Java/ASM: Index 0 out of bounds for length 0 even though list is 1024 elements long-CodePudding

I'm trying to understand ASM and am currently stuck at chapter 2.2.4 in the handbook. This should be a simple Java question though.

The example code I'm trying to understand has this line in it called byte[] b1 = ...;. Since this is not a complete statement (I even tried it out!) I replaced it with byte[] b1 = new byte[1024];.

When I compile however I get this weird error message:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0

called three lines later (classReader.accept(classWriter, 0);). Here's the full code and the full error message:

import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassWriter;

public class ModifyClassExample {
    public static void main(String[] args) {
        byte[] b1 = new byte[1024];
        ClassWriter classWriter = new ClassWriter(0);
        ClassReader classReader = new ClassReader(b1);
        classReader.accept(classWriter, 0);
        byte[] b2 = classWriter.toByteArray();
    }
}

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0
    at org.objectweb.asm.ClassReader.readStringish(ClassReader.java:3726)
    at org.objectweb.asm.ClassReader.readClass(ClassReader.java:3741)
    at org.objectweb.asm.ClassReader.accept(ClassReader.java:454)
    at org.objectweb.asm.ClassReader.accept(ClassReader.java:424)
    at ModifyClassExample.main(ModifyClassExample.java:9)

Process finished with exit code 1

I tried reducing the size of the byte array. That's why I know this error pops up at array lenghts of 14 .

Needless to say I need more than that.

CodePudding user response：

You're not providing any class data in the byte array b1 so it will not work. You can however construct a ClassReader from a standard java class such as

ClassReader classReader = new ClassReader("java.lang.Runnable");.

I'd recommend creating a test class with various constructs and use that as your test subject.

Or use the example in 2.2.3 to generate the data - the last line produces the byte array.