是否可以为不同的数据类型在类级别配置Jackson自定义反序列化器？

Question

我需要对一个长而复杂的json进行反序列化，为此我编写了一组java类来映射数据，并且我必须为许多不同类型的字段（包括String、Boolean、BigDecimal等）编写自定义反序列化器。

我知道我可以用相应的自定义反序列化器注释java类中的所有字段（如下所示），但是我需要注释所有类中的几乎所有字段。

@JsonDeserialize(using = CustomBooleanJsonDeserializer.class)
private boolean active;

@Bean
public Module customDeserializersModule() {
    SimpleModule module = new SimpleModule();
    module.addDeserializer(Boolean.class, new CustomBooleanJsonDeserializer());
    // add other custom deserializers 
    return module;
}

@RequestMapping(method = RequestMethod.POST, value = "/data")
public ResponseEntity<ServerInfo> register(@RequestBody DataMapper data) {
   // DataMapper is the target POJO class of the json's deserialization
}

@JsonDeserialize(using = CustomStringJsonDeserializer.class, forType = String.class)
@JsonDeserialize(using = CustomBooleanJsonDeserializer.class, forType = Boolean.class)
@JsonDeserialize(using = CustomBigDecimalJsonDeserializer.class, forType = BigDecimal.class)
public class DataMapper implements Serializable {
    // obviously, @JsonDeserialize doesn't have a forType method
}

或者可以通过某种方式为DataMapper类实现自定义反序列化，该类定义如何根据每个字段的数据类型反序列化每个字段（而不必注释每个字段）：

@JsonDeserialize(using = DataMapperJsonDeserializer.class)
public class DataMapper implements Serializable {
    // How can I implement the DataMapperJsonDeserializer with these 
    // characteristics? I know about the ContextualDeserializer interface, 
    // but I don't know how to use it without annotating each field.
}

或者某种将模块的效果限制在一个包或一组类上的方法：

module.restrictedTo(/*some package or set of classes*/);
// com.fasterxml.jackson.databind.Module doesn't have a restrictedTo method

Answer 1

您可以尝试将SimpleModule与ContextualDeserializer接口一起使用。第一个可用于包装默认反序列化程序，第二个可用于检查类型配置-检查注释。

让我们从注释开始：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@interface ForceCustomDeserializer {
}

我假设您只有一个特定类型的自定义实现，但如果不是，请扩展上面的注释并提供一些额外的信息，以便使用正确的反序列化程序。例如，下面我们可以看到两个自定义反序列化程序，它们额外记录一些信息并运行默认的反序列化。使用基本反序列化程序是因为如果您有一些额外的配置，我们不会松动它。

class CustomBoolDeserializer extends StdScalarDeserializer<Boolean> implements ContextualDeserializer {

    private NumberDeserializers.BooleanDeserializer base;

    public CustomBoolDeserializer(NumberDeserializers.BooleanDeserializer base) {
        super(Boolean.class);
        this.base = base;
    }

    @Override
    public Boolean deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        System.out.println("Custom BooleanDeserializer ....");

        return base.deserialize(p, ctxt);
    }

    @Override
    public JsonDeserializer<?> createContextual(DeserializationContext ctxt, BeanProperty property) {
        Class<?> parent = property.getMember().getDeclaringClass();
        ForceCustomDeserializer annotation = parent.getAnnotation(ForceCustomDeserializer.class);

        return annotation == null ? base : this;
    }
}

class CustomStringDeserializer extends StringDeserializer implements ContextualDeserializer {

    private final StringDeserializer base;

    public CustomStringDeserializer(StringDeserializer base) {
        this.base = base;
    }

    @Override
    public String deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        System.out.println("Custom StringDeserializer ....");

        return base.deserialize(p, ctxt);
    }

    @Override
    public JsonDeserializer<?> createContextual(DeserializationContext ctxt, BeanProperty property) {
        Class<?> parent = property.getMember().getDeclaringClass();
        ForceCustomDeserializer annotation = parent.getAnnotation(ForceCustomDeserializer.class);

        return annotation == null ? base : this;
    }
}

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.BeanProperty;
import com.fasterxml.jackson.databind.DeserializationConfig;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.deser.BeanDeserializerModifier;
import com.fasterxml.jackson.databind.deser.ContextualDeserializer;
import com.fasterxml.jackson.databind.deser.std.NumberDeserializers;
import com.fasterxml.jackson.databind.deser.std.StdScalarDeserializer;
import com.fasterxml.jackson.databind.deser.std.StringDeserializer;
import com.fasterxml.jackson.databind.module.SimpleModule;

import java.io.File;
import java.io.IOException;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

public class JsonApp {

    public static void main(String[] args) throws Exception {
        File jsonFile = new File("./resource/test.json").getAbsoluteFile();

        SimpleModule forcedCustomModule = new SimpleModule();
        forcedCustomModule.setDeserializerModifier(new BeanDeserializerModifier() {
            @Override
            public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config, BeanDescription beanDesc, JsonDeserializer<?> deserializer) {
                if (deserializer instanceof StringDeserializer) {
                    // wrap with yours or return new deserializer
                    return new CustomStringDeserializer((StringDeserializer) deserializer);
                }
                if (deserializer instanceof NumberDeserializers.BooleanDeserializer) {
                    // wrap with yours or return new deserializer
                    return new CustomBoolDeserializer((NumberDeserializers.BooleanDeserializer) deserializer);
                }
                // override for other types

                return deserializer;
            }
        });

        ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(forcedCustomModule);

        System.out.println(mapper.readValue(jsonFile, Pojo.class));
    }
}

@ForceCustomDeserializer
class Pojo {

    private String name;
    private boolean bool;

    // getters, setters, toString
}

{
  "name": "Jackson",
  "bool": true
}

Custom StringDeserializer ....
Custom BooleanDeserializer ....
Pojo{name='Jackson', bool=true}

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