1、springboot自动配置的原理初探
以下注解都在springboot的自动化配置包中:spring-boot-autoconfigure。读者朋友可以跟着一下步骤走一遍,应该对自动配置就有一定的认知了。
1.springboot程序的入口是在启动类,该类有个关键注解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 { //略…… }
2.打开SpringBootApplication注解,上面有个关键注解EnableAutoConfiguration
@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @AutoConfigurationPackage @Import(AutoConfigurationImportSelector.class) public @interface EnableAutoConfiguration { //…… }
3.EnableAutoConfiguration上有个@Import(AutoConfigurationImportSelector.class),注意AutoConfigurationImportSelector,
@Import作用创建一个AutoConfigurationImportSelector的bean对象,并且加入IoC容器
//org.springframework.boot.autoconfigure.AutoConfigurationImportSelector //此处只贴了关键方法 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; }
4.AutoConfigurationImportSelector类中的getCandidateConfigurations方法代码如上,其调用了SpringFactoriesLoader的loadFactoryNames方法,来获取
configurations,此configurations列表其实就是要被自动花配置的类。SpringFactoriesLoader的两个重要方法如下:
//org.springframework.core.io.support.SpringFactoriesLoader //只贴了两个关键方法 public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories"; //此方法返回的是即将要被自动化配置的类的全限定类名,是从META-INF/spring.factories配置的,配置文件中有个org.springframework.boot.autoconfigure.EnableAutoConfiguration 其后面可配置多个想被自动花配置的类 public static List<String> loadFactoryNames(Class<?> factoryType, @Nullab等le ClassLoader classLoader) { String factoryTypeName = factoryType.getName(); return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, 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));//META-INF/spring.factories 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 factoryTypeName = ((String) entry.getKey()).trim(); for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) { result.add(factoryTypeName, factoryImplementationName.trim()); } } } cache.put(classLoader, result); return result; } catch (IOException ex) { throw new IllegalArgumentException("Unable to load factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex); } }
5.举例分析,我们在spring.factories中可以看到org.springframework.boot.autoconfigure.EnableAutoConfiguration后有一个org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,说明springboot希望redis能够自动化配置。接着我们打开RedisAutoConfiguration源码查看。此处我故意没复制源码,用的截图,可以看到截图直接有报错,编译错误,错误的原因是我们还没添加spring-boot-starter-data-redis的依赖。**这里有个问题,为什么明明代码都报错,Cannot resolve symbol xxx(未找到类),但是我们的项目依然可以启动?不信你建立一个简单的springboot项目,只添加web依赖,手动打开RedisAutoConfiguration,发现是报红错的,但是你启动项目,发现没任何问题,why??**这个问题后面再解答,先接着看自动配置的问题。
6.先把RedisAutoConfiguration源码复制出来方便我写注释,上面用截图主要是让大家看到报错
@Configuration(proxyBeanMethods = false) @ConditionalOnClass(RedisOperations.class) @EnableConfigurationProperties(RedisProperties.class) @Import({ LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class }) public class RedisAutoConfiguration { @Bean @ConditionalOnMissingBean(name = "redisTemplate") public RedisTemplate<Object, Object> redisTemplate(RedisConnectionFactory redisConnectionFactory) throws UnknownHostException { RedisTemplate<Object, Object> template = new RedisTemplate<>(); template.setConnectionFactory(redisConnectionFactory); return template; } @Bean @ConditionalOnMissingBean public StringRedisTemplate stringRedisTemplate(RedisConnectionFactory redisConnectionFactory) throws UnknownHostException { StringRedisTemplate template = new StringRedisTemplate(); template.setConnectionFactory(redisConnectionFactory); return template; } }
看源码可知RedisAutoConfiguration上有一个Configuration和ConditionalOnClass注解,先分析这两个。首先Configuration注解,代表这是个Java config配置类,和spring配置bean的xml文件是一个作用,都是用来实例化bean的,**但是注意还有个@ConditionalOnClass(RedisOperations.class)注解,这个注解的作用是当RedisOperations.class这个类被找到后才会生效,如果没找到此类,那么整个RedisAutoConfiguration就不会生效。**所以当我们引入了redis的依赖,springboot首先会通过RedisAutoConfiguration的方法redisTemplate给我们设置一个默认的redis配置,当然这个方法上也有个注解@ConditionalOnMissingBean(name = "redisTemplate"),就是当我们没有手动配redisTemplate这个bean它才会调用这个默认的方法,注入一个redisTemplate到IoC容器,所以一般情况我们都是手动配置这个redisTemplate,方便我们设置序列化器,如下:
@Configuration public class RedisConfig { /** * 设置 redisTemplate 的序列化设置 * * @param redisConnectionFactory * @return */ @Bean public RedisTemplate<String, Object> redisTemplate(RedisConnectionFactory redisConnectionFactory) { // 1.创建 redisTemplate 模版 RedisTemplate<String, Object> template = new RedisTemplate<>(); // 2.关联 redisConnectionFactory template.setConnectionFactory(redisConnectionFactory); // 3.创建 序列化类 Jackson2JsonRedisSerializer jackson2JsonRedisSerializer = new Jackson2JsonRedisSerializer(Object.class); ObjectMapper om = new ObjectMapper(); // 4.设置可见度 om.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.ANY); // 5.启动默认的类型 om.enableDefaultTyping(ObjectMapper.DefaultTyping.NON_FINAL); // 6.序列化类,对象映射设置 jackson2JsonRedisSerializer.setObjectMapper(om); // 7.设置 value 的转化格式和 key 的转化格式 template.setValueSerializer(jackson2JsonRedisSerializer); template.setKeySerializer(new StringRedisSerializer()); template.afterPropertiesSet(); return template; } }
RedisAutoConfiguration上还有一下两个注解,作用是从配置文件读取redis相关的信息,ip、端口、密码等
@EnableConfigurationProperties(RedisProperties.class) @Import({ LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class })
2. 补充扩展(解释为什么引用的包都报红错了,项目还能启动)
所有的@Condition注解(包括衍生的)其实都对应一个具体的实现,这个实现类里面有个判断方法叫做matches,返回的是个布尔类型判断值。
打开ConditionalOnClass源码如下,其Conditional注解传递的是个OnClassCondition.class,这就其对应的判断类,也就是说,当我们使用ConditionalOnClass注解时,其实际上调用的是OnClassCondition来判断的
@Target({ ElementType.TYPE, ElementType.METHOD }) @Retention(RetentionPolicy.RUNTIME) @Documented @Conditional(OnClassCondition.class) public @interface ConditionalOnClass { /** * The classes that must be present. Since this annotation is parsed by loading class * bytecode, it is safe to specify classes here that may ultimately not be on the * classpath, only if this annotation is directly on the affected component and * <b>not</b> if this annotation is used as a composed, meta-annotation. In order to * use this annotation as a meta-annotation, only use the {@link #name} attribute. * @return the classes that must be present */ Class<?>[] value() default {}; /** * The classes names that must be present. * @return the class names that must be present. */ String[] name() default {}; }
ConditionalOnClass类图如下,它继承了condition接口
打开Condition接口如下,查看注释,注释中有说明 **条件判断是在bean定义即将注册到容器之前进行的,**看过springIoC源码的同学应该知道,spring创建一个对象的过程是当服务启动后,先读取xml配置文件(或者通过注解),根据配置文件先定义一个BeanDefinition,然后把这个bean给放到容器(在spring中实际就是一个Map),然后在根据bean定义,通过反射创建真正的对象。反射会触发类加载,当condition条件不满足时,根据如下注释可知,bean定义后续都被拦截了,连注册都不行,所以自然就不可能通过反射创建对象,不反射自然不会触发类加载,不触发类加载那么RedisAutoConfiguration当然啊不会加载,它不加载,那么即使它里面引用了一个不存在的类也不会有啥问题。
上面说的很绕,表达的不是很好,要想看懂以上部分需要掌握两方面的知识:
- 类加载原理,推荐看周志明老师的《深入理解JVM虚拟机》
- spring IoC容器创建bean的原理,推荐《spring揭秘》,详细看看IoC部分
3、又一个问题
spring-boot-autoconfigure.jar这个包中的RedisAutoConfiguration都报红色错误了,那么spring官方是怎么打包出来spring-boot-autoconfigure.jar的??怎么给我们提供了一个报错的包呢
//TODO
总结
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原文链接:https://blog.csdn.net/chen462488588/article/details/120769282