假设您的 SOR 或外部服务正在变慢，那么大量线程将开始等待其响应。在这种情况下，这些线程将显示相同的堆栈跟踪。
假设一个线程获得了一个锁并且它从未释放，那么在同一执行路径中的其他几个线程将进入阻塞状态，显示相同的堆栈跟踪。
如果循环（for 循环、while 循环、do..while 循环）条件未终止，则执行该循环的多个线程将显示相同的堆栈跟踪。
当出现上述任何一种情况时，应用程序的性能和可用性都会受到质疑。

重点关注：Troubleshooting Tools、Monitoring Tools
https://docs.oracle.com/en/java/javase/11/tools/tools-and-command-reference.html

https://programmer.ink/think/jvm-series-jvm-tuning-jps-jstat-jmap-jhat-jstack-jinfo.html

https://www.hollischuang.com/archives/110

线程状态产生的原因

runnable:状态一般为RUNNABLE。
in Object.wait():等待区等待,状态为WAITING或TIMED_WAITING。
waiting for monitor entry:进入区等待,状态为BLOCKED。
waiting on condition:等待区等待、被park。
sleeping:休眠的线程,调用了Thread.sleep()。

https://jakubstransky.com/2017/12/19/hotspot-jvm-internal-threads/

Tomcat线程模型

NIO: synchronous and non blocking. It can support large concurrency better than traditional bio. This mode is adopted by default after Tomcat 8.0.

NiO

https://developpaper.com/nio-optimization-principle-and-tomcat-thread-model/

Tomcat’s NiO model has more poller rolesAcceptor、PollerandHandler worker thread pool。 Are these three roles very familiar? If poller is replaced by reactor, is it the reactor model. Yes, Tomcat’s NiO model is based onMaster slave reactor modelIt’s just a change of name.

AcceptorThe: accepter thread is specifically responsible for establishing network connections（accept）。 After the new connection is created, instead of directly using the worker thread to process the request, the request is first sent to the poller buffer queue.
Poller: in poller, a selector object is maintained. After poller takes the connection from the buffer queue, it registers in the selector and blocks to wait for read and write ready（Read wait ready, send wait ready）。
Handlers: traverse the selector to find out the ready IO operations in the selector and hand them over to the worker thread for processing（Read memory read, decode, process, encode, send memory write）。

https://www.cnblogs.com/grey-wolf/p/13740845.html

调优
https://tomcat.apache.org/tomcat-9.0-doc/config/http.html
https://www.datadoghq.com/blog/tomcat-architecture-and-performance/