可以在Java或Kotlin中以编程方式获取局部变量名称吗？

Question

It is possible to get local variable names programmatically in Java or Kotlin?

For example, this code

public void printOneTwo() {
    int one = 1;
    int two = 2;

    printLocalVariables(one, two);
}

private void printLocalVariables(Objects... variables) {
    //code
}

//result: "one, two"

I need to convert variable one to string "one"

Answer 1

Theoretically, it is possible.

You can use a byte code parser like ASM to extract a method's local variable table from the class file of the declaring class. This table contains the names of all local variables and an offset for their validity to identify the declaration order.

Do however note that this attribute is optional. A compiler can omitt adding this debug information.

Answer 2

No, it's not possible. Only the variable values are passed, no data that could give you the name is available.

Answer 3

In the case like your example one and two will be inlined in the call site and leave no trail in the compiled code. Ups.

If all you want is easy logging, and you are using InteliJ (you should!), then use code templates. They allow one to use the local name, so you'll type one.log<tab> and get one.log("one") , for example

Answer 4

This is an interesting question. But What is the purpose? Is it just an exercise?

Yes, getting local variable names programmatically is possible under certain conditions.

1. According to some definitions, in java, function parameters are a kind of local variable. You can get those , if the code was compiled using debug information, with java reflection, method.getParameterTypes()

2. If you have a .class file compiled with debug information, you can read it the local variables from it, for example using asm's ClassReader. The following code looks at the locals of class A's function f1

Here is the code:

import java.io.File;

import java.io.FileInputStream;

import java.io.FileNotFoundException;

import java.io.IOException;

import java.util.List;


import org.objectweb.asm.ClassReader;

import org.objectweb.asm.ClassVisitor;

import org.objectweb.asm.tree.ClassNode;

import org.objectweb.asm.tree.LocalVariableNode;

import org.objectweb.asm.tree.MethodNode;

public class Example {
    public static void main(String[] args) throws FileNotFoundException, IOException {
        ClassReader reader = new ClassReader(new FileInputStream(new File("A.class")));

        final ClassNode classNode = new ClassNode();
        reader.accept((ClassVisitor) classNode, 0);

        for (final MethodNode mn : (List<MethodNode>)classNode.methods) {
            if ( mn.name.equals("f1"))
                for (LocalVariableNode n : (List<LocalVariableNode>)mn.localVariables) {
                    System.out.println(n.name);
                }
        }
    }
}

3. I'm not sure about this, but I think you could also do it using the LocalVariable class of the Java Debug Interface