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 */ @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
/** * <b>function:</b>将java对象转换成json字符串 * * 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":"lsy_@126.com"} ObjectMapper {"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"lsy_@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 * * 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("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 * 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":"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
@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。
@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
@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。
@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。