Reflection: Constant variables within a class loaded via reflection

Question

I have a class which has a bunch of Constant Strings.

I need to load this class via reflection and retrieve those constants. I can get up to:

controllerClass = Class.forName(constantsClassName);
Object someclass = controllerClass.newInstance();

but I am confused on how to retrieve the fields in this class.

possible duplicate of [Accessing Java static final ivar value through reflection](http://stackoverflow.com/questions/850148/accessing-java-static-final-ivar-value-through-reflection) — Dave Newton, Nov 14 '11 at 17:00
Two quick questions: 1. Are those constants private? 2. Are those constants static? — thkala, Nov 14 '11 at 17:02

score 5 · Accepted Answer · answered Nov 14 '11 at 17:00

5

A quick sample on accessing fields --

Field[] fields = controllerClass.getDeclaredFields();

for ( Field field : fields ) {
   field.setAccessible(true);
  System.out.println(field.get(someClass));

}

answered Nov 14 '11 at 17:00

Kal

24,724
7
65
65

1

Just one catch to beware of: several classes, e.g. Boolean, have at least one static final Boolean member. That is, recursive definition. If you want to create a recursive 'class printer', be sure to check that !field.equals(parent) – Meymann Feb 19 '13 at 13:40

score 2 · Answer 2 · edited Jul 28 '19 at 10:31

Assuming these constants are in static fields:

import java.lang.reflect.*;

public class Reflect {
  public static final String CONSTANT_1 = "1";
  public static final String CONSTANT_2 = "2";
  public static final String CONSTANT_3 = "3";

  public static void main(String[] args) throws Exception {
    Class clazz = Class.forName("Reflect");
    Field[] fields = clazz.getDeclaredFields();
    for(Field f: fields) {
      // for fields that are not visible (e.g. private)
      f.setAccessible(true);

      // note: get(null) for static field
      System.err.printf("%s: %s\n",f, (String)f.get(null) );
    }
  }
}

The output is:

$ java Reflect
public static final java.lang.String Reflect.CONSTANT_1: 1
public static final java.lang.String Reflect.CONSTANT_2: 2
public static final java.lang.String Reflect.CONSTANT_3: 3

Note that to get the value of a static field, you supply null as the arg.

score 2 · Answer 3 · answered Nov 14 '11 at 17:19

Here's a little sample:

import java.lang.reflect.Field;

public class Test {
    public static class X {
        public static int Y = 1;
        private static int Z = 2;

        public int x = 3;
        private int y = 4;
    }

    public static Object getXField(String name, X object) {
        try {
            Field f = X.class.getDeclaredField(name);

            f.setAccessible(true);

            return f.get(object);
        } catch (Exception e) {
            e.printStackTrace();

            return null;
        }
    }

    public static void main(String[] args) {
        System.out.println(Test.getXField("Y", null));
        System.out.println(Test.getXField("Z", null));

        System.out.println(Test.getXField("x", new X()));
        System.out.println(Test.getXField("y", new X()));
    }
}

Running this little program outputs:

A few observations:

For static fields the supplied object to Field.get() can be null.
For brevity, I used an exception catch-all with the base Exception class - you should use explicit exception classes in your code.
While Field.get() usually works as expected, the same cannot be said for Field.set() and its friends. More specifically it will happily change the value of a constant (e.g. a final field, or a private field that is never modified in the class methods), but due to constant inlining the old value may remain in use.

score 0 · Answer 4 · answered Nov 14 '11 at 16:59

0

You get to know about the modifiers via the class and not the object reference.

http://download.oracle.com/javase/tutorial/reflect/class/classModifiers.html

answered Nov 14 '11 at 16:59

Sid Malani

2,078
1
13
13

Reflection: Constant variables within a class loaded via reflection

4 Answers4

Linked