jackson学习

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ObjectMapper类是Jackson库的主要类。它提供一些功能将转换成Java对象匹配JSON结构，反之亦然。它使用JsonParser和JsonGenerator的实例实现JSON实际的读/写。

Jackson学习一之对象与JSON互相转化

新建基本Student类:

Java代码

1. package com.jingshou.pojo;

3. import java.util.Date;

5. public class Student {

6. private int id;

7. private String name;

8. private Date birthDay;

10. public int getId() {

11. return id;

12. }

13.

14. public void setId(int id) {

15. this.id = id;

16. }

17.

18. public String getName() {

19. return name;

20. }

21.

22. public void setName(String name) {

23. this.name = name;

24. }

25.

26. public Date getBirthDay() {

27. return birthDay;

28. }

29.

30. public void setBirthDay(Date birthDay) {

31. this.birthDay = birthDay;

32. }

33.

34. @Override

35. public String toString() {

36. return "Student [birthDay=" + birthDay + ", id=" + id + ", name="

37. + name + "]";

38. }

39.

40. }

班级类:

Java代码

1. package com.jingshou.pojo;

3. import java.util.List;

5. public class Class {

6. private String cname;

7. private List<Student> members;

8. public String getCname() {

9. return cname;

10. }

11. public void setCname(String cname) {

12. this.cname = cname;

13. }

14. public List<Student> getMembers() {

15. return members;

16. }

17. public void setMembers(List<Student> members) {

18. this.members = members;

19. }

20.

21. public String toString(){

22. return "The Class Name is: " + cname + " " + "The members are: " + members;

23.

24. }

25.

26.

27. }

测试代码:

Java代码

1. package com.jingshou.jackson;

3. import java.io.IOException;

4. import java.util.ArrayList;

5. import java.util.Date;

6. import java.util.List;

8. import com.fasterxml.jackson.databind.ObjectMapper;

9. import com.jingshou.pojo.Class;

10. import com.jingshou.pojo.Student;

11.

12. public class JacksonTest1 {

13.

14. public static void main(String[] args) throws IOException {

15. Student student1 = new Student();

16. student1.setId(5237);

17. student1.setName("jingshou");

18. student1.setBirthDay(new Date());

19.

20. ObjectMapper mapper = new ObjectMapper();

21.

22. // Convert object to JSON string

23. String Json = mapper.writeValueAsString(student1);

24. System.out.println("Change Object to JSON String: " + Json);

25.

26. // Convert Json string to Object

27. Student student2 = mapper.readValue(Json, Student.class);

28. System.out.println(student2);

29.

30. // Create a student list add it to Bj

31. Student student3 = new Student();

32. student3.setId(5117);

33. student3.setName("saiya");

34. student3.setBirthDay(new Date());

35.

36. List<Student> stuList = new ArrayList<Student>();

37. stuList.add(student1);

38. stuList.add(student3);

39.

40. Class bj = new Class();

41. bj.setCname("五年二班");

42. bj.setMembers(stuList);

43.

44. String json2 = mapper.writeValueAsString(bj);

45. System.out.println("The JSON from Class is: " + json2);

46.

47. Class bj2 = mapper.readValue(json2, Class.class);

48. System.out.println(bj2);

49.

50.

51. }

52.

53. }

运行结果:

Text代码

1. Change Object to JSON String: {"id":5237,"name":"jingshou","birthDay":1389513906622}

2. Student [birthDay=Sun Jan 12 16:05:06 CST 2014, id=5237, name=jingshou]

3. The JSON from Class is: {"cname":"五年二班","members":[{"id":5237,"name":"jingshou","birthDay":1389513906622},{"id":5117,"name":"saiya","birthDay":1389513906859}]}

4. The Class Name is: 五年二班 The members are: [Student [birthDay=Sun Jan 12 16:05:06 CST 2014, id=5237, name=jingshou], Student [birthDay=Sun Jan 12 16:05:06 CST 2014, id=5117, name=saiya]]

小结:

· 转换的关键就是ObjectMapper

· 在使用ObjectMapper对象readValue或者writeValue的时候,JSON可以以字符串的形式传入,也可以以文件的形式传入,官方的例子是让将JSON保存在文件中

· 明显JSON字串中的key与Java类中的field名字应该要相同

· Student类如果有自定义构造函数的时候,进行JSON->对象转换会出错,接下去再研究这个问题..

· Date域是以long类型输出的,有必要的话,需要进行格式转换

Jackson学习二之集合类对象与JSON互相转化

本篇主要演示如何使用Jackson对List, Map和数组与JSON互相转换.

Java代码

1. package com.jingshou.jackson;

3. import java.io.IOException;

4. import java.util.ArrayList;

5. import java.util.Arrays;

6. import java.util.Date;

7. import java.util.HashMap;

8. import java.util.List;

9. import java.util.Map;

10.

11. import com.fasterxml.jackson.databind.ObjectMapper;

12. import com.jingshou.pojo.Student;

13.

14. public class JacksonTest2 {

15.

16. public static void main(String[] args) throws IOException {

17. Student student1 = new Student();

18. student1.setId(5237);

19. student1.setName("jingshou");

20. student1.setBirthDay(new Date());

21.

22. Student student3 = new Student();

23. student3.setId(5117);

24. student3.setName("saiya");

25. student3.setBirthDay(new Date());

26.

27. ObjectMapper mapper = new ObjectMapper();

28.

29. //Convert between List and JSON

30. List<Student> stuList = new ArrayList<Student>();

31. stuList.add(student1);

32. stuList.add(student3);

33. String jsonfromList = mapper.writeValueAsString(stuList);

34. System.out.println(jsonfromList);

35.

36. //List Type is not required here.

37. List stuList2 = mapper.readValue(jsonfromList, List.class);

38. System.out.println(stuList2);

39. System.out.println("************************************");

40.

41. //Convert Map to JSON

42. Map<String, Object> map = new HashMap<String, Object>();

43. map.put("studentList", stuList);

44. map.put("class", "ClassName");

45. String jsonfromMap = mapper.writeValueAsString(map);

46. System.out.println(jsonfromMap);

47.

48. Map map2 = mapper.readValue(jsonfromMap, Map.class);

49. System.out.println(map2);

50. System.out.println(map2.get("studentList"));

51. System.out.println("************************************");

52.

53. //Convert Array to JSON

54. Student[] stuArr = {student1, student3};

55. String jsonfromArr = mapper.writeValueAsString(stuArr);

56. System.out.println(jsonfromArr);

57. Student[] stuArr2 = mapper.readValue(jsonfromArr, Student[].class);

58. System.out.println(Arrays.toString(stuArr2));

59. }

60.

61. }

运行结果:

Text代码

1. [{"id":5237,"name":"jingshou","birthDay":1389528275987},{"id":5117,"name":"saiya","birthDay":1389528275987}]

2. [{id=5237, name=jingshou, birthDay=1389528275987}, {id=5117, name=saiya, birthDay=1389528275987}]

3. ************************************

4. {"class":"ClassName","studentList":[{"id":5237,"name":"jingshou","birthDay":1389528275987},{"id":5117,"name":"saiya","birthDay":1389528275987}]}

5. {class=ClassName, studentList=[{id=5237, name=jingshou, birthDay=1389528275987}, {id=5117, name=saiya, birthDay=1389528275987}]}

6. [{id=5237, name=jingshou, birthDay=1389528275987}, {id=5117, name=saiya, birthDay=1389528275987}]

7. ************************************

8. [{"id":5237,"name":"jingshou","birthDay":1389528275987},{"id":5117,"name":"saiya","birthDay":1389528275987}]

9. [Student [birthDay=Sun Jan 12 20:04:35 CST 2014, id=5237, name=jingshou], Student [birthDay=Sun Jan 12 20:04:35 CST 2014, id=5117, name=saiya]]

再举一例实际应用:

小米网站注册页面输入邮件地址后,服务器提交的Ajax请求是:

https://account.xiaomi.com/pass/user@externalIdBinded?externalId=9999999%40qq.com&type=EM

服务器的返回是: &&&START&&&{"result":"ok","description":"成功","data":{"userId":-1},"code":0}

我们可以尝试用Map去读取后面那一段JSON

Java代码

1. package com.jingshou.jackson;

3. import java.io.IOException;

4. import java.util.Map;

6. import com.fasterxml.jackson.core.JsonParseException;

7. import com.fasterxml.jackson.databind.JsonMappingException;

8. import com.fasterxml.jackson.databind.ObjectMapper;

10. public class JacksonTest3 {

11.

12. public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException {

13. String json = "{\"result\":\"ok\",\"description\":\"成功\",\"data\":{\"userId\":-1},\"code\":0}";

14. ObjectMapper mapper = new ObjectMapper();

15. Map map = mapper.readValue(json, Map.class);

16. //输出 {result=ok, description=成功, data={userId=-1}, code=0}

17. System.out.println(map);

18. //输出{userId=-1}

19. Map dataMap = (Map) map.get("data");

20. System.out.println(dataMap);

21. }

22.

23. }

可见以Key-Value形式的JSON字符串,都可以直接使用Map成功读取出来

Jackson学习三之使用Tree Model读取以及更新JSON

依然是使用ObjectMapper,可以以类似读取DOM的方式读取JSON,请看例子:

Java代码

1. package com.jingshou.jackson;

3. import java.io.IOException;

4. import java.net.MalformedURLException;

5. import java.net.URL;

6. import java.util.Iterator;

7. import org.apache.commons.io.IOUtils;

8. import com.fasterxml.jackson.databind.JsonNode;

9. import com.fasterxml.jackson.databind.ObjectMapper;

10.

11. public class JacksonTest4 {

12.

13. public static void main(String[] args) throws MalformedURLException, IOException {

14. String url = "http://m.weather.com.cn/data/101210101.html";

15. String json = IOUtils.toString(new URL(url));

16. System.out.println(json);

17.

18. // Get rootNode and weather node;

19. ObjectMapper m = new ObjectMapper();

20. JsonNode rootNode = m.readTree(json);

21. JsonNode weaNode = rootNode.get("weatherinfo");

22. System.out.println(weaNode);

23. System.out.println(weaNode.get("city"));

24.

25. System.out.println("***************************************");

26.

27. // Use Iterator to traverse to tree

28. Iterator<String> keys = weaNode.fieldNames();

29. while(keys.hasNext()){

30. String fieldName = keys.next();

31. System.out.println(fieldName + ": " + weaNode.get(fieldName).textValue());

32. }

33.

34. }

35.

36. }

运行结果:

Java代码

1. {"weatherinfo":{"city":"杭州","city_en":"hangzhou","date_y":"2014年1月12日","date":"","week":"星期日","fchh":"11","cityid":"101210101"......}

2. "杭州"

3. ***************************************

4. city: 杭州

5. city_en: hangzhou

6. date_y: 2014年1月12日

7. date:

8. week: 星期日

9. fchh: 11

10. cityid: 101210101

11. temp1: 5℃~0℃

12. temp2: 5℃~0℃

13. temp3: 6℃~-2℃

14. ........

以下是新建以及更新Tree Model的例子:

Java代码

1. package com.jingshou.jackson;

3. import com.fasterxml.jackson.core.JsonProcessingException;

4. import com.fasterxml.jackson.databind.JsonNode;

5. import com.fasterxml.jackson.databind.ObjectMapper;

6. import com.fasterxml.jackson.databind.node.ObjectNode;

8. public class JacksonTest5 {

10. public static void main(String[] args) throws JsonProcessingException {

11. ObjectMapper mapper = new ObjectMapper();

12. JsonNode rootNode = mapper.createObjectNode(); // will be of type ObjectNode

13. ((ObjectNode) rootNode).put("name", "jingshou");

14. ((ObjectNode) rootNode).put("id", 523);

15. ((ObjectNode) rootNode).put("age", 30);

16. System.out.println(rootNode);

17.

18. String json = mapper.writeValueAsString(rootNode);

19. System.out.println(json);

20. ((ObjectNode) rootNode).remove("id");

21. System.out.println(rootNode);

22.

23. }

24.

25. }

运行结果:

Java代码

1. {"name":"jingshou","id":523,"age":30}

2. {"name":"jingshou","id":523,"age":30}

3. {"name":"jingshou","age":30}

Jackson学习四之使用Streaming API创建以及读取JSON

Java代码

1. package com.jingshou.jackson;

3. import java.io.File;

4. import java.io.IOException;

6. import com.fasterxml.jackson.core.JsonEncoding;

7. import com.fasterxml.jackson.core.JsonFactory;

8. import com.fasterxml.jackson.core.JsonGenerator;

9. import com.fasterxml.jackson.core.JsonParser;

10. import com.fasterxml.jackson.core.JsonToken;

11.

12. public class JacksonTest6 {

13.

14. public static void main(String[] args) throws IOException {

15. JsonFactory jfactory = new JsonFactory();

16.

17. /*** write to file ***/

18. JsonGenerator jGenerator = jfactory.createGenerator(new File(

19. "c:\\user.json"), JsonEncoding.UTF8);

20. jGenerator.writeStartObject(); // {

21.

22. jGenerator.writeStringField("name", "mkyong"); // "name" : "mkyong"

23. jGenerator.writeNumberField("age", 29); // "age" : 29

24.

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

26. jGenerator.writeStartArray(); // [

27.

28. jGenerator.writeString("msg 1"); // "msg 1"

29. jGenerator.writeString("msg 2"); // "msg 2"

30. jGenerator.writeString("msg 3"); // "msg 3"

31.

32. jGenerator.writeEndArray(); // ]

33.

34. jGenerator.writeEndObject(); // }

35. jGenerator.close();

36.

37. /*** read from file ***/

38. JsonParser jParser = jfactory.createParser(new File("c:\\user.json"));

39. // loop until token equal to "}"

40. while (jParser.nextToken() != JsonToken.END_OBJECT) {

41.

42. String fieldname = jParser.getCurrentName();

43. if ("name".equals(fieldname)) {

44.

45. // current token is "name",

46. // move to next, which is "name"'s value

47. jParser.nextToken();

48. System.out.println(jParser.getText()); // display mkyong

49.

50. }

51.

52. if ("age".equals(fieldname)) {

53.

54. // current token is "age",

55. // move to next, which is "name"'s value

56. jParser.nextToken();

57. System.out.println(jParser.getIntValue()); // display 29

58.

59. }

60.

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

62.

63. jParser.nextToken(); // current token is "[", move next

64.

65. // messages is array, loop until token equal to "]"

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

67.

68. // display msg1, msg2, msg3

69. System.out.println(jParser.getText());

70.

71. }

72.

73. }

74.

75. }

76. jParser.close();

77.

78. }

79.

80. }

小结:

· 对于性能要求高的程序,推荐使用流API,否则使用其他方法

· 不管是创建JsonGenerator还是JsonParser,都是使用JsonFactory

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