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这里分类和汇总了欣宸的全部原创(含配套源码):https://github.com/zq2599/blog_demos

关于jackson-core

  1. 本文主要内容是==jackson-core库==,这是个低阶API库,提供流式解析工具==JsonParser==,流式生成工具==JsonGenerator==;
  2. 在日常的序列化和反序列化处理中,最常用的是==jackson-annotations==和==jackson-databind==,而jackson-core由于它提供的API过于基础,我们大多数情况下是用不上的;
  3. 尽管jackson-databind负责序列化和反序列化处理,但它的底层实现是调用了jackson-core的API;
  4. 本着万丈高楼平地起的原则,本文咱们通过实战了解神秘的jackson-core,了解整个jackson的序列化和反序列化基本原理;

    源码下载

  5. 如果您不想编码,可以在GitHub下载所有源码,地址和链接信息如下表所示(https://github.com/zq2599/blog_demos):
名称 链接 备注
项目主页 https://github.com/zq2599/blog_demos 该项目在GitHub上的主页
git仓库地址(https) https://github.com/zq2599/blog_demos.git 该项目源码的仓库地址,https协议
git仓库地址(ssh) git@github.com:zq2599/blog_demos.git 该项目源码的仓库地址,ssh协议
  1. 这个git项目中有多个文件夹,本章的应用在==jacksondemo==文件夹下,如下图红框所示:
    在这里插入图片描述

    创建父子工程

    创建名为==jacksondemo==的maven工程,这是个父子结构的工程,其pom.xml内容如下:

    <?xml version="1.0" encoding="UTF-8"?>
    <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 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <properties>
        <java.version>1.8</java.version>
    </properties>
    <groupId>com.bolingcavalry</groupId>
    <artifactId>jacksondemo</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>pom</packaging>
    <modules>
        <module>core</module>
        <module>beans</module>
        <module>databind</module>
    </modules>
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>com.fasterxml.jackson.core</groupId>
                <artifactId>jackson-databind</artifactId>
                <version>2.11.0</version>
                <scope>compile</scope>
            </dependency>
            <dependency>
                <groupId>org.slf4j</groupId>
                <artifactId>slf4j-log4j12</artifactId>
                <version>1.7.25</version>
                <scope>compile</scope>
            </dependency>
            <dependency>
                <groupId>commons-io</groupId>
                <artifactId>commons-io</artifactId>
                <version>2.7</version>
                <scope>compile</scope>
            </dependency>
            <dependency>
                <groupId>org.apache.commons</groupId>
                <artifactId>commons-lang3</artifactId>
                <version>3.10</version>
                <scope>compile</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>
    </project>

    新增子工程beans

  2. 在父工程jscksondemo下新增名为==beans==的子工程,这里面是一些常量和Pojo类;
  3. 增加定义常量的类Constant.java:
    
    package com.bolingcavalry.jacksondemo.beans;

public class Constant {
/**

  • 该字符串的值是个网络地址,该地址对应的内容是个JSON
    */
    public final static String TEST_JSON_DATA_URL = "https://raw.githubusercontent.com/zq2599/blog_demos/master/files/twitteer_message.json";
    /**
  • 用来验证反序列化的JSON字符串
    */
    public final static String TEST_JSON_STR = "{\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" +
    "}";
    /**
  • 用来验证序列化的TwitterEntry实例
    */
    public final static TwitterEntry TEST_OBJECT = new TwitterEntry();
    /**
  • 准备好TEST_OBJECT对象的各个参数
    */
    static {
    TEST_OBJECT.setId(123456L);
    TEST_OBJECT.setFromUserId(101);
    TEST_OBJECT.setToUserId(102);
    TEST_OBJECT.setText("this is a message for serializer test");
    TEST_OBJECT.setLanguageCode("zh");
    }}
    
    3. 增加一个Pojo,对应的是一条推特消息:
    ```java
    package com.bolingcavalry.jacksondemo.beans;
    /**
    * @Description: 推特消息bean
    * @author: willzhao E-mail: zq2599@gmail.com
    * @date: 2020/7/4 16:24
    */
    public class TwitterEntry {
    /**
  • 推特消息id
    */
    long id;
    /**
  • 消息内容
    */
    String text; /**
  • 消息创建者
    */
    int fromUserId;
    /**
  • 消息接收者
    */
    int toUserId;
    /**
  • 语言类型
    */
    String languageCode; public long getId() {
    return id;
    } public void setId(long id) {
    this.id = id;
    } public String getText() {
    return text;
    } public void setText(String text) {
    this.text = text;
    } public int getFromUserId() {
    return fromUserId;
    } public void setFromUserId(int fromUserId) {
    this.fromUserId = fromUserId;
    } public int getToUserId() {
    return toUserId;
    } public void setToUserId(int toUserId) {
    this.toUserId = toUserId;
    } public String getLanguageCode() {
    return languageCode;
    } public void setLanguageCode(String languageCode) {
    this.languageCode = languageCode;
    } public TwitterEntry() {
    } public String toString() {
    return "[Tweet, id: "+id+", text='"+text+"', from: "+fromUserId+", to: "+toUserId+", lang: "+languageCode+"]";
    }}
    4. 以上就是准备工作了,接下来开始实战jackson-core;
    ### JsonFactory线程安全吗?
    1. JsonFactory是否是线程安全的,这是编码前要弄清楚的问题,因为==JsonParser==和==JsonGenerator==的创建都离不开JsonFactory;
    2. 如下图红框所示,jackson官方文档中明确指出==JsonFactory是线程安全的==,可以放心的作为全局变量给多线程同时使用:
    ![在这里插入图片描述](https://s4.51cto.com/images/blog/202201/20234340_61e9832c6885a13963.png?x-oss-process=image/watermark,size_14,text_QDUxQ1RP5Y2a5a6i,color_FFFFFF,t_100,g_se,x_10,y_10,shadow_20,type_ZmFuZ3poZW5naGVpdGk=)
    3. 官方文档地址:http://fasterxml.github.io/jackson-core/javadoc/2.11/
    ### jackson-core实战
    1. 新建子工程==core==,pom.xml如下:
    ```xml
    <?xml version="1.0" encoding="UTF-8"?>
    <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">
    <parent>
        <artifactId>jacksondemo</artifactId>
        <groupId>com.bolingcavalry</groupId>
        <version>1.0-SNAPSHOT</version>
        <relativePath>../pom.xml</relativePath>
    </parent>
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.bolingcavalry</groupId>
    <artifactId>core</artifactId>
    <name>core</name>
    <description>Demo project for jackson core use</description>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <source>8</source>
                    <target>8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
    <dependencies>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-log4j12</artifactId>
        </dependency>
        <dependency>
            <groupId>commons-io</groupId>
            <artifactId>commons-io</artifactId>
        </dependency>
        <dependency>
            <groupId>org.apache.commons</groupId>
            <artifactId>commons-lang3</artifactId>
        </dependency>
        <dependency>
            <groupId>com.bolingcavalry</groupId>
            <artifactId>beans</artifactId>
            <version>${project.version}</version>
        </dependency>
    </dependencies>
    </project>
    1. 新建StreamingDemo类,这里面是调用jackson-core的API进行序列化和反序列化的所有demo,如下:
      
      package com.bolingcavalry.jacksondemo.core;

import com.bolingcavalry.jacksondemo.beans.TwitterEntry;
import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.apache.commons.io.IOUtils;
import org.apache.commons.lang3.StringUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.net.URL;

/**

  • @Description: jackson低阶方法的使用
  • @author: willzhao E-mail: zq2599@gmail.com
  • @date: 2020/7/4 15:50
    */
    public class StreamingDemo {

    private static final Logger logger = LoggerFactory.getLogger(StreamingDemo.class);

    JsonFactory jsonFactory = new JsonFactory();

    /**

    /**

    • 用来验证反序列化的JSON字符串
      */
      final static String TEST_JSON_STR = "{\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" +
      "}";

    /**

    • 用来验证序列化的TwitterEntry实例
      */
      final static TwitterEntry TEST_OBJECT = new TwitterEntry();

    /**

    • 准备好TEST_OBJECT对象的各个参数
      */
      static {
      TEST_OBJECT.setId(123456L);
      TEST_OBJECT.setFromUserId(101);
      TEST_OBJECT.setToUserId(102);
      TEST_OBJECT.setText("this is a message for serializer test");
      TEST_OBJECT.setLanguageCode("zh");
      }

    /**

    • 反序列化测试(JSON -> Object),入参是JSON字符串
    • @param json JSON字符串
    • @return
    • @throws IOException
      */
      public TwitterEntry deserializeJSONStr(String json) throws IOException {

      JsonParser jsonParser = jsonFactory.createParser(json);

      if (jsonParser.nextToken() != JsonToken.START_OBJECT) {
      jsonParser.close();
      logger.error("起始位置没有大括号");
      throw new IOException("起始位置没有大括号");
      }

      TwitterEntry result = new TwitterEntry();

      try {
      // Iterate over object fields:
      while (jsonParser.nextToken() != JsonToken.END_OBJECT) {

          String fieldName = jsonParser.getCurrentName();
      
          logger.info("正在解析字段 [{}]", jsonParser.getCurrentName());
      
          // 解析下一个
          jsonParser.nextToken();
      
          switch (fieldName) {
              case "id":
                  result.setId(jsonParser.getLongValue());
                  break;
              case "text":
                  result.setText(jsonParser.getText());
                  break;
              case "fromUserId":
                  result.setFromUserId(jsonParser.getIntValue());
                  break;
              case "toUserId":
                  result.setToUserId(jsonParser.getIntValue());
                  break;
              case "languageCode":
                  result.setLanguageCode(jsonParser.getText());
                  break;
              default:
                  logger.error("未知字段 '" + fieldName + "'");
                  throw new IOException("未知字段 '" + fieldName + "'");
          }
      }

      } catch (IOException e) {
      logger.error("反序列化出现异常 :", e);
      } finally {
      jsonParser.close(); // important to close both parser and underlying File reader
      }

      return result;
      }

    /**

    • 反序列化测试(JSON -> Object),入参是JSON字符串
    • @param url JSON字符串的网络地址
    • @return
    • @throws IOException
      */
      public TwitterEntry deserializeJSONFromUrl(String url) throws IOException {
      // 从网络上取得JSON字符串
      String json = IOUtils.toString(new URL(TEST_JSON_DATA_URL), JsonEncoding.UTF8.name());

      logger.info("从网络取得JSON数据 :\n{}", json);

      if(StringUtils.isNotBlank(json)) {
      return deserializeJSONStr(json);
      } else {
      logger.error("从网络获取JSON数据失败");
      return null;
      }
      }

    /**

    • 序列化测试(Object -> JSON)
    • @param twitterEntry
    • @return 由对象序列化得到的JSON字符串
      */
      public String serialize(TwitterEntry twitterEntry) throws IOException{
      String rlt = null;
      ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
      JsonGenerator jsonGenerator = jsonFactory.createGenerator(byteArrayOutputStream, JsonEncoding.UTF8);

      try {
      jsonGenerator.useDefaultPrettyPrinter();

      jsonGenerator.writeStartObject();
      jsonGenerator.writeNumberField("id", twitterEntry.getId());
      jsonGenerator.writeStringField("text", twitterEntry.getText());
      jsonGenerator.writeNumberField("fromUserId", twitterEntry.getFromUserId());
      jsonGenerator.writeNumberField("toUserId", twitterEntry.getToUserId());
      jsonGenerator.writeStringField("languageCode", twitterEntry.getLanguageCode());
      jsonGenerator.writeEndObject();

      } catch (IOException e) {
      logger.error("序列化出现异常 :", e);
      } finally {
      jsonGenerator.close();
      }

      // 一定要在
      rlt = byteArrayOutputStream.toString();

      return rlt;
      }

    public static void main(String[] args) throws Exception {

    StreamingDemo streamingDemo = new StreamingDemo();
    
    // 执行一次对象转JSON操作
    logger.info("********************执行一次对象转JSON操作********************");
    String serializeResult = streamingDemo.serialize(TEST_OBJECT);
    logger.info("序列化结果是JSON字符串 : \n{}\n\n", serializeResult);
    
    // 用本地字符串执行一次JSON转对象操作
    logger.info("********************执行一次本地JSON反序列化操作********************");
    TwitterEntry deserializeResult = streamingDemo.deserializeJSONStr(TEST_JSON_STR);
    logger.info("\n本地JSON反序列化结果是个java实例 : \n{}\n\n", deserializeResult);
    
    // 用网络地址执行一次JSON转对象操作
    logger.info("********************执行一次网络JSON反序列化操作********************");
    deserializeResult = streamingDemo.deserializeJSONFromUrl(TEST_JSON_DATA_URL);
    logger.info("\n网络JSON反序列化结果是个java实例 : \n{}", deserializeResult);
    
    ObjectMapper a;

    }
    }

    
    3. 上述代码可见JsonParser负责将JSON解析成对象的变量值,核心是循环处理JSON中的所有内容;
    4. JsonGenerator负责将对象的变量写入JSON的各个属性,这里是开发者自行决定要处理哪些字段;
    5. 不论是JsonParser还是JsonGenerator,大家都可以感觉到工作量很大,需要开发者自己动手实现对象和JSON字段的关系映射,实际应用中不需要咱们这样辛苦的编码,jackson的另外两个库(annonation的databind)已经帮我们完成了大量工作,上述代码只是揭示最基础的jackson执行原理;
    6. 执行StreamingDemo类,得到结果如下,序列化和反序列化都成功了:
    ![在这里插入图片描述](https://s4.51cto.com/images/blog/202201/20234340_61e9832c7b61047210.png?x-oss-process=image/watermark,size_14,text_QDUxQ1RP5Y2a5a6i,color_FFFFFF,t_100,g_se,x_10,y_10,shadow_20,type_ZmFuZ3poZW5naGVpdGk=)
    - 以上就是jackson-core的基本功能,咱们了解了jackson最底层的工作原理,接下来的文章会继续实践更多操作;