There's one thing worthy noting here is observation expression. The observation expression supports OGNL grammar, for example, you can come up a expression like this `"{params,returnObj}"`. All OGNL expressions are supported as long as they are legal to the grammar.
Thanks for `advice`'s data structure, it is possible to observe from varieties of different angles. Inside `advice` parameter, all necessary information for notification can be found.
Pls. refer to [core parameters in expression](advice-class.md) for more details.
* Pls. also refer to [https://github.com/alibaba/arthas/issues/71](https://github.com/alibaba/arthas/issues/71) for more advanced usage
* OGNL official site: [https://commons.apache.org/proper/commons-ognl/language-guide.html](https://commons.apache.org/proper/commons-ognl/language-guide.html)
Many times what we are interested is the exact trace result when the method call takes time over one particular period. It is possible to achieve this in Arthas, for example: `trace *StringUtils isBlank '#cost>100'` means trace result will only be output when the executing time exceeds 100ms.
`trace` is handy to help discovering and locating the performance flaws in your system, but pls. note Arthas can only trace the first level method call each time.
> Only the call path which's time cost is higher than `10ms` will be shown. This feature is handy to focus on what's needed to focus when troubleshoot.
* Here Arthas provides the similar functionality JProfile and other commercial software provide. Compared to these professional softwares, Arthas doesn't deduce the time cost `trace` itself takes, therefore it is not as accurate as these softwares offer. More classes and methods on the calling path, more inaccurate `trace` output is, but it is still helpful for diagnostics where the bottleneck is.
* "[12.033735ms]" means the method on the node takes `12.033735` ms.
* "[min=0.005428ms,max=0.094064ms,total=0.105228ms,count=3] demo:call()" means aggregating all same method calls into one single line. The minimum time cost is `0.005428` ms, the maximum time cost is `0.094064` ms, and the total time cost for all method calls (`3` times in total) to "demo:call()" is `0.105228ms`. If "throws Exception" appears in this line, it means some exceptions have been thrown from this method calls.
The trace command will only trace the subcalls in the method to the trace, and will not trace down multiple layers. Because traces are expensive, multi-layer traces can lead to a lot of classes and methods that ultimately have to be traced.
You can use the regular expression to match multiple classes and methods on the path to achieve a multi-layer trace effect to some extent.
