Spring启动java.lang.NullPointerException：null

Question

我正在尝试Spring-boot使用 Hibernate 和 REST -API 构建 CRUD 应用程序。但是，当我尝试运行该应用程序时，一切正常，但控制台显示以下错误

java.lang.NullPointerException: null
    at io.javabrains.EmployerController.getAllEmployers(EmployerController.java:20) ~[classes/:na]

我尝试更改该值，但没有成功

雇主服务.java

package io.javabrains;

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

import org.springframework.stereotype.Service;

import io.javabrains.Entity.Employer;

@Service
public class EmployerService {

    private Repository repository;

    public List<Employer>getAllEmployers(){
        List<Employer>employers = new ArrayList<>();
        repository.findAll()
        .forEach(employers::add);
        return employers;

    }

    public void addEmployer(Employer employer) {
        repository.save(employer);
    }    
}

雇主控制器.java

package io.javabrains;

import java.util.List;

import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

import io.javabrains.Entity.Employer;

@RestController
public class EmployerController {

    private EmployerService service;


     @RequestMapping("/employer") 
     public List<Employer>getAllEmployers()
     {
         return  service.getAllEmployers();
}
    /*
     * @RequestMapping("/employer/{id}") public Employer getEmployer(@PathVariable
     * int id) { return service.getEmployer(id); }
     */

    @RequestMapping(method=RequestMethod.POST,value="/employer/create")
    public void addEmployer(@RequestBody Employer employer) {
        service.addEmployer(employer);  
    }
}

....

Answer 1

在分析给定的代码片段时，发生了空指针异常，因为您的代码没有要求 spring 依赖注入器将其 EmployerService作为依赖项注入到EmployerController，因此它不会将EmployerServicebean 类注入到引用中private EmployerService employerService;，因此它在 中为 null EmployerController。您可以通过在中添加@Autowire注释引用来要求 Spring 依赖注入器注入依赖项private EmployerService service;EmployerController

将您的更新EmployerService为以下内容即可

package io.javabrains;

import java.util.List;

import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

import io.javabrains.Entity.Employer;

@RestController
public class EmployerController {

  //UPDATE : Autowiring
  @Autowired
  private EmployerService employerService;


  @RequestMapping("/employer")
  public List < Employer > getAllEmployers() {
    return service.getAllEmployers();
  }
  /*
   * @RequestMapping("/employer/{id}") public Employer getEmployer(@PathVariable
   * int id) { return employerService.getEmployer(id); }
   */

  @RequestMapping(method = RequestMethod.POST, value = "/employer/create")
  public void addEmployer(@RequestBody Employer employer) {
    employerService.addEmployer(employer);
  }
}

而且，Service尝试访问时也会出现同样的问题repository也会出现同样的问题。

更新EmployeeService.java代码包含@autorwired逻辑：

package io.javabrains;

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

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

import io.javabrains.Entity.Employer;

@Service
public class EmployerService {

    @Autowired
    private Repository repository;

    public List<Employer>getAllEmployers(){
        List<Employer>employers = new ArrayList<>();
        repository.findAll()
        .forEach(employers::add);
        return employers;

    }

    public void addEmployer(Employer employer) {
        repository.save(employer);
    }    
}