/*
    * Copyright 2013 The Netty Project
    *
    * The Netty Project licenses this file to you under the Apache License,
    * version 2.0 (the "License"); you may not use this file except in compliance
    * with the License. You may obtain a copy of the License at:
    *
    *   https://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  
  package io.netty.util;
  
  import io.netty.util.internal.EmptyArrays;
  import io.netty.util.internal.ObjectUtil;
  import io.netty.util.internal.SystemPropertyUtil;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  import org.jetbrains.annotations.Nullable;
  
  import java.lang.ref.ReferenceQueue;
  import java.lang.ref.WeakReference;
  import java.lang.reflect.Method;
  import java.util.Arrays;
  import java.util.HashSet;
  import java.util.Set;
  import java.util.concurrent.ConcurrentHashMap;
  import java.util.concurrent.ThreadLocalRandom;
  import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
  import java.util.concurrent.atomic.AtomicReference;
  import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
  
  import static io.netty.util.internal.StringUtil.EMPTY_STRING;
  import static io.netty.util.internal.StringUtil.NEWLINE;
  import static io.netty.util.internal.StringUtil.simpleClassName;
  
  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 String PROP_TRACK_CLOSE = "io.netty.leakDetection.trackClose";
      private static final boolean DEFAULT_TRACK_CLOSE = true;
  
      private static final int TARGET_RECORDS;
      static final int SAMPLING_INTERVAL;
      private static final boolean TRACK_CLOSE;
  
      /**
       * 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, Level.DISABLED.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);
         TRACK_CLOSE = SystemPropertyUtil.getBoolean(PROP_TRACK_CLOSE, DEFAULT_TRACK_CLOSE);
 
         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) {
         ResourceLeakDetector.level = ObjectUtil.checkNotNull(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 = ConcurrentHashMap.newKeySet();
 
     private final ReferenceQueue<Object> refQueue = new ReferenceQueue<>();
     private final Set<String> reportedLeaks = ConcurrentHashMap.newKeySet();
 
     private final String resourceType;
     private final int samplingInterval;
 
     /**
      * Will be notified once a leak is detected.
      */
     private volatile LeakListener leakListener;
 
     /**
      * @deprecated use {@link ResourceLeakDetectorFactory#newResourceLeakDetector(Class, int, long)}.
      */
     @Deprecated
     public ResourceLeakDetector(Class<?> resourceType) {
         this(simpleClassName(resourceType));
     }
 
     /**
      * @deprecated use {@link ResourceLeakDetectorFactory#newResourceLeakDetector(Class, int, long)}.
      */
     @Deprecated
     public ResourceLeakDetector(String resourceType) {
         this(resourceType, DEFAULT_SAMPLING_INTERVAL, Long.MAX_VALUE);
     }
 
     /**
      * @deprecated Use {@link ResourceLeakDetector#ResourceLeakDetector(Class, int)}.
      * <p>
      * This should not be used directly by users of {@link ResourceLeakDetector}.
      * Please use {@link ResourceLeakDetectorFactory#newResourceLeakDetector(Class)}
      * or {@link ResourceLeakDetectorFactory#newResourceLeakDetector(Class, int, long)}
      *
      * @param maxActive This is deprecated and will be ignored.
      */
     @Deprecated
     public ResourceLeakDetector(Class<?> resourceType, int samplingInterval, long maxActive) {
         this(resourceType, samplingInterval);
     }
 
     /**
      * This should not be used directly by users of {@link ResourceLeakDetector}.
      * Please use {@link ResourceLeakDetectorFactory#newResourceLeakDetector(Class)}
      * or {@link ResourceLeakDetectorFactory#newResourceLeakDetector(Class, int, long)}
      */
     @SuppressWarnings("deprecation")
     public ResourceLeakDetector(Class<?> resourceType, int samplingInterval) {
         this(simpleClassName(resourceType), samplingInterval, Long.MAX_VALUE);
     }
 
     /**
      * @deprecated use {@link ResourceLeakDetectorFactory#newResourceLeakDetector(Class, int, long)}.
      * <p>
      * @param maxActive This is deprecated and will be ignored.
      */
     @Deprecated
     public ResourceLeakDetector(String resourceType, int samplingInterval, long maxActive) {
         this.resourceType = ObjectUtil.checkNotNull(resourceType, "resourceType");
         this.samplingInterval = samplingInterval;
     }
 
     /**
      * Creates a new {@link ResourceLeak} which is expected to be closed via {@link ResourceLeak#close()} when the
      * related resource is deallocated.
      *
      * @return the {@link ResourceLeak} or {@code null}
      * @deprecated use {@link #track(Object)}
      */
     @Deprecated
     public final ResourceLeak open(T obj) {
         return track0(obj, false);
     }
 
     /**
      * 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, false);
     }
 
     /**
      * Creates a new {@link ResourceLeakTracker} which is expected to be closed via
      * {@link ResourceLeakTracker#close(Object)} when the related resource is deallocated.
      *
      * Unlike {@link #track(Object)}, this method always returns a tracker, regardless
      * of the detection settings.
      *
      * @return the {@link ResourceLeakTracker}
      */
     @SuppressWarnings("unchecked")
     public ResourceLeakTracker<T> trackForcibly(T obj) {
         return track0(obj, true);
     }
 
     @SuppressWarnings("unchecked")
     private DefaultResourceLeak track0(T obj, boolean force) {
         Level level = ResourceLeakDetector.level;
         if (force ||
                 level == Level.PARANOID ||
                 (level != Level.DISABLED && ThreadLocalRandom.current().nextInt(samplingInterval) == 0)) {
             reportLeak();
             return new DefaultResourceLeak(obj, refQueue, allLeaks, getInitialHint(resourceType));
         }
         return null;
     }
 
     private void clearRefQueue() {
         for (;;) {
             DefaultResourceLeak ref = (DefaultResourceLeak) refQueue.poll();
             if (ref == null) {
                 break;
             }
             ref.dispose();
         }
     }
 
     /**
      * When the return value is {@code true}, {@link #reportTracedLeak} and {@link #reportUntracedLeak}
      * will be called once a leak is detected, otherwise not.
      *
      * @return {@code true} to enable leak reporting.
      */
     protected boolean needReport() {
         return logger.isErrorEnabled();
     }
 
     private void reportLeak() {
         if (!needReport()) {
             clearRefQueue();
             return;
         }
 
         // Detect and report previous leaks.
         for (;;) {
             DefaultResourceLeak ref = (DefaultResourceLeak) refQueue.poll();
             if (ref == null) {
                 break;
             }
 
             if (!ref.dispose()) {
                 continue;
             }
 
             String records = ref.getReportAndClearRecords();
             if (reportedLeaks.add(records)) {
                 if (records.isEmpty()) {
                     reportUntracedLeak(resourceType);
                 } else {
                     reportTracedLeak(resourceType, records);
                 }
 
                 LeakListener listener = leakListener;
                 if (listener != null) {
                     listener.onLeak(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 https://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 https://netty.io/wiki/reference-counted-objects.html for more information.",
                 resourceType, PROP_LEVEL, Level.ADVANCED.name().toLowerCase(), simpleClassName(this));
     }
 
     /**
      * @deprecated This method will no longer be invoked by {@link ResourceLeakDetector}.
      */
     @Deprecated
     protected void reportInstancesLeak(String resourceType) {
     }
 
     /**
      * Create a hint object to be attached to an object tracked by this record. Similar to the additional information
      * supplied to {@link ResourceLeakTracker#record(Object)}, will be printed alongside the stack trace of the
      * creation of the resource.
      */
     protected Object getInitialHint(String resourceType) {
         return null;
     }
 
     /**
      * Set leak listener. Previous listener will be replaced.
      */
     public void setLeakListener(LeakListener leakListener) {
         this.leakListener = leakListener;
     }
 
     public interface LeakListener {
 
         /**
          * Will be called once a leak is detected.
          */
         void onLeak(String resourceType, String records);
     }
 
     @SuppressWarnings("deprecation")
     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<?>, TraceRecord> headUpdater =
                 (AtomicReferenceFieldUpdater)
                         AtomicReferenceFieldUpdater.newUpdater(DefaultResourceLeak.class, TraceRecord.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 TraceRecord 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,
                 Object initialHint) {
             super(referent, refQueue);
 
             assert referent != null;
 
             this.allLeaks = allLeaks;
 
             // 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, initialHint == null ?
                     new TraceRecord(TraceRecord.BOTTOM) : new TraceRecord(TraceRecord.BOTTOM, initialHint));
         }
 
         @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) {
                 TraceRecord oldHead;
                 TraceRecord prevHead;
                 TraceRecord newHead;
                 boolean dropped;
                 do {
                     if ((prevHead = oldHead = headUpdater.get(this)) == null ||
                             oldHead.pos == TraceRecord.CLOSE_MARK_POS) {
                         // already closed.
                         return;
                     }
                     final int numElements = oldHead.pos + 1;
                     if (numElements >= TARGET_RECORDS) {
                         final int backOffFactor = Math.min(numElements - TARGET_RECORDS, 30);
                         dropped = ThreadLocalRandom.current().nextInt(1 << backOffFactor) != 0;
                         if (dropped) {
                             prevHead = oldHead.next;
                         }
                     } else {
                         dropped = false;
                     }
                     newHead = hint != null ? new TraceRecord(prevHead, hint) : new TraceRecord(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, TRACK_CLOSE ? new TraceRecord(true) : 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) {
                 synchronized (ref) {
                     // Empty synchronized is ok: https://stackoverflow.com/a/31933260/1151521
                 }
             }
         }
 
         @Override
         public @Nullable Throwable getCloseStackTraceIfAny() {
             TraceRecord head = headUpdater.get(this);
             if (head != null && head.pos == TraceRecord.CLOSE_MARK_POS) {
                 return head;
             }
             return null;
         }
 
         @Override
         public String toString() {
             TraceRecord oldHead = headUpdater.get(this);
             return generateReport(oldHead);
         }
 
         String getReportAndClearRecords() {
             TraceRecord oldHead = headUpdater.getAndSet(this, null);
             return generateReport(oldHead);
         }
 
         private String generateReport(TraceRecord oldHead) {
             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 != TraceRecord.BOTTOM; oldHead = oldHead.next) {
                 String s = oldHead.toString();
                 if (seen.add(s)) {
                     if (oldHead.next == TraceRecord.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();
         }
     }
 
     private static final AtomicReference<String[]> excludedMethods =
             new AtomicReference<String[]>(EmptyArrays.EMPTY_STRINGS);
 
     public static void addExclusions(Class clz, String ... methodNames) {
         Set<String> nameSet = new HashSet<String>(Arrays.asList(methodNames));
         // Use loop rather than lookup. This avoids knowing the parameters, and doesn't have to handle
         // NoSuchMethodException.
         for (Method method : clz.getDeclaredMethods()) {
             if (nameSet.remove(method.getName()) && nameSet.isEmpty()) {
                 break;
             }
         }
         if (!nameSet.isEmpty()) {
             throw new IllegalArgumentException("Can't find '" + nameSet + "' in " + clz.getName());
         }
         String[] oldMethods;
         String[] newMethods;
         do {
             oldMethods = excludedMethods.get();
             newMethods = Arrays.copyOf(oldMethods, oldMethods.length + 2 * methodNames.length);
             for (int i = 0; i < methodNames.length; i++) {
                 newMethods[oldMethods.length + i * 2] = clz.getName();
                 newMethods[oldMethods.length + i * 2 + 1] = methodNames[i];
             }
         } while (!excludedMethods.compareAndSet(oldMethods, newMethods));
     }
 
     private static class TraceRecord extends Throwable {
         private static final long serialVersionUID = 6065153674892850720L;
         public static final int BOTTOM_POS = -1;
         public static final int CLOSE_MARK_POS = -2;
 
         private static final TraceRecord BOTTOM = new TraceRecord(false) {
             private static final long serialVersionUID = 7396077602074694571L;
 
             // Override fillInStackTrace() so we not populate the backtrace via a native call and so leak the
             // Classloader.
             // See https://github.com/netty/netty/pull/10691
             @Override
             public Throwable fillInStackTrace() {
                 return this;
             }
         };
 
         private final String hintString;
         private final TraceRecord next;
         private final int pos;
 
         TraceRecord(TraceRecord next, Object hint) {
             // This needs to be generated even if toString() is never called as it may change later on.
             hintString = hint instanceof ResourceLeakHint ? ((ResourceLeakHint) hint).toHintString() : hint.toString();
             this.next = next;
             this.pos = next.pos + 1;
         }
 
         TraceRecord(TraceRecord next) {
            hintString = null;
            this.next = next;
            this.pos = next.pos + 1;
         }
 
         // Used to terminate the stack
         private TraceRecord(boolean closeMarker) {
             hintString = null;
             next = null;
             pos = closeMarker ? CLOSE_MARK_POS : BOTTOM_POS;
         }
 
         @Override
         public String toString() {
             StringBuilder buf = new StringBuilder(2048);
             if (hintString != null) {
                 buf.append("\tHint: ").append(hintString).append(NEWLINE);
             }
 
             // Append the stack trace.
             StackTraceElement[] array = getStackTrace();
             // Skip the first three elements.
             out: for (int i = 3; i < array.length; i++) {
                 StackTraceElement element = array[i];
                 // Strip the noisy stack trace elements.
                 String[] exclusions = excludedMethods.get();
                 for (int k = 0; k < exclusions.length; k += 2) {
                     // Suppress a warning about out of bounds access
                     // since the length of excludedMethods is always even, see addExclusions()
                     if (exclusions[k].equals(element.getClassName())
                             && exclusions[k + 1].equals(element.getMethodName())) {
                         continue out;
                     }
                 }
 
                 buf.append('\t');
                 buf.append(element.toString());
                 buf.append(NEWLINE);
             }
             return buf.toString();
         }
     }
 }