最近项目要实现一种需求，对于后端返回给前端的json格式的一种规范，不允许缺少字段和字段值都为null，所以琢磨了一下如何进行将springboot的Jackson序列化自定义一下，先看看如何实现，再去看源码
第一步：写配置类
@Configuration
public class WebConfiguration extends WebMvcConfigurationSupport {
@Override
protected void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
converters.stream().filter(c -> c instanceof MappingJackson2HttpMessageConverter)
.map(c ->(MappingJackson2HttpMessageConverter)c)
.forEach(c->{
ObjectMapper mapper = c.getObjectMapper();
// 为mapper注册一个带有SerializerModifier的Factory，此modifier主要做的事情为：当序列化类型为array，list、set时，当值为空时，序列化成[]
mapper.setSerializerFactory(mapper.getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));
c.setObjectMapper(mapper);
});
}
}
第二步：编写值为null时的自定义序列化
/**
* @title: MyBeanSerializerModifier
* @Author junyu
* 旧巷里有一个穿着白衬衫笑起来如太阳般温暖我的少年。
* 记忆里有一个穿着连衣裙哭起来如孩子般讨人喜的女孩。
* 他说，哪年树弯了腰，人见了老，桃花落了白发梢，他讲的笑话她还会笑，那便是好。
* 她说，哪年国改了号，坟长了草，地府过了奈何桥，她回头看时他还在瞧，就不算糟。
* @Date: 2020/9/12 16:44
* @Version 1.0
*/
public class MyBeanSerializerModifier extends BeanSerializerModifier {
private MyNullStringJsonSerializer myNullStringJsonSerializer;
private MyNullArrayJsonSerializer MyNullArrayJsonSerializer;
private MyNullObjectJsonSerializer MyNullObjectJsonSerializer;
private MyNullJsonSerializer myNullJsonSerializer;
public MyBeanSerializerModifier(){
myNullStringJsonSerializer = new MyNullStringJsonSerializer();
MyNullArrayJsonSerializer = new MyNullArrayJsonSerializer();
MyNullObjectJsonSerializer = new MyNullObjectJsonSerializer();
myNullJsonSerializer = new MyNullJsonSerializer();
}
@Override
public List<BeanPropertyWriter> changeProperties(SerializationConfig config, BeanDescription beanDesc,
List<BeanPropertyWriter> beanProperties) {
// 循环所有的beanPropertyWriter
beanProperties.forEach(writer ->{
// 判断字段的类型
if (isArrayType(writer)) {
//给writer注册一个自己的nullSerializer
writer.assignNullSerializer(MyNullArrayJsonSerializer);
} else if (isObjectType(writer)) {
writer.assignNullSerializer(MyNullObjectJsonSerializer);
} else if (isStringType(writer)) {
writer.assignNullSerializer(myNullStringJsonSerializer);
} else if (isPrimitiveType(writer)) {
writer.assignNullSerializer(myNullJsonSerializer);
}
});
return beanProperties;
}
// 判断是否是boolean类型
private boolean isPrimitiveType(BeanPropertyWriter writer) {
Class<?> clazz = writer.getType().getRawClass();
return clazz.isPrimitive();
}
// 判断是否是string类型
private boolean isStringType(BeanPropertyWriter writer) {
Class<?> clazz = writer.getType().getRawClass();
return clazz.equals(String.class);
}
// 判断是否是对象类型
private boolean isObjectType(BeanPropertyWriter writer) {
Class<?> clazz = writer.getType().getRawClass();
return !clazz.isPrimitive() && !clazz.equals(String.class)
&& clazz.isAssignableFrom(Object.class);
}
// 判断是否是集合类型
protected boolean isArrayType(BeanPropertyWriter writer) {
Class<?> clazz = writer.getType().getRawClass();
return clazz.isArray() || clazz.equals(List.class) || clazz.equals(Set.class);
}
class MyNullJsonSerializer extends JsonSerializer<Object>{
@Override
public void serialize(Object value, JsonGenerator jgen, SerializerProvider serializers) throws IOException {
if (value == null) {
jgen.writeNull();
}
}
}
class MyNullStringJsonSerializer extends JsonSerializer<Object>{
@Override
public void serialize(Object value, JsonGenerator jgen, SerializerProvider serializers) throws IOException {
if (value == null) {
jgen.writeString(StringUtils.EMPTY);
}
}
}
class MyNullArrayJsonSerializer extends JsonSerializer<Object>{
@Override
public void serialize(Object value, JsonGenerator jgen, SerializerProvider serializers) throws IOException {
if (value == null) {
jgen.writeStartArray();
jgen.writeEndArray();
}
}
}
class MyNullObjectJsonSerializer extends JsonSerializer<Object>{
@Override
public void serialize(Object value, JsonGenerator jgen, SerializerProvider serializers) throws IOException {
if (value == null) {
jgen.writeStartObject();
jgen.writeEndObject();
}
}
}
}
这样基本配置就完事了，现在可以试试效果了，自己定义一个bean用来返回，定义一个简单的controller去接受访问就行了，博主就不进行写这两个类了。返回结果如下
这是我的项目需求需要实现的，大家可以根据的自己的需求去改写MyBeanSerializerModifier这个类。还有另一种实现方式：不继承
@Configuration
public class WebConfiguration {
@Bean
public MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter(){
MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter = new MappingJackson2HttpMessageConverter();
ObjectMapper mapper = mappingJackson2HttpMessageConverter.getObjectMapper();
mapper.setSerializerFactory(mapper.getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));
mappingJackson2HttpMessageConverter.setObjectMapper(mapper);
return mappingJackson2HttpMessageConverter;
}
}
这种方法也是可以设置成功的，主要是不是继承了WebMvcConfigurationSupport类，毕竟这个类有很多可以自定义的方法，用起来顺手而已。
第一个问题：为什么继承WebMvcConfigurationSupport后，要重写extendMessageConverters方法；
第二个问题：为什么继承WebMvcConfigurationSupport后，再去生成@Bean的MappingJackson2HttpMessageConverter，却不生效；
第三个问题：为什么不继承WebMvcConfigurationSupport时，生成@Bean的MappingJackson2HttpMessageConverter是生效的；
这几个问题，都需要我们进入源码观察，废活不多说，我们来进入源码的世界。解决问题之前必须搞清楚在哪里进行了序列化。
第一步：我们要弄清楚在哪里进行的Jackson序列化，看这里https://www.processon.com/embed/5f5c6464f346fb7afd55448b，从返回请求开始的序列化基本流程就在这里了，虽然图有点low，但是清楚的记录的每一步，我们主要看一下下面的源码
/*
/**********************************************************
/* Field serialization methods
/**********************************************************
*/
//序列化每一个字段
protected void serializeFields(Object bean, JsonGenerator gen, SerializerProvider provider)
throws IOException
{
final BeanPropertyWriter[] props;
if (_filteredProps != null && provider.getActiveView() != null) {
props = _filteredProps;
} else {
props = _props;
}
int i = 0;
try {
for (final int len = props.length; i < len; ++i) {
BeanPropertyWriter prop = props[i];
if (prop != null) { // can have nulls in filtered list
//关键就在这一步进行的序列化，而为什么BeanPropertyWriter是数组，我们一会解释
prop.serializeAsField(bean, gen, provider);
}
}
if (_anyGetterWriter != null) {
_anyGetterWriter.getAndSerialize(bean, gen, provider);
}
} catch (Exception e) {
String name = (i == props.length) ? "[anySetter]" : props[i].getName();
wrapAndThrow(provider, e, bean, name);
} catch (StackOverflowError e) {
// 04-Sep-2009, tatu: Dealing with this is tricky, since we don't have many
// stack frames to spare... just one or two; can't make many calls.
// 10-Dec-2015, tatu: and due to above, avoid "from" method, call ctor directly:
//JsonMappingException mapE = JsonMappingException.from(gen, "Infinite recursion (StackOverflowError)", e);
JsonMappingException mapE = new JsonMappingException(gen, "Infinite recursion (StackOverflowError)", e);
String name = (i == props.length) ? "[anySetter]" : props[i].getName();
mapE.prependPath(new JsonMappingException.Reference(bean, name));
throw mapE;
}
}
既然已经找到了在哪里要进行序列化，那我们看看是如何实现的：
/**
* Method called to access property that this bean stands for, from within
* given bean, and to serialize it as a JSON Object field using appropriate
* serializer.
*/
@Override
public void serializeAsField(Object bean, JsonGenerator gen,
SerializerProvider prov) throws Exception {
// inlined 'get()'
final Object value = (_accessorMethod == null) ? _field.get(bean)
: _accessorMethod.invoke(bean, (Object[]) null);
// Null handling is bit different, check that first
if (value == null) {
//看到这里大家应该就知道null值是如何进行序列化 的了，如果不配置的话，默认是返回null
//因为_nullSerializer是有默认值的，大家看一看这个类的初始化
//那我们要是改一下_nullSerializer的这个默认类，让每一个字段调用我们自己的_nullSerializer不就可以了吗，
//yes、我们就这么干
if (_nullSerializer != null) {
gen.writeFieldName(_name);
_nullSerializer.serialize(null, gen, prov);
}
return;
}
// then find serializer to use
JsonSerializer<Object> ser = _serializer;
if (ser == null) {
Class<?> cls = value.getClass();
PropertySerializerMap m = _dynamicSerializers;
ser = m.serializerFor(cls);
if (ser == null) {
ser = _findAndAddDynamic(m, cls, prov);
}
}
// and then see if we must suppress certain values (default, empty)
if (_suppressableValue != null) {
if (MARKER_FOR_EMPTY == _suppressableValue) {
if (ser.isEmpty(prov, value)) {
return;
}
} else if (_suppressableValue.equals(value)) {
return;
}
}
// For non-nulls: simple check for direct cycles
if (value == bean) {
// three choices: exception; handled by call; or pass-through
if (_handleSelfReference(bean, gen, prov, ser)) {
return;
}
}
gen.writeFieldName(_name);
if (_typeSerializer == null) {
ser.serialize(value, gen, prov);
} else {
ser.serializeWithType(value, gen, prov, _typeSerializer);
}
}
那我们来解决第一个问题：为什么继承WebMvcConfigurationSupport后，要重写extendMessageConverters方法？
不知道大家记得不记得我们请求过来的时候，如果我们配置类集成了WebMvcConfigurationSupport类，dispatchservlet处理handle请求的ha，其实就是RequestMappingHandlerAdapter类，这个类是在WebMvcConfigurationSupport配置的，看源码：
@Bean
public RequestMappingHandlerAdapter requestMappingHandlerAdapter(
@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,
@Qualifier("mvcConversionService") FormattingConversionService conversionService,
@Qualifier("mvcValidator") Validator validator) {
RequestMappingHandlerAdapter adapter = createRequestMappingHandlerAdapter();
adapter.setContentNegotiationManager(contentNegotiationManager);
adapter.setMessageConverters(getMessageConverters());
adapter.setWebBindingInitializer(getConfigurableWebBindingInitializer(conversionService, validator));
adapter.setCustomArgumentResolvers(getArgumentResolvers());
adapter.setCustomReturnValueHandlers(getReturnValueHandlers());
if (jackson2Present) {
adapter.setRequestBodyAdvice(Collections.singletonList(new JsonViewRequestBodyAdvice()));
adapter.setResponseBodyAdvice(Collections.singletonList(new JsonViewResponseBodyAdvice()));
}
AsyncSupportConfigurer configurer = new AsyncSupportConfigurer();
configureAsyncSupport(configurer);
if (configurer.getTaskExecutor() != null) {
adapter.setTaskExecutor(configurer.getTaskExecutor());
}
if (configurer.getTimeout() != null) {
adapter.setAsyncRequestTimeout(configurer.getTimeout());
}
adapter.setCallableInterceptors(configurer.getCallableInterceptors());
adapter.setDeferredResultInterceptors(configurer.getDeferredResultInterceptors());
return adapter;
}
adapter.setMessageConverters(getMessageConverters());当大家看到这个方法的时候，应该就会想到我们的默认jackson转换器：MappingJackson2HttpMessageConverter,我们看看这个getMessageConverters()有什么幺蛾子：
protected final List<HttpMessageConverter<?>> getMessageConverters() {
if (this.messageConverters == null) {
this.messageConverters = new ArrayList<>();
configureMessageConverters(this.messageConverters);
if (this.messageConverters.isEmpty()) {
addDefaultHttpMessageConverters(this.messageConverters);
}
extendMessageConverters(this.messageConverters);
}
return this.messageConverters;
}
protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
//这些都不用管，跟我们的需求没啥作用，我们只看关键的部分，在下面
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);
}
//解析我们返回值的转换器就是在这里生成的
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()));
}
}
我们的MappingJackson2HttpMessageConverter类就是这里初始化的，初始化的时候默认的_nullSerializer也会被初始化，大家肯定说这已经初始化完了，该咋办，大家应该看到了extendMessageConverters(this.messageConverters);这个方法就是用来重写实现的了，这回知道我们继承WebMvcConfigurationSupport后，为什么要重写extendMessageConverters，我们的配置类遍历已经获取到的convert，然后对我们想要的转换器进行修改添加，那修改完了，是在哪里起作用的呢，我们再来看一看源码：
在序列化之前有一些方法是可以进行修改操作的，在调用writeWithMessageConverters方法的时候：
protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
.......
MediaType selectedMediaType = null;
MediaType contentType = outputMessage.getHeaders().getContentType();
boolean isContentTypePreset = contentType != null && contentType.isConcrete();
if (isContentTypePreset) {
if (logger.isDebugEnabled()) {
logger.debug("Found 'Content-Type:" + contentType + "' in response");
}
selectedMediaType = contentType;
}
else {
HttpServletRequest request = inputMessage.getServletRequest();
List<MediaType> acceptableTypes = getAcceptableMediaTypes(request);
//这里进行自定义操作修改MappingJackson2HttpMessageConverter
List<MediaType> producibleTypes = getProducibleMediaTypes(request, valueType, targetType);
.......
if (selectedMediaType != null) {
selectedMediaType = selectedMediaType.removeQualityValue();
//这这里进行选择我们的MappingJackson2HttpMessageConverter去自定义序列化
for (HttpMessageConverter<?> converter : this.messageConverters) {
GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
(GenericHttpMessageConverter<?>) converter : null);
if (genericConverter != null ?
((GenericHttpMessageConverter) converter).canWrite(targetType, valueType, selectedMediaType) :
converter.canWrite(valueType, selectedMediaType)) {
body = getAdvice().beforeBodyWrite(body, returnType, selectedMediaType,
(Class<? extends HttpMessageConverter<?>>) converter.getClass(),
inputMessage, outputMessage);
if (body != null) {
Object theBody = body;
LogFormatUtils.traceDebug(logger, traceOn ->
"Writing [" + LogFormatUtils.formatValue(theBody, !traceOn) + "]");
addContentDispositionHeader(inputMessage, outputMessage);
if (genericConverter != null) {
genericConverter.write(body, targetType, selectedMediaType, outputMessage);
}
else {
((HttpMessageConverter) converter).write(body, selectedMediaType, outputMessage);
}
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Nothing to write: null body");
}
}
return;
}
}
}
if (body != null) {
Set<MediaType> producibleMediaTypes =
(Set<MediaType>) inputMessage.getServletRequest()
.getAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
if (isContentTypePreset || !CollectionUtils.isEmpty(producibleMediaTypes)) {
throw new HttpMessageNotWritableException(
"No converter for [" + valueType + "] with preset Content-Type '" + contentType + "'");
}
throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
}
}
我们一直追踪getProducibleMediaTypes后，最终发现会调用BeanSerializerFactory的constructBeanOrAddOnSerializer，就是这里进行修改操作的。
protected JsonSerializer<Object> constructBeanOrAddOnSerializer(SerializerProvider prov,
JavaType type, BeanDescription beanDesc, boolean staticTyping)
throws JsonMappingException
{
// 13-Oct-2010, tatu: quick sanity check: never try to create bean serializer for plain Object
// 05-Jul-2012, tatu: ... but we should be able to just return "unknown type" serializer, right?
if (beanDesc.getBeanClass() == Object.class) {
return prov.getUnknownTypeSerializer(Object.class);
// throw new IllegalArgumentException("Cannot create bean serializer for Object.class");
}
final SerializationConfig config = prov.getConfig();
BeanSerializerBuilder builder = constructBeanSerializerBuilder(beanDesc);
builder.setConfig(config);
// First: any detectable (auto-detect, annotations) properties to serialize?
List<BeanPropertyWriter> props = findBeanProperties(prov, beanDesc, builder);
if (props == null) {
props = new ArrayList<BeanPropertyWriter>();
} else {
props = removeOverlappingTypeIds(prov, beanDesc, builder, props);
}
// [databind#638]: Allow injection of "virtual" properties:
prov.getAnnotationIntrospector().findAndAddVirtualProperties(config, beanDesc.getClassInfo(), props);
// [JACKSON-440] Need to allow modification bean properties to serialize:
if (_factoryConfig.hasSerializerModifiers()) {
for (BeanSerializerModifier mod : _factoryConfig.serializerModifiers()) {
props = mod.changeProperties(config, beanDesc, props);
}
}
.......//此处省略
}
大家看一下props = mod.changeProperties(config, beanDesc, props);我们在配置类里面可是为我们的MappingJackson2HttpMessageConverter配置了 withSerializerModifier方法，并且设置了我们的MyBeanSerializerModifier并且继承BeanSerializerModifier重写了 changeProperties，所以会调用我们的changeProperties方法，进行修改null值的序列化类，我们也返回了一个list类型的BeanPropertyWriter，所以知道为什么那个BeanPropertyWriter在解析时，会是个数组类型的了吧，因为不同字段解析是不一样的。
剩下的解释一下为什么单独配置并设置实例化@bean的MappingJackson2HttpMessageConverter也是好使的呢，大家可以看一下JacksonHttpMessageConvertersConfiguration类的源码，里面明确写了@ConditionalOnMissingBean注解，如果我们自己进行配置了，这个注入后就是一个备胎，以我们的为准，这个不多说
我们再来解析一下第二个问题：为什么继承WebMvcConfigurationSupport后，再去生成@Bean的MappingJackson2HttpMessageConverter，却不生效，这需要跟第三个问题一起解决：为什么不继承WebMvcConfigurationSupport时，生成@Bean的MappingJackson2HttpMessageConverter是生效的。
我们知道当我们继承WebMvcConfigurationSupport后，有一个配置会自动失效，就是自动注入的一个mvc配置，可以看看@SpringBootApplication注解里面有个@EnableAutoConfiguration注解，会引入一个AutoConfigurationImportSelector类。这个类就会扫描org.springframework.boot:spring-boot-autoconfigure下的spring.factories文件，这里面有一个我们默认的mvn配置也是继承了WebMvcConfigurationSupport，叫WebMvcAutoConfiguration，我们来看一下源码：
@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class })
//注意此处有一个ConditionalOnMissingBean注解，所以如果我们自己继承后，就相当于已经存在WebMvcConfigurationSupport类，
//就会走我们自己的配置类，此配置会失效
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
@AutoConfigureAfter({ DispatcherServletAutoConfiguration.class, TaskExecutionAutoConfiguration.class,
ValidationAutoConfiguration.class })
public class WebMvcAutoConfiguration {
.....
@Configuration(proxyBeanMethods = false)
public static class EnableWebMvcConfiguration extends DelegatingWebMvcConfiguration implements ResourceLoaderAware {
private final ResourceProperties resourceProperties;
private final WebMvcProperties mvcProperties;
private final ListableBeanFactory beanFactory;
private final WebMvcRegistrations mvcRegistrations;
private ResourceLoader resourceLoader;
public EnableWebMvcConfiguration(ResourceProperties resourceProperties,
ObjectProvider<WebMvcProperties> mvcPropertiesProvider,
ObjectProvider<WebMvcRegistrations> mvcRegistrationsProvider, ListableBeanFactory beanFactory) {
this.resourceProperties = resourceProperties;
this.mvcProperties = mvcPropertiesProvider.getIfAvailable();
this.mvcRegistrations = mvcRegistrationsProvider.getIfUnique();
this.beanFactory = beanFactory;
}
//如果我们不继承的话，处理请求的RequestMappingHandlerAdapter就会在这里生成
//会调用DelegatingWebMvcConfiguration里面的 requestMappingHandlerAdapter方法，
@Bean
@Override
public RequestMappingHandlerAdapter requestMappingHandlerAdapter(
@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,
@Qualifier("mvcConversionService") FormattingConversionService conversionService,
@Qualifier("mvcValidator") Validator validator) {
RequestMappingHandlerAdapter adapter = super.requestMappingHandlerAdapter(contentNegotiationManager,
conversionService, validator);
adapter.setIgnoreDefaultModelOnRedirect(
this.mvcProperties == null || this.mvcProperties.isIgnoreDefaultModelOnRedirect());
return adapter;
}
.....
}
不知道大家是否还记得getMessageConverters()方法里面添加默认messageConverters的时候之前，会调用一个configureMessageConverters(this.messageConverters);方法，我们的DelegatingWebMvcConfiguration 就已经重写了这个方法，所以我们如果不继承WebMvcConfigurationSupport，将会把我们的@bean形式存在的MappingJackson2HttpMessageConverter将会被扫描到
@Override
protected void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
this.configurers.configureMessageConverters(converters);
}
//会添加我们的convert
@Override
public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
for (WebMvcConfigurer delegate : this.delegates) {
delegate.configureMessageConverters(converters);
}
}
现在我们配置的自定义jackson序列化已经生效了，但是，你仔细看我的流程图会发现，其实调用序列化的时候走的是RequestResponseBodyMethodProcessor的handleReturnValue方法
@Override
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
mavContainer.setRequestHandled(true);
ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
// Try even with null return value. ResponseBodyAdvice could get involved.
//这里进入序列化流程
writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
}
最后在序列化的时候也是从这个类或则父类里面的一个属性：messageConverters
public abstract class AbstractMessageConverterMethodArgumentResolver implements HandlerMethodArgumentResolver {
private static final Set<HttpMethod> SUPPORTED_METHODS =
EnumSet.of(HttpMethod.POST, HttpMethod.PUT, HttpMethod.PATCH);
private static final Object NO_VALUE = new Object();
protected final Log logger = LogFactory.getLog(getClass());
//这个属性取值的
protected final List<HttpMessageConverter<?>> messageConverters;
protected final List<MediaType> allSupportedMediaTypes;
private final RequestResponseBodyAdviceChain advice;
...
}
于是，小伙伴们就疑惑了，这我们自定义的在RequestMappingHandlerAdapter里面呢，跟这个类也没关系啊，属性是咋设置进来的呢？我们再看看RequestMappingHandlerAdapter的源码，你会发现，RequestMappingHandlerAdapter这个类实现了InitializingBean类，也就说明，创建RequestMappingHandlerAdapter的时候会调用afterPropertiesSet方法，至于为啥，看源码吧：（不是主要流程）
//在createBean的时候会调用这个方法，看看是否实现了InitializingBean
protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)
throws Throwable {
boolean isInitializingBean = (bean instanceof InitializingBean);
if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
if (logger.isTraceEnabled()) {
logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
}
if (System.getSecurityManager() != null) {
try {
AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
((InitializingBean) bean).afterPropertiesSet();
return null;
}, getAccessControlContext());
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
//在这里进行调用的，
((InitializingBean) bean).afterPropertiesSet();
}
}
if (mbd != null && bean.getClass() != NullBean.class) {
String initMethodName = mbd.getInitMethodName();
if (StringUtils.hasLength(initMethodName) &&
!(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
!mbd.isExternallyManagedInitMethod(initMethodName)) {
invokeCustomInitMethod(beanName, bean, mbd);
}
}
}
那我们看看RequestMappingHandlerAdapter的afterPropertiesSet方法都干了些啥吧。
@Override
public void afterPropertiesSet() {
// Do this first, it may add ResponseBody advice beans
initControllerAdviceCache();
if (this.argumentResolvers == null) {
List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
}
if (this.initBinderArgumentResolvers == null) {
List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();
this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
}
if (this.returnValueHandlers == null) {
//是在这里生成的类
List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
}
}
private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {
List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>();
// Single-purpose return value types
handlers.add(new ModelAndViewMethodReturnValueHandler());
handlers.add(new ModelMethodProcessor());
handlers.add(new ViewMethodReturnValueHandler());
handlers.add(new ResponseBodyEmitterReturnValueHandler(getMessageConverters(),
this.reactiveAdapterRegistry, this.taskExecutor, this.contentNegotiationManager));
handlers.add(new StreamingResponseBodyReturnValueHandler());
handlers.add(new HttpEntityMethodProcessor(getMessageConverters(),
this.contentNegotiationManager, this.requestResponseBodyAdvice));
handlers.add(new HttpHeadersReturnValueHandler());
handlers.add(new CallableMethodReturnValueHandler());
handlers.add(new DeferredResultMethodReturnValueHandler());
handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));
// Annotation-based return value types
handlers.add(new ModelAttributeMethodProcessor(false));
//看到这个类了吗？生成的时候将RequestMappingHandlerAdapter里面的转换器设置进去了
handlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(),
this.contentNegotiationManager, this.requestResponseBodyAdvice));
// Multi-purpose return value types
handlers.add(new ViewNameMethodReturnValueHandler());
handlers.add(new MapMethodProcessor());
// Custom return value types
if (getCustomReturnValueHandlers() != null) {
handlers.addAll(getCustomReturnValueHandlers());
}
// Catch-all
if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {
handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));
}
else {
handlers.add(new ModelAttributeMethodProcessor(true));
}
return handlers;
}
讲到这里，不知道大家理解了多少，这些都是博主遇到需求后，自己问自己的问题，自己通过源码回答问题的，也希望大家能理解源码。还有一篇源码文章在路上：为什么我们的项目里出现两个配置类继承WebMvcConfigurationSupport时，只有一个会生效。我在网上找了半天都是说结果的，没有人分析源码到底是为啥，博主准备讲解一下，到希望可以帮 大家 ！