@Autowired如何与JavaConfig

Question

我正在学习Spring Annotation，我想知道每个注释如何与xml配置相似。 无论如何，我想在没有任何xml文件的情况下使用Spring。所以我开始使用JavaConfig。

为了告诉Spring，我们正在使用JavaConfig，我们正在使用@Configuration。

现在我正在研究@Bean和@Autowired是如何工作的。

@Bean用于定义与xml配置类似的bean

<bean></bean>

下面是我的示例代码，它可以很好地测试@Configuration和@Bean：

            package com.example.service;

            public class Greeting {
               public Greeting(){
                   System.out.println("In Greeting Constructor");
               }

               public String getMessage(){
                   return "Hello Greeting Message";
               }
            }


            package com.example.config;

            import org.springframework.context.annotation.Bean;
            import org.springframework.context.annotation.Configuration;

            import com.example.service.Greeting;

            @Configuration
            public class AnotherConfig {
                @Bean
                public Greeting getGreet(){
                    return new Greeting();
                }
            }   


            package com.example;

            import org.springframework.context.annotation.AnnotationConfigApplicationContext;
            import com.example.config.AnotherConfig;
            import com.example.service.Greeting;

            public class HelloApp {
                public static void main(String[] args) {
                    AnnotationConfigApplicationContext context1 = new AnnotationConfigApplicationContext(AnotherConfig.class);
                    Greeting obj3 = (Greeting)context1.getBean("getGreet");
                    System.out.println("obj3 ("+obj3+")");
                    System.out.println(obj3.getMessage());
                }
            }

            Output:
            In Greeting Constructor
            obj3 (com.example.service.Greeting@3043fe0e)
            Hello Greeting Message

现在我想在上面的例子中使用@Autowired：

            package com.example.service;

            public class Greeting {
               public Greeting(){
                   System.out.println("In Greeting Constructor");
               }

               public String getMessage(){
                   return "Hello Greeting Message";
               }
            }


            package com.example.config;

            import org.springframework.beans.factory.annotation.Autowired;
            import org.springframework.context.annotation.Bean;
            import org.springframework.context.annotation.Configuration;

            import com.example.service.Greeting;

            @Configuration
            public class AnotherConfig {

                @Autowired
                Greeting greeting;

                @Bean
                public Greeting getGreet(){
                    return this.greeting;
                }
            }       


            package com.example;

            import org.springframework.context.annotation.AnnotationConfigApplicationContext;
            import com.example.config.AnotherConfig;
            import com.example.service.Greeting;

            public class HelloApp {
                public static void main(String[] args) {
                    AnnotationConfigApplicationContext context1 = new AnnotationConfigApplicationContext(AnotherConfig.class);
                    Greeting obj3 = (Greeting)context1.getBean("getGreet");
                    System.out.println("obj3 ("+obj3+")");
                    System.out.println(obj3.getMessage());
                }
            }

            Error: Not working!!!

            Oct 15, 2016 2:49:04 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh
            INFO: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@439f5b3d: startup date [Sat Oct 15 02:49:04 SGT 2016]; root of context hierarchy
            Oct 15, 2016 2:49:05 AM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
            INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@7d417077: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,anotherConfig,org.springframework.context.annotation.ConfigurationClassPostProcessor.importAwareProcessor,getGreet]; root of factory hierarchy
            Exception in thread "main" obj3 (null)
            java.lang.NullPointerException
                at com.example.HelloApp.main(HelloApp.java:12)

我用谷歌搜索了一个他们提到的例子@AnnotationDrivenConfig是@Autowired工作所必需的，（http://www.basilv.com/psd/blog/2009/java-based-configuration-of-spring-dependency-injection） 我使用的是Spring 3.2，但是@AnnotationDrivenConfig没有解析，似乎无法使用？!!

所以，我尝试过ComponentScan的其他选项：

            package com.example.service;

            import org.springframework.stereotype.Component;

            @Component
            public class Greeting {
               public Greeting(){
                   System.out.println("In Greeting Constructor");
               }

               public String getMessage(){
                   return "Hello Greeting Message";
               }
            }


            package com.example.config;

            import org.springframework.beans.factory.annotation.Autowired;
            import org.springframework.context.annotation.Bean;
            import org.springframework.context.annotation.ComponentScan;
            import org.springframework.context.annotation.Configuration;

            import com.example.service.Greeting;

            @Configuration
            @ComponentScan("com.example.service")
            public class AnotherConfig {

                @Autowired  
                @Bean
                public Greeting getGreet(Greeting greeting){
                    return greeting;
                }
            }   


            package com.example;

            import org.springframework.context.annotation.AnnotationConfigApplicationContext;
            import com.example.config.AnotherConfig;
            import com.example.service.Greeting;

            public class HelloApp {
                public static void main(String[] args) {
                    AnnotationConfigApplicationContext context1 = new AnnotationConfigApplicationContext(AnotherConfig.class);
                    Greeting obj3 = (Greeting)context1.getBean("getGreet");
                    System.out.println("obj3 ("+obj3+")");
                    System.out.println(obj3.getMessage());
                }
            }

            Output: Working with construtor injection

            In Greeting Constructor
            obj3 (com.example.service.Greeting@7a9273a8)
            Hello Greeting Message

但为什么同样的事情不适用于属性设置器？

            package com.example.config;

            import org.springframework.beans.factory.annotation.Autowired;
            import org.springframework.context.annotation.Bean;
            import org.springframework.context.annotation.ComponentScan;
            import org.springframework.context.annotation.Configuration;

            import com.example.service.Greeting;

            @Configuration
            @ComponentScan("com.example.service")
            public class AnotherConfig {

                @Autowired  
                Greeting greeting;

                @Bean
                public Greeting getGreet(){
                    return this.greeting;
                }
            }   

            Error:
            Oct 15, 2016 2:54:41 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh
            INFO: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@439f5b3d: startup date [Sat Oct 15 02:54:41 SGT 2016]; root of context hierarchy
            Oct 15, 2016 2:54:42 AM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
            INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@1134affc: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,anotherConfig,org.springframework.context.annotation.ConfigurationClassPostProcessor.importAwareProcessor,greeting,getGreet]; root of factory hierarchy
            In Greeting Constructor
            obj3 (null)
            Exception in thread "main" java.lang.NullPointerException
                at com.example.HelloApp.main(HelloApp.java:12)