Json_JackSon_lesson1 Jackson的 序列化与反序列化

JackSon 支持三种序列化 与 三种反序列化 方式 ：

下面分别对 反序列化  与 序列化 进行讲解

3种 反序列化方式：

1.从文件中的json串 反序列化

2.从url 中获取字符串 进行 反序列化

3.从字符串进行反序列化

示例：

MyValue.java

package model;//Note: can use getters/setters as well; here we just use public fields directly:public class MyValue {public String name;public int age;// NOTE: if using getters/setters, can keep fields `protected` or `private`//public MyValue(String name, int age) {//// TODO Auto-generated constructor stub//this.name = name;//this.age = age;//}@Overridepublic String toString() {// TODO Auto-generated method stubreturn "name: "+ this.name + "\n"+"age: " + this.age + "\n";}}

反序列化 代码片段

ObjectMapper mapper = new ObjectMapper();//Json 转换为 Java类, 反序列化//** 方式一 : 从文件中读取MyValue fromFile = mapper.readValue(new File("data.json"), MyValue.class);System.out.println(fromFile);//** 方式二 : 从url中的返回值中读取// MyValue fromUrl = mapper.readValue(new URL("http://some.com/api/entry.json"), MyValue.class);// System.out.println(fromUrl);//** 方式三 : 从字符串中读取//jackson 不支持 单引号 ， 如："{'name':'Szh', 'age':24}"//会报错//Exception in thread "main" com.fasterxml.jackson.core.JsonParseException: Unexpected character (''' (code 39)): was expecting double-quote to start field name// at [Source: {'name':'Szh', 'age':24}; line: 1, column: 3]MyValue fromjson = mapper.readValue("{\"name\":\"Szh\", \"age\":24}", MyValue.class);System.out.println(fromjson);

额外教程： list, map, pojo 的反序列化

Map<String, Integer> scoreByName = mapper.readValue(jsonSource, Map.class);List<String> names = mapper.readValue(jsonSource, List.class);// and can obviously write out as well// mapper.writeValue(new File("names.json"), names);// as long as JSON structure matches, and types are simple. If you have POJO values, you need to indicate actual type (note: this is NOT needed for POJO properties with List etc types):Map<String, ResultValue> results = mapper.readValue(jsonSource,   new TypeReference<Map<String, ResultValue>>() { } );// why extra work? Java Type Erasure will prevent type detection otherwise

注意：

使用 jackson 包将 json字符串进行反序列化时，如果定义了内部类的话，这个内部类必须用 static 修饰，否则序列化会出错。具体的原因见Jackson and Inner Classes: yes, you can use, but they must be STATIC inner classes.

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3种 序列化方式：

1.将java对象进行序列化, 将序列化后的串写入到文件中。

2.按照字节数组的形式返回json串

3.按照字符数组的形式返回json串

序列化 代码片段

ObjectMapper mapper = new ObjectMapper();//Java类 转换为 Json串 , 序列化MyValue serializeClass = new MyValue();serializeClass.age = 22;serializeClass.name = "sxHJ";//方法一 : 将序列化后的串写入到文件中mapper.writeValue(new File("result.json"), serializeClass);//方式二: 按照字节数组的形式返回json串byte[] jsonBytes = mapper.writeValueAsBytes(serializeClass);System.out.println(jsonBytes);//方式三: 按照字符数组的形式返回json串String jsonString = mapper.writeValueAsString(serializeClass);System.out.println(jsonString);

完整代码 以及结果：

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">  <modelVersion>4.0.0</modelVersion>  <groupId>JacksonTest</groupId>  <artifactId>JacksonTest</artifactId>  <version>0.0.1-SNAPSHOT</version>  <name>jackson</name>      <properties><jackson.version>2.8.4</jackson.version>  </properties>    <dependencies><dependency>  <groupId>com.fasterxml.jackson.core</groupId>  <artifactId>jackson-databind</artifactId>  <version>${jackson.version}</version></dependency><dependency> <!-- Note: core-annotations version x.y.0 is generally compatible with  (identical to) version x.y.1, x.y.2, etc. --> <groupId>com.fasterxml.jackson.core</groupId> <artifactId>jackson-annotations</artifactId> <version>${jackson.version}</version></dependency><dependency> <groupId>com.fasterxml.jackson.core</groupId> <artifactId>jackson-core</artifactId> <version>${jackson.version}</version></dependency>  </dependencies>      <build>    <sourceDirectory>src</sourceDirectory>    <plugins>      <plugin>        <artifactId>maven-compiler-plugin</artifactId>        <version>3.1</version>        <configuration>          <source>1.8</source>          <target>1.8</target>        </configuration>      </plugin>    </plugins>  </build>  </project>

代码

import java.io.File;import java.io.IOException;import model.MyValue;import com.fasterxml.jackson.core.JsonParseException;import com.fasterxml.jackson.databind.JsonMappingException;import com.fasterxml.jackson.databind.ObjectMapper;public class JacksonFirst {public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException {ObjectMapper mapper = new ObjectMapper();//Json 转换为 Java类, 反序列化//** 方式一 : 从文件中读取MyValue fromFile = mapper.readValue(new File("data.json"), MyValue.class);System.out.println(fromFile);//** 方式二 : 从url中的返回值中读取// MyValue fromUrl = mapper.readValue(new URL("http://some.com/api/entry.json"), MyValue.class);// System.out.println(fromUrl);//** 方式三 : 从字符串中读取//jackson 不支持 单引号 ， 如："{'name':'Szh', 'age':24}"//会报错//Exception in thread "main" com.fasterxml.jackson.core.JsonParseException: Unexpected character (''' (code 39)): was expecting double-quote to start field name// at [Source: {'name':'Szh', 'age':24}; line: 1, column: 3]MyValue fromjson = mapper.readValue("{\"name\":\"Szh\", \"age\":24}", MyValue.class);System.out.println(fromjson);//Java类 转换为 Json串 , 序列化MyValue serializeClass = new MyValue();serializeClass.age = 22;serializeClass.name = "sxHJ";//方法一 : 将序列化后的串写入到文件中mapper.writeValue(new File("result.json"), serializeClass);//方式二: 按照字节数组的形式返回json串byte[] jsonBytes = mapper.writeValueAsBytes(serializeClass);System.out.println(jsonBytes);//方式三: 按照字符数组的形式返回json串String jsonString = mapper.writeValueAsString(serializeClass);System.out.println(jsonString);}}