在有限的资源环境中测试Java

I'm writing java that will run in an environment with little memory. My machine has loads of memory, so I need to limit the heap size (is this the only limit I can apply?). I've been using "-Xmx100m" as a VM parameter in eclipse, but when I run the application I can see in the task manager that the memory used goes well above 100MB. Am I using this incorrectly? I'm writing java that will run in an environmen