assertEquals("b", actualList.get(1));

BibEntries in tests

  • Use the assertEquals methods in BibtexEntryAssert to check that the correct BibEntry is returned.

Files and folders in tests

  • If you need a temporary file in tests, then add the following Annotation before the class:

    class TestClass{
      void setUp(@TempDirectory.TempDir Path temporaryFolder){

    to the test class. A temporary file is now created by Files.createFile(path). Using this pattern automatically ensures that the test folder is deleted after the tests are run. See the junit-pioneer doc for more details.

Loading Files from Resources

Sometimes it is necessary to load a specific resource or to access the resource directory

Path resourceDir = Paths.get(MSBibExportFormatTestFiles.class.getResource("MsBibExportFormatTest1.bib").toURI()).getParent();

When the directory is needed, it is important to first point to an actual existing file. Otherwise the wrong directory will be returned.

Preferences in tests

If you modify preference, use following pattern to ensure that the stored preferences of a developer are not affected:

Or even better, try to mock the preferences and insert them via dependency injection.

public void getTypeReturnsBibLatexArticleInBibLatexMode() {
     // Mock preferences
     PreferencesService mockedPrefs = mock(PreferencesService.class);
     GeneralPreferences mockedGeneralPrefs = mock(GeneralPReferences.class);
     // Switch to BibLatex mode

     // Now test
     EntryTypes biblatexentrytypes = new EntryTypes(mockedPrefs);
     assertEquals(BibLatexEntryTypes.ARTICLE, biblatexentrytypes.getType("article"));

To test that a preferences migration works successfully, use the mockito method verify. See PreferencesMigrationsTest for an example.

Background on Java testing

In JabRef, we mainly rely to basic JUnit tests to increase code coverage. There are other ways to test:

Type Techniques Tool (Java) Kind of tests Used In JabRef
Functional Dynamics, black box, positive and negative JUnit-QuickCheck Random data generation No, not intended, because other test kinds seem more helpful.
Functional Dynamics, black box, positive and negative GraphWalker Model-based No, because the BibDatabase doesn’t need to be tests
Functional Dynamics, black box, positive and negative TestFX GUI Tests Yes
Functional Dynamics, white box, negative PIT Mutation No
Functional Dynamics, white box, positive and negative Mockito Mocking Yes
Non-functional Dynamics, black box, positive and negative JETM, Apache JMeter Performance (performance testing vs load testing respectively) No
Structural Static, white box CheckStyle Constient formatting of the source code Yes
Structural Dynamics, white box SpotBugs Reocurreing bugs (based on experience of other projects) No


Global variables should be avoided. Try to pass them as dependency.

“Special Fields”

keywords sync

Database.addDatabaseChangeListener does not work as the DatabaseChangedEvent does not provide the field information. Therefore, we have to use BibtexEntry.addPropertyChangeListener(VetoableChangeListener listener)

Working with BibTeX data

Working with authors

You can normalize the authors using org.jabref.model.entry.AuthorList.fixAuthor_firstNameFirst(String). Then the authors always look nice. The only alternative containing all data of the names is org.jabref.model.entry.AuthorList.fixAuthor_lastNameFirst(String). The other fix... methods omit data (like the von parts or the junior information).


  • Benchmarks can be executed by running the jmh gradle task (this functionality uses the JMH Gradle plugin)
  • Best practices:
    • Read test input from @State objects
    • Return result of calculations (either explicitly or via a BlackHole object)
  • List of examples

Measure performance

Try out the YourKit JAva Profiler.


When creating an equals method follow:

  1. Use the == operator to check if the argument is a reference to this object. If so, return true.
  2. Use the instanceof operator to check if the argument has the correct type. If not, return false.
  3. Cast the argument to the correct type.
  4. For each “significant” field in the class, check if that field of the argument matches the corresponding field of this object. If all these tests succeed, return true otherwise, return false.
  5. When you are finished writing your equals method, ask yourself three questions: Is it symmetric? Is it transitive? Is it consistent?

Also, note:

  • Always override hashCode when you override equals (hashCode also has very strict rules) (Item 9 ofEffective Java)
  • Don’t try to be too clever
  • Don’t substitute another type for Object in the equals declaration

Files and Paths

Always try to use the methods from the nio-package. For interoperability, they provide methods to convert between file and path. Mapping between old methods and new methods


The following expressions can be used in FXML attributes, according to the official documentation

Type Expression Value point to Remark
Location @image.png path relative to the current FXML file  
Resource %textToBeTranslated key in ResourceBundle  
Attribute variable $idOfControl or $variable named control or variable in controller (may be path in the namespace) resolved only once at load time
Expression binding ${expression} expression, for example textField.text changes to source are propagated
Bidirectional expression binding #{expression} expression changes are propagated in both directions (not yet implemented in JavaFX, see feature request)
Event handler #nameOfEventHandler name of the event handler method in the controller  
Constant <text><Strings fx:constant="MYSTRING"/></text> constant (here MYSTRING in the Strings class)  

JavaFX Radio Buttons example

All radio buttons that should be grouped together need to have a ToggleGroup defined in the FXML code Example:

                <ToggleGroup fx:id="citeToggleGroup"/>
                <RadioButton fx:id="inPar" minWidth="-Infinity" mnemonicParsing="false"
                             text="%Cite selected entries between parenthesis" toggleGroup="$citeToggleGroup"/>
                <RadioButton fx:id="inText" minWidth="-Infinity" mnemonicParsing="false"
                             text="%Cite selected entries with in-text citation" toggleGroup="$citeToggleGroup"/>
                <Label minWidth="-Infinity" text="%Extra information (e.g. page number)"/>
                <TextField fx:id="pageInfo"/>

JavaFX Dialogs

All dialogs should be displayed to the user via DialogService interface methods. DialogService provides methods to display various dialogs (including custom ones) to the user. It also ensures the displayed dialog opens on the correct window via initOwner() (for cases where the user has multiple screens). The following code snippet demonstrates how a custom dialog is displayed to the user:

dialogService.showCustomDialog(new DocumentViewerView());

If an instance of DialogService is unavailable within current class/scope in which the dialog needs to be displayed, DialogService can be instantiated via the code snippet shown as follows:

DialogService dialogService = Injector.instantiateModelOrService(DialogService.class);