举例讲解Java的Jackson库中ObjectMapper类的使用

ObjectMapper类是Jackson库的主要类。它提供一些功能将转换成Java对象匹配JSON结构，反之亦然。它使用JsonParser和JsonGenerator的实例实现JSON实际的读/写。

类声明
以下是org.codehaus.jackson.map.ObjectMapper类的声明：

public class ObjectMapper
  extends ObjectCodec
   implements Versioned

嵌套类

构造函数

这个类继承了以下类方法：
java.lang.Object

例子
测试类基本代码如下

/*
 * @project java
 * @package
 * @file Jackson.java
 * @version 1.0 

 */ 

public class Jackson {
  /*
   *
   * Class Descripton goes here.
   *
   * @class Jackson
   * @version 1.0
   */
  public static JsonGenerator jsonGenerator = null;
  private static ObjectMapper mapper = new ObjectMapper();
  public static void main(String[] args) {
    Student student = new Student();
    student.setIsstudent(true);
    student.setUid(1000);
    student.setUname("xiao liao");
    student.setUpwd("123");
    student.setNumber(12); 

    Map<String, Student> stuMap = new HashMap<String, Student>();
    stuMap.put("1", student);
    stuMap.put("2", student); 

    List<Object> stuList = new ArrayList<Object>();
    List<Student> stuList1 = new ArrayList<Student>();
    stuList1.add(student);
    student= new Student();
    student.setIsstudent(false);
    student.setUid(200);
    student.setUname("xiao mi");
    stuList1.add(student); 

    stuList.add(student);
    stuList.add("xiao xin");
    stuList.add("xiao er");
    stuList.add(stuMap); 

    //readJson2List();
    try {
      //readJson2Array();
      //writeArray2Json(array);
      //writeJson2List();
      //writeEntity2Json(student);
      writeJson2Entity();
      //writeMap2Json(stuMap);
      //writeList2Json(stuList1); 

    } catch (IOException e) {
      e.printStackTrace();
    }
  }
   /**
   *
   * <code>writeEntity2Json</code>
   * @description: TODO(实体类转换成json)
   * @param object
   * @throws IOException
   */
   public static void writeEntity2Json(Object object) throws IOException {
      mapper.writeValue( new File("D:\\developSoft\\aaadowload\\testjson1\\lib\\aa.txt"),object );
      mapper.writeValue( System.out,object ); 

   }
   /**
   *
   * <code>writeArray2Json</code>
   * @description: TODO(数组转换成json数组)
   * @param object
   * @throws IOException
   */
   public static void writeArray2Json(Object object) throws IOException { 

     // writeValue具有和writeObject相同的功能
     mapper.writeValue( new File("D:\\developSoft\\aaadowload\\testjson1\\lib\\aa.txt"),object );
     mapper.writeValue(System.out,object ); 

   }
   /**
   *
   * <code>writeMap2Json</code>
   * @description: TODO(map对象转换成json对象)
   * @param object
   * @throws IOException
   * @since  2011-11-8   廖益平
   */
   public static void writeMap2Json(Object object) throws IOException { 

     System.out.println("使用ObjectMapper-----------");
     // writeValue具有和writeObject相同的功能
     System.out.println("==>"+mapper.writeValueAsString(object));
     mapper.writeValue( new File("D:\\developSoft\\aaadowload\\testjson1\\lib\\aamap.txt"),object );
     mapper.writeValue( System.out , object );
   }
   /**
   *
   * <code>writeList2Json</code>
   * @description: TODO(list转换成json)
   * @param object
   * @throws IOException
   */
   public static void writeList2Json(Object object) throws IOException {
     System.out.println("==>"+mapper.writeValueAsString(object));
     mapper.writeValue( new File("D:\\developSoft\\aaadowload\\testjson1\\lib\\aamap.txt"),object );
     mapper.writeValue( System.out , object );
   }
   /**
   *
   * <code>writeJson2Entity</code>
   * @description: TODO(json转换成实体)
   * @throws IOException
   */
   public static void writeJson2Entity() throws IOException {
     System.out.println("json串转换成entity-------------");
//    File file = new File("D:\\developSoft\\aaadowload\\testjson1\\lib\\aa.txt");
//    FileInputStream inputStream = new FileInputStream(file);
//    Student student = mapper.readValue(inputStream,Student.class);
//    System.out.println(student.toString());
     //漂亮输出
     //mapper.defaultPrettyPrintingWriter().writeValueAsString(value); 

     String json = "{\"uid\":1000,\"uname\":\"xiao liao\",\"upwd\":\"123\",\"number\":12.0,\"isstudent\":true}";
     Student student1 = mapper.readValue(json,Student.class);
     System.out.println("json2:"+student1.toString());
   }
   /**
   *
   * <code>writeJson2List</code>
   * @description: TODO(json专程list对象)
   * @throws IOException
   */
   public static void writeJson2List() throws IOException {
     System.out.println("json串转换成entity-------------");
     File file = new File("D:\\developSoft\\aaadowload\\testjson1\\lib\\aa.txt");
     FileInputStream inputStream = new FileInputStream(file);
     Student student = mapper.readValue(inputStream,Student.class);
     System.out.println(student.toString()); 

     String json = "[{\"uid\":1000,\"uname\":\"xiao liao\",\"upwd\":\"123\",\"number\":12.0,\"isstudent\":true},{\"uid\":200,\"uname\":\"xiao mi\",\"upwd\":null,\"number\":0.0,\"isstudent\":false}]";
     List<LinkedHashMap<String, Object>> s= mapper.readValue(json,List.class);
     for (int i = 0; i < s.size(); i++) {
       LinkedHashMap<String, Object> link = s.get(i);
       Set<String> key = link.keySet();
       for (Iterator iterator = key.iterator(); iterator.hasNext();) {
        String string = (String) iterator.next();
        System.out.println(string+"==>"+link.get(string)); 

      }
       System.out.println("json:"+i+""+s.get(i).toString()); 

    }
   }
   /**
    * JSON转换为List对象
    */
   public static void readJson2List() {
    String json = "[{\"uid\":1,\"uname\":\"www\",\"number\":234,\"upwd\":\"456\"},"
     + "{\"uid\":5,\"uname\":\"tom\",\"number\":3.44,\"upwd\":\"123\"}]";
    try {
    List<LinkedHashMap<String, Object>> list = mapper.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();
    }
   }
   /**
    * JSON转换为List对象
    */
   public static void readJson2Array() {
     String json = "[{\"uid\":1,\"uname\":\"www\",\"number\":234,\"upwd\":\"456\"},"
       + "{\"uid\":5,\"uname\":\"tom\",\"number\":3.44,\"upwd\":\"123\"}]";
     try {
       Student[] students = mapper.readValue(json, Student[].class);
       for (Student student : students) {
        System.out.println(">"+student.toString());
      }
     } catch (JsonParseException e) {
       e.printStackTrace();
     } catch (JsonMappingException e) {
       e.printStackTrace();
     } catch (IOException e) {
       e.printStackTrace();
     }
   } 

}

打印结果 ：

串转换成entity-------------
json2:uid=1000,name=xiao liao,upwd=123,number=12.0,isStudent=true

writeMap2Json -----------
{"2":{"uid":1000,"uname":"xiao liao","upwd":"123","number":12.0,"isstudent":true},"1":{"uid":1000,"uname":"xiao liao","upwd":"123","number":12.0,"isstudent":true}}

readJson2Array------------------
>uid=1,name=www,upwd=456,number=234.0,isStudent=false
>uid=5,name=tom,upwd=123,number=3.44,isStudent=false
writeMap2Json -----------
{"2":{"uid":1000,"uname":"xiao liao","upwd":"123","number":12.0,"isstudent":true},"1":{"uid":1000,"uname":"xiao liao","upwd":"123","number":12.0,"isstudent":true}}

大家逐个自己试试吧  ，上面也是我的测试代码