Spring Boot 项目中@Autowireing 的评估顺序是什么答案

【问题标题】：What is order of evaluation for @Autowiring in SpringBoot projectSpring Boot 项目中@Autowireing 的评估顺序是什么
【发布时间】：2018-04-24 21:21:13
【问题描述】：

我正在尝试编写一个简单的 SprintBoot REST 控制器以在 Websphere Liberty 上运行，但 @Autowire 存在问题。以下是相关的（我认为）代码：

@CrossOrigin
@RestController
@RequestMapping(path = "userSetting")
public class RESTInterface {

    @Autowired
    private UserSettingDAO userSettingDAO;

    @RequestMapping(path = "/getUserSetting", method = { RequestMethod.GET }, produces = MediaType.APPLICATION_JSON_UTF8_VALUE)
    public String getUserSetting(
            @RequestParam(value="user_id", required=true ) String user_id, 
            @RequestParam(value="app_id", required=true )  String app_id,
            @RequestParam(value="dashboard_name", required=true ) String dashboard_name 
            ) {
        if ( userSettingDAO == null ) {
            System.out.println( "userSettingDAO is null" );
    ...

////////////////////////////////////////////////////////////////////////////////

@Component
 public class UserSettingDAO {

    @Autowired
    private JdbcTemplate jdbcTemplate;

    @Autowired
    private DataSource dataSource;

     public UserSettingDAO() {
        System.out.println( "dataSource is " + dataSource );
        System.out.println( "jdbcTemplate is " + jdbcTemplate );

当我将应用程序部署到 Liberty 时，它似乎可以正常启动。当我到达适当的端点时，我在console.log 中看到以下内容：

UserSettingDAO.jdbcTemplate = null
dataSource is null
jdbcTemplate is null
2018-04-24 16:54:19.887  INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/userSetting/getUserSetting],methods=[GET],produces=[application/json;charset=UTF-8]}" onto public java.lang.String com.ui.usersetting.restinterface.RESTInterface.getUserSetting(java.lang.String,java.lang.String,java.lang.String)
2018-04-24 16:54:19.890  INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/userSetting/upsertUserSetting],methods=[POST],produces=[application/json;charset=UTF-8]}" onto public void com.ui.usersetting.restinterface.RESTInterface.upsertUserSetting(java.lang.String,java.lang.String,java.lang.String,java.lang.String)
2018-04-24 16:54:19.891  INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/userSetting/deleteUserSetting],methods=[DELETE],produces=[application/json;charset=UTF-8]}" onto public void com.ui.usersetting.restinterface.RESTInterface.deleteUserSetting(java.lang.String,java.lang.String,java.lang.String)
2018-04-24 16:54:19.893  INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/userSetting/hello],methods=[GET],produces=[application/json;charset=UTF-8]}" onto public java.lang.String com.ui.usersetting.restinterface.RESTInterface.hello()
2018-04-24 16:54:19.924  INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error],produces=[text/html]}" onto public org.springframework.web.servlet.ModelAndView org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.errorHtml(javax.servlet.http.HttpServletRequest,javax.servlet.http.HttpServletResponse)
2018-04-24 16:54:19.925  INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error]}" onto public org.springframework.http.ResponseEntity<java.util.Map<java.lang.String, java.lang.Object>> org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.error(javax.servlet.http.HttpServletRequest)
2018-04-24 16:54:20.166  INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerAdapter : Looking for @ControllerAdvice: org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@6a5997f7: startup date [Tue Apr 24 16:54:15 EDT 2018]; root of context hierarchy
2018-04-24 16:54:21.386  INFO 8807 --- [cutor-thread-11] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup
2018-04-24 16:54:21.389  INFO 8807 --- [cutor-thread-11] o.s.j.e.a.AnnotationMBeanExporter        : Bean with name 'dataSource' has been autodetected for JMX exposure
2018-04-24 16:54:21.396  INFO 8807 --- [cutor-thread-11] o.s.j.e.a.AnnotationMBeanExporter        : Located MBean 'dataSource': registering with JMX server as MBean [com.zaxxer.hikari:name=dataSource,type=HikariDataSource]

我的问题是：为什么两个println 语句说它们各自的变量为空？这是否与 UserSettingDAO 构造函数出现在要执行的日志中在有关dataSource 的行出现在日志中的事实有关？

应该做什么才能正确初始化这些变量？

【问题讨论】：

您的 application.properties 中是否配置了spring.datasource.url？查看docs.spring.io/spring-boot/docs/current/reference/html/… 以供参考
是的，还有spring.datasource.driver-class-name、spring.datasource.username 和spring.datasource.password。
我真的建议在 Liberty 上使用 Java EE API，例如 JAX-RS、CDI、JPA，而不是 Spring Boot。它也通过工具得到更好的集成和支持。

标签： spring-boot websphere-liberty open-liberty

【解决方案1】：

在调用 Spring 组件的构造函数时，任何 @Autowired 对象仍将为 null。这是因为spring容器需要先初始化一个类的实例，然后才能将值注入@Autowired字段。

我不知道 Spring 是如何实现注入的，但大多数 Java 注入都是这样工作的：

构造 bean 类的新实例，通常通过调用默认 ctor（或初始化动态生成的子类代理）
获取需要注入bean的对象
使用反射/代理，在 bean 上设置字段

为了验证这一点，我在UserSettingsDAO 上创建了一个利用@AutoWired 字段的方法：

@Component
public class UserSettingDAO {

   @Autowired
   private JdbcTemplate jdbcTemplate;

   @Autowired
   private DataSource ds;

   public UserSettingDAO() {
       System.out.println("ctor/jdbcTemplate is " + jdbcTemplate );
       System.out.println("ctor/datasource is: " + ds);
   }

   public void doStuff() {
       System.out.println("doStuff/jdbcTemplate is " + jdbcTemplate );
       System.out.println("doStuff/datasource is: " + ds);
   }
}

如果我们将这个 DAO 注入到另一个类中并使用它，我们会看到在构造 UserSettingDAO 之后，这些字段被初始化：

@CrossOrigin
@RestController
public class RESTInterface {

    @Autowired
    private UserSettingDAO dao;

    @RequestMapping(path = "/jdbc", method = { RequestMethod.GET }, produces = MediaType.APPLICATION_JSON_UTF8_VALUE)
    public String jdbc() {
        return dao.doStuff();
    }
}

检查日志，这将产生以下输出：

ctor/jdbcTemplate is null
ctor/datasource is: null
doStuff/jdbcTemplate is org.springframework.jdbc.core.JdbcTemplate@4eb46bed
doStuff/datasource is: com.ibm.ws.rsadapter.jdbc.WSJdbcDataSource@727d23c5

【讨论】：