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ObjectOutputStream and ObjectInputStream errors

Posted on 2009-07-06
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Last Modified: 2013-12-22
Hi,
Below are two snippets of code. The first one (starting with while(true)) is suppose to wait for a connection on a serversocket, get an ObjectOutputStream and ObjectInputStream for the socket it returns,  transmit an object of type "message", and get back the same type of object.

The second snippet is suppose to connect to the program running the first snippet, get a message, and transmit back a message. These two parts are suppose to work as mirror images of each other, IE the output stream of one is the input stream of the other.

The flow is as follows:
1 First snippet runs, waits for accept.
2 Second snippet runs, connects to the port where the first waits
3 First accepts, opens a socket, gets an ObjectOutputStream and sends a message
4 Second gets message from its ObjectInputStream (the other end of the first's output) and transmits back a message on its outputstream
5 First gets message from second on its InputStream which is the other end of the seconds's output stream

Basically, this is a handshake senario, however, when I run both programs, the first snippet gives the following error:
java.io.EOFException
      at java.io.ObjectInputStream$PeekInputStream.readFully(ObjectInputStream.java:2281)
      at java.io.ObjectInputStream$BlockDataInputStream.readShort(ObjectInputStream.java:2750)
      at java.io.ObjectInputStream.readStreamHeader(ObjectInputStream.java:780)
      at java.io.ObjectInputStream.<init>(ObjectInputStream.java:280)
      at dimes.comm2gui.GUIConnectorThread.run(GUIConnectorBean.java:165)
(line 165 is marked with ---> in the code below)

On the other side of the socket, the second snippet (which is suppose to receive the message after it connects)  gives this error:
java.lang.ClassNotFoundException: dimes.comm2gui.common.Message
      at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
      at java.security.AccessController.doPrivileged(Native Method)
      at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
      at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
      at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
      at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
      at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)
      at java.lang.Class.forName0(Native Method)
      at java.lang.Class.forName(Class.java:247)
      at java.io.ObjectInputStream.resolveClass(ObjectInputStream.java:604)
      at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1575)
      at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1496)
      at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1732)
      at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1329)
      at java.io.ObjectInputStream.readObject(ObjectInputStream.java:351)
      at dimes.comm2agent.AgentConnectorBean.initConenction(AgentConnectorBean.java:86)
(Line 86 is marked with a --->)

I assume both of these problems are part of the same basic issue. But I don't know what it is. From what I've been reading the trouble seems to be with the Object Header not being read completely or something of that nature. I attempted to flush the Outputstream after I start it, in the hope that it may transmit the header, but that didn't seem to work.

Any ideas?
Thanks

private ServerSocket comm;
	private Socket lComm;
	private GUIConnectorBean bean;
	private String xml;
	private ObjectOutputStream oOStream;
	private ObjectInputStream iOStream;
	private Message Current;
	private GUICommunicator Communicator;
 
while(true){
			try {
				
				lComm=comm.accept(); //Wait for connection from GUI
				oOStream = new ObjectOutputStream(lComm.getOutputStream());
				oOStream.flush();
				Message msg = new Message(MessageTypes.DIMESACK, -1, "DIMES", new Object[]{bean.getAgentVersion()});
				oOStream.writeObject(msg);
				
				InputStream iStream = lComm.getInputStream();
----->			iOStream = new ObjectInputStream(iStream);
				
				Current = (Message)iOStream.readObject();
				if (MessageTypes.DIMESACK==Current.getType());
				else continue;
				bean.setObjectOutputStream(oOStream);
				Communicator = GUICommunicator.getInstance();
				bean.setConnectionTimer();
				break;
				
				
			} catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (ClassNotFoundException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} 
		}
--------------------------------------------------------------------------------------------------------------
	private Socket commSocket = new Socket();
	private InetAddress localHost;
	private Logger logger;
	private Thread iP;
	private ObjectInputStream iOStream;
	private ObjectOutputStream oOStream;
 
 
				commSocket.connect(new InetSocketAddress(localHost, CONNECT_PORT),SOCKET_TIMEOUT);
				iOStream = new ObjectInputStream(commSocket.getInputStream());
				
--->				Message msg = (Message) iOStream.readObject();
				if (!(MessageTypes.DIMESACK==msg.getType())) return false;
				
				inputString = msg.getAttrib();
oOStream = new ObjectOutputStream(commSocket.getOutputStream());
					oOStream.writeObject(new Message(MessageTypes.DIMESACK, -1, "", null)); //Send an empty ACK message to Agent to finish handshake
					iP = new Thread (new inputProcessor(iOStream, logger)); 
					iP.start();
					return true;

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Question by:bored_shiva
2 Comments
 
LVL 16

Accepted Solution

by:
imladris earned 500 total points
ID: 24785985
The critical error looks to be the ClassNotFound exception for Message. This causes the receiver to not run at all; which causes the sender to shutdown with an End Of File.

A ClassNotFound exception is an indication that the JVM can't find the class it needs; Message in this case. The JVM looks at each directory on the classpath. The classpath is specified by an environment variable, or as a commandline argument. Also, the JVM shows the Message class as being in a package:

dimes.comm2gui.common.Message

so it will be looking at each directory in the class path, and then for subdirectories of dimes\comm2gui\common and hoping to find a Message.class file there. The exception indicates that it has failed to find the needed class.
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Author Closing Comment

by:bored_shiva
ID: 31600123
I actually ended up finding this out myself, but you're absolutely right. I had a copy of the class in a different location, with the proper include and all, but still apparently not enough to satisfy the JVM. Once I changed the location to be identical in both places, the program(s) worked like a charm.  
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