Unload class (to free static references)?

My problem: apache fop has a bug that it has a static Map field in a class that increases constantly. Is it possible to "unload" this class to ensure freeing up the references from this static field? And subsequent calls would reload the class when needed?

Those streams often are created within a method
and there's no reference (variable) to it outside
the method (e.g. in the class or super class)?Not sure that reflection will provide you with a work-around for this; although, you could write your own classloader to load JFreeChart, and have it provide your own implementation of ImageInputStream which retains references to un-closed streams so they can be killed off later.
We're definitely on the way to the land of ick with this sort of hack, though... I think maintaining your own fork of the source code and keeping it in sync with the main project might actually turn out to be easier. That or beating the dev's around the head to get your fixes accepted.
Dave.

Similar Messages

How to Unload Classes?

i have created custom classloader. now i want to unload classes those are loaded by custom classloader.
i know that the classes will be unloaded when its classloader is unloaded. so i want to know how to unload cusotm classloader.
In ClassLoader class there is one static inner class it has unload method . but we can not access that method from custom classloader.

as a rule of thumb,
an object of may get marked for gc once no reachable references to it exist in the system.
a type (class) gets unloaded once:
1) no reachable references to it exist in the system;
2) there are no instance of that type left in the system or all the instances are unreachable (which is essencially the same as 1)
a classloader gets unloaded once there are no reachable references to it exist in the system.
reachable is the keyword here. Otherwise, if you had two classes A & B references each other, they would never be marked for gc. But as long as no other reference to either A or B exist, they both can be gc'ed. The same applies to classloaders (after all, they are just regular objects). Even though type T holds a reference to its classloader, if nothing refers to T, it's reference to the classloader doesn't count.

How can I write an instance of a class in a static variable

Hi !
I have an instance of a class
Devisen dev = new Devisen();
In an other class I have a static method and I need there the content of some variables from dev.
public static void abc()
{ String text=dev.textfield.getText()
I get the errormessage, that the I cannot use the Not-static variable dev in a static variable.
I understand that I cannot reference to the class Devisen because Devisen is not static I so I had to reference to an instance. But an instance is the same as a class with static methodes. (I think so)
Is there a possibility, if I am in a static method, to call the content of a JTextField of an instance of a class ?
Thank you Wolfgang

Hallo, here is more code for my problem:
class Login {
   Devisen dev=new Devisen();
class Devisen {
   JTextField field2;
   if (!Check.check_field2()) return; // if value not okay than return
class Check {
   public static void check_field2()
     HOW TO GET THE CONTENT OF field2 HERE ?
One solution ist to give the instance to the static function, with the keyword "this"
if (!Check.check_field2(this)) return;and get the instance
public static void check_field2(Devisen dev)BUT is that a problem for memory to give every method an instance of the class ? I have 50 fields to control and I dont want do give every check_method an instance of Devisen, if this is a problem for performance.
Or do I only give the place where the existing instance is.
Hmm...?
Thank you Wolfgang

How do you make a static reference to a method? I've included code.

I'm sorry but this is a cross post. This should be here but it is also in the 100% pure Java forum. It won't happen again.
Now...
Why doesn't this work? How do I use the method add(int a, int b)?
ERROR - "Can't make static reference to method int add(int, int) in testClass"
interface testInterface{
    static String sString = "TESTING";
    public int add(int a, int b);
class testClass implements testInterface{
    public int add(int a, int b){
        return a+b;
    public static void main(String argv[]){
        int sum = add(3,4);    // here's the error
        System.out.println("test");
        System.out.println( sum );
}Again, I apologize for the cross post.

hi,
this seems to be pretty easy, isn't it?
Oh c'mon! You try to invoke a non-static method from within a static method. Solution: Create a specific instance of class testClass:
testClass test=new testClass();
test.add(3,4);best regards, Michael

Can't make static reference to method while it is static

Hello, can somebody please help me with my problem. I created a jsp page wich includes a .java file I wrote. In this JSP I called a method in the class I created. It worked but when I made the method static and adjusted the calling of the method it started to complain while i didnt make an instance of the class. the error is:Can't make static reference to method
here is the code for the class and jsp:
public class PhoneCheckHelper {
public static String checkPhoneNumber(String phoneNumber) {
String newPhoneNumber ="";
for(int i=0; i<phoneNumber.length(); i++) {
char ch = phoneNumber.charAt(i);
if(Character.isDigit(ch)) {
newPhoneNumber += String.valueOf(ch);
return newPhoneNumber;
<html>
<head>
<title>phonecheck_handler jsp pagina</title>
<%@page import="java.util.*,com.twofoldmedia.text.*, java.lang.*" %>
</head>
<body>
<input type="text" value="<%= PhoneCheckHelper.checkPhoneNumber(request.getParameter("phonenumberfield")) %>">
</body>
</html>

Go over to the "New to Java" forum where that message is frequently explained. Do a search if you don't see it in the first page of posts.

Can't make static reference to method

hi all,
pls help me in the code i'm getting
" can't make static reference to method....."
kindly help me
the following code gives the error:
import java.rmi.*;
import java.rmi.Naming;
import java.rmi.RemoteException;
import java.io.*;
import java.io.IOException;
import java.io.LineNumberReader;
public class client
static String vcno;
static String vpin;
static String vamt;
static String vordid;
static String vptype;
//static String vreq;
static String vreq = "1";
static String vorgid;
static String vm1;
static String vs1;
static String vqty1;
static String vm2;
static String vs2;
static String vqty2;
static String vm3;
static String vs3;
static String vqty3;
static String vm4;
static String vs4;
static String vqty4;
public static void main(String args[])
try
ServerIntf serverintf=(ServerIntf)Naming.lookup("rmi://shafeeq/server");
int c;
StringBuffer sb;
FileReader f1=new FileReader("c:/testin.txt");
sb=new StringBuffer();
int line=0;
while ((c=f1.read())!=-1) {
sb.append((char)c);
LineNumberReader inLines = new LineNumberReader(f1);
String inputline;
String test;
while((inputline=inLines.readLine())!=null)
line=inLines.getLineNumber();
switch(line)
case 1: {
vcno = inLines.readLine();
System.out.println(vcno);
case 2: {
vpin = inLines.readLine();
System.out.println(vpin);
case 3: {
vptype = inLines.readLine();
System.out.println(vptype);
case 4: {
vamt = inLines.readLine();
System.out.println(vamt);
case 5: {
vordid = inLines.readLine();
System.out.println(vordid);
case 6: {
vorgid = inLines.readLine();
System.out.println(vorgid);
     case 7: {
vm1 = inLines.readLine();
System.out.println(vm1);
     case 8: {
vs1 = inLines.readLine();
System.out.println(vs1);
     case 9: {
vqty1 = inLines.readLine();
System.out.println(vqty1);
     case 10: {
vm2 = inLines.readLine();
System.out.println(vm2);
     case 11: {
vs2 = inLines.readLine();
System.out.println(vs2);
case 12: {
vqty2 = inLines.readLine();
System.out.println(vqty2);
case 13: {
vm3 = inLines.readLine();
System.out.println(vm3);
     case 14: {
vs3 = inLines.readLine();
System.out.println(vs3);
case 15: {
vqty3 = inLines.readLine();
System.out.println(vqty3);
case 16: {
vm4 = inLines.readLine();
System.out.println(vm4);
case 17: {
vs4 = inLines.readLine();
System.out.println(vs4);
case 18: {
vqty4 = inLines.readLine();
System.out.println(vqty4);
f1.close();
FileWriter f2=new FileWriter("c:/testout.txt");
String t;
t=ServerIntf.add(vcno,vpin,vamt,vordid,vptype,vreq,vorgid,vm1,vs1,vqty1,vm2,vs2, vqty2,vm3,vs3,vqty3,vm4,vs4,vqty4);
String str1 = " >>";
str1 = t + str1;
f2.write(str1);
System.out.println('\n'+"c:/testout.txt File updated");
f2.close();
catch(Exception e)
System.out.println("Error " +e);

Yes, ServerIntf is the interface type. The instance serverIntf. You declared it somewhere at the top of the routine.
So what you must do is call t=serverIntf.add(...)This is probably just a mistype.

STATIC REFERENCE

THE CODE IS ATTACHED. I NEED TO SOLVE THE ERROS.
THIS IS THE ERRORS AFTER COMPILE
C:\My Documents\Java\Assignment\Assign3\PwdClient.java:47: Can't make static reference to method void setLayout(java.awt.LayoutManager) in class java.awt.Container.
                    setLayout(new FlowLayout());
                    ^
C:\My Documents\Java\Assignment\Assign3\PwdClient.java:48: Can't make static reference to method void setBackground(java.awt.Color) in class java.awt.Component.
                    setBackground(Color.green);
                    ^
C:\My Documents\Java\Assignment\Assign3\PwdClient.java:53: Can't make static reference to method java.awt.Component add(java.awt.Component) in class java.awt.Container.
                    add(l);
                    ^
C:\My Documents\Java\Assignment\Assign3\PwdClient.java:58: Can't make static reference to method java.awt.Component add(java.awt.Component) in class java.awt.Container.
                    add(name);
                    ^
C:\My Documents\Java\Assignment\Assign3\PwdClient.java:61: Can't make a static reference to nonstatic variable tf_name in class PwdClient.
                    tf_name = new TextField(20);
                    ^
5 errors
//HERE IS THE CODE
import java.io.*;
import java.net.*;
import javax.swing.*;
import java.util.*;
import java.awt.*;
import java.awt.event.*;
class PwdClient extends Frame
     private TextField tf_name;
     private TextField tf_password;
     private String input_str_name;
     private String input_str_password;
     private String str_to_pass;
     //private String str;
     //private String str_name;
     //private String str_password;
     //private Button button1;
     private final static int buffsize = 512;
     public static void main(String args[])
          try
                    String server = args[0];
                    int port = Integer.parseInt(args[1]);
                    //create socket
                    Socket s = new Socket(server, port);
                    //readresult from server
                    InputStream is = s.getInputStream();
                    DataInputStream dis = new DataInputStream(is);
                    OutputStream os = s.getOutputStream();
                    DataOutputStream dos = new DataOutputStream(os);
                    InputStreamReader isr = new InputStreamReader(System.in);
                    BufferedReader br = new BufferedReader(isr);
                    //Frame frm = new Frame( "Client Server" );
                    setLayout(new FlowLayout());
                    setBackground(Color.green);
                    String str = "Enter Name and Password:";
                    Label l= new Label( str, Label.CENTER);
                    l.setBackground(Color.red);
                    add(l);
                    String str_name = "Name:";
                    Label name = new Label( str_name, Label.CENTER);
                    name.setBackground(Color.red);
                    add(name);
                    //Name textfield User input for name
                    tf_name = new TextField(20);
                    add(tf_name);
                    String str_password = "Password:";
                    Label password = new Label( str_password, Label.CENTER);
                    password.setBackground(Color.red);
                    add(password);
                    //Password Textfield (user input for password)
                    tf_password = new TextField(20);
                    tf_password.setEchoChar('*');
                    add(tf_password);
                    Button Button1 = new Button("Submit");
                    add(Button1);
                    Button1.addActionListener( new ActionListener()
                         public void actionPerformed(ActionEvent ae)
                              input_str_name = tf_name.getText();
                              tf_name.setText(new String(""));
                              input_str_password = tf_password.getText();
                              tf_password.setText(new String(""));
                              str_to_pass = input_str_name+"/"+input_str_password;
                              dos.writeBytes(str_to_pass);
                              String received_from_server = brr.readLine();
                              l.setText(received_from_server, Label.CENTER);
                    frm.setSize( 300, 200 );
                    frm.show();
                    frm.addWindowListener( new WindowAdapter()
                         public void windowClosing(WindowEvent e)
                         { System.exit(0);}
          }//closes the try
          catch(Exception e){}

Put the code you have in static main in a constructor, or make an instance of PwdClient in main that you can call those methods on. You can't call non-static methods directly from a static method (main). You need to instantiate the class.
public class PwdClient extends JFrame {
public PwdClient() {
    // the code from your main method
public static void main(String[] args) {
    new PwdClient(); // makes a new instance of PwdClient
}

Static reference; how am I instantiating this incorrectly?

Hi, all:
I'm getting a static reference error, but I don't know how to fix it. I'm trying to generate a list of objects, and I can't tell where the problem is located. I've chopped all the irrelevant stuff out of this class so you can see what's going on:
public class VonNeumannGridNet {
     public int nCols = -1;
     public int nNodes;
     public int nRows = -1;
     public int radius = -1;
     public ArrayList <Node> nodeList;
     public Class <Node> nodeClass;
     public Class <Edge> edgeClass;
     public VonNeumannGridNet ( Class<Node> node, Class <Edge> edge, int cols, int rows, int connectRadius) {
          nodeClass = node;
          edgeClass = edge;
          nCols = cols;
          nRows = rows;
          nNodes = nCols * nRows;
          radius = connectRadius;
     public ArrayList<Node> makeVonNeumannGridNet(Class<Node> node, Class<Edge> edge,
               int cols, int rows, int connectRadius) throws IllegalAccessException, InstantiationException     {
          return nodeList;
     public static List <Node> makeVonNeumannGridNet(int cols, int rows, int connectRadius,
               double rewireProb, Class <Node> node, Class <Edge> edge) {
          ArrayList <Node> list = makeVonNeumannGridNet(node, edge, cols, rows, connectRadius); //ERROR HERE
          ArrayList <Node> network = new ArrayList <Node> (NetUtilities.randomRewireSymmetric(list, rewireProb));
          return network;
public Class <Node> getNodeClass() {     return nodeClass;}
public void setNodeClass(Class <Node> node)     {nodeClass = node;}
public Class <Edge> getEdgeClass()     { return edgeClass;}
public void setEdgeClass(Class <Edge> edge) {edgeClass = edge;}
}The error happens when I try to "makeVonNeumannGridNet"; how is that a static reference?

Geesh what a worthless discussion.
After all, the cause of your problem is that you're trying to access an instance method from within a static (non-instance!) method. The possible solutions might be obvious:
1) make both methods non-static.
2) make both methods static.
3) make the calling method non-static and make the method-to-be-called static.
4) instantiate the class having the instance method and invoke that method using the reference.
Read a good Java book/tutorial and learn about static vs. non-static. Don't use them randomly, but use them when the design and behaviour require.

Error: Cannot make a static reference to the non-static method

Below is a java code. I attempt to call method1 and method2 and I got this error:
Cannot make a static reference to the non-static method but If I add the keyword static in front of my method1 and method2 then I would be freely call the methods with no error!
Anyone has an idea about such error? and I wrote all code in the same one file.
public class Lab1 {
     public static void main(String[] args) {
          method1(); //error
     public void method1 ()
     public void method2 ()
}

See the Search Forums at the left of the screen?
If you had searched with "Cannot make a static reference to the non-static method"
http://search.sun.com/search/onesearch/index.jsp?qt=Cannot+make+a+static+reference+to+the+non-static+method+&rfsubcat=siteforumid%3Ajava54&col=developer-forums
you would have the answer. Almost every question you will ask has already been asked and answered.

How do I make a static reference to a method? Sample Code Included

Why doesn't this work? How do I use the method add(int a, int b)?
ERROR - "Can't make static reference to method int add(int, int) in testClass"
interface testInterface{
    static String sString = "TESTING";
    int add(int a, int b);
class testClass implements testInterface{
    public int add(int a, int b){
        return a+b;
    public static void main(String argv[]){
        int sum = add(3,4);    // here's the error
        System.out.println("test");
        System.out.println( sum );
}

Why doesn't this work?Because you can't call a non-static method like add (which operates on the object called this) from a static method like main (for which there is no this).
There are two ways to fix this:
(1) In main, create a TestClass object, and call add() for that object:
class TestClass implements TestInterface {
   public int add(int a, int b) {
      return a+b;
   public static void main(String[] args) {
      TestClass testObject = new TestClass();
      int sum = testObject.add(3, 4);
      System.out.println("test");
      System.out.println(sum);
}(2) Make add() static. This is the preferred approach, because add() doesn't really need a TestClass object.
class TestClass implements TestInterface {
   public static int add(int a, int b) {
      return a+b;
   // main is same as the original
}

Thread safety: exposing "this" via a "static" reference in constructor

please consider:
public class Foo {
public static List<Foo> list = new ArrayList<Foo>();
public int i = 1;
Foo() {
    Foo.list.add(this);
    i++;
    // ...... other stuff that does not effect "i"
}Exiting, I want "i" to always be 2. But what if a thread got the reference to "this" from the static List before the constructor exited and did:
((Foo) Foo.list.get(0)).i++;Is the only solution a synchronized wrapper for "list"?
Thanks.

A synchronized wrapper is not sufficient:
public static List<Foo> list = Collections.synchronizedList(new ArrayList<Foo>());This is because the list is now thread-safe, but what about the items in it?
Look at your constructor:
Foo.list.add(this);
i++;Another thread could access the list in between the execution of these two lines and see the value of i before i is incremented.

Why inner classes cannot have static declarations ?

Hi Friends,
When i tried to make static declarations on a inner class which is non static, i am getting compilation error saying "inner classes cannot have static declarations". I want to know reason behind this implementation.
Code which i have tried:
public class TestOuter
class TestInner{
static int i =10;
public static boolean validate(int a){
if(a==0)
System.out.println("Invalid data");
return false;
return true;
public static void main(String a[]){
boolean result = new TestOuter.TestInner.validate(0);
System.out.println("Result="+result);
Thanks,
Shiju V.

so I think if the
outer class is not static , then Inner class can't be
static as well. This is incorrect. An enclosed class can be indeed static while the outer is not, and vice versa.
The difference between static/non static in regards to enclosed classes is that the static ones are 'top-level' and cannot access the members of the enclosing class.
The effect of making an enclosed class static means there is only one instance of any variables, no matter how many instances of the outer class are created. In this situation how could the static inner class know which variables to access of its non static outer class. Of course the answer is that it could not know, and thus an static inner class cannot access instance variables of its enclosing class.
Now, regarding non-static inner classes, and trying to give a valid answer to the original post:
As with instance methods and variables, a non-static inner class is associated with an instance of its enclosing class and has direct access to that object's instance variables and methods.
TestOuter outer = new TestOuter();
TestOuter.TestInner inner = outer.new TestInner();Because an inner class is associated with an instance (inner class implicitly keeps a reference to the object of the enclosing class that created it), it cannot define any static members itself. Static members cannot access the this pointer.
So, in an ordinary (non-static) inner class, the link to the outer class object is achieved with a special this reference. A static inner class does not have that special this reference, nor would a static method/variable of an ordinary (non-static) inner class.

Open VI reference from a static reference

Hello guys,
To open VIs dynamically, instead of hardcoding the VI name in a string constant, I do this as shown below to keep a dependency to my VI, so if someone ever delete the VI from the project, move it, rename it, it will create a broken wire somewhere so we'll be able to fix it faster.
However, don't you think it's a bit weird to have the reference to the VI already, but we need to open a new reference, is there a way to get rid of the property node to get the VI path? Is the static VI reference should be closed after the other reference is opened?
Would it make sense to be able to right clic the static reference and specify the option to open a call and forget reference?
Is there a better practice?
Cheers,
Solved!
Go to Solution.

From the LabVIEW help:
vi path accepts a string containing the name of the VI that you want to reference or a path to the VI that you want to reference. If you wire a name string, the string must match the full delimited name of a VI in memory on that target. If you wire a path, LabVIEW searches for a VI in memory that you previously loaded from that path on the same target. If a matching VI is not found in memory, LabVIEW then tries to load the VI from that file on disk. An error occurs if LabVIEW cannot find the file or if the file conflicts with another VI in memory.
So, basically, if you use the string, you load the VI in memory and get an error if not found. If you use path, it will still use the VI in memory at that path, or attempt to load it. In your case, it won't make much difference. I typically use VI Name myself since I know it is in memory.

Service "No valuation class exists for account reference"

Hi,
in AC01 trx, I cannot create a Service.
The error is:
No valuation class exists for account reference
I tried with trx OMSK to link the account category reference to material type but won't work.
The error is always: No valuation class exists for account reference

Hi,
try also to check from SPRO, Materials Management, External Services Management, Service Master, "Define Service Category", you have to set the standard "Account category reference" 0006 or the one (custom) you have set in customizing also check (Valuation and Account Assignment trx OMSK)
Regards

Force Derived Class to Implement Static Method C#

So the situation is like, I have few classes, all of which have a standard CRUD methods but static. I want to create a base class which will be inherited so that it can force to implement this CRUD methods. But the problem is, the CRUD methods are static. So
I'm unable to create virtual methods with static (for obvious reasons). Is there anyway I can implement this without compromising on static.
Also, the signature of these CRUD methods are similar.
E.g. ClassA will have CRUD methods with these type of Signatures
public static List<ClassA> Get()
public static ClassA Get(int ID)
public static bool Insert(ClassA objA)
public static bool Update(int ID)
public static bool Delete(int ID)
ClassB will have CRUD signatures like
public static List<ClassB> Get()
public static ClassB Get(int ID)
public static bool Insert(ClassB objB)
public static bool Update(int ID)
public static bool Delete(int ID)
So I want to create a base class with exact similar signature, so that inherited derived methods will implement their own version.
For E.g. BaseClass will have CRUD methods like
public virtual static List<BaseClass> Get()
public virtual static BaseClassGet(int ID)
public virtual static bool Insert(BaseClass objBase)
public virtual static bool Update(int ID)
public virtual static bool Delete(int ID)
But the problem is I can't use virtual and static due to it's ovbious logic which will fail and have no meaning.
So is there any way out?
Also, I have few common variables (constants) which I want to declare in that base class so that I don't need to declare them on each derived class. That's why i can't go with interface also.
Anything that can be done with Abstract class?

Hi,
With static methods, this is absolutely useless.
Instead, you could use the "Singleton" pattern which restrict a class to have only one instance at a time.
To implement a class which has the singleton pattern principle, you make a sealed class with a private constructor, and the main instance which is to be accessed is a readonly static member.
For example :
sealed class Singleton
//Some methods
void Method1() { }
int Method2() { return 5; }
//The private constructor
private Singleton() { }
//And, most importantly, the only instance to be accessed
private static readonly _instance = new Singleton();
//The corresponding property for public access
public static Instance { get { return _instance; } }
And then you can access it this way :
Singleton.Instance.Method1();
Now, to have a "mold" for this, you could make an interface with the methods you want, and then implement it in a singleton class :
interface ICRUD<BaseClass>
List<BaseClass> GetList();
BaseClass Get(int ID);
bool Insert(BaseClass objB);
bool Update(int ID);
bool Delete(int ID);
And then an example of singleton class :
sealed class CRUDClassA : ICRUD<ClassA>
public List<ClassA> GetList()
//Make your own impl.
throw new NotImplementedException();
public ClassA Get(int ID)
//Make your own impl.
throw new NotImplementedException();
public bool Insert(ClassA objA)
//Make your own impl.
throw new NotImplementedException();
public bool Update(int ID)
//Make your own impl.
throw new NotImplementedException();
public bool Delete(int ID)
//Make your own impl.
throw new NotImplementedException();
private CRUDClassA() { }
private static readonly _instance = new CRUDClassA();
public static Instance { get { return _instance; } }
That should solve your problem, I think...
Philippe

Unload class (to free static references)?

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