在命令行中使用Eclipse MAT工具

最近應用在測試中出現Out Of Memory的問題， 通過jmap查看，發現JVM heap全用滿了。

有很多工具可以查看JVM堆的信息， 收費的比如JProfiler， YourKit，免費的如Oracle JDK自帶的visualvm, jhat和Eclipse MAT。

這個應用安裝在一臺AWS上，沒有圖形界面， 內存也比較小，想通過VNC遠程桌面啟動visualvm或者MAT不可能，通過jhat分析dump出來的snapshot(大約4.3G)也很慢，半天沒有分析完畢，這種辦法也放棄。

最后通過MAT的命令行工具分析了dump出來的snapshot,查找到OOM的元兇。

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

既然沒有辦法圖形化啟動visualvm和MAT,那么就使用MAT文件夾下的ParseHeapDump.sh, 特別適合分析大堆的信息。

首先你需要修改MemoryAnalyzer.ini中的Xmx值，確保有充足的硬盤空間(至少dump文件的兩倍)。

然后運行

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

會得到suspects, overview和top_components三個視圖的信息。

可以看到akka.dispatch.Dispatcher$$anon$1一個實例占用了2.4GB的內存，這就是罪魁禍首。這其實是akka dispatcher的mailbox中的java.util.concurrent.ConcurrentLinkedQueue，每個Node占用了81M的內存，

消息體太大了。

你也可以引用MAT的類，得到heap dump中的信息， 因為MAT使用Eclipse RCP框架， 基于osgi架構，使用起來不太方便，所以你可以別人抽取出來的MAT庫，如https://bitbucket.org/joebowbeer/andromat,

然后實現一個命令行程序,比如下面的例子就是輸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));

}

}

}

}