一、背景
最近给安卓端写接口，对于空数据会返回null值，安卓哥不乐意了，说我这边不支持null，你必须将null转其他初始值，于是我和安卓哥约定：对于常见的数据类型将他们的null值转为类型初始值，对于其他自定义的数据类型返回空串。
二、配置Jackson
环境准备：
- SpringBoot
- com.fasterxml.jackson
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonStreamContext;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;
import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
import java.io.IOException;
import java.lang.reflect.Field;
import java.util.Collection;
import java.util.Date;
import java.util.Map;
@Configuration
public class JacksonConfig {
private static final ThreadLocal<Boolean> IS_APP_REQUEST = ThreadLocal.withInitial(() -> false);
@Bean
@Primary
@ConditionalOnMissingBean(ObjectMapper.class)
public ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {
ObjectMapper objectMapper = builder.createXmlMapper(false).build();
objectMapper.getSerializerProvider().setNullValueSerializer(new JsonSerializer<Object>() {
@Override
public void serialize(Object value, JsonGenerator gen, SerializerProvider provider) throws IOException {
handleNullValue(gen);
// else if (IS_APP_REQUEST.get()) {
// handleNullValue(gen);
// } else {
// gen.writeNull();
// }
}
private void handleNullValue(JsonGenerator gen) throws IOException {
JsonStreamContext context = gen.getOutputContext();
if (context.inObject()) {
String fieldName = context.getCurrentName();
Object currentObj = context.getCurrentValue();
if (currentObj != null) {
Class<?> clazz = currentObj.getClass();
Field field = getField(clazz, fieldName);
if (field != null) {
setDefaultByType(field.getType(), gen);
return;
}
}
}
// 默认处理
gen.writeString("");
}
private Field getField(Class<?> clazz, String fieldName) {
try {
Field field = clazz.getDeclaredField(fieldName);
field.setAccessible(true);
return field;
} catch (NoSuchFieldException e) {
// 查找父类字段
Class<?> superClass = clazz.getSuperclass();
while (superClass != null) {
try {
Field field = superClass.getDeclaredField(fieldName);
field.setAccessible(true);
return field;
} catch (NoSuchFieldException ex) {
superClass = superClass.getSuperclass();
}
}
}
return null;
}
private void setDefaultByType(Class<?> type, JsonGenerator gen) throws IOException {
if (type.isPrimitive()) {
handlePrimitive(type, gen);
} else if (type == String.class) {
gen.writeString("");
} else if (Number.class.isAssignableFrom(type)) {
gen.writeNumber(0);
} else if (type == Boolean.class) {
gen.writeBoolean(false);
} else if (type == Date.class) {
gen.writeString("");
} else if (type.isArray() || Collection.class.isAssignableFrom(type)) {
gen.writeStartArray();
gen.writeEndArray();
} else if (Map.class.isAssignableFrom(type)) {
gen.writeStartObject();
gen.writeEndObject();
} else {
gen.writeString("");
// gen.writeStartObject();
// gen.writeEndObject();
}
}
private void handlePrimitive(Class<?> type, JsonGenerator gen) throws IOException {
if (type == int.class) {
gen.writeNumber(0);
} else if (type == long.class) {
gen.writeNumber(0L);
} else if (type == double.class) {
gen.writeNumber(0.0);
} else if (type == float.class) {
gen.writeNumber(0.0f);
} else if (type == boolean.class) {
gen.writeBoolean(false);
} else if (type == char.class) {
gen.writeString(String.valueOf('\0'));
} else {
gen.writeNull();
}
}
});
return objectMapper;
}
@Bean
public WebMvcConfigurer webMvcConfigurer() {
return new WebMvcConfigurer() {
@Override
public void addInterceptors(InterceptorRegistry registry) {
registry.addInterceptor(new HandlerInterceptor() {
@Override
public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) {
IS_APP_REQUEST.set(isAppRequest(request));
return true;
}
@Override
public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) {
IS_APP_REQUEST.remove();
}
});
}
};
}
/**
* 判断是否为移动端请求
*/
private boolean isAppRequest(HttpServletRequest request) {
String path = request.getRequestURI();
String userAgent = request.getHeader("User-Agent");
return path.startsWith("/client/c/app/") || path.startsWith("/auth/c/");
}
}
以上配置对于Satoken异常不会生效，这一点需要另外处理：
@Resource
private JacksonConfig jacksonConfig;
// Satoken异常处理
......
.setError(e -> {
// 由于过滤器中抛出的异常不进入全局异常处理，所以必须提供[异常处理函数]来处理[认证函数]里抛出的异常
// 在[异常处理函数]里的返回值，将作为字符串输出到前端，此处统一转为JSON输出前端
SaResponse saResponse = SaHolder.getResponse();
saResponse.setHeader(Header.CONTENT_TYPE.getValue(), ContentType.JSON + ";charset=" + CharsetUtil.UTF_8);
CommonResult<String> result = GlobalExceptionUtil.getCommonResult((Exception) e);
// json格式化
ObjectMapper objectMapper = jacksonConfig.jacksonObjectMapper(new Jackson2ObjectMapperBuilder());
try {
return objectMapper.writeValueAsString(result);
} catch (JsonProcessingException ex) {
throw new RuntimeException(ex);
}
});
三、总结
本文对于Jackson序列化配置支持将null值转为数据类型初始值，比如int->0;String->"";map/object->{}等等。 需要注意的是对于satoken这样的异常不走全局响应，需要单独处理响应内容。
还支持对特定的url进行响应处理，比如只对安卓端。
在本文之前笔者也试过各种办法，都不如这个好使，不愧是deepseek生成的代码，🐂🍺！
