如何使用多个Realm数据库？新手多实例配置与使用疑问

Hey there! Let's work through this together—since you're new to both programming and Realm, it totally makes sense to have questions about managing multiple databases. No worries at all about your question format or English, we're here to help!

The issue you're seeing is because when you create custom Realm configurations but don't set one as the default, Realm doesn't know which database to return when you call Realm.getDefaultInstance(). Here are two straightforward solutions to fix this:

Option 1: Set a Default Configuration #

If you want to keep using Realm.getDefaultInstance() for one of your databases (say, the most frequently used one), you can set its configuration as the default in your MyApplication class. This way you get the best of both worlds: easy access to your main database, and explicit access to the others.

Step 1: Configure in MyApplication #

public class MyApplication extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        // Initialize Realm first
        Realm.init(this);

        // Build your first database configuration
        RealmConfiguration myConfig = new RealmConfiguration.Builder()
            .name("myrealm.realm")
            .schemaVersion(2)
            .modules(new MyCustomSchema())
            .build();

        // Build your second database configuration
        RealmConfiguration otherConfig = new RealmConfiguration.Builder()
            .name("otherrealm.realm")
            .schemaVersion(5)
            .modules(new MyOtherSchema())
            .build();

        // Build your third and fourth configurations here too, following the same pattern

        // Set one configuration as the default (pick whichever you use most)
        Realm.setDefaultConfiguration(myConfig);
    }
}

Step 2: Access Databases in MainActivity #

// Get the default Realm instance (uses myConfig)
Realm defaultRealm = Realm.getDefaultInstance();

// Get other instances by passing their specific configuration
Realm otherRealm = Realm.getInstance(otherConfig);
// Repeat for your third and fourth databases with their respective configs

Option 2: Store Configurations as Static Variables #

If you don't want a default database and prefer to explicitly access all of them, you can store each configuration as a static variable in MyApplication. This saves you from rebuilding the configuration every time you need to access a database.

Step 1: Store Configurations in MyApplication #

public class MyApplication extends Application {
    // Static variables to hold all your configurations
    public static RealmConfiguration myConfig;
    public static RealmConfiguration otherConfig;
    public static RealmConfiguration thirdConfig;
    public static RealmConfiguration fourthConfig;

    @Override
    public void onCreate() {
        super.onCreate();
        Realm.init(this);

        // Initialize each configuration
        myConfig = new RealmConfiguration.Builder()
            .name("myrealm.realm")
            .schemaVersion(2)
            .modules(new MyCustomSchema())
            .build();

        otherConfig = new RealmConfiguration.Builder()
            .name("otherrealm.realm")
            .schemaVersion(5)
            .modules(new MyOtherSchema())
            .build();

        // Initialize thirdConfig and fourthConfig here with their own settings
    }
}

Step 2: Access Databases in MainActivity #

// Access each database using the static configuration variables
Realm myRealm = Realm.getInstance(MyApplication.myConfig);
Realm otherRealm = Realm.getInstance(MyApplication.otherConfig);
Realm thirdRealm = Realm.getInstance(MyApplication.thirdConfig);
Realm fourthRealm = Realm.getInstance(MyApplication.fourthConfig);

Important Reminders #

• Don't forget to register MyApplication in your AndroidManifest.xml—otherwise its onCreate method won't run:

  <application
      android:name=".MyApplication"
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:roundIcon="@mipmap/ic_launcher_round"
      android:supportsRtl="true"
      android:theme="@style/Theme.YourAppName">
      <!-- Your activities go here -->
  </application>
  

• Always close Realm instances when you're done with them (e.g., in your Activity's onDestroy method) to avoid memory leaks:

  @Override
  protected void onDestroy() {
      super.onDestroy();
      myRealm.close();
      otherRealm.close();
      // Close all other instances too
  }
  

• Using separate modules for each database is the right call—it keeps your schemas isolated so each database only contains the objects it needs.

内容的提问来源于stack exchange，提问作者JorGT