Redirecting Method Calls

Redirecting method calls, or just using a Redirect, is a way that you can make the JVM call your mixin method instead of the target method at a specified point. This is handy in situations like when you just want that one void to do something else, but don't want to overwrite it or do a head injection and cancellation. Redirects are very powerful, and can help development a ton.

Here is how you can use a basic redirect to change a System.out.println call into a call to your app's logger:

package my.mixins;

import code.TargetClass;
import code.Logger;

import org.spongepowered.asm.mixin.Mixin;
import org.spongepowered.asm.mixin.injection.At;
import org.spongepowered.asm.mixin.injection.Redirect;

@Mixin(TargetClass.class)
public class MixinTargetClass {
    @Redirect(
        // the method this function is called in
        method = "exampleVoid",
        // target the invocation of System.out.println
        at = @At(
            value = "INVOKE",
            target = "Ljava/lang/System;println(Ljava/lang/String;)V"
        )
    )
    private void println(String message) {
        Logger.info(message);
    }
}

In this example, we are targeting a void that looks like this:

package code;

public class TargetClass {
    public void exampleVoid() {
        System.out.println("Hey!");
    }
}

We can already notice a few things:

The redirect accepts the instance of the class being called, but in this instance it is excluded because println is a static method.
The redirect accepts the arguments given to the method in the At (like any mixin with an At value of INVOKE).
The return type of the mixin method matches that of the targeted.

Once you understand this, you get a powerful tool that you can use when you need to, that also saves time, effort, and avoids complexity!