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]