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What is meant by 'object serialization'? Can you please explain it with some examples?
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Serialization and Deserialization in Java August 1, 2017 April 9, 2019 codeNuclear Serialization: Serialization is a mechanism of converting the state of an object into a byte stream. Java provides a mechanism, called object serialization where an object can be represented as a sequence of bytes that includes the object's data as well as information about the object's type and the types of data stored in the object. In this post, we are going to explore an important Java concept frequently asked during Job’s interviews i.e. Java Serialization. We’ll exclusively talk about serialization and will explain its basic to advance level features with the help of code examples and sample project.
- Serialization is the conversion of the state of an object into a byte stream; deserialization does the opposite. Stated differently, serialization is the conversion of a Java object into a static stream (sequence) of bytes which can then be saved to a database or transferred over a network.
- May 21, 2019 1. Serialization is the conversion of the state of an object into a byte stream; deserialization does the opposite. Stated differently, serialization is the conversion of a Java object into a static stream (sequence) of bytes which can then be saved to a database or transferred over a network.
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13 Answers
Serialization is the conversion of an object to a series of bytes, so that the object can be easily saved to persistent storage or streamed across a communication link. The byte stream can then be deserialized - converted into a replica of the original object.
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You can think of serialization as the process of converting an object instance into a sequence of bytes (which may be binary or not depending on the implementation).
It is very useful when you want to transmit one object data across the network, for instance from one JVM to another.
In Java, the serialization mechanism is built into the platform, but you need to implement the Serializable interface to make an object serializable.
You can also prevent some data in your object from being serialized by marking the attribute as transient.
Finally you can override the default mechanism, and provide your own; this may be suitable in some special cases. To do this, you use one of the hidden features in java.
It is important to notice that what gets serialized is the 'value' of the object, or the contents, and not the class definition. Thus methods are not serialized.
Here is a very basic sample with comments to facilitate its reading:
When we run this program, the file 'o.ser' is created and we can see what happened behind.
If we change the value of: someInteger to, for example Integer.MAX_VALUE, we may compare the output to see what the difference is.
Here's a screenshot showing precisely that difference:
Can you spot the differences? ;)
There is an additional relevant field in Java serialization: The serialversionUID but I guess this is already too long to cover it.
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OscarRyzOscarRyz148k9999 gold badges350350 silver badges525525 bronze badges
Daring to answer 6 year old question, adding just a very high level understanding for people new to Java
What is Serialization?
Converting an object to bytes and bytes back to object (Deserialization).
when is serialization used?
When we want to Persist the Object.When we want the object to exist beyond the lifetime of the JVM.
Real World Example:
ATM: When the account holder tries to withdraw money from the server through ATM, the account holder information like withdrawl details will be serialized and sent to server where the details are deserialized and used to perform operations.
How serialization is performed in java.
- Implement
java.io.Serializable
interface (marker interface so no method to implement). - Persist the object: Use
java.io.ObjectOutputStream
class, a filter stream which is a wrapper around a lower-level byte stream (to write Object to file systems or transfer a flattened object across a network wire and rebuilt on the other side).writeObject(<<instance>>)
- to write an objectreadObject()
- to read an serialized Object
Remember:
When you serialize an object, only the object's state will be saved, not the object's class file or methods.
When you serialized a 2 byte object, you see 51 bytes serialized file.
Steps how the object is serialized and de-serialized.
Answer for: How did it convert to 51 bytes file?
- First writes the serialization stream magic data (STREAM_MAGIC= 'AC ED' and STREAM_VERSION=version of the JVM).
- Then it writes out the metadata of the class associated with an instance (length of the class, the name of the class, serialVersionUID).
- Then it recursively writes out the metadata of the superclass until it finds
java.lang.Object
. - Then starts with the actual data associated with the instance.
- Finally writes the data of objects associated with the instance starting from metadata to actual content.
If you are interested in more in dept information about Java Serialization please check this link.
Edit : One more good link to read.
This will answer a few frequent questions:
- How not to serialize any field in class.
Ans: use transient keyword - When child class is serialized does parent class get serialized?
Ans: No, If parent is not extending Serializable interface parents field don't get serialized. - When parent is serialized does child class get serialized?
Ans: Yes, by default child class also get serialized. - How to avoid child class from getting serialized?
Ans: a. Override writeObject and readObject method and throwNotSerializableException
.b. also you can mark all fields transient in child class. - Some system-level classes such as Thread, OutputStream and its subclasses, and Socket are not serializable.
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Serialization is taking a 'live' object in memory and converting it to a format that can be stored somewhere (eg. in memory, on disk) and later 'deserialized' back into a live object.
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I liked the way @OscarRyz presents. Although here i am continuing the story of serialization which was originally written by @amitgupta.
Even though knowing about the robot class structure and having serialized data Earth's scientist were not able to deserialize the data which can make robots working.
Mars's scientists were waiting for the complete payment. Once the payment was done Mars's scientists shared the serialversionUID with Earth's scientists. Earth's scientist set it to robot class and everything became fine.
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My Two cents from my own blog:
Here is a detailed explanation of the Serialization: (my own blog)
Serialization:
Serialization is the process of persisting the state of an object. It is represented and stored in the form of a sequence of bytes. This can be stored in a file. The process to read the state of the object from the file and restoring it is called deserialization.
What is the need of Serialization?
In modern day architecture, there is always a need to store object state and then retrieve it. For example in Hibernate, to store a object we should make the class Serializable. What it does, is that once the object state is saved in the form of bytes it can be transferred to another system which can then read from the state and retrieve the class. The object state can come from a database or a different jvm or from a separate component. With the help of Serialization we can retrieve the Object state.
Code Example and explanation:
First let's have a look at the Item Class:
In the above code it can be seen that Item class implements Serializable.
This is the interface that enables a class to be serializable.
Now we can see a variable called serialVersionUID is initialized to Long variable. This number is calculated by the compiler based on the state of the class and the class attributes. This is the number that will help the jvm identify the state of an object when it reads the state of the object from file.
For that we can have a look at the official Oracle Documentation:
The serialization runtime associates with each serializable class a version number, called a serialVersionUID, which is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization. If the receiver has loaded a class for the object that has a different serialVersionUID than that of the corresponding sender's class, then deserialization will result in an InvalidClassException. A serializable class can declare its own serialVersionUID explicitly by declaring a field named 'serialVersionUID' that must be static, final, and of type long: ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L; If a serializable class does not explicitly declare a serialVersionUID, then the serialization runtime will calculate a default serialVersionUID value for that class based on various aspects of the class, as described in the Java(TM) Object Serialization Specification. However, it is strongly recommended that all serializable classes explicitly declare serialVersionUID values, since the default serialVersionUID computation is highly sensitive to class details that may vary depending on compiler implementations, and can thus result in unexpected InvalidClassExceptions during deserialization. Therefore, to guarantee a consistent serialVersionUID value across different java compiler implementations, a serializable class must declare an explicit serialVersionUID value. It is also strongly advised that explicit serialVersionUID declarations use the private modifier where possible, since such declarations apply only to the immediately declaring class--serialVersionUID fields are not useful as inherited members.
If you have noticed there is another keyword we have used which is transient.
If a field is not serializable, it must be marked transient. Here we marked the itemCostPrice as transient and don't want it to be written in a file
Now let's have a look on how to write the state of an object in the file and then read it from there.
In the above we can see an example of serialization and deserialization of an object.
For that we used two classes. For serializing the object we have used ObjectOutputStream. We have used the method writeObject to write the object in the file.
For Deserializing we have used ObjectInputStream which reads from the object from the file. It uses readObject to read the object data from the file.
The output of the above code would be like:
Notice that itemCostPrice from deserialized object is null as it was not written.
We have already discussed the basics of Java Serialization in part I of this article.
Now let's discuss it deeply and how it works.
First let's start with the serialversionuid.
The serialVersionUID is used as a version control in a Serializable class.
If you do not explicitly declare a serialVersionUID, JVM will do it for you automatically, based on various properties of the Serializable class.
Java's Algorithm of Calculating serialversionuid (Read more details here)
- The class name.
- The class modifiers written as a 32-bit integer.
- The name of each interface sorted by name.
- For each field of the class sorted by field name (except private static and private transient fields: The name of the field. The modifiers of the field written as a 32-bit integer. The descriptor of the field.
- If a class initializer exists, write out the following: The name of the method, .
- The modifier of the method, java.lang.reflect.Modifier.STATIC, written as a 32-bit integer.
- The descriptor of the method, ()V.
- For each non-private constructor sorted by method name and signature: The name of the method, . The modifiers of the method written as a 32-bit integer. The descriptor of the method.
- For each non-private method sorted by method name and signature: The name of the method. The modifiers of the method written as a 32-bit integer. The descriptor of the method.
- The SHA-1 algorithm is executed on the stream of bytes produced by DataOutputStream and produces five 32-bit values sha[0.4]. The hash value is assembled from the first and second 32-bit values of the SHA-1 message digest. If the result of the message digest, the five 32-bit words H0 H1 H2 H3 H4, is in an array of five int values named sha, the hash value would be computed as follows:
Java's serialization algorithm
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The algorithm to serialize an object is described as below:
1. It writes out the metadata of the class associated with an instance.
2. It recursively writes out the description of the superclass until it finds java.lang.object.
3. Once it finishes writing the metadata information, it then starts with the actual data associated with the instance. But this time, it starts from the topmost superclass.
4. It recursively writes the data associated with the instance, starting from the least superclass to the most-derived class.
1. It writes out the metadata of the class associated with an instance.
2. It recursively writes out the description of the superclass until it finds java.lang.object.
3. Once it finishes writing the metadata information, it then starts with the actual data associated with the instance. But this time, it starts from the topmost superclass.
4. It recursively writes the data associated with the instance, starting from the least superclass to the most-derived class.
Things To Keep In Mind:
- Static fields in a class cannot be serialized.
- If the serialversionuid is different in the read class it will throw a
InvalidClassException
exception. - If a class implements serializable then all its sub classes will also be serializable.
- If a class has a reference of another class, all the references must be Serializable otherwise serialization process will not be performed. In such case, NotSerializableException is thrown at runtime.
Eg:
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Serialization means persisting objects in java. If you want to save the state of the object and want to rebuild the state later (may be in another JVM) serialization can be used.
Note that the properties of an object is only going to be saved. If you want to resurrect the object again you should have the class file, because the member variables only will be stored and not the member functions.
eg:
The Searializable is a marker interface which marks that your class is serializable. Marker interface means that it is just an empty interface and using that interface will notify the JVM that this class can be made serializable.
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Serialization is the process of converting an object's state to bits so that it can be stored on a hard drive. When you deserialize the same object, it will retain its state later. It lets you recreate objects without having to save the objects' properties by hand.
Serialization Example In Java Word
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Serialization is the process of saving an object in a storage medium (such as a file, or a memory buffer) or to transmit it over a network connection in binary form. The serialized objects are JVM independent and can be re-serialized by any JVM. In this case the 'in memory' java objects state are converted into a byte stream. This type of the file can not be understood by the user. It is a special types of object i.e. reused by the JVM (Java Virtual Machine). This process of serializing an object is also called deflating or marshalling an object.
The object to be serialized must implement
java.io.Serializable
Interface. Default serialization mechanism for an object writes the class of the object, the class signature, and the values of all non-transient and non-static fields.ObjectOutput
interface extends the DataOutput
interface and adds methods for serializing objects and writing bytes to the file. The ObjectOutputStream
extends java.io.OutputStream
and implements ObjectOutput
interface. It serializes objects, arrays, and other values to a stream. Thus the constructor of ObjectOutputStream
is written as:Above code has been used to create the instance of the
ObjectOutput
class with the ObjectOutputStream( )
constructor which takes the instance of the FileOuputStream
as a parameter. The
ObjectOutput
interface is used by implementing the ObjectOutputStream
class. The ObjectOutputStream
is constructed to serialize the object. Deserializing an Object in java
The opposite operation of the serialization is called deserialization i.e. to extract the data from a series of bytes is s known as deserialization which is also called inflating or unmarshalling.
ObjectInputStream
extends java.io.InputStream
and implements ObjectInput
interface. It deserializes objects, arrays, and other values from an input stream. Thus the constructor of ObjectInputStream
is written as:Above code of the program creates the instance of the
ObjectInputStream
class to deserialize that file which had been serialized by the ObjectInputStream
class. Goat simulator pc download. The above code creates the instance using the instance of the FileInputStream
class which holds the specified file object which has to be deserialized because the ObjectInputStream()
constructor needs the input stream.SinduSindu
Serialization is the process of turning a Java object into byte array and then back into object again with its preserved state. Useful for various things like sending objects over network or caching things to disk.
Read more from this short article which explains programming part of the process quite well and then move over to to Serializable javadoc. You may also be interested in reading this related question.
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Return the file as an Object : http://www.tutorialspoint.com/java/java_serialization.htm
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Java Object Serialization
Serialization
is a mechanism to transform a graph of Java objects into an array of bytes for storage(to disk file
) or transmission(across a network
), then by using deserialization we can restore the graph of objects.Graphs of objects are restored correctly using a reference sharing mechanism. But before storing, check whether serialVersionUID from input-file/network and .class file serialVersionUID are the same. If not, throw a java.io.InvalidClassException
.Each versioned class must identify the original class version for which it is capable of writing streams and from which it can read. For example, a versioned class must declare:
Java Serialization Tutorial
serialVersionUID Syntax
serialVersionUID is essential to the serialization process. But it is optional for the developer to add it into the java source file. If a serialVersionUID is not included, the serialization runtime will generate a serialVersionUID and associate it with the class. The serialized object will contain this serialVersionUID along with other data.
Note - It is strongly recommended that all serializable classes explicitly declare a serialVersionUID,
since the default serialVersionUID computation is highly sensitive to class details that may vary depending on compiler implementations
, and can thus result in unexpected serialVersionUID conflicts during deserialization, causing deserialization to fail.A Java object is only serializable. if a class or any of its superclasses implements either the java.io.Serializable interface or its subinterface, java.io.Externalizable.
- A class must implement java.io.Serializable interface in order to serialize its object successfully. Serializable is a marker interface and used to inform the compiler that the class implementing it has to be added serializable behavior. Here Java Virtual Machine (JVM) is responsible for its automatic serialization.transient Keyword:
java.io.Serializable interface
While serializing an object, if we don't want certain data members of the object to be serialized we can use the transient modifier. The transient keyword will prevent that data member from being serialized.- Fields declared as transient or static are ignored by the serialization process.
TRANSIENT & VOLATILE - Implementing the Externalizable interface allows the object to assume complete control over the contents and format of the object's serialized form. The methods of the Externalizable interface, writeExternal and readExternal, are called to save and restore the objects state. When implemented by a class they can write and read their own state using all of the methods of ObjectOutput and ObjectInput. It is the responsibility of the objects to handle any versioning that occurs.
- Only objects that support the java.io.Serializable or java.io.Externalizable interface can be
written to
/read from
streams. The class of each serializable object is encoded including the class name and signature of the class, the values of the object's fields and arrays, and the closure of any other objects referenced from the initial objects.
Serializable Example For Files
Serializable Example Over Network
Distributing object's state across different address spaces, either in different processes on the same computer, or even in multiple computers connected via a network, but which work together by sharing data and invoking methods.
@see
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|*| Serializing a class : Converting an object to bytes and bytes back to object (Deserialization).
|=> Object-Serialization is process of converting the state of an object into steam of bytes.
- |-> Implement when you want the object to exist beyond the lifetime of the JVM.
- |-> Serialized Object can be stored in Database.
- |-> Serializable-objects cannot be read and understood by humans so we can acheive security.
Java Serializable Id
|=> Object-Deserialization is the process of getting the state of an object and store it into an object(java.lang.Object).
- |-> Before storing its state it checks whether serialVersionUID form input-file/network and .class file serialVersionUID are same.
  If not throw java.io.InvalidClassException.
|=> A Java object is only serializable, if its class or any of its superclasses
- implements either the java.io.Serializable interface or
- its subinterface, java.io.Externalizable.
|=> Static fields in a class cannot be serialized.
|=> If you do not want to serialise a variable of a class use transient keyword
|=> If a class implements serializable then all its sub classes will also be serializable.
|=> If a class has a reference of another class, all the references must be Serializable otherwise serialization process will not be performed. In such case,
NotSerializableException is thrown at runtime.
PangNotSerializableException is thrown at runtime.
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Serialization Example In Java Api
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Java provides a mechanism, called object serialization where an object can be represented as a sequence of bytes that includes the object's data as well as information about the object's type and the types of data stored in the object.
Json Serialization Example In Java
After a serialized object has been written into a file, it can be read from the file and deserialized that is, the type information and bytes that represent the object and its data can be used to recreate the object in memory.
Most impressive is that the entire process is JVM independent, meaning an object can be serialized on one platform and deserialized on an entirely different platform.
Classes ObjectInputStream and ObjectOutputStream are high-level streams that contain the methods for serializing and deserializing an object.
The ObjectOutputStream class contains many write methods for writing various data types, but one method in particular stands out −
The above method serializes an Object and sends it to the output stream. Similarly, the ObjectInputStream class contains the following method for deserializing an object −
This method retrieves the next Object out of the stream and deserializes it. The return value is Object, so you will need to cast it to its appropriate data type.
To demonstrate how serialization works in Java, I am going to use the Employee class that we discussed early on in the book. Suppose that we have the following Employee class, which implements the Serializable interface −
Example
Notice that for a class to be serialized successfully, two conditions must be met −
- The class must implement the java.io.Serializable interface.
- All of the fields in the class must be serializable. If a field is not serializable, it must be marked transient.
If you are curious to know if a Java Standard Class is serializable or not, check the documentation for the class. The test is simple: If the class implements java.io.Serializable, then it is serializable; otherwise, it's not.
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Serializing an Object
The ObjectOutputStream class is used to serialize an Object. The following SerializeDemo program instantiates an Employee object and serializes it to a file.
When the program is done executing, a file named employee.ser is created. The program does not generate any output, but study the code and try to determine what the program is doing.
Note − When serializing an object to a file, the standard convention in Java is to give the file a .ser extension.
Example
Deserializing an Object
The following DeserializeDemo program deserializes the Employee object created in the SerializeDemo program. Study the program and try to determine its output −
Serializable Example In Java
Example
This will produce the following result −
Output
Here are following important points to be noted −
What Is Serialization In Java
- The try/catch block tries to catch a ClassNotFoundException, which is declared by the readObject() method. For a JVM to be able to deserialize an object, it must be able to find the bytecode for the class. If the JVM can't find a class during the deserialization of an object, it throws a ClassNotFoundException.
- Notice that the return value of readObject() is cast to an Employee reference.
- The value of the SSN field was 11122333 when the object was serialized, but because the field is transient, this value was not sent to the output stream. The SSN field of the deserialized Employee object is 0.