Disallowing the dynamic loading of agents by default

[Ron Pressler]

Hi.

In JDK 21 we intend to disallow the dynamic loading of agents by default. This
will affect tools that use the Attach API to load an agent into a JVM some time
after the JVM has started [1]. There is no change to any of the mechanisms that
load an agent at JVM startup (-javaagent/-agentlib on the command line or the
Launcher-Agent-Class attribute in the main JAR's manifest).

This change in default behavior was proposed in 2017 as part of JEP 261 [2][3]. 
At that time the consensus was to switch to this default not in JDK 9 but in a 
later release to give tool maintainers sufficient time to inform their users. 
To allow the dynamic loading of agents, users will need to specify 
-XX:+EnableDynamicAgentLoading on the command line.

I'll post a draft JEP for review shortly.

-- Ron

[1]: https://docs.oracle.com/en/java/javase/19/docs/api/jdk.attach/com/sun/tools/attach/package-summary.html
[2]: https://openjdk.org/jeps/261
[3]: https://mail.openjdk.org/pipermail/jigsaw-dev/2017-April/012040.html