Is ArrayList(Collection c) constructor Thread-Safe?

Hi,
Say I have the following statement
public List getList() {
return new ArrayList(myList);
}Assuming I also have other methods which use/modify myList, does this method above need to be synchronized?
Looking into the ArrayList contructor, this eventually calls a static native method System.arraycopy, is this implicitly synchronized as it is native or do I need to take care of this myself?
Thanks

The constructor new ArrayList(List) is thread safe because no other thread can access the ArrayList until it because visable to another thread.
The myList might not be thread safe. i.e. is possible you could be copying the myList while another thread is modifying it? If so then myList should be synchronized.
Using Vector makes no difference as the list with an issue is myList. Even if myList was a Vector you would have same problem as more than one method is accessed so you would have race condition.
   public ArrayList(Collection c) throws NullPointerException {
        size = c.size();
        // Allow 10% room for growth
        elementData = new Object[
                (int) Math.min((size * 110L) / 100, Integer.MAX_VALUE)];
        c.toArray(elementData);
    }Note: the c.size() can change between c.size() and c.toArray() being called or while c.toArray() is called. The race condition could mean the collection not being copies or an exception being thrown even if all the methods on the collection are synchronized.
If you think Vector is so different.
    public Vector(Collection c) throws NullPointerException {
        elementCount = c.size();
        // 10% for growth
        elementData = new Object[
                (int) Math.min((elementCount * 110L) / 100, Integer.MAX_VALUE)];
        c.toArray(elementData);
    }The only real solution is if myList can be modifed by another thread is to synchronize it and add the following.
public List getList() {
   synchronized(myList) {
         return new ArrayList(myList);
}

Similar Messages

Java.lang.reflect.Constructor thread safe ?

When you have a Constructor object in Java.
Is calling the newInstance method on the same Constructor object from multiple threads safe.
Or is this not thread safe leading to wrongly constructed objects?

ejp wrote:
And as Constructors are immutable, it's hard to see how it's going to get altered.Constructors are basically immutable, but they have at least one state which can make the difference between newInstance working or failing.
As you can see in the following example, setAccessible() is not something you would want to set to different values in different threads.
import java.lang.reflect.Constructor;
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        cons.setAccessible(false);
        try {
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JDK 1.4.2 Collections.unmodifiableMap() thread safe?

Hello,
doesn anyone know if the call to Collections.unmodifiableMap() is thread safe.
Thanks
Marc

What do you mean?
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Thread-safe design pattern for encapsulating Collections?

Hi,
This must be a common problem, but I just can't get my head around it.
Bascially I'm reading in data from a process, and creating/updating data structuers from this data. I've got a whloe bunch of APTProperty objects (each with a name/value) stored in a sychronized HashMap encapsulated by a class called APTProperties. It has methods such as:
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Or should I just have a method called
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Hope this makes sense.
Keith

In that case, I think you need to provide your own locking mechanism.
public class APTProperties {
the add, get and remove methods call lock(), executes its own logic, then call unlock()
//your locking mechanism
private Thread owner; //dont need to be volatile due to synchronized access
public synchronized void lock() {
while(owner != null) {
wait();
th = Thread.currentThread();
public synzhronized void unlock() {
if(owner == currentThread()){
owner = null;
notifyAll();
}else {
throw new IllegalMonitorStateException("Thread: "+ Thread.currentThread() + "does not own the lock");
Now you dont gain a lot from this code, since a client has to use it as
objAPTProperties.lock();
Iterator ite = objAPTProperties.propertyIterator();
... do whatever needed
objAPTProperties.unlock();
But if you know how the iterator will be used, you can make the client unaware of the locking mechanism. Lets say, your clients will always go upto the end thru the iterator. In that case, you can use a decorator iterator to handle the locking
public class APTProperties {
public Iterator getThreadSafePropertyIterator() {
private class ThreadSafeIterator implements Iterator {
private iterator itr;
private boolean locked;
private boolean doneIterating;
public ThreadSafeIterator(Iterator itr){
this.itr = itr;
public boolean hasNext() {
return doneIterating ? false : itr.hasNext();
public Object next() {
lockAsNecessary();
Object obj = itr.next();
unlockAsNecessary();
return obj;
public void remove() {
lockAsNecessary();
itr.remove();
unlockAsNecessary();
private void lockAsNecessary() {
if(!locked) {
lock();
locked = true;
private void unlcokAsNecessary() {
if(!hasNext()) {
unlock();
doneIterating = true;
}//APTProperties ends
The code is right out of my head, so it may have some logic problem, but the basic idea should work.

Is XMLProcessor Thread Safe?

I was wondering if the XMLProcessor.processXSL(xsl, null) method is thread safe?
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null

is vector thread safe ?
is arraylist thread safe ?
i want to use any one of thsese without
Synchronised keyword
Vector is synchronized but ArrayList is not. This means that two different threads cannot access the same method in a Vector at the same time. An ArrayList can easily be made synchronized and will then behave as a Vector.
Now synchronized methods don't necessarily lead to thread-safe code. That depends on how the Vector/ArrayList is used. Have a look at the new concurrency package in Java 5. It contains lots of readymade collections with different behaviours. Maybe one will suit your needs and be thread-safe with your usage right out of the box.

Is Vector thread-safe in J2ME?

I understand java.util.Vector is thread-safe in J2SE. How about in Vector in J2ME? I did not find any documentation about this.
Any help is appreciated.

is vector thread safe ?
is arraylist thread safe ?
i want to use any one of thsese without
Synchronised keyword
Vector is synchronized but ArrayList is not. This means that two different threads cannot access the same method in a Vector at the same time. An ArrayList can easily be made synchronized and will then behave as a Vector.
Now synchronized methods don't necessarily lead to thread-safe code. That depends on how the Vector/ArrayList is used. Have a look at the new concurrency package in Java 5. It contains lots of readymade collections with different behaviours. Maybe one will suit your needs and be thread-safe with your usage right out of the box.

SSRS - Is there a multi thread safe way of displaying information from a DataSet in a Report Header?

In order to dynamically display data in the Report Header based in the current record of the Dataset, we started using Shared Variables, we initially used ReportItems!SomeTextbox.Value, but we noticed that when SomeTextbox was not rendered in the body
(usually because a comment section grow to occupy most of the page if not more than one page), then the ReportItem printed a blank/null value.
So, a method was defined in the Code section of the report that would set the value to the shared variable:
public shared Params as String
public shared Function SetValues(Param as String ) as String
Params = Param
Return Params
End Function
Which would be called in the detail section of the tablix, then in the header a textbox would hold the following expression:
=Code.Params
This worked beautifully since, it now didn't mattered that the body section didn't had the SetValues call, the variable persited and the Header displayed the correct value. Our problem now is that when the report is being called in different threads with
different data, the variable being shared/static gets modified by all the reports being run at the same time.
So far I've tried several things:
- The variables need to be shared, otherwise the value set in the Body can't be seen by the header.
- Using Hashtables behaves exactly like the ReportItem option.
- Using a C# DLL with non static variables to take care of this, didn't work because apparently when the DLL is being called by the Body generates a different instance of the DLL than when it's called from the header.
So is there a way to deal with this issue in a multi thread safe way?
Thanks in advance!

Hi Angel,
Per my understanding that you want to dynamic display the group data in the report header, you have set page break based on the group, so when click to the next page, the report hearder will change according to the value in the group, when you are using
the shared variables you got the multiple thread safe problem, right?
I have tested on my local environment and can reproduce the issue, according to the multiple safe problem the better way is to use the harshtable behaves in the custom code, you have mentioned that you have tryied touse the harshtable but finally got
the same result as using the ReportItem!TextBox.Value, the problem can be cuased by the logic of the code that not works fine.
Please reference to the custom code below which works fine and can get all the expect value display on every page:
Shared ht As System.Collections.Hashtable = New System.Collections.Hashtable
Public Function SetGroupHeader( ByVal group As Object _
,ByRef groupName As String _
,ByRef userID As String) As String
Dim key As String = groupName & userID
If Not group Is Nothing Then
Dim g As String = CType(group, String)
If Not (ht.ContainsKey(key)) Then
' must be the first pass so set the current group to group
ht.Add(key, g)
Else
If Not (ht(key).Equals(g)) Then
ht(key) = g
End If
End If
End If
Return ht(key)
End Function
Using this exprssion in the textbox of the reportheader:
=Code.SetGroupHeader(ReportItems!Language.Value,"GroupName", User!UserID)
Links belowe about the hashtable and the mutiple threads safe problem for your reference:
http://stackoverflow.com/questions/2067537/ssrs-code-shared-variables-and-simultaneous-report-execution
http://sqlserverbiblog.wordpress.com/2011/10/10/using-custom-code-functions-in-reporting-services-reports/
If you still have any problem, please feel free to ask.
Regards
Vicky Liu

Thread safe Timed Set

Hello,
I'm trying to create a thread safe set that also contains a time when it was updated last. It's a cache of a table from the db. I am using CopyOnWriteArraySet as the set is going to be modified rarely, items from set are never modified or removed individually.
There are couple of questions, is the an existing collection that I can use to achieve the same in JDK 6 (not external libraries)? Is there a better choice over CopyOnWriteArraySet? I tried searching but couldn't find anything.
If not, would the following implementation suffice my requirements?
Any suggestions greatly appreciated.
Thanks in advance,
-t
public class CacheSet<T> extends CopyOnWriteArraySet<T> {
private static final long serialVersionUID = 1L;
private long lastSyncTime = 0;
* @param lastSyncTime
*          the lastSyncTime to set
public synchronized void setLastSyncTime(long lastSyncTime) {
this.lastSyncTime = lastSyncTime;
* @return the lastSyncTime
public synchronized long getLastSyncTime() {
return lastSyncTime;
}

if you are caching the contents of a table, i assume you are only ever replacing the set. i would suggest something like this:
public class TableData<T>
private final Set<T> _tableData;
private final long _timestamp;
public TableData(Set<T> data, long timestamp) {
    _tableData = Collections.unmodifiableSet(data);
    _timestamp = timestamp;
// normal accessors here
public class TableCache<T>
private volatile TableData<T> _curTable;
public void update(TableData<T> newTable)
    _curTable = newTable;
public TableData<T> get()
    return _curTable;
}the basic idea is that the TableData class is immutable (e.g. when you update the cache, you create a new set and new TableData instance), so you don't need to worry about concurrency issues (you can pass it a normal HashSet or whatever). thread visibility issues are resolved by using volatile on the TableCache class.
Edited by: jtahlborn on Apr 17, 2010 10:47 AM

Thread safe logging class, using final and static

Hi,
I'm looking through a logging class (which writes entries to a text file and so on) written by someone else, and I'm trying to determine whether it's thread-safe...
Essentially the class looks like this:
public final class Logger {
private static FileOutputStream fout;
private static PrinterWriter pout;
private static final long MaxLength = 100000;
static {
/* basically checks size of logfile, if bigger than 100k, then copy it as bak, and start again */
public void write(String msg) {
/* write entry into log file */
public void writeWithTimeStamp(string msg) {
/* write entry with time stamp into log file */
Now, I could be wrong, but I don't think that the class is thread-safe is it? in order to be thread-safe, I would have to use synchronized before write() and writeWithTimeStamp(), right?
My confusion arises from the use of the keyword "final" at the top of the class. This class isn't being inherited (or rather, there's no reason for the class not to be inheritable)... so what is it's purpose, if there is one?!
And my other question concerns the static block. But I need to describe the current setup first: The Logger class is being use by a file server. File server basically sits there, accepts socket connections from the outside, and takes text files and output them into a local directory. When a new socket connection is created, a new thread is created. As it stands right now, each thread instantiates its own Logger object which seems weird! Bringing me to my question which was, if each thread instantiates its own Logger object, the static block is only ran ONCE, regardless of how many threads (and therefore Logger objects) are created, right??
And wouldn't it be a better idea to simply create only ONE Logger object and pass it by reference to all newly created threads as they are needed so that all threads access the same and only Logger object? (and therefore utilize sychronization)
Thanks!

In JDK 1.4, there are already classes written that do all of that work. Check out the docs for the java.util.logging package. Specifically, check out FileHandler (which can rotate log files if they are greater than a specified size) and LogRecord (which is a standardized way to store logging entries). I believe the class is threadsafe, it doesn't say it isn't. Try it and see. If not, you can easily make a threadsafe wrapper for it (or any class for that matter).
The online documentation is at:
http://java.sun.com/j2se/1.4/docs/api/index.html
If you are curious, the simplest way to make any class threadsafe, if you don't want to or are not able to modify the class code itself is to define another class that calls methods of the original class, but that is synchronized.
Example:
    // NonThreadSafe.java
    public class NonThreadSafe {
        public NonThreadSafe (String s) {
            // Constructor
        public void SomeMethod (int param) {
            // Do some important stuff here...
        public int AnotherMethod (boolean param) {
            // Do some more important stuff here...
    // ThreadSafeWrapper.java
    public class ThreadSafeWrapper {
        protected NonThreadSafe nts;
        public ThreadSafeWrapper (String s) {
            nts = new NonThreadSafe(s);
        public synchronized void SomeMethod (int param) {
            nts.SomeMethod(param);
        public synchronized int AnotherMethod (boolean param) {
            return nts.AnotherMethod(param);
        public NonThreadSafe GetNonThreadSafe () {
            return nts;
    };Unfortunately, ThreadSafeWrapper isn't derived from NonThreadSafe, so you are somewhat limited in what you can do with it compared to what you could do with NonThreadSafe. AFAIK, you can't override unsynchronized methods with synchronized ones.
Another thing you could do is just write a method that writes to your logging class, and synchronize just that method. For example:
    // ThreadSafeLogger.java
    public class ThreadSafeLogger {
        public static synchronized void LogMessage (MyLogger log, String msg) {
            log.writeString(msg);
    // In another thread, far, far away:
        ThreadSafeLogger.LogMessage(theLog, "Blah");
        ...Hope that helps.
Jason Cipriani
[email protected]
[email protected]

JTextArea: Not thread safe

I am working on a lobby system with a chat are using Networking, and recently I noticed that occasionally my JTextArea does not append the data I give it, even if I have a System.out.println() directly before which shows me what it is being passed, and it is being passed correct Strings, it is just not appending them.
    Thread updateText = new Thread() {
        public void run()
            for(;;)
                try {
                    for (int x = 0; x < texts.size(); x++) {
                        System.out.println("ADDED: "+texts.get(x));
                        textArea.append(texts.get(x));
                    texts.clear();
                    scrollToEnd(textArea);
                    Thread.sleep(100);
                } catch (InterruptedException ex) {
                    Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
    };I have an ArrayList texts, which keeps track of what is being typed into the Chat Area. This code displays correctly What I want it to, i.e. "ADDED: Hello guys, etc.", however the JTextArea does not update. Is there something to make the JTextArea thread-safe because I have heard on several sites that JTextAreas are not thread-safe.
Any help would be greatly appreciated.

I changed my code to this: and now it does not work at all, i.e., the JTextArea never updates:
    Thread updateText = new Thread() {
        public void run()
            for(;;)
                try {
                    SwingUtilities.invokeAndWait(update);
                } catch (InterruptedException ex) {
                    Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
                } catch (InvocationTargetException ex) {
                    Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
    Runnable update = new Runnable() {
        public void run()
            try {
                texts.clear();
                scrollToEnd(textArea);
                for (int x = 0; x < texts.size(); x++) {
                    System.out.println("ADDED: " + texts.get(x));
                    textArea.append(texts.get(x));
                Thread.sleep(100);
            } catch (InterruptedException ex) {
                Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
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Hashtable really thread safe or is there a bug in it ?

Hashtable is said to be thread-safe, so I think it should be true.
So I just want someone to tell me what is wrong with my following observation:
I downloaded the source code of jdk 1.3.1 and read the file Hashtable.java, that contains the implementation code of Hashtables.
Lots of methods (like get(), put(), contains(), ...) are declared public and synchronized. So far, all is good.
But the function size() is defined as follows:
public int size() {
return count;
where count is an instance variable defined as follows:
* The total number of entries in the hash table.
private transient int count;
As the function size() is not synchronized, it can retrieve the value of "count" when another method is in the process of updating it.
For instance, the method put() (that is synchronized) increments the variable "count" with this instruction:
count++;
But we know that operation on variable of type "int" are atomic, so this issue doesn't harm the thread safety of the code.
But there is another issue with multithreading, that deals with caching of data in the working memory of each thread.
Suppose the following scenario:
1) Thread A calls method size(). This method reads the value of "count" and put it in the working memory of thread A.
2) Thread B calls method put(). This method increments "count" using instruction "count++".
3) Thread A calls method size() again. This method reads the value of "count" from the working memory of the thread, and this cached value may not be the updated value by thread B.
As size() is not synchronized, or "count" is not declared "volatile", multiple threads may retrieve stale value, because the content of the thread working memory is not in sync with main memory.
So, here I see a problem.
Is there something wrong in this observation ?
But

Is there something wrong in this observation ?Unfortunately not, as far as I can tell.
Recent analysis of java's synchronized support has revealed the danger in such unsynchronized code blocks, so it may have been a simple case that it wasn't realised that size is not thread-safe.
If this is an issue, use Collections.synchronizedMap - in Sun's JDK 1.3, at least, all of the methods are synchronized, except for iterators.
Regards,
-Troy

Thread Safe Testing

I havn't written much mult-threaded code but I realise it's important when many web round-trips are required.
How do I design, test and verify that I have a thread-safe application? I don't like relying on the testing statistics of chance unless the odds of failure are 0.0001%!
Is there a correct way to verify a thread-safe application?

I agree it is a lofty, though sadly in all probability too time consuming, goal to achieve. However, there are a few pitfalls to watch out for that should get most of the common, pernicious thread safety issues:
Synchronize data that must be shared. This will involved performance bottle-necks. If the data is read-mostly, consider a Singleton cache. Though pitfalls will always remain here. Data sharing among threads, IMO, is the single greatest issue.
Use static methods and instances wisely. If an object has instance variables that are used from method to method call (either within the class or from the caller), then state exists for that object. It is inherently a candidate to look at in terms of thead-safety. This class should be created for each logical unit of work that depends on its state. So, do not reuse these classes among threads (or at least provide an init() or reset() method to clear the state). The same applies to static methods. You can safely use a static method from multiple threads if it does not modify other static variables (e.g., changing the state of a static variable).
Within the Collections API, pay attention to which types are synchronized and which are not. The Javadocs will specify.
Try to write methods thread-safe. Have them only use the arguments provided in the method's signature.
Watch for relatively simple race conditions that can easily be spotted.
static private Singleton INSTANCE;
private Singleton() {
    super();
    // Now I will fetch values from a database, perhaps taking seconds
static final public Singleton getInstance() {
      if (INSTANCE == null) {
         INSTANCE = new Singleton(); // what if this takes some time, e.g. database query to get cache values?
      return INSTANCE;
}Not good. Here's a possible refactoring:
private static final Singleton INSTANCE; // bonus, we can now make it final
static {
   INSTANCE = new SIngleton(); // Occurs only when class is loaded, atomic
public static final Singleton getInstance() {
return INSTANCE; // no race condition
}The above would be even more applicable and pronounced if you offered a static refresh() method on the instance, say to refresh its cache of data from a database.
Seems easy, right? The rub is that you need to verify that a given method does not call other methods that are not thread-safe. If every class you design follows the above precepts, then you will probably only be chasing down rarer thread-safety issues.
- Saish

Is this thread safe?

I've got a HttpServletRequest instance variable in my servlet that I use in my model to retrieve parameters. The servlet looks like:
public class MyServlet extends HttpServlet implements Controller {
HttpServletRequest req;
public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    processRequest(req, resp);
public void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    processRequest(req, resp);
public void processRequest(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    this.req = req;
    MyModel model = new MyModel(this);
    model.retrieve();
public String getParameter(String parameter) {
    return req.getParameter(parameter);
}The Controller interface has only one method - public String getParameter(String parameter)
In MyModel because I pass an instance of Controller into the constructor I can do - controller.getParameter(parameter);
Is what I'm doing thread safe?

Actually, a static nested class is more appropriate than an inner class as there is no implicit outer-class reference to maintain:
public class MyServlet extends HttpServlet {
    public void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        processRequest(req, resp);
    public void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        processRequest(req, resp);
    public void processRequest(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        MyModel model = new MyModel(new MyController(req));
        model.retrieve();
    private static class MyController implements Controller {
        private HttpServletRequest req;
        MyController(HttpServletRequest req) {
            this.req = req;
        public String getParameter(String parameter) {
            return req.getParameter(parameter);
}This code is now thread safe because there is a different instance of MyController per thread. Note that if an instance of MyController is accessed by multiple threads, then its thread safety is dependent upon the thread safety of HttpServletRequest. However, based on the sort of thing I assume you are trying to do, I doubt you will be sharing instances between threads.

Is Executor thread-safe

Hello,
Suppose you have an Executor to execute tasks (one of the standard implementations provided by the JDK).
Is there any risk if you have multiple threads that use the executor to submit or execute tasks?.
In other words: Is Executor thread-safe?
I'm not talking about the working threads that execute the tasks.
I'm talking about the threads that send the tasks to execution.
Thank you.

JoseLuis wrote:
can you please tell me where can I find that kind of facts, because I could not find it in the javadoc.Well, it isn't a requirement for ExecutorServices, but it how the implementations provided by the java.util.concurrent package operate. That fact is stated in the API for ThreadPoolExecutor in several places:
In the class description:
Queuing
Any BlockingQueue may be used to transfer and hold submitted tasks.Then, all the Constructors that take work queues only take BlockingQueues, and the getQueue() method returns a BlockingQueue.
Finally, there are only 2 implementations of the ExecutorService provided, ThreadPoolExecutor, and ScheduledThreadPoolExecutor, which subclasses ExecutorService (and does not indicate different rules for Queues).
>
if you can tell me about a book or official document it would be great.
thanx again

SortedSet thread-safe howto?

Hello,
i m trying to acces SortedSet with multiple thread, but i can't
here is my code :
public class Test {
static SortedSet queue = Collections.synchronizedSortedSet((SortedSet) new TreeSet());
static Vector v = new Vector();
public static Vector getV() { return v;}
public static SortedSet getQueue () {
return queue;
public static void main(String[] args) throws Exception {
launchFrame();
for (int i=0; i<2; i++) {
new SimpleThread2("thread"+i).start();
public static void end() {
System.out.println(v.size()+" lalala "+queue.size());
class SimpleThread2 extends Thread {
int j = 0;
public SimpleThread2(String str) {
super(str);
public void run() {
for (int i = 0; i < 10; i++) {
try {
SortedSet s = Test.getQueue();
synchronized (s) {
s.add(new String(""+i));
Test.getV().add(""+i);
} catch (Exception ex) {ex.printStackTrace();}
Test.end();
here is my result :
10 lalala 10
20 lalala 10
as u can see Vector is thread safe but SortedSet not....

That has nothing to do with Thread-safe or not. A Set be it sorted or not always only contains at most one reference to a specific Object as long as equals() and compareTo() are implemented properly.
In the second run you add identical Strings to both the SortedSet and the Vector. In the SortedSet, they replace the old entries and in the Vector, they are appended as the Vector is a List and allows multiple identical entries.

Is ArrayList(Collection c) constructor Thread-Safe?

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