jackson学习之二：jackson-core

傷城~ 2022-12-06 15:16 198阅读 0赞

### 欢迎访问我的GitHub ###

> 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][https_github.com_zq2599_blog_demos]

### 系列文章汇总 ###

*  [jackson学习之一：基本信息][jackson]
 *  [jackson学习之二：jackson-core][jackson_jackson-core]
 *  [jackson学习之三：常用API操作][jackson_API]
 *  [jackson学习之四：WRAP\_ROOT\_VALUE（root对象）][jackson_WRAP_ROOT_VALUE_root]
 *  [jackson学习之五：JsonInclude注解][jackson_JsonInclude]
 *  [jackson学习之六：常用类注解][jackson 1]
 *  [jackson学习之七：常用Field注解][jackson_Field]
 *  [jackson学习之八：常用方法注解][jackson 2]
 *  [jackson学习之九：springboot整合(配置文件)][jackson_springboot]
 *  [jackson学习之十(终篇)：springboot整合(配置类)][jackson_springboot 1]

### 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 ###

1.  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)：

<table> 
 <thead> 
  <tr> 
   <th align="left">名称</th> 
   <th align="left">链接</th> 
   <th align="left">备注</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left">项目主页</td> 
   <td align="left">https://github.com/zq2599/blog_demos</td> 
   <td align="left">该项目在GitHub上的主页</td> 
  </tr> 
  <tr> 
   <td align="left">git仓库地址(https)</td> 
   <td align="left">https://github.com/zq2599/blog_demos.git</td> 
   <td align="left">该项目源码的仓库地址，https协议</td> 
  </tr> 
  <tr> 
   <td align="left">git仓库地址(ssh)</td> 
   <td align="left">git@github.com:zq2599/blog_demos.git</td> 
   <td align="left">该项目源码的仓库地址，ssh协议</td> 
  </tr> 
 </tbody> 
</table>

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][2020090516314153.png_pic_center]

### 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 ###

1.  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；
2.  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");
        }}

1.  增加一个Pojo，对应的是一条推特消息：

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+"]";
        }}

1.  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][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JvbGluZ19jYXZhbHJ5_size_16_color_FFFFFF_t_70_pic_center]
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 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();
    
        /** * 该字符串的值是个网络地址，该地址对应的内容是个JSON */
        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 */
        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;
        }
    }

1.  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；
2.  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；
3.  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；
4.  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][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JvbGluZ19jYXZhbHJ5_size_16_color_FFFFFF_t_70_pic_center 1]

*  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；

### 欢迎关注公众号：程序员欣宸 ###

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[https_github.com_zq2599_blog_demos]: https://github.com/zq2599/blog_demos
[jackson]: https://blog.csdn.net/boling_cavalry/article/details/107135958
[jackson_jackson-core]: https://blog.csdn.net/boling_cavalry/article/details/108571629
[jackson_API]: https://blog.csdn.net/boling_cavalry/article/details/108192174
[jackson_WRAP_ROOT_VALUE_root]: https://blog.csdn.net/boling_cavalry/article/details/108298858
[jackson_JsonInclude]: https://blog.csdn.net/boling_cavalry/article/details/108412558
[jackson 1]: https://blog.csdn.net/boling_cavalry/article/details/108333324
[jackson_Field]: https://blog.csdn.net/boling_cavalry/article/details/108589494
[jackson 2]: https://blog.csdn.net/boling_cavalry/article/details/108433330
[jackson_springboot]: https://blog.csdn.net/boling_cavalry/article/details/108460433
[jackson_springboot 1]: https://blog.csdn.net/boling_cavalry/article/details/108559056
[2020090516314153.png_pic_center]: /images/20221123/42c166f6328f4945a2a01e49271db4bb.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JvbGluZ19jYXZhbHJ5_size_16_color_FFFFFF_t_70_pic_center]: /images/20221123/96c5a745e6b744c49351d63d33433962.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JvbGluZ19jYXZhbHJ5_size_16_color_FFFFFF_t_70_pic_center 1]: /images/20221123/41e6a970a6e5499799e29bb257e3da39.png