Linux – Why CPU Usage Reported by Top Command Exceeds 100%

You should add a snapshot of tops output to your question so we can see exactly what you see.

The most common discrepancy in processor use indicators is some counting "I/O wait" time as well as "actually CPU busy" time. I/O wait time is when a process would be actively doing something, so could potentially have used all the time-slice the scheduler has given it, but has yielded as it is blocked waiting for I/O operations to complete.

Short running processes have an effect too: if there are many processes starting, doing something small, then closing, they may not be seen at all by top or similar (especially if you don't have top updating often) because they did their thing and stopped between updates. Even though they were not present when top took its samples (so don't appear in the tasks list) they overall counts in the /proc filesystem will still show since the last sample the CPU was busy for X scheduler ticks and idle for Y.

The scale used by top is 100% when a core is fully used. Or when one core is 20% and a second one is 80%. This lead to strange results on multicore computers because it easely can exceed 100%.
If you have 8 cores, then top can display from 0% (idle system) to 800% (full power).

Your program is just using your 4 cores with hyperthreading (so 8 virtual cores) at maximum capacity. So top gives you nearly 8 x 100% = 800%.

You can't explicitly tell the JVM to use a single thread, it will use multiple threads to do its work, even if you only use a single thread in your application.

You can use the htop utility as a better top alternative. You'll see your program using all of the 8 cores.