(三)SpringBoot启动过程的分析-创建应用程序上下文
阅读原文时间:2022年01月10日阅读:4

-- 以下内容均基于2.1.8.RELEASE版本

紧接着上一篇(二)SpringBoot启动过程的分析-环境信息准备,本文将分析环境准备完毕之后的下一步操作:ApplicationContext的创建。

// SpringApplication.java

ConfigurableApplicationContext context = createApplicationContext();

protected ConfigurableApplicationContext createApplicationContext() {
    // ①
    Class<?> contextClass = this.applicationContextClass;
    if (contextClass == null) {
        try {
            switch (this.webApplicationType) {
            case SERVLET:
                // ②
                contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
                break;
            case REACTIVE:
                // ③
                contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
                break;
            default:
                // ④
                contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
            }
        }
        catch (ClassNotFoundException ex) {
            throw new IllegalStateException("Unable create a default ApplicationContext, " + "please specify an ApplicationContextClass", ex);
        }
    }
    // ⑤
    return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}

创建ApplicationContext时可以指定需要创建的上下文类型,默认是根据当前应用类型来创建。在SpringApplication.java中指定了几种默认类型,他们分别是默认的非Web应用上下文容器、默认的Web应用上下文容器和响应式Web上下文容器

public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context." + "annotation.AnnotationConfigApplicationContext";

public static final String DEFAULT_SERVLET_WEB_CONTEXT_CLASS = "org.springframework.boot." + "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";

public static final String DEFAULT_REACTIVE_WEB_CONTEXT_CLASS = "org.springframework." + "boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext";

① - 是否有指定的ApplicationContext,若有直接实例化

② - 实例化Web应用上下文容器

③ - 实例化响应式Web应用上下文容器

④ - 实例化非Web应用上下文容器

⑤ - 使用无参构造函数对指定的上下文容器进行实例化

AnnotationConfigServletWebServerApplicationContext

对于常规的以SpringBoot开发的web应用来说,AnnotationConfigServletWebServerApplicationContext将会是默认的上下文容器。它可以解析@Configuration等JSR-330定义的注解,查看它的包路径,可以看到,它是属于SpringBoot下的上下文容器。

public class AnnotationConfigServletWebServerApplicationContext extends ServletWebServerApplicationContext implements AnnotationConfigRegistry {

    private final AnnotatedBeanDefinitionReader reader;

    private final ClassPathBeanDefinitionScanner scanner;

    private final Set<Class<?>> annotatedClasses = new LinkedHashSet<>();

    private String[] basePackages;

    public AnnotationConfigServletWebServerApplicationContext() {
        // ①
        this.reader = new AnnotatedBeanDefinitionReader(this);
        // ②
        this.scanner = new ClassPathBeanDefinitionScanner(this);
    }
}

① - 构造AnnotatedBeanDefinitionReader,传入当前对象作为Bean注册器

② - 构造ClassPathBeanDefinitionScanner,传入当前对象作为Bean注册器

它内部的无参构造方法就干了两件事:初始化reader和scanner。他们两个的存在使当前的上下文容器支持以带有@Configuration注解的类作为加载Bean入口,也支持以指定的basePackage来作为加载Bean的入口,同时还支持注解的过滤。

用编程方式注册带注解的Bean到容器中,也就是基于注解配置的Bean的注册。

public class AnnotatedBeanDefinitionReader {

    // ①
    private final BeanDefinitionRegistry registry;

    // ②
    private BeanNameGenerator beanNameGenerator = new AnnotationBeanNameGenerator();

    // ③
    private ScopeMetadataResolver scopeMetadataResolver = new AnnotationScopeMetadataResolver();

    // ④
    private ConditionEvaluator conditionEvaluator;

    // ⑤
    public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry) {
        this(registry, getOrCreateEnvironment(registry));
    }
    // ...省略部分代码
}

① - BeanDefinition注册器

② - BeanName生成器

③ - Bean的作用域元数据解析器

④ - 条件注解评估器,用于判断是否跳过被条件注解修饰的类的加载

⑤ - 通过Bean注册器来构造Reader

注册内定的PostProcessor

在初始化AnnotatedBeanDefinitionReader的时候,将一些用于作基础处理的类处理器优先注册到容器内部,不太明白也没关系,照着代码看下去,看完几个就明白是干什么的了。

public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {
    Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
    Assert.notNull(environment, "Environment must not be null");
    this.registry = registry;
    this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);
    // 注册一些固定的内部指定的类到容器中
    AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
}

public static void registerAnnotationConfigProcessors(BeanDefinitionRegistry registry) {
    registerAnnotationConfigProcessors(registry, null);
}

public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors( registry, @Nullable Object source) {

    DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);

    if (beanFactory != null) {
        // ①
        if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {
            beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
        }
        // ②
        if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
            beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
        }
    }

    // ③
    Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<>(8);

    if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
        RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
        def.setSource(source);
        beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
    }

    if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
        RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
        def.setSource(source);
        beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
    }

    // Check for JSR-250 support, and if present add the CommonAnnotationBeanPostProcessor.
    if (jsr250Present && !registry.containsBeanDefinition(COMMON_ANNOTATION_PROCESSOR_BEAN_NAME)) {
        RootBeanDefinition def = new RootBeanDefinition(CommonAnnotationBeanPostProcessor.class);
        def.setSource(source);
        beanDefs.add(registerPostProcessor(registry, def, COMMON_ANNOTATION_PROCESSOR_BEAN_NAME));
    }

    // Check for JPA support, and if present add the PersistenceAnnotationBeanPostProcessor.
    if (jpaPresent && !registry.containsBeanDefinition(PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME)) {
        RootBeanDefinition def = new RootBeanDefinition();
        try {
            def.setBeanClass(ClassUtils.forName(PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME, AnnotationConfigUtils.class.getClassLoader()));
        }
        catch (ClassNotFoundException ex) {
            throw new IllegalStateException("Cannot load optional framework class: " + PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME, ex);
        }
        def.setSource(source);
        beanDefs.add(registerPostProcessor(registry, def, PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME));
    }

    if (!registry.containsBeanDefinition(EVENT_LISTENER_PROCESSOR_BEAN_NAME)) {
        RootBeanDefinition def = new RootBeanDefinition(EventListenerMethodProcessor.class);
        def.setSource(source);
        beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_PROCESSOR_BEAN_NAME));
    }

    if (!registry.containsBeanDefinition(EVENT_LISTENER_FACTORY_BEAN_NAME)) {
        RootBeanDefinition def = new RootBeanDefinition(DefaultEventListenerFactory.class);
        def.setSource(source);
        beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_FACTORY_BEAN_NAME));
    }

    return beanDefs;
}

① - 给BeanFactory添加AnnotationAwareOrderComparator,用于处理Bean的排序

② - 给BeanFactory添加ContextAnnotationAutowireCandidateResolver,用于处理@Lazy注解和@Qualifier注解

③ - 注册指定的类到容器

SpringBoot默认优先注册了以下PostProcessor类,并以不同的名称来称呼这些类:

左边的为BeanDefinition的名称,右边为对应的类,名称的前缀都统一是org.springframework.context.annotation

其实看到这里就明白许多,单纯的就是Spring固定了一些类要首先注册进去容器,也就是在处理注解的Reader准备完毕的时候,它已经准备好了一些后续要使用的类,他们用于在不同阶段来处理不同的事情。至于注册的这些类分别可以完成什么工作,将在他们真正开始执行的时候再进行分析。至此AnnotatedBeanDefinitionReader的初始化完成了。

和上面的Reader不同的是它用于扫描类路径上的Bean,通过给定的basePackage路径来扫描类。它可以使用可配置的过滤器来检测候选类,已确定他们将是否被加载。默认的过滤器包括Spring框架的@Component、@Controller、@Service、@Repository

// 构造方法一
public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry) {
    this(registry, true);
}

// 构造方法二
public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters) {
    this(registry, useDefaultFilters, getOrCreateEnvironment(registry));
}

// 构造方法三
public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters,
        Environment environment) {

    this(registry, useDefaultFilters, environment,
            (registry instanceof ResourceLoader ? (ResourceLoader) registry : null));
}

// 构造方法四
public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters, Environment environment, @Nullable ResourceLoader resourceLoader) {

    Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
    this.registry = registry;
    // 使用默认过滤器,从构造方法一直接传入true,代表它是默认的。
    if (useDefaultFilters) {
        // 注册默认过滤器
        registerDefaultFilters();
    }
    // 设置环境对象和资源加载器
    setEnvironment(environment);
    setResourceLoader(resourceLoader);
}

通过多个重载的构造方法来构造ClassPathBeanDefinitionScanner对象。最后一个实际使用的构造方法传入了类注册器、是否使用默认过滤器、环境对象、资源加载器。若传入的是一个普通的BeanDefinitionRegistry(实现了BeanDefinitionRegistry接口但没有实现ResourceLoader接口的类)那么默认的资源加载器将会是org.springframework.core.io.support.PathMatchingResourcePatternResolver.

注册默认注解过滤器

为@Component注册过滤器, 还会隐式的注册@Controller、@Service、@Repository。为何说是隐式的呢?查看这几个注解便知。

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Controller {

    @AliasFor(annotation = Component.class)
    String value() default "";
}

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Service {

    @AliasFor(annotation = Component.class)
    String value() default "";
}

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Repository {

    @AliasFor(annotation = Component.class)
    String value() default "";

}

他同时使用了@Component注解,并将别名设置为Component.class

// ClassPathScanningCandidateComponentProvider.java

protected void registerDefaultFilters() {
    // 新增Component注解过滤器(包含Controller、Service、Repository)
    this.includeFilters.add(new AnnotationTypeFilter(Component.class));
    ClassLoader cl = ClassPathScanningCandidateComponentProvider.class.getClassLoader();
    try {
        // 如果可用的话,支持JSR-250的ManagedBean注解过滤器
        this.includeFilters.add(new AnnotationTypeFilter(((Class<? extends Annotation>) ClassUtils.forName("javax.annotation.ManagedBean", cl)), false));
        logger.trace("JSR-250 'javax.annotation.ManagedBean' found and supported for component scanning");
    }
    catch (ClassNotFoundException ex) {
        // JSR-250 1.1 API (as included in Java EE 6) not available - simply skip.
    }
    try {
        // 如果可用的话,支持JSR-330支持的Named注解过滤器
        this.includeFilters.add(new AnnotationTypeFilter(((Class<? extends Annotation>) ClassUtils.forName("javax.inject.Named", cl)), false));
        logger.trace("JSR-330 'javax.inject.Named' annotation found and supported for component scanning");
    }
    catch (ClassNotFoundException ex) {
        // JSR-330 API not available - simply skip.
    }
}

在上下文容器创建完毕且未刷新的情况下,内部已经优先注册了一批PostProcessor,毫无疑问,他们将会在容器接下来的操作中扮演重要角色。这里只需要理解容器在程序运行的不同阶段会有不同的执行逻辑。大概知晓它在创建的时候做了什么,在后续文章中会逐步分析。