Why Java’s ConcurrentHashMap Can Freeze Your CPU and How to Fix It
This article explains the hidden multithreading bugs in JDK 7 HashMap and JDK 8 ConcurrentHashMap that can cause 100% CPU usage, demonstrates how to reproduce them, and offers practical ways to avoid or resolve the issue.
In JDK 7 the HashMap implementation can enter an infinite loop during resize when accessed concurrently, leading to 100% CPU usage and even data loss.
Although HashMap is not thread‑safe, developers often unintentionally place it in multithreaded contexts; the usual remedy is to switch to ConcurrentHashMap, but JDK 8 introduced its own bug that also caused CPU spikes, which was fixed in JDK 9 (see bug ID JDK‑8062841).
Running the following sample program reproduces the JDK 8 ConcurrentHashMap bug: the process stays in a running state while one thread consumes nearly all CPU resources.
Using top -Hp [pid] reveals a thread with close to 100% CPU; jstack -l [pid] shows the offending thread (nid=0x7ab9, decimal 31417) stuck inside computeIfAbsent.
The core problem is the recursive use of computeIfAbsent, which can also be observed in a Fibonacci example that similarly drives CPU to 100%.
To avoid this issue, do not call computeIfAbsent recursively; alternatively, downgrade to a segmented‑lock implementation or upgrade to JDK 9 where the bug is resolved.
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