Why does Java have transient fields

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Because not all variables are of a serializable nature

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Simplified example code for transient-keyword    import java io     class NameStore implements Serializable       private String firstName  lastName      private transient String fullName       public NameStore  String fName  String lName           this firstName   fName          this lastName   lName          buildFullName               private void buildFullName                assume building fullName is compuational memory intensive          this fullName   this firstName         this lastName             public String toString            return  First Name       this firstName                 nLast Name       this lastName                 nFull Name       this fullName             private void readObject ObjectInputStream inputStream              throws IOException  ClassNotFoundException               inputStream defaultReadObject            buildFullName             public class TransientExample      public static void main String args    throws Exception           ObjectOutputStream o   new ObjectOutputStream new FileOutputStream  ns             o writeObject new NameStore  Steve    Jobs             o close             ObjectInputStream in   new ObjectInputStream new FileInputStream  ns             NameStore ns    NameStore in readObject            System out println ns

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Before understanding the transient keyword  one has to understand the concept of serialization  If the reader knows about serialization  please skip the first point   What is serialization   Serialization is the process of making the object s state persistent  That means the state of the object is converted into a stream of bytes to be used for persisting  e g  storing bytes in a file  or transferring  e g  sending bytes across a network   In the same way  we can use the deserialization to bring back the object s state from bytes  This is one of the important concepts in Java programming because serialization is mostly used in networking programming  The objects that need to be transmitted through the network have to be converted into bytes  For that purpose  every class or interface must implement the Serializable interface  It is a marker interface without any methods   Now what is the transient keyword and its purpose   By default  all of object s variables get converted into a persistent state  In some cases  you may want to avoid persisting some variables because you don t have the need to persist those variables  So you can declare those variables as transient  If the variable is declared as transient  then it will not be persisted  That is the main purpose of the transient keyword   I want to explain the above two points with the following example   package javabeat samples   import java io FileInputStream  import java io FileOutputStream  import java io IOException  import java io ObjectInputStream  import java io ObjectOutputStream  import java io Serializable   class NameStore implements Serializable      private String firstName      private transient String middleName      private String lastName       public NameStore  String fName  String mName  String lName           this firstName   fName          this middleName   mName          this lastName   lName             public String toString            StringBuffer sb   new StringBuffer 40           sb append  First Name               sb append this firstName           sb append  Middle Name               sb append this middleName           sb append  Last Name               sb append this lastName           return sb toString             public class TransientExample      public static void main String args    throws Exception           NameStore nameStore   new NameStore  Steve    Middle   Jobs            ObjectOutputStream o   new ObjectOutputStream new FileOutputStream  nameStore                writing to object         o writeObject nameStore           o close                reading from object         ObjectInputStream in   new ObjectInputStream new FileInputStream  nameStore             NameStore nameStore1    NameStore in readObject            System out println nameStore1             And the output will be the following   First Name   Steve Middle Name   null Last Name   Jobs   Middle Name is declared as transient  so it will not be stored in the persistent storage   Source

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Because not all variables are of a serializable nature   Serialization and Deserialization are symmetry processes  if not you can t expect the result to be determined  in most cases  undetermined values are meaningless  Serialization and Deserialization are idempotent  it means you can do  serialization as many time as you want  and the result is the same   So if the Object can exists on memory but not on disk  then the Object can t be serializable  because the machine can t restore the memory map when deserialization  Fro example  you can t serialize a Stream object  You can not serialize a Connection object  because it s state also dependent on the remote site

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Serialization systems other than the native java one can also use this modifier  Hibernate  for instance  will not persist fields marked with either  Transient or the transient modifier  Terracotta as well respects this modifier   I believe the figurative meaning of the modifier is  this field is for in-memory use only  don t persist or move it outside of this particular VM in any way  Its non-portable   i e  you can t rely on its value in another VM memory space  Much like volatile means you can t rely on certain memory and thread semantics

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As per google transient meaning     lasting only for a short time  impermanent   Now if you want to make anything transient in java use transient keyword   Q  where to use transient    A  Generally in java we can save data to files by acquiring them in variables and writing those variables to files  this process is known as Serialization  Now if we want to avoid variable data to be written to file  we would make that variable as transient   transient int result 10    Note  transient variables cannot be local

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transient is used to indicate that a class field doesn t need to be serialized  Probably the best example is a Thread field  There s usually no reason to serialize a Thread  as its state is very  flow specific

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Before I respond to this question  I must explain to you the SERIALIZATION  because if you understand what it means serialization in science computer you can easily understand this keyword   Serialization When an object is transferred through the network   saved on physical media file       the object must be  serialized   Serialization converts byte status object series  These bytes are sent on the network saved and the object is re-created from these bytes  Example  public class Foo implements Serializable     private String attr1   private String attr2           Now IF YOU WANT TO do NOT TRANSFERT SAVED field of this object SO  you can use keyword transient   private transient attr2    Example

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My small contribution    What is a transient field  Basically  any field modified with the transient keyword is a transient field  Why are transient fields needed in Java  The transient keyword gives you some control over the serialization process and allows you to exclude some object properties from this process  The serialization process is used to persist Java objects  mostly so that their states can be preserved while they are transferred or inactive  Sometimes  it makes sense not to serialize certain attributes of an object  Which fields should you mark transient  Now we know the purpose of the transient keyword and transient fields  it s important to know which fields to mark transient  Static fields aren t serialized either  so the corresponding keyword would also do the trick  But this might ruin your class design  this is where the transient keyword comes to the rescue  I try not to allow fields whose values can be derived from others to be serialized  so I mark them transient  If you have a field called interest whose value can be calculated from other fields  principal  rate  amp  time   there is no need to serialize it  Another good example is with article word counts  If you are saving an entire article  there s really no need to save the word count  because it can be computed when article gets  deserialized   Or think about loggers  Logger instances almost never need to be serialized  so they can be made transient

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It s needed when you don t want to share some sensitive data that go with serialization

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A transient variable is a variable that may not be serialized    One example of when this might be useful that comes to mind is  variables that make only sense in the context of a specific object instance and which become invalid once you have serialized and deserialized the object  In that case it is useful to have those variables become null instead so that you can re-initialize them with useful data when needed

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The transient keyword in Java is used to indicate that a field should not be  part of the serialization  which means saved  like to a file  process   From the Java Language Specification  Java SE 7 Edition  Section 8 3 1 3  transient Fields      Variables may be marked transient to   indicate that they are not part of the   persistent state of an object    For example  you may have fields that are derived from other fields  and should only be done so programmatically  rather than having the state be persisted via serialization    Here s a GalleryImage class which contains an image and a thumbnail derived from the image   class GalleryImage implements Serializable       private Image image      private transient Image thumbnailImage       private void generateThumbnail                    Generate thumbnail             private void readObject ObjectInputStream inputStream              throws IOException  ClassNotFoundException               inputStream defaultReadObject            generateThumbnail                  In this example  the thumbnailImage is a thumbnail image that is generated by invoking the generateThumbnail method   The thumbnailImage field is marked as transient  so only the original image is serialized rather than persisting both the original image and the thumbnail image  This means that less storage would be needed to save the serialized object   Of course  this may or may not be desirable depending on the requirements of the system -- this is just an example    At the time of deserialization  the readObject method is called to perform any operations necessary to restore the state of the object back to the state at which the serialization occurred  Here  the thumbnail needs to be generated  so the readObject method is overridden so that the thumbnail will be generated by calling the generateThumbnail method   For additional information  the Discover the secrets of the Java Serialization API article  which was originally available on the Sun Developer Network  has a section which discusses the use of and presents a scenario where the transient keyword is used to prevent serialization of certain fields

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To allow you to define variables that you don t want to serialize    In an object you may have information that you don t want to serialize persist  perhaps a reference to a parent factory object   or perhaps it doesn t make sense to serialize  Marking these as  transient  means the serialization mechanism will ignore these fields

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Simply put  the transient java keyword protect fields from the been Serialize as their non-transient fields counter parts   In this code snippet our abstract class BaseJob implement Serializable interface  we extends from BaseJob but we need not serialize the remote and local data sources  serialize only organizationName and isSynced fields   public abstract class BaseJob implements Serializable     public void ShouldRetryRun        public class SyncOrganizationJob extends BaseJob       public String organizationName     public Boolean isSynced      Inject transient RemoteDataSource remoteDataSource      Inject transient LocalDaoSource localDataSource      public SyncOrganizationJob String organizationName         super new           Params BACKGROUND  groupBy GROUP  requireNetwork   persist            this organizationName   organizationName        this isSynced isSynced

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A field which is declare with transient modifier it will not take part in serialized process  When an object is serialized saved in any state   the values of its transient fields are ignored in the serial representation  while the field other than transient fields will take part in serialization process  That is the main purpose of the transient keyword

[java] Why does Java have transient fields?

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