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

User · Answer

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

User · Answer

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

User · Answer

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 lt Object gt  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

User · Answer

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

User · Answer

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   lt init gt     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   lt init gt     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

User · Answer

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

User · Answer

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 lt Foo gt  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

User · Answer

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

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

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