Jackson ObjectMapper readValue過程

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1.整體調用棧

2.看一下調用棧的兩個方法

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　　resolve 方法中通過 Iterator i$ = this._beanProperties.iterator() 遍歷屬性的所有子屬性，緩存對應的 deserializer。觀察調用棧的方法，可以發現是循環調用的。

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3.比如尋找自定義的 LocalDateTime類的序列化實現類，看方法調用棧最上邊的方法

　　

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　　如果沒有找到用戶自定義的反序列化工具，則去找默認的標準反序列化工具

　　

　　

　　

  deser = NumberDeserializers.find(rawType, clsName);
  deser = DateDeserializers.find(rawType, clsName);?

　　可以看一下?com.fasterxml.jackson.databind.deser.std.DateDeserializers 和?com.fasterxml.jackson.databind.deser.std.NumberDeserializers，遍豁然開朗。

4.加入對應類型序列化工具類

import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.JavaType;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.module.SimpleModule;
import java.lang.reflect.Type;
import java.math.BigDecimal;
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;
import xxx.utils.json.deserializers.LocalDateDeserializer;
import xxx.utils.json.deserializers.LocalDateTimeDeserializer;
import xxx.utils.json.deserializers.LocalTimeDeserializer;
import xxx.utils.json.serializers.BigDecimalSerializer;
import xxx.utils.json.serializers.LocalDateSerializer;
import xxx.utils.json.serializers.LocalDateTimeSerializer;
import xxx.utils.json.serializers.LocalTimeSerializer;public class JacksonHelper {private static final SimpleModule module = initModule();private static final ObjectMapper mapper;private static final ObjectMapper prettyMapper;public JacksonHelper() {}private static SimpleModule initModule() {return (new SimpleModule()).addSerializer(BigDecimal.class, new BigDecimalSerializer()).addSerializer(LocalTime.class, new LocalTimeSerializer()).addDeserializer(LocalTime.class, new LocalTimeDeserializer()).addSerializer(LocalDate.class, new LocalDateSerializer()).addDeserializer(LocalDate.class, new LocalDateDeserializer()).addSerializer(LocalDateTime.class, new LocalDateTimeSerializer()).addDeserializer(LocalDateTime.class, new LocalDateTimeDeserializer());}public static JavaType genJavaType(Type type) {return getMapper().getTypeFactory().constructType(type);}public static ObjectMapper getMapper() {return mapper;}public static ObjectMapper getPrettyMapper() {return prettyMapper;}static {mapper = (new ObjectMapper()).registerModule(module).configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false).configure(DeserializationFeature.READ_UNKNOWN_ENUM_VALUES_AS_NULL, true).configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);prettyMapper = mapper.copy().configure(SerializationFeature.INDENT_OUTPUT, true);}
}

MAPPER = JacksonHelper.getMapper().registerModule((new SimpleModule(LocalDateTimeDeserializer2.class.getName())).addDeserializer(LocalDateTime.class, new LocalDateTimeDeserializer2()));MAPPER.setSerializationInclusion(JsonInclude.Include.NON_NULL);

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import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonNode;
import java.io.IOException;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;public class LocalDateTimeDeserializer extends JsonDeserializer<LocalDateTime> {public LocalDateTimeDeserializer() {}public LocalDateTime deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException {String dateTimeStr = ((JsonNode)jp.getCodec().readTree(jp)).asText();return LocalDateTime.parse(dateTimeStr, DateTimeFormatter.ISO_LOCAL_DATE_TIME);}
}

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonNode;
import java.io.IOException;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;public class LocalDateTimeDeserializer2 extends JsonDeserializer<LocalDateTime> {public LocalDateTimeDeserializer2() {}public LocalDateTime deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException {String dateTimeStr = ((JsonNode)jp.getCodec().readTree(jp)).asText();DateTimeFormatter df = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");return LocalDateTime.parse(dateTimeStr, df);}
}

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　　可見，ObjectMapper registerModule 最后注冊的module會優先被發現。例如上邊首先 第一個 Module加入了一個LocalDateTime反序列化工具類LocalDateTimeDeserializer，接著第二個Module加入了LocalDateTime反序列化工具類LocalDateTimeDeserializer2，最后得到的反序列化工具類是LocalDateTimeDeserializer2。

5.java.util.Date日期類型解析

　　日期格式：yyyy-MM-dd HH:mm:ss

　　第一種方法：MAPPER.setConfig(MAPPER.getDeserializationConfig().with(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss")));

　　第二種方法：自定義反序列化 MAPPER.registerModule((new SimpleModule(Date.class.getName())).addDeserializer(Date.class, DateDeserializer2.dateDeserializer));

import com.fasterxml.jackson.databind.deser.std.DateDeserializers;
import java.text.DateFormat;
import java.text.SimpleDateFormat;

public class DateDeserializer2 extends DateDeserializers.DateDeserializer{public static final DateDeserializer2 dateDeserializer = new DateDeserializer2(DateDeserializers.DateDeserializer.instance, new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"), "yyyy-MM-dd HH:mm:ss");public DateDeserializer2() {}public DateDeserializer2(DateDeserializers.DateDeserializer base, DateFormat df, String formatString) {super(base, df, formatString);}
}

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　　默認的Date解析通過?DateDeserializers.DateDeserializer，時間的格式化處理是調用自己的StdDateFormat類來實現日期格式化

　　

　　而StdDateFormat定義的格式化如下

　　

　　DeserializationContext中為啥可以獲取StdDateFormat（objectMapper readValue 時會創建DeserializationContext, 注入DeserializationConfig【包含BaseSettings(包含DateFormat)】）

　　

　　

　　

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