從服務類呼叫方法時，SpringBoot@AutowiredNullPointerException

我正在學習 Spring Boot。我試圖通過呼叫它的方法從服務類的 ArrayList 中獲取一個值。但是我得到了一個 NullPointerException。我添加了@Autowired 和@Service 注釋，它不起作用。我還查找了一些有關 Loc 容器的主題，但似乎沒有幫助。

這是我的主控制器：

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";
    }
}

這是我的服務類：

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;
    }
}

這是模型類產品：

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  
                '}';
    }
}

編輯：能夠通過在 MainController 中添加建構式來修復它

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

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

uj5u.com熱心網友回復：

在注入依賴之前呼叫服務方法。這就是您獲得 NPE 的原因。嘗試使用建構式依賴注入

@Controller
public class MainController {

public MainController(ProductService productService) {
    this.productService = productService;
}

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";
}
}

或者在控制器方法里面呼叫service方法

@Controller
public class MainController {

@Autowired
private ProductService productService;

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

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

uj5u.com熱心網友回復：

您的產品永遠不會在產品服務類中初始化，我建議您在 ProductService 類中嘗試這個

    @Service
   public class ProductService {
   ArrayList<Product> products;
 ProductService()
{
        this.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;
                         } 
                    }

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