Java Reflection: How to get the name of a variable?

Question

Using Java Reflection, is it possible to get the name of a local variable? For example, if I have this:

Foo b = new Foo();
Foo a = new Foo();
Foo r = new Foo();

is it possible to implement a method that can find the names of those variables, like so:

public void baz(Foo... foos)
{
    for (Foo foo: foos) {
        // Print the name of each foo - b, a, and r
        System.out.println(***); 
    }
}

EDIT: This question is different from Is there a way in Java to find the name of the variable that was passed to a function? in that it more purely asks the question about whether one can use reflection to determine the name of a local variable, whereas the other question (including the accepted answer) is more focused on testing values of variables.

Olimpia

All great answers! Thanks to everyone for replying and commenting - this has been an interesting and insightful discussion. – Olimpia

I found this Detail Tutorial for Reflection – Gethsemane

It is possible. See my [gist][1]. Works for JDK 1.1 to JDK 7. [1]:gist.github.com/2011728 – Hesler

possible duplicate of Is there a way in Java to find the name of the variable that was passed to a function? – Gambill

Not a duplicate, and updated my question to explain why. If anything, that other question is a duplicate (or special case) of this one! – Olimpia

Answer 1

As of Java 8, some local variable name information is available through reflection. See the "Update" section below.

Complete information is often stored in class files. One compile-time optimization is to remove it, saving space (and providing some obsfuscation). However, when it is is present, each method has a local variable table attribute that lists the type and name of local variables, and the range of instructions where they are in scope.

Perhaps a byte-code engineering library like ASM would allow you to inspect this information at runtime. The only reasonable place I can think of for needing this information is in a development tool, and so byte-code engineering is likely to be useful for other purposes too.

---

Update: Limited support for this was added to Java 8. Parameter (a special class of local variable) names are now available via reflection. Among other purposes, this can help to replace @ParameterName annotations used by dependency injection containers.

Damning

Answer 2

It is not possible at all. Variable names aren't communicated within Java (and might also be removed due to compiler optimizations).

EDIT (related to comments):

If you step back from the idea of having to use it as function parameters, here's an alternative (which I wouldn't use - see below):

public void printFieldNames(Object obj, Foo... foos) {
    List<Foo> fooList = Arrays.asList(foos);
    for(Field field : obj.getClass().getFields()) {
         if(fooList.contains(field.get()) {
              System.out.println(field.getName());
         }
    }
}

There will be issues if a == b, a == r, or b == r or there are other fields which have the same references.

EDIT now unnecessary since question got clarified

Darken

Then how do you explain this: java.sun.com/javase/6/docs/api/java/lang/reflect/Field.html ? – Pilch

-1: I think you've misunderstood. @David is after fields, not local variables. Local variables are indeed unavailable through the Reflection API. – Bigley

I think Pourquoi Litytestdata is right. Obviously fields can't be optimized away, so Marcel J. must be thinking of local variables. – Morganatic

@David: You need to edit to clarify that you meant fields rather than local variables. The original question gives code that declares b, a, and r as local variables. – Ga

please don't penalize someone for not reading minds when the original poster meant one thing but asked something else. :( – Ga

I agree that he was talking about fields, but the method he provides as an example clearly states that he will use them as parameters like in baz(b,a,r); which simply won't work. – Darken

@Jason S: He said both "instance variable" and "field". Those declarations could be local variables, or they could be package-private fields, or he could have been making them up on the spot and simply forgot the access modifiers. They don't directly contradict the actual question as asked. – Morganatic

I did mean local variables, and I've edited the question to reflect that. I thought it might not be possible to get variable names, but I figured I'd ask SO before considering it impossible. – Olimpia

Downvote rescinded. Apologies for misunderstanding and thanks for clarifying. – Bigley

Answer 3

(Edit: two previous answers removed, one for answering the question as it stood before edits and one for being, if not absolutely wrong, at least close to it.)

If you compile with debug information on (javac -g), the names of local variables are kept in the .class file. For example, take this simple class:

class TestLocalVarNames {
    public String aMethod(int arg) {
        String local1 = "a string";
        StringBuilder local2 = new StringBuilder();
        return local2.append(local1).append(arg).toString();
    }
}

After compiling with javac -g:vars TestLocalVarNames.java, the names of local variables are now in the .class file. javap's -l flag ("Print line number and local variable tables") can show them.

javap -l -c TestLocalVarNames shows:

class TestLocalVarNames extends java.lang.Object{
TestLocalVarNames();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      5      0    this       LTestLocalVarNames;

public java.lang.String aMethod(int);
  Code:
   0:   ldc     #2; //String a string
   2:   astore_2
   3:   new     #3; //class java/lang/StringBuilder
   6:   dup
   7:   invokespecial   #4; //Method java/lang/StringBuilder."<init>":()V
   10:  astore_3
   11:  aload_3
   12:  aload_2
   13:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   16:  iload_1
   17:  invokevirtual   #6; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
   20:  invokevirtual   #7; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   23:  areturn

  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      24      0    this       LTestLocalVarNames;
   0      24      1    arg       I
   3      21      2    local1       Ljava/lang/String;
   11      13      3    local2       Ljava/lang/StringBuilder;
}

The VM spec explains what we're seeing here:

§4.7.9 The LocalVariableTable Attribute:

The LocalVariableTable attribute is an optional variable-length attribute of a Code (§4.7.3) attribute. It may be used by debuggers to determine the value of a given local variable during the execution of a method.

The LocalVariableTable stores the names and types of the variables in each slot, so it is possible to match them up with the bytecode. This is how debuggers can do "Evaluate expression".

As erickson said, though, there's no way to access this table through normal reflection. If you're still determined to do this, I believe the Java Platform Debugger Architecture (JPDA) will help (but I've never used it myself).

Morganatic

Uh-oh, erickson posted while I was editing and now I'm contradicting him. Which probably means I'm wrong. – Morganatic

By default, javac puts a local variable table in the class for each method to assist debugging. Use the -l option to javap to see the local variable table. – Damning

Not by default, it seems. I had to use javac -g:vars to get it. (I've been trying to edit this answer for the past three hours, but as I said my network connection is having problems, which makes it hard to research.) – Morganatic

You're right, sorry about that. It's line numbers that are "on" by default. – Damning

Answer 4

import java.lang.reflect.Field;


public class Test {

 public int i = 5;
 
 public Integer test = 5;
 
 public String omghi = "der";
 
 public static String testStatic = "THIS IS STATIC";
 
 public static void main(String[] args) throws IllegalArgumentException, IllegalAccessException {
  Test t = new Test();
  for(Field f : t.getClass().getFields()) {
   System.out.println(f.getGenericType() +" "+f.getName() + " = " + f.get(t));
  }
 }

}

Biller

getDeclaredFields() can be used in case you want private field’s names as well – Phase

Answer 5

You can do like this:

Field[] fields = YourClass.class.getDeclaredFields();
//gives no of fields
System.out.println(fields.length);         
for (Field field : fields) {
    //gives the names of the fields
    System.out.println(field.getName());   
}

Backfire

Your answer works fine to get all the fieads. To get only one field, when I use: YourClass.class.getDeclaredField("field1"); I get NullPointer. What is the problem in using it? How I should use getDeclaredField method? – Afroasian

Answer 6

All you need to do is make an array of fields and then set it to the class you want like shown below.

Field fld[] = (class name).class.getDeclaredFields();   
for(Field x : fld)
{System.out.println(x);}

For example if you did

Field fld[] = Integer.class.getDeclaredFields();
          for(Field x : fld)
          {System.out.println(x);}

you would get

public static final int java.lang.Integer.MIN_VALUE
public static final int java.lang.Integer.MAX_VALUE
public static final java.lang.Class java.lang.Integer.TYPE
static final char[] java.lang.Integer.digits
static final char[] java.lang.Integer.DigitTens
static final char[] java.lang.Integer.DigitOnes
static final int[] java.lang.Integer.sizeTable
private static java.lang.String java.lang.Integer.integerCacheHighPropValue
private final int java.lang.Integer.value
public static final int java.lang.Integer.SIZE
private static final long java.lang.Integer.serialVersionUID

Jabon

Answer 7

update @Marcel Jackwerth's answer for general.

and only working with class attribute, not working with method variable.

/**
 * get variable name as string
 * only work with class attributes
 * not work with method variable
 *
 * @param headClass variable name space
 * @param vars      object variable
 * @throws IllegalAccessException
 */
public static void printFieldNames(Object headClass, Object... vars) throws IllegalAccessException {
    List<Object> fooList = Arrays.asList(vars);
    for (Field field : headClass.getClass().getFields()) {
        if (fooList.contains(field.get(headClass))) {
            System.out.println(field.getGenericType() + " " + field.getName() + " = " + field.get(headClass));
        }
    }
}

Willie

Answer 8

see this example :

PersonneTest pt=new PersonneTest();
System.out.println(pt.getClass().getDeclaredFields().length);
Field[]x=pt.getClass().getDeclaredFields();
System.out.println(x[1].getName());

Earthnut