springboot 接口返回字符串带引号的问题解决

时间：2023-02-02 10:13:14|栏目：JAVA代码|点击：次

框架版本

springboot 2.2.1.RELEASE
fastjson 1.2.78

maven pom.xml parent 和 dependencies

 <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.2.1.RELEASE</version>
    </parent>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.12</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-websocket</artifactId>
        </dependency>
        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>fastjson</artifactId>
            <version>1.2.78</version>
        </dependency>
    </dependencies>

问题

如下代码，返回结果是 "hello world"，添加了一个双引号。

@RestController
@Slf4j
public class Hello1Controller {

    @GetMapping(value = "/printHello")
    @ResponseBody
    public String printHello() {
        return "hello world";
    }
}

排查

不管返回什么的结果，肯定经过MessageConvert进行消息的输出，debug看使用了哪一个MessageConvert

选择MessageConvert的方法是
org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor#writeWithMessageConverters(T, org.springframework.core.MethodParameter, org.springframework.http.server.ServletServerHttpRequest, org.springframework.http.server.ServletServerHttpResponse)

关键代码

        if (selectedMediaType != null) {
            selectedMediaType = selectedMediaType.removeQualityValue();
             // 打个断点在此处
            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;
                }
            }
        }

debug发现我的messageConvert只有一个，是com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter。那肯定是经过FastJson将String 写到输出流。

debug查看FastJsonHttpMessageConverter具体的write 代码，可以看到此时输出字符串添加了引号。

方法：com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter#writeInternal

写个fastjson 转化字符串试试，如下结果输出是 "hello world"，添加了引号。

import com.alibaba.fastjson.JSON;

public class JsonTest {
    public static void main(String[] args) {
        System.out.println(JSON.toJSONString("hello world"));
        // 
    }
}

问题所在

import com.alibaba.fastjson.serializer.SerializerFeature;
import com.alibaba.fastjson.support.config.FastJsonConfig;
import com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.MediaType;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.web.servlet.config.annotation.EnableWebMvc;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

import java.math.RoundingMode;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.List;

@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer  {

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        FastJsonHttpMessageConverter fastJsonConverter = new FastJsonHttpMessageConverter();
        FastJsonConfig config = new FastJsonConfig();
        config.setCharset(StandardCharsets.UTF_8);
        //设置允许返回为null的属性
        config.setSerializerFeatures(SerializerFeature.WriteMapNullValue);
       
        fastJsonConverter.setFastJsonConfig(config);
        List<MediaType> list = new ArrayList<>();
        list.add(MediaType.APPLICATION_JSON);
        fastJsonConverter.setSupportedMediaTypes(list);
        converters.add(fastJsonConverter);
    }

}

我配置了FastJsonHttpMessageConverter ，导致只使用FastJsonHttpMessageConverter 进行消息转换。

解决方法

添加StringHttpMessageConverter ，让其转化String。注意顺序，StringHttpMessageConverter 要在FastJsonHttpMessageConverter 之前。

 @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        FastJsonHttpMessageConverter fastJsonConverter = new FastJsonHttpMessageConverter();
        FastJsonConfig config = new FastJsonConfig();
        config.setCharset(StandardCharsets.UTF_8);
        //设置允许返回为null的属性
        config.setSerializerFeatures(SerializerFeature.WriteMapNullValue);

        fastJsonConverter.setFastJsonConfig(config);
        List<MediaType> list = new ArrayList<>();
        list.add(MediaType.APPLICATION_JSON);
        fastJsonConverter.setSupportedMediaTypes(list);

        StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
        stringHttpMessageConverter.setSupportedMediaTypes(list);
        converters.add(stringHttpMessageConverter);

        converters.add(fastJsonConverter);
    }