Static Initializer in PK Class
What's the purpose of this line:
static{
Class c = StringPK.class;
in pk classes generated by kodo's pk tool?
Dave Ford
Smart Soft - The Developer Training Company
http://www.smart-soft.com
This allows us a chance to force class loading. Each PersistenceCapable
class registers with the core JDO classes, which in turn allows us to
probe the registered class for base information stored during the
enhancement process.
Otherwise, it is possible that we would have no idea of what class the
object-id resolves to without searching your entire classpath for
..metadata files, as the spec calls for no specific method to return the
matching PersistenceCapable class for a given object-id.
Dave Ford wrote:
What's the purpose of this line:
static{
Class c = StringPK.class;
in pk classes generated by kodo's pk tool?
Dave Ford
Smart Soft - The Developer Training Company
http://www.smart-soft.com
Stephen Kim
[email protected]
SolarMetric, Inc.
http://www.solarmetric.com
Similar Messages
-
Problems with static member variables WAS: Why is the static initializer instantiating my class?!
Hi,
I have been hunting down a NullPointerException for half a day to come to
the following conclusion.
My constructor calls a method which uses static variables. Since an intance
of my class is created in the static block when the class is loaded, those
statics are probably not fully initialized yet and the constructor called
from the static block has those null pointer problems.
I've considered moving the initialization of the static variables from the
declaration to the static block. But your code is inserted BEFORE any other
code. Therefore not solving my problem.
Two questions:
1) what would be a solution to my problem? How can I make sure my static
variables are initialized before the enhancer generated code in the static
block calls my constructor? Short of decompiling, changing the code and
recompiling.
2) Why is the enhancing code inserted at the beginning of the static block
and not at the end? The enhancements would be more transparent that way if
the static variables are initialized in the static block.
Thanks,
EricHi Eric,
JDO calls the no-args constructor. Your application should regard this constructor as belonging
primarily to JDO. For example, you would not want to initialize persistent fields to nondefault
values since that effort is wasted by JDO's later initilization to persistent values. Typically all
you want to initialize in the no-args constructor are the transactional and unmanaged fields. This
rule means that you need initialization after construction if your application uses the no-args
constructor and wants persistent fields initialized. On the other hand, if your application really
uses constructors with arguments, and you're initializing persistent fields in the no-args
constructor either unintentionally through field initializers or intentionally as a matter of
consistency, you will find treating the no-args constructor differently helpful.
On the other hand, if Kodo puts its static initializer code first as you report, then it is a bug.
Spec Section 20.16: "The generated static initialization code is placed after any user-defined
static initialization code."
David Ezzio
Eric Borremans wrote:
>
Hi,
I have been hunting down a NullPointerException for half a day to come to
the following conclusion.
My constructor calls a method which uses static variables. Since an intance
of my class is created in the static block when the class is loaded, those
statics are probably not fully initialized yet and the constructor called
from the static block has those null pointer problems.
I've considered moving the initialization of the static variables from the
declaration to the static block. But your code is inserted BEFORE any other
code. Therefore not solving my problem.
Two questions:
1) what would be a solution to my problem? How can I make sure my static
variables are initialized before the enhancer generated code in the static
block calls my constructor? Short of decompiling, changing the code and
recompiling.
2) Why is the enhancing code inserted at the beginning of the static block
and not at the end? The enhancements would be more transparent that way if
the static variables are initialized in the static block.
Thanks,
Eric -
Is static initialization of a class varaible guaranteed to occur only once?
In other words, if two threads attempt to create an instance of this class, would the static initialization of ConnectionFactory occur once?
public ConnectionSource {
private static ConnectionFactory factory = new ConnectionFactory();
I need exactly one instance of ConnectionFactory.
SAFif ConnectionSource is unloaded (because there are no references to any instances on ConnectionSource, and no reference to the Class object itself), then a different ConnectionFactory would be created when the class is reloaded. This may or may not affect your program execution.
Hackmann -
Why is the static initializer instantiating my class?!
It seems that the static { ... } blocks for my enhanced classes actually
create instances of those classes. Gack! It seems, furthermore, that
this is due to the JDO spec itself, whose JDOImplHelper.registerClass()
requires an instance of the class being registered. Double gack!
This is causing a problem for me, because I've put a sequence generator in
my class's constructor. Now merely **loading** a PC class will cause my
sequence to be incremented (not great), or will throw an exception if the
environment isn't set up for the sequence generator (terrible). This
latter problem happens if I enhance my classes, then try to enhance them
again -- the enhancer tries to load my class, the static { ... } block
tries to load the sequence generator, and the whole enhancer blows up.
Questions:
* Why is this necessary? Why does JDOImplHelper.registerClass() take an
instance of the class being registered? Could the call to
JDOImplHelper.registerClass() happen the first time the ctor is called,
instead of inside a static { ... } block?
* Given that the above questions probably have reasonable answers, how
should I work around this problem?
Thanks,
PaulHi Patrick,
Do you understand why jdoNewInstance and jdoNewObjectIdInstance are not static? And if that could
be done, would that void the need to register the class when loaded?
David
Patrick Linskey wrote:
>
On 5/28/02 12:39 PM, "Paul Cantrell" <[email protected]> wrote:
Questions:
* Why is this necessary? Why does JDOImplHelper.registerClass() take an
instance of the class being registered? Could the call to
JDOImplHelper.registerClass() happen the first time the ctor is called,
instead of inside a static { ... } block?The JDO spec inserts some utility methods into PersistenceCapable classes
for which it needs a method around.
These utility methods are the jdoNewInstance() methods and the
jdoNewObjectIdInstance() methods. These methods improve performance by
allowing PersistenceCapable objects and application ID OID classes to be
created without using reflection.
This class registration must occur at class initialization time, because
your first operation on the class might very well be to look up objects of
that class from the data store, in which the no-args constructor might not
be called.
* Given that the above questions probably have reasonable answers, how
should I work around this problem?Your best bet is probably to not do the sequence generation in your no-args
constructor. One option would be to create a constructor with a marker
boolean that designates if sequence generation should occur; another
probably more elegant solution would be to create a factory method that uses
the default constructor and then assigns an ID, or fetches an ID and then
invokes a non-default constructor with that ID.
-Patrick
Patrick Linskey [email protected]
SolarMetric Inc. http://www.solarmetric.com -
I have an IMac 10.5.8 and I am having problems with Java. I have tried todownload Java for Mac OS X 10.5 Update 4 and it looks as if it is doingsuccessfully, but I cannot see where it has put it and get this message when trying to launch Java preferences:
Cannotlaunch Java application
A static initializer of the main class threw an exception:java.lang.NullPointerException"
Couldyou please help?Hmm, only way I know would be a relatively painless Archive & Install, which gives you a new/old OS, but can preserve all your files, pics, music, settings, etc., as long as you have plenty of free disk space and no Disk corruption, and is relatively quick & painless...
http://docs.info.apple.com/article.html?artnum=107120
Just be sure to select Preserve Users & Settings. -
To use a static variable in another class,but report NullPointerException
when TableMain is running,I run testRecord so that let TableMain add a occur informatin and
happened time in a row in TableMain,but when I run testRecord,java report a NullPointerException and I dont know how to solve this problem,thanks for helping me to check my code;(error report info is in end)
import java.awt.event.*;
import javax.swing.table.*;
import java.io.*;
public class TableMain extends JFrame{
JTable table;
static OwnModel model;
String[] columnHeader={"occur","time"};
class OwnModel extends DefaultTableModel{
public OwnModel(Object[] columnNames,int numRows){
super(columnNames,numRows);
public boolean isCellEditable(int row,int column){
return false;
public TableMain(){
model=new OwnModel(columnHeader,0);
table=new JTable(model);
JScrollPane scroll=new JScrollPane(table);
JButton save=new JButton("save record");
JButton delete=new JButton("delete record");
JPanel buttons=new JPanel();
buttons.add(save);
buttons.add(delete);
JLabel sign=new JLabel("occur record");
Container cp=getContentPane();
cp.add(BorderLayout.NORTH,sign);
cp.add(BorderLayout.CENTER,scroll);
cp.add(BorderLayout.SOUTH,buttons);
this.setSize(300,500);
this.setVisible(true);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
public static void main(String[] args){
new TableMain();
import java.util.Vector;
import java.util.Calendar;
import java.text.SimpleDateFormat;
public class testRecord{
public static void main(String[] args){
SimpleDateFormat simpledf=new SimpleDateFormat("yyyy/MM/dd hh:mm:ss");
String tableName="friends";
Calendar occurtime=Calendar.getInstance();
Vector record=new Vector();
record.add("send 'desc' sql, "+"operated table is :"+tableName);
record.add(simpledf.format(occurtime.getTime()));
System.out.println("model is "+Guide.model);
System.out.println("record is "+record);
Guide.model.addRow(record);
error report:
model is null
record is [send 'desc' sql,operated table is :friends, 2004/05/26 11:39:03]
Exception in thread "main" java.lang.NullPointerException
at testRecord.main(testRecord.java:14)but I just use this constructor once,never use twice
with same jvm;I thinks my idea is not too very badIt's a public constructor (if I'm looking in the right
place) - it could be called from anywhere. To
intialize a static variable outside the declaration,
use a static initializer:private static String whatever;static {
whatever = "whatever";
}Or simply (in this case):private static String whatever = "whatever"; -
Help required on java.lang.StackOverFlowError and static initializer
I wanted to create an instance of a class that contains another instance of the same class. So I wrote:
class A {
A z = new A ();
void display () {
System.out.println ("Hello World");
public static void main (String [] args) {
A y = new A();
y.display ();
}During execution I got java.lang.StackOverFlowError. But if I put a static initializer, it works fine. Here is the code using static initializer.
class A {
static{
A z = new A ();
void display () {
System.out.println ("Hello World");
public static void main (String [] args) {
A y = new A();
y.display ();
}Could anyone please help me to understand the logic why "java.lang.StackOverFlowError" is happening here and how the same program runs fine by putting a static initializer ?
Regards,
ShambhuCould anyone please help me to understand the logic
why "java.lang.StackOverFlowError" is happening hereWhen you instantiate an A object with A y = new A () then A z = new A () also gets executed inside the A class, which in it's turn executes A z = new A () again, and again, and again...
and how the same program runs fine by putting a
static initializer ?Because the static block gets executed only once.
The use of class- and instance variables is explained in more detail here:
http://java.sun.com/docs/books/tutorial/java/javaOO/classvars.html -
Can we have try/catch in a static block in a class?
hi All
i have a question about put a try/catch block in a static block in a class to catch exceptions that maybe thrown from using System.xxxx(). in my custom class, i have a static block to initialize some variables using System.xxx(). in case of any error/exception, i need to be able to catch it and let the caller know about it. i tried to put a try/catch block in the static block, and tried to rethrow the exception. but it is not allowed, how would i handle situation like this? thanks for your help and advise in advance.You could just swallow the exception inside try/catch
block, and instead of throwing it out, just set a
static variable to allow checking from outside
whether the initialization succeeded, or check within
the constructor / methods of this class for
successful initialization, and throw the exception
then. You could even save that exception in a static
variable for later.Ouch, ouch, you're hurting my brain. This would allow someone to ignore a (presumably) fatal error. Throw a RuntimeException as indicated. You can wrap a checked exception in an unchecked one if need be. -
JNDI lookup in Static initializer
Hi,
Is there a problem looking up home interfaces in a static initializer in
WL6.1?
I have a class X which runs in my application client. It has a static
initializer that looks up a home interface. The call to Context.lookup()
times out with weblogic.rjvm.PeerGoneException: No message was received for:
'240' seconds.
The strange thing is, if I force the static initializer to run myself (by
instantiating X directly), then it runs ok. I only get the above problem if
the VM causes it to run (by loading a class Y that references X).
Also, this behaviour is only apparent in the client. In the app server, X
behaves correctly. I'm passing in JNDI properties as system properties on
the command line to the client.
Any help appreciated.
Cheers,
ManishCheck the class loader and thread dump during the place where it works and
the one where it doesn't ... that could help track down why it works that
way.
Peace,
Cameron Purdy
Tangosol Inc.
<< Tangosol Server: How Weblogic applications are customized >>
<< Download now from http://www.tangosol.com/download.jsp >>
"Manish Shah" <[email protected]> wrote in message
news:[email protected]..
Hi,
Is there a problem looking up home interfaces in a static initializer in
WL6.1?
I have a class X which runs in my application client. It has a static
initializer that looks up a home interface. The call to Context.lookup()
times out with weblogic.rjvm.PeerGoneException: No message was receivedfor:
'240' seconds.
The strange thing is, if I force the static initializer to run myself (by
instantiating X directly), then it runs ok. I only get the above problemif
the VM causes it to run (by loading a class Y that references X).
Also, this behaviour is only apparent in the client. In the app server, X
behaves correctly. I'm passing in JNDI properties as system properties on
the command line to the client.
Any help appreciated.
Cheers,
Manish -
How to reference a static variable before the static initializer runs
I'm anything but new to Java. Nevertheless, one discovers something new ever' once n a while. (At least I think so; correct me if I'm wrong in this.)
I've long thought it impossible to reference a static variable on a class without the class' static initializer running first. But I seem to have discovered a way:
public class Foo {
public static final SumClass fooVar; // by default initialized to null
static {
fooVar = new SumClass();
public class Bar {
public static final SumClass barVar;
static {
barVar = Foo.fooVar; // <<<--- set to null !
}Warning: Speculation ahead.
Normally the initial reference to Foo would cause Foo's class object to instantiate, initializing Foo's static variables, then running static{}. But apparently a static initializer cannot be triggered from within another static initializer. Can anyone confirm?
How to fix/avoid: Obviously, one could avoid use of the static initializer. The illustration doesn't call for it.
public class Foo {
public static final SumClass fooVar = new SumClass(); // either this ..
public class Bar {
public static final SumClass barVar = Foo.fooVar; // .. or this would prevent the problem
}But there are times when you need to use it.
So what's an elegant way to avoid the problem?DMF. wrote:
jschell wrote:
But there are times when you need to use it. I seriously doubt that.
I would suppose that if one did "need" to use it it would only be once in ones entire professional career.Try an initializer that requires several statements. Josh Bloch illustrates one in an early chapter of Effective Java, IIRC.
Another classic usage is for Singletons. You can make one look like a Monostate and avoid the annoying instance() invocation. Sure, it's not the only way, but it's a good one.
What? You only encounter those once in a career? We must have very different careers. ;)
So what's an elegant way to avoid the problem? Redesign. Not because it is elegant but rather to correct the error in the design.<pff> You have no idea what my design looks like; I just drew you a couple of stick figures.If it's dependent on such things as when a static initializer runs, it's poor. That's avoidable. Mentioning a case where such a dependency is used, that's irrelevant. It can be avoided. I know this is the point where you come up with a series of unfortunate coincidences that somehow dictate that you must use such a thing, but the very fact that you're pondering the problem with the design is a design problem. By definition.
Besides, since what I was supposing to be a problem wasn't a problem, your "solution" isn't a solution. Is it?Well, you did ask the exact question "So what's an elegant way to avoid the problem?". If you didn't want it answered, you should have said so. I'm wondering if there could be any answer to that question that wouldn't cause you to respond in such a snippy manner. Your design is supposedly problematic, as evidenced by your question. I fail to see why the answer "re-design" is unacceptable. Maybe "change the way the Java runtime initializes classes" would have been better?
This thread is bizarre. Why ask a question to which the only sane answer, you have already ruled out? -
Static initialization of java code included in the Imported Archives jars?
Hi
Does SAP PI 7.0 support static initialization of java code included in the Imported Archives jars?
Static block of a java class included in the Imported Archive jars throws a Runtime Exception called ExceptionInInitializationError while trying to run.
Regards,
RDSHave you tested the code outside PI?
-
Require the definition of static members in derived classes
What I would like to do is require the definition of static variable with a particular signature in all implementing/extending classes of T.
I would have thought this could be done in the following way:
public template T
{public static final Object obj;}To me, this says that all classes implementing this interface must define a member with this exact signature. This is not the case.
Basically, I'd like to replicate the behavior I see when I extend a serializable class. I get this warning : "The serializable class X does not declare a static final serialVersionUID field of type long".
Anyone know how to do this? I've googled many search strings, and looks on the forums extensively. I could potentially use code weaving (Aspects), but this adds another level of complexity to my project.
Thanks in advance,
LukeThere is a definite use for it.
I have an ObjectFactory A. Upon initializing a copy of this object, it scans the given package for all classes. Each class is inspected via Reflection. If there is a variable with a particular signature, in my case public static final ObjectFactory.Constructor, we get a copy of the static variable and add it to a TreeMap.
Then, whenever someone calls ObjectFactory.createInstance("class name"), I do a lookup, and invoke a method of the Contructor object to return an instance of that object. Also, the ObjectFactory allows the programmer to list all "extensions" loaded in this manner.
The reason I'm doing this instead of simply putting a static initializer in the "extension" class that would register the extension with the ObjectFactory is:
I want to control when and where extensions are registered.
Does this makes sense?
The only thing I am missing, is how to warn (other than through documentation) the programmer that they need to define this field, exactly in the same manner as classes extended from serialized classes receive a warning that "The serializable class X does not declare a static final serialVersionUID field of type long".
I could do this using Aspect oriented programming, but that adds a layer of complexity that I do not need.
Here is the code:
public class ObjectFactory<T extends Object>
private TreeMap<String,Constructor<T>> cons=new TreeMap<String,Constructor<T>>();
public ObjectFactory(String pkg)
ClassLoader cld=Thread.currentThread().getContextClassLoader();
if(cld==null)
throw new NullPointerException();
String path=pkg.replace(".","/");
URL tmp;
Enumeration<URL> resources;
File dir;
try
resources=cld.getResources(path);
while(resources.hasMoreElements())
try
tmp=resources.nextElement();
dir=new File(URLDecoder.decode(tmp.getPath(), "UTF-8"));
if(dir.exists())
String[] files=dir.list();
for(String file:files)
try
if(file.endsWith(".class"))
Class<?> c=Class.forName(pkg + '.' + file.substring(0, file.length() - 6));
try
Field f=c.getField("constructor");
//TODO add type checking
Constructor<T> con=Constructor.class.cast(f.get(null));
synchronized(cons)
cons.put(con.getName(),con);
System.out.println(con.getName());
catch(IllegalAccessException ignored)
catch(NoSuchFieldException ignored)
catch(ClassNotFoundException e)
catch(UnsupportedEncodingException e)
catch(IOException e)
public interface Constructor<T>
public T createInstance();
public String getName();
public T createInstance(String name) throws FactoryNotFound
Constructor<T> c=cons.get(name);
if(c==null)
throw new FactoryNotFound();
return c.createInstance();
import ObjectFactory.Constructor;
public final class Extension
public static final String desc="";
public static final String name="Test01";
public static final Constructor<Object> constructor=new Constructor<Object>()
public Object createInstance()
return new Fighter();
public String getName()
return name;
} -
hi guys,
can anyone explain what might cause the following error?
i have the following classes
class BonesPlayLevel {
// execution reaches here
ObjFigure.initJointedModel();
class ObjFigure {
public static void initJointedModel() {
// execution does not reach here
initJointedModelList(myVecs);
and when i try and run i get the following error message:
java.lang.Error: Static initializer: java.lang.NullPointerException
java.land.Error
cheers
spobits ok i understand how to debug, and how the stack trace works.
i am running this in JBuilder, here is the output produced
D:\Nokia\Devices\Series_60_MIDP_Concept_SDK_Beta_0_3_1_Nokia_edition\bin\emulator.exe -classpath "E:\javaApps\MIDlets\mlj3d_exbones\classes;D:\MLJ3D\v1.0.1dev\jars\mljFrame.jar;D:\MLJ3D\v1.0.1dev\jars\mljRender.jar;" -Xdevice:Series_60_MIDP_Concept_SDK_Beta_0_3_1_Nokia_edition -Xdescriptor:"E:\javaApps\MIDlets\mlj3d_exbones\jad-temp\ExBones.jad"
java.lang.Error: Static initializer: java/lang/NullPointerException -
Instance initializer and static initializer blocks
Hi guys,
I read about the above mentioned in the JLS and also in a book before, but I still don't quite understand, what is the use of these. I sort of have a rough idea, but not exactly. I mean, what is the purpose of the instance initializer and static initializer blocks, how can it be useful? I understand I can execute pieces of code that will initialize instance and static variables accordingly, but how is it different then to using a constructor to initialize these fields? Are these pieces of code executed before any constructor is executed, or when otherwise?
Sorry for my noob, I'm learning.
PR.Static initializers are useful for initializing a class when the initialization is more complex than simply setting a single variable, or when that initialization can throw a checked exception.
public class Foo {
private static final Bar bar;
static {
try {
bar = new Bar();
bar.doSomeInitializationStuff();
catch (SomeCheckedExceptionThatBarThrows e) {
throw new ExceptionInInitializerError(e);
}Here we could not do the two-step new Bar() + doSomeInit() stuff in the line where we declare the bar variable. Additionally, assuming that one or both of those can throw a checked exception, we could not do that on the declaration line; we need the static initializer to wrap that in the appropriate unchecked exception.
This allows us to do more complex class initialization when the class is loaded than we could do with a simple variable initialization.
Instance initializers are useful if you want to perform the same steps in every constructor and don't want to have to repeat the code in each constructor. Instance initializers are executed as the first step of each constructor (or maybe it's after any super() calls, I forget). -
Help please with a Static Initializer for ImageIcons inside Jar Files
At the moment I'm playing around displaying XML using A JTree
I am subclassing DefaultMutableTreeNode, and want it to have some default Icons set up....
Sort of like this:
public class XNode extends DefaultMutableTreeNode
public static final ImageIcon icon;
public static ImageIcon getIcon()
return XNode.icon;
}The only thing is that for the life of me after trying various things and searching these forums and google:
I can't work out how to write the static Initializer for the ImageIcon.
The ImageIcon will need to be created using a URL, cos it will be inside my jar file.....
the usual...
URL url = this.getClass().getResource("/images/Exit16.gif");
ImageIcon Icon = new ImageIcon( url );but I wanted these Icons to be class members..............
Could someone help?
}DrClap " I don't understand why you put Class.forName in there either"
A: Because I don't really know what I'm doing.
I will try that suggestion.
At present I have this:
static ImageIcon loadIcon;
static
try
loadIcon = new ImageIcon( Class.forName("cis.editor.xml.nodes.AlNode").getResource("/images/Exit16.gif"));
catch( ClassNotFoundException cnfe )
System.out.println("ClassNotFound: " + cnfe.getMessage() );
public static final ImageIcon defaultIcon = loadIcon;
static
try
loadIcon = new ImageIcon( Class.forName("cis.editor.xml.nodes.AlNode").getResource("/images/tree_folder_major.gif"));
catch( ClassNotFoundException cnfe )
System.out.println("ClassNotFound: " + cnfe.getMessage() );
public static final ImageIcon commentIcon = loadIcon;
//.......... ANd so OnWhy? Because do far it was the only way I could get it to compile.
And It works..
However it's a real abortion.. codewise.
I need to load about 16 Icons that the various subclasses can 'use'
DefaultTreeCellRenderer - must do somethng similar because it has a bunch of Icons to "Pick from",
Only How is that done 'properly' ?
Maybe you are looking for
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