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Technical details about 1.17/Java 16 issues


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Hi, I'm new to the forum, but a longtime programmer who recently got into Minecraft mods through my son (I've worked with Java on and off over the years).  I'm curious if there are any technical details around getting Forge to work with Minecraft 1.17/Java 16?

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Each version of Forge targets a single version of Minecraft, so we have Forge versions for 1.15, Forge versions for 1.16... the first version of Forge for 1.17 is still being worked on

This is my Forum Signature, I am currently attempting to transform it into a small guide for fixing easier issues using spoiler blocks to keep things tidy.


As the most common issue I feel I should put this outside the main bulk:

The only official source for Forge is https://files.minecraftforge.net, and the only site I trust for getting mods is CurseForge.

If you use any site other than these, please take a look at the StopModReposts project and install their browser extension, I would also advise running a virus scan.


For players asking for assistance with Forge please expand the spoiler below and read the appropriate section(s) in its/their entirety.


Logs (Most issues require logs to diagnose):


Please post logs using one of the following sites (Thank you Lumber Wizard for the list):

https://gist.github.com/100MB Requires member (Free)

https://pastebin.com/: 512KB as guest, 10MB as Pro ($$$)

https://hastebin.com/: 400KB

Do NOT use sites like Mediafire, Dropbox, OneDrive, Google Drive, or a site that has a countdown before offering downloads.


What to provide:

...for Crashes and Runtime issues:

Minecraft 1.14.4 and newer:

Post debug.log

Older versions:

Please update...


...for Installer Issues:

Post your installer log, found in the same place you ran the installer

This log will be called either installer.log or named the same as the installer but with .log on the end

Note for Windows users:

Windows hides file extensions by default so the installer may appear without the .jar extension then when the .log is added the log will appear with the .jar extension


Where to get it:

Mojang Launcher: When using the Mojang launcher debug.log is found in .minecraft\logs.


Curse/Overwolf: If you are using the Curse Launcher, their configurations break Forge's log settings, fortunately there is an easier workaround than I originally thought, this works even with Curse's installation of the Minecraft launcher as long as it is not launched THROUGH Twitch:

  1. Make sure you have the correct version of Forge installed (some packs are heavily dependent on one specific build of Forge)
  2. Make a launcher profile targeting this version of Forge.
  3. Set the launcher profile's GameDir property to the pack's instance folder (not the instances folder, the folder that has the pack's name on it).
  4. Now launch the pack through that profile and follow the "Mojang Launcher" instructions above.






or alternately, 


Fallback ("No logs are generated"):

If you don't see logs generated in the usual place, provide the launcher_log.txt from .minecraft


Server Not Starting:


If your server does not start or a command window appears and immediately goes away, run the jar manually and provide the output.


Reporting Illegal/Inappropriate Adfocus Ads:


Get a screenshot of the URL bar or copy/paste the whole URL into a thread on the General Discussion board with a description of the Ad.

Lex will need the Ad ID contained in that URL to report it to Adfocus' support team.


Posting your mod as a GitHub Repo:


When you have an issue with your mod the most helpful thing you can do when asking for help is to provide your code to those helping you. The most convenient way to do this is via GitHub or another source control hub.

When setting up a GitHub Repo it might seem easy to just upload everything, however this method has the potential for mistakes that could lead to trouble later on, it is recommended to use a Git client or to get comfortable with the Git command line. The following instructions will use the Git Command Line and as such they assume you already have it installed and that you have created a repository.


  1. Open a command prompt (CMD, Powershell, Terminal, etc).
  2. Navigate to the folder you extracted Forge’s MDK to (the one that had all the licenses in).
  3. Run the following commands:
    1. git init
    2. git remote add origin [Your Repository's URL]
      • In the case of GitHub it should look like: https://GitHub.com/[Your Username]/[Repo Name].git
    3. git fetch
    4. git checkout --track origin/master
    5. git stage *
    6. git commit -m "[Your commit message]"
    7. git push
  4. Navigate to GitHub and you should now see most of the files.
    • note that it is intentional that some are not synced with GitHub and this is done with the (hidden) .gitignore file that Forge’s MDK has provided (hence the strictness on which folder git init is run from)
  5. Now you can share your GitHub link with those who you are asking for help.

[Workaround line, please ignore]


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  • 3 weeks later...
On 7/11/2021 at 6:20 AM, 0xx06 said:

what are you asking? it doesnt really make sense at all

My apologies for the delayed response, work has been a bit crazy.

Anyways, there was a comment about how, with Java 16 being used with Minecraft 1.17 (instead of Java 8), it broke reflection.  Which I assume is what Forge uses to patch itself in at runtime (examining Minecraft's Java classes and adding the necessary infrastructure for mods to work).  I'm curious to know the technical details of how they're working around that problem as I enjoy low-level technical work like that.

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2 minutes ago, diesieben07 said:

Forge doesn't use reflection to patch Minecraft.

The main blocker to Java 16 was that Forge used reflection into some JDK internals during the loading process (mainly regarding jar files). Java 16 blocks this by default (you can no longer reflect into JDK internals).

Thanks for the clarification on that.  I see that there's a beta release of Forge now for 1.17, so I'll take a look at how it all works under the hood.

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14 minutes ago, diesieben07 said:

Forge doesn't use reflection to patch Minecraft.

btw, the reason I thought that is because I used to program in Objective-C a lot, where you had the ability to reroute existing functions at runtime (method swizzling).  Which was a great way to create a patcher that keeps working over time if things don't change too much.  I wasn't sure if the same thing was possible using reflection in Java (haven't delved into JVM internals in years).

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48 minutes ago, diesieben07 said:

You can't change what things do. For that Forge uses bytecode patching.

Yeah, that would be the only other way to do it.  Is there an easy way to search out the locations you need to patch in situations where not much has changed (aside from the offset) in a new Minecraft version?  i.e. find the same functions when they've simply moved a bit?  Just thinking from past experience with finding function offsets in C/C++ dynamic libraries as they change.  I really should just look at the code (use the source Luke).  Damn work getting in the way of personal curiosities!

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Thanks for bearing with me and answering my questions @diesieben07.  The MCPConfig scripts/tools look like a good place to start down the rabbit hole when I get more time.  Funny enough, it might actually help my work since I've been swearing at Gradle a lot lately for being the most convoluted build system since the GNU Autotools. 😂

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