Consider the following example:

    String color()
}

interface ColorPicker {
    fun color(): String
}

Given a common interface called ColorPicker that is implemented by multiple classes.

The Primary Bean

import io.micronaut.context.annotation.Primary;
import javax.inject.Singleton;
@Primary
@Singleton
class Green implements ColorPicker {
    @Override
    public String color() {
        return "green";
    }
}

The Primary Bean

import io.micronaut.context.annotation.Primary
import javax.inject.Singleton
@Primary
@Singleton
class Green: ColorPicker {
    override fun color(): String {
        return "green"
    }

The Green bean is a ColorPicker, but is annotated with @Primary.

Another Bean of the Same Type

@Singleton
public class Blue implements ColorPicker {
    @Override
    public String color() {
        return "blue";
    }
}

import javax.inject.Singleton
@Singleton
class Blue implements ColorPicker {
    @Override
    String color() {
        return "blue"
    }
}

Another Bean of the Same Type

The Blue bean is also a ColorPicker and hence you have two possible candidates when injecting the ColorPicker interface. Since Green is the primary it will always be favoured.

@Controller("/testPrimary")
public class TestController {
    protected final ColorPicker colorPicker;
    public TestController(ColorPicker colorPicker) { (1)
    @Get
    public String index() {
        return colorPicker.color();
    }
}

@Controller("/test")
class TestController {
    protected final ColorPicker colorPicker
    TestController(ColorPicker colorPicker) { (1)
        this.colorPicker = colorPicker
    }
    @Get
    String index() {
        colorPicker.color()
    }
}

@Controller("/test")
class TestController(val colorPicker: ColorPicker) { (1)
    @Get
    fun index(): String {
        return colorPicker.color()
    }

If multiple possible candidates are present and no @Primary is defined then a will be thrown.