java - Dao Room数据库无法访问

Question

所以我创建了这个数据库，由于某种原因我收到一条错误消息:Cannot access database on the main thread since it may potentially lock the UI for a long period of time.日志显示错误位于:wordList = (ArrayList<Note>) wDb.noteDao().getAllNotes(); ，但我看不出我做错了什么？

ListActivity.java

    private NoteDatabase wDb;
    ArrayList<Note> wordList;


    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        s1 = getResources().getStringArray(R.array.PL);//Kald denne først
        startService(new Intent(ListActivity.this,WordLeanerService.class).putExtra("inputExtra", s1));
        setupConnectionToWLservice();

        LoadData(); //Load SharedPreferences
        setContentView(R.layout.activity_main);
        s2 = getResources().getStringArray(R.array.DC);
        s3 = getResources().getStringArray(R.array.DC2);

        recyclerView = findViewById(R.id.recyclerView);
        Exit = findViewById(R.id.CloseApp);
        editSearch = findViewById(R.id.EditText);
        searchBtn = findViewById(R.id.search_button);
        getData(); //get set data from EditActivity and set Arrays

        wDb = NoteDatabase.getInstance(getApplicationContext());
        wordList = (ArrayList<Note>) wDb.noteDao().getAllNotes();
        if(wordList.isEmpty()){
            for (int i = 0; i < images.length; i++) {
                Note nWord = new Note(s1[i], s3[i], s2[i], images[i], items[i], Notes[i]);
                wDb.noteDao().insert(nWord);
            }
        }
    }

Note.java

package com.example.word_learner_app;

import androidx.room.ColumnInfo;
import androidx.room.Entity;
import androidx.room.PrimaryKey;

@Entity(tableName = "note_table")
public class Note {
    @PrimaryKey(autoGenerate = true)
    private int id;
    @ColumnInfo(name = "wordTitle")
    private String title;
    @ColumnInfo(name = "description")
    private String description;
    @ColumnInfo(name = "prounciation")
    private String prounciation;
    @ColumnInfo(name = "image")
    private int Image;
    @ColumnInfo(name = "rating")
    private String rating;
    @ColumnInfo(name = "notes")
    private String notes;

    public Note(String title, String description, String prounciation, int Image, String rating, String notes) {
        this.title = title;
        this.description = description;
        this.prounciation = prounciation;
        this.Image = Image;
        this.rating = rating;
        this.notes = notes;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getId() {
        return id;
    }

    public String getTitle() {
        return title;
    }

    public String getDescription() {
        return description;
    }

    public String getProunciation() {
        return prounciation;
    }

    public int getImage() {
        return Image;
    }

    public String getRating() {
        return rating;
    }

    public String getNotes() {
        return notes;
    }
}

NoteDao.java

package com.example.word_learner_app;

import androidx.room.Dao;
import androidx.room.Delete;
import androidx.room.Insert;
import androidx.room.Query;
import androidx.room.Update;

import java.util.List;

@Dao
public interface NoteDao {
    @Insert
    void insert(Note note);

    @Update
    void update(Note note);

    @Delete
    void delete(Note note);

    @Query("SELECT * FROM note_table")
    List<Note> getAllNotes();
}

NoteDatabase.java

package com.example.word_learner_app;

import android.content.Context;

import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;

@Database(entities = {Note.class}, version = 1, exportSchema = false)
public abstract class NoteDatabase extends RoomDatabase {

    private static NoteDatabase instance;

    public abstract NoteDao noteDao();

    public static synchronized NoteDatabase getInstance(Context contexte){
        if(instance == null){
            instance = Room.databaseBuilder(contexte.getApplicationContext(),
                    NoteDatabase.class, "note_database")
                    .fallbackToDestructiveMigration()
                    .build();
        }
        return instance;
    }
}

Logcat

2020-03-27 16:40:06.697 18007-18007/? E/ord_learner_ap: Unknown bits set in runtime_flags: 0x8000
2020-03-27 16:40:08.226 18007-18007/com.example.word_learner_app E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.word_learner_app, PID: 18007
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.word_learner_app/com.example.word_learner_app.ListActivity}: java.lang.IllegalStateException: Cannot access database on the main thread since it may potentially lock the UI for a long period of time.
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.IllegalStateException: Cannot access database on the main thread since it may potentially lock the UI for a long period of time.
        at androidx.room.RoomDatabase.assertNotMainThread(RoomDatabase.java:267)
        at androidx.room.RoomDatabase.query(RoomDatabase.java:323)
        at androidx.room.util.DBUtil.query(DBUtil.java:83)
        at com.example.word_learner_app.NoteDao_Impl.getAllNotes(NoteDao_Impl.java:158)
        at com.example.word_learner_app.ListActivity.onCreate(ListActivity.java:67)
        at android.app.Activity.performCreate(Activity.java:7802)
        at android.app.Activity.performCreate(Activity.java:7791)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7356) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) 

Answer 1

您尝试执行的操作在 Android 中未经授权，您尝试从 MainThread 获取数据库中的数据，这可能会导致应用程序卡住。这就是为什么建议使用协程来执行此类功能。

编辑:在java中你可以使用callback就像这个codelabs