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Is the "ConcurrentHashMap" possible to hang?

First, I will summarize what I have found so far.

  • This answer suggests that changing the concurrencyLevel parameter of the ConcurrentHashMap constructor may help. I tried this and my code was still hanging.
  • The answers here suggest that this might be a runtime error.

What am I trying to do:

  • I have 10 worker threads working together with the main thread. Worker threads will have to process many arrays to find the index of the maximum element in the array (if there are several maximum values, the first occurrence will be used). Among these "many arrays" some of them may be duplicated, so I try to avoid these full scans of arrays in order to speed up the program.
  • The controller class contains ConcurrentHashMap , which maps the hash values ​​of the arrays to the corresponding indexes of the maximum element.
  • Workflows will first ask the controller class for the displayed index before attempting to calculate the index by performing a full array scan. In the latter case, the new settlement index will be placed on the card.
  • The main thread does not access the hash map.

What happened:

  • My code will hang after 70,000 ~ 130,000 calls to getMaxIndex() . This score is obtained by putting the log line in getMaxIndex() so that it is not exactly accurate.
  • Using my processor will gradually increase by ~ 6 seconds, and then it will drop to ~ 10% after reaching a maximum of ~ 100%. I have a lot of unused memory left. (Does this sound like a dead end?)
  • If the code does not use the card, it works just fine (see getMaxIndex() version 2 below).
  • I tried adding a synchronized signature to getMaxIndex() and using a regular HashMap instead, which also didn't work.
  • I also tried using different initialCapacity values ​​(e.g. 50,000 and 100,000). Does not work.

Here is my code:

 // in the controller class int getMaxIndex(@NotNull double[] arr) { int hash = Arrays.hashCode(arr); if(maxIndices.containsKey(hash)) { return maxIndices.get(hash); } else { int maxIndex = IntStream.range(0, arr.length) .reduce((a, b) -> arr[a] < arr[b] ? b : a) .orElse(-1); // -1 to let program crash maxIndices.put(hash, maxIndex); return maxIndex; } }

The worker thread will call getMaxIndex() as follows: return remaining[controller.getMaxIndex(arr)]; , remaining is just another int array.

getMaxIndex() v2:

 int getMaxIndex(@NotNull double[] arr) { return IntStream.range(0, arr.length) .reduce((a, b) -> arr[a] < arr[b] ? b : a) .orElse(-1); // -1 to let program crash }

JVM information in case it matters:

 java version "1.8.0_151" Java(TM) SE Runtime Environment (build 1.8.0_151-b12) Java HotSpot(TM) 64-Bit Server VM (build 25.151-b12, mixed mode)

EDIT : landfill; I used Phaser to synchronize worker threads, so some of them seem to be waiting in phase, but pool-1-thread-2 , pool-1-thread-10 , pool-1-thread-11 and pool-1-thread-12 does not seem to expect a phaser.

 Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.151-b12 mixed mode): "Attach Listener" #23 daemon prio=9 os_prio=0 tid=0x00007f0c54001000 nid=0x4da2 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "pool-1-thread-13" #22 prio=5 os_prio=0 tid=0x00007f0c8c2cb800 nid=0x4d5e waiting on condition [0x00007f0c4eddd000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x000000076e792f40> (a java.util.concurrent.Phaser$QNode) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.Phaser$QNode.block(Phaser.java:1140) at java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3323) at java.util.concurrent.Phaser.internalAwaitAdvance(Phaser.java:1067) at java.util.concurrent.Phaser.arriveAndAwaitAdvance(Phaser.java:690) at Ant.call(Ant.java:77) at Ant.call(Ant.java:10) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-12" #21 prio=5 os_prio=0 tid=0x00007f0c8c2ca000 nid=0x4d5d waiting on condition [0x00007f0c4eede000] java.lang.Thread.State: TIMED_WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x0000000775518738> (a java.util.concurrent.SynchronousQueue$TransferStack) at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215) at java.util.concurrent.SynchronousQueue$TransferStack.awaitFulfill(SynchronousQueue.java:460) at java.util.concurrent.SynchronousQueue$TransferStack.transfer(SynchronousQueue.java:362) at java.util.concurrent.SynchronousQueue.poll(SynchronousQueue.java:941) at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1073) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-11" #20 prio=5 os_prio=0 tid=0x00007f0c8c2c8000 nid=0x4d5c waiting on condition [0x00007f0c4efdf000] java.lang.Thread.State: TIMED_WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x0000000775518738> (a java.util.concurrent.SynchronousQueue$TransferStack) at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215) at java.util.concurrent.SynchronousQueue$TransferStack.awaitFulfill(SynchronousQueue.java:460) at java.util.concurrent.SynchronousQueue$TransferStack.transfer(SynchronousQueue.java:362) at java.util.concurrent.SynchronousQueue.poll(SynchronousQueue.java:941) at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1073) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-10" #19 prio=5 os_prio=0 tid=0x00007f0c8c2c6000 nid=0x4d5b waiting on condition [0x00007f0c4f0e0000] java.lang.Thread.State: TIMED_WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x0000000775518738> (a java.util.concurrent.SynchronousQueue$TransferStack) at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215) at java.util.concurrent.SynchronousQueue$TransferStack.awaitFulfill(SynchronousQueue.java:460) at java.util.concurrent.SynchronousQueue$TransferStack.transfer(SynchronousQueue.java:362) at java.util.concurrent.SynchronousQueue.poll(SynchronousQueue.java:941) at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1073) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-9" #18 prio=5 os_prio=0 tid=0x00007f0c8c2c4800 nid=0x4d5a waiting on condition [0x00007f0c4f1e1000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x000000076e7c74f8> (a java.util.concurrent.Phaser$QNode) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.Phaser$QNode.block(Phaser.java:1140) at java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3323) at java.util.concurrent.Phaser.internalAwaitAdvance(Phaser.java:1067) at java.util.concurrent.Phaser.arriveAndAwaitAdvance(Phaser.java:690) at Ant.call(Ant.java:77) at Ant.call(Ant.java:10) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-8" #17 prio=5 os_prio=0 tid=0x00007f0c8c2c2800 nid=0x4d59 waiting on condition [0x00007f0c4f2e2000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x000000076e64fb78> (a java.util.concurrent.Phaser$QNode) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.Phaser$QNode.block(Phaser.java:1140) at java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3323) at java.util.concurrent.Phaser.internalAwaitAdvance(Phaser.java:1067) at java.util.concurrent.Phaser.arriveAndAwaitAdvance(Phaser.java:690) at Ant.call(Ant.java:77) at Ant.call(Ant.java:10) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-7" #16 prio=5 os_prio=0 tid=0x00007f0c8c2c1000 nid=0x4d58 waiting on condition [0x00007f0c4f3e3000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x000000076e8b44c8> (a java.util.concurrent.Phaser$QNode) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.Phaser$QNode.block(Phaser.java:1140) at java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3323) at java.util.concurrent.Phaser.internalAwaitAdvance(Phaser.java:1067) at java.util.concurrent.Phaser.arriveAndAwaitAdvance(Phaser.java:690) at Ant.call(Ant.java:77) at Ant.call(Ant.java:10) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-6" #15 prio=5 os_prio=0 tid=0x00007f0c8c2bf800 nid=0x4d57 waiting on condition [0x00007f0c4f4e4000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x000000076e5b4500> (a java.util.concurrent.Phaser$QNode) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.Phaser$QNode.block(Phaser.java:1140) at java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3323) at java.util.concurrent.Phaser.internalAwaitAdvance(Phaser.java:1067) at java.util.concurrent.Phaser.arriveAndAwaitAdvance(Phaser.java:690) at Ant.call(Ant.java:77) at Ant.call(Ant.java:10) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-5" #14 prio=5 os_prio=0 tid=0x00007f0c8c2bd800 nid=0x4d56 waiting on condition [0x00007f0c4f5e5000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x000000076e836958> (a java.util.concurrent.Phaser$QNode) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.Phaser$QNode.block(Phaser.java:1140) at java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3323) at java.util.concurrent.Phaser.internalAwaitAdvance(Phaser.java:1067) at java.util.concurrent.Phaser.arriveAndAwaitAdvance(Phaser.java:690) at Ant.call(Ant.java:77) at Ant.call(Ant.java:10) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-4" #13 prio=5 os_prio=0 tid=0x00007f0c8c2bc000 nid=0x4d55 waiting on condition [0x00007f0c4f6e6000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x000000076e4f4cf0> (a java.util.concurrent.Phaser$QNode) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.Phaser$QNode.block(Phaser.java:1140) at java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3323) at java.util.concurrent.Phaser.internalAwaitAdvance(Phaser.java:1067) at java.util.concurrent.Phaser.arriveAndAwaitAdvance(Phaser.java:690) at Ant.call(Ant.java:77) at Ant.call(Ant.java:10) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-3" #12 prio=5 os_prio=0 tid=0x00007f0c8c2ba000 nid=0x4d54 waiting on condition [0x00007f0c4f7e7000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x000000076e40abb8> (a java.util.concurrent.Phaser$QNode) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.Phaser$QNode.block(Phaser.java:1140) at java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3323) at java.util.concurrent.Phaser.internalAwaitAdvance(Phaser.java:1067) at java.util.concurrent.Phaser.arriveAndAwaitAdvance(Phaser.java:690) at Ant.call(Ant.java:77) at Ant.call(Ant.java:10) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-2" #11 prio=5 os_prio=0 tid=0x00007f0c8c2b8800 nid=0x4d53 waiting on condition [0x00007f0c4f8e8000] java.lang.Thread.State: TIMED_WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x0000000775518738> (a java.util.concurrent.SynchronousQueue$TransferStack) at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215) at java.util.concurrent.SynchronousQueue$TransferStack.awaitFulfill(SynchronousQueue.java:460) at java.util.concurrent.SynchronousQueue$TransferStack.transfer(SynchronousQueue.java:362) at java.util.concurrent.SynchronousQueue.poll(SynchronousQueue.java:941) at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1073) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-1" #10 prio=5 os_prio=0 tid=0x00007f0c8c2b5800 nid=0x4d52 waiting on condition [0x00007f0c4f9e9000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x000000076e486ab0> (a java.util.concurrent.Phaser$QNode) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.Phaser$QNode.block(Phaser.java:1140) at java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3323) at java.util.concurrent.Phaser.internalAwaitAdvance(Phaser.java:1067) at java.util.concurrent.Phaser.arriveAndAwaitAdvance(Phaser.java:690) at Ant.call(Ant.java:77) at Ant.call(Ant.java:10) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "Service Thread" #9 daemon prio=9 os_prio=0 tid=0x00007f0c8c200800 nid=0x4d50 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "C1 CompilerThread2" #8 daemon prio=9 os_prio=0 tid=0x00007f0c8c1fd800 nid=0x4d4f waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "C2 CompilerThread1" #7 daemon prio=9 os_prio=0 tid=0x00007f0c8c1f8800 nid=0x4d4e waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "C2 CompilerThread0" #6 daemon prio=9 os_prio=0 tid=0x00007f0c8c1f7800 nid=0x4d4d waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Monitor Ctrl-Break" #5 daemon prio=5 os_prio=0 tid=0x00007f0c8c1fb000 nid=0x4d4c runnable [0x00007f0c781b4000] java.lang.Thread.State: RUNNABLE at java.net.SocketInputStream.socketRead0(Native Method) at java.net.SocketInputStream.socketRead(SocketInputStream.java:116) at java.net.SocketInputStream.read(SocketInputStream.java:171) at java.net.SocketInputStream.read(SocketInputStream.java:141) at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284) at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326) at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178) - locked <0x000000077550ecb0> (a java.io.InputStreamReader) at java.io.InputStreamReader.read(InputStreamReader.java:184) at java.io.BufferedReader.fill(BufferedReader.java:161) at java.io.BufferedReader.readLine(BufferedReader.java:324) - locked <0x000000077550ecb0> (a java.io.InputStreamReader) at java.io.BufferedReader.readLine(BufferedReader.java:389) at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:64) "Signal Dispatcher" #4 daemon prio=9 os_prio=0 tid=0x00007f0c8c181000 nid=0x4d49 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Finalizer" #3 daemon prio=8 os_prio=0 tid=0x00007f0c8c14d800 nid=0x4d42 in Object.wait() [0x00007f0c78564000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x0000000775500d08> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143) - locked <0x0000000775500d08> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164) at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209) "Reference Handler" #2 daemon prio=10 os_prio=0 tid=0x00007f0c8c149000 nid=0x4d41 in Object.wait() [0x00007f0c78665000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x0000000775500d48> (a java.lang.ref.Reference$Lock) at java.lang.Object.wait(Object.java:502) at java.lang.ref.Reference.tryHandlePending(Reference.java:191) - locked <0x0000000775500d48> (a java.lang.ref.Reference$Lock) at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153) "main" #1 prio=5 os_prio=0 tid=0x00007f0c8c00c800 nid=0x4d35 waiting on condition [0x00007f0c91f77000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x000000076dd5e268> (a java.util.concurrent.FutureTask) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.FutureTask.awaitDone(FutureTask.java:429) at java.util.concurrent.FutureTask.get(FutureTask.java:191) at java.util.concurrent.AbstractExecutorService.invokeAll(AbstractExecutorService.java:244) at ConcurrentACS.loop(ConcurrentACS.java:138) at ConcurrentACS.compute(ConcurrentACS.java:165) at ConcurrentACS.main(ConcurrentACS.java:192) "VM Thread" os_prio=0 tid=0x00007f0c8c141800 nid=0x4d3f runnable "GC task thread#0 (ParallelGC)" os_prio=0 tid=0x00007f0c8c022000 nid=0x4d37 runnable "GC task thread#1 (ParallelGC)" os_prio=0 tid=0x00007f0c8c024000 nid=0x4d38 runnable "GC task thread#2 (ParallelGC)" os_prio=0 tid=0x00007f0c8c025800 nid=0x4d39 runnable "GC task thread#3 (ParallelGC)" os_prio=0 tid=0x00007f0c8c027800 nid=0x4d3a runnable "VM Periodic Task Thread" os_prio=0 tid=0x00007f0c8c205800 nid=0x4d51 waiting on condition JNI global references: 272
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2 answers

is it possible for ConcurrentHashMap hang?

The short answer is no, if by “freezing” you mean a program cycle or a dead end. If you mean that you found a race condition (error) in this code that could cause it to crash during normal JVM and system execution, I seriously doubt it.

I suspect that something else is happening, and just because you are using CHM in a dangling version should not imply that the class has an error. I would like to use stack dumps or a profiler to show that the code is locked on the CHM line before I use any fault.

Is it possible to call CHM several times per second so that the performance of your program will suffer because of this? Sure. But he would not be dependent on being stuck or stuck.

My processor usage will gradually increase by ~ 6 seconds, and then it will reach ~ 10% after reaching a maximum of ~ 100%. I have a lot of unused memory left. (Does this sound like a dead end?)

Now the published stack trace shows that no thread is blocked in the CHM code, so it does not look like a problem. The performance curve you are talking about seems to be due to the fact that you are using fork / join thread-pool, it starts X threads initially, but then some of them finish their tasks and shut down. This can be expected. This has nothing to do with CHM.

 if(maxIndices.containsKey(hash)) { return maxIndices.get(hash);

Just a quick comment. This code makes 2 CHM calls instead of something like:

 Integer maxIndex = maxIndices.get(hash); if (maxIndex != null) { return maxIndex; } ...

But it is simply inefficient and will not cause errors. In addition, it is important to recognize that race conditions in your code means that multiple threads can get null for the index and calculate the index value. But also this is not a mistake that will lead to a "freeze".

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The first version is not thread safe because your validation sequence is not atomic. Try using this implementation:

  private final Map<Integer, Integer> maxIndices = new ConcurrentHashMap<>(); int getMaxIndex(final double[] arr) { // make sure the content of the arr can't be modified concurrently // otherwise create a copy of the array in this method int hash = Arrays.hashCode(arr); return maxIndices.computeIfAbsent(hash, key -> IntStream.range(0, arr.length).reduce((a, b) -> arr[a] < arr[b] ? b : a).orElse(-1)); }
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Source: https://habr.com/ru/post/1273455/

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