fastjson2-2.0.46解析时java空泛型对象如何解析为一对大括号

Question

相关jar包版本

<properties>
        <maven.compiler.source>17</maven.compiler.source>
        <maven.compiler.target>17</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <spring.cloud.alibaba.version>2022.0.0.0</spring.cloud.alibaba.version>
        <spring.cloud.version>4.0.0</spring.cloud.version>
    </properties>
......
<!-- fastjson -->
        <dependency>
            <groupId>com.alibaba.fastjson2</groupId>
            <artifactId>fastjson2</artifactId>
            <version>2.0.46</version>
        </dependency>
        <dependency>
            <groupId>com.alibaba.fastjson2</groupId>
            <artifactId>fastjson2-extension-spring5</artifactId>
            <version>2.0.46</version>
        </dependency>
......

FastJsonConfiguration相关代码

@Configuration
public class FastJsonConfiguration implements WebMvcConfigurer {
    @Bean
    public HttpMessageConverter<?> fastJsonHttpMessageConverter() {
        FastJsonHttpMessageConverter fastJsonHttpMessageConverter = new FastJsonHttpMessageConverter();
        FastJsonConfig fastJsonConfig = fastJsonHttpMessageConverter.getFastJsonConfig();
        fastJsonConfig.setCharset(StandardCharsets.UTF_8);
        fastJsonConfig.setReaderFeatures(
                JSONReader.Feature.SupportSmartMatch,
                JSONReader.Feature.FieldBased,
                JSONReader.Feature.TrimString);

        fastJsonConfig.setWriterFeatures(
                JSONWriter.Feature.WriteLongAsString,
                JSONWriter.Feature.WriteNullListAsEmpty,
                JSONWriter.Feature.WriteNullStringAsEmpty,
               JSONWriter.Feature.WriteNullBooleanAsFalse,
               JSONWriter.Feature.NullAsDefaultValue,
                JSONWriter.Feature.PrettyFormat
                );

        fastJsonConfig.setDateFormat("yyyy-MM-dd HH:mm:ss");
        fastJsonHttpMessageConverter.setDefaultCharset(StandardCharsets.UTF_8);
        List<MediaType> mediaTypeList = new ArrayList<>();
        mediaTypeList.add(MediaType.APPLICATION_JSON);
        fastJsonHttpMessageConverter.setSupportedMediaTypes(mediaTypeList);

        return fastJsonHttpMessageConverter;
    }

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        converters.add(0, fastJsonHttpMessageConverter());
    }
}

Controller返回的泛型对象

.......
public class MyResult<T> {
    private final boolean result;
    private final String code;
    private final String msg;
    private final T body;
......
}

目前返回结果中，body为null时

{
    "body": null,
        "msg": "",
        "code": "",
    "result": true
}

期望返回

{
    "body": {},
        "msg": "",
        "code": "",
    "result": true
}

Answer 1

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在Fastjson2中，要实现当body为null时返回空对象{}而非null，你需要自定义一个FastJsonConfig的处理器，并在序列化过程中处理空泛型对象。以下是修改后的代码示例：

@Configuration
public class FastJsonConfiguration implements WebMvcConfigurer {

    // ... 其他配置保持不变 ...

    @Bean
    public HttpMessageConverter<?> fastJsonHttpMessageConverter() {
        FastJsonHttpMessageConverter fastJsonHttpMessageConverter = new FastJsonHttpMessageConverter();
        
        // 创建并配置FastJsonConfig
        FastJsonConfig fastJsonConfig = new FastJsonConfig();
        fastJsonConfig.setCharset(StandardCharsets.UTF_8);
        // 设置日期格式等其他配置
        fastJsonConfig.setDateFormat("yyyy-MM-dd HH:mm:ss");

        // 自定义SerializerFeature来处理空泛型对象
        SerializerFeature[] serializerFeatures = new SerializerFeature[]{
                SerializerFeature.WriteDateUseDateFormat,
                SerializerFeature.WriteMapNullValue, // 为了写出null值（可选，根据需求）
                SerializerFeature.PrettyFormat,
                // 添加以下特性以处理空泛型对象
                new SerializerFeature() {
                    @Override
                    public boolean isEnabled(SerializerFeatureContext context) {
                        // 判断当前字段是否是MyResult类中的body字段并且其值为null
                        if (context.getPropertyName().equals("body") && context.get(Object.class).equals(null)) {
                            return true;
                        }
                        return false;
                    }

                    @Override
                    public void writeBefore(SerializerFeatureContext context) throws IOException {
                        // 当满足条件时，写入一个空JSON对象
                        context.out.write("{}");
                        context.setWriteStopped(true); // 停止后续的序列化过程
                    }
                }
        };
        fastJsonConfig.setSerializerFeatures(serializerFeatures);

        List<MediaType> mediaTypeList = new ArrayList<>();
        mediaTypeList.add(MediaType.APPLICATION_JSON);
        fastJsonHttpMessageConverter.setSupportedMediaTypes(mediaTypeList);
        fastJsonHttpMessageConverter.setFastJsonConfig(fastJsonConfig);

        return fastJsonHttpMessageConverter;
    }

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        converters.add(0, fastJsonHttpMessageConverter());
    }
}

上述代码创建了一个自定义的SerializerFeature，它会在序列化MyResult类的body字段且值为null时输出空JSON对象{}。

然而，Fastjson2可能没有提供直接设置特定字段为空对象的功能，因此这个解决方案可能需要额外的开发工作，或者考虑在业务层对返回结果进行预处理，确保body字段不为null而是返回一个空对象实例。

请查阅Fastjson2官方文档以确认是否有更方便的方式来实现这一需求。若Fastjson2本身不支持这种精细化控制，则建议通过扩展或替换序列化逻辑来达到目的。