SpringCloud中通过Feign接口实现服务的远程调用，对于数据的序列化和反序列化依旧借助了HttpMessageConverter（消息转换器）
加载流程
项目启动后加载改容器：
@Configuration(proxyBeanMethods = false)
public class FeignClientsConfiguration {
@Autowired
private ObjectFactory<HttpMessageConverters> messageConverters;
@Bean
@ConditionalOnMissingBean
public Decoder feignDecoder() {
return new OptionalDecoder(
new ResponseEntityDecoder(new SpringDecoder(this.messageConverters)));
}
@Bean
@ConditionalOnMissingBean
@ConditionalOnMissingClass("org.springframework.data.domain.Pageable")
public Encoder feignEncoder() {
return new SpringEncoder(this.messageConverters);
}
}
会加载Spring容器中的HttpMessageConverters对象。
public class HttpMessageConvertersAutoConfiguration {
static final String PREFERRED_MAPPER_PROPERTY = "spring.http.converters.preferred-json-mapper";
@Bean
@ConditionalOnMissingBean
public HttpMessageConverters messageConverters(ObjectProvider<HttpMessageConverter<?>> converters) {
return new HttpMessageConverters(converters.orderedStream().collect(Collectors.toList()));
}
}
上面的代码会读取Spring容器中所有的HttpMessageConverter对象，开始构造HttpMessageConverters对象。
public class HttpMessageConverters implements Iterable<HttpMessageConverter<?>> {
private final List<HttpMessageConverter<?>> converters;
//构造方法，此时传入的消息转换器为Spring容器中的
public HttpMessageConverters(Collection<HttpMessageConverter<?>> additionalConverters) {
this(true, additionalConverters);
}
//参数1为true：表示会加载默认的消息转换器
public HttpMessageConverters(boolean addDefaultConverters, Collection<HttpMessageConverter<?>> converters) {
//核心方法getCombinedConverters，将自定义消息转换器和默认的消息转换器排序。
//由于参数为true，那么会调用getDefaultConverters()获取默认的消息转换器。
List<HttpMessageConverter<?>> combined = getCombinedConverters(converters,
addDefaultConverters ? getDefaultConverters() : Collections.emptyList());
combined = postProcessConverters(combined);
this.converters = Collections.unmodifiableList(combined);
}
//如果自定义转换器可以替换默认转换器，那么自定义转换器放在可替换默认转换器前，若不能替换，那么放在列表最前的位置。
private List<HttpMessageConverter<?>> getCombinedConverters(Collection<HttpMessageConverter<?>> converters,
List<HttpMessageConverter<?>> defaultConverters) {
List<HttpMessageConverter<?>> combined = new ArrayList<>();
List<HttpMessageConverter<?>> processing = new ArrayList<>(converters);
for (HttpMessageConverter<?> defaultConverter : defaultConverters) {
Iterator<HttpMessageConverter<?>> iterator = processing.iterator();
while (iterator.hasNext()) {
HttpMessageConverter<?> candidate = iterator.next();
if (isReplacement(defaultConverter, candidate)) {
combined.add(candidate);
iterator.remove();
}
}
combined.add(defaultConverter);
if (defaultConverter instanceof AllEncompassingFormHttpMessageConverter) {
configurePartConverters((AllEncompassingFormHttpMessageConverter) defaultConverter, converters);
}
}
combined.addAll(0, processing);
return combined;
}
//判断自定义的消息转换器是否可以替换默认的消息转换器
private boolean isReplacement(HttpMessageConverter<?> defaultConverter, HttpMessageConverter<?> candidate) {
for (Class<?> nonReplacingConverter : NON_REPLACING_CONVERTERS) {
if (nonReplacingConverter.isInstance(candidate)) {
return false;
}
}
return ClassUtils.isAssignableValue(defaultConverter.getClass(), candidate);
}
//获取默认的消息转换器
private List<HttpMessageConverter<?>> getDefaultConverters() {
List<HttpMessageConverter<?>> converters = new ArrayList<>();
//判断是否引入了WebMvcConfigurationSupport类的包，mvc一般会加载
if (ClassUtils.isPresent("org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport",
null)) {
converters.addAll(new WebMvcConfigurationSupport() {
public List<HttpMessageConverter<?>> defaultMessageConverters() {
return super.getMessageConverters();
}
}.defaultMessageConverters());
}
else {
//使用RestTemplate中定义的消息转换器。
converters.addAll(new RestTemplate().getMessageConverters());
}
//重排序，将xml消息转换器放在最后
reorderXmlConvertersToEnd(converters);
return converters;
}
//将xml的消息转换器放在最后
private void reorderXmlConvertersToEnd(List<HttpMessageConverter<?>> converters) {
List<HttpMessageConverter<?>> xml = new ArrayList<>();
for (Iterator<HttpMessageConverter<?>> iterator = converters.iterator(); iterator.hasNext();) {
HttpMessageConverter<?> converter = iterator.next();
if ((converter instanceof AbstractXmlHttpMessageConverter)
|| (converter instanceof MappingJackson2XmlHttpMessageConverter)) {
xml.add(converter);
iterator.remove();
}
}
converters.addAll(xml);
}
}
主要的逻辑是getDefaultConverters()方法，new WebMvcConfigurationSupport()对象会创建默认的消息转换器。
public class WebMvcConfigurationSupport implements ApplicationContextAware, ServletContextAware {
//判断是否引入了改依赖
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);
}
//核心方法
protected final List<HttpMessageConverter<?>> getMessageConverters() {
if (this.messageConverters == null) {
this.messageConverters = new ArrayList<>();
configureMessageConverters(this.messageConverters);
if (this.messageConverters.isEmpty()) {
//增加默认的消息转换器
addDefaultHttpMessageConverters(this.messageConverters);
}
extendMessageConverters(this.messageConverters);
}
return this.messageConverters;
}
//创建默认的消息转换器
protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
messageConverters.add(new ByteArrayHttpMessageConverter());
messageConverters.add(new 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()));
}
}
}
影响范围
当我们向Spring容器注册两个消息转换器对象时：
@Service
public class NotNullMappingJackson2HttpMessageConverter extends MappingJackson2HttpMessageConverter {
@Override
public ObjectMapper getObjectMapper() {
return super.getObjectMapper().setSerializationInclusion(JsonInclude.Include.NON_NULL);
}
}
一个自定义转换器是实现了MappingJackson2HttpMessageConverter接口。
理论上优先级仅仅比MappingJackson2HttpMessageConverter高。
@Configuration
public class MessageConverterConfig2 {
@Bean
public FastJsonHttpMessageConverter fastJsonHttpMessageConverter() {
//1.需要定义一个convert转换消息的对象;
FastJsonHttpMessageConverter fastJsonHttpMessageConverter = new FastJsonHttpMessageConverter();
//2.添加fastJson的配置信息，比如：是否要格式化返回的json数据;
FastJsonConfig fastJsonConfig = new FastJsonConfig();
fastJsonConfig.setSerializerFeatures(
SerializerFeature.WriteMapNullValue,
SerializerFeature.WriteNullNumberAsZero,
SerializerFeature.QuoteFieldNames,
SerializerFeature.WriteNullStringAsEmpty,
SerializerFeature.DisableCircularReferenceDetect,
SerializerFeature.WriteNullListAsEmpty,
SerializerFeature.WriteDateUseDateFormat);
//3处理中文乱码问题
List<MediaType> fastMediaTypes = new ArrayList<>();
fastMediaTypes.add(MediaType.APPLICATION_JSON);
//4.在convert中添加配置信息.
fastJsonHttpMessageConverter.setSupportedMediaTypes(fastMediaTypes);
fastJsonHttpMessageConverter.setFastJsonConfig(fastJsonConfig);
return fastJsonHttpMessageConverter;
}
}
FastJsonHttpMessageConverter继承了AbstractHttpMessageConverter类。
由于不能替换默认的消息转换器，那么会放在列表之上。
启动项目，查看断点：
image.png
