4、Spring Boot 自動配置原理

阿新 • • 發佈：2018-11-20

add adp over 生成修改 more servlet func details

1.4 Spring Boot 自動配置原理

簡介

spring boot自動配置功能可以根據不同情況來決定spring配置應該用哪個，不應該用哪個，舉個例子：

Spring的JdbcTemplate是不是在Classpath裏面？如果是，並且DataSource也存在，就自動配置一個JdbcTemplate的Bean
Thymeleaf是不是在Classpath裏面？如果是，則自動配置Thymeleaf的模板解析器、視圖解析器、模板引擎

那個這個是怎麽實現的呢？原因就在於它利用了Spring的條件化配置，條件化配置允許配置存在於應用中，但是在滿足某些特定條件前會忽略這些配置。

要實現條件化配置我們要用到@Conditional條件化註解。

本篇從如下幾個方面進行展開：

@Conditional註解
spring boot 的條件化配置詳解
spring boot 自動配置源碼分析
自己動手實現spring boot starter pom

1.4.1 @Conditional 註解

@Conditional是由Spring 4提供的一個新特性，用於根據特定條件來控制Bean的創建行為。

1.4.2 Spring Boot 的條件化配置詳解

在spring boot項目中會存在一個名為spring-boot-autoconfigure的jar包，條件化配置就是在這個jar裏面實現的，它用到了如下的條件化註解，這些註解都是以@ConditionalOn開頭的，他們都是應用了@Conditional的組合註解：

@ConditionalOnBean ------------------------ 配置了某個特定的Bean
@ConditionalMissingBean ------------------- 沒有配置特定的Bean
@ConditionalOnCalss ----------------------- Classpath裏有指定的類
@ConditionalOnMissingCalss ---------------- Classpath缺少指定的類
@ConditionalOnExpression ------------------ 給定的Spring Expression Language(SpEL)表達式計算

@ConditionalOnJava ------------------------ Java的版本匹配特定值或者一個範圍值
@ConditionalOnProperty -------------------- 指定的配置屬性要有一個明確的值
@ConditionalOnResource -------------------- ClassPath裏有指定的資源
@ConditionalOnWebApplication -------------- 這是一個Web應用程序
@ConditionalOnNotWebApplication ----------- 這不是一個Web應用程序

以JdbcTemplateAutoConfiguration為例，它裏面有這段代碼：

@Bean
@Primary
@ConditionalOnMissingBean(JdbcOperations.class)
public JdbcTemplate jdbcTemplate() {
    return new JdbcTemplate(this.dataSource);
}

只有在不存在JdbcOperations（如果查看JdbcTemplate的源碼，你會發現JdbcTemplate類實現了JdbcOperations接口）實例的時候，才會初始化一個JdbcTemplate 的Bean。

1.4.3 Spring Boot 自動配置源碼分析

spring boot項目的啟動類用的註解--@SpringBootApplication是一個組合註解，其中@EnableAutoConfiguration是自動配置相關的。

查看@EnableAutoConfiguration註解源碼（本文基於spring-boot-starter-parent 2.1.0.RELEASE）：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

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

    String[] excludeName() default {};

}

而這個@EnableAutoConfiguration註解裏面有個@Import註解導入了EnableAutoConfigurationImportSelector用來實現具體的功能。

進入AutoConfigurationImportSelector.java裏面有個方法selectImports（）調用了方法getAutoConfigurationEntry()，然後getAutoConfigurationEntry()方法又調用了getCandidateConfigurations（），進而調用了SpringFactoriesLoader.loadFactoryNames（）方法

@Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        }
        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                .loadMetadata(this.beanClassLoader);
        AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
                autoConfigurationMetadata, annotationMetadata);
        return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    }

    /**
     * Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata}
     * of the importing {@link Configuration @Configuration} class.
     * @param autoConfigurationMetadata the auto-configuration metadata
     * @param annotationMetadata the annotation metadata of the configuration class
     * @return the auto-configurations that should be imported
     */
    protected AutoConfigurationEntry getAutoConfigurationEntry(
            AutoConfigurationMetadata autoConfigurationMetadata,
            AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        }
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        List<String> configurations = getCandidateConfigurations(annotationMetadata,
                attributes);
        configurations = removeDuplicates(configurations);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = filter(configurations, autoConfigurationMetadata);
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationEntry(configurations, exclusions);
    }

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
            AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
                getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
        Assert.notEmpty(configurations,
                "No auto configuration classes found in META-INF/spring.factories. If you "
                        + "are using a custom packaging, make sure that file is correct.");
        return configurations;
    }

在SpringFactoriesLoader.loadFactoryNames（）方法裏面，會調用loadSpringFactories（），然後查詢META-INF/spring.factories這個配置文件

    // 所要查詢的資源文件
    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

    public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
    }
    
    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        MultiValueMap<String, String> result = cache.get(classLoader);
        if (result != null) {
            return result;
        }

        try {
            Enumeration<URL> urls = (classLoader != null ?
                    classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            result = new LinkedMultiValueMap<>();
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                UrlResource resource = new UrlResource(url);
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                for (Map.Entry<?, ?> entry : properties.entrySet()) {
                    String factoryClassName = ((String) entry.getKey()).trim();
                    for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
                        result.add(factoryClassName, factoryName.trim());
                    }
                }
            }
            cache.put(classLoader, result);
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load factories from location [" +
                    FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }

SpringFactoriesLoader屬於Spring框架私有的一種擴展方案，其主要功能就是從指定的配置文件META-INF/spring.factories加載配置。

SpringFactoriesLoader.loadFactoryNames方法會掃描具有META-INF/spring.factories文件的jar包，而我們的spring-boot-autoconfigure.jar裏面就有一個這樣的文件，此文件中聲明了具體有哪些自動配置：

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

# 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.OnBeanCondition,org.springframework.boot.autoconfigure.condition.OnClassCondition,org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition

# 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.CloudServiceConnectorsAutoConfiguration,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.CassandraReactiveDataAutoConfiguration,org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,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.jdbc.JdbcRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,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.RedisReactiveAutoConfiguration,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.elasticsearch.rest.RestClientAutoConfiguration,org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,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.http.HttpMessageConvertersAutoConfiguration,org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,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.groovy.template.GroovyTemplateAutoConfiguration,org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,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.mongo.embedded.EmbeddedMongoAutoConfiguration,org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,org.springframework.boot.autoconfigure.reactor.core.ReactorCoreAutoConfiguration,org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,org.springframework.boot.autoconfigure.security.servlet.SecurityRequestMatcherProviderAutoConfiguration,org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerAutoConfiguration,org.springframework.boot.autoconfigure.security.oauth2.resource.reactive.ReactiveOAuth2ResourceServerAutoConfiguration,org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration,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.client.RestTemplateAutoConfiguration,org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,org.springframework.boot.autoconfigure.web.reactive.function.client.ClientHttpConnectorAutoConfiguration,org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration

# 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,org.springframework.boot.autoconfigure.session.NonUniqueSessionRepositoryFailureAnalyzer

# 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.servlet.JspTemplateAvailabilityProvider

我們上面提到的JdbcTemplateAutoConfiguration自動配置類就在裏面。

1.4.4 自己動手實現spring boot starter pom

接下來，我們就來寫一個簡單的spring boot starter pom。

新建starter maven項目 spring-boot-starter-autoconfigure
修改pom文件

<?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">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.hw</groupId>
    <artifactId>spring-boot-starter-autoconfigure</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>spring-boot-starter-autoconfigure</name>
    <description>Demo project for Spring Boot Autoconfigure</description>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <!-- 這裏需要引入spring boot的自動配置作為依賴 -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-autoconfigure</artifactId>
            <version>2.1.0.RELEASE</version>
        </dependency>

    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

屬性配置 HelloServiceProperties.java


package com.hw.springbootstarterautoconfigure.conifg;

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

/**
 * @Description TODO
 * @Author hw
 * @Date 2018/11/18 22:57
 * @Version 1.0
 */

/**
 * @ConfigurationProperties 自動匹配application.properties文件中hello.msg的值，然後賦值給類屬性msg，這裏的msg默認值為“spring boot”
 */
@ConfigurationProperties(prefix = "hello")
public class HelloServiceProperties {

    private static final String MSG = "spring boot";

    private String msg = MSG;

    public String getMsg() {
        return msg;
    }

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

}

判定依據類HelloService.java，後面的代碼會依據此類是否存在，來決定是否生產對應的Bean

package com.hw.springbootstarterautoconfigure.conifg;

/**
 * @Description TODO
 * @Author hw
 * @Date 2018/11/18 23:00
 * @Version 1.0
 */
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;
    }
}

自動配置類 HelloServiceAutoConfiguration.java，根據HelloServiceProperties提供的參數，並通過@ConditionalOnClass(HelloService.class)判定HelloService這個類在Classpath中是否存在，存在並且還沒有對應的Bean，就生成對應的helloService Bean

package com.hw.springbootstarterautoconfigure.conifg;

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;

/**
 * @Description TODO
 * @Author hw
 * @Date 2018/11/18 23:01
 * @Version 1.0
 */
@Configuration
@EnableConfigurationProperties(HelloServiceProperties.class)
@ConditionalOnClass(HelloService.class)
@ConditionalOnProperty(prefix = "hello", matchIfMissing = true, value = "enabled")
public class HelloServiceAutoConfiguration {

    @Autowired
    HelloServiceProperties helloServiceProperties;

    @Bean
    @ConditionalOnMissingBean(HelloService.class)
    public HelloService helloService() {
        HelloService service = new HelloService();
        service.setMsg(helloServiceProperties.getMsg());
        return service;
    }
}

註冊配置，需要到META-INF/spring.factories文件中註冊改自動配置類：在src/main/source目錄下新建該文件，然後進行配置。

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.hw.springbootstarterautoconfigure.conifg.HelloServiceAutoConfiguration

實現controller HelloController.java

package com.hw.springbootstarterautoconfigure.web;

import com.hw.springbootstarterautoconfigure.conifg.HelloService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

/**
 * @Description TODO
 * @Author hw
 * @Date 2018/11/18 23:07
 * @Version 1.0
 */
@RestController
public class HelloController {

    /**
     * 代碼中沒有配置這個helloService Bean，但是自動配置能夠幫忙實例化，因此可以直接註入
     */
    @Autowired
    HelloService helloService;

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

啟動項目訪問/helloService 接口。得到 hello spring boot
在application.properties裏面配置hello.msg=hw，然後再次訪問/helloService接口。得到 hello hw
本文完整代碼：Spring Boot自動配置

4、Spring Boot 自動配置原理

add adp over 生成修改 more servlet func details 1.4 Spring Boot 自動配置原理簡介 spring boot自動配置功能可以根據不同情況來決定spring配置應該用哪個，不應該用哪個，舉個例子： Spring的J

4、Spring Boot 自動配置原理

1.4 Spring Boot 自動配置原理

簡介

1.4.1 @Conditional 註解

1.4.2 Spring Boot 的條件化配置詳解

1.4.3 Spring Boot 自動配置源碼分析

1.4.4 自己動手實現spring boot starter pom

4、Spring Boot 自動配置原理

Spring Boot自動配置原理、實戰

Spring Boot自動配置原理（轉）

Spring boot 自動配置原理

【Spring Boot】（22）、Spring Boot啟動配置原理

Spring Boot自動配置原理

【SpringBoot】spring-boot-自動配置原理

Spring Boot 自動配置原理application.properties

【Spring Boot】（13）、Spring Boot自動配置SpringMVC

我理解的spring boot自動配置原理

Spring Boot自動配置原理與實踐（一）

spring boot自動配置的實現原理——@Conditional

如何編寫Spring-Boot自動配置

006-Spring Boot自動配置

Spring boot 啟動配置原理

spring boot 自動配置

程式設計實戰篇——Spring Boot 自動配置實現

Spring boot自動配置原始碼解析

Spring Boot 自動配置（auto-configurtion）揭祕

Spring Boot自動配置

4、Spring Boot 自動配置原理

1.4 Spring Boot 自動配置原理

簡介

1.4.1 @Conditional 註解

1.4.2 Spring Boot 的條件化配置詳解

1.4.3 Spring Boot 自動配置源碼分析

1.4.4 自己動手實現spring boot starter pom

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