自定义spring boot starter三部曲之三:源码分析spring.factories加载过程

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本文是《自定义spring boot starter三部曲》系列的终篇,前文中我们开发了一个starter并做了验证,发现关键点在于spring.factories的自动加载能力,让应用只要依赖starter的jar包即可,今天我们来分析Spring和Spring boot源码,了解spring.factories自动加载原理;

三部曲文章链接

  1. 《自定义spring boot starter三部曲之一:准备工作》
  2. 《自定义spring boot starter三部曲之二:实战开发》
  3. 《自定义spring boot starter三部曲之三:源码分析spring.factories加载过程》

版本情况

本文中涉及到的库的版本:

  1. Spring boot :1.5.9.RELEASE;
  2. JDK :1.8.0_144

初步分析

先回顾customizeservicestarter模块中spring.factories文件的内容:

    org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.bolingcavalry.customizeservicestarter.CustomizeConfiguration

从上述内容可以确定今天源码学习目标:

  1. spring容器如何处理配置类;
  2. spring boot配置类的加载情况;
  3. spring.factories中的EnableAutoConfiguration配置何时被加载?
  4. spring.factories中的EnableAutoConfiguration配置被加载后做了什么处理;

spring容器如何处理配置类

  1. ConfigurationClassPostProcessor类的职责是处理配置类;
  2. ConfigurationClassPostProcessor是BeanDefinitionRegistryPostProcessor接口的实现类,它的postProcessBeanDefinitionRegistry方法在容器初始化阶段会被调用(BeanDefinitionRegistryPostProcessor接口的更多细节请参考《spring4.1.8扩展实战之六:注册bean到spring容器(BeanDefinitionRegistryPostProcessor接口)》);
  3. postProcessBeanDefinitionRegistry方法又调用processConfigBeanDefinitions方法处理具体业务;
  4. processConfigBeanDefinitions方法中通过ConfigurationClassParser类来处理Configuration注解,如下图:
    20191102100793\_1.png
  5. 如上图红框所示,所有被Configuration注解修饰过的类,都会被parser.parse(candidates)处理,即ConfigurationClassParser类的parse方法;
  6. parse方法中调用processDeferredImportSelectors方法做处理:找到Configuration类中的Import注解,对于Import注解的值,如果实现了ImportSelector接口,就调用其selectImports方法,将返回的名称实例化:
    private void processDeferredImportSelectors() {
            //这里就是Configuration注解中的Import注解的值,
            //例如EnableAutoConfiguration注解的源码中,Import注解的值是EnableAutoConfigurationImportSelector.class
            List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
            this.deferredImportSelectors = null;
            Collections.sort(deferredImports, DEFERRED_IMPORT_COMPARATOR);

            for (DeferredImportSelectorHolder deferredImport : deferredImports) {
                ConfigurationClass configClass = deferredImport.getConfigurationClass();
                try {
                    //以EnableAutoConfiguration注解为例,其Import注解的值为EnableAutoConfigurationImportSelector.class,
                    //那么此处就是在调用EnableAutoConfigurationImportSelector的selectImports方法,返回了一个字符串数组
                    String[] imports = deferredImport.getImportSelector().selectImports(configClass.getMetadata());
                    //字符串数组中的每个字符串都代表一个类,此处做实例化
                    processImports(configClass, asSourceClass(configClass), asSourceClasses(imports), false);
                }
                catch (BeanDefinitionStoreException ex) {
                    throw ex;
                }
                catch (Throwable ex) {
                    throw new BeanDefinitionStoreException(
                            "Failed to process import candidates for configuration class [" +
                            configClass.getMetadata().getClassName() + "]", ex);
                }
            }
        }

小结一下spring容器配置类的逻辑:

  1. 找出配置类;
  2. 找出配置类中的Import注解;
  3. Import注解的值是class,如果该class实现了ImportSelector接口,就调用其selectImports方法,将返回的名称实例化;

有了上面的结论就可以结合Spring boot的源码来分析加载了哪些数据了;

spring boot配置类的加载情况

  1. 我们的应用使用了SpringBootApplication注解,看此注解的源码,使用了EnableAutoConfiguration注解:
    @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 {
        ......
  1. EnableAutoConfiguration注解中,通过Import注解引入了EnableAutoConfigurationImportSelector.class:
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @AutoConfigurationPackage
    @Import(EnableAutoConfigurationImportSelector.class)
    public @interface EnableAutoConfiguration {
        ......
  1. 看EnableAutoConfigurationImportSelector的源码:
    /** * {@link DeferredImportSelector} to handle {@link EnableAutoConfiguration * auto-configuration}. This class can also be subclassed if a custom variant of * {@link EnableAutoConfiguration @EnableAutoConfiguration}. is needed. * * @deprecated as of 1.5 in favor of {@link AutoConfigurationImportSelector} * @author Phillip Webb * @author Andy Wilkinson * @author Stephane Nicoll * @author Madhura Bhave * @since 1.3.0 * @see EnableAutoConfiguration */
    @Deprecated
    public class EnableAutoConfigurationImportSelector
            extends AutoConfigurationImportSelector {

        @Override
        protected boolean isEnabled(AnnotationMetadata metadata) {
            if (getClass().equals(EnableAutoConfigurationImportSelector.class)) {
                return getEnvironment().getProperty(
                        EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class,
                        true);
            }
            return true;
        }

    }

上述源码有三处重点需要关注:
第一,EnableAutoConfigurationImportSelector是AutoConfigurationImportSelector的子类;
第二,EnableAutoConfigurationImportSelector已经被废弃了,不推荐使用;
第三,文档中已经写明废弃原因:从1.5版本开始,其特性由父类AutoConfigurationImportSelector实现;

  1. 查看AutoConfigurationImportSelector的源码,重点关注selectImports方法,该方法的返回值表明了哪些类会被实例化:
    @Override
        public String[] selectImports(AnnotationMetadata annotationMetadata) {
            if (!isEnabled(annotationMetadata)) {
                return NO_IMPORTS;
            }
            try {
                //将所有spring-autoconfigure-metadata.properties文件中的键值对保存在autoConfigurationMetadata中
                AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                        .loadMetadata(this.beanClassLoader);
                AnnotationAttributes attributes = getAttributes(annotationMetadata);
                //取得所有配置类的名称
                List<String> configurations = getCandidateConfigurations(annotationMetadata,
                        attributes);
                configurations = removeDuplicates(configurations);
                configurations = sort(configurations, autoConfigurationMetadata);
                Set<String> exclusions = getExclusions(annotationMetadata, attributes);
                checkExcludedClasses(configurations, exclusions);
                configurations.removeAll(exclusions);
                configurations = filter(configurations, autoConfigurationMetadata);
                fireAutoConfigurationImportEvents(configurations, exclusions);
                return configurations.toArray(new String[configurations.size()]);
            }
            catch (IOException ex) {
                throw new IllegalStateException(ex);
            }
        }
  1. 通过上述代码可以发现,getCandidateConfigurations方法的调用是个关键,它返回的字符串都是即将被实例化的类名,来看此方法源码:
    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;
        }
  1. getCandidateConfigurations方法中,调用了静态方法SpringFactoriesLoader.loadFactoryNames,上面提到的SpringFactoriesLoader.loadFactoryNames方法是关键,看看官方文档对此静态方法的描述,如下图红框所示,该方法会在spring.factories文件中寻找指定接口对应的实现类的全名(包名+实现类):
    20191102100793\_2.png
  2. 在getCandidateConfigurations方法中,调用SpringFactoriesLoader.loadFactoryNames的时候传入的指定类型是getSpringFactoriesLoaderFactoryClass方法的返回值:
    protected Class<?> getSpringFactoriesLoaderFactoryClass() {
            return EnableAutoConfiguration.class;
        }

现在可以梳理一下了:

  1. spring boot应用启动时使用了EnableAutoConfiguration注解;
  2. EnableAutoConfiguration注解通过import注解将EnableAutoConfigurationImportSelector类实例化,并且将其selectImports方法返回的类名实例化后注册到spring容器;
  3. EnableAutoConfigurationImportSelector的selectImports方法返回的类名,来自spring.factories文件内的配置信息,这些配置信息的key等于EnableAutoConfiguration;

现在真相大白了:只要我们在spring.factories文件内配置了EnableAutoConfiguration,那么对于的类就会被实例化后注册到spring容器;

至此,《自定义spring boot starter三部曲》系列就完结了,希望实战加源码分析的三篇文章,能帮助您理解和实现自定义starter这种简单快捷的扩展方式;


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