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：

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("[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

/**
 * 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":"[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

 */
@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
 */
@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
 */
@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>[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。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。