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95 lines
3.6 KiB
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95 lines
3.6 KiB
Plaintext
redefine
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========
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> Load the external `*.class` files to re-define the loaded classes in JVM.
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Reference: [Instrumentation#redefineClasses](https://docs.oracle.com/javase/8/docs/api/java/lang/instrument/Instrumentation.html#redefineClasses-java.lang.instrument.ClassDefinition...-)
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> Notes: Re-defined classes cannot be restored. There are chances that redefining may fail due to some reasons, for example: there's new field introduced in the new version of the class, pls. refer to JDK's documentation for the limitations.
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> The `reset` command is not valid for classes that have been processed by `redefine`. If you want to reset, you need `redefine` the original bytecode.
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> The `redefine` command will conflict with the `jad`/`watch`/`trace`/`monitor`/`tt` commands. After executing `redefine`, if you execute the above mentioned command, the bytecode of the class will be reset.
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> The reason is that in the JDK `redefine` and `retransform` are different mechanisms. When two mechanisms are both used to update the bytecode, only the last modified will take effect.
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### Options
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|Name|Specification|
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|---:|:---|
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|`[c:]`|hashcode of the class loader|
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|`[p:]`|absolute path of the external `*.class`, multiple paths are separated with 'space'|
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### Usage
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```bash
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redefine /tmp/Test.class
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redefine -c 327a647b /tmp/Test.class /tmp/Test$Inner.class
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```
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### Use with the jad/mc command
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```bash
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jad --source-only com.example.demo.arthas.user.UserController > /tmp/UserController.java
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mc /tmp/UserController.java -d /tmp
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redefine /tmp/com/example/demo/arthas/user/UserController.class
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```
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* Use `jad` command to decompile bytecode, and then you can use other editors, such as vim to modify the source code.
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* `mc` command to compile the modified code
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* Load new bytecode with `redefine` command
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### Tips for uploading .class files to the server
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The `mc` command may fail. You can modify the code locally, compile it, and upload it to the server. Some servers do not allow direct uploading files, you can use the `base64` command to bypass.
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1. Convert the `.class` file to base64 first, then save it as result.txt
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```bash
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Base64 < Test.class > result.txt
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```
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2. Login the server, create and edit `result.txt`, copy the local content, paste and save
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3. Restore `result.txt` on the server to `.class`
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```
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Base64 -d < result.txt > Test.class
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```
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4. Use the md5 command to verify that the `.class` files are consistent.
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### Restrictions of the redefine command
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* New field/method is not allowed
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* The function that is running, no exit can not take effect, such as the new `System.out.println` added below, only the `run()` function will take effect.
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```java
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public class MathGame {
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public static void main(String[] args) throws InterruptedException {
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MathGame game = new MathGame();
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while (true) {
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game.run();
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TimeUnit.SECONDS.sleep(1);
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// This doesn't work because the code keeps running in while
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System.out.println("in loop");
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}
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}
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public void run() throws InterruptedException {
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// This works because the run() function ends completely every time
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System.out.println("call run()");
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try {
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int number = random.nextInt();
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List<Integer> primeFactors = primeFactors(number);
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print(number, primeFactors);
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} catch (Exception e) {
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System.out.println(String.format("illegalArgumentCount:%3d, ", illegalArgumentCount) + e.getMessage());
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}
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}
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``` |