spring boot 是如何加载jackson的？

Spring Boot 自动引入jackson: 通过：Spring-Boot-starter-web

Jackson自动配置

这里的configurations是读取的这里：

通过反射加载JacksonAutoConfiguration:

public void configure(ObjectMapper objectMapper) {
        Assert.notNull(objectMapper, "ObjectMapper must not be null");

        MultiValueMap<Object, Module> modulesToRegister = new LinkedMultiValueMap<>();
        if (this.findModulesViaServiceLoader) {
            ObjectMapper.findModules(this.moduleClassLoader).forEach(module -> registerModule(module, modulesToRegister));
        }
        else if (this.findWellKnownModules) {
            registerWellKnownModulesIfAvailable(modulesToRegister);
        }

        if (this.modules != null) {
            this.modules.forEach(module -> registerModule(module, modulesToRegister));
        }
        if (this.moduleClasses != null) {
            for (Class<? extends Module> moduleClass : this.moduleClasses) {
                registerModule(BeanUtils.instantiateClass(moduleClass), modulesToRegister);
            }
        }
        List<Module> modules = new ArrayList<>();
        for (List<Module> nestedModules : modulesToRegister.values()) {
            modules.addAll(nestedModules);
        }
        objectMapper.registerModules(modules);

        if (this.dateFormat != null) {
            objectMapper.setDateFormat(this.dateFormat);
        }
        if (this.locale != null) {
            objectMapper.setLocale(this.locale);
        }
        if (this.timeZone != null) {
            objectMapper.setTimeZone(this.timeZone);
        }

        if (this.annotationIntrospector != null) {
            objectMapper.setAnnotationIntrospector(this.annotationIntrospector);
        }
        if (this.propertyNamingStrategy != null) {
            objectMapper.setPropertyNamingStrategy(this.propertyNamingStrategy);
        }
        if (this.defaultTyping != null) {
            objectMapper.setDefaultTyping(this.defaultTyping);
        }
        if (this.serializationInclusion != null) {
            objectMapper.setSerializationInclusion(this.serializationInclusion);
        }

        if (this.filters != null) {
            objectMapper.setFilterProvider(this.filters);
        }

        this.mixIns.forEach(objectMapper::addMixIn);

        if (!this.serializers.isEmpty() || !this.deserializers.isEmpty()) {
            SimpleModule module = new SimpleModule();
            addSerializers(module);
            addDeserializers(module);
            objectMapper.registerModule(module);
        }

        this.visibilities.forEach(objectMapper::setVisibility);

        customizeDefaultFeatures(objectMapper);
        this.features.forEach((feature, enabled) -> configureFeature(objectMapper, feature, enabled));

        if (this.handlerInstantiator != null) {
            objectMapper.setHandlerInstantiator(this.handlerInstantiator);
        }
        else if (this.applicationContext != null) {
            objectMapper.setHandlerInstantiator(
                    new SpringHandlerInstantiator(this.applicationContext.getAutowireCapableBeanFactory()));
        }
    }

上面的类中会用到一些yml中的一些配置，是在下面的代码中获取：

在WebMvcConfigurationSupport中可以看到：

public class WebMvcConfigurationSupport implements ApplicationContextAware, ServletContextAware {

    private static final boolean romePresent;

    private static final boolean jaxb2Present;

    private static final boolean jackson2Present;

    private static final boolean jackson2XmlPresent;

    private static final boolean jackson2SmilePresent;

    private static final boolean jackson2CborPresent;

    private static final boolean gsonPresent;

    private static final boolean jsonbPresent;

    static {
        ClassLoader classLoader = WebMvcConfigurationSupport.class.getClassLoader();
        romePresent = ClassUtils.isPresent("com.rometools.rome.feed.WireFeed", classLoader);
        jaxb2Present = ClassUtils.isPresent("javax.xml.bind.Binder", classLoader);
        jackson2Present = ClassUtils.isPresent("com.fasterxml.jackson.databind.ObjectMapper", classLoader) &&
                        ClassUtils.isPresent("com.fasterxml.jackson.core.JsonGenerator", classLoader);
        jackson2XmlPresent = ClassUtils.isPresent("com.fasterxml.jackson.dataformat.xml.XmlMapper", classLoader);
        jackson2SmilePresent = ClassUtils.isPresent("com.fasterxml.jackson.dataformat.smile.SmileFactory", classLoader);
        jackson2CborPresent = ClassUtils.isPresent("com.fasterxml.jackson.dataformat.cbor.CBORFactory", classLoader);
        gsonPresent = ClassUtils.isPresent("com.google.gson.Gson", classLoader);
        jsonbPresent = ClassUtils.isPresent("javax.json.bind.Jsonb", classLoader);
    }
}

protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
        StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
        stringHttpMessageConverter.setWriteAcceptCharset(false);  // see SPR-7316

        messageConverters.add(new ByteArrayHttpMessageConverter());
        messageConverters.add(stringHttpMessageConverter);
        messageConverters.add(new ResourceHttpMessageConverter());
        messageConverters.add(new ResourceRegionHttpMessageConverter());
        try {
            messageConverters.add(new SourceHttpMessageConverter<>());
        }
        catch (Throwable ex) {
            // Ignore when no TransformerFactory implementation is available...
        }
        messageConverters.add(new AllEncompassingFormHttpMessageConverter());

        if (romePresent) {
            messageConverters.add(new AtomFeedHttpMessageConverter());
            messageConverters.add(new RssChannelHttpMessageConverter());
        }

        if (jackson2XmlPresent) {
            Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.xml();
            if (this.applicationContext != null) {
                builder.applicationContext(this.applicationContext);
            }
            messageConverters.add(new MappingJackson2XmlHttpMessageConverter(builder.build()));
        }
        else if (jaxb2Present) {
            messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
        }

        if (jackson2Present) {
            Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.json();
            if (this.applicationContext != null) {
                builder.applicationContext(this.applicationContext);
            }
            messageConverters.add(new MappingJackson2HttpMessageConverter(builder.build()));
        }
        else if (gsonPresent) {
            messageConverters.add(new GsonHttpMessageConverter());
        }
        else if (jsonbPresent) {
            messageConverters.add(new JsonbHttpMessageConverter());
        }

        if (jackson2SmilePresent) {
            Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.smile();
            if (this.applicationContext != null) {
                builder.applicationContext(this.applicationContext);
            }
            messageConverters.add(new MappingJackson2SmileHttpMessageConverter(builder.build()));
        }
        if (jackson2CborPresent) {
            Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.cbor();
            if (this.applicationContext != null) {
                builder.applicationContext(this.applicationContext);
            }
            messageConverters.add(new MappingJackson2CborHttpMessageConverter(builder.build()));
        }
    }

在使用Jackson进行序列化和反序列化时主要是使用MappingJackson2SmileHttpMessageConverter进行的，下一篇详细介绍一下。