Frank Krueger Frank Krueger - 24 days ago 9
Java Question

How do I read a private field in Java?

I have a poorly designed class in a 3rd-party

JAR
and I need to access one of its private fields. For example,
why should I need to choose private field is it necessary?

class IWasDesignedPoorly {
private Hashtable stuffIWant;
}

IWasDesignedPoorly obj = ...;


How can I use reflection to get the value of
stuffIWant
?

Answer

In order to access private fields, you need to get them from the class's declared fields and then make them accessible:

Field f = obj.getClass().getDeclaredField("stuffIWant"); //NoSuchFieldException
f.setAccessible(true);
Hashtable iWantThis = (Hashtable) f.get(obj); //IllegalAccessException

EDIT: as has been commented by aperkins, both accessing the field, setting it as accessible and retrieving the value will all throw Exceptions, although the only checked exceptions you need to be mindful of are commented above.

The NoSuchFieldException would be thrown if you asked for a field by a name which did not correspond to a declared field.

obj.getClass().getDeclaredField("misspelled"); //will throw NoSuchFieldException

The IllegalAccessException would be thrown if the field was not accessible (for example, if it is private and has not been made accessible via missing out the f.setAccessible(true) line.

The RuntimeExceptions which may be thrown are either SecurityExceptions (if the JVM's SecurityManager will not allow you to change a field's accessibility), or IllegalArgumentExceptions, if you try and access the field on an object not of the field's class's type:

f.get("BOB"); //will throw IllegalArgumentException, as String is of the wrong type
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