聊聊netty的ResourceLeakDetector

Posted 2021-04-30 码匠的流水账

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序

本文主要研究一下netty的ResourceLeakDetector

LEAK异常


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枚举定义了四种不同的leak detection级别，分别是DISABLED、SIMPLE、ADVANCED、PARANOID；默认level为SIMPLE；可以使用-Dio.netty.leakDetection.level=advanced来进行设置
ResourceLeakDetector的静态代码块会读取io.netty.noResourceLeakDetection系统属性，如果显示设置为false，则变更defaultLevel为DISABLED；如果没有设置，则默认disabled为false，defaultLevel为SIMPLE；ResourceLeakDetector还有TARGET_RECORDS(io.netty.leakDetection.targetRecords)及SAMPLING_INTERVAL(io.netty.leakDetection.samplingInterval)两个属性，其中targetRecords默认为4，samplingInterval默认为128
ResourceLeakDetector提供了track方法用于创建ResourceLeakTracker；track方法内部调用track0方法，如果level为PARANOID则立即调用reportLeak，创建DefaultResourceLeak，否则利用随机数来判断(PlatformDependent.threadLocalRandom().nextInt(samplingInterval)) == 0)是否调用reportLeak并创建DefaultResourceLeak；reportLeak方法有个for循环，不断从refQueue取DefaultResourceLeak，然后调用reportUntracedLeak或者reportTracedLeak进行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定义的私有静态类，它继承了WeakReference类，同时实现了ResourceLeakTracker(定义了record、close方法)接口；record方法内部调用的是record0方法，它会更新newHead为新的Record；close方法会移除allLeaks，allLeaks由ResourceLeakDetector创建DefaultResourceLeak时传入，每创建一个DefaultResourceLeak，DefaultResourceLeak会把自己加入到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继承了WrappedByteBuf，它的构造器要求传入ResourceLeakTracker
SimpleLeakAwareByteBuf覆盖了WrappedByteBuf的retainedSlice、retainedDuplicate、readRetainedSlice方法，它们内部都会调用unwrappedDerived方法，unwrappedDerived方法在unwrappedDerived对象是AbstractPooledDerivedByteBuf类型时会调用AbstractByteBuf.leakDetector.track进行track
SimpleLeakAwareByteBuf也覆盖了WrappedByteBuf的release方法，在调用父类release成功时会再调用closeLeak方法，使用leak.close(trackedByteBuf)来释放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继承了SimpleLeakAwareByteBuf，它对方法进行了覆盖，这些覆盖的方法要么内部通过recordLeakNonRefCountingOperation调用leak.record，要么直接调用leak.record

小结

ResourceLeakDetector使用Level枚举定义了四种不同的leak detection级别，分别是DISABLED、SIMPLE、ADVANCED、PARANOID；默认level为SIMPLE；可以使用-Dio.netty.leakDetection.level=advanced来进行设置；ResourceLeakDetector还有TARGET_RECORDS(io.netty.leakDetection.targetRecords)及SAMPLING_INTERVAL(io.netty.leakDetection.samplingInterval)两个属性，其中targetRecords默认为4，samplingInterval默认为128
ResourceLeakDetector提供了track方法用于创建ResourceLeakTracker；track方法内部调用track0方法，如果level为PARANOID则立即调用reportLeak，创建DefaultResourceLeak，否则利用随机数来判断(PlatformDependent.threadLocalRandom().nextInt(samplingInterval)) == 0)是否调用reportLeak并创建DefaultResourceLeak；reportLeak方法有个for循环，不断从refQueue取DefaultResourceLeak，然后调用reportUntracedLeak或者reportTracedLeak进行error
DefaultResourceLeak是ResourceLeakDetector定义的私有静态类，它继承了WeakReference类，同时实现了ResourceLeakTracker(定义了record、close方法)接口；record方法内部调用的是record0方法，它会更新newHead为新的Record；close方法会移除allLeaks，allLeaks由ResourceLeakDetector创建DefaultResourceLeak时传入，每创建一个DefaultResourceLeak，DefaultResourceLeak会把自己加入到allLeaks中
SimpleLeakAwareByteBuf继承了WrappedByteBuf，它的构造器要求传入ResourceLeakTracker；SimpleLeakAwareByteBuf覆盖了WrappedByteBuf的retainedSlice、retainedDuplicate、readRetainedSlice方法，它们内部都会调用unwrappedDerived方法，unwrappedDerived方法在unwrappedDerived对象是AbstractPooledDerivedByteBuf类型时会调用AbstractByteBuf.leakDetector.track进行track；SimpleLeakAwareByteBuf也覆盖了WrappedByteBuf的release方法，在调用父类release成功时会再调用closeLeak方法，使用leak.close(trackedByteBuf)来释放trackedByteBuf
AdvancedLeakAwareByteBuf继承了SimpleLeakAwareByteBuf，它对方法进行了覆盖，这些覆盖的方法要么内部通过recordLeakNonRefCountingOperation调用leak.record，要么直接调用leak.record；另外有SimpleLeakAwareCompositeByteBuf与AdvancedLeakAwareCompositeByteBuf，它们对leak detect的支持类似SimpleLeakAwareByteBuf与AdvancedLeakAwareByteBuf

doc

Netty 的资源泄露探测机制
A Netty ByteBuf Memory Leak Story and the Lessons Learned
In 4.0.23.Final, Seeing io.netty.util.ResourceLeakDetector - LEAK: ByteBuf.release() was not called before it's garbage-collected #2774

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