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

 2019-11-02 21:24  阅读(1061)
文章分类:Spring boot

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 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 { ......

  2. 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 { ......

  3. 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 configurations = getCandidateConfigurations(annotationMetadata, attributes); configurations = removeDuplicates(configurations); configurations = sort(configurations, autoConfigurationMetadata); Set 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); } }

  2. 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 getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) { List 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; }

  3. 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

  4. 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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