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What is the difference between Serializable and Externalizable in Java?
(11 answers)
Closed 9 years ago.
I read that
Externalizable provides us writeExternal() and readExternal() method which gives us flexibility to control java serialization mechanism instead of relying on Java's default serialization.
But If i implement Serializable and override readObject(), writeObject(), then does not it also means the same that I am customizing serialization process?
How does it differ?
THanks.
Difference between Externalizable and Serializable
In case of Serializable, default serialization process is used. while in case of Externalizable custom Serialization process is used which is implemented by application.
JVM gives call back to readExternel() and writeExternal() of java.io.Externalizalbe interface for restoring and writing objects into persistence.
Externalizable interface provides complete control of serialization process to application.
readExternal() and writeExternal() supersede any specific implementation of writeObject and readObject methods.
Though Externalizable provides complete control, it also presents challenges to serialize super type state and take care of default values in case of transient variable and static variables in Java. If used correctly Externalizable interface can improve performance of serialization process.
So go for Externalizable interface
When you have special requirements for the serialization of an object. For example, you may have some security-sensitive parts of the object, like passwords, which you do not want to keep and transfer somewhere. Or, it may be worthless to save a particular object referenced from the main object because its value will become worthless after restoring.
Official docs on Bean Persistence
Implement writeObject when you need to exercise greater control over what gets serialized when you need to serialize objects that default serialization cannot handle, or when you need to add data to the serialization stream that is not an object data member. Implement readObject to reconstruct the data stream you wrote with writeObject.
Use the Externalizable interface when you need complete control over your bean's serialization (for example, when writing and reading a specific file format). To use the Externalizable interface you need to implement two methods: readExternal and writeExternal. Classes that implement Externalizable must have a no-argument constructor.
Related
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Although the Serializable interface in Java has no methods, no fields, it can achieve its function. How?
(6 answers)
Closed 7 years ago.
I know what is serialization and why it is used, but my question:
Why is serialization a marker interface?
What is the potential benefit of not having writeObject, readObject in Serializable interface because when we do serialization we finally override these 2 methods?
How does readResolve ensure that the object created during deserialization is not the new object. I know the below and it is returning the same object during deserialization but who will call this readResolve method internally?
private Object readResolve() throws java.io.ObjectStreamException {
return INSTANCE;
}
Because there needs to be some explicit way of declaring a class to be serializable. The framework can't just assume all classes to be serializable, as there are many kinds of objects that will stop working if their fields are written to disk and are later reloaded from there (e.g. FileInputStream, which relies on an open operating system file handle which might no longer exist when the object is deserialized). The modern way of having such a declaration would be an annotation, but those didn't exist in Java at the time serialization was added.
You won't necessarily need to override them - if the default behavior of the serializer is good enough; you don't need to do anything but implement Serializable.
The serialization framework calls it when an object has been completely deserialized. At this time, the object is allowed to inspect its own contents, and if it decides that it should instead be represented by another instance, it can return that instance instead (if not, it returns this). Whatever comes out of this method is returned to the code that requested the deserialization. If a preexisting object was returned, the new object created by the deserializer will not be seen by anyone and will eventually be garbage collected.
Marker Interfaces are used to tell JVM to perform specific tasks. they don't have any method. Serializable is also a marker interface.
Serialization is the process of flattening the objects. when you implement serializable interface in a class, it tells JVM to serialize its object i.e. it has to be converted into stream.
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RMI marshal and serialization
(3 answers)
Closed 9 years ago.
I need some clarification on serialization in java. If i am serializing an object and i want to return that object over RMI do i just implement serializable on my object's custom class? I've seen objectoutputstream and objectinputstream but i don't know if i need to use those as i don't quite undersand their usage. My second question is how would i go about making an arraylist serializable? My third question is maybe related to my first question but how do i marshal an object before returning it? does serializing the object marshals it in the process?
Lets call the class in question Sentence:
public class Sentence implements Serializable {
}
And then i would be creating an instance of that class and returning that object from some other class
Yes: implementing Serializable makes your object serializable, unless it references an object that is not serializable. Object streams are used internally by RMI to send and receive objects. You don't need to use them if using RMI. If you want to send an object using sockets, or save it to a file, then you can use them.
ArrayList is already serializable. Look at its javadoc. You don't have to do anything.
You simply return the object from your RMI method, and RMI will serialize it for you. Serialization and marshalling are two words that basically mean the same thing. In the same way, if an RMI methods takes an argument, RMI will serialize the object passed as argument, send it to the RMI server, which will deserialize it and then call your actual method. It's all done for you by RMI.
Serializable is what's called a marker interface; it just tells Java that your class has some ability without requiring you to implement any particular methods. Implementing it in your class tells the JVM that it's okay (meaningful) to take the fields of your Java object and convert them to a packaged form for use later or on another computer.
That's all you have to do if all of the fields in your class are Serializable. If you have fields that aren't Serializable, such as fields that hold network connections or native resources, you'll need to mark them transient (ignored by serialization) and take care of setting them back up when your class is deserialized by overriding readObject and/or writeObject.
We don't need it if we're implementing Serializable. So why this difference? How does it relate to the actual mechanism of Serialization?
A thorough explanation (although the grammar of the article might be improved) can be found on http://www.jusfortechies.com/java/core-java/externalization.php . The short answer, for future reference in case the linked page goes away:
Externalizable is an interface extending Serializable. Contrary to Serializable, though, objects are not restored by just reading the serialized bytestream, but the public constructor is called and only once the object is thus created, its state is restored. This makes restoring more efficient.
Edit: See also What is the difference between Serializable and Externalizable in Java? .
This is primarily used for caching purposes. In order to deserialize across streams, you will need to spell out how you want your object to be deserialized, hence the two methods provided by the contract in Externalizable interface: writeExternal and readExternal. Note that Externalizable extends Serializable, so you don't necessarily need to implement Serializable interface (although it's a marker interface and there are no methods to be actually implemented).
For a sample implementation, have a look at MimeType.
A public no-arg constructor is needed while using Externalizable interface.
Because in case of Serializable
readObject reads the required information from the ObjectInputStream
Serialization uses reflection mechanism to get the necessary fields and their corresponding values.
Serializable serializes all the data members (except static and transient).
But in case of Externalizable
No reflection mechanism used.
User doesn't serializes all data members.That's why to fetch values of the members which are not externalized public no arg constructor is required.
In Java we implement an interface Serializable that defines no method (also called a marker interface).
But suppose I have an interface without any method just like a Serializable interface, can I make it work just like that, meaning that I would be able to use my interface instead of the Serializable?
Thanks
Only Serializable will mark an object as being compatible with Java's built-in serialization machinery.
You can create other empty interfaces, but they won't mean the same thing. Each interface is distinct, even if it defines the same set of methods.
No. If you want to be able to use Java Serialization, your objects need to implement Serializable.
If you want to use other serialization tools, (ie: Hibernate, SimpleXML, XStream), that is always a possibility, but those generally involve adding annotations, xml files, or other configurations.
If you want your object be serializable it must implement java.io.Serializable.
You could also create your own interface or class that extend from Serializable and then another class/interface extend from that one.
No. Serializable is a special interface, and you can't use a different one with the built-in serialization mechanism. You could roll your own entire system, but it'd be a big job.
If you don't implement Serializable you will not be able to use Java serialization to persist it. But you are more than welcome to implement your own persistence that is not relying on Java serialization.
Serializable is called a marker, but only because classes like ObjectOutputStream and ObjectInputStream use the reflection API to check for it. There is no special magic there; if you wanted to implement your own alternative serialization strategy, you could also create your own interface and use that as the marker you wished to respect. Although, you should consider whether it would actually be better design to use the existing Java marker (Serializable). I believe some other alternatives to the built-in Object stream do this.
If you merely wish to take complete control over how your objects are serialized into bytes, you can do so without rewriting the ObjectXXXStream classes, by using Serializable subclass Externalizable. And in fact, this is commonly done, since the automatic serialization afforded by Serializable ungracefully dies with the smallest change to the SerialVersionUID, which, unless supplied, is calculated automatically from various signatures of the class - making Serializable unsuitable for must persistent storage, since most kinds of changes (i.e. during a routine upgrade) to the underlying class will prevent reloading the serialized data.
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Closed 10 years ago.
Possible Duplicate:
What is the difference between Serializable and Externalizable in Java?
What is the difference between Serializable and Externalizable interface?
Serializable allows an object to be written out in a standard format; there's some control over part of the process, but mostly it's automatic. Externalizable provides much more complete control (e.g., allowing an object that is a table of numbers to be written out as CSV-format data).
Serializable doesn't require you to write any methods (though it might be a good idea) since it is a marker interface. Externalizable requires the writing of methods.
Serializability of a class is enabled by the class implementing the java.io.Serializable
interface. Classes that do not implement this interface will not have any of their state serialized or deserialized.
Externalizable Only the identity of the class of an Externalizable instance is written in the serialization stream and it is the responsibility of the class to save and restore the contents of its instances.