Java Language Reflection API Getting and Setting fields
Example
Using the Reflection API, it is possible to change or get the value of a field at runtime. For example, you could use it in an API to retrieve different fields based on a factor, like the OS. You can also remove modifiers like final to allow modifing fields that are final.
To do so, you will need to use the method Class#getField() in a way such as the one shown below:
// Get the field in class SomeClass "NAME".
Field nameField = SomeClass.class.getDeclaredField("NAME");
// Get the field in class Field "modifiers". Note that it does not
// need to be static
Field modifiersField = Field.class.getDeclaredField("modifiers");
// Allow access from anyone even if it's declared private
modifiersField.setAccessible(true);
// Get the modifiers on the "NAME" field as an int.
int existingModifiersOnNameField = nameField.getModifiers();
// Bitwise AND NOT Modifier.FINAL (16) on the existing modifiers
// Readup here https://en.wikipedia.org/wiki/Bitwise_operations_in_C
// if you're unsure what bitwise operations are.
int newModifiersOnNameField = existingModifiersOnNameField & ~Modifier.FINAL;
// Set the value of the modifiers field under an object for non-static fields
modifiersField.setInt(nameField, newModifiersOnNameField);
// Set it to be accessible. This overrides normal Java
// private/protected/package/etc access control checks.
nameField.setAccessible(true);
// Set the value of "NAME" here. Note the null argument.
// Pass null when modifying static fields, as there is no instance object
nameField.set(null, "Hacked by reflection...");
// Here I can directly access it. If needed, use reflection to get it. (Below)
System.out.println(SomeClass.NAME);
Getting fields is much easier. We can use Field#get() and its variants to get its value:
// Get the field in class SomeClass "NAME".
Field nameField = SomeClass.class.getDeclaredField("NAME");
// Set accessible for private fields
nameField.setAccessible(true);
// Pass null as there is no instance, remember?
String name = (String) nameField.get(null);
Do note this:
When using Class#getDeclaredField, use it to get a field in the class itself:
class HackMe extends Hacked {
public String iAmDeclared;
}
class Hacked {
public String someState;
}
Here, HackMe#iAmDeclared is declared field.
However, HackMe#someState is not a declared field as it is inherited from its superclass, Hacked.
