Code Howtos

Oliver Kopp edited this page Jan 22, 2018 · 67 revisions

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

Table of Contents

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 simplier 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 differnt parameter types.

Posting an object

post(object) posts an object trough 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 occure in JabRef.

For example: Every time an entry was added to the database a new EntryAddedEvent is sent trough 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 Apache Commons Logging. All log messages are passed internally to Java's java.util.logging which handles any filterting, formatting and writing of log messages.

  • Obtaining a Logger for a Class:

    private static final Log LOGGER = LogFactory.getLog(<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);

Using Localization correctly

(More information about this topic from the translator side is provided [here] ( about the JabRef interface and [here] ( about the JabRef help files)

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 optain 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.
  2. Tests fail. In the test output a snippet is generated which must be added to the English translation file. There is also a snippet generated for the non-English files, but this is irrelevant.
  3. Add snippet to English translation file located at src/main/resources/l10n/
  4. With gradlew localizationUpdate the "KEY" is added to the other translation files as well (you can use gradlew localizationUpdateExtended for extended output). Crowdin will add them as required.
  5. Tests are green again.

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 themself are independent of any logic and therefore easy to test.

Example: (PageNumbersFormatter)[]

Drag and Drop

org.jabref.external.DroppedFileHandler.handleDroppedfile(String, ExternalFileType, boolean, BibtexEntry) FileListEditor sets a TransferHandler inherited from FileListEditorTransferHandler. There, at importData, a DroppedFileHandler is instantiated and handleDroppedfile called.

Get the JabRef frame panel

org.jabref.JabRefFrame jrf = JabRef.jrf;
org.jabref.BasePanel basePanel = JabRef.jrf.basepanel();

Get Absolute Filename or Path

File f = FileUtil.expandFilename(basePanel.getDatabaseContext(), path, JabRefPreferences.getInstance().getFileDirectoryPreferences()).get(); 

String path Can be the files name or a relative path to it.

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.

Globals.prefs is a global variable storing a link to the preferences form.

Globals.prefs.getTYPE(key) returns the value of the given configuration key. TYPE has to be replaced by Boolean, Double, Int, ByteArray. If a string is to be put, the method name is only “get”.

To store the configuration keys in constants, one has two options

  • as constant in the own class
  • as constant in

There are JabRef classes existing, where the strings are hard-coded and where constants are not used. That way of configuration should be avoided.

When adding a new preference, following steps have to be taken:

  • add a constant for the configuration key
  • in put a “defaults.put(<configuration key>, <value>)” statement

When accessing a preference value, the method Globals.prefs.getTYPE(key) has to be used.

Defaults should go into the model package. See here for the comment.

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

Test Cases

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

we would have five tests containing a single assert statement and named accordingly (formatDoesNotChangeStringWithoutBraces, formatDoesNotRemoveSingleBrace, formatRemovesDoubleBracesAtBeginning, 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 add @Test(expected=ExpectedException.class) to the test method.

Lists in tests

  • Use Assert.assertEquals(Collections.emptyList(), actualList); instead of Assert.assertEquals(0, actualList.size()); to test whether a list is empty.
  • Similarly, use Assert.assertEquals(Arrays.asList("a", "b"), actualList); to compare lists instead of
         Assert.assertEquals(2, actualList.size());
         Assert.assertEquals("a", actualList.get(0));
         Assert.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
         public TemporaryFolder testFolder = new TemporaryFolder();

to the test class. A temporary file is now created by File tempFile = testFolder.newFile("file.txt");. Using this pattern automatically ensures that the test folder is deleted after the tests are run. See the blog of Gary Gregory for more details.

Loading Files from Resources:

Sometimes it is necessary to load a specific resource as a File:


This returns an object. To avoid problems with whitespaces or any other special characters, the File creation should always be done with the Paths-Class.

File f = Paths.get(url.toUri()).toFile(); 
//concrete example
File auxFile = Paths.get(AuxCommandLineTest.class.getResource("paper.aux").toURI()).toFile(); 

For more information see discussions at

Preferences in tests

If Globals.prefs are not initialized in a test case, try to add

public static void setUp() {
    Globals.prefs = JabRefPreferences.getInstance();

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

private JabRefPreferences backup;

public void setUp() {
    prefs = JabRefPreferences.getInstance();
    backup = prefs;

public void tearDown() {
    //clean up preferences to default state

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

public void getTypeReturnsBibLatexArticleInBibLatexMode() {
     // Mock preferences
     JabrefPreferences mockedPrefs = mock(JabrefPreferences.class);        
     // Switch to BibLatex mode

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


Global variables are OK here. Global variables are NOT OK everywhere else, especially model and logic.

Find out issues with the EDT

If someone wants to find out more, add the following first in main or start in JabRefMain:

RepaintManager.setCurrentManager(new CheckingRepaintManager());

then it is just to work with JabRef until an exception happens.


UI for Preferences

  • JabRefFrame.preferences() shows the preferences
  • class: PrefsDialog3

Designing GUI Confirmation dialogs

  1. Avoid asking questions
  2. Be as concise as possible
  3. Identify the item at risk
  4. Name your buttons for the actions

More information:

"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


When creating an equalsmethod 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)
  • 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)