Jackson 框架使用说明，轻易转换JSON

Jackson 框架

使用说明

Jackson可以轻松的将Java对象转换成json对象和xml文档，同样也可以将json、xml转换成Java对象。

前面有介绍过json-lib这个框架，在线博文：http://www.cnblogs.com/duanxz/archive/2013/05/03/3056174.html
相比json-lib框架，Jackson所依赖的jar包较少，简单易用并且性能也要相对高些。而且Jackson社区相对比较活跃，更新速度也比较快。

 

一、准备工作

1、 下载依赖库jar包

Jackson的jar all下载地址：http://jackson.codehaus.org/1.7.6/jackson-all-1.7.6.jar

然后在工程中导入这个jar包即可开始工作

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

因为下面的程序是用junit测试用例运行的，所以还得添加junit的jar包。版本是junit-4.2.8

如果你需要转换xml，那么还需要stax2-api.jar

2、 测试类基本代码如下(下面的代码还不能直接运行）

package com.dxz.json;

import java.io.IOException;
import org.codehaus.jackson.JsonEncoding;
import org.codehaus.jackson.JsonGenerator;
import org.codehaus.jackson.map.ObjectMapper;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
/**
 * <b>function:</b>Jackson 将java对象转换成JSON字符串，也可以将JSON字符串转换成java对象
 * jar-lib-version: jackson-all-1.6.2
 * @createDate 2010-11-23 下午04:54:53
 * @file JacksonTest.java
 */
@SuppressWarnings("unchecked")
public class JacksonTest{
    private JsonGenerator jsonGenerator = null;
    private ObjectMapper objectMapper = null;
    private AccountBean bean = null;
    
    @Before
    public void init() {
        bean = new AccountBean();
        bean.setAddress("china-Guangzhou");
        bean.setEmail("[email protected]");
        bean.setId(1);
        bean.setName("duanxz");
        
        objectMapper = new ObjectMapper();
        try {
            jsonGenerator = objectMapper.getJsonFactory().createJsonGenerator(System.out, JsonEncoding.UTF8);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    
    @After
    public void destory() {
        try {
            if (jsonGenerator != null) {
                jsonGenerator.flush();
            }
            if (!jsonGenerator.isClosed()) {
                jsonGenerator.close();
            }
            jsonGenerator = null;
            objectMapper = null;
            bean = null;
            System.gc();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

3、 所需要的JavaEntity

AccountBean.java

/**
 * AccountBean
 */
package com.dxz.json;

public class AccountBean {
private int id;
private String name;
    private String email;
    private String address;
    private Birthday birthday;
    
    
    @Override
    public String toString() {
        return this.name + "#" + this.id + "#" + this.address + "#" + this.birthday + "#" + this.email;
    }
    
    //getter、setter
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public Birthday getBirthday() {
        return birthday;
    }

    public void setBirthday(Birthday birthday) {
        this.birthday = birthday;
    }
}

Birthday.java

package com.dxz.json;

public class Birthday {
    private String birthday;
    
    public Birthday(String birthday) {
        super();
        this.birthday = birthday;
    }
 
    //getter、setter
 
    public Birthday() {}
    
    @Override
    public String toString() {
        return this.birthday;
    }
}

二、Java对象转换成JSON

1、 JavaBean(Entity/Model)转换成JSON（JacksonTest中的方法）

/**
     * <b>function:</b>将java对象转换成json字符串
     * @author hoojo
     * @createDate 2010-11-23 下午06:01:10
     */
    @Test
    public void writeEntityJSON() {
        
        try {
            System.out.println("jsonGenerator");
            //writeObject可以转换java对象，eg:JavaBean/Map/List/Array等
            jsonGenerator.writeObject(bean);    
            System.out.println();
            
            System.out.println("ObjectMapper");
            //writeValue具有和writeObject相同的功能
            objectMapper.writeValue(System.out, bean);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

该方法执行结果：

jsonGenerator
{"email":"[email protected]","birthday":null,"address":"china-Guangzhou","name":"duanxz","id":1}
ObjectMapper
{"email":"[email protected]","birthday":null,"address":"china-Guangzhou","name":"duanxz","id":1}

上面分别利用JsonGenerator的writeObject方法和ObjectMapper的writeValue方法完成对Java对象的转换，二者传递的参数及构造的方式不同；JsonGenerator的创建依赖于ObjectMapper对象。也就是说如果你要使用JsonGenerator来转换JSON，那么你必须创建一个ObjectMapper。但是你用ObjectMapper来转换JSON，则不需要JSONGenerator。

• objectMapper的writeValue方法可以将一个Java对象转换成JSON。这个方法的参数一，需要提供一个输出流，转换后可以通过这个流来输出转换后的内容。或是提供一个File，将转换后的内容写入到File中。当然，这个参数也可以接收一个JSONGenerator，然后通过JSONGenerator来输出转换后的信息。第二个参数是将要被转换的Java对象。如果用三个参数的方法，那么是一个Config。这个config可以提供一些转换时的规则，过指定的Java对象的某些属性进行过滤或转换等。

 

2、 将Map集合转换成Json字符串

/**
     * <b>function:</b>将map转换成json字符串
     * @createDate 2010-11-23 下午06:05:26
     */
    @Test
    public void writeMapJSON() {
          try {
                Map<String, Object> map = new HashMap<String, Object>();
                map.put("name", bean.getName());
                map.put("account", bean);
                bean = new AccountBean();
                bean.setAddress("china-Beijin");
                bean.setEmail("[email protected]");
                map.put("account2", bean);
                System.out.println("writeMapJSON*****jsonGenerator");
                jsonGenerator.writeObject(map);
                System.out.println("");
                System.out.println("writeMapJSON*****objectMapper");
                objectMapper.writeValue(System.out, map);
          } catch (IOException e) {
                e.printStackTrace();
          }
    }

转换后结果如下：

writeMapJSON*****jsonGenerator
{"account2":{"email":"[email protected]","birthday":null,"address":"china-Beijin","name":null,"id":0},"name":"duanxz","account":{"email":"[email protected]","birthday":null,"address":"china-Guangzhou","name":"duanxz","id":1}}
writeMapJSON*****objectMapper
{"account2":{"email":"[email protected]","birthday":null,"address":"china-Beijin","name":null,"id":0},"name":"duanxz","account":{"email":"[email protected]","birthday":null,"address":"china-Guangzhou","name":"duanxz","id":1}}

3、 将List集合转换成json

/**
     * <b>function:</b>将list集合转换成json字符串
     */
    @Test
    public void writeListJSON() {
        try {
            List<AccountBean> list = new ArrayList<AccountBean>();
            list.add(bean);
            bean = new AccountBean();
            bean.setId(2);
            bean.setAddress("address2");
            bean.setEmail("email2");
            bean.setName("haha2");
            list.add(bean);
            System.out.println("jsonGenerator");
            // list转换成JSON字符串
            jsonGenerator.writeObject(list);
            System.out.println();
            System.out.println("ObjectMapper");
            // 用objectMapper直接返回list转换成的JSON字符串
            System.out.println("1###" + objectMapper.writeValueAsString(list));
            System.out.print("2###");
            // objectMapper list转换成JSON字符串
            objectMapper.writeValue(System.out, list);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

结果如下：

jsonGenerator
[{"email":"[email protected]","birthday":null,"address":"china-Guangzhou","name":"duanxz","id":1},{"email":"email2","birthday":null,"address":"address2","name":"haha2","id":2}]
ObjectMapper
1###[{"email":"[email protected]","birthday":null,"address":"china-Guangzhou","name":"duanxz","id":1},{"email":"email2","birthday":null,"address":"address2","name":"haha2","id":2}]
2###[{"email":"[email protected]","birthday":null,"address":"china-Guangzhou","name":"duanxz","id":1},{"email":"email2","birthday":null,"address":"address2","name":"haha2","id":2}]

外面就是多了个[]中括号；同样Array也可以转换，转换的JSON和上面的结果是一样的，这里就不再转换了。~.~

4、下面来看看jackson提供的一些类型，用这些类型完成json转换；如果你使用这些类型转换JSON的话，那么你即使没有JavaBean(Entity)也可以完成复杂的Java类型的JSON转换。下面用到这些类型构建一个复杂的Java对象，并完成JSON转换。

@Test
    public void writeOthersJSON() {
        try {
            String[] arr = { "a", "b", "c" };
            System.out.println("jsonGenerator");
            String str = "hello world jackson!";
            // byte
            jsonGenerator.writeBinary(str.getBytes());
            // boolean
            jsonGenerator.writeBoolean(true);
            // null
            jsonGenerator.writeNull();
            // float
            jsonGenerator.writeNumber(2.2f);
            // char
            jsonGenerator.writeRaw("c");
            // String
            jsonGenerator.writeRaw(str, 5, 10);
            // String
            jsonGenerator.writeRawValue(str, 5, 5);
            // String
            jsonGenerator.writeString(str);
            jsonGenerator.writeTree(JsonNodeFactory.instance.POJONode(str));
            System.out.println();
            // Object
            jsonGenerator.writeStartObject();// {
            jsonGenerator.writeObjectFieldStart("user");// user:{
            jsonGenerator.writeStringField("name", "jackson");// name:jackson
            jsonGenerator.writeBooleanField("sex", true);// sex:true
            jsonGenerator.writeNumberField("age", 22);// age:22
            jsonGenerator.writeEndObject();// }
            jsonGenerator.writeArrayFieldStart("infos");// infos:[
            jsonGenerator.writeNumber(22);// 22
            jsonGenerator.writeString("this is array");// this is array
            jsonGenerator.writeEndArray();// ]
            jsonGenerator.writeEndObject();// }
            AccountBean bean = new AccountBean();
            bean.setAddress("address");
            bean.setEmail("email");
            bean.setId(1);
            bean.setName("haha");
            // complex Object
            jsonGenerator.writeStartObject();// {
            jsonGenerator.writeObjectField("user", bean);// user:{bean}
            jsonGenerator.writeObjectField("infos", arr);// infos:[array]
            jsonGenerator.writeEndObject();// }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

运行后，结果如下：

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

怎么样？构造的json字符串和输出的结果是一致的吧。关键看懂用JSONGenerator提供的方法，完成一个Object的构建。

三、JSON转换成Java对象

1、 将json字符串转换成JavaBean对象

@Test
    public void readJson2Entity() {
          String json = "{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}";
          try {
                AccountBean acc = objectMapper.readValue(json, AccountBean.class);
                System.out.println(acc.getName());
                System.out.println(acc);
          } catch (JsonParseException e) {
                e.printStackTrace();
          } catch (JsonMappingException e) {
                e.printStackTrace();
          } catch (IOException e) {
                e.printStackTrace();
          }
    }

很简单，用到了ObjectMapper这个对象的readValue这个方法，这个方法需要提供2个参数。第一个参数就是解析的JSON字符串，第二个参数是即将将这个JSON解析吃什么Java对象，Java对象的类型。当然，还有其他相同签名方法，如果你有兴趣可以一一尝试使用方法，当然使用的方法和当前使用的方法大同小异。运行后，结果如下：

haha
haha#1#address#null#email

2、 将json字符串转换成List<Map>集合

/**
     * <b>function:</b>json字符串转换成list<map>
     */
    @Test
    public void readJson2List() {
        String json = "[{\"address\": \"address2\",\"name\":\"haha2\",\"id\":2,\"email\":\"email2\"},{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}]";
        try {
            List<LinkedHashMap<String, Object>> list = objectMapper.readValue(
                    json, List.class);
            System.out.println(list.size());
            for (int i = 0; i < list.size(); i++) {
                Map<String, Object> map = list.get(i);
                Set<String> set = map.keySet();
                for (Iterator<String> it = set.iterator(); it.hasNext();) {
                    String key = it.next();
                    System.out.println(key + ":" + map.get(key));
                }
            }
        } catch (JsonParseException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

尝试过将上面的JSON转换成List，然后List中存放AccountBean，但结果失败了。但是支持Map集合。因为你转成List.class，但是不知道List存放何种类型。只好默然Map类型。因为所有的对象都可以转换成Map结合，运行后结果如下：

2
address:address2
name:haha2
id:2
email:email2
address:address
name:haha
id:1
email:email

3、 Json字符串转换成Array数组，由于上面的泛型转换不能识别到集合中的对象类型。所有这里用对象数组，可以解决这个问题。只不过它不再是集合，而是一个数组。当然这个不重要，你可以用Arrays.asList将其转换成List即可。

/**
     * <b>function:</b>json字符串转换成Array
     */
    @Test
    public void readJson2Array() {
        String json = "[{\"address\": \"address2\",\"name\":\"haha2\",\"id\":2,\"email\":\"email2\"},{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}]";
        try {
            AccountBean[] arr = objectMapper.readValue(json,
                    AccountBean[].class);
            System.out.println(arr.length);
            for (int i = 0; i < arr.length; i++) {
                System.out.println(arr[i]);
            }
        } catch (JsonParseException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

 

运行后的结果：

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

4、 Json字符串转换成Map集合

/**
     * <b>function:</b>json字符串转换Map集合
     */
    @Test
    public void readJson2Map() {
        String json = "{\"success\":true,\"A\":{\"address\": \"address2\",\"name\":\"haha2\",\"id\":2,\"email\":\"email2\"},\"B\":{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}}";
        try {
            Map<String, Map<String, Object>> maps = objectMapper.readValue(
                    json, Map.class);
            System.out.println(maps.size());
            Set<String> key = maps.keySet();
            Iterator<String> iter = key.iterator();
            while (iter.hasNext()) {
                String field = iter.next();
                System.out.println(field + ":" + maps.get(field));
            }
        } catch (JsonParseException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

 

运行后结果如下：

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

四、Jackson对XML的支持

Jackson也可以完成java对象到xml的转换，转换后的结果要比json-lib更直观，不过它依赖于stax2-api.jar这个jar包。

/**  * <b>function:</b>java对象转换成xml文档  * 需要额外的jar包 stax2-api.jar  * @author hoojo  * @createDate 2010-11-23 下午06:11:21  */ @Test public void writeObject2Xml() {       //stax2-api-3.0.2.jar       System.out.println("XmlMapper");       XmlMapper xml = new XmlMapper();             try {             //javaBean转换成xml             //xml.writeValue(System.out, bean);             StringWriter sw = new StringWriter();             xml.writeValue(sw, bean);             System.out.println(sw.toString());             //List转换成xml             List<AccountBean> list = new ArrayList<AccountBean>();             list.add(bean);             list.add(bean);             System.out.println(xml.writeValueAsString(list));                         //Map转换xml文档             Map<String, AccountBean> map = new HashMap<String, AccountBean>();             map.put("A", bean);             map.put("B", bean);             System.out.println(xml.writeValueAsString(map));       } catch (JsonGenerationException e) {             e.printStackTrace();       } catch (JsonMappingException e) {             e.printStackTrace();       } catch (IOException e) {             e.printStackTrace();       } }

运行上面的方法，结果如下：

XmlMapper <unknown><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>[email protected]</email></unknown> <unknown><unknown><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>[email protected]</email></unknown><email><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>[email protected]</email></email></unknown> <unknown><A><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>[email protected]</email></A><B><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>[email protected]</email></B></unknown>

看结果，根节点都是unknown 这个问题还没有解决，由于根节点没有转换出来，所有导致解析xml到Java对象，也无法完成。