Java unit testing: how to measure memory footprint for method call

Question

Assuming I have a class that does some heavy processing, operating with several collections. What I want to do is to make sure that such operation can't lead to out-of-memory or even better I want to set a threshold of how much memory it can use.

class MyClass() {    public void myMethod()    {       for(int i=0; i<10000000; i++)       {          // Allocate some memory, may be several collections       }    } }  class MyClassTest {    @Test    public void myMethod_makeSureMemoryFootprintIsNotBiggerThanMax()    {       new MyClass().myMethod();        // How do I measure amount of memory it may try to allocate?    } } 

What is the right approach to do this? Or this is not possible/not feasible?

Answer 1

I can think of several options:

• Finding out how much memory your method requires via a microbenchmark (i.e. jmh).
• Building allocation strategies based on heuristic estimation. There are several open source solutions implementing class size estimation i.e. ClassSize. A much easier way could be utilizing a cache which frees rarely used objects (i.e. Guava's Cache). As mentioned by @EnnoShioji, Guava's cache has memory-based eviction policies.

You can also write your own benchmark test which counts memory. The idea is to

1. Have a single thread running.
2. Create a new array to store your objects to allocate. So these objects won't be collected during GC run.
3. System.gc(), memoryBefore = runtime.totalMemory() - runtime.freeMemory()
4. Allocate your objects. Put them into the array.
5. System.gc(), memoryAfter = runtime.totalMemory() - runtime.freeMemory()

This is a technique I used in my lightweight micro-benchmark tool which is capable of measuring memory allocation with byte-precision.

Answer 2

You can use profiler (for ex. JProfiler) for view memory usage by classes. Or , how mentioned Areo, just print memory usage:

    Runtime runtime = Runtime.getRuntime();     long usedMemoryBefore = runtime.totalMemory() - runtime.freeMemory();     System.out.println("Used Memory before" + usedMemoryBefore);         // working code here     long usedMemoryAfter = runtime.totalMemory() - runtime.freeMemory();     System.out.println("Memory increased:" + (usedMemoryAfter-usedMemoryBefore));