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Jackson文档翻译-Streaming Model examples

发表于 分类于 ， Valine：
Jackson 流媒体模型示例

原文

本 Jackson 教程向你展示如何使用 JsonGenerator 把 JSON 字符串和 JSON 数组写入到文件中，以及，使用 JsonParser 来解析它们。

Jackson Streaming APIs

  • JsonGenerator – Write JSON
  • JsonParser – Parse JSON

注意

Jackson 流媒体模型是在 data-binding 和 Tree Model 这两个底层处理模型之上建立的。它在 JSON 解析和生成上拥有最好的性能和控制。

1. JSON 数据写入 - JsonGenerator

1.1 把 JSON 写入到文件中

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// JacksonExample1.java

package com.mkyong;

import com.fasterxml.jackson.core.JsonEncoding;
import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.File;
import java.io.IOException;

public class JacksonExample1 {

    public static void main(String[] args) {

        ObjectMapper mapper = new ObjectMapper();

        try (JsonGenerator jGenerator =
                     mapper.getFactory().createGenerator(
                             new File("c:\\projects\\user.json")
                             , JsonEncoding.UTF8)) {
            
            jGenerator.writeStartObject();                                  // {

            jGenerator.writeStringField("name", "mkyong");  				// "name" : "mkyong"
            jGenerator.writeNumberField("age", 38);         				// "age" : 38

            jGenerator.writeFieldName("messages");                          // "messages" :

            jGenerator.writeStartArray();                                   // [

            jGenerator.writeString("msg 1");                            		// "msg 1"
            jGenerator.writeString("msg 2");                            		// "msg 2"
            jGenerator.writeString("msg 3");                            		// "msg 3"

            jGenerator.writeEndArray();                                     // ]

            jGenerator.writeEndObject();                                    // }

        } catch (JsonGenerationException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}

Output

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c:\\projects\\user.json

{"name":"mkyong","age":38,"messages":["msg 1","msg 2","msg 3"]}

2. JSON 数组写入 - JsonGenerator

2.1 把 JSON 数组写入到文件中

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// JacksonExample2.java
        
package com.mkyong;

import com.fasterxml.jackson.core.JsonEncoding;
import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.File;
import java.io.IOException;

public class JacksonExample2 {

    public static void main(String[] args) {

        ObjectMapper mapper = new ObjectMapper();

        try (JsonGenerator jGenerator =
                     mapper.getFactory().createGenerator(
                             new File("c:\\projects\\user2.json")
                             , JsonEncoding.UTF8)) {

            // pretty print
            jGenerator.useDefaultPrettyPrinter();

            // start array
            jGenerator.writeStartArray();                                   // [

            jGenerator.writeStartObject();                                  // {

            jGenerator.writeStringField("name", "mkyong");  				// "name" : "mkyong"
            jGenerator.writeNumberField("age", 38);         				// "age" : 38

            jGenerator.writeFieldName("messages");                          // "messages" :

            jGenerator.writeStartArray();                                   // [

            jGenerator.writeString("msg 1");                            	// "msg 1"
            jGenerator.writeString("msg 2");                            	// "msg 2"
            jGenerator.writeString("msg 3");                            	// "msg 3"

            jGenerator.writeEndArray();                                     // ]

            jGenerator.writeEndObject();                                    // }

            // next object, pls

            jGenerator.writeStartObject();                                  // {

            jGenerator.writeStringField("name", "lap");  					// "name" : "lap"
            jGenerator.writeNumberField("age", 5);         					// "age" : 5

            jGenerator.writeFieldName("messages");                          // "messages" :

            jGenerator.writeStartArray();                                   // [

            jGenerator.writeString("msg a");                            	// "msg a"
            jGenerator.writeString("msg b");                            	// "msg b"
            jGenerator.writeString("msg c");                            	// "msg c"

            jGenerator.writeEndArray();                                     // ]

            jGenerator.writeEndObject();                                    // }

            jGenerator.writeEndArray();                                     // ]

        } catch (JsonGenerationException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}

Output

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c:\\projects\\user2.json

[ 
	{
	  "name" : "mkyong",
	  "age" : 38,
	  "messages" : [ "msg 1", "msg 2", "msg 3" ]
	}, {
	  "name" : "lap",
	  "age" : 5,
	  "messages" : [ "msg a", "msg b", "msg c" ]
	} 
]

3. 读取 JSON - JsonParser

Token(标记)

在 Jackson 流媒体中，JSON 字符串会被分割成一个标记列表（token list），每个标记(token)都会被增量处理。

  • Token 1 = {
  • Token 2 = name
  • Token 3 = mkyong
  • Token 4 = }

3.1 JsonParser 解析 JSON 文件的例子：

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c:\\projects\\user.json

{"name":"mkyong","age":38,"messages":["msg 1","msg 2","msg 3"]}

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// JacksonExample3.java

package com.mkyong;

import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;
import com.fasterxml.jackson.databind.JsonMappingException;

import java.io.File;
import java.io.IOException;

public class JacksonExample3 {

    public static void main(String[] args) {

        try (JsonParser jParser = new JsonFactory()
                .createParser(new File("c:\\projects\\user.json"));) {

            // loop until token equal to "}"
            while (jParser.nextToken() != JsonToken.END_OBJECT) {

                String fieldname = jParser.getCurrentName();

                if ("name".equals(fieldname)) {
                    // current token is "name",
                    // move to next, which is "name"'s value
                    jParser.nextToken();
                    System.out.println(jParser.getText());
                }

                if ("age".equals(fieldname)) {
                    jParser.nextToken();
                    System.out.println(jParser.getIntValue());
                }

                if ("messages".equals(fieldname)) {

                    if (jParser.nextToken() == JsonToken.START_ARRAY) {
                        // messages is array, loop until token equal to "]"
                        while (jParser.nextToken() != JsonToken.END_ARRAY) {
                            System.out.println(jParser.getText());
                        }
                    }

                }

            }

        } catch (JsonGenerationException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

}

Output

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mkyong
38
msg 1
msg 2
msg 3

4. 读取 JSON 数组 - JsonParser

4.1 JsonParser 解析 JSON 数组文件的例子：

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// c:\\projects\\user2.json

[
    {
    "name" : "mkyong",
    "age" : 38,
    "messages" : [ "msg 1", "msg 2", "msg 3" ]
    }, {
    "name" : "lap",
    "age" : 5,
    "messages" : [ "msg a", "msg b", "msg c" ]
    }
]

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// JacksonExample4.java

package com.mkyong;

import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;
import com.fasterxml.jackson.databind.JsonMappingException;

import java.io.File;
import java.io.IOException;

public class JacksonExample4 {

    public static void main(String[] args) {

        try (JsonParser jParser = new JsonFactory()
                .createParser(new File("c:\\projects\\user2.json"));) {

            // JSON array?
            if (jParser.nextToken() == JsonToken.START_ARRAY) {

                while (jParser.nextToken() != JsonToken.END_ARRAY) {

                    // loop until token equal to "}"
                    while (jParser.nextToken() != JsonToken.END_OBJECT) {

                        String fieldname = jParser.getCurrentName();
                        if ("name".equals(fieldname)) {
                            // current token is "name",
                            // move to next, which is "name"'s value
                            jParser.nextToken();
                            System.out.println(jParser.getText());
                        }

                        if ("age".equals(fieldname)) {
                            jParser.nextToken();
                            System.out.println(jParser.getIntValue());
                        }

                        if ("messages".equals(fieldname)) {

                            //jParser.nextToken(); // current token is "[", move next
                            if (jParser.nextToken() == JsonToken.START_ARRAY) {
                                // messages is array, loop until token equal to "]"
                                while (jParser.nextToken() != JsonToken.END_ARRAY) {
                                    System.out.println(jParser.getText());
                                }
                            }

                        }

                    }

                }
            }

        } catch (JsonGenerationException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

}

Output

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mkyong
38
msg 1
msg 2
msg 3
lap
5
msg a
msg b
msg c

Note
More Jackson examples