当接口作为参数提供时，java依赖项注入不起作用

Question

这是我的控制器类

@Controller
public class HomeController{

    @RequestMapping("/")
    public String home(MyTest test){
        test.draw();
        return "homePage";
    }
}

在将MyTest（接口）作为参数传递给home方法时，Spring不会注入其实现类，而是抛出异常

SEVERE: Servlet.service() for servlet [dispatcher] in context with path [/spring-mvc-demo] threw exception [Request processing failed; nested exception is java.lang.IllegalStateException: No primary or default constructor found for interface com.managers.MyTest] with root cause
java.lang.NoSuchMethodException: com.managers.MyTest.<init>()
    at java.lang.Class.getConstructor0(Unknown Source)
    at java.lang.Class.getDeclaredConstructor(Unknown Source)
    at org.springframework.web.method.annotation.ModelAttributeMethodProcessor.createAttribute(ModelAttributeMethodProcessor.java:209)
    at org.springframework.web.servlet.mvc.method.annotation.ServletModelAttributeMethodProcessor.createAttribute(ServletModelAttributeMethodProcessor.java:84)
    at org.springframework.web.method.annotation.ModelAttributeMethodProcessor.resolveArgument(ModelAttributeMethodProcessor.java:132)
    at org.springframework.web.method.support.HandlerMethodArgumentResolverComposite.resolveArgument(HandlerMethodArgumentResolverComposite.java:124)
    at org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues(InvocableHandlerMethod.java:161)
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:131)
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:102)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:871)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:777)
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:991)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:925)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:978)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:870)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:855)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:936)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1004)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1686)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

但是在直接传递实现类即MyTestImpl时，它工作得很好

@RequestMapping("/")
    public String home(MyTestImpl test){
        test.draw();
        return "homePage";
    }

如果出现接口，请在此说明异常原因。 下面是MyTest实现类

@Component
public class MyTestImpl implements MyTest{

    @Override
    public void draw() {
        System.out.println("inside test");

    }
}

春天。xml

<context:component-scan base-package="com.controllers,com.ManagerImpl" />
    <mvc:annotation-driven/>
    <bean
        class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/WEB-INF/view/" />
        <property name="suffix" value=".jsp" />
    </bean>

Answer 1

是您的组件扫描值覆盖接口所在的包。使用@Autowired注释进行注射

Answer 2

你所做的是正确的，只是你需要告诉Spring如何反序列化你的接口（也就是说，需要哪个具体的类实现）

    @JsonDeserialize(as=MyTestImpl.class)
    public interface MyTest  {
    ...

另外，假设您正在使用Post方法，请将控制器更改为：

@PostMapping(value = "/",
            consumes = MediaType.APPLICATION_JSON_UTF8_VALUE,
            produces = MediaType.APPLICATION_JSON_UTF8_VALUE)
    public String home(MyTest test){
        test.draw();
        return "homePage";
    }

（“products”参数当然可以是text/html或您返回的任何内容）

Answer 3

首先，我想指出，你们混合了不同的术语

如前所述，自动布线是spring的核心概念。如前所述，@Autowire应该在setters上完成，对于非配置bean*，下面是相关的短语：

Marks a constructor, field, setter method or config method as to be autowired by Spring's dependency injection facilities.

其次，这里您试图将bean（@Component）注入到MVC控制器中，更具体地说，是注入bean的接口

如果仔细查看文档（here），您会发现Spring支持各种参数，并且会尽可能地将它们传递给控制器方法

这里的问题是，Spring无法知道将什么传递给您的方法。 当调用控制器时，spring将看到这个参数属于最后一类，即^{：

如果一个方法参数与上述任何一个参数都不匹配，默认情况下，如果它是由BeanUtils#isSimpleProperty确定的简单类型，它将被解析为@RequestParam，否则将被解析为@ModelAttribute

因此，Spring会将其视为一个“模型属性”，但它无法知道如何映射它，因为接口不能被实例化，因此您的错误。p>

现在，根据您的问题，您可以：

作为@Controller的成员自动连接bean或接口，例如：

@Controller
public class HomeController{

    //might need @Qualifier if more than one implementation
    @Autowire
    private MyTest test; 

    @RequestMapping("/")
    public String home(){
        test.draw();
        return "homePage";
    }
}

或者将您的实现传递给您的方法控制器

*Although i haven't found anything clear in the spring documentation that states what clearly is a config method, i'm pretty sure that does not apply to @Controller methods.

Answer 4

你这样做是完全错误的。这是一个工作代码

@Controller
public class HomeController{
  private final MyTest test;

   @Autowired
   public HomeController(MyTest test) {
      this.test = test;
   }

   @RequestMapping("/")
   public String home(){
      test.draw();
      return "homePage";
   }
}

用@RequestMapping注释的方法的参数应该/将以Pathvariable或RequestParams的形式出现，或者仅以HttpServletRequest对象本身的形式出现。这不是自动关联实例的方式

依赖项注入在构造函数、字段和接口级别工作。不在方法参数级别。希望一切都清楚