Using BCEL to do bytecode modification on a simple application I have, I encountered problems with thread safety. As soon as I got 2 threads instrumenting in parallel, the java Verifier begins to complains about generated method signatures.
I struggled half a day to find it out. Thread safety problem is not that easy to isolate. After some googling I found the following post on BCEL mailing lists
It appears that some of BCEL static class (org.apache.bcel.generic.Type and org.apache.bcel.classfile.Utiity at least), which should provides static method to compute human understandable signature from bytecode and vice-versa are miscoded.
If I understand that doing a thread safe application can be difficult, I am really thinking BCEL guy were lazy this time with a snip like
// sample purpose - pseudo code
package org.apache.bcel.classfile;
public abstract class Utility {
public static int consumed_chars;
public static String[] methodSignatureArgumentTypes(String) {
loop {
// read and set consumed_chars field
consumed_chars++;
}
}
public static String methodSignatureToString(String) {
loop {
// read and set consumed_chars field
consumed_chars++;
}
}
}
As we can see its obvious it is non thread safe because its bad designed: two static methods are using the same static field as a temporary storage to perform string operations.
I had to wrap the consumed_chars in a ThreadLocal and replace all calls. It works fine, but I cannot be sure I covered all the bad design BCEL put in.
Guy, ok to say BCEL is not designed for thread safety in the FAQ. But please try to avoid such ugly coding.
Another option for me could be to add some synchronized in my instrumentation application - but it would be a hell from a performance point of view.
Considering BCEL is used in app server modules (npulse, willy technology), it should be thread safe. In app server there is lots of classloading ocuring concurenlty (stub / skel generation, CMP impl at deployment time ...)
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