java的Jackson框架实现轻易转换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。

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
 * @file JacksonTest.java
 * @package com.hoo.test
 * @project Spring3
 * @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
 */
@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

/**
 * <b>function:</b>将map转换成json字符串
 * @author hoojo

 */
@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("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

/**
 * <b>function:</b>将list集合转换成json字符串
 * @author hoojo
 */
@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":"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。

@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
 */
@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
 */
@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 */
@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
 */
@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>hoojo_@126.com</email></unknown>
<unknown><unknown><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></unknown>
<email><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></email></unknown>
<unknown><A><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></A>
<B><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</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。

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