Why is @Required set for setters and not fields?

If I have the following class:

package com.example;

import org.springframework.beans.factory.annotation.Required;

public class Customer 
{
    private Person person;

    public Person getPerson() {
        return person;
    }
    @Required
    public void setPerson(Person person) {
        this.person = person;
    }
}

Why in Spring is the @Required tag tied to the setter, and not the field, like this?

package com.example;

import org.springframework.beans.factory.annotation.Required;

public class Customer 
{
    @Required
    private Person person;

    public Person getPerson() {
        return person;
    }
    public void setPerson(Person person) {
        this.person = person;
    }
}

The dependency is that the property is set, but doesn't need to be tied down to one specific setter that populates that property or field, unless I'm missing something.

Solution

Traditionally a bean had properties defined as a getter and/or setter. The private field can be left out (sometimes name "virtual" field). In the new Java EE standard beans are more loose, and might also have a property defined as public field.

Nevertheless Spring pre-dates the latest standard, and having the annotation on a method i.o. a field has the advantage that access can be intercepted via AOP (simple byte code manipulation).