How to monitor Java memory usage?

I would say that the consultant is right in the theory, and you are right in practice. As the saying goes:

In theory, theory and practice are the same. In practice, they are not.

The Java spec says that System.gc suggests to call garbage collection. In practice, it just spawns a thread and runs right away on the Sun JVM.

Although in theory you could be messing up some finely tuned JVM implementation of garbage collection, unless you are writing generic code intended to be deployed on any JVM out there, don't worry about it. If it works for you, do it.

You can take a look at stagemonitor. It is a open source java (web) application performance monitor. It captures response time metrics, JVM metrics, request details (including a call stack captured by the request profiler) and more. The overhead is very low.

Optionally, you can use the great timeseries database graphite with it to store a long history of datapoints that you can look at with fancy dashboards.

Example:

Take a look at the project website to see screenshots, feature descriptions and documentation.

Note: I am the developer of stagemonitor

There are tools that let you monitor the VM's memory usage. The VM can expose memory statistics using JMX. You can also print GC statistics to see how the memory is performing over time.

Invoking System.gc() can harm the GC's performance because objects will be prematurely moved from the new to old generations, and weak references will be cleared prematurely. This can result in decreased memory efficiency, longer GC times, and decreased cache hits (for caches that use weak refs). I agree with your consultant: System.gc() is bad. I'd go as far as to disable it using the command line switch.

If you want to really look at what is going on in the VM memory you should use a good tool like VisualVM. This is Free Software and it's a great way to see what is going on.

Nothing is really "wrong" with explicit gc() calls. However, remember that when you call gc() you are "suggesting" that the garbage collector run. There is no guarantee that it will run at the exact time you run that command.