Jackson 框架，輕易轉換JSON

時間 2019-11-10

標籤 jackson 框架輕易轉換 json 欄目 JavaScript 简体版

原文原文鏈接

相比json-lib框架，Jackson所依賴的jar包較少，簡單易用而且性能也要相對高些。並且Jackson社區相對比較活躍，更新速度也比較快。java

1、準備工做node

一、下載依賴庫jar包json

Jackson的jar all下載地址：http://jackson.codehaus.org/1.7.6/jackson-all-1.7.6.jarapi

而後在工程中導入這個jar包便可開始工做數組

官方示例：http://wiki.fasterxml.com/JacksonInFiveMinutesapp

由於下面的程序是用junit測試用例運行的，因此還得添加junit的jar包。版本是junit-4.2.8框架

若是你須要轉換xml，那麼還須要stax2-api.jaride

二、測試類基本代碼以下性能

 1 package com.hoo.test;
 2 
 3 import java.io.IOException;
 4 import java.io.StringWriter;
 5 import java.util.ArrayList;
 6 import java.util.HashMap;
 7 import java.util.Iterator;
 8 import java.util.LinkedHashMap;
 9 import java.util.List;
10 import java.util.Map;
11 import java.util.Set;
12 import org.codehaus.jackson.JsonEncoding;
13 import org.codehaus.jackson.JsonGenerationException;
14 import org.codehaus.jackson.JsonGenerator;
15 import org.codehaus.jackson.JsonParseException;
16 import org.codehaus.jackson.map.JsonMappingException;
17 import org.codehaus.jackson.map.ObjectMapper;
18 import org.codehaus.jackson.node.JsonNodeFactory;
19 import org.codehaus.jackson.xml.XmlMapper;
20 import org.junit.After;
21 import org.junit.Before;
22 import org.junit.Test;
23 import com.hoo.entity.AccountBean;
24 /**
25  * <b>function:</b>Jackson 將java對象轉換成JSON字符串，也能夠將JSON字符串轉換成java對象
26  * jar-lib-version: jackson-all-1.6.2
27  * jettison-1.0.1
28  * @file JacksonTest.java
29  * @package com.hoo.test
30  * @project Spring3
31  * @blog http://blog.csdn.net/IBM_hoojo
32  * @email hoojo_@126.com
33  * @version 1.0
34  */
35 @SuppressWarnings("unchecked")
36 public class JacksonTest {
37     private JsonGenerator jsonGenerator = null;
38     private ObjectMapper objectMapper = null;
39     private AccountBean bean = null;
40     
41     @Before
42     public void init() {
43         bean = new AccountBean();
44         bean.setAddress("china-Guangzhou");
45         bean.setEmail("hoojo_@126.com");
46         bean.setId(1);
47         bean.setName("hoojo");        
48         objectMapper = new ObjectMapper();
49         try {
50             jsonGenerator = objectMapper.getJsonFactory().createJsonGenerator(System.out, JsonEncoding.UTF8);
51         } catch (IOException e) {
52             e.printStackTrace();
53         }
54     }
55     
56     @After
57     public void destory() {
58         try {
59             if (jsonGenerator != null) {
60                 jsonGenerator.flush();
61             }
62             if (!jsonGenerator.isClosed()) {
63                 jsonGenerator.close();
64             }
65 
66             jsonGenerator = null;
67             objectMapper = null;
68             bean = null;
69             System.gc();
70         } catch (IOException e) {
71             e.printStackTrace();
72         }
73     }
74 }

三、所須要的JavaEntity測試

 1 package com.hoo.entity;
 2  
 3 public class AccountBean {
 4     private int id;
 5     private String name;
 6     private String email;
 7     private String address;
 8     private Birthday birthday;   
 9     //getter、setter
10     
11     @Override
12     public String toString() {
13         return this.name + "#" + this.id + "#" + this.address + "#" + this.birthday + "#" + this.email;
14     }
15 }
16 
17 //Birthday類
18 package com.hoo.entity;
19  
20 public class Birthday {
21     private String birthday;    
22     public Birthday(String birthday) {
23         super();
24         this.birthday = birthday;
25     } 
26     //getter、setter
27 
28     public Birthday() {}   
29 
30     @Override
31     public String toString() {
32         return this.birthday;
33     }
34 }

2、Java對象轉換成JSON

一、 JavaBean(Entity/Model)轉換成JSON

 1 /**
 2  * <b>function:</b>將java對象轉換成json字符串
 3  */
 4 @Test
 5 public void writeEntityJSON() {  
 6     try {
 7         System.out.println("jsonGenerator");
 8         //writeObject能夠轉換java對象，eg:JavaBean/Map/List/Array等
 9         jsonGenerator.writeObject(bean);    
10         System.out.println();        
11         System.out.println("ObjectMapper");
12         //writeValue具備和writeObject相同的功能
13         objectMapper.writeValue(System.out, bean);
14 
15     } catch (IOException e) {
16         e.printStackTrace();
17     }
18 }

運行後結果以下：

1 jsonGenerator
2 {"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}
3 
4 ObjectMapper
5 {"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}

上面分別利用JsonGenerator的writeObject方法和ObjectMapper的writeValue方法完成對Java對象的轉換，兩者傳遞的參數及構造的方式不一樣；JsonGenerator的建立依賴於ObjectMapper對象。也就是說若是你要使用 JsonGenerator來轉換JSON，那麼你必須建立一個ObjectMapper。可是你用ObjectMapper來轉換JSON，則不須要 JSONGenerator。

objectMapper的writeValue方法能夠將一個Java對象轉換成JSON。這個方法的參數一，須要提供一個輸出流，轉換後能夠通過這個流來輸出轉換後的內容。或是提供一個File，將轉換後的內容寫入到File中。固然，這個參數也能夠接收一個JSONGenerator，而後通過JSONGenerator來輸出轉換後的信息。第二個參數是將要被轉換的Java對象。若是用三個參數的方法，那麼是一個Config。這個 config能夠提供一些轉換時的規則，過指定的Java對象的某些屬性進行過濾或轉換等。

二、將Map集合轉換成Json字符串

 1 /**
 2  * <b>function:</b>將map轉換成json字符串
 3  */
 4 @Test
 5 public void writeMapJSON() {
 6     try {
 7         Map<String, Object> map = new HashMap<String, Object>();
 8         map.put("name", bean.getName());
 9         map.put("account", bean);
10         bean = new AccountBean();
11         bean.setAddress("china-Beijin");
12         bean.setEmail("hoojo@qq.com");
13         map.put("account2", bean);        
14         System.out.println("jsonGenerator");
15         jsonGenerator.writeObject(map);
16         System.out.println("");        
17         System.out.println("objectMapper");
18         objectMapper.writeValue(System.out, map);
19     } catch (IOException e) {
20         e.printStackTrace();
21     }
22 }

轉換後結果以下：

1 jsonGenerator
2 {"account2":{"address":"china-Beijin","name":null,"id":0,"birthday":null,"email":"hoojo@qq.com"},"name":"hoojo",
3 "account":{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}}
4 
5 objectMapper
6 {"account2":{"address":"china-Beijin","name":null,"id":0,"birthday":null,"email":"hoojo@qq.com"},"name":"hoojo",
7 "account":{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}}

三、將List集合轉換成json

 1 /**
 2  * <b>function:</b>將list集合轉換成json字符串
 3  */
 4 @Test
 5 public void writeListJSON() {
 6     try {
 7         List<AccountBean> list = new ArrayList<AccountBean>();
 8         list.add(bean);       
 9         bean = new AccountBean();
10         bean.setId(2);
11         bean.setAddress("address2");
12         bean.setEmail("email2");
13         bean.setName("haha2");
14         list.add(bean);        
15         System.out.println("jsonGenerator");
16         //list轉換成JSON字符串
17         jsonGenerator.writeObject(list);
18         System.out.println();
19         System.out.println("ObjectMapper");
20         //用objectMapper直接返回list轉換成的JSON字符串
21         System.out.println("1###" + objectMapper.writeValueAsString(list));
22         System.out.print("2###");
23         //objectMapper list轉換成JSON字符串
24         objectMapper.writeValue(System.out, list);
25     } catch (IOException e) {
26         e.printStackTrace();
27     }
28 }

結果以下：

1 jsonGenerator
2 [{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"},
3 {"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]
4 
5 ObjectMapper
6 1###[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"},
7 {"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]
8 2###[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"},
9 {"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]

外面就是多了個[]中括號；一樣Array也能夠轉換，轉換的JSON和上面的結果是同樣的，這裏就再也不轉換了。~.~

四、下面來看看jackson提供的一些類型，用這些類型完成json轉換；若是你使用這些類型轉換JSON的話，那麼你即便沒有 JavaBean(Entity)也能夠完成複雜的Java類型的JSON轉換。下面用到這些類型構建一個複雜的Java對象，並完成JSON轉換。

 1 @Test
 2 public void writeOthersJSON() {
 3     try {
 4         String[] arr = { "a", "b", "c" };
 5         System.out.println("jsonGenerator");
 6         String str = "hello world jackson!";
 7         //byte
 8         jsonGenerator.writeBinary(str.getBytes());
 9         //boolean
10         jsonGenerator.writeBoolean(true);
11         //null
12         jsonGenerator.writeNull();
13         //float
14         jsonGenerator.writeNumber(2.2f);
15         //char
16         jsonGenerator.writeRaw("c");
17         //String
18         jsonGenerator.writeRaw(str, 5, 10);
19         //String
20         jsonGenerator.writeRawValue(str, 5, 5);
21         //String
22         jsonGenerator.writeString(str);
23          jsonGenerator.writeTree(JsonNodeFactory.instance.POJONode(str));
24         System.out.println();
25         
26         //Object
27         jsonGenerator.writeStartObject();//{
28         jsonGenerator.writeObjectFieldStart("user");//user:{
29         jsonGenerator.writeStringField("name", "jackson");//name:jackson
30         jsonGenerator.writeBooleanField("sex", true);//sex:true
31         jsonGenerator.writeNumberField("age", 22);//age:22
32         jsonGenerator.writeEndObject();//}       
33         jsonGenerator.writeArrayFieldStart("infos");//infos:[
34         jsonGenerator.writeNumber(22);//22
35         jsonGenerator.writeString("this is array");//this is array
36         jsonGenerator.writeEndArray();//]        
37         jsonGenerator.writeEndObject();//}               
38 
39         AccountBean bean = new AccountBean();
40         bean.setAddress("address");
41         bean.setEmail("email");
42         bean.setId(1);
43         bean.setName("haha");
44 
45         //complex Object
46         jsonGenerator.writeStartObject();//{
47             jsonGenerator.writeObjectField("user", bean);//user:{bean}
48             jsonGenerator.writeObjectField("infos", arr);//infos:[array]
49             jsonGenerator.writeEndObject();//
50         }        
51     } catch (Exception e) {
52         e.printStackTrace();
53     }
54 }

運行後，結果以下：

1 jsonGenerator
2 "aGVsbG8gd29ybGQgamFja3NvbiE=" true null 2.2c world jac  worl "hello world jackson!" "hello world jackson!"
3  {"user":{"name":"jackson","sex":true,"age":22},"infos":[22,"this is array"]} 
4 {"user":{"address":"address","name":"haha","id":1,"birthday":null,"email":"email"},"infos":["a","b","c"]}

怎麼樣？構造的json字符串和輸出的結果是一致的吧。關鍵看懂用JSONGenerator提供的方法，完成一個Object的構建。

3、JSON轉換成Java對象

一、將json字符串轉換成JavaBean對象

 1 @Test
 2 public void readJson2Entity() {
 3     String json = "{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}";
 4     try {
 5         AccountBean acc = objectMapper.readValue(json, AccountBean.class);
 6         System.out.println(acc.getName());
 7         System.out.println(acc);
 8     } catch (JsonParseException e) {
 9         e.printStackTrace();
10     } catch (JsonMappingException e) {
11         e.printStackTrace();
12     } catch (IOException e) {
13         e.printStackTrace();
14     }
15 }

很簡單，用到了ObjectMapper這個對象的readValue這個方法，這個方法須要提供2個參數。第一個參數就是解析的JSON字符串，第二個參數是即將將這個JSON解析吃什麼Java對象，Java對象的類型。固然，還有其餘相同簽名方法，若是你有興趣能夠一一嘗試使用方法，固然使用的方法和當前使用的方法大同小異。運行後，結果以下：

1 haha
2 haha#1#address#null#email

二、將json字符串轉換成List<Map>集合

 1 @Test
 2 public void readJson2List() {
 3     String json = "[{\"address\": \"address2\",\"name\":\"haha2\",\"id\":2,\"email\":\"email2\"},"+
 4                 "{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}]";
 5     try {
 6         List<LinkedHashMap<String, Object>> list = objectMapper.readValue(json, List.class);
 7         System.out.println(list.size());
 8         for (int i = 0; i < list.size(); i++) {
 9             Map<String, Object> map = list.get(i);
10             Set<String> set = map.keySet();
11             for (Iterator<String> it = set.iterator();it.hasNext();) {
12                 String key = it.next();
13                 System.out.println(key + ":" + map.get(key));
14             }
15         }
16     } catch (JsonParseException e) {
17         e.printStackTrace();
18     } catch (JsonMappingException e) {
19         e.printStackTrace();
20     } catch (IOException e) {
21         e.printStackTrace();
22     }
23 }

嘗試過將上面的JSON轉換成List，而後List中存放AccountBean，但結果失敗了。可是支持Map集合。由於你轉成List.class，可是不知道List存放何種類型。只好默然Map類型。由於全部的對象均可以轉換成Map結合，運行後結果以下：

1 2
2 address:address2
3 name:haha2
4 id:2
5 email:email2
6 address:address
7 name:haha
8 id:1
9 email:email

三、 Json字符串轉換成Array數組，因爲上面的泛型轉換不能識別到集合中的對象類型。全部這裏用對象數組，能夠解決這個問題。只不過它再也不是集合，而是一個數組。固然這個不重要，你能夠用Arrays.asList將其轉換成List便可。

 1 @Test
 2 public void readJson2Array() {
 3     String json = "[{\"address\": \"address2\",\"name\":\"haha2\",\"id\":2,\"email\":\"email2\"},"+
 4             "{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}]";
 5     try {
 6         AccountBean[] arr = objectMapper.readValue(json, AccountBean[].class);
 7         System.out.println(arr.length);
 8         for (int i = 0; i < arr.length; i++) {
 9             System.out.println(arr[i]);
10         }        
11     } catch (JsonParseException e) {
12         e.printStackTrace();
13     } catch (JsonMappingException e) {
14         e.printStackTrace();
15     } catch (IOException e) {
16         e.printStackTrace();
17     }
18 }

運行後的結果：

1 2
2 haha2#2#address2#null#email2
3 haha#1#address#null#email

四、 Json字符串轉換成Map集合

 1 @Test
 2 public void readJson2Map() {
 3     String json = "{\"success\":true,\"A\":{\"address\": \"address2\",\"name\":\"haha2\",\"id\":2,\"email\":\"email2\"},"+
 4                 "\"B\":{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}}";
 5     try {
 6         Map<String, Map<String, Object>> maps = objectMapper.readValue(json, Map.class);
 7         System.out.println(maps.size());
 8         Set<String> key = maps.keySet();
 9         Iterator<String> iter = key.iterator();
10         while (iter.hasNext()) {
11             String field = iter.next();
12             System.out.println(field + ":" + maps.get(field));
13         }
14     } catch (JsonParseException e) {
15         e.printStackTrace();
16     } catch (JsonMappingException e) {
17         e.printStackTrace();
18     } catch (IOException e) {
19         e.printStackTrace();
20     }
21 }

運行後結果以下：

1 3
2 success:true
3 A:{address=address2, name=haha2, id=2, email=email2}
4 B:{address=address, name=haha, id=1, email=email}

4、Jackson對XML的支持

Jackson也能夠完成java對象到xml的轉換，轉換後的結果要比json-lib更直觀，不過它依賴於stax2-api.jar這個jar包。

 1 /**
 2  * <b>function:</b>java對象轉換成xml文檔
 3  * 須要額外的jar包 stax2-api.jar
 4  */
 5 @Test
 6 public void writeObject2Xml() {
 7     //stax2-api-3.0.2.jar
 8     System.out.println("XmlMapper");
 9     XmlMapper xml = new XmlMapper();   
10     try {
11         //javaBean轉換成xml
12         //xml.writeValue(System.out, bean);
13         StringWriter sw = new StringWriter();
14         xml.writeValue(sw, bean);
15         System.out.println(sw.toString());
16         //List轉換成xml
17         List<AccountBean> list = new ArrayList<AccountBean>();
18         list.add(bean);
19         list.add(bean);
20         System.out.println(xml.writeValueAsString(list));        
21         //Map轉換xml文檔
22         Map<String, AccountBean> map = new HashMap<String, AccountBean>();
23         map.put("A", bean);
24         map.put("B", bean);
25         System.out.println(xml.writeValueAsString(map));
26     } catch (JsonGenerationException e) {
27         e.printStackTrace();
28     } catch (JsonMappingException e) {
29         e.printStackTrace();
30     } catch (IOException e) {
31         e.printStackTrace();
32     }
33 }

運行上面的方法，結果以下：

1 XmlMapper
2 
3 <unknown><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></unknown>
4 <unknown><unknown><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></unknown>
5 <email><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></email></unknown>
6 <unknown><A><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></A>
7 <B><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></B></unknown>

看結果，根節點都是unknown 這個問題尚未解決，因爲根節點沒有轉換出來，全部致使解析xml到Java對象，也沒法完成。

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