arthas/tutorials/katacoda/case-jad-mc-redefine-en/case-jad-mc-redefine.md

This case introduces the ability to dynamically update code via the jad/mc/redefine command.

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"}

This logic will be modified by redefine command below.

Use jad command to decompile UserController

jad --source-only com.example.demo.arthas.user.UserController > /tmp/UserController.java{{execute T2}}

The result of jad command will be saved in the /tmp/UserController.java file.

Then open Terminal 3, use vim to edit /tmp/UserController.java:

vim /tmp/UserController.java{{execute T3}}

For example, when the user id is less than 1, it also returns normally without throwing an exception:

    @GetMapping(value={"/user/{id}"})
    public User findUserById(@PathVariable Integer id) {
        logger.info("id: {}", (Object)id);
        if (id != null && id < 1) {
			return new User(id, "name" + id);
            // throw new IllegalArgumentException("id < 1");
        }
        return new User(id.intValue(), "name" + id);
    }

Use sc command to find the ClassLoader that loads the UserController

sc -d *UserController | grep classLoaderHash{{execute T2}}

$ sc -d *UserController | grep classLoaderHash
 classLoaderHash   1be6f5c3

It can be found that it is loaded by spring boot LaunchedURLClassLoader@1be6f5c3.

Note that the hashcode changes, you need to check the current ClassLoader information first, and extract the hashcode corresponding to the ClassLoader.

if you use-c, you have to manually type hashcode by -c <hashcode>.

For classloader with only one instance, it can be specified by --classLoaderClass using class name, which is more convenient to use.

The value of --classloaderclass is the class name of classloader. It can only work when it matches a unique classloader instance. The purpose is to facilitate the input of general commands. However, -c <hashcode> is dynamic.

mc

After saving /tmp/UserController.java, compile with the mc (Memory Compiler) command and specify the ClassLoader with the --classLoaderClass option:

mc --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader /tmp/UserController.java -d /tmp{{execute T2}}

$ mc --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader /tmp/UserController.java -d /tmp
Memory compiler output:
/tmp/com/example/demo/arthas/user/UserController.class
Affect(row-cnt:1) cost in 346 ms

You can also execute mc -c <classLoaderHash> /tmp/UserController.java -d /tmp，using -c to specify ClassLoaderHash:

$ mc -c 1be6f5c3 /tmp/UserController.java -d /tmp

redefine

Then reload the newly compiled UserController.class with the redefine command:

redefine /tmp/com/example/demo/arthas/user/UserController.class{{execute T2}}

$ redefine /tmp/com/example/demo/arthas/user/UserController.class
redefine success, size: 1

Check the results of the hotswap code

After the redefine command is executed successfully, visit https://HOST_SUBDOMAIN-80-KATACODA_HOST.environments.katacoda.com/user/0 again.

The result is:

{
  "id": 0,
  "name": "name0"
}