Difference Between Serializable and Externalizable in Java Serialization
The process of writing the state of an object to a file is called serialization, but strictly speaking, it is the process of converting an object from java supported form into a file-supported form or network-supported form by using fileOutputStream and objectOutputStream classes we can implement serialization.
But we can serialize only serializable objects. An object is said to be serializable if and only if the corresponding class implements a Serializable interface. Serializable interface is present in java.io package, and it doesn’t contain any method and hence it is a marker interface. If we are trying to serializable a non-serializable object then we will get Runtime Exception saying notSerializableException.
Java
Object has been serialized Object has been deserialized Name:Ram Age:34 2364
Externalizable
In serialization, everything is taken care of by JVM and the programmer doesn’t have any control. In serialization, it is always possible to solve the total object to file, and it is not possible to save part of the object which may create performance problems. To overcome this problem we should go for externalization.
The main advantage of Externalizable over serialization is, everything is taken care of by the programmer and JVM doesn’t have any control. Based on our requirements we can save either the total object or part of the object which improves the performance of the system. To provide the generalizable ability for any java object, it’s a mandate for the corresponding class to implement a generalizable interface.
This interface defines two methods as follows:
public void writeExternal( ObjectOutput obj ) throws IOException
This method will be executed automatically at the time of serialization within this method we have to write code to save the required variable to the file.
public void readExternal(ObjectInput in )throws IOException , ClassNotFoundException
This method will be executed automatically at the time of deserialization. Within this method, we have to write code to read the required variable from the file and assign it to the current object. But strictly speaking at the time of deserialization JVM will create a separate new object by executing a public no-argument constructor and then on that object, JVM will call the readExternal method.
Hence, every Externalizable implemented class should compulsorily contain a public no-argument constructor otherwise we will get Runtime Exception saying InvalidClassException.
Java
Public no-argument constructor Name : Ram Age : 35
Now, we are done discussing both of them, let us conclude all the differences in a table for a better reach of understanding.
| Serializable | Externalizable |
|---|---|
| A serializable interface is used to implement serialization. | An externalizable interface used to implement Externalization |
| Serializable is a marker interface i.e. it does not contain any method. | The externalizable interface is not a marker interface and thus it defines two methods writeExternal() and readExternal(). |
| Serializable interface passes the responsibility of serialization to JVM and the programmer has no control over serialization, and it is a default algorithm. | The externalizable interface provides all serialization responsibilities to a programmer and hence JVM has no control over serialization. |
| Serialization using a serializable interface has bad performance. | Serialization using an externalizable interface has better performance. |
| Default serialization does not require any no-arg constructor. | A public no-arg constructor is required while using an Externalizable interface. |
| It is hard to analyze and modify class structure because any change in structure may break serialization. | It is relatively easy to analyze and modify class structure because of complete control over serialization logic. |
| Using a serializable interface we save the total object to a file, and it is not possible to save part of the object. | Base on our requirements we can save either the total object or part of the object. |
| Transient keywords play an important role here. | Transient keywords won’t play any role. |
Interface Serializable
Warning: Deserialization of untrusted data is inherently dangerous and should be avoided. Untrusted data should be carefully validated according to the «Serialization and Deserialization» section of the Secure Coding Guidelines for Java SE. Serialization Filtering describes best practices for defensive use of serial filters.
Classes that do not implement this interface will not have any of their state serialized or deserialized. All subtypes of a serializable class are themselves serializable. The serialization interface has no methods or fields and serves only to identify the semantics of being serializable.
It is possible for subtypes of non-serializable classes to be serialized and deserialized. During serialization, no data will be written for the fields of non-serializable superclasses. During deserialization, the fields of non-serializable superclasses will be initialized using the no-arg constructor of the first (bottommost) non-serializable superclass. This constructor must be accessible to the subclass that is being deserialized. It is an error to declare a class Serializable if this is not the case; the error will be detected at runtime. A serializable subtype may assume responsibility for saving and restoring the state of a non-serializable supertype’s public, protected, and (if accessible) package-access fields. See the Java Object Serialization Specification, section 3.1, for a detailed specification of the deserialization process, including handling of serializable and non-serializable classes.
When traversing a graph, an object may be encountered that does not support the Serializable interface. In this case the NotSerializableException will be thrown and will identify the class of the non-serializable object.
Classes that require special handling during the serialization and deserialization process must implement special methods with these exact signatures:
private void writeObject(java.io.ObjectOutputStream out) throws IOException private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException; private void readObjectNoData() throws ObjectStreamException;
The writeObject method is responsible for writing the state of the object for its particular class so that the corresponding readObject method can restore it. The default mechanism for saving the Object’s fields can be invoked by calling out.defaultWriteObject. The method does not need to concern itself with the state belonging to its superclasses or subclasses. State is saved by writing the individual fields to the ObjectOutputStream using the writeObject method or by using the methods for primitive data types supported by DataOutput.
The readObject method is responsible for reading from the stream and restoring the classes fields. It may call in.defaultReadObject to invoke the default mechanism for restoring the object’s non-static and non-transient fields. The defaultReadObject method uses information in the stream to assign the fields of the object saved in the stream with the correspondingly named fields in the current object. This handles the case when the class has evolved to add new fields. The method does not need to concern itself with the state belonging to its superclasses or subclasses. State is restored by reading data from the ObjectInputStream for the individual fields and making assignments to the appropriate fields of the object. Reading primitive data types is supported by DataInput.
The readObjectNoData method is responsible for initializing the state of the object for its particular class in the event that the serialization stream does not list the given class as a superclass of the object being deserialized. This may occur in cases where the receiving party uses a different version of the deserialized instance’s class than the sending party, and the receiver’s version extends classes that are not extended by the sender’s version. This may also occur if the serialization stream has been tampered; hence, readObjectNoData is useful for initializing deserialized objects properly despite a «hostile» or incomplete source stream.
Serializable classes that need to designate an alternative object to be used when writing an object to the stream should implement this special method with the exact signature:
ANY-ACCESS-MODIFIER Object writeReplace() throws ObjectStreamException;
This writeReplace method is invoked by serialization if the method exists and it would be accessible from a method defined within the class of the object being serialized. Thus, the method can have private, protected and package-private access. Subclass access to this method follows java accessibility rules.
Classes that need to designate a replacement when an instance of it is read from the stream should implement this special method with the exact signature.
ANY-ACCESS-MODIFIER Object readResolve() throws ObjectStreamException;
This readResolve method follows the same invocation rules and accessibility rules as writeReplace.
Enum types are all serializable and receive treatment defined by the Java Object Serialization Specification during serialization and deserialization. Any declarations of the special handling methods discussed above are ignored for enum types.
Record classes can implement Serializable and receive treatment defined by the Java Object Serialization Specification, Section 1.13, «Serialization of Records». Any declarations of the special handling methods discussed above are ignored for record types.
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 Object Serialization Specification. This specification defines the serialVersionUID of an enum type to be 0L. However, it is strongly recommended that all serializable classes other than enum types 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 InvalidClassException s 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. Array classes cannot declare an explicit serialVersionUID, so they always have the default computed value, but the requirement for matching serialVersionUID values is waived for array classes.
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