官方推薦
在官方文檔中， Spring官方建議我們正確使用“default” 包，將Main Application Class放置在“default” 包下。推薦結構如下：
com
+- example
+- myapplication
+- Application.java
|
+- customer
| +- Customer.java
| +- CustomerController.java
| +- CustomerService.java
| +- CustomerRepository.java
|
+- order
+- Order.java
+- OrderController.java
+- OrderService.java
+- OrderRepository.java在這個結構的基礎上，Main函數強烈建議放置在com.example.myapplication這個"default" 包路徑下。這種情況下，Main方法的類只需要如下書寫即可：
package com.example.myapplication;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
@SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
}原因探究
這個結構有什麼好處呢？可以看到上面啓動類中，類上面只有一個註解@SpringBootApplication，這個註解提供了多個能力。請看下面的代碼：
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {...}官方文檔對@SpringBootApplication有做比較詳細的介紹，點這裏查看。
@EnableAutoConfiguration: 開啓自動配置。@ComponentScan: 開啓包路徑掃描，默認情況下會掃描當前包和子包的@Service,@Controller，@Component等註解。@Configuration: 允許在上下文中註冊其他的Bean或者導入其他配置類。
強調Main Class一定要放在“default” 包下，就是因爲只有這樣，@ComponentScan才能掃描到所有的註解。
再看@SpringBootApplication的內容代碼，使用@SpringBootApplication的屬性exclude,excludeName,scanBasePackages, scanBasePackageClasses, nameGenerator, proxyBeanMethods就可以完成@EnableAutoConfiguration, @ComponentScan以及@Configuration的屬性自定義。
/**
* Exclude specific auto-configuration classes such that they will never be applied.
* @return the classes to exclude
*/
@AliasFor(annotation = EnableAutoConfiguration.class)
Class<?>[] exclude() default {};
/**
* Exclude specific auto-configuration class names such that they will never be
* applied.
* @return the class names to exclude
* @since 1.3.0
*/
@AliasFor(annotation = EnableAutoConfiguration.class)
String[] excludeName() default {};
/**
* Base packages to scan for annotated components. Use {@link #scanBasePackageClasses}
* for a type-safe alternative to String-based package names.
* <p>
* <strong>Note:</strong> this setting is an alias for
* {@link ComponentScan @ComponentScan} only. It has no effect on {@code @Entity}
* scanning or Spring Data {@link Repository} scanning. For those you should add
* {@link org.springframework.boot.autoconfigure.domain.EntityScan @EntityScan} and
* {@code @Enable...Repositories} annotations.
* @return base packages to scan
* @since 1.3.0
*/
@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
String[] scanBasePackages() default {};
/**
* Type-safe alternative to {@link #scanBasePackages} for specifying the packages to
* scan for annotated components. The package of each class specified will be scanned.
* <p>
* Consider creating a special no-op marker class or interface in each package that
* serves no purpose other than being referenced by this attribute.
* <p>
* <strong>Note:</strong> this setting is an alias for
* {@link ComponentScan @ComponentScan} only. It has no effect on {@code @Entity}
* scanning or Spring Data {@link Repository} scanning. For those you should add
* {@link org.springframework.boot.autoconfigure.domain.EntityScan @EntityScan} and
* {@code @Enable...Repositories} annotations.
* @return base packages to scan
* @since 1.3.0
*/
@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
Class<?>[] scanBasePackageClasses() default {};
/**
* The {@link BeanNameGenerator} class to be used for naming detected components
* within the Spring container.
* <p>
* The default value of the {@link BeanNameGenerator} interface itself indicates that
* the scanner used to process this {@code @SpringBootApplication} annotation should
* use its inherited bean name generator, e.g. the default
* {@link AnnotationBeanNameGenerator} or any custom instance supplied to the
* application context at bootstrap time.
* @return {@link BeanNameGenerator} to use
* @see SpringApplication#setBeanNameGenerator(BeanNameGenerator)
* @since 2.3.0
*/
@AliasFor(annotation = ComponentScan.class, attribute = "nameGenerator")
Class<? extends BeanNameGenerator> nameGenerator() default BeanNameGenerator.class;
/**
* Specify whether {@link Bean @Bean} methods should get proxied in order to enforce
* bean lifecycle behavior, e.g. to return shared singleton bean instances even in
* case of direct {@code @Bean} method calls in user code. This feature requires
* method interception, implemented through a runtime-generated CGLIB subclass which
* comes with limitations such as the configuration class and its methods not being
* allowed to declare {@code final}.
* <p>
* The default is {@code true}, allowing for 'inter-bean references' within the
* configuration class as well as for external calls to this configuration's
* {@code @Bean} methods, e.g. from another configuration class. If this is not needed
* since each of this particular configuration's {@code @Bean} methods is
* self-contained and designed as a plain factory method for container use, switch
* this flag to {@code false} in order to avoid CGLIB subclass processing.
* <p>
* Turning off bean method interception effectively processes {@code @Bean} methods
* individually like when declared on non-{@code @Configuration} classes, a.k.a.
* "@Bean Lite Mode" (see {@link Bean @Bean's javadoc}). It is therefore behaviorally
* equivalent to removing the {@code @Configuration} stereotype.
* @since 2.2
* @return whether to proxy {@code @Bean} methods
*/
@AliasFor(annotation = Configuration.class)
boolean proxyBeanMethods() default true;小結
Spring Boot官方推薦的工程結構在大部分情況下都適用，用這個結構也是最保險的。如果有特殊情況，務必要記得配置@ComponentScan的包路徑。