Field injection is not recommended

Reference documents ：https://www.cnblogs.com/wang-yaz/p/9340156.html
Reference documents ：http://www.pianshen.com/article/5023380349/
First of all, we need to understand the use @Autowired Three ways to inject :

@Service("accountEmailService")
 2 public class AccountEmailServiceImpl implements AccountEmailService{
 3 
 4 
 5     /**   Injection by constructor ---begin  **/
 6     private JavaMailSender javaMailSender;
 7 
 8     @Autowired
 9     public AccountEmailServiceImpl(JavaMailSender javaMailSender){
10         this.javaMailSender = javaMailSender;
11     }
12  
13 
14     /**  adopt set Methods to inject ---begin **/
15     private JavaMailSender javaMailSender;
16     @Autowired
17     public void setJavaMailSender(JavaMailSender javaMailSender){
18         this.javaMailSender = javaMailSender;
19     }
20    
21 
22     /**  adopt field Inject  **/
23     @Autowired
24     private JavaMailSender javaMailSender;
25 

Spring It is recommended to use constructor injection ：
Spring Team recommends: “Always use constructor based dependency injection in your beans. Always use assertions for mandatory dependencies”.
because field Injection may occur NullPointerException

class MyComponent {

  @Inject MyCollaborator collaborator;

  public void myBusinessMethod() {
    collaborator.doSomething(); // -> NullPointerException
  }
}

However, constructor injection is not abandoned because the code is not concise enough . In fact, we have been using attribute injection without error . So there are two solutions ：
The first one is ： take “@Autowired” Replace notes with “@Resouce” annotation .
@Resource yes JSR-250 Notes in the specification , at present Spring The specification is supported [email protected] It's based on Bean To match the name of Bean, If you can't get it, match it according to the type ; and @Autowired Match according to type , Passing the name requires Spring Of @Qualifier Annotations to cooperate .

The second kind ： modify setting In the face field Inspection of injection :setting->editor->code style->inspection-> spring->spring core-> code, Remove the hook ：