java - Jackson:字段类型限制

Question

如果传递的 pojo 包含意外类型的字段，是否可以让 jackson 在写入 json 时抛出异常？我想与自定义序列化程序一起提供预期类型。

Answer 1

Jackson 将所有属性包装在 com.fasterxml.jackson.databind.ser.BeanPropertyWriter 对象中，并且它们负责序列化值。我们可以使用自定义实现来装饰这些对象:

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationConfig;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.json.JsonMapper;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.ser.BeanPropertyWriter;
import com.fasterxml.jackson.databind.ser.BeanSerializerModifier;

import java.util.List;
import java.util.stream.Collectors;

public class JsonTypeApp {

    public static void main(String[] args) throws Exception {
        SimpleModule fieldRestrictionModule = new SimpleModule("FieldRestriction");
        fieldRestrictionModule.setSerializerModifier(new BeanSerializerModifier() {
            @Override
            public List<BeanPropertyWriter> changeProperties(SerializationConfig config, BeanDescription beanDesc, List<BeanPropertyWriter> beanProperties) {
                return beanProperties.stream().map(p -> new PrimitivesOnlyBeanPropertyWriter(p)).collect(Collectors.toList());
            }
        });

        ObjectMapper mapper = JsonMapper.builder()
                .addModule(fieldRestrictionModule)
                .build();

        System.out.println(mapper.writeValueAsString(new Test()));
    }
}

class PrimitivesOnlyBeanPropertyWriter extends BeanPropertyWriter {

    public PrimitivesOnlyBeanPropertyWriter(BeanPropertyWriter writer) {
        super(writer);
    }

    @Override
    public void serializeAsField(Object bean, JsonGenerator gen, SerializerProvider prov) throws Exception {
        if (getType().isPrimitive()) {
            super.serializeAsField(bean, gen, prov);
        } else {
            throw new IllegalStateException(getType().getTypeName() + " is not allowed!");
        }
    }
}

如果Test类仅包含原始字段对象将被正确序列化。在其他情况下 - 将抛出java.lang.IllegalStateException。