Mixin Examples

The Fabric Wiki's Mixin segments is under heavy reviews, pages on the topic are subject to major edits or rewrites. This page in particular is noted as being unreliable to learn Mixin as a tool from. Learning by example should be used sparingly, as it may lead to a lack of understanding. Prioritize, if you are unable to learn through documentation, presenting your intention in detail in a Mixin support channel on the Fabric or SpongePowered Discord servers and discussing with experienced devs there.

Mixin Examples

This is a collection of frequently used mixins. This page is intended as a cheat sheet. See Mixin Introduction if you haven't already.

Mixing into a private inner class

Use the targets parameter and a $ sign to get the inner class.

@Mixin(targets = "net.minecraft.client.render.block.BlockModelRenderer$AmbientOcclusionCalculator")
public class AmbientOcclusionCalculatorMixin {
    // do your stuff here
}

Access the this instance of the class your mixin is targeting

Note: Double casting this should be avoided when possible. If you intend to use a method or field from the target class, use @Shadow. If the method or field is from a parent of the target class, have your mixin extend the direct parent of the target class.

Mixin:

@Mixin(TargetClass.class)
public class MyMixin extends EveryThingThatTargetClassExtends implements EverythingThatTargetClassImplements {
  @Inject(method = "foo()V", at = @At("HEAD"))
  private void injected(CallbackInfo ci) {
    TargetClass thisObject = (TargetClass)(Object)this;
  }
}

Injecting into the head of a static block

Mixin:

@Inject(method = "<clinit>", at = @At("HEAD"))
private void injected(CallbackInfo ci) {
    doSomething3();
}

Result:

static {
+   injected(new CallbackInfo(“<clinit>”, false));
    doSomething1();
    doSomething2();
}

Injecting into the head of a method

Mixin:

@Inject(method = "foo()V", at = @At("HEAD"))
private void injected(CallbackInfo ci) {
  doSomething4();
}

Result:

  public void foo() {
+   injected(new CallbackInfo("foo", false));
    doSomething1();
    doSomething2();
    doSomething3();
  }

Injecting into the tail of a method

Mixin:

@Inject(method = "foo()V", at = @At("TAIL"))
private void injected(CallbackInfo ci) {
  doSomething4();
}

Result:

  public void foo() {
    doSomething1();
    if (doSomething2()) {
      return;
    }
    doSomething3();
+   injected(new CallbackInfo("foo", false));
  }

Injecting into the returns of a method

Mixin:

@Inject(method = "foo()V", at = @At("RETURN"))
private void injected(CallbackInfo ci) {
  doSomething4();
}

Result:

  public void foo() {
    doSomething1();
    if (doSomething2()) {
+     injected(new CallbackInfo("foo", false));
      return;
    }
    doSomething3();
+   injected(new CallbackInfo("foo", false));
  }

Injecting into the point before a method call

Mixin:

@Inject(method = "foo()V", at = @At(value = "INVOKE", target = "La/b/c/Something;doSomething()V"))
private void injected(CallbackInfo ci) {
  doSomething3();
}

Result:

  public void foo() {
    doSomething1();
    Something something = new Something();
+   injected(new CallbackInfo("foo", false));
    something.doSomething();
    doSomething2();
  }

Injecting into the point after a method call

Mixin:

@Inject(method = "foo()V", at = @At(value = "INVOKE", target = "La/b/c/Something;doSomething()V", shift = At.Shift.AFTER))
private void injected(CallbackInfo ci) {
  doSomething3();
}

Result:

  public void foo() {
    doSomething1();
    Something something = new Something();
    something.doSomething();
+   injected(new CallbackInfo("foo", false));
    doSomething2();
  }

Injecting into the point with shift amount

Mixin:

@Inject(method = "foo()V", at = @At(value = "INVOKE", target = "La/b/c/Something;doSomething()V", shift = At.Shift.BY, by = 2))
private void injected(CallbackInfo ci) {
  doSomething3();
}

Result:

  public void foo() {
    doSomething1();
    Something something = new Something();
    something.doSomething();
    doSomething2();
+   injected(new CallbackInfo("foo", false));
  }

Injecting with a slice

Mixin:

@Inject(
  method = "foo()V",
  at = @At(
    value = "INVOKE",
    target = "La/b/c/Something;doSomething()V"
  ),
  slice = @Slice(
    from = @At(value = "INVOKE", target = "La/b/c/Something;doSomething2()V"),
    to = @At(value = "INVOKE", target = "La/b/c/Something;doSomething3()V")
  )
)
private void injected(CallbackInfo ci) {
  doSomething5();
}

Result:

  public class Something {
    public void foo() {
      this.doSomething1();
+     // It will not inject into here because this is outside of the slice section
      this.doSomething();
      this.doSomething2();
+     injected(new CallbackInfo("foo", false));
      this.doSomething();
      this.doSomething3();
+     // It will not inject into here because this is outside of the slice section
      this.doSomething();
      this.doSomething4();
    }
  }

Injecting and cancelling

Mixin:

@Inject(method = "foo()V", at = @At("HEAD"), cancellable = true)
private void injected(CallbackInfo ci) {
  ci.cancel();
}

Result:

  public void foo() {
+   CallbackInfo ci = new CallbackInfo("foo", true);
+   injected(ci);
+   if (ci.isCancelled()) return;
    doSomething1();
    doSomething2();
    doSomething3();
  }

Injecting and cancelling with a return value

Mixin:

@Inject(method = "foo()I;", at = @At("HEAD"), cancellable = true)
private void injected(CallbackInfoReturnable<Integer> cir) {
  cir.setReturnValue(3);
}

Result:

  public int foo() {
+   CallbackInfoReturnable<Integer> cir = new CallbackInfoReturnable<Integer>("foo", true);
+   injected(cir);
+   if (cir.isCancelled()) return cir.getReturnValue();
    doSomething1();
    doSomething2();
    doSomething3();
    return 10;
  }

Capturing local values

Capture locals without MixinExtras

Mixin:

@Inject(method = "foo()V", at = @At(value = "TAIL"), locals = LocalCapture.CAPTURE_FAILHARD)
private void injected(CallbackInfo ci, TypeArg1 arg1) {
  //CAPTURE_FAILHARD: If the calculated locals are different from the expected values, throws an error.
  arg1.doSomething4();
}

Result:

  public void foo() {
    TypeArg1 arg1 = getArg1();
    arg1.doSomething1();
    arg1.doSomething2();
    TypeArg2 arg2 = getArg2();
    arg2.doSomething3();
+   injected(new CallbackInfo("foo", false), arg1);
  }

Capture locals with MixinExtras

See the oficial MixinExtra's Wiki.

MixinExtras required Fabric Loader 0.15 or above, or you have to manually specify it in build.gradle.

If there are multiple locals with that type, you have to specify ordinal or it will throw an error.

the use of @Local is recommended over LocalCapture

Mixin:

@Inject(method = "foo()V", at = @At(value = "TAIL"))
private void injected(CallbackInfo ci, @Local TypeArg2 arg2) {
  arg2.doSomething4();
}

Result:

  public void foo() {
    TypeArg1 arg1 = getArg1();
    arg1.doSomething1();
    arg1.doSomething2();
    TypeArg2 arg2 = getArg2();
    arg2.doSomething3();
+   injected(new CallbackInfo("foo", false), arg2);
  }

Capturing one of multiple locals of a type

Mixin:

@Inject(method = "foo()V", at = @At(value = "TAIL"))
private void injected(CallbackInfo ci, @Local(ordinal = 2) TypeArg arg) {
  arg.doSomething4();
}

Result:

  public void foo() {
    TypeArg arg1 = getArg1();
    TypeArg arg2 = getArg2();
    TypeArg arg3 = getArg3();
    TypeArg arg4 = getArg4();
    doSomething();
+   injected(new CallbackInfo("foo", false), arg3);
  }

Modifying locals

This requires MixinExtras.

Mixin:

  @Inject(method = "foo()V", at = @At(value = "INVOKE", target = "doSomething()V", shift = At.Shift.AFTER))
  private static void injected(CallbackInfo ci, @Local LocalRef<String> localRef) {
    localRef.set(localRef.get() + " - modified")
  }

Result:

  public void foo() {
    String s = "example string";
    doSomething();
+   s = s + " - modified";
    doSomething2(s);
  }

Modifying a return value

Mixin:

@Inject(method = "foo()I;", at = @At("RETURN"), cancellable = true)
private void injected(CallbackInfoReturnable<Integer> cir) {
  cir.setReturnValue(cir.getReturnValue() * 3);
}

Result:

  public int foo() {
    doSomething1();
    doSomething2();
-   return doSomething3() + 7;
+   int i = doSomething3() + 7;
+   CallbackInfoReturnable<Integer> cir = new CallbackInfoReturnable<Integer>("foo", true, i);
+   injected(cir);
+   if (cir.isCancelled()) return cir.getReturnValue();
+   return i;
  }

Redirecting a method call

Mixin:

@Redirect(method = "foo()V", at = @At(value = "INVOKE", target = "La/b/c/Something;doSomething(I)I"))
private int injected(Something something, int x) {
  return x + 3;
}

Result:

  public void foo() {
    doSomething1();
    Something something = new Something();
-   int i = something.doSomething(10);
+   int i = injected(something, 10);
    doSomething2();
  }

Redirecting a get field

Mixin:

@Redirect(method = "foo()V", at = @At(value = "FIELD", target = "La/b/c/Something;aaa:I", opcode = Opcodes.GETFIELD))
private int injected(Something something) {
  return 12345;
}

Result:

  public class Something {
    public int aaa;
    public void foo() {
      doSomething1();
-     if (this.aaa > doSomething2()) {
+     if (injected(this) > doSomething2()) {
        doSomething3();
      }
      doSomething4();
    }
  }

Redirecting a put field

Mixin:

@Redirect(method = "foo()V", at = @At(value = "FIELD", target = "La/b/c/Something;aaa:I", opcode = Opcodes.PUTFIELD))
private void injected(Something something, int x) {
  something.aaa = x + doSomething5();
}

Result:

  public class Something {
    public int aaa;
    public void foo() {
      doSomething1();
-     this.aaa = doSomething2() + doSomething3();
+     inject(this, doSomething2() + doSomething3());
      doSomething4();
    }
  }

Modifying an argument

Mixin:

@ModifyArg(method = "foo()V", at = @At(value = "INVOKE", target = "La/b/c/Something;doSomething(ZIII)V"), index = 2)
private int injected(int x) {
  return x * 3;
}

Result:

  public void foo() {
    doSomething1();
    Something something = new Something();
-   something.doSomething(true, 1, 4, 5);
+   something.doSomething(true, 1, injected(4), 5);
    doSomething2();
  }

Modifying multiple arguments

Mixin:

@ModifyArgs(method = "foo()V", at = @At(value = "INVOKE", target = "La/b/c/Something;doSomething(IDZ)V"))
private void injected(Args args) {
    int a0 = args.get(0);
    double a1 = args.get(1);
    boolean a2 = args.get(2);
    args.set(0, a0 + 3);
    args.set(1, a1 * 2.0D);
    args.set(2, !a2);
}

Result:

  public void foo() {
    doSomething1();
    Something something = new Something();
-   something.doSomething(3, 2.5D, true);
+   // Actually, synthetic subclass of Args is generated at runtime,
+   // but we omit the details to make it easier to understand the concept.
+   Args args = new Args(new Object[] { 3, 2.5D, true });
+   injected(args);
+   something.doSomething(args.get(0), args.get(1), args.get(2));
    doSomething2();
  }

Modifying a parameter

Mixin:

@ModifyVariable(method = "foo(ZIII)V", at = @At("HEAD"), ordinal = 1)
private int injected(int y) {
  return y * 3;
}

Result:

  public void foo(boolean b, int x, int y, int z) {
+   y = injected(y);
    doSomething1();
    doSomething2();
    doSomething3();
  }

Modifying a local variable on an assignment

Mixin:

@ModifyVariable(method = "foo()V", at = @At("STORE"), ordinal = 1)
private double injected(double x) {
  return x * 1.5D;
}

Result:

  public void foo() {
    int i0 = doSomething1();
    double d0 = doSomething2();
-   double d1 = doSomething3() + 0.8D;
+   double d1 = injected(doSomething3() + 0.8D);
    double d2 = doSomething4();
  }

Modifying a constant

Mixin:

@ModifyConstant(method = "foo()V", constant = @Constant(intValue = 4))
private int injected(int value) {
  return ++value;
}

Result:

  public void foo() {
-   for (int i = 0; i < 4; i++) {
+   for (int i = 0; i < injected(4); i++) {
      doSomething(i);
    }
  }

Table of Contents

Mixin Examples

Mixing into a private inner class

Access the this instance of the class your mixin is targeting

Injecting into the head of a static block

Injecting into the head of a method

Injecting into the tail of a method

Injecting into the returns of a method

Injecting into the point before a method call

Injecting into the point after a method call

Injecting into the point with shift amount

Injecting with a slice

Injecting and cancelling

Injecting and cancelling with a return value

Capturing local values

Capture locals without MixinExtras

Capture locals with MixinExtras

Capturing one of multiple locals of a type

Modifying locals

Modifying a return value

Redirecting a method call

Redirecting a get field

Redirecting a put field

Modifying an argument

Modifying multiple arguments

Modifying a parameter

Modifying a local variable on an assignment

Modifying a constant