@Detailed explanation of postconstruct annotation

Initialization mode one ：@PostConstruct annotation

Assuming that the class UserController There's a member variable UserService By **@Autowired** modification , that UserService The injection is in UserController After the construction method of .

If you want to UserController Object generation completes some initialization operations , But these initialization operations depend on the injected objects , Then it can't be implemented in the constructor （ps：spring Initialization exception at startup ）, for example ：

public class UserController {
	@Autowired
	private UserService userService;

	public UserController() {
		//  call userService Custom initialization method for , here userService by null, Report errors 
		userService.userServiceInit();
	}
}

therefore , have access to @PostConstruct Annotation to complete initialization ,@PostConstruct The method of annotation will be in UserService Automatically called after injection .

public class UserController {
	@Autowired
	private UserService userService;

	public UserController() {
	}

	//  Initialization method 
	@PostConstruct
	public void init(){
		userService.userServiceInit();
	}
}

** summary ：** Class initialization call sequence ：

（1） Construction method Constructor

（2）@Autowired

（3）@PostConstruct

Initialization mode two ： Realization InitializingBean Interface

In addition to using annotations to complete initialization , It can also be achieved by InitializingBean Complete class initialization

public class UserController implements InitializingBean {
	@Autowired
	private UserService userService;

	public UserController() {
	}

	//  Initialization method 
	@Override
	public void afterPropertiesSet() throws Exception {
		userService.userServiceInit();
	}
}

The more common ones are SqlSessionFactoryBean, It is achieved through InitializingBean Finished initializing .

@Override
public void afterPropertiesSet() throws Exception {
 // buildSqlSessionFactory() Is the core method to complete initialization , Must be executed after the constructor call 
 this.sqlSessionFactory = buildSqlSessionFactory(); 
}