繼續WebMvcConfigurerAdapter類，重寫configureMessageConverters方法

import java.nio.charset.Charset;
import java.util.List;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;
import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter;
import com.alibaba.fastjson.serializer.SerializerFeature;
import com.alibaba.fastjson.support.config.FastJsonConfig;
import com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter4;

/**
 * webmvc配置
 */
@Configuration
public class WebMvcConfigurer extends WebMvcConfigurerAdapter {
	@Autowired
	private RequestMappingHandlerAdapter handlerAdapter;

	/**
	 * 使用阿里 FastJson 作為JSON MessageConverter
	 */
	@Override
	public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
		FastJsonHttpMessageConverter4 converter = new FastJsonHttpMessageConverter4();
		FastJsonConfig config = new FastJsonConfig();
		config.setDateFormat("yyyy-MM-dd HH:mm:ss");
		config.setSerializerFeatures(SerializerFeature.WriteMapNullValue, // 保留空的欄位
				SerializerFeature.WriteNullStringAsEmpty, // String null -> ""
				SerializerFeature.WriteNullNumberAsZero);// Number null -> 0
		converter.setFastJsonConfig(config);
		converter.setDefaultCharset(Charset.forName("UTF-8"));
		converters.add(converter);
	}
}