Getting the name of a method parameter

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?

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.

I have found another solution after marking this question as answered. The solution is Paranamer.

Example:

 Method method = Foo.class.getMethod(...);

 Paranamer paranamer = new CachingParanamer();

 String[] parameterNames = paranamer.lookupParameterNames(method) // throws ParameterNamesNotFoundException if not found

 // or ...

 parameterNames = paranamer.lookupParameterNames(method, false) // will return null if not found

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