Unable to access java fields in clojure

Question

I'm having an issue where I can see the fields of a Java class/object, but I can't actually access them. I can see the fields in two ways. Using this following code.

=>(require '[clojure.reflect :as r])    
=>(use '[clojure.pprint :only [print-table]])    
=>(print-table (sort-by :name (:members (r/reflect  myClass))))

And also, by creating an instance of the object. Let's say the fields are an int denoted a, a string denoted word , and a String ArrayList denoted mylist.

=>myObj
#<myClass 1 hello [world]>

In both cases, I can see that these fields exist. However, when I run the following code, I get the following error.

=>(. myObj mylist)
IllegalArgumentException No matching field found: mylist for class myClass clojure.lang.Reflector.getInstanceField (Reflector.java:271)

Does anyone have any idea what is going on?

In response to Nicolas Modrzyk's answer, I run (.-myFieild myObject) and get IllegalArgumentException No matching field found: myField for class myClass clojure.lang.Reflector.invokeNoArgInstanceMember (Reflector.java:308)

Additionally, these fields are not private. I have the Java source code in front of me.

They're probably private fields. You can't access them from clojure, just like you can't access them from java. — amalloy
– amalloy, Commented Jun 29, 2015 at 22:56
It would be very helpful if the question included at least part of the output from the call to print-table so we can tell what the actual field names are — Arthur Ulfeldt
– Arthur Ulfeldt, Commented Jun 29, 2015 at 22:56
Are the fields public? If not, the default access in Java is package. Your clojure code is probably running in a different one, then has no access. Please show us the Java code. — Thumbnail
– Thumbnail, Commented Jul 1, 2015 at 10:19

Nicolas Modrzyk · Accepted Answer · 2015-06-30 00:36:27Z

3

The correct notation for accessing java fields is slightly different:

   (.-fieldName instance)

Here is a full example with the java class File and its private field path:

    (require '[clojure.reflect :as r])   
    (use '[clojure.pprint :only [print-table]])    

    (import '[java.io File])
    (def f (File. "test.txt"))

    ; access a public static field
    (. File separator)
    ; "/"

    (print-table 
      (sort-by :name (:members (r/reflect  File))))

    (.-path f)
    ; java.lang.IllegalArgumentException: 
    ; No matching field found: path for class java.io.File

    (def field (.getDeclaredField File "path"))
    ; you need the below if the field is private
    (.setAccessible field true)

    ; get the value
    (.get field f)
    ; "test.txt"

answered Jun 30, 2015 at 0:36

Nicolas Modrzyk

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1 Comment

Mars Over a year ago

The - is not required, but is recommended.

Jordan · Accepted Answer · 2015-07-02 16:27:03Z

0

Thumbnail's comment was correct. I wasn't aware of Java's default access being package. I have since accessed the fields through reflection. Thank you everyone for your help.

answered Jul 2, 2015 at 16:27

Jordan

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