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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;
 
/**
 * function: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 hoojo_@126.com
 * @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("hoojo_@126.com");
        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

/**
 * function: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":"hoojo_@126.com"}
ObjectMapper
{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}

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

/**
 * function: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 map = new HashMap();
        map.put("name", bean.getName());
        map.put("account", bean);
        bean = new AccountBean();
        bean.setAddress("china-Beijin");
        bean.setEmail("hoojo@qq.com");
        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":"hoojo@qq.com"},"name":"hoojo",
"account":{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}}
objectMapper
{"account2":{"address":"china-Beijin","name":null,"id":0,"birthday":null,"email":"hoojo@qq.com"},"name":"hoojo",
"account":{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}}

3、 将List集合转换成json

/**
 * function: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 list = new ArrayList();
        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":"hoojo_@126.com"},
{"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]
ObjectMapper
1###[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"},
{"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]
2###[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"},
{"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。

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集合

/**
 * function:json字符串转换成list
 * @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> list = objectMapper.readValue(json, List.class);
        System.out.println(list.size());
        for (int i = 0; i < list.size(); i++) {
            Map map = list.get(i);
            Set set = map.keySet();
            for (Iterator 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。

/**
 * function: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

/**
 * function: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> maps = objectMapper.readValue(json, Map.class);
        System.out.println(maps.size());
        Set key = maps.keySet();
        Iterator 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。

/**
 * function: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 list = new ArrayList();
        list.add(bean);
        list.add(bean);
        System.out.println(xml.writeValueAsString(list));
        
        //Map转换xml文档
        Map map = new HashMap();
        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-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>hoojo_@126.comemail>unknown>
<unknown><unknown><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>hoojo_@126.comemail>unknown>
<email><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>hoojo_@126.comemail>email>unknown>
<unknown><A><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>hoojo_@126.comemail>A>
<B><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>hoojo_@126.comemail>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。