Can anybody please tell why we need to serializable object for passing one activity to another activity in android? Android is following Java syntax. In java we can pass object to another class without serializable.
Thanks
In ordinary java programs passing parameters(Object type), is kind of create a new handler to the object and giving to another method (In regular words passing the reference by value).
But when it comes in android, passing object references from activity to activity, where their states have to be persisted, is a serious headache.
One way you can do is create a static object in the first activity and access from the second, though this seems to be a easiest way, there is no guarantee that the system maintains the activity in the memory. Therefore the second activity may loose the object reference.
Other way, and the mostly recommended way is serializing(Kind of flatten the object) the object and pass with the intent as extra. In android there are two ways to serialize.
Implement the java's serializable interface
Implement the android's parcelable interface
However, on the android, there is a serious performance hit that comes with using serializable, the solution is using parcelable.
You can find a pretty good tutorial and explanation on android parcelable implementation here.
We need to understand following concepts before getting to the answer:
Android uses Binder for inter-process process. It is required even for simple app because the OS and the apps run in different processes.
Marshalling:
A procedure for converting higher level application data structures into parcels for purpose of embedding into Binder transaction
Unmarshalling
A procedure for reconstructing higher-level application data-structures from parcels received though binder transactions.
You can consider Intents as higher level abstraction of Binder
Based on the documentation following is the way how intent communication occurs:
Activity A creates an Intent with an action description and passes
it to startActivity().
The Android System searches all apps for an intent filter that
matches the intent. When a match is found,
the system starts the matching activity (Activity B) by invoking
its onCreate() method and passing it the Intent.
Why Parcelable or Serializable
IPC (Inter Process Communication) requires data in Intent to be Marshalled and unMarshalled. Binder provides built-in support for marshalling many common data-types. However when we define custom object, it would impact this process and the final object received might be corrupted during the process.
When you define custom object, you need to be responsible for providing this marshalling and unmarshalling which is achieved through Parcelable and Serializable (Since comparison between these two would be another topic I won't discuss much here). Both of these provide mechanisms to perform marshalling and unmarshalling. This is the reason why you need to use Parcelable or Serializable.
Using Parcelable you write the custom code for marshalling and unmarshalling the object thereby you gain complete control over the process.
Serializable is a marker interface, which implies the user cannot marshall the data according to their requirements and its done on JVM, which doesn't give any control at your side.
Disclaimer: Description above is my understanding for the rationale behind the need for serialization based on some
documentation
There are basically two questions in your question, so let's decouple it.
Why marshall in a Parcelable instead of passing an object reference directly?
It's obvious faster and more memory efficient to reference objects rather than marshall/unmarshall them. So you shouldn't use Parcelable when you can pass the object directly.
However, there are situations where you may not have access to the object reference.
in Intent because the process that handles the Intent may not be the process that emitted the Intent (it's an inter-process communication)
in Activity lifecycle, for instance in onRestoreState(), because the whole app may have been killed by memkiller when the user wants to resume it.
everywhere else where Android frameworks requires
In IPC, why use Parcelable rather than Serializable like Java does?
That's only a performance optimization.
If We want to pass object from Activity to to Another Activity . We need to save the passing state.
//to pass :
intent.putExtra("MyClass", obj);
// to retrieve object in second Activity
getIntent().getSerializableExtra("MyClass");
Related
I have a GAE backend and an Android client. I have generated the client library successfully and that works fine. Now, I'm trying to send an object via an intent from one activity to another in Android. To my horror I noticed that the generated GAE models are final classes that don't implement parcelable or serializable interfaces. Because of that I decided to try and use gson to parse them to json string and then back to the original object. That however fails on fields like DateTime for some reason. My next try was to just let the classes implement serializable (bad idea to touch generated classes!!) but the issue with that was that since these classes extend AbstractMap the serializable output is a HashMap rather than my original object! Now I'm thinking about letting it implement Parcelable but that's a lot of work and sounds like a very bad idea since these models are generated and could change (thus removing my parcelable work.)
I know that people here love code so here is my problem written in code:
Activity A (MainActivity) is trying to send a "UserScore" object (generated from GAE) to activity B.
Intent intent = new Intent(MainActivity.this, ResultsActivity.class);
intent.putExtra(ResultsActivity.BUNDLE_USERSCORE, userScore);
startActivityForResult(intent, REQUEST_COUNTDOWN);
This would've given my error since userScore isn't parcelable or serializable but in this case I made it serializable. Activity B (ResultsActivity) receives:
Object extra = getIntent().getSerializableExtra(BUNDLE_USERSCORE);
Now you would expect the extra object to be of the type UserScore but since it was sent via intent and is a subclass of AbstractMap it is in fact a HashMap.
So, my question is: has anybody encountered this issue and found a way to properly send GAE models via intents in android?
To pass data from an Activity to a Fragment, naturally, I think of using a Bundle. The problem in my case is that the Object I need to pass is a Google Cloud Endpoint message, which is neither Parcelable nor Serializable. So how might I pass the data?
For clarity, a Google Cloud Endpoint message is a POJO that is used to pass data to and from endpoint methods. I assume they would be Serializable, but to my surprise they aren't.
You can:
Subclass and implement Serializable or Parcelable on that class
Or use any other strategy to pass data inside of the app:
Singleton class holding any memory cache you may use
If the fragment is inside of the activity you can access the fragment from the activity and pass the variable in
In the worst of cases you can always use SharedPreferences or any kind of disk persistence
I'm developing an Android app which stores a TreeSet object using serialization in a file. I need to serialize the object on close of my app and deserialize on launch because I need an access during a complete runtime. Unfortunately it's quite unpredictable on which activity the user will start/end so it's unclrear to me where are the right places to put this serialization and deserialization calls so that the object is available during the complete runtime.
thanks in advance for any ideas
Well, depending on how performance dependant your app is and how complex your serialization process is I'd read on startup in your main activities onCreate() and write everytime a value is being set. Simple and unefficient, but safe.
I have a class in my Android app that I've made Parcelable so that it can be passed between Activities.
I would like to be able to save this object to the filesystem. It seems that since I've already implemented Parcelable, it would make sense to pipe the output of this to the filesystem and read it back later.
Is there a correct way to do this? Or must I implement both Parcelable and Serialiazble if I want to both pass the object between Activities and also save it to the filesystem?
From http://developer.android.com/reference/android/os/Parcel.html
Parcel is not a general-purpose serialization mechanism. This class (and the corresponding Parcelable API for placing arbitrary objects into a Parcel) is designed as a high-performance IPC transport. As such, it is not appropriate to place any Parcel data in to persistent storage: changes in the underlying implementation of any of the data in the Parcel can render older data unreadable.
For this problem, I did the following:
Implemented Serializable in my object
Added a toJSON() method to convert the object to a JSON object
Used a custom JSONSerializer to write the JSON objects to a file
Added a constructor that takes a JSON object as a parameter, used by the custom JSONSerializer
It ended up being pretty simple...I can paste some sample code if needed.
I have a Network Client class that is receiving a large binary block and parsing it into a usable Java object. The Network Client is on a separate thread from the app's View. What is the best way to make this object available to the View? I've come up with the following solutions, but I feel like none of them are the correct one:
Create the object in the Network Client and let the View access it directly
I would send a small message in a Handler telling the View that the data has been updated
Con: requires that I synchronize the object between the threads to ensure that the Network Client doesn't replace the object while the View is accessing it
Serialize (Parcel?) the object in the Network Client and send it through a Handler to the View
Pro: there are no questions of ownership of the data
Con: would probably be a huge performance drain on the app
Create a reference to the object and pass that to the View
I come from a C++ background, and I'm not sure if this is even possible in Java. I C++, I could just send the View a pointer to the object and let it take care of it. That seems like something Java wouldn't let me do. Is this feasible?
Are any of these solutions advisable, or should I approach the problem in a completely different way?
If you don't want to keep downloading when the activity is in the background, then use non-blocking IO, not threads.
If you do want to keep downloading when the activity is in the background, you probably want to use a service. You can make the object Parcelable or so; I think the underlying service implementation passes pointers around if your activity and service are within the same process (I think they are by default, but ICBW).
If the object is really big and you don't feel comfortable returning it with a get method, maybe you could put its contents into an SQLite database and optionally expose it as a ContentProvider. You could also send an Intent and either cause the View to then go and grab the payload or attach it to the Intent.
Look at the application class subclassing this class and referencing this within your manifest will enable you to store the reference to the service/download controller at a central position that will be available in every activity of your app. This enables you to keep the data in memory and reduce the need of recreating the big object if you need it in more places then just one activity.
For the download you can use a local service that communicates with your activity through a binder object. Keep in mind that a service is not a thread. If you want have the download running in the background you need to create a thread in the oncreate method of your service.
Also keep in mind that it is good practice to have an annotation show the user that a service is doing something and let him access the service and cancel it or view it status.