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jackson学习之三:常用API

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内容:所有原创文章分类和汇总,及配套源码,涉及Java、Docker、Kubernetes、DevOPS等;

全系列源码下载整个《jackson学习》系列的源码都可以在GitHub下载,地址和链接信息如下表所示(

):这个git项目中有多个文件夹,《K8S官方java客户端》系列的源码在jacksondemo文件夹下,如下图红框所示:本篇概览

本文是《jackson学习》系列的第三篇,前面咱们学习了jackson的低阶API,知道了底层原理,本篇开始学习平时最常用的基本功能,涉及内容如下:

体验最常用的操作,内容如下图所示:介绍常用的可配置属性,以便按需要来设置;接下来进入快速浏览的环节,咱们一起先把各个API过一遍;单个对象序列化

先看常用的序列化API:

对象转字符串:

String jsonStr = mapper.writeValueAsString(twitterEntry);
对象转文件:
mapper.writeValue(new File("twitter.json"), twitterEntry);
对象转byte数组:
byte[] array = mapper.writeValueAsBytes(twitterEntry);
单个对象反序列化字符串转对象:
TwitterEntry tFromStr = mapper.readValue(objectJsonStr, TwitterEntry.class);
文件转对象:
TwitterEntry tFromFile = mapper.readValue(new File("twitter.json"), TwitterEntry.class);
byte数组转对象:
TwitterEntry tFromBytes = mapper.readValue(array, TwitterEntry.class);
字符串网络地址转对象:
String testJsonDataUrl = ";;TwitterEntry tFromUrl = mapper.readValue(new URL(testJsonDataUrl), TwitterEntry.class);
集合序列化HashMap转字符串:
String mapJsonStr = mapper.writeValueAsString(map);
集合反序列化字符串转HashMap:
Map<String, Object> mapFromStr = mapper.readValue(mapJsonStr, new TypeReference<Map<String, Object>>() {});
JsonNode如果您不想使用XXX.class来做反序列化,也能使用JsonNode来操作:
JsonNode jsonNode = mapper.readTree(mapJsonStr);String name = jsonNode.get("name").asText();int age = jsonNode.get("age").asInt();String city = jsonNode.get("addr").get("city").asText();String street = jsonNode.get("addr").get("street").asText();
时间字段格式化对于Date字段,默认的反序列化是时间戳,可以修改配置:
mapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd hh:mm:ss"));dateMapStr = mapper.writeValueAsString(dateMap);
JSON数组的反序列化

假设jsonArrayStr是个json数组格式的字符串:

JSON数组转对象数组:

TwitterEntry[] twitterEntryArray = mapper.readValue(jsonArrayStr, TwitterEntry[].class);
JSON数组转对象集合(ArrayList):
List<TwitterEntry> twitterEntryList = mapper.readValue(jsonArrayStr, new TypeReference<List<TwitterEntry>>() {});
完整代码上述所有常用API用法的完整代码如下:
package com.bolingcavalry.jacksondemo.databind;import com.bolingcavalry.jacksondemo.beans.TwitterEntry;import com.fasterxml.jackson.core.type.TypeReference;import com.fasterxml.jackson.databind.JsonNode;import com.fasterxml.jackson.databind.ObjectMapper;import org.slf4j.Logger;import org.slf4j.LoggerFactory;import java.io.File;import java.net.URL;import java.text.SimpleDateFormat;import java.util.*;public class SimpleDemo {    private static final Logger logger = LoggerFactory.getLogger(SimpleDemo.class);    public static void main(String[] args) throws Exception {        ObjectMapper mapper = new ObjectMapper();        logger.info("以下是序列化操作");        // 对象 -> 字符串        TwitterEntry twitterEntry = new TwitterEntry();        twitterEntry.setId(123456L);        twitterEntry.setFromUserId(101);        twitterEntry.setToUserId(102);        twitterEntry.setText("this is a message for serializer test");        twitterEntry.setLanguageCode("zh");        String jsonStr = mapper.writeValueAsString(twitterEntry);        logger.info("序列化的字符串:{}", jsonStr);        // 对象 -> 文件        mapper.writeValue(new File("twitter.json"), twitterEntry);        // 对象 -> byte数组        byte[] array = mapper.writeValueAsBytes(twitterEntry);        logger.info("\n\n以下是反序列化操作");        // 字符串 -> 对象        String objectJsonStr = "{\n" +                "  \"id\":1125687077,\n" +                "  \"text\":\"@stroughtonsmith You need to add a \\\"Favourites\\\" tab to TC/iPhone. Like what TwitterFon did. I can't WAIT for your Twitter App!! :) Any ETA?\",\n" +                "  \"fromUserId\":855523, \n" +                "  \"toUserId\":815309,\n" +                "  \"languageCode\":\"en\"\n" +                "}";        TwitterEntry tFromStr = mapper.readValue(objectJsonStr, TwitterEntry.class);        logger.info("从字符串反序列化的对象:{}", tFromStr);        // 文件 -> 对象        TwitterEntry tFromFile = mapper.readValue(new File("twitter.json"), TwitterEntry.class);        logger.info("从文件反序列化的对象:{}", tFromStr);        // byte数组 -> 对象        TwitterEntry tFromBytes = mapper.readValue(array, TwitterEntry.class);        logger.info("从byte数组反序列化的对象:{}", tFromBytes);        // 字符串网络地址 -> 对象        String testJsonDataUrl = ";;        TwitterEntry tFromUrl = mapper.readValue(new URL(testJsonDataUrl), TwitterEntry.class);        logger.info("从网络地址反序列化的对象:{}", tFromUrl);        logger.info("\n\n以下是集合序列化操作");        Map<String, Object> map = new HashMap<>();        map.put("name", "tom");        map.put("age", 11);        Map<String, String> addr = new HashMap<>();        addr.put("city","深圳");        addr.put("street", "粤海");        map.put("addr", addr);        String mapJsonStr = mapper.writeValueAsString(map);        logger.info("HashMap序列化的字符串:{}", mapJsonStr);        logger.info("\n\n以下是集合反序列化操作");        Map<String, Object> mapFromStr = mapper.readValue(mapJsonStr, new TypeReference<Map<String, Object>>() {});        logger.info("从字符串反序列化的HashMap对象:{}", mapFromStr);        // JsonNode类型操作        JsonNode jsonNode = mapper.readTree(mapJsonStr);        String name = jsonNode.get("name").asText();        int age = jsonNode.get("age").asInt();        String city = jsonNode.get("addr").get("city").asText();        String street = jsonNode.get("addr").get("street").asText();        logger.info("用JsonNode对象和API反序列化得到的数:name[{}]、age[{}]、city[{}]、street[{}]", name, age, city, street);        // 时间类型格式        Map<String, Object> dateMap = new HashMap<>();        dateMap.put("today", new Date());        String dateMapStr = mapper.writeValueAsString(dateMap);        logger.info("默认的时间序列化:{}", dateMapStr);        // 设置时间格式        mapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd hh:mm:ss"));        dateMapStr = mapper.writeValueAsString(dateMap);        logger.info("自定义的时间序列化:{}", dateMapStr);        System.out.println(objectJsonStr);        // json数组        String jsonArrayStr = "[{\n" +                "  \"id\":1,\n" +                "  \"text\":\"text1\",\n" +                "  \"fromUserId\":11, \n" +                "  \"toUserId\":111,\n" +                "  \"languageCode\":\"en\"\n" +                "},\n" +                "{\n" +                "  \"id\":2,\n" +                "  \"text\":\"text2\",\n" +                "  \"fromUserId\":22, \n" +                "  \"toUserId\":222,\n" +                "  \"languageCode\":\"zh\"\n" +                "},\n" +                "{\n" +                "  \"id\":3,\n" +                "  \"text\":\"text3\",\n" +                "  \"fromUserId\":33, \n" +                "  \"toUserId\":333,\n" +                "  \"languageCode\":\"en\"\n" +                "}]";        // json数组 -> 对象数组        TwitterEntry[] twitterEntryArray = mapper.readValue(jsonArrayStr, TwitterEntry[].class);        logger.info("json数组反序列化成对象数组:{}", Arrays.toString(twitterEntryArray));        // json数组 -> 对象集合        List<TwitterEntry> twitterEntryList = mapper.readValue(jsonArrayStr, new TypeReference<List<TwitterEntry>>() {});        logger.info("json数组反序列化成对象集合:{}", twitterEntryList);    }}
执行结果如下:
C:\jdk\bin\java.exe -javaagent:C:\sofware\JetBrains\IntelliJIDEA\lib\idea_rt.jar=64570:C:\sofware\JetBrains\IntelliJIDEA\bin -Dfile.encoding=UTF-8 -classpath C:\jdk\jre\lib\charsets.jar;C:\jdk\jre\lib\deploy.jar;C:\jdk\jre\lib\ext\access-bridge-64.jar;C:\jdk\jre\lib\ext\cldrdata.jar;C:\jdk\jre\lib\ext\dnsns.jar;C:\jdk\jre\lib\ext\jaccess.jar;C:\jdk\jre\lib\ext\jfxrt.jar;C:\jdk\jre\lib\ext\localedata.jar;C:\jdk\jre\lib\ext\nashorn.jar;C:\jdk\jre\lib\ext\sunec.jar;C:\jdk\jre\lib\ext\sunjce_provider.jar;C:\jdk\jre\lib\ext\sunmscapi.jar;C:\jdk\jre\lib\ext\sunpkcs11.jar;C:\jdk\jre\lib\ext\zipfs.jar;C:\jdk\jre\lib\javaws.jar;C:\jdk\jre\lib\jce.jar;C:\jdk\jre\lib\jfr.jar;C:\jdk\jre\lib\jfxswt.jar;C:\jdk\jre\lib\jsse.jar;C:\jdk\jre\lib\management-agent.jar;C:\jdk\jre\lib\plugin.jar;C:\jdk\jre\lib\resources.jar;C:\jdk\jre\lib\rt.jar;D:\github\blog_demos\jacksondemo\databind\target\classes;C:\Users\12167\.m2\repository\com\fasterxml\jackson\core\jackson-databind\2.11.0\jackson-databind-2.11.0.jar;C:\Users\12167\.m2\repository\com\fasterxml\jackson\core\jackson-annotations\2.11.0\jackson-annotations-2.11.0.jar;C:\Users\12167\.m2\repository\com\fasterxml\jackson\core\jackson-core\2.11.0\jackson-core-2.11.0.jar;C:\Users\12167\.m2\repository\org\slf4j\slf4j-log4j12\1.7.25\slf4j-log4j12-1.7.25.jar;C:\Users\12167\.m2\repository\org\slf4j\slf4j-api\1.7.25\slf4j-api-1.7.25.jar;C:\Users\12167\.m2\repository\log4j\log4j\1.2.17\log4j-1.2.17.jar;C:\Users\12167\.m2\repository\commons-io\commons-io\2.7\commons-io-2.7.jar;C:\Users\12167\.m2\repository\org\apache\commons\commons-lang3\3.10\commons-lang3-3.10.jar;D:\github\blog_demos\jacksondemo\beans\target\classes com.bolingcavalry.jacksondemo.databind.SimpleDemo2020-08-28 07:53:01 INFO  SimpleDemo:27 - 以下是序列化操作2020-08-28 07:53:01 INFO  SimpleDemo:38 - 序列化的字符串:{"id":123456,"text":"this is a message for serializer test","fromUserId":101,"toUserId":102,"languageCode":"zh"}2020-08-28 07:53:01 INFO  SimpleDemo:47 - 以下是反序列化操作2020-08-28 07:53:01 INFO  SimpleDemo:60 - 从字符串反序列化的对象:[Tweet, id: 1125687077, text='@stroughtonsmith You need to add a "Favourites" tab to TC/iPhone. Like what TwitterFon did. I can't WAIT for your Twitter App!! :) Any ETA?', from: 855523, to: 815309, lang: en]2020-08-28 07:53:01 INFO  SimpleDemo:64 - 从文件反序列化的对象:[Tweet, id: 1125687077, text='@stroughtonsmith You need to add a "Favourites" tab to TC/iPhone. Like what TwitterFon did. I can't WAIT for your Twitter App!! :) Any ETA?', from: 855523, to: 815309, lang: en]2020-08-28 07:53:01 INFO  SimpleDemo:68 - 从byte数组反序列化的对象:[Tweet, id: 123456, text='this is a message for serializer test', from: 101, to: 102, lang: zh]2020-08-28 07:53:04 INFO  SimpleDemo:74 - 从网络地址反序列化的对象:[Tweet, id: 112233445566, text='this is a message from zq2599's github', from: 201, to: 202, lang: en]2020-08-28 07:53:04 INFO  SimpleDemo:77 - 以下是集合序列化操作2020-08-28 07:53:04 INFO  SimpleDemo:90 - HashMap序列化的字符串:{"name":"tom","addr":{"city":"深圳","street":"粤海"},"age":11}2020-08-28 07:53:04 INFO  SimpleDemo:92 - 以下是集合反序列化操作2020-08-28 07:53:04 INFO  SimpleDemo:94 - 从字符串反序列化的HashMap对象:{name=tom, addr={city=深圳, street=粤海}, age=11}2020-08-28 07:53:04 INFO  SimpleDemo:103 - 用JsonNode对象和API反序列化得到的数:name[tom]、age[11]、city[深圳]、street[粤海]2020-08-28 07:53:04 INFO  SimpleDemo:111 - 默认的时间序列化:{"today":1598572384838}2020-08-28 07:53:04 INFO  SimpleDemo:116 - 自定义的时间序列化:{"today":"2020-08-28 07:53:04"}{  "id":1125687077,  "text":"@stroughtonsmith You need to add a \"Favourites\" tab to TC/iPhone. Like what TwitterFon did. I can't WAIT for your Twitter App!! :) Any ETA?",  "fromUserId":855523,   "toUserId":815309,  "languageCode":"en"}2020-08-28 07:53:04 INFO  SimpleDemo:145 - json数组反序列化成对象数组:[[Tweet, id: 1, text='text1', from: 11, to: 111, lang: en], [Tweet, id: 2, text='text2', from: 22, to: 222, lang: zh], [Tweet, id: 3, text='text3', from: 33, to: 333, lang: en]]2020-08-28 07:53:04 INFO  SimpleDemo:149 - json数组反序列化成对象集合:[[Tweet, id: 1, text='text1', from: 11, to: 111, lang: en], [Tweet, id: 2, text='text2', from: 22, to: 222, lang: zh], [Tweet, id: 3, text='text3', from: 33, to: 333, lang: en]]Process finished with exit code 0
还会产生名为twitter.json的文件,内容如下:
{"id":123456,"text":"this is a message for serializer test","fromUserId":101,"toUserId":102,"languageCode":"zh"}
常用配置

下面是平时可能用到的自定义配置项目:

序列化结果格式化:

mapper.enable(SerializationFeature.INDENT_OUTPUT);
空对象不要抛出异常:
mapper.disable(SerializationFeature.FAIL_ON_EMPTY_BEANS);
Date、Calendar等序列化为时间格式的字符串(如果不执行以下设置,就会序列化成时间戳格式):
mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
反序列化时,遇到未知属性不要抛出异常:
mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
反序列化时,空字符串对于的实例属性为null:
mapper.enable(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT);
允许C和C++样式注释:
mapper.configure(JsonParser.Feature.ALLOW_COMMENTS, true);
允许字段名没有引号(可以进一步减小json体积):
mapper.configure(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES, true);
允许单引号:
mapper.configure(JsonParser.Feature.ALLOW_SINGLE_QUOTES, true);
特殊配置:在json对象最外层再包裹一层最后要说的是个特殊配置,先来看看正常情况一个普通的序列化结果:
{  "id" : 1,  "text" : "aabbcc",  "fromUserId" : 456,  "toUserId" : 0,  "languageCode" : "zh"}
接下来咱们做两件事,首先,是给上述json对应的实例类添加一个注解,如下图红框:其次,执行以下配置:
mapper.enable(SerializationFeature.WRAP_ROOT_VALUE);
然后再次执行TwitterEntry实例的序列化,得到的结果如下,可见和之前的序列化结果相比,之前的整个json都变成了一个value,此value对应的key就是注解JsonRootName的value属性:
{  "aaa" : {    "id" : 1,    "text" : "aabbcc",    "fromUserId" : 456,    "toUserId" : 0,    "languageCode" : "zh"  }}
至此,开发中常用的API和配置都已经介绍完成,希望能给您带来一些参考,接下来的章节,咱们一起去了解另一个常用操作:jackson注解;欢迎关注我的公众号:程序员欣宸

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