Using reflection to change static final File.separatorChar for unit testing?

by

From the documentation for Field.set:

If the underlying field is final, the method throws an IllegalAccessException unless setAccessible(true) has succeeded for this field and this field is non-static.

So at first it seems that you are out of luck, since File.separatorChar is static. Surprisingly, there is a way to get around this: simply make the static field no longer final through reflection.

I adapted this solution from javaspecialist.eu:

static void setFinalStatic(Field field, Object newValue) throws Exception {
    field.setAccessible(true);

    // remove final modifier from field
    Field modifiersField = Field.class.getDeclaredField("modifiers");
    modifiersField.setAccessible(true);
    modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);

    field.set(null, newValue);
}

I’ve tested it and it works:

setFinalStatic(File.class.getField("separatorChar"), '#');
System.out.println(File.separatorChar); // prints "#"

Do exercise extreme caution with this technique. Devastating consequences aside, the following actually works:

setFinalStatic(Boolean.class.getField("FALSE"), true);
System.out.format("Everything is %s", false); // "Everything is true"

Important update: the above solution does not work in all cases. If the field is made accessible and read through Reflection before it gets reset, an IllegalAccessException is thrown. It fails because the Reflection API creates internal FieldAccessor objects which are cached and reused (see the java.lang.reflect.Field#acquireFieldAccessor(boolean) implementation).
Example test code which fails:

Field f = File.class.getField("separatorChar"); f.setAccessible(true); f.get(null);
// call setFinalStatic as before: throws IllegalAccessException

More Related Contents:

  1. Get declared fields of java.lang.reflect.Fields in jdk12
  2. Is it bad practice to use Reflection in Unit testing? [duplicate]
  3. How to assign class level data in a TestNG unit test class
  4. BufferedWriter not writing everything to its output file
  5. How to write a UTF-8 file with Java?
  6. Reading a .txt file using Scanner class in Java
  7. Can I get information about the local variables using Java reflection?
  8. How do I add a resources folder to my Java project in Eclipse
  9. Is this the best way to rewrite the content of a file in Java?
  10. Overriding Binding in Guice
  11. Write a file in UTF-8 using FileWriter (Java)?
  12. How to tell why a file deletion fails in Java?
  13. java.security.AccessControlException: Access denied (java.io.FilePermission
  14. mock or stub for chained call
  15. Java reflection: how to get field value from an object, not knowing its class
  16. What are mock objects in Java?
  17. Set field value with reflection
  18. Write I/O file to shared network drive using credentials
  19. How to prove that HashMap in java is not thread-safe
  20. Reflection for Class of generic parameter in Java?
  21. How does FileLock work?
  22. Read/write .txt file with special characters
  23. In Java, when I call OutputStream.close() do I always need to call OutputStream.flush() before?
  24. Injecting @Autowired private field during testing
  25. Instantiate a class object with constructor that accepts a string parameter?
  26. File.exists() returns false when file exists
  27. Java introspection and reflection
  28. How can I improve performance of Field.set (perhap using MethodHandles)?
  29. ElementType.LOCAL_VARIABLE annotation type
  30. Wrong number of arguments error when invoking a method

Leave a Comment