使用GSON库转换Java对象为JSON对象的进阶实例详解

对list和map等结构的常用转换操作基本上可以满足我们处理的绝大多数需求,但有时项目中对json有特殊的格式规定.比如下面的json串解析:

[{"tablename":"students","tabledata":[{"id":1,"name":"李坤","birthday":"jun 22, 2012 9:54:49 pm"},{"id":2,"name":"曹贵生","birthday":"jun 22, 2012 9:54:49 pm"},{"id":3,"name":"柳波","birthday":"jun 22, 2012 9:54:49 pm"}]},{"tablename":"teachers","tabledata":[{"id":1,"name":"米老师","title":"教授"},{"id":2,"name":"丁老师","title":"讲师"}]}]

分析之后我们发现普通的方式都不好处理上面的json串.请看本文是如何处理的吧:

实体类:

import java.util.date; 
 
public class student { 
  private int id; 
  private string name; 
  private date birthday; 
 
  public int getid() { 
    return id; 
  } 
 
  public void setid(int id) { 
    this.id = id; 
  } 
 
  public string getname() { 
    return name; 
  } 
 
  public void setname(string name) { 
    this.name = name; 
  } 
 
  public date getbirthday() { 
    return birthday; 
  } 
 
  public void setbirthday(date birthday) { 
    this.birthday = birthday; 
  } 
 
  @override 
  public string tostring() { 
    return "student [birthday=" + birthday + ", id=" + id + ", name=" 
        + name + "]"; 
  } 
 
} 

public class teacher { 
  private int id; 
 
  private string name; 
 
  private string title; 
 
  public int getid() { 
    return id; 
  } 
 
  public void setid(int id) { 
    this.id = id; 
  } 
 
  public string getname() { 
    return name; 
  } 
 
  public void setname(string name) { 
    this.name = name; 
  } 
 
  public string gettitle() { 
    return title; 
  } 
 
  public void settitle(string title) { 
    this.title = title; 
  } 
 
  @override 
  public string tostring() { 
    return "teacher [id=" + id + ", name=" + name + ", title=" + title 
        + "]"; 
  } 
 
} 

注意这里定义了一个tabledata实体类:

import java.util.list; 
 
public class tabledata { 
 
  private string tablename; 
 
  private list tabledata; 
 
  public string gettablename() { 
    return tablename; 
  } 
 
  public void settablename(string tablename) { 
    this.tablename = tablename; 
  } 
 
  public list gettabledata() { 
    return tabledata; 
  } 
 
  public void settabledata(list tabledata) { 
    this.tabledata = tabledata; 
  } 
} 

测试类:
(仔细看将json转回为对象的实现,这里经过两次转化,第一次转回的结果是map不是我们所期望的对象,对map再次转为json后再转为对象,我引用的是gson2.1的jar处理正常,好像使用gson1.6的jar会报错,所以建议用最新版本)

import java.util.arraylist; 
import java.util.date; 
import java.util.list; 
 
import com.google.gson.gson; 
import com.google.gson.reflect.typetoken; 
 
public class gsontest5 { 
 
  /** 
   * @param args 
   */ 
  public static void main(string[] args) { 
    // 对象转为json-->start 
    student student1 = new student(); 
    student1.setid(1); 
    student1.setname("李坤"); 
    student1.setbirthday(new date()); 
 
    student student2 = new student(); 
    student2.setid(2); 
    student2.setname("曹贵生"); 
    student2.setbirthday(new date()); 
 
    student student3 = new student(); 
    student3.setid(3); 
    student3.setname("柳波"); 
    student3.setbirthday(new date()); 
 
    list<student> stulist = new arraylist<student>(); 
    stulist.add(student1); 
    stulist.add(student2); 
    stulist.add(student3); 
 
    teacher teacher1 = new teacher(); 
    teacher1.setid(1); 
    teacher1.setname("米老师"); 
    teacher1.settitle("教授"); 
 
    teacher teacher2 = new teacher(); 
    teacher2.setid(2); 
    teacher2.setname("丁老师"); 
    teacher2.settitle("讲师"); 
    list<teacher> teacherlist = new arraylist<teacher>(); 
    teacherlist.add(teacher1); 
    teacherlist.add(teacher2); 
 
    tabledata td1 = new tabledata(); 
    td1.settablename("students"); 
    td1.settabledata(stulist); 
 
    tabledata td2 = new tabledata(); 
    td2.settablename("teachers"); 
    td2.settabledata(teacherlist); 
 
    list<tabledata> tdlist = new arraylist<tabledata>(); 
    tdlist.add(td1); 
    tdlist.add(td2); 
    gson gson = new gson(); 
    string s = gson.tojson(tdlist); 
 
    system.out.println(s); 
    // 结果:[{"tablename":"students","tabledata":[{"id":1,"name":"李坤","birthday":"jun 22, 2012 10:44:16 am"},{"id":2,"name":"曹贵生","birthday":"jun 22, 2012 10:44:16 am"},{"id":3,"name":"柳波","birthday":"jun 22, 2012 10:44:16 am"}]},{"tablename":"teachers","tabledata":[{"id":1,"name":"米老师","title":"教授"},{"id":2,"name":"丁老师","title":"讲师"}]}] 
    // 对象转为json-->end 
 
    // ///////////////////////////////////////////////////////////////////// 
 
    // 将json转为数据-->start 
    list<tabledata> tabledatas2 = gson.fromjson(s, 
        new typetoken<list<tabledata>>() { 
        }.gettype()); 
    for (int i = 0; i < tabledatas2.size(); i++) { 
      tabledata entitydata = tabledatas2.get(i); 
      string tablename = entitydata.gettablename(); 
      list tabledata = entitydata.gettabledata(); 
      string s2 = gson.tojson(tabledata); 
      // system.out.println(s2); 
      // system.out.println(entitydata.getdata()); 
      if (tablename.equals("students")) { 
        system.out.println("students"); 
        list<student> retstulist = gson.fromjson(s2, 
            new typetoken<list<student>>() { 
            }.gettype()); 
        for (int j = 0; j < retstulist.size(); j++) { 
          system.out.println(retstulist.get(j)); 
        } 
 
      } else if (tablename.equals("teachers")) { 
        system.out.println("teachers"); 
        list<teacher> rettchrlist = gson.fromjson(s2, 
            new typetoken<list<teacher>>() { 
            }.gettype()); 
        for (int j = 0; j < rettchrlist.size(); j++) { 
          system.out.println(rettchrlist.get(j)); 
        } 
      } 
    } 
 
    // json转为对象-->end 
  } 
} 

输出结果:

[{"tablename":"students","tabledata":[{"id":1,"name":"李坤","birthday":"jun 22, 2012 10:04:12 pm"},{"id":2,"name":"曹贵生","birthday":"jun 22, 2012 10:04:12 pm"},{"id":3,"name":"柳波","birthday":"jun 22, 2012 10:04:12 pm"}]},{"tablename":"teachers","tabledata":[{"id":1,"name":"米老师","title":"教授"},{"id":2,"name":"丁老师","title":"讲师"}]}] 
students 
student [birthday=fri jun 22 22:04:12 cst 2012, id=1, name=李坤] 
student [birthday=fri jun 22 22:04:12 cst 2012, id=2, name=曹贵生] 
student [birthday=fri jun 22 22:04:12 cst 2012, id=3, name=柳波] 
teachers 
teacher [id=1, name=米老师, title=教授] 
teacher [id=2, name=丁老师, title=讲师] 

注册typeadapter及处理enum类型

枚举类型给我们的程序带来了好处,如何用gson来实现与json的互转呢?请看本文.

本文重点掌握如何自己写一个typeadapter及注册typeadapter和处理enum类型.

实体类:

public enum packagestate { 
  play, update, updating, download, downloading, 
} 

public class packageitem { 
  private string name; 
  private packagestate state; 
  private string size; 
 
  public string getname() { 
    return name; 
  } 
 
  public void setname(string name) { 
    this.name = name; 
  } 
 
  public packagestate getstate() { 
    return state; 
  } 
 
  public void setstate(packagestate state) { 
    this.state = state; 
  } 
 
  public string getsize() { 
    return size; 
  } 
 
  public void setsize(string size) { 
    this.size = size; 
  } 
 
  @override 
  public string tostring() { 
    return "packageitem [name=" + name + ", size=" + size + ", state=" 
        + state + "]"; 
  } 
} 

自己写一个转换器实现jsonserializer<t>接口和jsondeserializer<t>接口:

import java.lang.reflect.type; 
 
import com.google.gson.jsondeserializationcontext; 
import com.google.gson.jsondeserializer; 
import com.google.gson.jsonelement; 
import com.google.gson.jsonparseexception; 
import com.google.gson.jsonprimitive; 
import com.google.gson.jsonserializationcontext; 
import com.google.gson.jsonserializer; 
 
public class enumserializer implements jsonserializer<packagestate>, 
    jsondeserializer<packagestate> { 
 
  // 对象转为json时调用,实现jsonserializer<packagestate>接口 
  @override 
  public jsonelement serialize(packagestate state, type arg1, 
      jsonserializationcontext arg2) { 
    return new jsonprimitive(state.ordinal()); 
  } 
 
  // json转为对象时调用,实现jsondeserializer<packagestate>接口 
  @override 
  public packagestate deserialize(jsonelement json, type typeoft, 
      jsondeserializationcontext context) throws jsonparseexception { 
    if (json.getasint() < packagestate.values().length) 
      return packagestate.values()[json.getasint()]; 
    return null; 
  } 
 
} 

测试类:

import com.google.gson.gson; 
import com.google.gson.gsonbuilder; 
 
public class gsontest6 { 
 
  public static void main(string[] args) { 
    gsonbuilder gsonbuilder = new gsonbuilder(); 
    gsonbuilder.registertypeadapter(packagestate.class, 
        new enumserializer()); 
    gson gson = gsonbuilder.create(); 
    packageitem item = new packageitem(); 
    item.setname("item_name"); 
    item.setsize("500m"); 
    item.setstate(packagestate.updating);// 这个 state是枚举值 
 
    string s = gson.tojson(item); 
    system.out.println(s); 
 
    system.out.println("--------------------------------"); 
 
    packageitem retitem = gson.fromjson(s, packageitem.class); 
    system.out.println(retitem); 
  } 
} 

输出结果(结果中已经将state的对应枚举类型转为了int类型):

{"name":"item_name","state":2,"size":"500m"} 
-------------------------------- 
packageitem [name=item_name, size=500m, state=updating]