He can swap out the stereo, add different wheels and tires, change the exhaust system — whatever.
Answer: If you want your applications to perform, the application server instance's JVM should not swap out of physical memory.
You can swap out one system and replace it with another.
Writing your software with inversion of control in mind also makes unit testing much, much easier, because you can swap out real implementations for mocks in your tests.
Get the oil checked, swap out the brakes, check the exhaust etc.
You could even swap out the batteries while charging others with the phone.