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Thinking in Java
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Finding the class

You might wonder what’s necessary for an object to be recovered from its serialized state. For example, suppose you serialize an object and send it as a file or through a network to another machine. Could a program on the other machine reconstruct the object using only the contents of the file?

The best way to answer this question is (as usual) by performing an experiment. The following file goes in the subdirectory for this chapter:

//: c12:Alien.java
// A serializable class.
import java.io.*;
public class Alien implements Serializable {} ///:~


The file that creates and serializes an Alien object goes in the same directory:

//: c12:FreezeAlien.java
// Create a serialized output file.
// {Clean: X.file}
import java.io.*;

public class FreezeAlien {
  // Throw exceptions to console:
  public static void main(String[] args) throws Exception {
    ObjectOutput out = new ObjectOutputStream(
      new FileOutputStream("X.file"));
    Alien zorcon = new Alien();
    out.writeObject(zorcon);
  }
} ///:~


Rather than catching and handling exceptions, this program takes the quick-and-dirty approach of passing the exceptions out of main( ), so they’ll be reported on the console.

Once the program is compiled and run, it produces a file called X.file in the c12 directory. The following code is in a subdirectory called xfiles:

//: c12:xfiles:ThawAlien.java
// Try to recover a serialized file without the
// class of object that's stored in that file.
// {ThrowsException}
import java.io.*;

public class ThawAlien {
  public static void main(String[] args) throws Exception {
    ObjectInputStream in = new ObjectInputStream(
      new FileInputStream(new File("..", "X.file")));
    Object mystery = in.readObject();
    System.out.println(mystery.getClass());
  }
} ///:~


Even opening the file and reading in the object mystery requires the Class object for Alien; the JVM cannot find Alien.class (unless it happens to be in the Classpath, which it shouldn’t be in this example). You’ll get a ClassNotFoundException. (Once again, all evidence of alien life vanishes before proof of its existence can be verified!) The JVM must be able to find the associated .class file.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire