Cannot figure out how to save this field into database. You can consider adding a type converter for

• 一.场景：使用Room框架，报错，Cannot figure out how to save this field into database. You can consider adding a type converter for it。在ClassMomentInfo类中包含了两个成员变量，mMediaBeans和mClazzs，运行时候提示以上错误。错误代码如下：

@Entity(tableName = "classMomentInfo")
public class ClassMomentInfo {
    

    @PrimaryKey(autoGenerate=true)
    private long uid;
    
   //省略代码...

    //ROOM不支持直接存储集合
    //@TypeConverters(MediaBeanTypeConverter.class) 
    private List<MediaBean> mMediaBeans;
    
    //@TypeConverters(StringTypeConverter.class)
    private List<String> mClazzs;

     //省略代码...
}

• 二.原因，Room不支持直接存储列表的功能
• 三.解决方案
• 3.1.新建类XxxConverter，代码如下：

import android.arch.persistence.room.TypeConverter;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import java.lang.reflect.Type;
import java.util.Collections;
import java.util.List;

/** * describe:使用Room框架,room不支持对象中直接存储集合 * https://stackoverflow.com/questions/53812636/android-error-cannot-figure-out-how-to-save-this-field-into-database-you-can */
public class StringTypeConverter {
    

    Gson gson = new Gson();

    @TypeConverter
    public List<String> stringToSomeObjectList(String data) {
    
        if (data == null) {
    
            return Collections.emptyList();
        }

        Type listType = new TypeToken<List<String>>() {
    }.getType();

        return gson.fromJson(data, listType);
    }

    @TypeConverter
    public String someObjectListToString(List<String> someObjects) {
    
        return gson.toJson(someObjects);
    }

}

• 3.2.接着在ClassMomentInfo类中，在成员变量mClazzs上方添加"@TypeConverters(StringTypeConverter.class)";
• 3.3.需要注意的是：在ClassMomentInfo类中，还有另外一个成员变量mMediaBeans，由于该成员变量存储的是MediaBean类型，所以需要单独创建类似StringTypeConverter的类，且需要在mMediaBeans上方加上“@TypeConverters(MediaBeanTypeConverter.class)”。此时，新建MediaBeanTypeConverter类，代码如下：

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import com.pj.teacherlocal.entity.work.MediaBean;
import java.lang.reflect.Type;
import java.util.Collections;
import java.util.List;

public class MediaBeanTypeConverter {
    

    Gson gson = new Gson();

    @TypeConverter
    public List<MediaBean> stringToSomeObjectList(String data) {
    
        if (data == null) {
    
            return Collections.emptyList();
        }

        Type listType = new TypeToken<List<MediaBean>>() {
    }.getType();

        return gson.fromJson(data, listType);
    }

    @TypeConverter
    public String someObjectListToString(List<MediaBean> someObjects) {
    
        return gson.toJson(someObjects);
    }

}

• 3.4.综上，处理方案已经很明显了。
• 四.为了避免有些小伙伴疑惑，我将类ClassMomentInfo和类MediaBean的完整代码贴出来。

ClassMomentInfo

import android.arch.persistence.room.Embedded;
import android.arch.persistence.room.Entity;
import android.arch.persistence.room.PrimaryKey;
import android.arch.persistence.room.TypeConverters;

import com.pj.teacherlocal.entity.CurrentTeacherInfo;
import com.pj.teacherlocal.utils.MediaBeanTypeConverter;
import com.pj.teacherlocal.utils.StringTypeConverter;

import java.util.List;

@Entity(tableName = "classMomentInfo")
public class ClassMomentInfo {
    

    @PrimaryKey(autoGenerate=true)
    private long uid;

    private String content;

    private String date;

    //ROOM不支持直接存储集合
    @TypeConverters(MediaBeanTypeConverter.class)                     
    private List<MediaBean> mMediaBeans;
    @TypeConverters(StringTypeConverter.class)
    private List<String> mClazzs;

    //关联另外一个对象
    @Embedded
    private CurrentTeacherInfo mCurrentTeacherInfo;

    public long getUid() {
    
        return uid;
    }

    public void setUid(long uid) {
    
        this.uid = uid;
    }

    public String getContent() {
    
        return content;
    }

    public void setContent(String content) {
    
        this.content = content;
    }

    public String getDate() {
    
        return date;
    }

    public void setDate(String date) {
    
        this.date = date;
    }

    public List<MediaBean> getMediaBeans() {
    
        return mMediaBeans;
    }

    public void setMediaBeans(List<MediaBean> mediaBeans) {
    
        mMediaBeans = mediaBeans;
    }

    public List<String> getClazzs() {
    
        return mClazzs;
    }

    public void setClazzs(List<String> clazzs) {
    
        mClazzs = clazzs;
    }

    public CurrentTeacherInfo getCurrentTeacherInfo() {
    
        return mCurrentTeacherInfo;
    }

    public void setCurrentTeacherInfo(CurrentTeacherInfo currentTeacherInfo) {
    
        mCurrentTeacherInfo = currentTeacherInfo;
    }

    @Override
    public String toString() {
    
        return "ClassMomentInfo{" +
                "uid=" + uid +
                ", content='" + content + '\'' +
                ", date='" + date + '\'' +
                ", mMediaBeans=" + mMediaBeans +
                ", mClazzs=" + mClazzs +
                ", mCurrentTeacherInfo=" + mCurrentTeacherInfo +
                '}';
    }

}

MediaBean

public class MediaBean {
    

    private String url;

    public String getUrl() {
    
        return url;
    }

    public void setUrl(String url) {
    
        this.url = url;
    }

    @Override
    public String toString() {
    
        return "MediaBean{" +
                "url='" + url + '\'' +
                '}';
    }

}