I am trying to use a video decode library (org.bytedeco) in the mod. The code can be called in the source code and  compiled correctly, but when i enter the game, it throws ClassNotFoundException org.opencv.core.Core and NoClassDefFoundError org/opencv/core/Core. I just implemented 4 libs like this:

    implementation 'org.bytedeco:ffmpeg:5.0-1.5.7'
    implementation 'org.bytedeco:javacpp:1.5.7'
    implementation 'org.bytedeco:javacv:1.5.7'
    implementation 'org.bytedeco:opencv:4.5.5-1.5.7'

And considering the runtime dependency, i also used jarJar:

    implementation(group : 'org.bytedeco', name : 'ffmpeg', version : '[5.0-1.5.7,6.0)' ){
        jarJar.pin(it,'5.0-1.5.7')
    }
    implementation(group : 'org.bytedeco', name : 'javacpp', version : '[1.5.7,1.6)' ){
        jarJar.pin(it,'1.5.7')
    }
    implementation(group : 'org.bytedeco', name : 'javacv', version : '[1.5.7,1.6)' ){
        jarJar.pin(it,'1.5.7')
    }
    implementation(group : 'org.bytedeco', name : 'opencv', version : '[4.5.5-1.5.7,6.0)' ){
        jarJar.pin(it,'4.5.5-1.5.7')
    }

i don't know why the compilation can be correct but when i run the game it contains errors. 

or i need to add external jar libs in other ways...

The libraries you adding in the first example are not mods.  So forge won't load them.

You could of course change them into mods by adding a META-INF/mods.toml to them.

 

JarJar is not something I know a lot about, but I do know you have to enable it in your build.gradle

https://forge.gemwire.uk/wiki/Jar-in-Jar#Using_ForgeGradle_to_generate_a_Jar-In-Jar

Quote

To enable it, you need to call `jarJar.enable()` anywhere in your buildscript

 

Shading is another option mentioned on that wiki link, e.g.

https://imperceptiblethoughts.com/shadow/introduction/