When Java class is instance of Serializable

Question

I wonder when Java Class is instance of Serializable. As I know class is serializable only if it implements Serializable interface. I'm trying to use junit to generate entity class (from some kind of template) and check if it is serializable.

My generated class (which does NOT implement Serializable) looks like:

package test;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Table;
import javax.annotation.Generated;
import javax.persistence.Id;


@Entity
@Table( name = MyTestTableDto.TABLE_NAME )
public class MyTestTableDto 
{
    public static final String TABLE_NAME = "MY_TEST_TABLE";

    public static final String COLUMN_MY_ID_FIELD = "MY_ID_FIELD";
    public static final String FIELD_MY_ID_FIELD = "myIdField";

    @Id
    @Column( nullable = true, length = 14, name = COLUMN_MY_ID_FIELD )
    private Long myIdField;

    public Long getMyIdField()
    {
        return myIdField;
    }

    public void setMyIdField( Long aMyIdField )
    {
        this.myIdField = aMyIdField;
    }

}

Test is below:

 File generatedFile = new File( GEN_OUTPUT_DIR, File.separator + className + ".java" );

 assertTrue( generatedFile.getClass() instanceof Serializable ); //returns true

The result suggest that my generated class is instance of Serializable. My question is why? I think if it does not implements Serializable it should not be instance of it. I searched for an answer but I couldnt find anything.

Indispose

Answer 1

You do this:

File generatedFile = new File(...);

This created an instance of the class File.

generatedFile.getClass()

This got the Class object for the File object, an object of type Class<File>

The documentation of File describes the class:

All Implemented Interfaces: Serializable, Comparable

If you'd like to achieve that you want to parse a java source file to see if it implements Serializable, you should get an appropriate tool for this, build the abstract syntax tree of the file in question, and extract the information from there.

Or you could just do it like this, using the isAssignableFrom method of the Class class:

public boolean checkSerializable(Class<?> classToCheck) {
    return Serializable.class.isAssignableFrom(classToCheck);
}

And then check the class itself:

boolean isSerializable = checkSerializable(MyTestTableDto.class);

Intestinal

Answer 2

This is because you call instanceof on Class object, which is serializable.

getClass returns a Class<T> object, where T is the type of the class. Because Class is defined as

public final
class Class<T> implements java.io.Serializable,
                          java.lang.reflect.GenericDeclaration,
                          java.lang.reflect.Type,
                          java.lang.reflect.AnnotatedElement 
{
  ...
}

it's clearly serializable.

Empoison

The others are right. In his test he instanciates an Object of Type File and checks if that instance of File is Serializable or not. He does not check his own implementation – Feininger

@Feininger : generatedFile.getClass() returns a Class object. That specific Class object is tested with instanceof. The instanceof sees Class<File>, which is serializable, therefore it returns true. To test if the file is serializable, you need to type generatedFile instanceof Serializable, not generatedFile.getClass() instanceof Serializable. – Empoison

Answer 3

The statement

assertTrue( generatedFile.getClass() instanceof Serializable );

is returning true, because this will check whether the runtime class of this generatedFile Object is an instance of Serializable or not. And the class object is serializable.

Draught

Answer 4

From your code, the below call will return Class instance for File object, not MyTestTableDto

generatedFile.getClass()

(removed the wrong statement)

Carburetor