Instance Variable Are Not Thread Safe
Jul 10, 2014Implementation that proves that instance variables are not thread safe ....
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Implementing A Thread-safe Queue
Jul 10, 2014I have situation where a user can request java server to send a value to an embedded device, and if the device is asleep, that value needs to be stored in a queue until the device wakes up and sends a position to java server, at which point the server checks if there is a value in the queue and if there is, it then sends that value to the device. The maximum size of the queue is 1 for now. And if the user makes continuous requests to java server, the old value is removed and the new value is added.
Initially I was looking into BlockingQueue, but the problem with that is, well, it blocks. queue.put(value) will block if queue is full, and queue.take() will block if queue is empty. I can't have something that blocks. When the device responds to server, server checks if value is in queue, if it is not then the server carries on the rest of its responsibility. Thus, I then entertained ConcurrentLinkedQueue. While queue.offer(value) and queue.poll(); allow you to add and remove values respectively from the queue without blocking, it does not allow you to set a maximum size limit of the queue. My queue must have a maximum size limit and it has to always be the newest value the user submits to the queue (where old values are removed).So this is what I came up with:
Java Code: class Unit {
private List<Integer> cmdQueue;
public Unit(){
cmdQueue = Collections.synchronizedList(new LinkedList<Integer>());
[code]....
I use Collections.synchronizedList, as I do here, do I still need to use synchronize as I did above?
RMI :: Using RMI To Wrap A Process That Is Not Thread Safe
Apr 3, 2013I have a multi-threaded application but one function (A) I need to call, that is provided by a third party, is not thread safe. I need to make parallel calls to function (A) so my only option is to start multiple process that I can call from each of my main applications threads.
To do this I created a RMI interface between a the client and the remote process. On the server side I start multiple processes each assigned a different port. On the client side I have a queue manager that connects to the processes I started and builds a queue of them. It passes the client to the threads as needed so they can function in parallel.
I am new to Java development and have the following questions:
1. Each of the server processes require an init function to be run before providing the service. How do I setup the server processes to continue to run in memory listening for a a service request after running the init processes.
2. Is there a better implementation of what I am trying to do?
Servlets :: Static Variables In Thread-safe?
Jan 22, 2015Are the static variables in servlet thread-safe?
View Replies View RelatedEnterprise JavaBeans :: Does Container Managed Entity Manager Is Thread-safe In Stateless Session Bean?
May 8, 2014I read JEE6 doc and confused with : Does container managed entity manager (injected by PersistenceContext annotation) is thread-safe in stateless session bean in multiple-thread env?
See code below, if there are 2 requests to stateless sesion bean in 2 concurrent threads , is it using same Entity Manager Instance or not?
@Stateless(name = "HRFacade", mappedName = "HR_FACES_EJB_JPA-HRFacade-HRFacade")
public class HRFacadeBean implements HRFacade, HRFacadeLocal {
@Resource
SessionContext sessionContext;
@PersistenceContext(unitName = "HRFacade")
private EntityManager em;
[Code] .....
Accessing A Variable Of A Thread From Another Thread
Jul 16, 2014class A extends Thread
{
String str;
public void run( )
{
//random stuff
}
}
[Code]....
Servlets :: Thread Safety Should Not Use Any Variables Or Objects At Instance / Class Level
Jun 3, 2014As web server has multiple threads to serve client requests in Thread Pool & to ensure Thread Safety we should not use any variables or Objects at Instance/Class level.But in case of Session Variable which one is the Best Practice as the Session object is used by all the requests to have the same Session ID.
My Code :
public class MyServlet extends HttpServlet {
private static Logger log = Logger.getLogger(ClientRegistrationServlet.class);
private HttpSession session; /* This is used at Instance Level*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
[code]....
Instance Variable For Objects?
Feb 23, 2015I am working on a project and it asks me to "Provide appropriate names and data types for each of the instance variables. Maintain two GVdie objects" under class fields. I am unsure as to what is being asked when asking for two objects as instance variables and how I would write that...
View Replies View RelatedOne Object / One Instance Variable - Different Values?
Feb 5, 2015Let's pretend I'm working on an RPG. Like in all RPGs, there are items found all throughout the imaginary world. Each player and NPC can obtain an item. My question will concern those items.
In other words, I'd like to use instances of a class in multiple places of the code. Each instance will have its own, individual values of instance variables, with one obvious exception: itemQuantity should have a different value in playerInventory, npcInventory, etc. Also, I'd like a list of all items that can be found in the game. This list doesn't need itemQuantity at all.
class Items {
String itemName;
float itemWeight;
int itemQuantity;
[Code] ....
The question is: should I really make itemQuantity an instance variable of the Item class? It seems as though for each copy of the Item class I should create a separate copy with different value of itemQuantity, but that's not very efficient. Where is the error in my logic?
What's important is that there may be plenty items in a game and a player may be given power to create new items during the course of the game.
Variable In Interface Cannot Be Instance Scope?
May 5, 2014I'm just wondering why variables in interface can't be instance scope?
interface Test{
int a;
}
And then
Test test = new TestImpl();
test.a=13;
Yes, it violates OO, but I don't see why this is not possible? Since interface is not an implementation, therefore it can;t have any instance scope variable. I can't find the correlation of interface being abstract and being able to hold instance scope variable. There's gotta be another reason. I'm just curious about any programmatic limitation, not deliberate design constraint. the example of programmatic limitation is like when Java forbids multiple inheritance since when different parents have the exact same method, then the child will have trouble determining which method to run at runtime.
New Instance Variable Values Not Updating
May 6, 2015Alright, I have a JavaFX gui that is creating a new instance of data calculation to graph in a chart; however, the data is not updating each time the Platform.runLater() feature executes. Each time an event occurs, a new instance with the same variable name occurs. I use to get methods to retrieve the data I want, so shouldn't the values update each time the new instance is created? This is a very condensed version of what happens with the event, but this is what is not working correctly.
Event:
solarPlot = new SolarTracker();
solarPlot.getElevation();
solarPlot.getAzimuth();
Class constructor :
public SolarTracker() {
[Code] .....
Unable To Change Value Of Instance Variable
May 23, 2014Is there anything wrong on my code?
import java.util.*;
public class Card
{
private String description;
private String suit;
private int value;
private static final String [] cardName {"Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten",
"Jace","Queen","King","Joker"};
[Code]...
Difference Between Instance And Class Variable
Mar 25, 2014I'm having trouble to fully understand the difference between instance and class variables.
I though it was all about the static, i.e:
int age; // instance variable
static int age; // class variable
What's behind this?
Swing/AWT/SWT :: What Is Event-dispatching Thread For GUIs And Why Use It Over Main Thread
Sep 20, 2014I'm currently learning about Swing but I can't get my head round this piece of the code. Here is a simplified gui (not interested in the gui part but the execution)
public class SwingDemo implements ActionListener {
SwingDemo(){
JFrame jfrm = new JFrame("Simple gui pro");
//rest of code
public static void main(String[] args) {
new SwingDemo();
}
I get the above, create a new instance of SwingDemo in the main thread which starts up the gui through the constructor. However, then the tutorial says that I should avoid doing the above but do this instead:
public class SwingDemo implements ActionListener {
SwingDemo(){
JFrame jfrm = new JFrame("Simple gui pro");
//rest of code
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() { //why do this instead?
public void run(){
new SwingDemo();
}
});
}
}
Reading, it talks about an event-dispatching thread which has completely lost me... Why not just instantiate the object directly instead of creating another thread?
Passing 2 Sub Arrays Into Sorting Thread Then To A 3rd Thread Merge
Oct 17, 2014im having an issue with the 3rd thread that are supposed to merge the two sorted sub arrays , i pass the 2 subarrays to my runnable function sortlist and they are renamed IntSortList 1 and 2 and th1.start() and th1.join() are called and it works fine, but then i have another runnable constructor that takes IntSortList 1 and 2 but it does take a runnable. below is the code in my main,
Runnable InSortlist1 = new sortList(data2p1);
Runnable InSortlist1 = new sortList(data2p1);
Thread th1 = new Thread (IntSortlist1);
Thread th2 = new Thread (IntSortlist2);
try
{
th1.start();
th1.join();
[code]....
Converting Parent Instance To Child Instance?
Mar 7, 2014I've Parent and child(extends Parent) class To initialize the constructors, I'm injecting from google.juice#injector. Let me show the code,
Parent.class
public class Parent{
private Animal animal;
@inject
Parent(Animal animal){
this.animal = animal;
[code].....
When I do this, ClassCastException is happening. Why is it so? Is there any way to convert instance of parent to child instance.
Is It Safe To Set Font With Absolute Parameters
Feb 14, 2015I was wondering, if, for example I set font for my textarea like this:
area.setFont(new Font("Serif", Font.ITALIC, 18));
Is that last parameter, in this case 18, will be actually the same size for all users? Maybe I should avoid these absolute numbers?
How To Access One Thread From Another Thread
Oct 8, 2014How to access one thread from another thread?
View Replies View RelatedDifference In Variable Assignment Between Constructor And Variable Section
May 21, 2014Given the case I have an object which is assigned only once. What is the difference in doing:
public class MyClass {
private MyObj obj = new MyObj("Hello world");
private MyClass(){}
//...
}
and
public class MyClass {
private MyObj obj;
private MyClass(){
obj = new MyObj("Hello world");
}
//...
}
Is there a difference in performance or usability? (Given also the case there are no static calls expected) ....
Are Terms Local Variable And Member Variable Comparable
Oct 27, 2014The term "Local variable" is related to scope. That is a local variable is one which is defined in a certain block of code, and its scope is confined inside that block of code.And a "Member variable" is simple an instance variable.
I read in a discussion forum that when local variables are declared (example code below), their name reservation takes place in memory but they are not automatically initialized to anything. On the other hand, when member variables are declared, they are automatically initialized to null by default.
Java Code: public void myFunction () {
int [] myInt; // A local, member variable (because "static" keyword is not there) declared
} mh_sh_highlight_all('java');
So it seems that they are comparing local variables and member variables. While I think a member variable can also be be local in a block of code, isn't it?
Reference Variable - Create Another Variable And Set It Equal To First
Jan 11, 2015Given a reference variable : Car c1 = new Car();
when we create another variable and set it equal to the first : Car c2 = c1;
we're pointing c2 at the same car object that c1 points to (as opposed to pointing c2 at c1, which in turn points at the car). So if we have code like,
Car c1 = new Car();
Car[] cA = {c1, c1, c1, c1};
are we doing the same? Are we creating four *new* reference variables, each of which points at the same car (again as opposed to pointing them at c1 itself)? I think so, but want to make sure I'm understanding this correctly.
How To Use Value Of String Variable Cel1 As Variable Name
May 23, 2014I have a JFrame jf and JPanel jp on it. jp has five TextFields named cel1, cel2.. cel5. I wish to construct a String Cel + for loop index and run a for loop to reset the values of all the text fields using a single statement such as cel1.SetText("abc"). Similar things can be done in foxfro. How does one do it in java?
View Replies View RelatedHow To Delete A Thread
Nov 19, 2014Doesn't matter now, I solved it but I don't know how to delete a thread...
View Replies View RelatedAdd KeyListener To Thread
Feb 13, 2014I want to try and run a thread that starts running on start and quits after pressing a key.I cant seem to detect keypresses. ci cant seem to add code: Post denied. New posts are limited by number of URLs it may contain and checked if it doesn't contain forbidden words.
its just a code snippit without links or anything..this is a small part... but how do i add a keyListener in here that listends to a random key?The class implements KeyListener and the overrided method "keyPressed" sets the isRunning boolean to false... but the keyPressed method is never executed.
public static void main(String[] args) {
Thread thread = new Thread() {
public void run() {
while(isRunning){
System.out.println("running");
try {
sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
};
thread.start();
}
How To Start New Thread
Apr 27, 2014how to start a new thread
public class pracDraw extends JFrame {
private Color red=Color.red;
public int i;
private Color white = Color.white;
JPanel pr=new JPanel();
JTextField t=new JTextField();
[code]....
If u run it you can see that the JTextField (t1) is drawn on the panel along the line but it does so with a gray border. How do i eliminate that grey border and only draw the text field along the line and finally make the text field green after it reaches the end of line?
What Does Instance Mean
Jan 16, 2015in a set of instructions they keep referring to instance versions of things I've heard of before ie "private instance array of String references" (wtf is a string reference?) or "instance string variable" so what does all this mean?
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