我正在尝试使用Room Persistence
Library的示例。我创建了一个实体：

@Entity
public class Agent {
    @PrimaryKey
    public String guid;
    public String name;
    public String email;
    public String password;
    public String phone;
    public String licence;
}

创建了一个 DAO 类：

@Dao
public interface AgentDao {
    @Query("SELECT COUNT(*) FROM Agent where email = :email OR phone = :phone OR licence = :licence")
    int agentsCount(String email, String phone, String licence);

    @Insert
    void insertAgent(Agent agent);
}

创建数据库类：

@Database(entities = {Agent.class}, version = 1)
public abstract class AppDatabase extends RoomDatabase {
    public abstract AgentDao agentDao();
}

在 Kotlin 中使用以下子类公开数据库：

class MyApp : Application() {

    companion object DatabaseSetup {
        var database: AppDatabase? = null
    }

    override fun onCreate() {
        super.onCreate()
        MyApp.database =  Room.databaseBuilder(this, AppDatabase::class.java, "MyDatabase").build()
    }
}

在我的活动中实现了以下功能：

void signUpAction(View view) {
        String email = editTextEmail.getText().toString();
        String phone = editTextPhone.getText().toString();
        String license = editTextLicence.getText().toString();

        AgentDao agentDao = MyApp.DatabaseSetup.getDatabase().agentDao();
        //1: Check if agent already exists
        int agentsCount = agentDao.agentsCount(email, phone, license);
        if (agentsCount > 0) {
            //2: If it already exists then prompt user
            Toast.makeText(this, "Agent already exists!", Toast.LENGTH_LONG).show();
        }
        else {
            Toast.makeText(this, "Agent does not exist! Hurray :)", Toast.LENGTH_LONG).show();
            onBackPressed();
        }
    }

不幸的是，在执行上述方法时，它会因以下堆栈跟踪而崩溃：

    FATAL EXCEPTION: main
 Process: com.example.me.MyApp, PID: 31592
java.lang.IllegalStateException: Could not execute method for android:onClick
    at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
    at android.view.View.performClick(View.java:5612)
    at android.view.View$PerformClick.run(View.java:22288)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6123)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)
 Caused by: java.lang.reflect.InvocationTargetException
    at java.lang.reflect.Method.invoke(Native Method)
    at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
    at android.view.View.performClick(View.java:5612)聽
    at android.view.View$PerformClick.run(View.java:22288)聽
    at android.os.Handler.handleCallback(Handler.java:751)聽
    at android.os.Handler.dispatchMessage(Handler.java:95)聽
    at android.os.Looper.loop(Looper.java:154)聽
    at android.app.ActivityThread.main(ActivityThread.java:6123)聽
    at java.lang.reflect.Method.invoke(Native Method)聽
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)聽
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)聽
 Caused by: java.lang.IllegalStateException: Cannot access database on the main thread since it may potentially lock the UI for a long periods of time.
    at android.arch.persistence.room.RoomDatabase.assertNotMainThread(RoomDatabase.java:137)
    at android.arch.persistence.room.RoomDatabase.query(RoomDatabase.java:165)
    at com.example.me.MyApp.RoomDb.Dao.AgentDao_Impl.agentsCount(AgentDao_Impl.java:94)
    at com.example.me.MyApp.View.SignUpActivity.signUpAction(SignUpActivity.java:58)
    at java.lang.reflect.Method.invoke(Native Method)聽
    at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)聽
    at android.view.View.performClick(View.java:5612)聽
    at android.view.View$PerformClick.run(View.java:22288)聽
    at android.os.Handler.handleCallback(Handler.java:751)聽
    at android.os.Handler.dispatchMessage(Handler.java:95)聽
    at android.os.Looper.loop(Looper.java:154)聽
    at android.app.ActivityThread.main(ActivityThread.java:6123)聽
    at java.lang.reflect.Method.invoke(Native Method)聽
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)聽
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)聽

似乎该问题与在主线程上执行 db 操作有关。但是，上面链接中提供的示例测试代码不在单独的线程上运行：

@Test
    public void writeUserAndReadInList() throws Exception {
        User user = TestUtil.createUser(3);
        user.setName("george");
        mUserDao.insert(user);
        List<User> byName = mUserDao.findUsersByName("george");
        assertThat(byName.get(0), equalTo(user));
    }

我在这里有什么遗漏吗？我怎样才能让它在没有崩溃的情况下执行？请建议。