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] 

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