SpringBoot @Autowired从服务类调用方法时出现 NullPointerException

问：

我正在学习Spring Boot。我正在尝试通过调用其方法从服务类的 ArrayList 中获取值。但是我得到了一个NullPointerException。我同时添加了@Autowired和@Service注释，它不起作用。我还查找了一些有关 Loc 容器的主题，但似乎没有帮助。

这是我的MainController：

package com.example.ecommerce;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;

import java.util.ArrayList;

@Controller
public class MainController {



    @Autowired
    private ProductService productService;
    //I'm getting a NullPointerException here when I call the method "getAllProduct"
    ArrayList<Product> products = productService.getAllProduct();

    @GetMapping("/")
    public String home(){
        return "home";
    }

    @GetMapping("/products")
    public String product(Model model){
        model.addAttribute("products",products);
        return "product";
    }
}

这是我的服务类：

package com.example.ecommerce;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

@Service
public class ProductService {

    ArrayList<Product> products = new ArrayList<>(Arrays.asList(
            new Product("iPhone 4","Apple",1000),
            new Product("iPhone 5","Banana",2000),
            new Product("iPhone 6","Orange",3000)
    ));


    public ArrayList<Product> getAllProduct(){
        return products;
    }
}

下面是模型类 Product：

package com.example.ecommerce;


import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

public class Product {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
    private String name;
    private String brand;
    private double price;

    public Product() {
    }

    public Product(String name, String brand, double price) {
        this.name = name;
        this.brand = brand;
        this.price = price;
    }

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    @Override
    public String toString() {
        return "Product{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", brand='" + brand + '\'' +
                ", price=" + price +
                '}';
    }
}

编辑：能够通过在MainController中添加构造函数来修复它

    private final ProductService productService;
    private final ArrayList<Product> products;

    @Autowired
    public MainController(ProductService productService) {
        this.productService = productService;
        this.products = productService.getAllProduct();
    }