在命令行中使用Eclipse MAT工具

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。

使用MAT命令行工具

首先通过jstat或者jmap查看heap信息,比如通过jmap查看：

 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 (4096.0MB)
   NewSize                  = 1431306240 (1365.0MB)
   MaxNewSize               = 1431306240 (1365.0MB)
   OldSize                  = 2863661056 (2731.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)
Heap Usage:
PS Young Generation
Eden Space:
   capacity = 482344960 (460.0MB)
   used     = 468288384 (446.5946044921875MB)
   free     = 14056576 (13.4053955078125MB)
   97.08578358525816% used
From Space:
   capacity = 278921216 (266.0MB)
   used     = 0 (0.0MB)
   free     = 278921216 (266.0MB)
   0.0% used
To Space:
   capacity = 477102080 (455.0MB)
   used     = 0 (0.0MB)
   free     = 477102080 (455.0MB)
   0.0% used
PS Old Generation
   capacity = 2863661056 (2731.0MB)
   used     = 2863365080 (2730.7177352905273MB)
   free     = 295976 (0.28226470947265625MB)
   99.98966441927965% used
12340 interned Strings occupying 1051736 bytes.

最多的类的实例:

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

从分析来看猜测是akka actor mailbox里面的字符串消息太多了。

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

1	./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。
有问题的对象
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。

编写程序得到所需信息

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));
            }
        }
    }
}