How to read the value of a private field from a different class in Java?
class, field, java, private, reflection
Solution
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 can throw `Exception`s, 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 `RuntimeException`s which may be thrown are either `SecurityException`s (if the JVM's `SecurityManager` will not allow you to change a field's accessibility), or `IllegalArgumentException`s, 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
Problem
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`?