Final serialVersionUID SERIALVERSIONUID?
As the Java coding convention suggest, all final fields should be named with CAPITAL LETTERS. But serialVersionUID has lower case letters even it has to be final ... is Sun not using its own convention?
The convension was written after the name for the field was chosen.
Many features which were in version 1.0 including Vector, Hashtable, Enumeration don't sit will with convensions developed later.
Similar Messages
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Trying to understand serialVersionUID
so I'm coding up my own event using Eclipse (for pretty much the first time), and it gives me a warning of
The serializable class CaptureEvent does not declare a static final serialVersionUID field of type long
now ... first I notice that it's a warning, not an error. second, after looking up stuff on the api, it appears that the serialVersionUID can be any number I want - as long as it's unique? I asked a friend what the point of it is, and he said it's for objects that implement the Serializable interface to guarantee that it can only be accessed by one object at a time. (useful for threads, mostly, to prevent deadlock situations and such) That makes perfect sense to me, but I'm not sure how to go about implementing it. Do I simply have to make up any long I want? If that's the case, how do I know the one I pick will be unique? Does it need to be declared as public?
public static final long serialVersionUID = <anyLongIWant>L;
Please help me to understand this better. ^.^now ... first I notice that it's a warning, not an
error. second, after looking up stuff on the api, it
appears that the serialVersionUID can be any number I
want - as long as it's unique? it can be any number you want. it is used as a version number for (de)serialization. if you deserialize an object and the version of the serialized form is different from the one in the current class, you will get an error. you should start with any value, and change it only if you change your class to be incompatible with previously serialized versions, in other words when you want to get an error
I asked a friend what
the point of it is, and he said it's for objects that
implement the Serializable interface to guarantee
that it can only be accessed by one object at a time.
(useful for threads, mostly, to prevent deadlock
situations and such)that is absolute bollocks - this has nothing to do with threading. your friend is full of it
That makes perfect sense to
me,if you have indeed read the api docs, you should know that it makes no sense at all
but I'm not sure how to go about implementing
it. Do I simply have to make up any long I want?
If that's the case, how do I know the one I pick
will be unique? Does it need to be declared as
public?i think you should read the API docs again. -
SerialVersionUID in abstract classes
Hi,
i have a Question about the serialVersionUID for abstract classes.
first i will describe a simple example to show my problem.
abstract class A implements Serializable {
private int value;
public A(int value) {
this.value = value;
protected abstract void doSomething();
class B extends A {
private static final serialVersionUID = 8373629029L;
public B(int value) {
super(value);
protected void doSomething() {
//do something
} B is a ValueObjects on a Server and transmitted to a Client Application.
When i change the abstract super class A i get an exception on the client because the serialVersionUID does not mathc to the Client anymore
So i changed the " implements Serializable" Statement down to B. And tried the same. Then i get an InvalidClassException because with following hint "no valid constructor". So i also have to make A Serializable and also need a defined serialVersionUID for A.
I think there is something i don't unterstand in the serialization Mechanism in Java.
Why does it makes sense to adda serialVersionUID to an abstract class? And how do i generate the UID with the "serialver" tool deliversd with the JDK form a class which i cannot instanciate?
Edited by: kbj on Jan 23, 2009 7:14 AMkbj wrote:
So i would like to know what is the best practise in such a case?Classes designed for inheritance should rarely implement Serializable, and interfaces should rarely extend it. If a class or interface exists primarily to participate in a framework that requires all participants to implement Serializable then it makes sense to violate this rule.
So in your case you may want to provide a protected parameterless constructor and a protected initialization method if a client provided invariant is required ("value" in your case). Then all public or protected instance methods need to call a private method that checks that the class has been initialized (i.e. the subclass has been written so that it calls the protected initialization method and passes in the "value" invariant). If not then you should throw an IllegalStateException. The field you use to flag that the object has been initialized should be of type java.util.concurrent.atomic.AtomicReference.
The above is taken pretty much verbatim from chapter 11 of effective java second edition. The book also provides an example of the above you could use as a template. -
hey there!
I opened a file with Eclipse SDK 3.2.2, and a certain warning appears:
The serializable class xxx does not declare a static final serialVersionUID field of type long.
After hitting Ctrl-1, a few options appeared, one of them being Add generated serial version ID.
Clicking that, a new line appeared into my class:
private static final long serialVersionUID = -318066740353445032L;Well ,I thought that with this done, I solved the problem, but the warning still remains there..... what can I do??Answer found:
I had to rebuild that file again so Eclipse could interpret that problem as "solved". Easy as pie :) -
SerialVersionUID field warning issue
Hi all,
i'm using the jdk1.5.0 compiler in my java developement under Eclipse platform.
However i keep getting a strange warning on approximately each class in my applications:
The serializable class "MyClass" does not declare a static final serialVersionUID field of type long .
why i'm i getting this warning and what is it about ?
thanks.thanks for the two interesting replies : that solved the problem.
I need your further advice about warning issues :
i wrote an application in which i'm implementing dynamic java compilation using eclipse jdt package..
for warning settings i'm using :
String warningSetting = new String("allDeprecation,"
+ "assertIdentifier," + "charConcat,"
+ "conditionAssign," + "constructorName," + "deprecation,"
+ "fieldHiding," + "finalBound,"
+ "finally," + "indirectStatic," + "intfNonInherited,"
+ "localHiding," + "maskedCatchBlocks,"
+ "noEffectAssign," + "pkgDefaultMethod,"
+ "semicolon," + "specialParamHiding," + "staticReceiver,"
+ "syntheticAccess,"
+ "unnecessaryElse," + "uselessTypeCheck," + "unsafe,"
+ "unusedImport,"
+ "unusedThrown");however the client is complainig that the warning level is too high...
what are the warnings from above list that are not very critical so i can remove them to reduce warning level ?
thank you. -
Why should i declare a variable serialVersionUID?
hello all,
the following code giving warning saying that
The serializable class newFrame does not declare a static final
serialVersionUID field of type long.
why should i need to declare a static final
serialVersionUID variable? is their any resason
public class newFrame extends JFrame{
public ContentPaneTest() {
super("Content pane");
getContentpane().add(new JLabel("hello all"););
setVisible(true);
pack();
setDefaultCloseOperation(EXIT_ON_CLOSE);
public static void main(String args[]){
new newFrame();
}thanks
dayatwo options:
1) You could explicitly specify a serialVersionUID to remove the warnings.
2) If you just want to remove those specific warnings, you could do so by changing the compiler settings of your IDE (for Eclipse, "Potential programming problems" / "Serializable class without serialVersionUID").
serialVersionUID is used during deserialization to verify that the sender and receiver of a serialized object have compatible corresponding classes (for that object).
If you don't care about serialization, 2) would be your choice. -
ERROR: serializable class HelloComponent does not declare a static final
Hi everyone! I'm sorry but I'm a newbie to Java and I'm having some, I assume, basic problems learning to compile and run with javac. Here is my code:
import javax.swing.* ;
import java.awt.* ;
public class MyJava {
* @param args
public static void main(String[] args)
// TODO Auto-generated method stub
JFrame frame = new JFrame( "HelloJava" ) ;
HelloComponent hello = new HelloComponent() ;
frame.add( hello ) ;
frame.setSize( 300, 300 ) ;
frame.setVisible( true ) ;
class HelloComponent extends JComponent
public void paintComponent( Graphics g )
g.drawString( "Hello Java it's me!", 125, 95 ) ;
And here is the error:
1. WARNING in HelloJava.java
(at line 20)
class HelloComponent extends JComponent {
^^^^^^^^^^^^^^
The serializable class HelloComponent does not declare a static final serialVersionUID field of type long
ANY HELP WOULD BE GREAT! THANKS! =)Every time I extend GameLoop, it gives me the warning
serializable class X does not declare a static final serialVersionUID field of type long
This is just a warning. You do not need to fix this because the class you are writing will never be serialized by the gaming engine. If you really want the warning to go away, add the code to your class:
static final long serialVersionUID=0; -
The serializable class SpacePainter does not declare a static final serial
The serializable class SpacePainter does not declare a static final serialVersionUID field of type longWhat does this mean??? It appears as a warning in Eclipse but I have no idea what it is. It happens when I create a class the extends JFrame or JCompnent or JApplet. I finnally got it to stop with this:
static final long serialVersionUID = 1;Because your eclipse is configured that way. You can probably filter the warning. You don't have to implement the serialVersionUID, but you should if you really serialize the exceptions.
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Serializable class doesn�t declare static final serialVersion of type long
The serializable class SomeClassName does not declare a static final serialVersionUID field of type long
I have a lot of these warnings in my current application.
What does it really mean?
In what situations do I actually want/need to use this?
If I need it somehwere, do I just declare this field?Putting a version number on a serializable class is a precaution against the class definition changing in a way that confuses ObjectStreams when reading old serialisations.
You can just declare the varialble, initially make it 1 and crank it up in your source when you change the fields of the class. -
Stupid Question: Suppress Warnings vs Fixing What Ain't Broken
Hey all,
I'm getting about 70 warnings in my code, mostly because I'm using anonymous classes to set up Actions as shown in the code snip below. This generates the warning "The serializable class does not declare a static final serialVersionUID field of type long" every time I create an Action this way:
final Action defaultUpAction = am.get(im.get(KeyStroke.getKeyStroke("control UP")));
Action upAction = new AbstractAction(){ //warning generated here
public void actionPerformed(ActionEvent e){
defaultUpAction.actionPerformed(e);
lastAction = "UP";
};I understand what's causing the warning, and I even know how to fix it (or ignore the warning).
My question is, which is less of a hack? Do I just throw in a few SuppressWarnings annotations, or should I add a default serialVersionUID in each anonymous class? I'm not using the serialization functionality, but I might want to sometime in the future in which case suppressing the warnings would be a bad idea. Both solutions (adding a field I don't need or suppressing warnings instead of fixing them) seem a bit kludgey to me. Am I just being too touchy, or is there a better way?
ThanksThanks for the input, that does make quite a bit of sense. I suppose my hesitation comes from the feeling that although I was told by the higher-ups to "fix those warnings", all I'm doing is concealing them. I guess I shouldn't look at it that way, especially since I don't use serialization at all in any of my code (nobody else does, either). So in this case, simply suppressing the warnings probably makes more sense than adding fixes that won't be used anyway (and in fact may mislead future developers into thinking I'm using serialization somewhere).
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So im just starting java, about 12 weeks in.
So in my class we are getting started with objects. What i want to do is create a class called car, and have it create a new car and draw images loaded, and then have the tires rotate, there are 2 images,
the car base and the tires.
I know how to display images in applets, but i dont know how to define the images in the objects class and then draw them from the class.I get errors with this code from my car class
import java.applet.*;
public class car extends Applet{
* @param args
private String model;
private int passangers;
private double gas,speed;
private Image tire;
private MediaTracker tr;
tr = new MediaTracker(this);
tire = getImage(getCodeBase(), "tire.png");
tr.addImage(tire,0);
public car(String id, int pass, double tank)
model=id;
passangers=pass;
gas=tank;
public car()
model="";
passangers=0;
gas=0;
}I get errors on these lines
private Image tire;
private MediaTracker tr;
tr = new MediaTracker(this);
tire = getImage(getCodeBase(), "tire.png");
tr.addImage(tire,0);errors
Severity and Description Path Resource Location Creation Time Id
Image cannot be resolved to a type Car Game car.java line 13 1197077768237 1745
MediaTracker cannot be resolved to a type Car Game car.java line 14 1197077768238 1746
Return type for the method is missing Car Game car.java line 16 1197077768239 1750
Syntax error on token ";", { expected after this token Car Game car.java line 14 1197077768238 1747
Syntax error on token "(", delete this token Car Game car.java line 17 1197077768239 1752
Syntax error on token "0", invalid FormalParameter Car Game car.java line 17 1197077768239 1753
Syntax error on token(s), misplaced construct(s) Car Game car.java line 15 1197077768238 1748
Syntax error on token(s), misplaced construct(s) Car Game car.java line 16 1197077768239 1749
Syntax error on tokens, delete these tokens Car Game car.java line 16 1197077768239 1751
The serializable class car does not declare a static final serialVersionUID field of type long Car Game car.java line 4 1197077768237 1744
The serializable class main does not declare a static final serialVersionUID field of type long Applet main.java line 5 1196990860997 1682
The serializable class main does not declare a static final serialVersionUID field of type long Car Game main.java line 5 1197077768196 1743
The serializable class ShowImage does not declare a static final serialVersionUID field of type long Display JPEG/src ShowImage.java line 4 1196488911896 1393Edited by: jasonpeinko on Dec 7, 2007 5:42 PM -
Compiling with javac command gives error msg
HI,
I'm working on a linux based system and I need java for programming projects but when i want to compile a project, I get the error:
1. WARNING in Og05_08.java
(at line 8)
public class Og05_08 extends Applet implements ActionListener
^^^^^^^
The serializable class Og05_08 does not declare a static final serialVersionUID field of type long
1 problem (1 warning)I think it has something to do with the compiler and not with the programming.It's a warning, not an error. From the API:
The serialization runtime associates with each serializable class a version number, called a serialVersionUID, which is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization. If the receiver has loaded a class for the object that has a different serialVersionUID than that of the corresponding sender's class, then deserialization will result in an InvalidClassException. A serializable class can declare its own serialVersionUID explicitly by declaring a field named "serialVersionUID" that must be static, final, and of type long:
ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L;
If a serializable class does not explicitly declare a serialVersionUID, then the serialization runtime will calculate a default serialVersionUID value for that class based on various aspects of the class, as described in the Java(TM) Object Serialization Specification. However, it is strongly recommended that all serializable classes explicitly declare serialVersionUID values, since the default serialVersionUID computation is highly sensitive to class details that may vary depending on compiler implementations, and can thus result in unexpected InvalidClassExceptions during deserialization. Therefore, to guarantee a consistent serialVersionUID value across different java compiler implementations, a serializable class must declare an explicit serialVersionUID value. It is also strongly advised that explicit serialVersionUID declarations use the private modifier where possible, since such declarations apply only to the immediately declaring class--serialVersionUID fields are not useful as inherited members.
~ -
HashTable cannot be resolved to a Type
Hey Guys,
I want to use a HashTable to assign Integer object values to the to String keys defined by the getActionCommand() methods of an array of JButtons. Now I tried to construct a HashTable like this:
private HashTable<String, Integer> commandParser = new HashTable<String, Integer>();Eclipse gives me the error "HashTable cannot be resolved to a Type" .
I thought I had done it all as was stated in the APIs. What?s wrong here?
Sum1 hlp pls i cant figger ths oot ;-)
EDIT: While I?m here, I get a warning:
The serializable class BuildPane does not declare a static final serialVersionUID field of type long
for every one of my classes.
What is that about? I ignored it because it?s just a warning, but always safe, never sorry.
Edited by: AikmanAgain on Mar 29, 2008 5:29 AMAikmanAgain wrote:
Hey Guys,
I want to use a HashTable to assign Integer object values to the to String keys defined by the getActionCommand() methods of an array of JButtons. Now I tried to construct a HashTable like this:
private HashTable<String, Integer> commandParser = new HashTable<String, Integer>();Eclipse gives me the error "HashTable cannot be resolved to a Type" .
I thought I had done it all as was stated in the APIs. What?s wrong here?There is no class called HashTable in the JRE. Look again ... and remember Java is case sensitive.
The serializable class BuildPane does not declare a static final serialVersionUID field of type long
for every one of my classes.
What is that about? I ignored it because it?s just a warning, but always safe, never sorry.The serialVersionUID is a field that's used in serialization (as the name and the warning should have suggested).
Google for any serialization tutorial and you'll learn what it's good for. -
Hi,
I m new to j2ee
Plz help me
Whenever i run the servlet on eclipse it shows the error on the class name,
The error is below
The serializable class LoginForm does not declare a static final serialVersionUID field of type long
Some people said that on http://www.java.nnseek.com/search.php?q=serialVersionUID I can find about this problem. But this is a news conferences and I haven't time to find it.As stated, this is a warning that you can safely ignore, if you choose to do so.
serialVersionUID is meant for version maintenance in case you actually serialize objects.
Eclipse is only reminding you that it thinks it is a good practice to have one.
You can disable those specific warnings in the compiler preferences of your IDE. -
Repost-Best way of using connection pooling
I am reposting this, seems best suitable in this category.
I am using Eclipse 3.1 along with Tomcat 5.0, MySQL 4.1, J2EE1.4. I could set up the JNDI Dataresource connection pooling and tested with small test servlet. Now thinking of having common methods for getting connection / closing / commiting ....etc.
I wrote following. [Please let me know whether it is correct way of doing it - as i am not very sure]
package common;
import java.sql.*;
import javax.sql.*;
import javax.naming.*;
import org.apache.log4j.Logger;
public final class connectionManager {
private static Logger logger = Logger.getLogger(common.connectionManager.class);
public connectionManager() {}
public static Connection getConn () throws NamingException, SQLException
//JNDI DataSource connection pooling
Connection conn = null;
try{
Context initContext = new InitialContext();
Context envContext = (Context)initContext.lookup("java:/comp/env");
DataSource ds = (DataSource)envContext.lookup("jdbc/TQ3DB");
conn = ds.getConnection();
}catch (NamingException ne) {
new GlobalExceptionHandler(logger, ne);
conn = null;
throw new NamingException();
}catch (SQLException e){
new GlobalExceptionHandler(logger, e);
conn = null;
throw new SQLException();
return conn;
}//getConnection
public static void commit(Connection conn) throws SQLException
conn.commit();
public static void rollback(Connection conn) throws SQLException
conn.rollback();
public static void setAutoCommit(Connection conn, boolean autoCommit)
throws SQLException
conn.setAutoCommit(autoCommit );
public static void closeConnection(Connection conn) throws SQLException{
if (conn != null) {
conn.close();
conn = null;
}//closeConnection
public static void closeResources(ResultSet oRS, PreparedStatement pstmt) throws SQLException
if (oRS != null) {
oRS.close();
oRS = null;
if (pstmt != null) {
pstmt.close();
pstmt = null;
} // closeResources
}//ConnectionManager
I am having a login form which submits user name and password. I am checking this against the database. Following is the servlet to do that.
package login;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.sql.*;
import common.*;
public class loginServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request, response);
}//doGet
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException,IOException{
String userId = request.getParameter("userId");
String password = request.getParameter("password");
** call a method to validate the password which will return the
** User Name for authorized users and null string for un-authorised.
String uName = validateUser(userId, password);
//if uName is null .. user is not authorized.
if (uName == null){
//redirect to jsp page with error message
RequestDispatcher rd =
getServletContext().getRequestDispatcher("/jsps/mainmenu.jsp");
if (rd != null){
rd.forward(request,response);
else{
// the user is valid - create a seesion for this user.
HttpSession userSession = request.getSession(true);
// put the user name session variable.
userSession.setAttribute("userName", uName);
//redirect to Main menu page
RequestDispatcher rd =
getServletContext().getRequestDispatcher("/jsps/mainmenu.jsp");
if (rd != null){
rd.forward(request,response);
}// end of doPost
private String validateUser(String userId, String password)
throws SQLException{
String returnVal = null;
connectionManager cm = new connectionManager();
Connection conn = null;
PreparedStatement pstmt = null;
ResultSet oRS = null;
try{
//get the connection
conn = cm.getConn ();
//get records from user table for this user id and password
String sQry = "SELECT user_login FROM user "
+ "where user_login = ? AND user_pwd = ? ";
pstmt = conn.prepareStatement(sQry);
pstmt.setString(1, userId);
pstmt.setString(2, password);
oRS = pstmt.executeQuery();
//check for record
if (oRS.next()) {
returnVal = oRS.getString("user_login");
}else {returnVal = null;}
}catch (Exception e){
returnVal = null;
}finally{
cm.closeResources(oRS, pstmt);
cm.closeConnection(conn);
return returnVal;
}// end of servlet class
But i am unable to compile it and i am also getting lots of warnings.
I am getting error at line
1)String uName = validateUser(userId, password);
Unhandled exception type SQLException loginServlet.java TQ3/WEB-INF/src/login line
Following warnings:
2)For loginServlet Declaration
The serializable class DBTest does not declare a static final serialVersionUID field of type long loginServlet.java
3)The static method getConn() from the type connectionManager should be accessed in a static way
4)The static method closeResources(ResultSet, PreparedStatement) from the type connectionManager should be accessed in a static way
5)The static method closeConnection(Connection) from the type connectionManager should be accessed in a static way
Definitely I am doing it wrong but exactly where? I am having very strong doubt the way i am using connections is not the correct way. Pls help me.
regards
ManishaI am in a search of best way to use connection pooling. Initially was using simple JDBC call, then modified to JNDI, afterwards tried to have common class. Later came accross the idea of Singleton/Static. I wanted to have a common class which will handle all connection related issues and at the same time give good performance.
With due respect to all Java Gurus: i got all from web articles/tutorials/java forum etc. There is a long discussion regarding Singlet vs static in this forum. But finally got confused and could not figure out in my case which method shall i make use of, so tried both.
What I want is somebody pointing out flwas inside my 2 code snippets and guide me about which method shall i adopt in future.
Static way:
package common;
import java.sql.Connection;
import javax.sql.DataSource;
import java.sql.SQLException;
import javax.naming.InitialContext;
import javax.naming.NamingException;
public final class ConnectionManager_Static {
private static InitialContext ctx = null;
private static DataSource ds = null;
public ConnectionManager_Static(){ }
//as the staic method is updating static var i am synchonizing it
private static synchronized void getDatasource () throws NamingException, SQLException
if (ds == null){
ctx = new InitialContext();
ds = (DataSource)ctx.lookup("java:comp/env/jdbc/MySql");
//making getConnection() also static as it is not instance specific
public static Connection getConnection () throws NamingException, SQLException, Exception
Connection conn = null;
try{
if (ds == null) {getDatasource ();}
if (ds != null) {
conn = ds.getConnection();
}catch (Exception e){
throw new Exception("From ConnectionManager_Static",e);
return conn;
}//getConnection
}Singleton:
package common;
import java.sql.*;
import javax.sql.*;
import javax.naming.*;
public final class ConnectionManager_Singleton {
private static ConnectionManager_Singleton INSTANCE = null;
private DataSource datasource = null;
// Private constructor for singleton pattern
private ConnectionManager_Singleton() throws NamingException{
Context ctx = new InitialContext();
datasource = (DataSource)ctx.lookup("java:comp/env/jdbc/MySql");
//synchronized creator for multi-threading issues
//another if check to avoid multiple instantiation
private synchronized static void createInstance() throws NamingException{
if (INSTANCE == null) {
INSTANCE = new ConnectionManager_Singleton();
public static ConnectionManager_Singleton getInstance() throws NamingException {
if (INSTANCE == null) createInstance();
return INSTANCE;
public Connection getConnection() throws Exception
Connection con = null;
try{
con = datasource.getConnection();
}catch(Exception e){
throw new Exception("From connection manager singleton ", e);
return con;
}Sorry, It's becoming long.
Thanaks in advance,
Manisha
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