這篇來先通過學習分析Spring Boot的執行原理後，根據已掌握的知識來自定義一個start pom。

      Spring Boot關於自動配置的原始碼在spring-boot-autoconfigure-xxx.jar內。若想檢視有哪些自動配置，可以檢視這裡的原始碼。可以通過下面幾種方式檢視當前專案中已啟用和未啟用的自動配置的報告。

     1）執行jar包時增加--debug引數：

      java -jar xx.jar  --debug

      2）在application中設定屬性，debug=true：

spring:
  profiles:
    active: prod



book:
  author: jack
  name: hello world

author:
  name: jack
  age: 18

debug: true
 

      此時啟動，可以在控制檯輸出。已啟用的自動配置為：

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

   DispatcherServletAutoConfiguration matched:
      - @ConditionalOnClass found required class 'org.springframework.web.servlet.DispatcherServlet'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
      - @ConditionalOnWebApplication (required) found StandardServletEnvironment (OnWebApplicationCondition)

   DispatcherServletAutoConfiguration.DispatcherServletConfiguration matched:
      - @ConditionalOnClass found required class 'javax.servlet.ServletRegistration'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
      - Default DispatcherServlet did not find dispatcher servlet beans (DispatcherServletAutoConfiguration.DefaultDispatcherServletCondition)

   DispatcherServletAutoConfiguration.DispatcherServletRegistrationConfiguration matched:
      - @ConditionalOnClass found required class 'javax.servlet.ServletRegistration'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
      - DispatcherServlet Registration did not find servlet registration bean (DispatcherServletAutoConfiguration.DispatcherServletRegistrationCondition)

   DispatcherServletAutoConfiguration.DispatcherServletRegistrationConfiguration#dispatcherServletRegistration matched:
      - @ConditionalOnBean (names: dispatcherServlet; types: org.springframework.web.servlet.DispatcherServlet; SearchStrategy: all) found beans 'dispatcherServlet', 'dispatcherServlet' (OnBeanCondition)

   EmbeddedServletContainerAutoConfiguration matched:
      - @ConditionalOnWebApplication (required) found StandardServletEnvironment (OnWebApplicationCondition)

   EmbeddedServletContainerAutoConfiguration.EmbeddedTomcat matched:
      - @ConditionalOnClass found required classes 'javax.servlet.Servlet', 'org.apache.catalina.startup.Tomcat'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
      - @ConditionalOnMissingBean (types: org.springframework.boot.context.embedded.EmbeddedServletContainerFactory; SearchStrategy: current) did not find any beans (OnBeanCondition)
 

   未啟用的自動配置為：

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

   ActiveMQAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'javax.jms.ConnectionFactory', 'org.apache.activemq.ActiveMQConnectionFactory' (OnClassCondition)

   AopAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'org.aspectj.lang.annotation.Aspect', 'org.aspectj.lang.reflect.Advice' (OnClassCondition)

   ArtemisAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'javax.jms.ConnectionFactory', 'org.apache.activemq.artemis.jms.client.ActiveMQConnectionFactory' (OnClassCondition)

   BatchAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'org.springframework.batch.core.launch.JobLauncher', 'org.springframework.jdbc.core.JdbcOperations' (OnClassCondition)

   CacheAutoConfiguration:
      Did not match:
         - @ConditionalOnBean (types: org.springframework.cache.interceptor.CacheAspectSupport; SearchStrategy: all) did not find any beans (OnBeanCondition)
      Matched:
         - @ConditionalOnClass found required class 'org.springframework.cache.CacheManager'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)

   CacheAutoConfiguration.CacheManagerJpaDependencyConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class 'org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean' (OnClassCondition)
         - Ancestor org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration did not match (ConditionEvaluationReport.AncestorsMatchedCondition)

   CaffeineCacheConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'com.github.benmanes.caffeine.cache.Caffeine', 'org.springframework.cache.caffeine.CaffeineCacheManager' (OnClassCondition)

   CassandraAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class 'com.datastax.driver.core.Cluster' (OnClassCondition)

   CassandraDataAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'com.datastax.driver.core.Cluster', 'org.springframework.data.cassandra.core.CassandraAdminOperations' (OnClassCondition)

   CassandraRepositoriesAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'com.datastax.driver.core.Session', 'org.springframework.data.cassandra.repository.CassandraRepository' (OnClassCondition)

   CloudAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class 'org.springframework.cloud.config.java.CloudScanConfiguration' (OnClassCondition)

   CouchbaseAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'com.couchbase.client.java.CouchbaseBucket', 'com.couchbase.client.java.Cluster' (OnClassCondition)

   CouchbaseCacheConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'com.couchbase.client.java.Bucket', 'com.couchbase.client.spring.cache.CouchbaseCacheManager' (OnClassCondition)

   CouchbaseDataAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'com.couchbase.client.java.Bucket', 'org.springframework.data.couchbase.repository.CouchbaseRepository' (OnClassCondition)

   CouchbaseRepositoriesAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'com.couchbase.client.java.Bucket', 'org.springframework.data.couchbase.repository.CouchbaseRepository' (OnClassCondition)

   DataSourceAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class 'org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType' (OnClassCondition)

   DataSourceTransactionManagerAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'org.springframework.jdbc.core.JdbcTemplate', 'org.springframework.transaction.PlatformTransactionManager' (OnClassCondition)

   DeviceDelegatingViewResolverAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class 'org.springframework.mobile.device.view.LiteDeviceDelegatingViewResolver' (OnClassCondition)

   DeviceResolverAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'org.springframework.mobile.device.DeviceResolverHandlerInterceptor', 'org.springframework.mobile.device.DeviceHandlerMethodArgumentResolver' (OnClassCondition)

   DispatcherServletAutoConfiguration.DispatcherServletConfiguration#multipartResolver:
      Did not match:
         - @ConditionalOnBean (types: org.springframework.web.multipart.MultipartResolver; SearchStrategy: all) did not find any beans (OnBeanCondition)

   EhCacheCacheConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'net.sf.ehcache.Cache', 'org.springframework.cache.ehcache.EhCacheCacheManager' (OnClassCondition)
 

一：運作原理

       關於Spring Boot的運作原理，我們還是迴歸到@SpringBootApplication註解上來，這個註解是一個組合註解，它的核心功能是由@EnableAutoConfiguration註解提供的，下面看看@EnableAutoConfiguration的註解的原始碼如下：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package org.springframework.boot.autoconfigure;

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;
import org.springframework.context.annotation.Import;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({EnableAutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {};

    String[] excludeName() default {};
}

    這裡關鍵功能是@Import註解匯入的配置功能，

EnableAutoConfigurationImportSelector

使用SpringFactoriesLoader.loadFactoryNames方法來掃描具有META-INF/spring.factories檔案的jar包，spring-boot-autoconfigure-xxx.jar裡就有一個spring.factories檔案，此檔案聲明瞭有哪些自動配置，如下：

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

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnClassCondition

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\
org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\
org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\
org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,\
org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,\
org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,\
org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,\
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\
org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration

# Failure analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer

# Template availability providers
org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.web.JspTemplateAvailabilityProvider

二：核心註解

      開啟上面任意一個AutoConfiguration檔案，一般都有下面的條件註解，在spring-boot-autoconfigure-xxx.jar的org.springframework.boot.autocinfigure.condition包下，條件註解如下：

      @ConditionalOnBean：當容器裡有指定的Bean的條件下

      @ConditionalOnClass：當類路徑下有指定的類的條件下

      @ConditionalOnExpression：基於SpEL表示式作為判斷條件

      @ConditionalOnJava：基於JVM版本作為判斷條件

      @ConditionalOnJndi：在JNDI存在的條件下查詢指定的位置

      @@ConditionalOnMissingBean：當容器裡沒有指定Bean的情況下

     @ConditionalOnMissingClass：當類路徑下沒有指定的類的條件下

      @ConditionalOnNotWebApplication：當前專案不是Web專案的條件下

      @ConditionalOnProperty：指定的屬性是否有指定的值

     @ConditionalOnResource：類路徑是否有指定的值

      @ConditionalOnSingleCandidate：當指定bean在容器中只有一個，或者雖然有多個但是指定首先的Bean

      @ConditionalOnWebApplication：當前專案是Web專案的條件下

     這些註解都是組合了@Condition元註解，只是使用了不同的條件（Condition），

     下面看看@ConditionalOnWebApplication的原始碼：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package org.springframework.boot.autoconfigure.condition;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.context.annotation.Conditional;

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional({OnWebApplicationCondition.class})
public @interface ConditionalOnWebApplication {
}

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package org.springframework.boot.autoconfigure.condition;

import org.springframework.boot.autoconfigure.condition.ConditionMessage.Builder;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.annotation.Order;
import org.springframework.core.type.AnnotatedTypeMetadata;
import org.springframework.util.ClassUtils;
import org.springframework.util.ObjectUtils;
import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.support.StandardServletEnvironment;

@Order(-2147483628)
class OnWebApplicationCondition extends SpringBootCondition {
    private static final String WEB_CONTEXT_CLASS = "org.springframework.web.context.support.GenericWebApplicationContext";

    OnWebApplicationCondition() {
    }

    public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
        boolean required = metadata.isAnnotated(ConditionalOnWebApplication.class.getName());
        ConditionOutcome outcome = this.isWebApplication(context, metadata, required);
        if (required && !outcome.isMatch()) {
            return ConditionOutcome.noMatch(outcome.getConditionMessage());
        } else {
            return !required && outcome.isMatch() ? ConditionOutcome.noMatch(outcome.getConditionMessage()) : ConditionOutcome.match(outcome.getConditionMessage());
        }
    }

    private ConditionOutcome isWebApplication(ConditionContext context, AnnotatedTypeMetadata metadata, boolean required) {
        Builder message = ConditionMessage.forCondition(ConditionalOnWebApplication.class, new Object[]{required ? "(required)" : ""});
        if (!ClassUtils.isPresent("org.springframework.web.context.support.GenericWebApplicationContext", context.getClassLoader())) {
            return ConditionOutcome.noMatch(message.didNotFind("web application classes").atAll());
        } else {
            if (context.getBeanFactory() != null) {
                String[] scopes = context.getBeanFactory().getRegisteredScopeNames();
                if (ObjectUtils.containsElement(scopes, "session")) {
                    return ConditionOutcome.match(message.foundExactly("'session' scope"));
                }
            }

            if (context.getEnvironment() instanceof StandardServletEnvironment) {
                return ConditionOutcome.match(message.foundExactly("StandardServletEnvironment"));
            } else {
                return context.getResourceLoader() instanceof WebApplicationContext ? ConditionOutcome.match(message.foundExactly("WebApplicationContext")) : ConditionOutcome.noMatch(message.because("not a web application"));
            }
        }
    }
}

      從isWebApplication方法可以看出，判斷條件是：

     1）GenericWebApplicationContext是否在類路徑中

      2）容器裡是否為session的scope

      3）當前容器的Enviroment是否為StandardServletEnvironment

      4）當前的ResourceLoader是否為WebApplicationContext

      5）我們需要構造ConditionOutcome類的物件來幫助我們，最終通過ConditionOutcome.isMatch方法返回布林值來確定條件

三：實戰

   下面我們自己來寫一個starter pom，這意味著我們不僅有自動配的功能，而且具有更通用的藕合度更低的配置。為了方便理解，在這裡將Bean的屬性在application.yml中配置

    1，新建start的Maven專案，這裡我為了管理，把這個專案作為module，如圖所示：

修改pom.xml程式碼如下：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <artifactId>SpringCloudTwo</artifactId>
        <groupId>com.jack</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>

    <artifactId>com.jack.springboot-starter-hello</artifactId>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-autoconfigure</artifactId>
            <version>1.5.6.RELEASE</version>
        </dependency>
    </dependencies>

</project>

  2，屬性配置，程式碼如下：

package com.jack.springboot.starter.hello;

import org.springframework.boot.context.properties.ConfigurationProperties;

@ConfigurationProperties(prefix = "hello")
public class HelloServiceProperties {
    private static final String MSG = "world";
    private String msg = MSG;

    public static String getMSG() {
        return MSG;
    }

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }
}

在application.yml中通過hello.msg來設定，若不設定，預設為hello.msg=world

3,判斷依據類，程式碼如下：

package com.jack.springboot.starter.hello;

public class HelloService {
    private String msg;

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }
    public String sayHello(){
        return "Hello "+msg;
    }
}

    下面根據此類的存在與否來建立這個類的Bean，這個類可以是第三方的類庫

4，自動配置類，程式碼如下：

package com.jack.springboot.starter.hello;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
@EnableConfigurationProperties(HelloServiceProperties.class)
@ConditionalOnClass(HelloService.class)
@ConditionalOnProperty(prefix = "hello",value = "enabled",matchIfMissing = true)
public class HelloServiceAutoConfiguration {
    @Autowired
    private HelloServiceProperties helloServiceProperties;
    @Bean
    @ConditionalOnMissingBean(HelloService.class)
    public HelloService helloService(){
        HelloService helloService = new HelloService();
        helloService.setMsg(helloServiceProperties.getMsg());
        return helloService;
    }
}

    根據HelloServiceProperties提供的引數，並通過@ConditionalOnClass判斷HelloService這個類在此類路徑中是否存在，且當容器總沒有這個Bean的情況下自動配置這個Bean

5，註冊配置

    若想自動配置生效，需要註冊自動配置類，在src/main/resources下新建META-INF/spring.factories，結構如下：

   在spring.factories中填寫如下內容註冊：

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.jack.springboot.starter.hello.HelloServiceAutoConfiguration

6，測試

    在其他專案新增starter作為依賴，修改pom.xml新增依賴，程式碼如下：

<dependency>
            <groupId>com.jack</groupId>
            <artifactId>com.jack.springboot-starter-hello</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>

    我們可以在Maven的依賴中檢視依賴：

    在開發階段，我們引入的依賴是springboot-starter-hello這個專案，在starter穩定之後，我們可以將springboot-starter-hello通過mvn install安裝到本地庫，或將這個jar包發到Maven私服上。

測試程式碼如下；

package com.jack.controller;

import com.jack.springboot.starter.hello.HelloService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class SpringBootStarterController {

    @Autowired
    private HelloService helloService;
    @RequestMapping(value = "/start")
    public String start(){
        return helloService.sayHello();
    }
}

    在程式碼中可以直接注入到HelloService的Bean，但是在專案中我們並沒有配置這個Bean，這是通過自動注入配置完成的，在瀏覽器輸入http://localhost:9090/start，效果如下所示：

    這時在application.yml配置配置msg的內容：

server:
  port: 9090

hello:
  msg: jack

重啟，進行測試，效果如下：

如果在application.yml中開啟debug=true,則可以檢視到自動配置報告。