深度分析：SpringBoot 中自定义 starter 实例与原理

2023-04-15
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1 自定义启动器

本文首先提出以下开发需求：需要自定义一个启动器 spring-boot-football-starter，业务方引入这个启动器之后可以直接使用 Football 实例。

1.1 创建项目

1.1.1 项目名

java-front-football-starter

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1.1.2 springboot 版本号

<dependency>    <artifactId>spring-boot-dependencies</artifactId>    <groupId>org.springframework.boot</groupId>    <scope>import</scope>    <type>pom</type>    <version>2.1.7.RELEASE</version></dependency>

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1.2 创建 Football

@Getter@Setterpublic class Football {    private String a;    private String b;}

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1.3 创建配置类

@Getter@Setter@ConfigurationProperties(prefix = "football") // prefix不支持驼峰命名public class FootballProperties {    private boolean enable = true;    private String a;    private String b;}

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1.4 创建 AutoConfiguration

@Slf4j@Configuration@ConditionalOnClass(Football.class) // 只有当classpath中存在Football类时才实例化本类@EnableConfigurationProperties(FootballProperties.class)public class FootballAutoConfiguration {
    @Autowired    private FootballProperties footballProperties;
    @Bean    @ConditionalOnMissingBean(Football.class) // 只有当容器不存在其它Football实例时才使用此实例    public Football football() {        if (!footballProperties.isEnable()) {            String defaultValue = "default";            Football football = new Football();            football.setA(defaultValue);            football.setB(defaultValue);            log.info("==========football close bean={}==========", JSON.toJSONString(football));            return football;        }        Football football = new Football();        football.setA(footballProperties.getA());        football.setB(footballProperties.getB());        log.info("==========football open bean={}==========", JSON.toJSONString(football));        return football;    }}

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条件注解说明

@ConditionalOnBean只有在当前上下文中存在某个对象时才进行实例化Bean
@ConditionalOnClass只有classpath中存在class才进行实例化Bean
@ConditionalOnExpression只有当表达式等于true才进行实例化Bean
@ConditionalOnMissingBean只有在当前上下文中不存在某个对象时才进行实例化Bean
@ConditionalOnMissingClass某个class类路径上不存在时才进行实例化Bean
@ConditionalOnNotWebApplication当前应用不是WEB应用才进行实例化Bean

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1.5 spring.factories

工厂文件位置

- src/main/resources    - META-INF        -spring.factories

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工厂文件内容

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\com.java.front.football.starter.configuration.FootballAutoConfiguration

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2 测试启动器

2.1 引入依赖

<dependency>    <groupId>com.java.test</groupId>    <artifactId>java-front-football-starter</artifactId>    <version>1.0.0</version></dependency>

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2.2 application.yml

server:  port: 9999football:  enable: true  a: "aaa"  b: "bbb"

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2.3 TestController

@RestController@RequestMapping("/test/football")public class TestController {
    @Resource    private Football football;
    @GetMapping    public Football get() {        return football;    }}

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2.4 TestApplication

@SpringBootApplicationpublic class TestApplication {
    public static void main(String[] args) {        SpringApplication.run(TestApplication.class, args);    }}

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2.5 测试用例

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.DEFINED_PORT, classes = TestApplication.class)@RunWith(SpringJUnit4ClassRunner.class)public class TestControllerTest {
    @Test    public void test() {        String response = HttpUtil.get("http://localhost:9999/test/football");        Assert.assertTrue(!StringUtils.isEmpty(response));    }}

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2.6 测试结果

==========football open bean={"a":"aaa","b":"bbb"}==========

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3 原理分析

启动类

@SpringBootApplicationpublic class TestApplication {
    public static void main(String[] args) {        SpringApplication.run(TestApplication.class, args);    }}

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SpringBootApplication

@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 {    //......}

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EnableAutoConfiguration

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

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AutoConfigurationImportSelector

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
    protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,            AnnotationMetadata annotationMetadata) {        if (!isEnabled(annotationMetadata)) {            return EMPTY_ENTRY;        }        AnnotationAttributes attributes = getAttributes(annotationMetadata);        // 加载配置        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);    }}

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getCandidateConfigurations

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
    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;    }}

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loadFactoryNames

public final class SpringFactoriesLoader {
    // 工厂加载路径    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 {            // 根据路径加载工厂信息（FootballAutoConfiguration）            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);        }    }}

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4 延伸知识

4.1 spring 官方启动器

https://docs.spring.io/spring-boot/docs/2.1.7.RELEASE/reference/html/using-boot-build-systems.html#using-boot-starter

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英文介绍

Starters are a set of convenient dependency descriptors that you can include in your application. You get a one-stop shop for all the Spring and related technologies that you need without having to hunt through sample code and copy-paste loads of dependency descriptors. For example, if you want to get started using Spring and JPA for database access, include the spring-boot-starter-data-jpa dependency in your project

中文介绍

Starter 是一组方便的依赖描述符，你可以将其包含在应用程序中。你可以一站式获得所有所需的 Spring 和相关技术，而无需在示例代码中搜索并复制粘贴大量依赖描述符。如果你想要开始使用 Spring 和 JPA 进行数据库访问，请在项目中包含 spring-boot-starter-data-jpa 依赖项

4.2 spring-boot-starter

这个启动器是核心启动器，功能包括自动配置支持、日志记录和 YAML。任意引入一个启动器点击分析，最终发现引用核心启动器。

引用 spring-boot-starter-data-redis

<dependency>    <groupId>org.springframework.boot</groupId>    <artifactId>spring-boot-starter-data-redis</artifactId></dependency>

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发现引用 spring-boot-starter

<dependency>    <groupId>org.springframework.boot</groupId>    <artifactId>spring-boot-starter</artifactId>    <version>2.1.7.RELEASE</version>    <scope>compile</scope></dependency>

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发现引用 spring-boot-autoconfigure

<dependency>    <groupId>org.springframework.boot</groupId>    <artifactId>spring-boot-autoconfigure</artifactId>    <version>2.1.7.RELEASE</version>    <scope>compile</scope></dependency>

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工厂文件位置

/META-INF/spring.factories

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文件中指定 Redis 自动配置

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration

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自动装配类 RedisAutoConfiguration

@Configuration@ConditionalOnClass(RedisOperations.class)@EnableConfigurationProperties(RedisProperties.class) // 配置信息类@Import({ LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class })public class RedisAutoConfiguration {    //......}

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配置信息类 RedisProperties

@ConfigurationProperties(prefix = "spring.redis")public class RedisProperties {    private int database = 0;    private String url;    private String host = "localhost";    private String password;    private int port = 6379;    //......}

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application.yml 文件配置

spring    redis        host:xxx        port:xxx        password:xxx

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4.3 第三方启动器

如果使用第三方启动器，如何知道在 yml 文件中设置什么配置项？本章节以 mybatis 为例：

引入 mybatis-spring-boot-starter

dependency>    <groupId>org.mybatis.spring.boot</groupId>    <artifactId>mybatis-spring-boot-starter</artifactId>    <version>2.2.3</version></dependency>

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自动装配依赖

<dependency>    <groupId>org.mybatis.spring.boot</groupId>    <artifactId>mybatis-spring-boot-autoconfigure</artifactId></dependency>

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工厂文件位置

/META-INF/spring.factories

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文件中指定 MyBatis 自动配置

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration

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自动配置类 MybatisAutoConfiguration

@ConditionalOnClass({ SqlSessionFactory.class, SqlSessionFactoryBean.class })@ConditionalOnSingleCandidate(DataSource.class)@EnableConfigurationProperties(MybatisProperties.class) // 配置信息类@AutoConfigureAfter({ DataSourceAutoConfiguration.class, MybatisLanguageDriverAutoConfiguration.class })public class MybatisAutoConfiguration implements InitializingBean {    //......}

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配置信息类 MybatisProperties

@ConfigurationProperties(prefix = MybatisProperties.MYBATIS_PREFIX)public class MybatisProperties {    public static final String MYBATIS_PREFIX = "mybatis";    private String configLocation;    private String[] mapperLocations;    private String typeAliasesPackage;    private Class<?> typeAliasesSuperType;    //......}

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application.yml 文件配置

mybatis:  mapperLocations: classpath:db/sqlmappers/*.xml  configLocation: classpath:db/mybatis-config.xml

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5 文章总结

第一章节介绍如何自定义一个简单启动器，第二章节对自定义启动器进行测试，第三章节通过源码分析介绍启动器运行原理，第四章进行知识延伸介绍 spring 官方启动器，使用第三方启动器相关知识，希望本文对大家有所帮助。

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深度分析：SpringBoot 中自定义 starter 实例与原理

1 自定义启动器

1.1 创建项目

1.1.1 项目名

1.1.2 springboot 版本号

1.2 创建 Football

1.3 创建配置类

1.4 创建 AutoConfiguration

1.5 spring.factories

2 测试启动器

2.1 引入依赖

2.2 application.yml

2.3 TestController

2.4 TestApplication

2.5 测试用例

2.6 测试结果

3 原理分析

4 延伸知识

4.1 spring 官方启动器

4.2 spring-boot-starter

4.3 第三方启动器

5 文章总结

Java你猿哥

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