Java Transient Keyword Tutorial – A transient variable is a variable that can not be serialized

If you have a requirement to serialize an object then you have an option to skip serialize specific field by marking it as transient.

transient is a Java keyword which marks a member variable not to be serialized when it is persisted to stream of bytes.

When do you need to serialize an object?

When a Java object is transferred through the network, the object needs to be serialized. Serialization converts the object state to serial bytes.

Let’s take a look at transient Java example

1. First let’s create a POJO object CrunchifyTransientVariableObject which implements Serializable.
2. Create main class CrunchifyJavaTransientFieldTutorial.
   • Create an Object.
   • Write an Object to file. A file output stream is an output stream for writing data to a File or to a FileDescriptor.
   • Read an Object.
   • If field is transient – it will return 0 (for int), null (for string field) and so on.

CrunchifyTransientVariableObject.java

package crunchify.com.tutorial;

import java.io.Serializable;

// Serializability of a class is enabled by the class implementing the java.io.Serializable interface.
// 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 . describes best practices for defensive use of serial filters.
public class CrunchifyTransientVariableObject implements Serializable {

    private static final long serialVersionUID = 1L;

    private String companyName;

    private int employeeCount;

    // A transient variable is a variable that can not be serialized.
    // Variables may be marked transient to indicate that they are not part of the persistent state of an object.

    private transient int homeZip;
    // private int homeZip;

    public String getName() {
        return companyName;
    }

    public void setName(String companyName) {
        this.companyName = companyName;
    }

    public int getEmployeeCount() {
        return employeeCount;
    }

    public void setEmployeeCount(int employeeCount) {
        this.employeeCount = employeeCount;
    }

    public int getZip() {
        return homeZip;
    }

    public void setZip(int homeZip) {
        this.homeZip = homeZip;
    }

    public CrunchifyTransientVariableObject(String companyName, int employeeCount, int homeZip) {
        this.companyName = companyName;
        this.employeeCount = employeeCount;
        this.homeZip = homeZip;
    }

}

Here homeZip is a transient variable.

CrunchifyJavaTransientFieldTutorial.java

package crunchify.com.tutorial;

import java.io.*;

/**
 * @author Crunchify.com
 *
 *         Java Transient Keyword Tutorial - A transient variable is a variable that can not be serialized.
 *
 */
public class CrunchifyJavaTransientFieldTutorial {

    public static void main(String[] args) {

        CrunchifyTransientVariableObject crunchifyCompany = new CrunchifyTransientVariableObject("Crunchify.com", 5, 95112);

        // A file output stream is an output stream for writing data to a File or to a FileDescriptor.
        try (FileOutputStream crunchifyFOS = new FileOutputStream("crunchify.obj");
             ObjectOutputStream crunchifyOOS = new ObjectOutputStream(crunchifyFOS)) {
            crunchifyOOS.writeObject(crunchifyCompany);
            crunchifyOOS.flush();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        CrunchifyTransientVariableObject crunchifyResult = null;

        // A FileInputStream obtains input bytes from a file in a file system. What files are available depends on the host environment.
        try (FileInputStream crunchifyFOS = new FileInputStream("crunchify.obj");
             ObjectInputStream crunchifyOIS = new ObjectInputStream(crunchifyFOS)) {
            crunchifyResult = (CrunchifyTransientVariableObject) crunchifyOIS.readObject();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }

        System.out.println("Company Name: " + crunchifyResult.getName());
        System.out.println("Number of Employee: " + crunchifyResult.getEmployeeCount());
        System.out.println("Zip code: " + crunchifyResult.getZip());
    }

}

Run program:

With transient keyword:

/Library/Java/JavaVirtualMachines/jdk-14.0.2.jdk/Contents/Home/bin/java -javaagent
Company Name: Crunchify.com
Number of Employee: 5
Zip code: 0

Process finished with exit code 0

Without transient keyword:

Remove transient keyword in above program and re-run a program again.

/Library/Java/JavaVirtualMachines/jdk-14.0.2.jdk/Contents/Home/bin/java -javaagent
Company Name: Crunchify.com
Number of Employee: 5
Zip code: 95112

Process finished with exit code 0

As you see above, now we were able to see zip code 95112.

Please let me know if you see any issue serializing an object and transient variable.