spring boot 自定义starter

    spring boot提供了一系列的starter pom来简化maven的依赖加载，例如spring-boot-starter-web、spring-boot-starter-test等。

    在咱们定制本身的starter前先了解一下必须知道的概念。

一、@Enable*注释

    @Enable*注释并非新发明的注释，早在Spring 3框架就引入了这些注释，用这些注释替代XML配置文件。不少Spring开发者都知道@EnableTransactionManagement注释，它可以声明事务管理；@EnableWebMvc注释，它能启用Spring MVC；以及@EnableScheduling注释，它能够初始化一个调度器。 这些注释事实上都是简单的配置，经过@Import注释导入。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import({ EnableAutoConfigurationImportSelector.class,
        AutoConfigurationPackages.Registrar.class })
public @interface EnableAutoConfiguration {

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     */
    Class<?>[] exclude() default {};

}

  EnableAutoConfigurationImportSelector类使用了Spring Core包的SpringFactoriesLoader类的loadFactoryNamesof()方法。

    SpringFactoriesLoader会查询META-INF/spring.factories文件中包含的JAR文件。 

    当找到spring.factories文件后，SpringFactoriesLoader将查询配置文件命名的属性。在例子中，是org.springframework.boot.autoconfigure.EnableAutoConfiguration。 

    让咱们来看看spring-boot-autoconfigure JAR文件，它真的包含了一个spring.factories文件，内容以下：

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JmsTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration

    在这个文件中，能够看到一系列Spring Boot自动配置的列表。下面咱们来看这些配置的细节，以MongoAutoConfiguration为例：

@Configuration
@ConditionalOnClass(Mongo.class)
@EnableConfigurationProperties(MongoProperties.class)
public class MongoAutoConfiguration {

    @Autowired
    private MongoProperties properties;

    private Mongo mongo;

    @PreDestroy
    public void close() throws UnknownHostException {
        if (this.mongo != null) {
            this.mongo.close();
        }
    }

    @Bean
    @ConditionalOnMissingBean
    public Mongo mongo() throws UnknownHostException {
        this.mongo = this.properties.createMongoClient();
        return this.mongo;
    }

}

    这个类进行了简单的Spring配置，声明了MongoDB所需典型Bean。 
    这个类跟其它不少类同样，重度依赖于Spring Boot注释： 

• @ConditionOnClass激活一个配置，在类路径中只能存在一到几个这样的类。 
• @EnableConfigurationProperties自动映射一个POJO到Spring Boot配置文件（默认是application.properties文件）的属性集。 
• @ConditionalOnMissingBean启用一个Bean定义，但必须是这个Bean以前未定义过才有效。 
  还可使用@ AutoConfigureBefore注释、@AutoConfigureAfter注释来定义这些配置类的载入顺序。

二、@Conditional注释

    Spring Boot的强大之处在于使用了Spring 4框架的新特性：@Conditional注释，此注释使得只有在特定条件知足时才启用一些配置。 
    在Spring Boot的org.springframework.boot.autoconfigure.condition包中说明了使用@Conditional注释能给咱们带来什么，下面对这些注释作一个概述：

• @ConditionalOnBean
• @ConditionalOnClass
• @ConditionalOnExpression
• @ConditionalOnMissingBean
• @ConditionalOnMissingClass
• @ConditionalOnNotWebApplication
• @ConditionalOnResource
• @ConditionalOnWebApplication

    以@ConditionalOnExpression注释为例，它容许在Spring的EL表达式中写一个条件。

@Conditional(OnExpressionCondition.class)
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE, ElementType.METHOD })
public @interface ConditionalOnExpression {

    /**
     * The SpEL expression to evaluate. Expression should return {@code true} if the
     * condition passes or {@code false} if it fails.
     */
    String value() default "true";

}

在这个类中，咱们想利用@Conditional注释，条件在OnExpressionCondition类中定义：

public class OnExpressionCondition extends SpringBootCondition {

    @Override
    public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
        // ...
        // we first get a handle on the EL context via the ConditionContext

        boolean result = (Boolean) resolver.evaluate(expression, expressionContext);

        // ...
        // here we create a message the user will see when debugging

        return new ConditionOutcome(result, message.toString());
    }
}

在最后，@Conditional经过简单的布尔表达式（即ConditionOutcome方法）来决定。

 

三、应用程序上下文初始化器

    spring.factories还提供了第二种可能性，即定义应用程序的初始化。这使得咱们能够在应用程序载入前操纵Spring的应用程序上下文ApplicationContext。 
  特别是，能够在上下文建立监听器，使用ConfigurableApplicationContext类的addApplicationListener()方法。 
    AutoConfigurationReportLoggingInitializer监听到系统事件时，好比上下文刷新或应用程序启动故障之类的事件，Spring Boot能够执行一些工做。这有助于咱们以调试模式启动应用程序时建立自动配置的报告。 
    要以调试模式启动应用程序，可使用-Ddebug标识，或者在application.properties文件这添加属性debug= true。

四、调试Spring Boot自动配置

 Spring Boot的官方文档（http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#howto-troubleshoot-auto-configuration）有助于理解自动配置期间发生了什么。当以调试模式运行时，Spring Boot会产生一个报告，以下：

Positive matches:
-----------------

   MessageSourceAutoConfiguration
      - @ConditionalOnMissingBean (types: org.springframework.context.MessageSource; SearchStrategy: all) found no beans (OnBeanCondition)

   JmxAutoConfiguration
      - @ConditionalOnClass classes found: org.springframework.jmx.export.MBeanExporter (OnClassCondition)
      - SpEL expression on org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration: ${spring.jmx.enabled:true} (OnExpressionCondition)
      - @ConditionalOnMissingBean (types: org.springframework.jmx.export.MBeanExporter; SearchStrategy: all) found no beans (OnBeanCondition)

   DispatcherServletAutoConfiguration
      - found web application StandardServletEnvironment (OnWebApplicationCondition)
      - @ConditionalOnClass classes found: org.springframework.web.servlet.DispatcherServlet (OnClassCondition)


Negative matches:
-----------------

   DataSourceAutoConfiguration
      - required @ConditionalOnClass classes not found: org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType (OnClassCondition)

   DataSourceTransactionManagerAutoConfiguration
      - required @ConditionalOnClass classes not found: org.springframework.jdbc.core.JdbcTemplate,org.springframework.transaction.PlatformTransactionManager (OnClassCondition)

   MongoAutoConfiguration
      - required @ConditionalOnClass classes not found: com.mongodb.Mongo (OnClassCondition)

   FallbackWebSecurityAutoConfiguration
      - SpEL expression on org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration: !${security.basic.enabled:true} (OnExpressionCondition)

   SecurityAutoConfiguration
      - required @ConditionalOnClass classes not found: org.springframework.security.authentication.AuthenticationManager (OnClassCondition)

   EmbeddedServletContainerAutoConfiguration.EmbeddedJetty
      - required @ConditionalOnClass classes not found: org.eclipse.jetty.server.Server,org.eclipse.jetty.util.Loader (OnClassCondition)

   WebMvcAutoConfiguration.WebMvcAutoConfigurationAdapter#localeResolver
      - @ConditionalOnMissingBean (types: org.springframework.web.servlet.LocaleResolver; SearchStrategy: all) found no beans (OnBeanCondition)
      - SpEL expression: '${spring.mvc.locale:}' != '' (OnExpressionCondition)

   WebSocketAutoConfiguration
      - required @ConditionalOnClass classes not found: org.springframework.web.socket.WebSocketHandler,org.apache.tomcat.websocket.server.WsSci (OnClassCondition)

    对于每一个自动配置，能够看到它启动或失败的缘由。

    Spring Boot经过配置信息指出：特定配置项被选中的缘由、列出匹配到对应类的配置项（positive match）、不包括某个配置项的缘由（negative match）。

-------------demo-----------

五、建立starter项目

5.1 建立一个enable注解

package com.aa.cloud.annotation;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * @ClassName: EnableDemo 
 * @Description:  
 * @author weiyb 
 * @date 2017年9月20日 下午4:19:19 
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
public @interface EnableDemo {

}

5.2 服务类

package com.aa.cloud.service;

/**
 * @ClassName: UserService 
 * @Description:  
 * @author weiyb 
 * @date 2017年9月20日 下午4:08:51 
 */
public class DemoService {
	public void demoStarter() {
		System.out.println("hello world");
	}
}

5.3 自动配置类

package com.aa.cloud.auto.config;

import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import com.aa.cloud.annotation.EnableDemo;
import com.aa.cloud.service.DemoService;

/**
 * @ClassName: DemoAutoConfiguration 
 * @Description:  
 * @author weiyb 
 * @date 2017年9月20日 下午4:09:07 
 */
@Configuration
@ConditionalOnBean(annotation = EnableDemo.class)//条件判断，只有在使用了@EnableUser时，才会启动
public class DemoAutoConfiguration {
	@Bean
	public DemoService dbCountRunner() {
		return new DemoService();
	}
}

5.4 配置META-INF/spring.factories

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.aa.cloud.auto.config.DemoAutoConfiguration

六、建立demo starter test项目

6.1 启动类

package com.cloud;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

import com.aa.cloud.annotation.EnableDemo;

/**
 * @ClassName: ProviderApplication 
 * @Description:  
 * @author weiyb 
 * @date 2017年9月20日 下午4:08:42 
 */
@SpringBootApplication
@EnableDemo
public class ProviderApplication {
	public static void main(String[] args) {
		SpringApplication.run(ProviderApplication.class, args);
	}
}

6.2 测试类

package com.demo.starter.test;

import javax.annotation.Resource;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;

import com.aa.cloud.service.DemoService;
import com.cloud.ProviderApplication;

/**
 * @ClassName: DemoServiceTest 
 * @Description:  
 * @author weiyb 
 * @date 2017年9月20日 下午4:08:35 
 */
@RunWith(value = SpringJUnit4ClassRunner.class)
@SpringBootTest(classes = ProviderApplication.class)
public class DemoServiceTest {

	@Resource
	private DemoService userService;

	@Test
	public void demoStarter() {
		userService.demoStarter();
	}
}