Currently, visiting http://localhost/user/0 will return a 500 error: `curl http://localhost/user/0`{{execute T3}} ``` {"timestamp":1550223186170,"status":500,"error":"Internal Server Error","exception":"java.lang.IllegalArgumentException","message":"id < 1","path":"/user/0"} ``` But what are the specific parameters of the request, what is the exception stack? ### View the parameters/exception of UserController Execute in Arthas: `watch com.example.demo.arthas.user.UserController * '{params, throwExp}'`{{execute T2}} 1. The first argument is the class name, which supports wildcards. 2. The second argument is the function name, which supports wildcards. Visit `curl http://localhost/user/0`{{execute T3}} , the `watch` command will print the parameters and exception ```bash $ watch com.example.demo.arthas.user.UserController * '{params, throwExp}' Press Q or Ctrl+C to abort. Affect(class-cnt:1 , method-cnt:2) cost in 53 ms. ts=2019-02-15 01:35:25; [cost=0.996655ms] result=@ArrayList[ @Object[][isEmpty=false;size=1], @IllegalArgumentException[java.lang.IllegalArgumentException: id < 1], ] ``` The user can see that the actual thrown exception is `IllegalArgumentException`. The user can exit the watch command by typing `Q`{{execute T2}} or `Ctrl+C`. If the user want to expand the result, can use the `-x` option: `watch com.example.demo.arthas.user.UserController * '{params, throwExp}' -x 2`{{execute T2}} ```bash $ watch com.example.demo.arthas.user.UserController * '{params, throwExp}' -x 2 Press Q or Ctrl+C to abort. Affect(class count: 1 , method count: 2) cost in 190 ms, listenerId: 1 ts=2020-08-13 05:22:45; [cost=4.805432ms] result=@ArrayList[ @Object[][ @Integer[0], ], java.lang.IllegalArgumentException: id < 1 at com.example.demo.arthas.user.UserController.findUserById(UserController.java:19) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ... , ] ``` ### The return value expression In the above example, the third argument is the `return value expression`, which is actually an `ognl` expression that supports some built-in objects: * loader * clazz * method * target * params * returnObj * throwExp * isBefore * isThrow * isReturn You can use these built-in objects in the expressions. For example, return an array: `watch com.example.demo.arthas.user.UserController * '{params[0], target, returnObj}'`{{execute T2}} More references: https://arthas.aliyun.com/doc/en/advice-class.html ### The conditional expression The `watch` command supports conditional expressions in the fourth argument, such as: `watch com.example.demo.arthas.user.UserController * returnObj 'params[0] > 100'`{{execute T2}} When visit https://[[HOST_SUBDOMAIN]]-80-[[KATACODA_HOST]].environments.katacoda.com/user/1 , the `watch` command print nothing. When visit https://[[HOST_SUBDOMAIN]]-80-[[KATACODA_HOST]].environments.katacoda.com/user/101 , the `watch` command will print: ```bash $ watch com.example.demo.arthas.user.UserController * returnObj 'params[0] > 100' Press Q or Ctrl+C to abort. Affect(class-cnt:1 , method-cnt:2) cost in 47 ms. ts=2019-02-13 19:42:12; [cost=0.821443ms] result=@User[ id=@Integer[101], name=@String[name101], ] ``` ### Capture when an exception occurs The `watch` command supports the `-e` option, which means that only requests that throw an exception are caught: `watch com.example.demo.arthas.user.UserController * "{params[0],throwExp}" -e`{{execute T2}} ### Filter by cost The watch command supports filtering by cost, such as: `watch com.example.demo.arthas.user.UserController * '{params, returnObj}' '#cost>200'`{{execute T2}}