# Abstract Players and modpack developers sometimes find the game crashed with `ConcurrentModificationExceptions` (CME) and `IndexOutOfBoundsExceptions` (IOOBE), which only give stacktrace of current thread and are hard to trace which mod causes them. Here's one of the crashes which is caused by SimpleReloadInstance in 1.20.1 Forge: ![CME from SimpleReloadInstance](https://cdn.modrinth.com/data/cached_images/877cd1d1976b1969fffb619bdbd62b13244e02c8.png) So this is what the mod is developed for. Players who install this mod and add javaagent to JVM Argument correctly will receive a full log for modification history of a certain collection: ![log of this mod](https://cdn.modrinth.com/data/cached_images/d3d00f23694f96591fc77d8387540d7ce29c078a.png) # Usage First, add this jar to mods folder. Second, edit your Java Virtual Machine Argument in your launcher. Add `-javaagent:mods/CMESuckMyDuck-.jar=;;;`. Finally, run the game, play and wait until the crash happens. # Usage for Other Java Projects This mod can not only be used for Minecraft debugging, but also be utilized to debug other java projects. Similar to Minecraft. The only different step is that you should add gson and asm jar to classpath (`-cp`) before javaagent, and add our `CMESuckMyDuck-.jar` to classpath after javaagent. # Arguments ## A full name of the class, which has a container that you would like to monitor. Use `\` instead of `.` (a.k.a. the internal name of class). ## A field name of the container in target class, which you would like to monitor. For Forge, use SRG name. For Fabric, use intermediary name. For NeoForge, use official name. ## Currently, we only support three containers: `List`, `Set`, `Map`. This argument indicates the type of monitored container. ## `static` or `nonstatic`. This argument indicates the container is a static field or non-static field. # How to get and Let's start with an example. First, take a look at the stacktrace of CME/IOOBE: ![Replace this with a description](https://cdn.modrinth.com/data/cached_images/f4908f0fb44bc480d0fba4a1714bf9dbca4b8043.png) Second, read the source code of SoundEngine and confirm which container is facing this issue: ![Replace this with a description](https://cdn.modrinth.com/data/cached_images/26c2eefabbde9a9ce3aea62dbfdf1012d736c16d.png) Now we know we should monitor map `field_217942_m` (`instanceToChannel` in `SoundEngine`). Keep going. Third, install this mod, and add "`-javaagent:mods/CMESuckMyDuck-1.0.0.jar=net/minecraft/client/audio/SoundEngine;field_217942_m;Map;nonstatic`" to Java Virtual Machine Argument. Relaunch the game and wait for the next crash. Finally, open `CMESuckMyDuck.log` file and you will see which thread and which mod has concurrently modified the container. # Examples (JVM Arguments) ## ConcurrentModificationException from SoundEngine in Forge 1.16.5 Environment `-javaagent:mods/CMESuckMyDuck-1.0.0.jar=net/minecraft/client/audio/SoundEngine;field_217942_m;Map;nonstatic` ## ConcurrentModificationException from PotionBrewing in Forge 1.20.1 Environment `-javaagent:mods/CMESuckMyDuck-1.0.0.jar=net/minecraft/world/item/alchemy/PotionBrewing;f_43494_;List;static` ## ArrayIndexOutOfBoundsException from Zeta mod `-javaagent:CMESuckMyDuck-1.0.0.jar=org/violetmoon/zetaimplforge/event/ForgeZetaEventBus;convertedHandlers;Map;nonstatic` # Other options ## Log Level Use system property `-Dcme_suck_my_duck.log_level=` to set custom log level. Default 1, which means no debug message will be logged. Users can set it to 0 to output debug message, which is not recommended - query functions like Map#get, Set#containsAll will also be logged when set to 0, and make the file very very long. # Conclusion I like eating peking duck. It's so delicious! ![Photo by Polina Tankilevitch: https://www.pexels.com/photo/close-up-photo-of-savory-peking-duck-dish-on-a-ceramic-bowl-5848604/](https://cdn.modrinth.com/data/cached_images/d4ce4bba6f0bef8780cd5843390bf3358dd9b55b_0.webp)

CME is Bad

CME Sucks v1.0.1

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