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This case introduces the ability to dynamically update code via the `jad`/`mc`/`redefine` command.
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Currently, visiting http://localhost/user/0 will return a 500 error:
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`curl http://localhost/user/0`{{execute T3}}
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```
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{"timestamp":1550223186170,"status":500,"error":"Internal Server Error","exception":"java.lang.IllegalArgumentException","message":"id < 1","path":"/user/0"}
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```
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This logic will be modified by `redefine` command below.
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### Use jad command to decompile UserController
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`jad --source-only com.example.demo.arthas.user.UserController > /tmp/UserController.java`{{execute T2}}
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The result of jad command will be saved in the `/tmp/UserController.java` file.
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Then open `Terminal 3`, use `vim` to edit `/tmp/UserController.java`:
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`vim /tmp/UserController.java`{{execute T3}}
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For example, when the user id is less than 1, it also returns normally without throwing an exception:
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```java
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@GetMapping(value={"/user/{id}"})
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public User findUserById(@PathVariable Integer id) {
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logger.info("id: {}", (Object)id);
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if (id != null && id < 1) {
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return new User(id, "name" + id);
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// throw new IllegalArgumentException("id < 1");
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}
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return new User(id.intValue(), "name" + id);
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}
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```
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### Use sc command to find the ClassLoader that loads the UserController
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`sc -d *UserController | grep classLoaderHash`{{execute T2}}
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```bash
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$ sc -d *UserController | grep classLoaderHash
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|
classLoaderHash 1be6f5c3
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```
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It can be found that it is loaded by spring boot `LaunchedURLClassLoader@1be6f5c3`.
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Note that the hashcode changes, you need to check the current ClassLoader information first, and extract the hashcode corresponding to the ClassLoader.
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if you use`-c`, you have to manually type hashcode by `-c <hashcode>`.
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For classloader with only one instance, it can be specified by `--classLoaderClass` using class name, which is more convenient to use.
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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.
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### mc
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After saving `/tmp/UserController.java`, compile with the `mc` (Memory Compiler) command and specify the ClassLoader with the `--classLoaderClass` option:
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`mc --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader /tmp/UserController.java -d /tmp`{{execute T2}}
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|
```bash
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$ mc --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader /tmp/UserController.java -d /tmp
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|
Memory compiler output:
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|
/tmp/com/example/demo/arthas/user/UserController.class
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|
Affect(row-cnt:1) cost in 346 ms
|
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|
```
|
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|
|
You can also execute `mc -c <classLoaderHash> /tmp/UserController.java -d /tmp`，using `-c` to specify ClassLoaderHash:
|
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|
|
|
|
```bash
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|
$ mc -c 1be6f5c3 /tmp/UserController.java -d /tmp
|
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|
```
|
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|
|
### redefine
|
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Then reload the newly compiled `UserController.class` with the `redefine` command:
|
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|
|
|
|
`redefine /tmp/com/example/demo/arthas/user/UserController.class`{{execute T2}}
|
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|
|
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|
```
|
|
|
$ redefine /tmp/com/example/demo/arthas/user/UserController.class
|
|
|
redefine success, size: 1
|
|
|
```
|
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|
|
### 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.
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|
|
|
|
The result is:
|
|
|
|
|
|
```
|
|
|
{
|
|
|
"id": 0,
|
|
|
"name": "name0"
|
|
|
}
|
|
|
```
|