Analyzing a problem in WebSphere Application Server often requires looking at the javacore file. Once you are identifying an interesting thread there it often requires to look at the SystemOut/SystemErr log for entries originated from that thread. But how to find those? Well in WAS v8.5.5 it is quite easy as you can see in the the below example.
Looking at the javacore file locate the 3XMJAVALTHREAD eye-catcher line for the thread and take the value shown next to the getId: which is 0x7CB in our example.
3XMTHREADINFO "WebContainer : 16" J9VMThread:0x000000000A967100, j9thread_t:0x0000000027D4A6F0, java/lang/Thread:0x00000006E81BB378, state:R, prio=5
3XMJAVALTHREAD (java/lang/Thread getId:0x7CB, isDaemon:true)
3XMTHREADINFO1 (native thread ID:0x600, native priority:0x5, native policy:UNKNOWN, vmstate:R, vm thread flags:0x01000001)
3XMCPUTIME CPU usage total: 4274.125000000 secs, user: 3905.828125000 secs, system: 368.296875000 secs, current category="Application"
Now switch to the SystemOut/SystemErr log which by default lists the id of the thread after the timestamp. This thread id matches the id we identified before in the javacore ignoring leading zeros and the case for the hex characters:
[2017-05-10 06:32:22:735 CEST] 000007cb SRTServletRes W com.ibm.ws.webcontainer.srt.SRTServletResponse setIntHeader CWSRV8094W: WARNING: Cannot set header. Response already committed.