Regarding current JVMTI improvements

[Alan Bateman]

On 21/01/2016 10:27, Michael Rasmussen wrote:
> Hi
>
> Now that some changes are going into the JVMTI implementation in order to
> support modules, I just wanted to bring up the question I asked back in
> early December regarding support for setting the -Xpatch from JVMTI in
> the OnLoad phase, as well as the inability for JVMTI agents to instrument
> classes loaded before Start phase, due to CFLH events not being fired.
>
There is an issue with changing jdk.boot.class.path.append in the 
Agent_OnLoad as we discussed a few weeks back. Lois is looking this as 
JDK-8144730 [1]. A separate question is whether the JDK-specific 
property jdk.boot.class.path.append will be documented property. We do 
mention it in JEP 261 but it is not currently documented in the 
System.getProperties() implementation note - this is something 
potentially to discuss in the context of JDK-8136930 [2].

As regards the JVM TI spec and the CFLH event then it is a deliberate 
change to only post this event in the Start and Live phases and to have 
the Start phase be after the module system is initialized. To do it 
otherwise would break existing agents that instrument code that executes 
before the Live phase. We know this disadvantages agents that want to 
instrument classes that are loaded and used early in the VM startup and 
we will have a solution for this case once we have the "module-aware" 
agent support, tracked as JDK-8146454 [3], in jake. I think we will have 
to tie this to either a new JVM TI capability or a new "early start" 
phase. I don't expect any issues for agents that are interested in 
compilation events but there will be restrictions on agents that do 
instrumentation before the current Start phase. From your other mails 
then I understand it that you are patching classes in java.base so 
assuming you aren't patching those classes to invoke code outside of 
java.base then it should work with only minor updates to your agent.

-Alan

[1] https://bugs.openjdk.java.net/browse/JDK-8144730
[2] https://bugs.openjdk.java.net/browse/JDK-8136930
[3] https://bugs.openjdk.java.net/browse/JDK-8146454