Java - ObjectOutputStream putFields() method

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Description

The Java ObjectOutputStream putFields() method retrieves the object used to buffer persistent fields to be written to the stream. The fields will be written to the stream when writeFields method is called. putFields method is used inside a custom writeObject() method to manually define what fields should be serialized - instead of relying on default serialization.

• It gives you an ObjectOutputStream.PutField object.

• You can explicitly set values for the fields you want to serialize.

• It allows partial control of what gets written to the stream, useful in versioning or for skipping fields.

Declaration

Following is the declaration for java.io.ObjectOutputStream.putFields() method.

public ObjectOutputStream.PutField putFields()

Parameters

NA

Return Value

This method returns an instance of the class Putfield that holds the serializable fields.

Exception

IOException − If an I/O error has occurred.

Example - Usage of ObjectOutputStream putFields() method

The following example shows the usage of ObjectOutputStream putFields() method.

ObjectOutputStreamDemo.java

package com.tutorialspoint;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

public class ObjectOutputStreamDemo {
   public static void main(String[] args) {
      int i = 319874;
      
      try {
         // create a new file with an ObjectOutputStream
         FileOutputStream out = new FileOutputStream("test.txt");
         ObjectOutputStream oout = new ObjectOutputStream(out);

         // write something in the file
         oout.writeInt(i);
         oout.writeInt(1653984);

         // flush the stream
         oout.flush();

         // close the stream
         oout.close();

         // create an ObjectInputStream for the file we created before
         ObjectInputStream ois = new ObjectInputStream(new FileInputStream("test.txt"));

         // read and print an int
         System.out.println("" + ois.readInt());

         // read and print an int
         System.out.println("" + ois.readInt());
      } catch (Exception ex) {
         ex.printStackTrace();
      }
   }

   static public class Example implements Serializable {

      private void writeObject(ObjectOutputStream out) throws IOException {

         // write into the ObjectStreamField array the variable string
         ObjectOutputStream.PutField fields = out.putFields();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result −

319874
1653984

Example - Custom serialization using putFields() to exclude a field

The following example shows the usage of ObjectOutputStream putFields() method.

ObjectOutputStreamDemo.java

package com.tutorialspoint;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

public class ObjectOutputStreamDemo {
   public static void main(String[] args) throws IOException, ClassNotFoundException {
      try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("put1.ser"))) {
         Person person = new Person("Alice", 30);
         oos.writeObject(person);

         ObjectInputStream ois = new ObjectInputStream(new FileInputStream("put1.ser"));
         Person p1 = (Person)ois.readObject();
         System.out.println("Name= " +  p1.name + " age=" + p1.age);
      }
   }

   static class Person implements Serializable {
      private static final long serialVersionUID = 1L;

      String name;
      int age;

      public Person(String name, int age) {
         this.name = name;
         this.age = age;
      }

      private void writeObject(ObjectOutputStream oos) throws IOException {
         ObjectOutputStream.PutField fields = oos.putFields();
         fields.put("name", name);   // Only write name
         // Skip age
         oos.writeFields();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

Name= Alice age=0

Explanation

• We only serialize the name field and intentionally exclude age. That is why, after de-serialization, output shows 'age=0'.

• This is useful when you want to deprecate a field or hide it from future serialized output.

Example - Version-tolerant serialization - write extra placeholder field

The following example shows the usage of ObjectOutputStream putFields() method.

ObjectOutputStreamDemo.java

package com.tutorialspoint;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

public class ObjectOutputStreamDemo {
   public static void main(String[] args) throws IOException, ClassNotFoundException {
      try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("put2.ser"))) {
         Book book = new Book("Java Basics");
         oos.writeObject(book);

         ObjectInputStream ois = new ObjectInputStream(new FileInputStream("put2.ser"));
         Book b1 = (Book)ois.readObject();
         System.out.println(" New field 'version' added in writeObject. After deserialization, 'version' =  " + b1.version);
      }
   }

   static class Book implements Serializable {
      private static final long serialVersionUID = 1L;

      String title;
      int version = 2; // newer field

      public Book(String title) {
         this.title = title;
      }

      private void writeObject(ObjectOutputStream oos) throws IOException {
         ObjectOutputStream.PutField fields = oos.putFields();
         fields.put("title", title);
         fields.put("version", version); // manually add new field
         oos.writeFields();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

New field 'version' added in writeObject. After deserialization, 'version' =  2

Explanation

• We manually add a new version field for backward/forward.