How do I read a private field in Java?

Question

I have a poorly designed class in a 3rd-party JAR and I need to access one of its private fields. For example,

class IWasDesignedPoorly {
    private Hashtable stuffIWant;
}

IWasDesignedPoorly obj = ...;

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

Answer 1

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

Answer 2

As oxbow_lakes mentions, you can use reflection to get around the access restrictions (assuming your SecurityManager will let you).

That said, if this class is so badly designed that it makes you resort to such hackery, maybe you should look for an alternative. Sure this little hack might be saving you a few hours now, but how much will it cost you down the road?

Answer 3

Reflection isn't the only way.

An alternative solution is to extract the class from the .jar, decompile it using (say) Jode or Jad, change the field (or add an accessor), and recompile it against the original .jar. Then put the new .class ahead of the .jar in the classpath, or reinsert it in the .jar. (the jar utility allows you to extract and reinsert to an existing .jar)

This requires the .jar not to be signed, of course.

Answer 4

Use the Soot Java Optimization framework to directly modify the bytecode. http://www.sable.mcgill.ca/soot/

Soot is completely written in Java and works with new Java versions.

Answer 5

One other option that hasn't been mentioned yet: use Groovy. Groovy allows you to access private instance variables as a side affect of the design of the language. Whether or not you have a getter for the field, you can just use

def obj = new IWasDesignedPoorly()
def hashTable = obj.getStuffIWant()