java - 管理服务和应用程序之间的 Realm

Question

我正在开发一个 Android 应用程序，该应用程序在服务中接收一些 BLE 数据，然后将它们存储到 Realm 数据库中，并明显显示在 Activities 和 上 fragment 。我设置了一个名为 RealmManager 的 Singleton 类，它使用单个 Realm 实例(带有 getDefaultInstance)来执行事务和查询。如果我调用 RealmManager 方法将新数据存储到 Realm，则会引发异常，因为我在不同的线程上使用 Realm。在此示例中，我无法从服务和 Activity 中调用 RealmManager.sharedInstance().userWithEmail("pippo"):

public class RealmManager {

     private static Realm realmDB;
     private static RealmManager ourInstance;

     public RealmManager(){
       realmDB = Realm.getDefaultInstance();
     }

     public static RealmManager sharedInstance(){
        if(ourInstance == null)
           ourInstance = new RealmManager();
     }

    public User userWithEmail(String email){
        User user = realmDB.where(User.class).equalTo(USER_EMAIL, email).findFirst();
        return user;
    }
}

如果我在每个方法上实例化一个新的 Realm 实例，然后在最后调用 Realm.close，我无法跨应用程序使用 Realm 结果，因为 Realm 实例已关闭。在此示例中，我无法在调用者中使用user，因为实例已关闭:

public ArrayList<Patient> getPatientsForLoggedUser() {
       final Realm realmDB = 
       Realm.getInstance(RealmManager.getDefaultInstance();
       try {
           ArrayList<Patient> patients = new ArrayList<>(realmDB.where(Patient.class).equalTo("user.email", loggedUser().getEmail()).findAllSorted("firstName", Sort.ASCENDING));
           return patients;
       } finally {
           if(realmDB != null)
               realmDB.close();
       }
    }

如何使用 RealmManager 类从应用程序和服务中保存和检索所有应用程序的数据库数据？

Answer 1

你不能像这样创建一个单一的 Realm。

通常您需要ThreadLocal<Realm>为此，但 Realm 已经使用所谓的 RealmCache 在内部管理此问题，看起来有点像这样:

private static class RefAndCount {
    // The global Realm instance in this thread.
    private final ThreadLocal<BaseRealm> localRealm = new ThreadLocal<>();

    // How many local references to this Realm instance in this thread.
    private final ThreadLocal<Integer> localCount = new ThreadLocal<>();

    // How many threads have instances refer to this configuration.
    private int globalCount = 0;
}

private synchronized <E extends BaseRealm> E doCreateRealmOrGetFromCache(RealmConfiguration configuration) {
    RefAndCount refAndCount = refAndCountMap.get(RealmCacheType.valueOf(Realm.class));

    if (refAndCount.localRealm.get() == null) {
        // Creates a new local Realm instance
        BaseRealm realm = Realm.createInstance(this);
        refAndCount.localRealm.set(realm);
        refAndCount.localCount.set(0);

        refAndCount.globalCount++;
    }

    Integer refCount = refAndCount.localCount.get();
    refAndCount.localCount.set(refCount + 1);

    //noinspection unchecked
    return (E) refAndCount.localRealm.get();
}


synchronized void release(BaseRealm realm) {
    String canonicalPath = realm.getPath();
    RefAndCount refAndCount = refAndCountMap.get(RealmCacheType.valueOf(Realm.class));
    Integer refCount = refAndCount.localCount.get();
    if (refCount == null) {
        refCount = 0;
    }

    if (refCount <= 0) {
        RealmLog.warn("%s has been closed already. refCount is %s", canonicalPath, refCount);
        return;
    }

    refCount -= 1;

    if (refCount == 0) {
        refAndCount.localCount.set(null);
        refAndCount.localRealm.set(null);

        refAndCount.globalCount--;
        if (refAndCount.globalCount < 0) {
            // Should never happen.
            throw new IllegalStateException("Global reference counter of Realm" + canonicalPath + " got corrupted.");
        }

        realm.doClose();

        if (getTotalGlobalRefCount() == 0) {
            configuration = null;
        }
    } else {
        refAndCount.localCount.set(refCount);
    }
}

考虑到这一点，我们学到的是:

1.) Realm.getDefaultInstance() 增加本地计数，并打开一个新的全局线程本地 Realm 实例如果localCount 0 -> 1

2.) realm.close()减少本地计数

3.) 如果 localCount 1 -> 0 ，然后全局线程本地Realm实例被释放

4.) 所有这些方法都是 synchronized实现中

<小时/>

考虑到这一点，您可能不应该为所有线程创建全局 Realm 实例。主要是因为您要在现有的线程本地缓存之上创建线程本地缓存，这会变得很棘手。虽然你可以检查this question as an example from daniel christopher (gist)。

<小时/>

通常，您应该提供本地 Realm 实例并关闭它们。

documentation describes the best practices for managing Realm instance lifecycle ，尽管我个人更喜欢每个线程只保留 1 个 Realm 实例。