SpringBoot - SPI机制
概要
什么是SPI呢，全称是 Service Provider Interface 。简单翻译的话，就是服务提供者接口，是一种寻找服务实现的机制。这个是针对厂商或者插件的。
其实就是一个规范定义、或者说是实现的标准。
一、SPI思想
SPI的思想：系统里抽象的各个模块，往往有很多不同的实现方案，比如日志模块的方案，xml解析模块、jdbc模块的方案等。面向的对象的设计里，我们一般推荐模块之间基于接口编程，模块之间不对实现类进行硬编码。一旦代码里涉及具体的实现类，就违反了可拔插的原则，如果需要替换一种实现，就需要修改代码。为了实现在模块装配的时候能不在程序里动态指明，这就需要一种服务发现机制。java SPI 就是提供这样的一个机制：为某个接口寻找服务实现的机制。
直白一点来说就是：我定义了一个接口，但是不想固定死具体的实现类，因为那样如果要更换实现类就要改动源代码，这往往是不合适的。那么我也可以定义一个规范，在之后需要更换实现类或增加其他实现类时，遵守这个规范，我也可以动态的去发现这些实现类。
SpringBoot在启动的时候，会扫描所有jar包 resource/META-INF/spring.factories 文件，依据类的全限定名，利用反射机制将 Bean 装载进容器中。
这些监听器的实现类都是在spring.factories文件中配置好的，代码中通过getSpringFactoriesInstances方法获取，这种机制叫做SPI机制：通过本地的注册发现获取到具体的实现类，轻松可插拔。
SpringBoot默认情况下提供了两个spring.factories文件，分别是：
spring-boot-2.2.2.RELEASE.jar
spring-boot-autoconfigure-2.2.2.RELEASE.jar
二、Spring Boot中的类SPI扩展机制
在springboot的自动装配过程中，最终会加载META-INF/spring.factories文件，而加载的过程是由SpringFactoriesLoader加载的。从CLASSPATH下的每个Jar包中搜寻所有META-INF/spring.factories配置文件，然后将解析properties文件，找到指定名称的配置后返回。需要注意的是，其实这里不仅仅是会去ClassPath路径下查找，会扫描所有路径下的Jar包，只不过这个文件只会在Classpath下的jar包中。
三、源码分析
1. 入口方法 getSpringFactoriesInstances()
根据类型获取META-INF/spring.factories文件中对应的实现类
SpringApplication#getSpringFactoriesInstances
1 private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
2 return getSpringFactoriesInstances(type, new Class<?>[]{});
3 }
4
5 private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
6 ClassLoader classLoader = getClassLoader();
7 // Use names and ensure unique to protect against duplicates
8 //从META-INF/spring.factories中加载对应类型的类的自动配置类
9 Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
10 // 加载上来后反射实例化
11 List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
12 //对实例列表进行排序
13 AnnotationAwareOrderComparator.sort(instances);
14 return instances;
15 }
2. SpringFactoriesLoader
我们看看它的源码（精简）：
1 public abstract class SpringFactoriesLoader {
2
3 private static final Log logger = LogFactory.getLog(SpringFactoriesLoader.class);
4
5 public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
6
7
8 public static <T> List<T> loadFactories(Class<T> factoryClass, ClassLoader classLoader) {
9
10 }
11
12 public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
13
14 }
15 }
SpringFactoriesLoader类的两个核心方法：loadFactoryNames是用来寻找spring.factories文件中的Factory名称的，loadFactories方法是用来寻找类的。
1) loadFactoryNames 方法
在该方法里，首先拿到ClassLoader，然后加载FactoryNames，加载类型(type)为ApplicationContextInitializer，类加载器(classLoader)为刚刚拿到的类加载器，返回值放入一个Set中，为的是确保没有重复的FactoryName，这是因为在之后加载的两个spring.factories配置文件中有两个重复的FactoryName。
SpringFactoriesLoader#loadFactoryNames
1 public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
2 ClassLoader classLoaderToUse = classLoader;
3 if (classLoaderToUse == null) {
4 classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
5 }
6 String factoryTypeName = factoryType.getName();
7 return loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
8 }
可以看到，加载的配置文件在META-INF下，名称为spring.factories，该配置文件一共有两个，且配置文件中，每个段落第一行为Key，后边为value，读取时会通过key将所有的value拿出来。
在配置文件中我们发现，key和value都是包名加类名的字符串，因此Springboot在读取文件后，是通过反射生成的类。
spring-boot-2.2.2.RELEASE.jar 该配置文件内容如下：
1 # PropertySource Loaders 2 org.springframework.boot.env.PropertySourceLoader=\ 3 org.springframework.boot.env.PropertiesPropertySourceLoader,\ 4 org.springframework.boot.env.YamlPropertySourceLoader 5 6 # Run Listeners 7 org.springframework.boot.SpringApplicationRunListener=\ 8 org.springframework.boot.context.event.EventPublishingRunListener 9 10 # Error Reporters 11 org.springframework.boot.SpringBootExceptionReporter=\ 12 org.springframework.boot.diagnostics.FailureAnalyzers 13 14 # Application Context Initializers 15 org.springframework.context.ApplicationContextInitializer=\ 16 org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\ 17 org.springframework.boot.context.ContextIdApplicationContextInitializer,\ 18 org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\ 19 org.springframework.boot.rsocket.context.RSocketPortInfoApplicationContextInitializer,\ 20 org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer 21 22 # Application Listeners 23 org.springframework.context.ApplicationListener=\ 24 org.springframework.boot.ClearCachesApplicationListener,\ 25 org.springframework.boot.builder.ParentContextCloserApplicationListener,\ 26 org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\ 27 org.springframework.boot.context.FileEncodingApplicationListener,\ 28 org.springframework.boot.context.config.AnsiOutputApplicationListener,\ 29 org.springframework.boot.context.config.ConfigFileApplicationListener,\ 30 org.springframework.boot.context.config.DelegatingApplicationListener,\ 31 org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\ 32 org.springframework.boot.context.logging.LoggingApplicationListener,\ 33 org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener 34 35 # Environment Post Processors 36 org.springframework.boot.env.EnvironmentPostProcessor=\ 37 org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\ 38 org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,\ 39 org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor,\ 40 org.springframework.boot.reactor.DebugAgentEnvironmentPostProcessor 41 42 # Failure Analyzers 43 org.springframework.boot.diagnostics.FailureAnalyzer=\ 44 org.springframework.boot.diagnostics.analyzer.BeanCurrentlyInCreationFailureAnalyzer,\ 45 org.springframework.boot.diagnostics.analyzer.BeanDefinitionOverrideFailureAnalyzer,\ 46 org.springframework.boot.diagnostics.analyzer.BeanNotOfRequiredTypeFailureAnalyzer,\ 47 org.springframework.boot.diagnostics.analyzer.BindFailureAnalyzer,\ 48 org.springframework.boot.diagnostics.analyzer.BindValidationFailureAnalyzer,\ 49 org.springframework.boot.diagnostics.analyzer.UnboundConfigurationPropertyFailureAnalyzer,\ 50 org.springframework.boot.diagnostics.analyzer.ConnectorStartFailureAnalyzer,\ 51 org.springframework.boot.diagnostics.analyzer.NoSuchMethodFailureAnalyzer,\ 52 org.springframework.boot.diagnostics.analyzer.NoUniqueBeanDefinitionFailureAnalyzer,\ 53 org.springframework.boot.diagnostics.analyzer.PortInUseFailureAnalyzer,\ 54 org.springframework.boot.diagnostics.analyzer.ValidationExceptionFailureAnalyzer,\ 55 org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyNameFailureAnalyzer,\ 56 org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyValueFailureAnalyzer 57 58 # FailureAnalysisReporters 59 org.springframework.boot.diagnostics.FailureAnalysisReporter=\ 60 org.springframework.boot.diagnostics.LoggingFailureAnalysisReporterView Code spring-boot-autoconfigure-2.2.2.RELEASE.jar 该配置文件内容如下：
1 # Initializers 2 org.springframework.context.ApplicationContextInitializer=\ 3 org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\ 4 org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener 5 6 # Application Listeners 7 org.springframework.context.ApplicationListener=\ 8 org.springframework.boot.autoconfigure.BackgroundPreinitializer 9 10 # Auto Configuration Import Listeners 11 org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\ 12 org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener 13 14 # Auto Configuration Import Filters 15 org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\ 16 org.springframework.boot.autoconfigure.condition.OnBeanCondition,\ 17 org.springframework.boot.autoconfigure.condition.OnClassCondition,\ 18 org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition 19 20 # Auto Configure 21 org.springframework.boot.autoconfigure.EnableAutoConfiguration=\ 22 org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\ 23 org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\ 24 org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\ 25 org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\ 26 org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\ 27 org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\ 28 org.springframework.boot.autoconfigure.cloud.CloudServiceConnectorsAutoConfiguration,\ 29 org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\ 30 org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\ 31 org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\ 32 org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\ 33 org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\ 34 org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\ 35 org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\ 36 org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\ 37 org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\ 38 org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\ 39 org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\ 40 org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\ 41 org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\ 42 org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\ 43 org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\ 44 org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\ 45 org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRepositoriesAutoConfiguration,\ 46 org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveRestClientAutoConfiguration,\ 47 org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,\ 48 org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\ 49 org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\ 50 org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\ 51 org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,\ 52 org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,\ 53 org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\ 54 org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\ 55 org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\ 56 org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\ 57 org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\ 58 org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,\ 59 org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\ 60 org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\ 61 org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\ 62 org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\ 63 org.springframework.boot.autoconfigure.elasticsearch.rest.RestClientAutoConfiguration,\ 64 org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\ 65 org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\ 66 org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\ 67 org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\ 68 org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\ 69 org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\ 70 org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\ 71 org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration,\ 72 org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,\ 73 org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,\ 74 org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\ 75 org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\ 76 org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\ 77 org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\ 78 org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\ 79 org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\ 80 org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\ 81 org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\ 82 org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\ 83 org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\ 84 org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\ 85 org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\ 86 org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\ 87 org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\ 88 org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\ 89 org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\ 90 org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,\ 91 org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\ 92 org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\ 93 org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\ 94 org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\ 95 org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\ 96 org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\ 97 org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\ 98 org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\ 99 org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,\ 100 org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\ 101 org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\ 102 org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\ 103 org.springframework.boot.autoconfigure.rsocket.RSocketMessagingAutoConfiguration,\ 104 org.springframework.boot.autoconfigure.rsocket.RSocketRequesterAutoConfiguration,\ 105 org.springframework.boot.autoconfigure.rsocket.RSocketServerAutoConfiguration,\ 106 org.springframework.boot.autoconfigure.rsocket.RSocketStrategiesAutoConfiguration,\ 107 org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,\ 108 org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,\ 109 org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,\ 110 org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,\ 111 org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,\ 112 org.springframework.boot.autoconfigure.security.rsocket.RSocketSecurityAutoConfiguration,\ 113 org.springframework.boot.autoconfigure.security.saml2.Saml2RelyingPartyAutoConfiguration,\ 114 org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\ 115 org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\ 116 org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,\ 117 org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,\ 118 org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerAutoConfiguration,\ 119 org.springframework.boot.autoconfigure.security.oauth2.resource.reactive.ReactiveOAuth2ResourceServerAutoConfiguration,\ 120 org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\ 121 org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,\ 122 org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration,\ 123 org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\ 124 org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\ 125 org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\ 126 org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\ 127 org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration,\ 128 org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\ 129 org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,\ 130 org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\ 131 org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\ 132 org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,\ 133 org.springframework.boot.autoconfigure.web.reactive.function.client.ClientHttpConnectorAutoConfiguration,\ 134 org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,\ 135 org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\ 136 org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\ 137 org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\ 138 org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\ 139 org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\ 140 org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\ 141 org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,\ 142 org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,\ 143 org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,\ 144 org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,\ 145 org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration 146 147 # Failure analyzers 148 org.springframework.boot.diagnostics.FailureAnalyzer=\ 149 org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\ 150 org.springframework.boot.autoconfigure.flyway.FlywayMigrationScriptMissingFailureAnalyzer,\ 151 org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\ 152 org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer,\ 153 org.springframework.boot.autoconfigure.session.NonUniqueSessionRepositoryFailureAnalyzer 154 155 # Template availability providers 156 org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\ 157 org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,\ 158 org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,\ 159 org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,\ 160 org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,\ 161 org.springframework.boot.autoconfigure.web.servlet.JspTemplateAvailabilityProviderView Code
2) loadFactories 方法
SpringFactoriesLoader#loadFactories
1 private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
2 //到缓存中读取
3 MultiValueMap<String, String> result = cache.get(classLoader);
4 //如果存在则直接返回
5 if (result != null) {
6 return result;
7 }
8
9 try {
10 //获取所以jar包META-INF/spring.factories对应的URL
11 Enumeration<URL> urls = (classLoader != null ?
12 classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
13 ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
14 result = new LinkedMultiValueMap<>();
15 //遍历数据
16 while (urls.hasMoreElements()) {
17 URL url = urls.nextElement();
18 //获取到资源数据
19 UrlResource resource = new UrlResource(url);
20 //加载配置文件
21 Properties properties = PropertiesLoaderUtils.loadProperties(resource);
22 //遍历解析配置文件
23 for (Map.Entry<?, ?> entry : properties.entrySet()) {
24 String factoryTypeName = ((String) entry.getKey()).trim();
25 for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
26 //将spring.factories配置文件数据放进结果集
27 result.add(factoryTypeName, factoryImplementationName.trim());
28 }
29 }
30 }
31 //加入缓存
32 cache.put(classLoader, result);
33 //返回结果
34 return result;
35 }
36 catch (IOException ex) {
37 throw new IllegalArgumentException("Unable to load factories from location [" +
38 FACTORIES_RESOURCE_LOCATION + "]", ex);
39 }
40 }
读取完spring.factories后，把读取到的内容(13个key)存储到枚举类中，然后遍历枚举类，将里边内容都add到一个map(result)里边去,最后把classloader以及遍历的结果都放入cache中，提高加载资源的效率。
3. createSpringFactoriesInstances
目前已经取出所有的配置，但还没有进行初始化,该方法是实例化对象的
SpringApplication#createSpringFactoriesInstances
1 private <T> List<T> createSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes,
2 ClassLoader classLoader, Object[] args, Set<String> names) {
3 List<T> instances = new ArrayList<>(names.size());
4 for (String name : names) {
5 try {
6 Class<?> instanceClass = ClassUtils.forName(name, classLoader);
7 Assert.isAssignable(type, instanceClass);
8 //获取构造方法
9 Constructor<?> constructor = instanceClass.getDeclaredConstructor(parameterTypes);
10 //实例化对象
11 T instance = (T) BeanUtils.instantiateClass(constructor, args);
12 //加入instances实例列表
13 instances.add(instance);
14 } catch (Throwable ex) {
15 throw new IllegalArgumentException("Cannot instantiate " + type + " : " + name, ex);
16 }
17 }
18 return instances;
19 }
参考链接：
https://www.baiyp.ren/SpringBoot%E6%BA%90%E7%A0%81%E5%88%86%E6%9E%90-SPI%E6%9C%BA%E5%88%B6.html
标签：SpringBoot,boot,springframework,SPI,autoconfigure,diagnostics,org,机制,data From： https://www.cnblogs.com/hld123/p/18346628