When is Static variables memeory freed ?

When is the memory used by static variables freed by the GC?

When is the memory used by static variables freed by
the GC?Are you talking about the variables or the objects they point to?
class Foo {
static Bar bar = new Bar();
static void relaseBar () {
bar = null;
} The first call to releaseBar() makes the Bar object eligible for GC, so the space it occupies could be freed at any point, or never--it's totally up to the whims of GC.
The space taken up by he bar reference variable, which is NOT the same as the object--will be eligible for GC only if and when the class is unloaded. Like anything that becomes eligible for GC, the memory isn't explicitly freed until GC runs, which is not very predictable, and my never happen at all. For most purposes, however, you can effectively consider memory "freed" at the point when it become eligible for GC.

Similar Messages

"requires unreachable" warning emitted when using static variables and the singleton pattern

I'm implementing code contracts on an existing code base and I've come across this situation in a few places. Wherever we have additional logic with code contracts where a singleton instance is instantiated, all code contracts emit a "reference use
unreached", or "requires unreachable" warning.
The code below demonstrates this.
static string instance = null;
static string InstanceGetter()
if (instance == null)
instance = "initialized";
AdditionalLogic(instance); // requires unreachable warning on this line
return instance;
static void AdditionalLogic(string str)
Contract.Requires(str != null);
Console.WriteLine(str);

Would the class get unloaded even if the Singleton object has some valid/reachable references pointing to it?
On a different note, will all the objects/instances of a class get destroyed if the class gets unloaded.That's the other way round, really: instances cannot be garbage-collected as long as they are strongly reachable (at least one reference pointing to them), and classes cannot be unloaded as long as such an instance exists.
To support tschodt's point, see the JVM specifications: http://java.sun.com/docs/books/jvms/second_edition/html/Concepts.doc.html#32202
*2.17.8 Unloading of Classes and Interfaces*
A class or interface may be unloaded if and only if its class loader is unreachable. The bootstrap class loader is always reachable; as a result, system classes may never be unloaded.
Will the same case (Custom ClassLoader getting unloaded) occur if I write the singleton using the code wherein we lazily initialize the Singleton (and actually create a Singleton instance rather than it being a Class/static variable).You should check the meaning of this vocabulary ("object", "instance", "reference"): http://download.oracle.com/javase/tutorial/java/javaOO/summaryclasses.html
The difference between the lazy vs eager initializations is the time when an instance is created. Once it is created, it will not be garbage collected as long as it is reachable.
Now I'm wondering, whether being referenced by a static attribute of a class guarantees being reachabe. I'm afraid not (see the JLS definition of reachable: http://java.sun.com/docs/books/jls/third_edition/html/execution.html#44762). That is, again, unless the class has been loaded by the system class loader.

Alternatives to binding to static variables

As has been discussed before, flex does not dispatch a binding event when a static variable changes. I was wondering if there are any alternatives to simulate this functionality.
I have a class called UserDataManager which contains static properties i use throughout my application. One of these is an array which i need to access and change on various different components throughout my application. After changing this must then change the array on all the other components.
I suspect i could manage this by 2-way binding on an instance of UserDataManager and passing this instance to all the components on creation, however this seems like a computationly and time expensive workaround.
Are there any alternatives?
Thanks

Use a singleton pattern so your statics arent static.

When should I use static variable and when should not? Java essential

When should I use static variable and when should not? Java essential

Static => same value for all instances of the class.
Non-static => each instance can have its own value.
Which you need in which circumstances is completely up to you.

Slow performance when multiple threads access static variable

Originally, I was trying to keep track of the number of function calls for a specific function that was called across many threads. I initially implemented this by incrementing a static variable, and noticed some pretty horrible performance. Does anyone have an ideas?
(I know this code is "incorrect" since increments are not atomic, even with a volatile keyword)
Essentially, I'm running two threads that try to increment a variable a billion times each. The first time through, they increment a shared static variable. As expected, the result is wrong 1339999601 instead of 2 billion, but the funny thing is it takes about 14 seconds. Now, the second time through, they increment a local variable and add it to the static variable at the end. This runs correctly (assuming the final increment doesn't interleave which is highly unprobable) and runs in about a second.
Why the performance hit? I'm not even using volatile (just for refernce if I make the variable volatile runtime hits about 30 seconds)
Again I realize this code is incorrect, this is purely an interesting side-expirement.
package gui;
public class SlowExample implements Runnable
     public static void main(String[] args)
          SlowExample se1 = new SlowExample(1, true);
          SlowExample se2 = new SlowExample(2, true);
          Thread t1 = new Thread(se1);
          Thread t2 = new Thread(se2);
          try
               long time = System.nanoTime();
               t1.start();
               t2.start();
               t1.join();
               t2.join();
               time = System.nanoTime() - time;
               System.out.println(count + " - " + time/1000000000.0);
               Thread.sleep(100);
          catch (InterruptedException e)
               e.printStackTrace();
          count = 0;
          se1 = new SlowExample(1, false);
          se2 = new SlowExample(2, false);
          t1 = new Thread(se1);
          t2 = new Thread(se2);
          try
               long time = System.nanoTime();
               t1.start();
               t2.start();
               t1.join();
               t2.join();
               time = System.nanoTime() - time;
               System.out.println(count + " - " + time/1000000000.0);
          catch (InterruptedException e)
               e.printStackTrace();
           * Results:
           * 1339999601 - 14.25520115
           * 2000000000 - 1.102497384
     private static int count = 0;
     public int ID;
     boolean loopType;
     public SlowExample(int ID, boolean loopType)
          this.ID = ID;
          this.loopType = loopType;
     public void run()
          if (loopType)
               //billion times
               for (int a=0;a<1000000000;a++)
                    count++;
          else
               int count1 = 0;
               //billion times
               for (int a=0;a<1000000000;a++)
                    count1++;
               count += count1;
}

Peter__Lawrey wrote:
Your computer has different types of memory
- registers
- level 1 cache
- level 2 cache
- main memory.
- non CPU local main memory (if you have multiple CPUs with their own memory banks)
These memory types have different speeds. Depending on how you use a variable affects which memory it is placed in.Plus you have the hotspot compiler kicking in sometime during the run. In other words for some time the VM is interpreting the code and then all of a sudden its compiled and executing the code compiled. Reliable micro benchmarking in java is not easy. See [Robust Java benchmarking, Part 1: Issues|http://www.ibm.com/developerworks/java/library/j-benchmark1.html]

Using a static variable declared in an applet in another class

Hi guys,
I created an applet and i want to use one of the static variables declared in teh applet class in another class i have. however i get an error when i try to do that...
in my Return2 class i try to call the variable infoPanel (declared as a static JPanel in myApplet...myApplet is set up like so:
public class myApplet extends JApplet implements ActionListener, ListSelectionListener
here are some of the lines causing a problem in the Return2 class:
myApplet.infoPanel.removeAll();
myApplet.infoPanel.add(functionForm2.smgframeold);
myApplet.infoPanel.validate();
myApplet.infoPanel.repaint();
here are some of the errors i get
dummy/Return2.java [211:1] package myApplet does not exist
myApplet.infoPanel.removeAll();
^
dummy/Return2.java [212:1] package myApplet does not exist
myApplet.infoPanel.add(functionForm2.smgframeold);
^
dummy/Return2.java [213:1] package myApplet does not exist
myApplet.infoPanel.validate();
^
dummy/Return2.java [214:1] package myApplet does not exist
myApplet.infoPanel.repaint();
^
please help! thanks :)

I don't declare any packages though....i think it just doesn't recognize myApplet for some reason..
other errors i got compiling are:
dummy/Return2.java [82:1] cannot resolve symbol
symbol : variable myApplet
location: class Return2
updateDesc.setString(3, myApplet.staticName);
I Don't get why i'm getting this error cuase they worked fine when myApplet was a standalone application, not an applet.
myApplet is in the same folder as Return2 and it compiles properly.

Using Static Variable against Context Attribute for Holding IWDView

Dear Friends,
I have a method which is in another DC which has a parameter of the type IWDView. In my view, I will have an action which will call the method in another component by passing the value for the view parameter. Here, I can achieve this in 2 types. One is - I declare a static variable and assign the wdDoModifyView's view as parameter value and I can pass this variable as parameter whenever calling that method or the second way - create an attribute and assign the same wdDoModifyView's view parameter as its value. Whenever I call this method, I can pass this attribute as parameter. What is the difference between these two types of holding the value since I am storing the same value i.e., wdDoModifyView's view parameter. But when I trigger the action from different user sessions, the first type of code (using static variable) prints the same value in both the sessions for view.hashCode() and View.toString(), but the same is printing the different values when I pass the attribute which holds the view parameter.
Clarification on this is highly appreciated
The problem I face is when I use static variable to get the view instance and export the data using the UI element's id, the data belonging to different user sessions is mixed up where as when I use Context Attribute, the same problem doesn't arise. I want to know the reason why it is so. Is there any other place or way where I can get the current view instance of each session instead of wdDoModifyView?

Hi Sujai ,
As you have specified the problem that we face when we use static attributes, when end users are using the application .
Static means i have n number of objects but the static variable value will remain same every where.
when it is context attribute for every object i.e nth object you have a nth context attribute i mean nth copy of the context attribute.
so every user has a unique Iview parameter , when context is used and
when static is used , assume you have userA , his iview is set this intially and u have another user B , when he is using , since the variable is static and when you access this variable you will get the value of userA.
Regards
Govardan Raj

Non-static variable being used in static context

I am currently attempting to write a basic user login system using basic applets. I have two JTextFields named "userText" and "passText".
What i am attempting to do is use the ".getText()" method to get the text out of the JTextField and verifying the string against a string already in a file using the bufferedReader, etc.
However when i try to compare the string in the file with the one in the text field using the following code:
if ((line.compareTo(username)) == 0)
i get the following error...
"non-static variable being used in a static context"
Any ideas?

The static method doesn't know about instances of the class instead you pass the instance to the method:
static public void myMethod(MyClass instance, String var) {
if(instance.line.compareTo(var))
And then the call would be:
MyClass.myMethod(anInstance, userName);

Runtime error in linking with static variables....

Hi,
I am building a shared library which includes a compiled object generated from a class containing the static variables only and also I have another version of same class with same & some new static variables in it and using that to generate another shared library. Now when I tried to run a program
which links with both the library it core dumps when it tries to return from the program i.e when it finishes.
Can someone please help me explain why my program is behaving like that?? Can duplicate inculsion of static variables in different scope can cause problem ?? How can this be avoided ?
Also please be advised that the class with static variables gets generated at the compile time using a script which uses a DTD whose version (specification) can be different in both the libraries therefore I can't just seperate the common stuff into one class and specific into another.
Thanks.
Rajeev.

Not to worry...found the answer in another post. Seems like patches need to applied to the OS.

Main method hangs setting static variable

I have a very annoying intermittent problem. Once in a while (roughly 1 out of 30 times), my app hangs in the main method when assigning a value to a static variable.
public static void main(String[] args) {
//read args
//do some very trivial stuff (never hangs here)
someOtherNonStaticClass.aStaticInt = 100;//once in a while it hangs here???
}I have tried setting "-Dsun.java2d.noddraw=true" but this hasnt helped. I have tried using JRE 1.4.1 and 1.4.2 and they both have this problem. Some machines seem to be worse than others but this may just be chance.
When the app hangs, it appears in task manager as a javaw.exe process, but the app itself doesnt appear.
Does anyone have any ideas or suggestions for me to try? Its pretty annoying not have a stack trace to work with.

I have tried setting "-Dsun.java2d.noddraw=true" butWhy should it?
this hasnt helped. I have tried using JRE 1.4.1 and
1.4.2 and they both have this problem. Some machines
seem to be worse than others but this may just be
chance.
When the app hangs, it appears in task manager as a
javaw.exe process, but the app itself doesnt appear.That's normal. All Java apps would appear as javaw.exe. Because the JVM has the process.
Does anyone have any ideas or suggestions for me to
try? Yes. Stop accessing fields of another class. It's bad design to begin with, and it looks to me like you're breaking something someplace else by setting that value to 100 right then.

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";

Static Variable in webapplication

Are the values of static variables preserved when two different users try and access them? i.e.
If one web application user sitting in Chicago sets a static variable to 'true' from 'false'. Will the other user sitting in Omaha see it as 'true' or 'false' in his session?

Alright, I did this test.
1) On my local tomcat I created a mini webapp where I
declared a static which populates from a form field.
static String displayname = "";
displayname = request.getParameter("username");
and then displayed it on the html page.
2) I opened IE sessions of my application and in one
I gave the user name 'Tom' and other 'Janet' and hit
continue. (Tom before Janet)
3) The page that had 'Tom' displayed tom and other
one Janet. Then I refreshed the display page for the
tom instance and it still showed 'Tom'.
Conclusion: It appears that the statics are indeed
loyal to the class loader and not the JVM. Hence, for
multiple users each instance should have its own
static.
Please let me know if you disagree or see my test to
be wrong.
Thanks
Message was edited by:
Birthdayhitting the refesh button on your IE page doesn't show you what the value of the static variable in your web app is. You're headding for trouble.
EDIT: Ok well it depends on you implementation how the refresh will work, but if you have a static fields in a class on your web server I promise you it's not in any way associated with any session.

Non-static variable cant accessed from the static context..your suggestion

Once again stuck in my own thinking, As per my knowledge there is a general error in java.
i.e. 'Non-static variable cant accessed from static context....'
Now the thing is that, When we are declaring any variables(non-static) and trying to access it within the same method, Its working perfectly fine.
i.e.
public class trial{
�� public static void main(String ar[]){ ////static context
�� int counter=0; ///Non static variable
�� for(;counter<10;) {
�� counter++;
�� System.out.println("Value of counter = " + counter) ; ///working fine
�� }
�� }
Now the question is that if we are trying to declare a variable out-side the method (Non-static) , Then we defenately face the error' Non-static varialble can't accessed from the static context', BUT here within the static context we declared the non-static variable and accessed it perfectly.
Please give your valuable suggestions.
Thanks,
Jeff

Once again stuck in my own thinking, As per my
knowledge there is a general error in java.
i.e. 'Non-static variable cant accessed from static
context....'
Now the thing is that, When we are declaring any
variables(non-static) and trying to access it within
the same method, Its working perfectly fine.
i.e.
public class trial{
�� public static void
main(String ar[]){ ////static context
�� int counter=0; ///Non
static variable
�� for(;counter<10;) {
�� counter++;
��
System.out.println("Value
of counter = " + counter) ; ///working fine
�� }
�� }
w the question is that if we are trying to declare a
variable out-side the method (Non-static) , Then we
defenately face the error' Non-static varialble can't
accessed from the static context', BUT here within
the static context we declared the non-static
variable and accessed it perfectly.
Please give your valuable suggestions.
Thanks,
JeffHi,
You are declaring a variable inside a static method,
that means you are opening a static scope... i.e. static block internally...
whatever the variable you declare inside a static block... will be static by default, even if you didn't add static while declaring...
But if you put ... it will be considered as redundant by compiler.
More over, static context does not get "this" pointer...
that's the reason we refer to any non-static variables declared outside of any methods... by creating an object... this gives "this" pointer to static method controller.

Non-static variable from a static context

This is the error i get . If i understand the error correctly it says im using a static variable when i shouldnt be? Or is it the other way round? below the error is the actual code....
The error...
Googler.java:27: non-static variable this cannot be referenced from a static context
          submitButton.addActionListener(new ButtonHandler());The code...
          JButton submitButton = new JButton("Submit Query");
          submitButton.addActionListener(new ButtonHandler());

thanks for the response.
I have already tried what you said but I tried it again anyway and i get the same error more less...
Googler.java:28: non-static variable this cannot be referenced from a static context
          ButtonHandler buttonHandler = new ButtonHandler();here is part of my code
public class Googler
static JTextField input1, input2;
     public static void main(String[] args)
          JFrame myFrame = new JFrame("Googler v1.0");
          Container c = myFrame.getContentPane();
          JLabel lab1 = new JLabel("Enter Google Query:");
          JLabel lab2 = new JLabel("Enter Unique API Key:");
          input1 = new JTextField(15);
          input2 = new JTextField(15);
          JRadioButton radSearch = new JRadioButton("Search Query");
          JRadioButton radCached = new JRadioButton("Cached Query");
          JButton submitButton = new JButton("Submit Query");
          ButtonHandler buttonHandler = new ButtonHandler();
          submitButton.addActionListener(buttonHandler);
          ButtonGroup group = new ButtonGroup();
          group.add(radSearch);
          group.add(radCached);Ive tried declaring buttonHandler as a static variable and this dosn't work either. I've never had this problem before it must be something silly im missing...?
Thanks
Lee

Non-static variable from static context?

Hi,
I've created a program using swing components
and I've set up a addActionListener to a button,
button.addActionListener(this);
when try and compile I get the following error:
non-static variable this cannot be referenced from a
static context
button.addActionListener(this);
I've checked site and my notes I don't seem to have
done anything different from programs that have compiled
in the past.
I'm currently doing a programming course so I'm fairly
new to Java, try not to get to advanced on me :)
Thx in advance for any help.
Chris

Well what is declared static? If I remeber right this error means that you have a static method that is trying to access data it does not have access to. Static methods cannot access data that is intance data because they do not exist in the instance (not 100% sure about this but I believe it is true, at the very least I know they do not have access to any non-static data). Post some more of your code like where you declare this (like class def) and where you set up the button.

When is Static variables memeory freed ?

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