从ObjectInputStream读取一个对象（自定义）

Question

So, I don't know why the Client part of my project give me this type of error

Exception in thread "main" java.lang.ClassNotFoundException: Mining.FrequentPatternMiner at java.net.URLClassLoader$1.run(Unknown Source) at java.net.URLClassLoader$1.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at java.lang.Class.forName0(Native Method) at java.lang.Class.forName(Unknown Source) at java.io.ObjectInputStream.resolveClass(Unknown Source) at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source) at java.io.ObjectInputStream.readClassDesc(Unknown Source) at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source) at java.io.ObjectInputStream.readObject0(Unknown Source) at java.io.ObjectInputStream.readObject(Unknown Source) at JabberClient.main(JabberClient.java:81)

In the Server part, I cast the FrequentPatternMiner variable and then i give it to the ObjectOutputStream

ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());

FrequentPatternMiner fpMiner=new FrequentPatternMiner(dataTarget, minsup);  
fpMiner.salva("FP_"+nameFile+"_minSup"+minsup+".dat");
System.out.println("Frequent Patterns \n"+fpMiner);

out.flush();
out.writeObject((Object)fpMiner);

In the Client part, i do this

ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());
ObjectInputStream in = new ObjectInputStream(socket.getInputStream());

Object fpMiner=in.readObject();
(((ObjectInput)in).readObject());
System.out.println(fpMiner);

How would I fix my code? Can you help me?

Answer 1

The problem you have is that FrequentPatternMiner is not on your classpath.

What you can do is ctrl+t on Eclipse and look for FrequentPatternMiner to know what is the jar containing this class. Then ensure that your project is referencing this class.

If you are using maven you can use mvm dependency:tree goal to know if your project contain this jar.

Answer 2

The code is correct, but your premise isn't. You've said, that the class " FrequentPatternMiner must be unknown for the client". With that premise you can't serialize the class as you did, because serialization only transfers the data of the object and not it's implementation. You might want to look into DataTransferObjects (which has to be known on both client and server) or use a simple array to transfer the object.

Example using a simple array as "DTO"

Server:

ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());

// The following line assumes that dataTarget and minsup are types 
// that are serializable and known to the client
Object[] objArray = new Object[] { dataTarget, minsup };

out.flush();
out.writeObject(objArray);

Client:

ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());
ObjectInputStream in = new ObjectInputStream(socket.getInputStream());

Object[] objArray = (Object[])in.readObject();
System.out.println(objArray);

// FrequentPatternMiner must still known to the client, if you need more than 
// the constructor arguments (like methods of the FrequentPatternMiner) Object.
FrequentPatternMiner fpMiner=new FrequentPatternMiner(objArray[0], objArray[1]);

Further Idea

If you don't want that FrequentPatternMiner implementation/class is known to the client, you can try "Remoting" or "Remote Procedure Calls" (RPC). This is an advanced subject and there are a lot of libraries out there (and platforms) that provide this functionallity. They do basically this:

• You need an Interface for the Class on the client (like interface IPatternMiner { void doMine() }
• You connect server and client (depends on the library on how to do this) and obtain an instance of IPatternMiner
• You call a method on the interface and the implementation gets executed on the server (with the parameters pased in from the client). Note that file operations write on the file system on the server

Answer 3

Casting to Object doesn't have any magical properties. Everything can be cast to Object. It doesn't change the fact that what is written to the object stream is a FrequentPatternMiner, and if that class isn't available at the receiver, this exception will result.

Solution: deploy the class.

Similarly, casting an ObjectInputStream to ObjectInput accomplishes precisely nothing.

Answer 4

You need to tyecast.

System.out.println((FrequentPatternMiner )fpMiner);

Else how would the client know class of the Object that is being deserialized. Note it is not necessary to typecast just for using it in sysout . It will print using Objects toString() method.

Though your Exception says

Exception in thread "main" java.lang.ClassNotFoundException

You need to add this class on the client side too. Lastly check that serialVersionUID is same for the class in server and client.