Why can I cache method & field ids but not class?

Question

In my efforts to understand JNI, I've been making heavy use of http://java.sun.com/docs/books/jni/.
I'm having trouble reconciling some of the points it makes with regard to caching; in particular, section 4.4.1 has an example of caching a jmethodID in MyNewString().
I don't understand why this is valid, but the example in 5.5.1 which caches the jclass is invalid.
After all, these are both simply jobjects in the end, and isn't the jmethodID cached in 4.4.1 based off the jclass found the first time? Doesn't that become invalid when the jclass becomes invalid?
Thanks,
: jay

Answer 1

The point the author of that book is trying to make is that caching JNI local references of such data like Java method IDs and classes is not valid across successive JNI calls. If you want to cache something the first time you load it and reuse it later, create a global reference via NewGlobalRef(), make this your cached variable, and delete the local reference automatically obtained from the JVM when you first asked for that something. Then you can refer to that cached variable all you want.
Also beware of trying to use references across threads - this is illegal in JNI. (You might want to reread chatper 5 for this and more. My opinion is the book "Essential JNI" is a bit better though not up to date either.)
- Chris