How to Diagnose High CPU Usage in a Java Program (Three‑Step Guide)

When a Java program consumes excessive CPU (even reaching 100%), you can troubleshoot it in three steps: first, use the top command to find the process with the highest CPU usage; second, locate the specific thread inside that process; third, print the thread's stack trace for analysis.

Preparation

Create a Java program that performs intensive calculations:

package com.sample.core.cpu;

public class CpuFullExample {
    private static final int times = 100000000;
    public static void main(String[] args) {
        for (int i = 0; i < times; i++) {
            Math.pow(2, i);
        }
    }
}

Compile and run it on a Linux system:

java com.sample.core.cpu.CpuFullExample

Step 1: Find the Process

Run top and observe the process ID (PID) that consumes the most CPU, e.g., PID 2411 with 99% usage.

Step 2: Find the Thread

Use top -Hp 2411 to list threads of the process and identify the thread ID (TID) with the highest CPU, e.g., TID 2412.

top -Hp 2411

Convert the decimal thread ID to hexadecimal:

printf '%x
' 2412

The result is 96c .

Step 3: Print the Thread Stack

Use jstack to dump the stack of the process and filter for the thread using its hexadecimal ID:

jstack 2411 | grep -A 10 96c

The output shows the stack trace, indicating the problem occurs at line 6 of CpuFullExample , where the intensive calculation is performed.

By following these steps, you can clearly identify the exact location of the performance bottleneck in the Java code.

Credits: https://www.jianshu.com/p/a455ac8a3ef0