Spring 源码解析|消息转换

小知识,大挑战!本文正在参与“程序员必备小知识>”创作活动。

本文已参与「掘力星计划>」,赢取创作大礼包,挑战创作激励金。

消息转换器定义

我们可以通过重写 configureMessageConverters()方法(替换Spring MVC创建的默认转换器)或重写extendMessageConverters()方法(自定义默认转换器或向默认转换器添加其他转换器)在Java配置中自定义 HttpMessageConverter。

HttpMessageConverter

消息转换器 HttpMessageConverter 借口定义如下:

public interface HttpMessageConverter<T> {

	/**
	 * Indicates whether the given class can be read by this converter.
	 * @param clazz the class to test for readability
	 * @param mediaType the media type to read (can be {@code null} if not specified);
	 * typically the value of a {@code Content-Type} header.
	 * @return {@code true} if readable; {@code false} otherwise
	 */
	boolean canRead(Class<?> clazz, @Nullable MediaType mediaType);

	/**
	 * Indicates whether the given class can be written by this converter.
	 * @param clazz the class to test for writability
	 * @param mediaType the media type to write (can be {@code null} if not specified);
	 * typically the value of an {@code Accept} header.
	 * @return {@code true} if writable; {@code false} otherwise
	 */
	boolean canWrite(Class<?> clazz, @Nullable MediaType mediaType);

	/**
	 * Return the list of media types supported by this converter. The list may
	 * not apply to every possible target element type and calls to this method
	 * should typically be guarded via {@link #canWrite(Class, MediaType)
	 * canWrite(clazz, null}. The list may also exclude MIME types supported
	 * only for a specific class. Alternatively, use
	 * {@link #getSupportedMediaTypes(Class)} for a more precise list.
	 * @return the list of supported media types
	 */
	List<MediaType> getSupportedMediaTypes();

	/**
	 * Return the list of media types supported by this converter for the given
	 * class. The list may differ from {@link #getSupportedMediaTypes()} if the
	 * converter does not support the given Class or if it supports it only for
	 * a subset of media types.
	 * @param clazz the type of class to check
	 * @return the list of media types supported for the given class
	 * @since 5.3.4
	 */
	default List<MediaType> getSupportedMediaTypes(Class<?> clazz) {
		return (canRead(clazz, null) || canWrite(clazz, null) ?
				getSupportedMediaTypes() : Collections.emptyList());
	}

	/**
	 * Read an object of the given type from the given input message, and returns it.
	 * @param clazz the type of object to return. This type must have previously been passed to the
	 * {@link #canRead canRead} method of this interface, which must have returned {@code true}.
	 * @param inputMessage the HTTP input message to read from
	 * @return the converted object
	 * @throws IOException in case of I/O errors
	 * @throws HttpMessageNotReadableException in case of conversion errors
	 */
	T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
			throws IOException, HttpMessageNotReadableException;

	/**
	 * Write an given object to the given output message.
	 * @param t the object to write to the output message. The type of this object must have previously been
	 * passed to the {@link #canWrite canWrite} method of this interface, which must have returned {@code true}.
	 * @param contentType the content type to use when writing. May be {@code null} to indicate that the
	 * default content type of the converter must be used. If not {@code null}, this media type must have
	 * previously been passed to the {@link #canWrite canWrite} method of this interface, which must have
	 * returned {@code true}.
	 * @param outputMessage the message to write to
	 * @throws IOException in case of I/O errors
	 * @throws HttpMessageNotWritableException in case of conversion errors
	 */
	void write(T t, @Nullable MediaType contentType, HttpOutputMessage outputMessage)
			throws IOException, HttpMessageNotWritableException;

}
复制代码

自定义消息转换器

下面是一个简单的消息转换器定义,代码如下:

@Configuration
@EnableWebMvc
public class WebConfiguration implements WebMvcConfigurer {

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        Jackson2ObjectMapperBuilder builder = new Jackson2ObjectMapperBuilder()
                .indentOutput(true)
                .dateFormat(new SimpleDateFormat("yyyy-MM-dd"))
                .modulesToInstall(new ParameterNamesModule());
        converters.add(new MappingJackson2HttpMessageConverter(builder.build()));
        converters.add(new MappingJackson2XmlHttpMessageConverter(builder.createXmlMapper(true).build()));
    }
}
复制代码

在前面的示例中,Jackson2ObjectMapperBuilder 用于为 MappingJackson2HttpMessageConverter 和MappingJackson2XmlHttpMessageConverter 创建公共配置,启用缩进、自定义日期格式和jackson模块参数名称注册,这增加了对访问参数名称的支持(Java8中添加的一个特性)。

配置 FastJson 为默认 JSON 解析器

其实我们在项目最最常用的还是 FastJSON 作为 Spring MVC 的默认 JSON 解析器。具体的配置如下:

<bean id="mappingJacksonHttpMessageConverter"
		  class="com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter">
	<!--class="org.springframework.http.converter.json.MappingJacksonHttpMessageConverter">-->
		<property name="supportedMediaTypes">
			<list>
				<value>text/html;charset=UTF-8</value>
				<value>application/json</value>
				<value>application/xml;charset=UTF-8</value>
			</list>
		</property>
</bean>
复制代码

参考资料

猜你喜欢

转载自juejin.im/post/7019692901454069790

相关文章