arthas/tutorials/katacoda/case-watch-method-exception-en/case-watch-method-exception.md

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

$ 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}}

$ 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:

$ 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}}