Java - ObjectStreamClass lookup(Class<?> cl) method

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Description

The Java ObjectStreamClass lookup(Class<?> cl) method finds the descriptor for a class that can be serialized. Creates an ObjectStreamClass instance if one does not exist yet for class. Null is returned if the specified class does not implement java.io.Serializable or java.io.Externalizable.

• ObjectStreamClass.lookup(Class<?> c) returns an ObjectStreamClass that describes how the class c will be serialized.

• If the class doesn't implement Serializable, it returns null.

• You can then inspect things like −

  • serialVersionUID

  • serializable fields

  • class name

  • class reference

Declaration

Following is the declaration for java.io.ObjectStreamClass.lookup(Class<?> cl) method.

 public static ObjectStreamClass lookup(Class<?> cl) 

Parameters

cl − class for which to get the descriptor.

Return Value

This method returns the class descriptor for the specified class.

Exception

NA

Example - Usage of ObjectStreamClass lookup(Class<?> cl) method

The following example shows the usage of ObjectStreamClass lookup(Class<?> cl) method.

ObjectStreamClassDemo.java

 package com.tutorialspoint; import java.io.ObjectStreamClass; import java.util.Calendar; public class ObjectStreamClassDemo { public static void main(String[] args) { // create a new object stream class for Integers ObjectStreamClass osc = ObjectStreamClass.lookup(Integer.class); // get the name for Integers System.out.println("" + osc.getName()); // create a new object stream class for Calendar ObjectStreamClass osc2 = ObjectStreamClass.lookup(Calendar.class); // get the name for Calendar System.out.println("" + osc2.getName()); } } 

Output

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

 java.lang.Integer java.util.Calendar 

Example - Lookup a user-defined class and print its serialVersionUID

The following example shows the usage of ObjectStreamClass lookup(Class<?> cl) method. We're checking the serialVersionUID and confirm that the class is serializable.

ObjectStreamClassDemo.java

 package com.tutorialspoint; import java.io.ObjectStreamClass; import java.io.Serializable; public class ObjectStreamClassDemo { public static void main(String[] args) { ObjectStreamClass osc = ObjectStreamClass.lookup(Employee.class); if (osc != null) { System.out.println("Class: " + osc.getName()); System.out.println("serialVersionUID: " + osc.getSerialVersionUID()); } else { System.out.println("Employee is not serializable."); } } static class Employee implements Serializable { private static final long serialVersionUID = 100L; String name; int id; } } 

Output

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

 Class: com.tutorialspoint.ObjectStreamClassDemo$Employee serialVersionUID: 100 

Explanation

• lookup(Employee.class) returns metadata for the Employee class.

• This includes the name and serialVersionUID.

Example - Lookup a non-serializable class and handle the null result

The following example shows the usage of ObjectStreamClass lookup(Class<?> cl) method. We're detecting whether a class can be serialized.

ObjectStreamClassDemo.java

 package com.tutorialspoint; import java.io.ObjectStreamClass; public class ObjectStreamClassDemo { public static void main(String[] args) { ObjectStreamClass osc = ObjectStreamClass.lookup(Config.class); if (osc == null) { System.out.println("Config is NOT serializable."); } else { System.out.println("Config is serializable with UID: " + osc.getSerialVersionUID()); } } // Not implementing Serializable static class Config { String key; String value; } } 

Output

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

 Config is NOT serializable. 

Explanation

• Since Config doesn't implement Serializable, lookup() returns null.

• Always a good idea to check before proceeding with serialization logic.