Hi there!
We'd like to discuss a proposal for extending the current DCmd framework to
support Java level DCmd.
At present, DCmd only allows the VM to register commands, which can be called
through jcmd or JMX. It would be beneficial if the user could create their own
commands.
The idea of this extension originally came from our internal Java agent that
detects the misusage of Unsafe API.
This agent can collect the call sites that allocate or free direct memory in
the application(NMT could not do it IMO) to detect direct memory leaks.
In the beginning, it just prints all call sites, without any statistical
function, it's hard to use.
So we plan to use a way similar to jeprof (from jemalloc) to generate a report
file that aggregates all useful information.
During the implementation process, we found that we need a mechanism to notify
the agent to generate reports.
The common practice is:
a) Register a service port, triggered by an HTTP request
b) Triggered by signal
c) Generate reports periodically, or when the process exits
But these three ways have certain problems.
For a) we need to introduce a network component, will increase the complexity
of implementation
For b) we cannot pass parameters
For c) some files that may never be used will be generated
Essentially, this question is how to notify the application to do a certain
task, or in other words, how do we issue a command to the application. We
believe that other Java developers will also encounter similar problems.
(And sometimes there may be multiple unrelated dependent components in a Java
application that require such a mechanism.)
Naturally, we think that jcmd can already issue some commands registered in VM
to the application, why can't we extend to the java level?
This feature will be very useful for some lightweight tools, just like the
scenario we encountered, to notify the tools to perform certain operations.
In addition, this feature will also bring benefits to Java beginners.
For example, in the beginning, beginners may not use advanced log components,
but they will also encounter the need to output debug logs. They may write code
like this:
```
if (debug) {
System.out.println("...");
}
```
If developers can easily control the value of debug, it's attractive.
Like this:
```
Factory.register("MyApp.flipDebug", out -> debug = !debug);
jcmd <pid> MyApp.flipDebug
```
For mainstream framework, we can apply this feature to trigger some common
activities, such as health checks, graceful shutdown, and dynamic configuration
updates, But to be honest, these frameworks are very mature and stable, and for
compatibility purposes, it's hard to let them use this extension.
Comments welcome!
Thanks,
Denghui
