Jackson 框架，轻易转换JSON

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/hoojo/archive/2011/04/21/2023805.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

package com.hoo.test;

import java.io.IOException;

import java.io.StringWriter;

import java.util.ArrayList;

import java.util.HashMap;

import java.util.Iterator;

import java.util.LinkedHashMap;

import java.util.List;

import java.util.Map;

import java.util.Set;

import org.codehaus.jackson.JsonEncoding;

import org.codehaus.jackson.JsonGenerationException;

import org.codehaus.jackson.JsonGenerator;

import org.codehaus.jackson.JsonParseException;

import org.codehaus.jackson.map.JsonMappingException;

import org.codehaus.jackson.map.ObjectMapper;

import org.codehaus.jackson.node.JsonNodeFactory;

import org.codehaus.jackson.xml.XmlMapper;

import org.junit.After;

import org.junit.Before;

import org.junit.Test;

import com.hoo.entity.AccountBean;

/**

 * <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

 * jettison-1.0.1

 * @author hoojo

 * @createDate 2010-11-23 下午04:54:53

 * @file JacksonTest.java

 * @package com.hoo.test

 * @project Spring3

 * @blog http://blog.csdn.net/IBM_hoojo

 * @email [e-mail]

 * @version 1.0

 */

@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("[e-mail]");

        bean.setId(1);

        bean.setName("hoojo");

        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

package com.hoo.entity;

public class AccountBean {

    private int id;

    private String name;

    private String email;

    private String address;

    private Birthday birthday;

    //getter、setter

    @Override

    public String toString() {

        return this.name + "#" + this.id + "#" + this.address + "#" + this.birthday + "#" + this.email;

    }

}

Birthday

package com.hoo.entity;

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

/**

 * <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();

    }

}

运行后结果如下：

jsonGenerator

{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"[e-mail]"}

ObjectMapper

{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"[e-mail]"}

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

 * @author hoojo

 * @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("[e-mail]");

        map.put("account2", bean);

        System.out.println("jsonGenerator");

        jsonGenerator.writeObject(map);

        System.out.println("");

        System.out.println("objectMapper");

        objectMapper.writeValue(System.out, map);

    } catch (IOException e) {

        e.printStackTrace();

    }

}

转换后结果如下：

jsonGenerator

{"account2":{"address":"china-Beijin","name":null,"id":0,"birthday":null,"email":"[e-mail]"},

"name":"hoojo",

"account":{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,

"email":"[e-mail]"}}

objectMapper

{"account2":{"address":"china-Beijin","name":null,"id":0,"birthday":null,"email":"[e-mail]"},

"name":"hoojo",

"account":{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,

"email":"[e-mail]"}}

3、 将List集合转换成json

/**

 * <b>function:</b>将list集合转换成json字符串

 * @author hoojo

 * @createDate 2010-11-23 下午06:05:59

 */

@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

[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"[e-mail]"},

{"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]

ObjectMapper

1###[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"[e-mail]"},

{"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]

2###[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"[e-mail]"},

{"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]

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":{"address":"address","name":"haha","id":1,"birthday":null,"email":"email"},"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>

 * @author hoojo

 * @createDate 2010-11-23 下午06:12:01

 */

@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

 * @author hoojo

 * @createDate 2010-11-23 下午06:14:01

 */

@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

 * @author hoojo

 * @createDate Nov 27, 2010 3:00:06 PM

 */

@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>[e-mail]</email></unknown>

<unknown><unknown><address>china-Guangzhou</address><name>hoojo</name><id>1</id>
<birthday/><email>[e-mail]</email></unknown>

<email><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/>
<email>[e-mail]</email></email></unknown>

<unknown><A><address>china-Guangzhou</address><name>hoojo</name><id>1</id>
<birthday/><email>[e-mail]</email></A>

<B><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/>
<email>[e-mail]</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。