how to insert an 'assert' instruction ?

[nadz]

Hello everyone,

I am a beginner in the modification of java bytecode and i want to insert a 'assert(false)' instruction in the body of one method.

My main question is : if you had to add an 'assert(false)' instruction to a class file how would you proceed with Javassist ?

Here is what i tried so far but unfortunately it didn't work :
At first i tried with CtMethod.insertAfter("assert(false)";) method to simply add it to the body of one method but you can't insert an 'assert' with these methods (the 'assert' instruction is not recognized by javassist).

So in order to discover the "bytecode pattern" of the 'assert(false)' instruction I used a CodeIterator instance and I went trough the bytecode of this simple method :

public static int func(int param) {
 assert(false);
 return 10 + param;
 }

I finally isolate the following bytes (using the byteAt() method) for the 'assert(false)' instruction :

byteAt index 0 : 178 GETSTATIC
byteAt index 1 : 0
byteAt index 2 : 9 T_SHORT
byteAt index 3 : 154 IFNE
byteAt index 4 : 0
byteAt index 5 : 11 FCONST_O
byteAt index 6 : 187 NEW
byteAt index 7 : 0
byteAt index 8 : 10 LCONST_1 or T_INT
byteAt index 9 : 89 DUP
byteAt index 10 : 183 INVOKESPECIAL
byteAt index 11 : 0
byteAt index 12 : 11 T_LONG
byteAt index 13 : 191 ATHROW

Then i tried to add this pattern byte by byte to my class file using CodeIterator.insert() and unfortunately it correctly add the bytes to the class file but it didn't result in an 'assert(false)' instruction...

What am I doing wrong here ? Thank you very much for any help !

[chiba]

If you don't need to turn on/off assertions, you can easily insert by
doing this:

insertAfter("if (expr) throw new AssertionError();")

This is equivalent to:

assert expr;

If you want to turn on/off by the -ea option, you have to also insert
the code for checking whether or not the assertion is effective
something like:

if (Foo.class.desiredAssertionStatus())
if (expr)
throw new AssertionError();

[nadz]

Thanks you very much !

[chiba]

One more thing.

Calling Foo.class.desiredAssertionStatus() would take time.
The bytecode generated by javac calls this method in a
static initializer and the result is cached in a private static
field.