CME-Suck-My-Duck

Output all stack traces when modifying a certain collection for debugging ConcurrentModificationException and IndexOutOfBoundsException.

Usage for Minecraft

First, add this jar to mods folder.

Second, edit your Java Virtual Machine Argument in your launcher. Add -javaagent:mods/CMESuckMyDuck-<version>.jar=<class full name>;<field name>;<type>;<phase>.

Finally, run the game, play and wait until the crash happens.

Usage for 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-<version>.jar to classpath after javaagent.

Arguments

<class full name>

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).

<field name>

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.

<type>

Currently, we only support three containers: List, Set, Map. This argument indicates the type of monitored container.

<phase>

static or nonstatic. This argument indicates the container is a static field or non-static field.

Examples

ConcurrentModificationException from SoundEngine in Forge 1.20.1 Environment

-javaagent:mods/CMESuckMyDuck-1.0.0.jar=net/minecraft/client/sounds/SoundEngine;f_120229_;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=<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.

Experiments

Code (simulates concurrent modification):

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.*;

public class Main {
	private static final DateFormat DATE_FORMAT = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
	public static final List<Integer> LIST = new ArrayList<>();
	public static final Random RANDOM = new Random(42);

	private static void modify1() {
		int cnt = 8;

		Random random = new Random(RANDOM.nextInt());
		int a = -1;
		try {
			while (cnt > 0) {
				if (LIST.contains(a)) {
					LIST.remove(a);
					cnt -= 1;
				}
				a = random.nextInt(256);

				try {
					Thread.sleep(RANDOM.nextInt(100));
				} catch (InterruptedException e) {
					System.err.println("[" + DATE_FORMAT.format(new Date()) + "] " + Thread.currentThread().getName());
					e.printStackTrace();
				}
			}
		} catch (IndexOutOfBoundsException e) {
			System.err.println("[" + DATE_FORMAT.format(new Date()) + "] " + Thread.currentThread().getName());
			e.printStackTrace();
		}
	}

	private static void modify2() {
		int cnt = 32;

		Random random = new Random(RANDOM.nextInt());
		int a = random.nextInt(256);
		try {
			while(cnt > 0) {
				if(!LIST.contains(a)) {
					LIST.add(a);
					cnt -= 1;
				}
				a = random.nextInt(256);

				try {
					Thread.sleep(RANDOM.nextInt(100));
				} catch(InterruptedException e) {
					System.err.println("[" + DATE_FORMAT.format(new Date()) + "] " + Thread.currentThread().getName());
					e.printStackTrace();
				}
			}
		} catch (IndexOutOfBoundsException e) {
			System.err.println("[" + DATE_FORMAT.format(new Date()) + "] " + Thread.currentThread().getName());
			e.printStackTrace();
		}
	}

	public static void main(String[] args) throws InterruptedException {
		List<Integer> temp = new ArrayList<>();
		for(int i = 0; i < 200; ++i) {
			temp.add(i);
		}

		LIST.addAll(temp);
		Thread t2 = new Thread(Main::modify2, "modify-2");
		t2.start();
		Thread t1 = new Thread(Main::modify1, "modify-1");
		t1.start();
		Thread t3 = new Thread(Main::modify1, "modify-3");
		t3.start();
		try {
			for(int i: LIST) {
				System.out.println(i);
				t1.join();
				t2.join();
				t3.join();
			}
		} catch(ConcurrentModificationException e) {
			System.err.println("[" + DATE_FORMAT.format(new Date()) + "] " + Thread.currentThread().getName());
			e.printStackTrace();
		}

		System.out.println("size: " + LIST.size());
	}
}

w.o. our agent:

with our agent:

With the modification history of LIST, we can know which threads cause the crash.