This document provides an overview of tools and techniques for analyzing Java heap dumps. It discusses how to generate heap dumps using Ctrl-Break, XX:+HeapDumpOnOutOfMemoryError, or jmap. Jmap can also provide heap summaries. JHat allows analyzing heap dumps visually. Eclipse MAT and JConsole can also analyze heap dumps. Jstack prints stack traces, and can detect deadlocks. Sample code shows how to generate heap dumps and cause deadlocks.

1. HEAP DUMP
ANALYSIS BASICS

2. HEAP
The Java virtual machine
heap is the area of memory
used by the JVM for
dynamic memory allocation.

3. HEAP DUMP
Ctrl-break
-XX:+HeapDumpOnOutOfMemoryError
generate a heap dump when an allocation
from the Java heap or the permanent
generation cannot be satisfied
jmap -dump:format=b,file=dump.bin pid

4. JMAP
● Included with JDK
● On most systems (including PROD)
● Simple command line tool
● Provides heap dump, heap summary
and more

5. JMAP HEAP DUMP
$ jps -l -m
5918 GarbageCollection
5923 sun.tools.jps.Jps -l -m
$ jmap -histo 5918|less
num #instances #bytes class name
----------------------------------------------
1: 76 9158320 [I
2: 35 8913488 [J
3: 34 8913440 [D
4: 139264 3342336 BannerUser
5: 1161 2329648 [C
6: 512 2316744 [B
7: 34 2228768 [F
<... more stats ...>
// Integer
// Long
// Double
// Char
// Byte
// Float
http://docs.oracle.com/javase/7/docs/api/java/lang/Class.html#getName%28%29

6. $ sudo jmap -heap 5938
using thread-local object allocation.
Mark Sweep Compact GC
Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize = 199229440 (190.0MB)
NewSize = 1310720 (1.25MB)
MaxNewSize = 17592186044415 MB
OldSize = 5439488 (5.1875MB)
<...lots more stats...>
HEAP SUMARY

7. JHAT HEAP ANALYSIS
$ jmap -dump:format=b,file=dump.bin 5938
$ jhat ~/dump.bin

8. JHAT HEAP ANALYSIS

9. JHAT HEAP ANALYSIS

10. JCONSOLE

11. ECLIPSE MAT
The Eclipse Memory Analyzer is a fast
and feature-rich Java heap analyzer
that helps you find memory leaks and
reduce memory consumption.
http://www.eclipse.org/mat/

12. STACK DUMP
$ jstack -l 5722
<...shows you full stack trace...>
Found one Java-level deadlock:
=============================
"Thread-1":
waiting to lock monitor 0x00007fa7dd004ab0 (object
0x00000007ef2b1020, a java.lang.Object),
which is held by "Thread-0"
"Thread-0":
waiting to lock monitor 0x00007fa7dd007360 (object
0x00000007ef2b1030, a java.lang.Object),
which is held by "Thread-1"

13. APPENDIX

14. GarbageCollection.java
/**
* GarbageCollection.java
* Copyright (c) 2008 by Dr. Herong Yang, http://www.herongyang.com/
*/
class GarbageCollection {
public static void main(String[] a) {
int max = 10000;
int min = 16;
Object[] arr = new Object[min];
Runtime rt = Runtime.getRuntime();
System.out.println("Free/total memory:");
for (int m=0; m<max; m++) {
for (int n=0; n<min-1; n++) arr[min-n-1] = arr[min-n-2];
arr[0] = getOneMega();
System.out.println(rt.freeMemory()+" "+rt.totalMemory());
try {
Thread.sleep(1000/10);
} catch (InterruptedException e) {

15. SimpleDeadLock.java
/**
* SimpleDeadLock.java
* Copyright (c) 2008 by Dr. Herong Yang, http://www.herongyang.com/
*/
import java.util.*;
public class SimpleDeadLock extends Thread {
public static Object l1 = new Object();
public static Object l2 = new Object();
private int index;
public static void main(String[] a) {
Thread t1 = new Thread1();
Thread t2 = new Thread2();
t1.start();
t2.start();
}
private static class Thread1 extends Thread {
public void run() {
synchronized (l1) {