Getting the name of a method parameter

Question

In Java 6, imagine I have the following method signature:

public void makeSandwich(Bread slice1, Bread slice2, List<Filling> fillings, boolean mustard)

I would like to know, at runtime, the value that was passed on to slice2 or any other parameter, the important bit here is that I want to get the value by parameter name.

I know how to get the list of parameter types with getParameterTypes or getGenericParameterTypes.

Ideally I would like to get a list of parameter names instead of types. Is there a way to do so?

Answer 1

This is not possible. Class files do not contains the argument names, as you can see with your IDE's autocompletion when the source is not available.

Therefore, the reflection API is not able to give out parameter names.

Answer 2

Since Java 1.8, this can be done as long as the parameter names are in the class files. Using javac this is done passing the -parameters flag. From the javac help

-parameters    Generate metadata for reflection on method parameters

From IDEs you will need to look at the compiler settings.

If the parameter names are in the class files then here is an example of doing this

import java.lang.reflect.Method;
import java.lang.reflect.Parameter;

public class ParameterNamesExamples {

  public static void main(String[] args) throws Exception {
    Method theDoSomethingMethod = ExampleClass.class.getMethods()[0];
    // Now loop through the parameters printing the names
    for(Parameter parameter : theDoSomethingMethod.getParameters()) {
      System.out.println(parameter.getName());
    }
  }

  private class ExampleClass {
    public void doSomething(String myFirstParameter, String mySecondParameter) {
      // No-op
    }
  }
}

The output will depend on if the parameter names are in the class files. If they are the output is:

myFirstParameter
mySecondParameter

If not the output is:

arg0
arg1

More information on this from Oracle can be found at Obtaining Names of Method Parameters

Answer 3

You can simply assign the value of the parameter to another value

Bread slice2;

public void makeSandwich(Bread slice1, Bread slice2, List<Filling> fillings, boolean mustard) {
    this.slice2 = slice2;
    System.out.println(this.slice2.getSomething());
}

Answer 4

In addition to this answer: "Parameter names are available if you have told the compiler to include them"

If you're using Eclipse go to project -> properties -> Java Compiler -> check "Store information about method parameters (usable via reflection)

Answer 5

Do you own the code of the method? You could annotate the parameters and pass names as arguments @Param("slice1"). Later you will be able to get the annotation and extract parameter name from it.

Answer 6

Parameter names are available if you have told the compiler to include them (compile with debug information). Spring has ParameterNameDiscoverer which can help you obtain the names. The default implementation uses asm ClassReader to do so.

With javac you should include the -g argument to include debug information. With Eclipse I think it is there by default; it can be configured using the preferences: Java -> Compiler and then enable "Store information about method parameters (usable via reflection)" (see also this answer).

Some frameworks use this. For example spring-mvc has @RequestParam which defaults to the param name, if resolvable. It also supports explicit naming - @RequestParam("foo") in case no debug information is provided.