spring boot 是如何利用jackson进行序列化的？

接上一篇：spring boot 是如何利用jackson进行反序列化的？

@RestController
public class HelloController {

    @RequestMapping("/")
    public BillSearch hello(@RequestBody BillSearch search) {

        return search;
    }
}

返回的search是如何序列化json的？

上一篇说到RequestResponseBodyMethodProcessor这个类在json序列化和反序列化都中很重要：

protected <T> Object readWithMessageConverters(NativeWebRequest webRequest, MethodParameter parameter,
            Type paramType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {

        HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);
        Assert.state(servletRequest != null, "No HttpServletRequest");
        ServletServerHttpRequest inputMessage = new ServletServerHttpRequest(servletRequest);

        Object arg = readWithMessageConverters(inputMessage, parameter, paramType);
        if (arg == null && checkRequired(parameter)) {
            throw new HttpMessageNotReadableException("Required request body is missing: " +
                    parameter.getExecutable().toGenericString(), inputMessage);
        }
        return arg;
    }

上面是反序列化时用的。

@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.
        writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
    }

上面是序列化时用到的。

当然也可以F7一点一点的debug也会进入到这里。

然后继续：

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;
                }
            }

又走了这样的循环城，上篇中也介绍到了类型的循环，它就是要找到个们的jackson:

org.springframework.http.converter.json.MappingJackson2HttpMessageConverter

下面的步骤太多，我就把调用的堆栈截图出来：

看第一行，又是BeanSerielizer

public final void serialize(Object bean, JsonGenerator gen, SerializerProvider provider)
        throws IOException
    {
        if (_objectIdWriter != null) {
            gen.setCurrentValue(bean); // [databind#631]
            _serializeWithObjectId(bean, gen, provider, true);
            return;
        }
        gen.writeStartObject(bean);
        if (_propertyFilterId != null) {
            serializeFieldsFiltered(bean, gen, provider);
        } else {
            serializeFields(bean, gen, provider);
        }
        gen.writeEndObject();
    }

开始序列化每个属性：

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
                    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;
        }
    }

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) {
            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);
        }
    }

这里用到的序列化的类是com.fasterxml.jackson.databind.ser.std.EnumSerializer，是不是和上一篇很像，其实序列化和反序列化的原理和过程真的很像。

这里由于属性是用的是枚举，但序列化后是汉字：

如果想序列化成数字呢？

我的枚举是这样的：

@Getter
@AllArgsConstructor
//@JsonDeserialize(using = BaseEnumDeserializer.class)
public enum EnumBookingState implements BaseEnum {

    订购中(1),
    订购完成(2);
    //

//@JsonValue

private final int value; }

很简单，只需要在value上中上@JsonValue，但它的原理是什么呢？

同样是上面的截图，但是序列化的工具就是不一样了：

public void serialize(Object bean, JsonGenerator gen, SerializerProvider prov) throws IOException
    {
        try {
            Object value = _accessor.getValue(bean);
            if (value == null) {
                prov.defaultSerializeNull(gen);
                return;
            }
            JsonSerializer<Object> ser = _valueSerializer;
            if (ser == null) {
                Class<?> c = value.getClass();
                /* 10-Mar-2010, tatu: Ideally we would actually separate out type
                 *   serializer from value serializer; but, alas, there's no access
                 *   to serializer factory at this point...
                 */
                // let's cache it, may be needed soon again
                ser = prov.findTypedValueSerializer(c, true, _property);
            }
            ser.serialize(value, gen, prov);
        } catch (Exception e) {
            wrapAndThrow(prov, e, bean, _accessor.getName() + "()");
        }
    }

为什么打上了标签就用的是JsonValueSerializer？

protected final JsonSerializer<?> findSerializerByAnnotations(SerializerProvider prov,
            JavaType type, BeanDescription beanDesc)
        throws JsonMappingException
    {
        Class<?> raw = type.getRawClass();
        // First: JsonSerializable?
        if (JsonSerializable.class.isAssignableFrom(raw)) {
            return SerializableSerializer.instance;
        }
        // Second: @JsonValue for any type
        AnnotatedMember valueAccessor = beanDesc.findJsonValueAccessor();
        if (valueAccessor != null) {
            if (prov.canOverrideAccessModifiers()) {
                ClassUtil.checkAndFixAccess(valueAccessor.getMember(),
                        prov.isEnabled(MapperFeature.OVERRIDE_PUBLIC_ACCESS_MODIFIERS));
            }
            JsonSerializer<Object> ser = findSerializerFromAnnotation(prov, valueAccessor);
            return new JsonValueSerializer(valueAccessor, ser);
        }
        // No well-known annotations...
        return null;
    }

具体的调用调用太多，还是把堆栈信息拿出来：

整个的调用过程与反序列化的过程几乎一样。

下面这个类是一个主要的过程 ：

protected JsonSerializer<?> _createSerializer2(SerializerProvider prov,
            JavaType type, BeanDescription beanDesc, boolean staticTyping)
        throws JsonMappingException
    {
        JsonSerializer<?> ser = null;
        final SerializationConfig config = prov.getConfig();

        // Container types differ from non-container types
        // (note: called method checks for module-provided serializers)
        if (type.isContainerType()) {
            if (!staticTyping) {
                staticTyping = usesStaticTyping(config, beanDesc, null);
            }
            // 03-Aug-2012, tatu: As per [databind#40], may require POJO serializer...
            ser =  buildContainerSerializer(prov, type, beanDesc, staticTyping);
            // Will return right away, since called method does post-processing:
            if (ser != null) {
                return ser;
            }
        } else {
            if (type.isReferenceType()) {
                ser = findReferenceSerializer(prov, (ReferenceType) type, beanDesc, staticTyping);
            } else {
                // Modules may provide serializers of POJO types:
                for (Serializers serializers : customSerializers()) {
                    ser = serializers.findSerializer(config, type, beanDesc);
                    if (ser != null) {
                        break;
                    }
                }
            }
            // 25-Jun-2015, tatu: Then JsonSerializable, @JsonValue etc. NOTE! Prior to 2.6,
            //    this call was BEFORE custom serializer lookup, which was wrong.
            if (ser == null) {
                ser = findSerializerByAnnotations(prov, type, beanDesc);
            }
        }

        if (ser == null) {
            // Otherwise, we will check "primary types"; both marker types that
            // indicate specific handling (JsonSerializable), or main types that have
            // precedence over container types
            ser = findSerializerByLookup(type, config, beanDesc, staticTyping);
            if (ser == null) {
                ser = findSerializerByPrimaryType(prov, type, beanDesc, staticTyping);
                if (ser == null) {
                    // And this is where this class comes in: if type is not a
                    // known "primary JDK type", perhaps it's a bean? We can still
                    // get a null, if we can't find a single suitable bean property.
                    ser = findBeanSerializer(prov, type, beanDesc);
                    // Finally: maybe we can still deal with it as an implementation of some basic JDK interface?
                    if (ser == null) {
                        ser = findSerializerByAddonType(config, type, beanDesc, staticTyping);
                        // 18-Sep-2014, tatu: Actually, as per [jackson-databind#539], need to get
                        //   'unknown' serializer assigned earlier, here, so that it gets properly
                        //   post-processed
                        if (ser == null) {
                            ser = prov.getUnknownTypeSerializer(beanDesc.getBeanClass());
                        }
                    }
                }
            }
        }
        if (ser != null) {
            // [databind#120]: Allow post-processing
            if (_factoryConfig.hasSerializerModifiers()) {
                for (BeanSerializerModifier mod : _factoryConfig.serializerModifiers()) {
                    ser = mod.modifySerializer(config, beanDesc, ser);
                }
            }
        }
        return ser;
    }

这个方法名，就告诉我们答案了：findSerializerByAnnotations