java - org.springframework.beans.BeanInstantiationException : Failed to instantiate

Question

我刚刚开始学习Spring Boot。我的代码中有一个错误，内容为

Error creating bean with name 'alien' defined in file[E:\Programing\Java\boot\Project1\target\classes\com\example\demo\Alien.class]:Instantiation of bean failed; nested exception isorg.springframework.beans.BeanInstantiationException: Failed toinstantiate [com.example.demo.Alien]: Constructor threw exception;nested exception is java.lang.NullPointerException: Cannot invoke"com.example.demo.Laptop.compile()" because "this.laptop" is null

下面是我的代码

主类

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;

@SpringBootApplication
public class Project1Application {

    public static void main(String[] args) {
        ConfigurableApplicationContext context= SpringApplication.run(Project1Application.class, args);
        
        Alien a=context.getBean(Alien.class);
        a.show();
    }

}

外星人等级

package com.example.demo;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Component;

@Component
public class Alien {

    private int id;
    private String name;
    private String tech;
    @Autowired
    private Laptop laptop;
    
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getTech() {
        return tech;
    }
    public void setTech(String tech) {
        this.tech = tech;
    }
    
    public void show() {
        System.out.println("in show..........");
    }
    public Alien() {
        super();
        System.out.println("Object Created......");
        laptop.compile();
    }
    
}

笔记本电脑类

package com.example.demo;

import org.springframework.stereotype.Component;

@Component
public class Laptop {

    private int lid;
    private String brand;
    
    @Override
    public String toString() {
        return "Laptop [lid=" + lid + ", brand=" + brand + "]";
    }
    public int getLid() {
        return lid;
    }
    public void setLid(int lid) {
        this.lid = lid;
    }
    public String getBrand() {
        return brand;
    }
    public void setBrand(String brand) {
        this.brand = brand;
    }
    
    public void compile() {
        System.out.println("Compiling..");
    }
    
}

Answer 1

问题是您在 Alien 的构造函数中调用 laptop.compile()。当时，Spring 还没有注入(inject) Laptop bean，因此 this.laptop 为 null。

有几种方法可以解决这个问题。

首先，您可以将 laptop.compile() 移至一个单独的方法，并用 @PostConstruct 进行注释。例如:

@Component
public class Alien {
    private int id;
    private String name;
    private String tech;
    @Autowired
    private Laptop laptop;
    
    // Getters + Setters
    
    public void show() {
        System.out.println("in show..........");
    }

    // Add this method
    @PostConstruct
    public void initialize() {
        laptop.compile();
    }

    public Alien() {
        super();
        System.out.println("Object Created......");
    }
}

---

或者，您可以用构造函数注入(inject)替换字段注入(inject):

@Component
public class Alien {
    private int id;
    private String name;
    private String tech;
    private Laptop laptop; // Remove @Autowired
    
    // Getters + Setters
    
    public void show() {
        System.out.println("in show..........");
    }


    @Autowired // Put @Autowired here
    public Alien(Laptop laptop) {
        super();
        this.laptop = laptop;
        System.out.println("Object Created......");
        this.laptop.compile();
    }
}

如今，构造函数注入(inject)是注入(inject) Bean 的首选方式，因此这将是一个可行的解决方案。请注意，即使我在构造函数顶部添加了 @Autowired 注释，但如果这是您唯一的构造函数，则没有必要。