SQL Developer 18.2.0.183 takes up CPU resources when it is idle

you have 30-40 files open

you might not be doing anything, but our parser probably is

that's just a guess, what's it look like with 5 files open? 10? 0?

Another example of SQL Developer using lots of CPU when one might not expect:

SQL Developer is Hanging when comparing package body  

but probably different than your scenario since at least one database connection must be open.

I can confirm this situation happens from time to time.

OSX Sierra, SQL Developer 18.2.0.183

All tabs are closed, all connections are disconnected (using "Disconnect connections" option).

CPU is still used.

I noticed that on macos Mojave the CPU usage drops to near to 0% when the app is minimized, so I just keep it like that while not using it.

This also happened in previous versions, both of macos and SQL Developer

Yep. If I open SQL Developer and minimize it, it pegs a core (in my case, with 8 cores, 12-13% cpu) and will continue until I close it. It's been that way for as long as I can remember. Just a plain vanilla install. No extensions, nothing. This is my 3rd machine and 4th OS install with SQL Developer over 6 years and it's always been this way for me. Kind of ridiculous...

On Microsoft Windows [Version 10.0.16299.726] using Oracle JDK 1.8.0_171 with SQL Developer 18.3.0.277 Build 277.2354 (same happens with 18.2.0.183 build 183.1748).  

I'm experiencing a similar issue where one of my cores get pegged at 100% and it seems to be correlated to our firewall killing my connection.  Reconnecting always reduces the CPU usage down to normal levels.  I wish there was a keep alive or auto-reconnect function, the useless battery drain and jet engine noises as the CPU fan tries to keep things cool is driving me crazy.

I haven't spent a ton of time investigating but these 2 threads seem to involved. When the problem occurs both these threads are running at 100%.  When I reconnect "RMI TCP Connection(10)" is terminated, AWT-EventQueue-0 goes to a thread state of WAITING(parking).  A new "RMI TCP Connection(11)" threads start to run and the CPU usage is normal.

"RMI TCP Connection(10)-172.31.16.9" #182 daemon prio=5 os_prio=0 tid=0x000000001f5c8800 nid=0x2f78 runnable [0x000000003c6ce000]

   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 java.io.BufferedInputStream.fill(BufferedInputStream.java:246)

        at java.io.BufferedInputStream.read(BufferedInputStream.java:265)

        ...

"AWT-EventQueue-0" #16 prio=6 os_prio=0 tid=0x000000001b11e000 nid=0x47bc runnable [0x000000001bfed000]

   java.lang.Thread.State: RUNNABLE

        at sun.awt.windows.WComponentPeer.getLocationOnScreen(Native Method)

        at java.awt.Component.getLocationOnScreen_NoTreeLock(Component.java:2051)

        at java.awt.Component.getLocationOnScreen(Component.java:2036)

        - locked <0x00000000ce2116d8> (a java.awt.Component$AWTTreeLock)

        ...

I wish there was a keep alive or auto-reconnect function

If it won't get you in trouble with your system admins, you can always try this extension:

SQL Developer 4 keepalive

But kind in mind the warnings that have been posted on this forum over the years, like in SSH & DB Connections - Keeping them alive