ソフト参照と弱参照の処理で問題が発生しました。コードには、すべてのロジックをソフト参照と弱参照の間で切り替えるフラグがあります。弱い参照ではすべて正常に機能しているように見えますが、ソフト参照では一貫してOutOfMemoryErrorが発生します。これは、MacOSXのJDK7とJDK6、およびDebianのIcedTea6で発生します。ただし、G1コレクターを備えたJDK7は、ソフト参照で動作していることがわかったセットアップであり、他のすべて（シリアル/パラレルGC、-client / -serverなど）は例外をスローできませんでした。
コードは少し大きいですが、問題を残しながら可能な限り絞り込むようにしています。
その上に、問題を再現する方法をより詳細に説明する大規模なコメントを残しました。
/*
*
* Leakling.java
*
*
* Issue:
*
*
* This code throws OutOfMemoryError when using soft references, whereas weak references
* work ok. Moreover, with JDK7 G1 garbage collector soft references work as well. Other
* collectors seem to fail. Code was tested with MacOSX 10.8.2 JDKs 1.7.0_10-b18 and
* 1.6.0_37-b06-434-11M3909, with Debian 6.0 IcedTea6 1.8.13.
* Various command line options including -Xmx, -client/-server, -XX:+UseParallelOldGC,
* -XX:+UseSerialGC were tested.
*
*
* Examples:
*
*
* 1. Default options, weak references, this works, counters go up and down,
* but everything keeps going just as expected:
*
* java -Xmx50m Leakling \
* --loop-delay=10 --min-chunk-size=1000 --max-chunk-size=100000 --use-soft-references=false
*
*
* 2. Default options, soft references, this eventually throws the exception:
*
* java -Xmx50m Leakling \
* --loop-delay=10 --min-chunk-size=1000 --max-chunk-size=100000 --use-soft-references=true
*
*
* 3. G1 collector (IcedTea6 doesn't support it), weak references, this works, but it did anyway:
*
* java -XX:+UseG1GC -Xmx50m Leakling \
* --loop-delay=10 --min-chunk-size=1000 --max-chunk-size=100000 --use-soft-references=false
*
*
* 4. G1 collector, soft references, this works with JDK7.
* JDK6 seems to just stop after having hit memory limit (with no message whatsoever).
*
* java -XX:+UseG1GC -Xmx50m Leakling \
* --loop-delay=10 --min-chunk-size=1000 --max-chunk-size=100000 --use-soft-references=true
*
*
* jarek, 02.01.2013
*
*
*/
import java.lang.ref.*;
import java.util.*;
public class Leakling {
private static final String TAG = "memory-chunk-";
class Chunk {
final String name;
final int size;
final private byte[] mem;
Chunk(String name, int minSize, int maxSize, Random randomizer) {
int currSize = minSize;
if (maxSize > minSize) {
currSize += randomizer.nextInt(maxSize - minSize + 1);
}
this.size = currSize;
this.mem = new byte[currSize];
this.name = name;
log(this + " has been created (" + currSize + " bytes)");
}
@Override
public void finalize() throws Throwable {
log(this + " is finalizing");
}
@Override
public String toString() {
return name + " of " + getReadableMemorySize(size);
}
}
class WeakChunk extends WeakReference<Chunk> {
final String name;
public WeakChunk(Chunk chunk, ReferenceQueue<Chunk> queue) {
super(chunk, queue);
this.name = chunk.name;
}
@Override
public String toString() {
return "weak reference of " + name + " is " + ((get() == null) ? "null" : "alive");
}
}
class SoftChunk extends SoftReference<Chunk> {
final String name;
public SoftChunk(Chunk chunk, ReferenceQueue<Chunk> queue) {
super(chunk, queue);
this.name = chunk.name;
}
@Override
public String toString() {
return "soft reference of " + name + " is " + ((get() == null) ? "null" : "alive");
}
}
// Logging as implemented here gives extra timing info (secs.milis starting from the initialization).
private final long start = System.currentTimeMillis();
private final Formatter formatter = new Formatter(System.err);
private final String formatString = "%1$d.%2$03d %3$s\n";
// I found this be better synchronized...
synchronized void log(Object o) {
long curr = System.currentTimeMillis();
long diff = curr - start;
formatter.format(formatString, (int) (diff / 1000), (int) (diff % 1000), o.toString());
}
private final ArrayList<Chunk> allChunks = new ArrayList<Chunk>();
private final ReferenceQueue<Chunk> softReferences = new ReferenceQueue<Chunk>();
private final ReferenceQueue<Chunk> weakReferences = new ReferenceQueue<Chunk>();
private final HashSet<Reference<Chunk>> allReferences = new HashSet<Reference<Chunk>>();
private final Random randomizer = new Random();
private int loopDelay = 200;
private int minChunkSize = 100;
private int maxChunkSize = 1000;
private int chunkCounter = 0;
private boolean useSoftReferences = false;
private long minMemory = 10 * 1024 * 1024; // Default range is 10-30MB
private long maxMemory = 3 * minMemory;
private long usedMemory = 0;
private String getReadableMemorySize(long size) {
if (size >= 1024 * 1024) {
return (float) (Math.round((((float) size) / 1024f / 1024f) * 10f)) / 10f + "MB";
}
if (size >= 1024) {
return (float) (Math.round((((float) size) / 1024f) * 10f)) / 10f + "kB";
} else if (size > 0) {
return size + "B";
} else {
return "0";
}
}
private void allocMem() {
Chunk chunk = new Chunk(TAG + chunkCounter++, minChunkSize, maxChunkSize, randomizer);
allChunks.add(chunk);
Reference ref = useSoftReferences
? (new SoftChunk(chunk, softReferences)) : (new WeakChunk(chunk, weakReferences));
allReferences.add(ref);
log(ref);
usedMemory += chunk.size;
}
private void freeMem() {
if (allChunks.size() < 1) {
return;
}
int i = randomizer.nextInt(allChunks.size());
Chunk chunk = allChunks.get(i);
log("freeing " + chunk);
usedMemory -= chunk.size;
allChunks.remove(i);
}
private int statMem() throws Exception {
for (Reference ref; (ref = softReferences.poll()) != null;) {
log(ref);
allReferences.remove(ref);
}
for (Reference ref; (ref = weakReferences.poll()) != null;) {
log(ref);
allReferences.remove(ref);
}
int weakRefs = 0;
int softRefs = 0;
for (Iterator<Reference<Chunk>> i = allReferences.iterator(); i.hasNext();) {
Reference<Chunk> ref = i.next();
if (ref.get() == null) {
continue;
}
if (ref instanceof WeakChunk) {
weakRefs++;
}
if (ref instanceof SoftChunk) {
softRefs++;
}
}
log(allChunks.size() + " chunks, "
+ softRefs + " soft refs, "
+ weakRefs + " weak refs, "
+ getReadableMemorySize(usedMemory) + " used, "
+ getReadableMemorySize(Runtime.getRuntime().freeMemory()) + " free, "
+ getReadableMemorySize(Runtime.getRuntime().totalMemory()) + " total, "
+ getReadableMemorySize(Runtime.getRuntime().maxMemory()) + " max");
if (loopDelay > 1) {
Thread.sleep(loopDelay);
}
return (int)((100 * usedMemory) / maxMemory); // Return % of maxMemory being used.
}
public Leakling(String[] args) throws Exception {
for (String arg : args) {
if (arg.startsWith("--min-memory=")) {
minMemory = Long.parseLong(arg.substring("--min-memory=".length()));
} else if (arg.startsWith("--max-memory=")) {
maxMemory = Long.parseLong(arg.substring("--max-memory=".length()));
} else if (arg.startsWith("--min-chunk-size=")) {
minChunkSize = Integer.parseInt(arg.substring("--min-chunk-size=".length()));
} else if (arg.startsWith("--max-chunk-size=")) {
maxChunkSize = Integer.parseInt(arg.substring("--max-chunk-size=".length()));
} else if (arg.startsWith("--loop-delay=")) {
loopDelay = Integer.parseInt(arg.substring("--loop-delay=".length()));
} else if (arg.startsWith("--use-soft-references=")) {
useSoftReferences = Boolean.parseBoolean(arg.substring("--use-soft-references=".length()));
} else {
throw new Exception("Unknown command line option...");
}
}
}
public void run() throws Exception {
log("Mem test started...");
while(true) {
log("going up...");
do {// First loop allocates memory up to the given limit in a pseudo-random fashion.
// Randomized rate of allocations/frees is about 4:1 as per the 10>=8 condition.
if (randomizer.nextInt(10) >= 8) {
freeMem();
} else {
allocMem();
}
} while (statMem() < 90); // Repeat until 90% of the given mem limit is hit...
log("going down...");
do {// Now do the reverse. Frees are four times more likely than allocations are.
if (randomizer.nextInt(10) < 8) {
freeMem();
} else {
allocMem();
}
} while (usedMemory > minMemory);
}
}
public static void main(String[] args) throws Exception {
(new Leakling(args)).run();
}
}