jasmin hacking and verify error

Question

I'm playing with jasmin and I try to launch my .class file, which is supposed to perform simple string concatenation. My jasmin source looks like this:

.class public default_class
.super java/lang/Object
.method public static main([Ljava/lang/String;)V
.limit locals 1
.limit stack 1
invokestatic main_65428301()I
return
.end method
.method public static main_65428301()I
.limit locals 1
.limit stack 100
new java/lang/String
dup
ldc "foo"
invokestatic java/lang/String.valueOf(Ljava/lang/Object;)Ljava/lang/String;
invokespecial java/lang/StringBuilder(Ljava/lang/String;)V
ldc "bar"
invokevirtual java/lang/StringBuilder.append(Ljava/lang/String;)Ljava/lang/StringBuilder;
invokevirtual java/lang/String.toString()V
astore_0
iconst_0
ireturn
.end method

Now I'm able to run java -jar jasmin.jar and I get default_class.class. However, when I try to launch it like java default_class I get an error:

Exception in thread "main" java.lang.VerifyError: (class: default_class, method: main_65428301 signature: ()I) Illegal use of nonvirtual function call

What should I change in my assembly to get this to work?

Answer 1

In JVM, to create the object you have to first use new instruction and then call <init> method (constructor). You do not create new StringBuilder and call the wrong constructor name (should be java/lang/StringBuilder/<init>(Ljava/lang/String;)V).

I also see no reason to do:

new java/lang/String
dup

or

invokestatic java/lang/String.valueOf(Ljava/lang/Object;)Ljava/lang/String;

Answer 2

"The new instruction does not completely create a new instance; instance initialization is not completed until an instance initialization method has been invoked on the uninitialized instance."