Java 程序如何跟踪其运行期间使用的最大实际堆大小？

Question

I'm running a certain Java program, with its -Xmx higher than -Xms , ie its heap can grow. The heap size at execution end is (IIRC) not the maximum used during the run.

• How can I get the current heap size?
• How can I get the maximum heap size over the course of the run, other than periodically polling the current size "myself"?

Answer 1

Get current heap size:

public static long getHeapSize(){
    int mb = 1024*1024;

    //Getting the runtime reference from system
    Runtime runtime = Runtime.getRuntime();

    return ((runtime.totalMemory() - runtime.freeMemory()) / mb);
}

Answer 2

If you want to inspect it from your program itself, use the methods of the Runtime class:

Runtime.getRuntime().freeMemory()
Runtime.getRuntime().maxMemory()

If it is ok for you to inspect the heap size from outside your program, you should use JVisualVM. You can find it in the bin folder of your JDK. Simply start it, and attach to your java program to get insight into your programs heap usage. It will display a graph of your heap usage, making it easy to find the maximum heap size of the run of your program.

Answer 3

// Get current size of heap in bytes

long heapSize = Runtime.getRuntime().totalMemory(); 

// Get maximum size of heap in bytes. The heap cannot grow beyond this size.// Any attempt will result in an OutOfMemoryException.

long heapMaxSize = Runtime.getRuntime().maxMemory();

// Get amount of free memory within the heap in bytes. This size will increase // after garbage collection and decrease as new objects are created.

long heapFreeSize = Runtime.getRuntime().freeMemory(); 

Answer 4

The other answers provide the mechanics of doing via Runtime.getRuntime().totalMemory() and maxMemory(), but be VERY careful - the answer given will be true at a given instant only. After a GC, the totalMemory() will change (downwards!). You can't have a perfectly accurate view of "how many live objects there are in the system" at all times - as that's exactly what GC calculates, and is expensive.

Using JMX (See the GC bean, etc..) will help with that polling, but again, it's a sample over time.

So I'm not sure what you're actually trying to solve here...

Answer 5

See runtime info:

Runtime.getRuntime().maxMemory();
Runtime.getRuntime().totalMemory();
Runtime.getRuntime().freeMemory();

Answer 6

Combination of jps and jstat command is really powerful enough to track your java memory details.

Use the jps command to get the pid of your java process and use jstat $pid to get the details of your java program. Now you can run them in a loop and closely moniter the memory detatils of your java program.

You can find a bash implementation of this idea on github . One big advantage of this script is that you can start this script, and then run your java program. You will be able to see the memory details of your program from the very begning.