Read method parameter names

Java compiler doesn't store method parameter names in the class file and we can't retrieve them using reflection. Actually, the names are stored in the LocalVariableTable attribute of the method, if a class is compiled with debug symbols on (i.e., javac -g). The LocalVariableTable attribute is not accessible using reflection, but it can be read using the bytecode parsing library like ASM.

Paramo is a little tool that extracts method or constructor parameter names from bytecode debug information in runtime.

Paramo works only if debug information is available in class files (compile with: javac -g ).


Using Paramo is very simple: just pass Method or Constructor to the static method Paramo#resolveParameters. It returns MethodParameter[] array with method parameter information, or an empty array if the method/constructor does not have any parameter.

MethodParameter is a simple POJO that holds two information about the method parameter:

  • parameter name

  • bytecode signature, including the generic information!

MethodresolveParameters does not cache anything; every time invoked it will examine the bytecode again. For efficient usage, wrap it and cache results. {: .attn}

In contrast to the rest of Jodd, Paramo is written to be 'raw', without many helper methods. This is done by purpose, since getting parameter names is usually part of some higher-level logic, that should wrap it in the way that serves the best to that logic.


Let's say we have the following class:

public static class Foo {
    public Foo(String something) {}

    public void hello() {}

    public void two(String username, String password) {}

We will assume that we got references to Methods and Constructor using reflection (and with tool such ReflectUtil#findMethod). Now, let's read parameter names:

MethodParameter[] s = Paramo.resolveParameters(constructor);
System.out.println(s.length);           // 1
System.out.println(s[0].getName());     // something
MethodParameter[] s = Paramo.resolveParameters(helloMethod);
System.out.println(s.length);           // 0
MethodParameter[] s = Paramo.resolveParameters(twoMethod);
System.out.println(s.length);           // 2
System.out.println(s[0].getName());     // username
System.out.println(s[1].getName());     // password

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