聊聊netty的ResourceLeakDetector

Netty · 發表 2019-04-03 13:28:17

摘要：序本文主要研究一下netty的ResourceLeakDetector LEAK異常 2019-04-02 15:23:17.026 ERROR 1 --- [reactor-http-epoll-2] io.netty.util.ResourceLeakDetector...

序

本文主要研究一下netty的ResourceLeakDetector

LEAK異常

2019-04-02 15:23:17.026 ERROR 1 --- [reactor-http-epoll-2] io.netty.util.ResourceLeakDetector: LEAK: ByteBuf.release() was not called before it's garbage-collected. See http://netty.io/wiki/reference-counted-objects.html for more information.
Recent access records: 
#1:
io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:286)
io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:253)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1408)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:930)
io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:799)
io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:427)
io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:328)
io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:905)
java.base/java.lang.Thread.run(Thread.java:835)
#2:
io.netty.buffer.AdvancedLeakAwareByteBuf.forEachByte(AdvancedLeakAwareByteBuf.java:670)
io.netty.handler.codec.http.HttpObjectDecoder$HeaderParser.parse(HttpObjectDecoder.java:801)
io.netty.handler.codec.http.HttpObjectDecoder.readHeaders(HttpObjectDecoder.java:601)
io.netty.handler.codec.http.HttpObjectDecoder.decode(HttpObjectDecoder.java:227)
io.netty.handler.codec.http.HttpClientCodec$Decoder.decode(HttpClientCodec.java:202)
io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:502)
io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:441)
io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:278)
io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:253)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1408)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:930)
io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:799)
io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:427)
io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:328)
io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:905)
java.base/java.lang.Thread.run(Thread.java:835)
#3:
io.netty.buffer.AdvancedLeakAwareByteBuf.forEachByte(AdvancedLeakAwareByteBuf.java:670)
io.netty.handler.codec.http.HttpObjectDecoder$HeaderParser.parse(HttpObjectDecoder.java:801)
io.netty.handler.codec.http.HttpObjectDecoder.readHeaders(HttpObjectDecoder.java:581)
io.netty.handler.codec.http.HttpObjectDecoder.decode(HttpObjectDecoder.java:227)
io.netty.handler.codec.http.HttpClientCodec$Decoder.decode(HttpClientCodec.java:202)
io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:502)
io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:441)
io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:278)
io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:253)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1408)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:930)
io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:799)
io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:427)
io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:328)
io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:905)
java.base/java.lang.Thread.run(Thread.java:835)
#4:
io.netty.buffer.AdvancedLeakAwareByteBuf.forEachByte(AdvancedLeakAwareByteBuf.java:670)
io.netty.handler.codec.http.HttpObjectDecoder$HeaderParser.parse(HttpObjectDecoder.java:801)
io.netty.handler.codec.http.HttpObjectDecoder$LineParser.parse(HttpObjectDecoder.java:850)
io.netty.handler.codec.http.HttpObjectDecoder.decode(HttpObjectDecoder.java:208)
io.netty.handler.codec.http.HttpClientCodec$Decoder.decode(HttpClientCodec.java:202)
io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:502)
io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:441)
io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:278)
io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:253)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1408)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:930)
io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:799)
io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:427)
io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:328)
io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:905)
java.base/java.lang.Thread.run(Thread.java:835)
#5:
io.netty.buffer.AdvancedLeakAwareByteBuf.getUnsignedByte(AdvancedLeakAwareByteBuf.java:160)
io.netty.handler.codec.http.HttpObjectDecoder.skipControlCharacters(HttpObjectDecoder.java:566)
io.netty.handler.codec.http.HttpObjectDecoder.decode(HttpObjectDecoder.java:202)
io.netty.handler.codec.http.HttpClientCodec$Decoder.decode(HttpClientCodec.java:202)
io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:502)
io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:441)
io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:278)
io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:253)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1408)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:930)
io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:799)
io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:427)
io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:328)
io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:905)
java.base/java.lang.Thread.run(Thread.java:835)
#6:
Hint: 'reactor.left.httpCodec' will handle the message from this point.
io.netty.channel.DefaultChannelPipeline.touch(DefaultChannelPipeline.java:116)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:345)
io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1408)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:930)
io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:799)
io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:427)
io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:328)
io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:905)
java.base/java.lang.Thread.run(Thread.java:835)
#7:
Hint: 'DefaultChannelPipeline$HeadContext#0' will handle the message from this point.
io.netty.channel.DefaultChannelPipeline.touch(DefaultChannelPipeline.java:116)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:345)
io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:930)
io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:799)
io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:427)
io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:328)
io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:905)
java.base/java.lang.Thread.run(Thread.java:835)
Created at:
io.netty.buffer.PooledByteBufAllocator.newDirectBuffer(PooledByteBufAllocator.java:339)
io.netty.buffer.AbstractByteBufAllocator.directBuffer(AbstractByteBufAllocator.java:185)
io.netty.buffer.AbstractByteBufAllocator.directBuffer(AbstractByteBufAllocator.java:176)
io.netty.channel.unix.PreferredDirectByteBufAllocator.ioBuffer(PreferredDirectByteBufAllocator.java:53)
io.netty.channel.DefaultMaxMessagesRecvByteBufAllocator$MaxMessageHandle.allocate(DefaultMaxMessagesRecvByteBufAllocator.java:114)
io.netty.channel.epoll.EpollRecvByteAllocatorHandle.allocate(EpollRecvByteAllocatorHandle.java:77)
io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:784)
io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:427)
io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:328)
io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:905)
java.base/java.lang.Thread.run(Thread.java:835)
: 9 leak records were discarded because the leak record count is targeted to 4. Use system property io.netty.leakDetection.targetRecords to increase the limit.

ResourceLeakDetector

netty-common-4.1.33.Final-sources.jar!/io/netty/util/ResourceLeakDetector.java

public class ResourceLeakDetector<T> {

private static final String PROP_LEVEL_OLD = "io.netty.leakDetectionLevel";
private static final String PROP_LEVEL = "io.netty.leakDetection.level";
private static final Level DEFAULT_LEVEL = Level.SIMPLE;

private static final String PROP_TARGET_RECORDS = "io.netty.leakDetection.targetRecords";
private static final int DEFAULT_TARGET_RECORDS = 4;

private static final String PROP_SAMPLING_INTERVAL = "io.netty.leakDetection.samplingInterval";
// There is a minor performance benefit in TLR if this is a power of 2.
private static final int DEFAULT_SAMPLING_INTERVAL = 128;

private static final int TARGET_RECORDS;
static final int SAMPLING_INTERVAL;

/**
* Represents the level of resource leak detection.
*/
public enum Level {
/**
* Disables resource leak detection.
*/
DISABLED,
/**
* Enables simplistic sampling resource leak detection which reports there is a leak or not,
* at the cost of small overhead (default).
*/
SIMPLE,
/**
* Enables advanced sampling resource leak detection which reports where the leaked object was accessed
* recently at the cost of high overhead.
*/
ADVANCED,
/**
* Enables paranoid resource leak detection which reports where the leaked object was accessed recently,
* at the cost of the highest possible overhead (for testing purposes only).
*/
PARANOID;

/**
* Returns level based on string value. Accepts also string that represents ordinal number of enum.
*
* @param levelStr - level string : DISABLED, SIMPLE, ADVANCED, PARANOID. Ignores case.
* @return corresponding level or SIMPLE level in case of no match.
*/
static Level parseLevel(String levelStr) {
String trimmedLevelStr = levelStr.trim();
for (Level l : values()) {
if (trimmedLevelStr.equalsIgnoreCase(l.name()) || trimmedLevelStr.equals(String.valueOf(l.ordinal()))) {
return l;
}
}
return DEFAULT_LEVEL;
}
}

private static Level level;

private static final InternalLogger logger = InternalLoggerFactory.getInstance(ResourceLeakDetector.class);

static {
final boolean disabled;
if (SystemPropertyUtil.get("io.netty.noResourceLeakDetection") != null) {
disabled = SystemPropertyUtil.getBoolean("io.netty.noResourceLeakDetection", false);
logger.debug("-Dio.netty.noResourceLeakDetection: {}", disabled);
logger.warn(
"-Dio.netty.noResourceLeakDetection is deprecated. Use '-D{}={}' instead.",
PROP_LEVEL, DEFAULT_LEVEL.name().toLowerCase());
} else {
disabled = false;
}

Level defaultLevel = disabled? Level.DISABLED : DEFAULT_LEVEL;

// First read old property name
String levelStr = SystemPropertyUtil.get(PROP_LEVEL_OLD, defaultLevel.name());

// If new property name is present, use it
levelStr = SystemPropertyUtil.get(PROP_LEVEL, levelStr);
Level level = Level.parseLevel(levelStr);

TARGET_RECORDS = SystemPropertyUtil.getInt(PROP_TARGET_RECORDS, DEFAULT_TARGET_RECORDS);
SAMPLING_INTERVAL = SystemPropertyUtil.getInt(PROP_SAMPLING_INTERVAL, DEFAULT_SAMPLING_INTERVAL);

ResourceLeakDetector.level = level;
if (logger.isDebugEnabled()) {
logger.debug("-D{}: {}", PROP_LEVEL, level.name().toLowerCase());
logger.debug("-D{}: {}", PROP_TARGET_RECORDS, TARGET_RECORDS);
}
}

/**
* @deprecated Use {@link #setLevel(Level)} instead.
*/
@Deprecated
public static void setEnabled(boolean enabled) {
setLevel(enabled? Level.SIMPLE : Level.DISABLED);
}

/**
* Returns {@code true} if resource leak detection is enabled.
*/
public static boolean isEnabled() {
return getLevel().ordinal() > Level.DISABLED.ordinal();
}

/**
* Sets the resource leak detection level.
*/
public static void setLevel(Level level) {
if (level == null) {
throw new NullPointerException("level");
}
ResourceLeakDetector.level = level;
}

/**
* Returns the current resource leak detection level.
*/
public static Level getLevel() {
return level;
}

/** the collection of active resources */
private final Set<DefaultResourceLeak<?>> allLeaks =
Collections.newSetFromMap(new ConcurrentHashMap<DefaultResourceLeak<?>, Boolean>());

private final ReferenceQueue<Object> refQueue = new ReferenceQueue<Object>();
private final ConcurrentMap<String, Boolean> reportedLeaks = PlatformDependent.newConcurrentHashMap();

private final String resourceType;
private final int samplingInterval;

//......

/**
* Creates a new {@link ResourceLeakTracker} which is expected to be closed via
* {@link ResourceLeakTracker#close(Object)} when the related resource is deallocated.
*
* @return the {@link ResourceLeakTracker} or {@code null}
*/
@SuppressWarnings("unchecked")
public final ResourceLeakTracker<T> track(T obj) {
return track0(obj);
}

private DefaultResourceLeak track0(T obj) {
Level level = ResourceLeakDetector.level;
if (level == Level.DISABLED) {
return null;
}

if (level.ordinal() < Level.PARANOID.ordinal()) {
if ((PlatformDependent.threadLocalRandom().nextInt(samplingInterval)) == 0) {
reportLeak();
return new DefaultResourceLeak(obj, refQueue, allLeaks);
}
return null;
}
reportLeak();
return new DefaultResourceLeak(obj, refQueue, allLeaks);
}

private void reportLeak() {
if (!logger.isErrorEnabled()) {
clearRefQueue();
return;
}

// Detect and report previous leaks.
for (;;) {
@SuppressWarnings("unchecked")
DefaultResourceLeak ref = (DefaultResourceLeak) refQueue.poll();
if (ref == null) {
break;
}

if (!ref.dispose()) {
continue;
}

String records = ref.toString();
if (reportedLeaks.putIfAbsent(records, Boolean.TRUE) == null) {
if (records.isEmpty()) {
reportUntracedLeak(resourceType);
} else {
reportTracedLeak(resourceType, records);
}
}
}
}

/**
* This method is called when a traced leak is detected. It can be overridden for tracking how many times leaks
* have been detected.
*/
protected void reportTracedLeak(String resourceType, String records) {
logger.error(
"LEAK: {}.release() was not called before it's garbage-collected. " +
"See http://netty.io/wiki/reference-counted-objects.html for more information.{}",
resourceType, records);
}

/**
* This method is called when an untraced leak is detected. It can be overridden for tracking how many times leaks
* have been detected.
*/
protected void reportUntracedLeak(String resourceType) {
logger.error("LEAK: {}.release() was not called before it's garbage-collected. " +
"Enable advanced leak reporting to find out where the leak occurred. " +
"To enable advanced leak reporting, " +
"specify the JVM option '-D{}={}' or call {}.setLevel() " +
"See http://netty.io/wiki/reference-counted-objects.html for more information.",
resourceType, PROP_LEVEL, Level.ADVANCED.name().toLowerCase(), simpleClassName(this));
}

//......
}

ResourceLeakDetector使用Level列舉定義了四種不同的leak detection級別，分別是DISABLED、SIMPLE、ADVANCED、PARANOID；預設level為SIMPLE；可以使用-Dio.netty.leakDetection.level=advanced來進行設定
ResourceLeakDetector的靜態程式碼塊會讀取io.netty.noResourceLeakDetection系統屬性，如果顯示設定為false，則變更defaultLevel為DISABLED；如果沒有設定，則預設disabled為false，defaultLevel為SIMPLE；ResourceLeakDetector還有TARGET_RECORDS(io.netty.leakDetection.targetRecords )及SAMPLING_INTERVAL(io.netty.leakDetection.samplingInterval )兩個屬性，其中targetRecords預設為4，samplingInterval預設為128
ResourceLeakDetector提供了track方法用於建立ResourceLeakTracker；track方法內部呼叫track0方法，如果level為PARANOID則立即呼叫reportLeak，建立DefaultResourceLeak，否則利用隨機數來判斷(PlatformDependent.threadLocalRandom().nextInt(samplingInterval)) == 0 )是否呼叫reportLeak並建立DefaultResourceLeak；reportLeak方法有個for迴圈，不斷從refQueue取DefaultResourceLeak，然後呼叫reportUntracedLeak或者reportTracedLeak進行error

DefaultResourceLeak

netty-common-4.1.33.Final-sources.jar!/io/netty/util/ResourceLeakDetector.java

private static final class DefaultResourceLeak<T>
extends WeakReference<Object> implements ResourceLeakTracker<T>, ResourceLeak {

@SuppressWarnings("unchecked") // generics and updaters do not mix.
private static final AtomicReferenceFieldUpdater<DefaultResourceLeak<?>, Record> headUpdater =
(AtomicReferenceFieldUpdater)
AtomicReferenceFieldUpdater.newUpdater(DefaultResourceLeak.class, Record.class, "head");

@SuppressWarnings("unchecked") // generics and updaters do not mix.
private static final AtomicIntegerFieldUpdater<DefaultResourceLeak<?>> droppedRecordsUpdater =
(AtomicIntegerFieldUpdater)
AtomicIntegerFieldUpdater.newUpdater(DefaultResourceLeak.class, "droppedRecords");

@SuppressWarnings("unused")
private volatile Record head;
@SuppressWarnings("unused")
private volatile int droppedRecords;

private final Set<DefaultResourceLeak<?>> allLeaks;
private final int trackedHash;

DefaultResourceLeak(
Object referent,
ReferenceQueue<Object> refQueue,
Set<DefaultResourceLeak<?>> allLeaks) {
super(referent, refQueue);

assert referent != null;

// Store the hash of the tracked object to later assert it in the close(...) method.
// It's important that we not store a reference to the referent as this would disallow it from
// be collected via the WeakReference.
trackedHash = System.identityHashCode(referent);
allLeaks.add(this);
// Create a new Record so we always have the creation stacktrace included.
headUpdater.set(this, new Record(Record.BOTTOM));
this.allLeaks = allLeaks;
}

@Override
public void record() {
record0(null);
}

@Override
public void record(Object hint) {
record0(hint);
}

/**
* This method works by exponentially backing off as more records are present in the stack. Each record has a
* 1 / 2^n chance of dropping the top most record and replacing it with itself. This has a number of convenient
* properties:
*
* <ol>
* <li>The current record is always recorded. This is due to the compare and swap dropping the top most
*record, rather than the to-be-pushed record.
* <li>The very last access will always be recorded. This comes as a property of 1.
* <li>It is possible to retain more records than the target, based upon the probability distribution.
* <li>It is easy to keep a precise record of the number of elements in the stack, since each element has to
*know how tall the stack is.
* </ol>
*
* In this particular implementation, there are also some advantages. A thread local random is used to decide
* if something should be recorded. This means that if there is a deterministic access pattern, it is now
* possible to see what other accesses occur, rather than always dropping them. Second, after
* {@link #TARGET_RECORDS} accesses, backoff occurs. This matches typical access patterns,
* where there are either a high number of accesses (i.e. a cached buffer), or low (an ephemeral buffer), but
* not many in between.
*
* The use of atomics avoids serializing a high number of accesses, when most of the records will be thrown
* away. High contention only happens when there are very few existing records, which is only likely when the
* object isn't shared! If this is a problem, the loop can be aborted and the record dropped, because another
* thread won the race.
*/
private void record0(Object hint) {
// Check TARGET_RECORDS > 0 here to avoid similar check before remove from and add to lastRecords
if (TARGET_RECORDS > 0) {
Record oldHead;
Record prevHead;
Record newHead;
boolean dropped;
do {
if ((prevHead = oldHead = headUpdater.get(this)) == null) {
// already closed.
return;
}
final int numElements = oldHead.pos + 1;
if (numElements >= TARGET_RECORDS) {
final int backOffFactor = Math.min(numElements - TARGET_RECORDS, 30);
if (dropped = PlatformDependent.threadLocalRandom().nextInt(1 << backOffFactor) != 0) {
prevHead = oldHead.next;
}
} else {
dropped = false;
}
newHead = hint != null ? new Record(prevHead, hint) : new Record(prevHead);
} while (!headUpdater.compareAndSet(this, oldHead, newHead));
if (dropped) {
droppedRecordsUpdater.incrementAndGet(this);
}
}
}

boolean dispose() {
clear();
return allLeaks.remove(this);
}

@Override
public boolean close() {
if (allLeaks.remove(this)) {
// Call clear so the reference is not even enqueued.
clear();
headUpdater.set(this, null);
return true;
}
return false;
}

@Override
public boolean close(T trackedObject) {
// Ensure that the object that was tracked is the same as the one that was passed to close(...).
assert trackedHash == System.identityHashCode(trackedObject);

try {
return close();
} finally {
// This method will do `synchronized(trackedObject)` and we should be sure this will not cause deadlock.
// It should not, because somewhere up the callstack should be a (successful) `trackedObject.release`,
// therefore it is unreasonable that anyone else, anywhere, is holding a lock on the trackedObject.
// (Unreasonable but possible, unfortunately.)
reachabilityFence0(trackedObject);
}
}

/**
* Ensures that the object referenced by the given reference remains
* <a href="package-summary.html#reachability"><em>strongly reachable</em></a>,
* regardless of any prior actions of the program that might otherwise cause
* the object to become unreachable; thus, the referenced object is not
* reclaimable by garbage collection at least until after the invocation of
* this method.
*
* <p> Recent versions of the JDK have a nasty habit of prematurely deciding objects are unreachable.
* see: https://stackoverflow.com/questions/26642153/finalize-called-on-strongly-reachable-object-in-java-8
* The Java 9 method Reference.reachabilityFence offers a solution to this problem.
*
* <p> This method is always implemented as a synchronization on {@code ref}, not as
* {@code Reference.reachabilityFence} for consistency across platforms and to allow building on JDK 6-8.
* <b>It is the caller's responsibility to ensure that this synchronization will not cause deadlock.</b>
*
* @param ref the reference. If {@code null}, this method has no effect.
* @see java.lang.ref.Reference#reachabilityFence
*/
private static void reachabilityFence0(Object ref) {
if (ref != null) {
// Empty synchronized is ok: https://stackoverflow.com/a/31933260/1151521
synchronized (ref) { }
}
}

@Override
public String toString() {
Record oldHead = headUpdater.getAndSet(this, null);
if (oldHead == null) {
// Already closed
return EMPTY_STRING;
}

final int dropped = droppedRecordsUpdater.get(this);
int duped = 0;

int present = oldHead.pos + 1;
// Guess about 2 kilobytes per stack trace
StringBuilder buf = new StringBuilder(present * 2048).append(NEWLINE);
buf.append("Recent access records: ").append(NEWLINE);

int i = 1;
Set<String> seen = new HashSet<String>(present);
for (; oldHead != Record.BOTTOM; oldHead = oldHead.next) {
String s = oldHead.toString();
if (seen.add(s)) {
if (oldHead.next == Record.BOTTOM) {
buf.append("Created at:").append(NEWLINE).append(s);
} else {
buf.append('#').append(i++).append(':').append(NEWLINE).append(s);
}
} else {
duped++;
}
}

if (duped > 0) {
buf.append(": ")
.append(duped)
.append(" leak records were discarded because they were duplicates")
.append(NEWLINE);
}

if (dropped > 0) {
buf.append(": ")
.append(dropped)
.append(" leak records were discarded because the leak record count is targeted to ")
.append(TARGET_RECORDS)
.append(". Use system property ")
.append(PROP_TARGET_RECORDS)
.append(" to increase the limit.")
.append(NEWLINE);
}

buf.setLength(buf.length() - NEWLINE.length());
return buf.toString();
}
}

DefaultResourceLeak是ResourceLeakDetector定義的私有靜態類，它繼承了WeakReference類，同時實現了ResourceLeakTracker(定義了record、close方法 )介面；record方法內部呼叫的是record0方法，它會更新newHead為新的Record；close方法會移除allLeaks，allLeaks由ResourceLeakDetector建立DefaultResourceLeak時傳入，每建立一個DefaultResourceLeak，DefaultResourceLeak會把自己加入到allLeaks中

SimpleLeakAwareByteBuf

netty-netty-4.1.33.Final/buffer/src/main/java/io/netty/buffer/SimpleLeakAwareByteBuf.java

class SimpleLeakAwareByteBuf extends WrappedByteBuf {

/**
* This object's is associated with the {@link ResourceLeakTracker}. When {@link ResourceLeakTracker#close(Object)}
* is called this object will be used as the argument. It is also assumed that this object is used when
* {@link ResourceLeakDetector#track(Object)} is called to create {@link #leak}.
*/
private final ByteBuf trackedByteBuf;
final ResourceLeakTracker<ByteBuf> leak;

SimpleLeakAwareByteBuf(ByteBuf wrapped, ByteBuf trackedByteBuf, ResourceLeakTracker<ByteBuf> leak) {
super(wrapped);
this.trackedByteBuf = ObjectUtil.checkNotNull(trackedByteBuf, "trackedByteBuf");
this.leak = ObjectUtil.checkNotNull(leak, "leak");
}

SimpleLeakAwareByteBuf(ByteBuf wrapped, ResourceLeakTracker<ByteBuf> leak) {
this(wrapped, wrapped, leak);
}

//......

@Override
public boolean release() {
if (super.release()) {
closeLeak();
return true;
}
return false;
}

@Override
public boolean release(int decrement) {
if (super.release(decrement)) {
closeLeak();
return true;
}
return false;
}

private void closeLeak() {
// Close the ResourceLeakTracker with the tracked ByteBuf as argument. This must be the same that was used when
// calling DefaultResourceLeak.track(...).
boolean closed = leak.close(trackedByteBuf);
assert closed;
}

private ByteBuf unwrappedDerived(ByteBuf derived) {
// We only need to unwrap SwappedByteBuf implementations as these will be the only ones that may end up in
// the AbstractLeakAwareByteBuf implementations beside slices / duplicates and "real" buffers.
ByteBuf unwrappedDerived = unwrapSwapped(derived);

if (unwrappedDerived instanceof AbstractPooledDerivedByteBuf) {
// Update the parent to point to this buffer so we correctly close the ResourceLeakTracker.
((AbstractPooledDerivedByteBuf) unwrappedDerived).parent(this);

ResourceLeakTracker<ByteBuf> newLeak = AbstractByteBuf.leakDetector.track(derived);
if (newLeak == null) {
// No leak detection, just return the derived buffer.
return derived;
}
return newLeakAwareByteBuf(derived, newLeak);
}
return newSharedLeakAwareByteBuf(derived);
}

//......
}

SimpleLeakAwareByteBuf繼承了WrappedByteBuf，它的構造器要求傳入ResourceLeakTracker
SimpleLeakAwareByteBuf覆蓋了WrappedByteBuf的retainedSlice、retainedDuplicate、readRetainedSlice方法，它們內部都會呼叫unwrappedDerived方法，unwrappedDerived方法在unwrappedDerived物件是AbstractPooledDerivedByteBuf型別時會呼叫AbstractByteBuf.leakDetector.track進行track
SimpleLeakAwareByteBuf也覆蓋了WrappedByteBuf的release方法，在呼叫父類release成功時會再呼叫closeLeak方法，使用leak.close(trackedByteBuf)來釋放trackedByteBuf

AdvancedLeakAwareByteBuf

netty-netty-4.1.33.Final/buffer/src/main/java/io/netty/buffer/AdvancedLeakAwareByteBuf.java

final class AdvancedLeakAwareByteBuf extends SimpleLeakAwareByteBuf {

private static final String PROP_ACQUIRE_AND_RELEASE_ONLY = "io.netty.leakDetection.acquireAndReleaseOnly";
private static final boolean ACQUIRE_AND_RELEASE_ONLY;

private static final InternalLogger logger = InternalLoggerFactory.getInstance(AdvancedLeakAwareByteBuf.class);

static {
ACQUIRE_AND_RELEASE_ONLY = SystemPropertyUtil.getBoolean(PROP_ACQUIRE_AND_RELEASE_ONLY, false);

if (logger.isDebugEnabled()) {
logger.debug("-D{}: {}", PROP_ACQUIRE_AND_RELEASE_ONLY, ACQUIRE_AND_RELEASE_ONLY);
}

ResourceLeakDetector.addExclusions(
AdvancedLeakAwareByteBuf.class, "touch", "recordLeakNonRefCountingOperation");
}

AdvancedLeakAwareByteBuf(ByteBuf buf, ResourceLeakTracker<ByteBuf> leak) {
super(buf, leak);
}

AdvancedLeakAwareByteBuf(ByteBuf wrapped, ByteBuf trackedByteBuf, ResourceLeakTracker<ByteBuf> leak) {
super(wrapped, trackedByteBuf, leak);
}

static void recordLeakNonRefCountingOperation(ResourceLeakTracker<ByteBuf> leak) {
if (!ACQUIRE_AND_RELEASE_ONLY) {
leak.record();
}
}

//......

@Override
public ByteBuf order(ByteOrder endianness) {
recordLeakNonRefCountingOperation(leak);
return super.order(endianness);
}

@Override
public ByteBuf slice() {
recordLeakNonRefCountingOperation(leak);
return super.slice();
}

@Override
public ByteBuf slice(int index, int length) {
recordLeakNonRefCountingOperation(leak);
return super.slice(index, length);
}

//......

@Override
public ByteBuf retain() {
leak.record();
return super.retain();
}

@Override
public ByteBuf retain(int increment) {
leak.record();
return super.retain(increment);
}

@Override
public boolean release() {
leak.record();
return super.release();
}

@Override
public boolean release(int decrement) {
leak.record();
return super.release(decrement);
}

@Override
public ByteBuf touch() {
leak.record();
return this;
}

@Override
public ByteBuf touch(Object hint) {
leak.record(hint);
return this;
}

//......
}

AdvancedLeakAwareByteBuf繼承了SimpleLeakAwareByteBuf，它對方法進行了覆蓋，這些覆蓋的方法要麼內部通過recordLeakNonRefCountingOperation呼叫leak.record，要麼直接呼叫leak.record

小結

ResourceLeakDetector使用Level列舉定義了四種不同的leak detection級別，分別是DISABLED、SIMPLE、ADVANCED、PARANOID；預設level為SIMPLE；可以使用-Dio.netty.leakDetection.level=advanced來進行設定；ResourceLeakDetector還有TARGET_RECORDS(io.netty.leakDetection.targetRecords )及SAMPLING_INTERVAL(io.netty.leakDetection.samplingInterval )兩個屬性，其中targetRecords預設為4，samplingInterval預設為128
ResourceLeakDetector提供了track方法用於建立ResourceLeakTracker；track方法內部呼叫track0方法，如果level為PARANOID則立即呼叫reportLeak，建立DefaultResourceLeak，否則利用隨機數來判斷(PlatformDependent.threadLocalRandom().nextInt(samplingInterval)) == 0 )是否呼叫reportLeak並建立DefaultResourceLeak；reportLeak方法有個for迴圈，不斷從refQueue取DefaultResourceLeak，然後呼叫reportUntracedLeak或者reportTracedLeak進行error
DefaultResourceLeak是ResourceLeakDetector定義的私有靜態類，它繼承了WeakReference類，同時實現了ResourceLeakTracker(定義了record、close方法 )介面；record方法內部呼叫的是record0方法，它會更新newHead為新的Record；close方法會移除allLeaks，allLeaks由ResourceLeakDetector建立DefaultResourceLeak時傳入，每建立一個DefaultResourceLeak，DefaultResourceLeak會把自己加入到allLeaks中
SimpleLeakAwareByteBuf繼承了WrappedByteBuf，它的構造器要求傳入ResourceLeakTracker；SimpleLeakAwareByteBuf覆蓋了WrappedByteBuf的retainedSlice、retainedDuplicate、readRetainedSlice方法，它們內部都會呼叫unwrappedDerived方法，unwrappedDerived方法在unwrappedDerived物件是AbstractPooledDerivedByteBuf型別時會呼叫AbstractByteBuf.leakDetector.track進行track；SimpleLeakAwareByteBuf也覆蓋了WrappedByteBuf的release方法，在呼叫父類release成功時會再呼叫closeLeak方法，使用leak.close(trackedByteBuf)來釋放trackedByteBuf
AdvancedLeakAwareByteBuf繼承了SimpleLeakAwareByteBuf，它對方法進行了覆蓋，這些覆蓋的方法要麼內部通過recordLeakNonRefCountingOperation呼叫leak.record，要麼直接呼叫leak.record；另外有SimpleLeakAwareCompositeByteBuf與AdvancedLeakAwareCompositeByteBuf，它們對leak detect的支援類似SimpleLeakAwareByteBuf與AdvancedLeakAwareByteBuf