最近应用在测试中出现Out Of Memory的问题， 通过jmap查看，发现JVM heap全用满了。

有很多工具可以查看JVM堆的信息， 收费的比如JProfiler， YourKit，免费的如Oracle JDK自带的visualvm, jhat和Eclipse MAT。

这个应用安装在一台AWS上，没有图形界面， 内存也比较小，想通过VNC远程桌面启动visualvm或者MAT不可能，通过jhat分析dump出来的snapshot(大约4.3G)也很慢，半天没有分析完毕，这种办法也放弃。

最后通过MAT的命令行工具分析了dump出来的snapshot,查找到OOM的元凶。

JVM version is 25.31 -b07

using thread-local object allocation.

Parallel GC with 4 thread(s)

Heap Configuration:

MinHeapFreeRatio = 0

MaxHeapFreeRatio = 100

MaxHeapSize = 4294967296 ( 4096.0 MB)

NewSize = 1431306240 ( 1365.0 MB)

MaxNewSize = 1431306240 ( 1365.0 MB)

OldSize = 2863661056 ( 2731.0 MB)

NewRatio = 2

SurvivorRatio = 8

MetaspaceSize = 21807104 ( 20.796875 MB)

CompressedClassSpaceSize = 1073741824 ( 1024.0 MB)

MaxMetaspaceSize = 17592186044415 MB

G1HeapRegionSize = 0 ( 0.0 MB)

Heap Usage:

PS Young Generation

Eden Space:

capacity = 482344960 ( 460.0 MB)

used = 468288384 ( 446.5946044921875 MB)

free = 14056576 ( 13.4053955078125 MB)

97.08578358525816 % used

From Space:

capacity = 278921216 ( 266.0 MB)

used = 0 ( 0.0 MB)

free = 278921216 ( 266.0 MB)

0.0 % used

To Space:

capacity = 477102080 ( 455.0 MB)

used = 0 ( 0.0 MB)

free = 477102080 ( 455.0 MB)

0.0 % used

PS Old Generation

capacity = 2863661056 ( 2731.0 MB)

used = 2863365080 ( 2730.7177352905273 MB)

free = 295976 ( 0.28226470947265625 MB)

99.98966441927965 % used

12340 interned Strings occupying 1051736 bytes.

num #instances #bytes class name

----------------------------------------------

1: 21606534 1530253752 [C

2: 21606239 518549736 java.lang.String

3: 19198980 460775520 scala.collection.immutable.ListSet$Node

4: 4568546 109645104 scala.collection.immutable.HashSet$HashSetCollision1

5: 103739 63212992 [B

6: 1487034 53464560 [Lscala.collection.immutable.HashSet;

7: 1487034 35688816 scala.collection.immutable.HashSet$HashTrieSet

8: 1350368 32408832 scala.collection.immutable.$colon$colon

9: 1090897 26181528 scala.collection.immutable.HashSet$HashSet1

10: 200035 17603080 akka.actor.ActorCell

11: 100536 8042880 java.lang.reflect.Constructor

12: 500026 8000416 scala.runtime.ObjectRef

既然没有办法图形化启动visualvm和MAT,那么就使用MAT文件夹下的ParseHeapDump.sh, 特别适合分析大堆的信息。

首先你需要修改MemoryAnalyzer.ini中的Xmx值，确保有充足的硬盘空间(至少dump文件的两倍)。

然后运行

./ParseHeapDump.sh heap.bin org.eclipse.mat.api:suspects org.eclipse.mat.api:overview org.eclipse.mat.api:top_components

会得到suspects, overview和top_components三个视图的信息。

可以看到akka.dispatch.Dispatcher$$anon$1一个实例占用了2.4GB的内存，这就是罪魁祸首。这其实是akka dispatcher的mailbox中的java.util.concurrent.ConcurrentLinkedQueue，每个Node占用了81M的内存，

消息体太大了。

你也可以引用MAT的类，得到heap dump中的信息， 因为MAT使用Eclipse RCP框架， 基于osgi架构，使用起来不太方便，所以你可以别人抽取出来的MAT库，如https://bitbucket.org/joebowbeer/andromat,

然后实现一个命令行程序,比如下面的例子就是输出所有的字符串的值：

package com.colobu.mat;    import org.eclipse.mat.SnapshotException;    import org.eclipse.mat.parser.model.PrimitiveArrayImpl;    import org.eclipse.mat.snapshot.ISnapshot;    import org.eclipse.mat.parser.internal.SnapshotFactory;    import org.eclipse.mat.snapshot.model.IClass;    import org.eclipse.mat.snapshot.model.IObject;    import org.eclipse.mat.util.ConsoleProgressListener;    import org.eclipse.mat.util.IProgressListener;    import java.io.File;    import java.io.IOException;    import java.util.Collection;    import java.util.HashMap;    public class Main {    public static void main (String[] args) throws SnapshotException, IOException {    String arg = args[args.length - 1 ];    String fileName = arg;    IProgressListener listener = new ConsoleProgressListener(System.out);    SnapshotFactory sf = new SnapshotFactory();    ISnapshot snapshot = sf.openSnapshot( new File(fileName),    new HashMap<String, String>(), listener);    System.out.println(snapshot.getSnapshotInfo());    System.out.println();    String[] classNames = { "java.lang.String" };    for (String name : classNames) {    Collection<IClass> classes = snapshot.getClassesByName(name, false );    if (classes == null || classes.isEmpty()) {    System.out.println(String.format( "Cannot find class %s in heap dump" , name));    continue ;    }    assert classes.size() == 1 ;    IClass clazz = classes.iterator().next();    int [] objIds = clazz.getObjectIds();    long minRetainedSize = snapshot.getMinRetainedSize(objIds, listener);    System.out.println(String.format( "%s instances = %d, retained size >= %d" , clazz.getName(), objIds.length, minRetainedSize));    for ( int i = 0 ; i < objIds.length; i++) {    IObject str = snapshot.getObject(objIds[i]);    String address = Long.toHexString(snapshot.mapIdToAddress(objIds[i]));    PrimitiveArrayImpl chars = (PrimitiveArrayImpl) str.resolveValue( "value" );    String value = new String(( char []) chars.getValueArray());    System.out.println(String.format( "id=%d, address=%s, value=%s" , objIds[i], address, value));    }    }    }    }