Kelly O'Hair's Blog

JVM TI Agents Article

Just a plug (and additional reference) on my December 2006 article on JVM TI at http://java.sun.com/developer/technicalArticles/J2SE/jvm_ti.

I really had not expected this article to be very popular, but I was assuming that only people writing JVM TI agents would be interested. It appears there is quite a bit of general curiosity on VM agents. Of course the Java Lobby and Sun System News links helped too. :^)

Big thanks to Janice for all her help with this article! It would never have read as well without her help.

Just for reference, there is also a JVMPI to JVM TI conversion article at: http://java.sun.com/developer/technicalArticles/Programming/jvmpitransition/.

-kto

Comments

• Hi Kelly, I hope it's a good opportunity for a technical question -
  I'm trying to add a new field to java.lang.Object. Obviously it doesn't work (or I wouldn't be asking), but is there any documented set of limitations on byte code instrumentation? I see that the java_crw_demo avoids instrumenting some classes and methods, but it doesn't have a reference to where this set of methods and classes come from.

  
  Adding a field to Object is probably a very bad idea, I'm not sure who knows what about an Object. Normally adding fields or even methods is considered a change to the object or it's behavior. BCI is usually used to instrument the existing bytecodes. I'd look at http://java.sun.com/javase/6/docs/platform/jvmti/jvmti.html#RedefineClasses
  for a few details on the redefinition limitations.

  -kto

  Posted by: genadyb on January 18, 2007 at 03:35 AM

• I've read this link, but redefining class won't help, as it does not permit me to add data members. So I'm using class load hook to add the field.
  Now I've already figured that modifying java.lang.Object it's a bad idea :) But I'm trying some kind of analysis that needs to associate a data member with each objects. I don't want to (and can't) use tags because I want to access this data member from java code. So I can't add this member to Object. But what other classes are also dangerous? I guess java.util.Date is ok, but is java.lang.Thread is ok too?

  Thanks!
  Genady
  (me@genady.org)

  
  In general, I don't go down this path of adding any field or methods. But I understand your situation, just don't have any concrete answers for you. This is somewhat uncharted territory and you'll need to experiment. You might try asking this question on the hotspot mailing alias listed at https://openjdk.dev.java.net/hotspot/. Sorry I'm not much help on this.

  
  -kto
  

  Posted by: genadyb on January 19, 2007 at 02:08 PM

• I've read this link, but redefining class won't help, as it does not permit me to add data members. So I'm using class load hook to add the field. Now I've already figured that modifying java.lang.Object it's a bad idea :) But I'm trying some kind of analysis that needs to associate a data member with each objects. I don't want to (and can't) use tags because I want to access this data member from java code. So I can't add this member to Object. But what other classes are also dangerous? I guess java.util.Date is ok, but is java.lang.Thread is ok too? Thanks! Genady (me@genady.org) In general, I don't go down this path of adding any field or methods. But I understand your situation, just don't have any concrete answers for you. This is somewhat uncharted territory and you'll need to experiment. You might try asking this question on the hotspot mailing alias listed at https://openjdk.dev.java.net/hotspot/. Sorry I'm not much help on this.

  -kto

  
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  Posted by: davidlagato on May 25, 2007 at 12:32 AM

This work is licensed under a Creative Commons License.