如何监视Java程序的内存使用情况？

Question

I just proposed an algorithm, and I want to prove its superiority compared with another algorithm in terms of time&space consumption. I will implement my algorithm in Java, does anyone know how to monitor the memory consumption of a Java program when the program is executing? Thanks.

Answer 1

from JDK 6 onwards you have a tool in the bin directory which monitors CPU, memory consumption as well as the number of threads spawned. its called visualvm . Just start it and then start your java process. You will see the java process in the tool on the left hand side. Double click on the process and view the statistics. Hope this helps. :)

Answer 2

While the tools mentioned in other answers will tell you how much memory Java allocated to your heap, this can only be taken as an upper bound on your program's memory consumption - the JVM does not garbage collect objects in the moment they're no longer needed, it can even increase heap size instead of garbage collecting and only perform garbage collection when it reaches the given heap limit ( -Xmx ).

So, the graphs you'll see may reach much higher than what the program really needs.

One way to deal with this is to lower the heap size limit ( -Xmx ) incrementally until the program crashes on OutOfMemory exception.

Another way can be used if you know at which point in your algorithm it will consume the most memory - place a breakpoint at that point, make a heap dump and then examine it to see how much living objects occupy.

Answer 3

You can use

long memory = runtime.totalMemory() - runtime.freeMemory();

This place also gives you runtime so that might be pretty useful for what you're trying to do as well! http://www.vogella.com/tutorials/JavaPerformance/article.html#runtimeinfo

Answer 4

Answer is found with a quick Google search.

Use the totalMemory( ) and freeMemory( ) methods [...] The java.lang.Runtime.totalMemory()

from http://crunchify.com/java-runtime-get-free-used-and-total-memory-in-java/

so

long totalmem = java.lang.Runtime.totalMemory();
long freemem  = java.lang.Runtime.freeMemory();
long consump  = totalmem - freemem;