I am trying to use arrayList across multiple threads in which 2 threads add elements to it and one thread just retrieve the first elements. I know I can use the syncronizedList but I wanted to see if this impleentation is right. Basically I add all my array manipulation in a synhcronized method
public void synchronized addElem(String str){
String s = str.trim();
myArray.add(s);
}
Is this ok?
It is not enough to synchronize writing, you need to synchronize reading as well. Otherwise, a read that happens concurrently with a write may return inconsistent data, or trigger exceptions:
public synchronized String getFirst() {
if (myArray.size() != 0)
return myArray.get(0);
return null;
}
You could also use Collections.synchronizedList
List<String> syncList = Collections.synchronizedList(new ArrayList<String>());
No. You'd need to synchronize on ALL times you access the list, as well as make sure you don't pass any references to it elsewhere. That's why the synchronized classes exist- to save you the effort. This is part of the answer, but not sufficient.
You can also used a synchronized block as synchronized(myArray) { // logic}. This is preferred over a synchronized method if your method is too long and will hold up the required object for too long. synchronized block, on the other hand, will keep the object locked only as long as it is needed.
Related
Here is a short bit of text from the Oracle Java Tutorials:
“Synchronized Statements
Another way to create synchronized code is with synchronized statements. Unlike synchronized methods, synchronized statements must specify the object that provides the intrinsic lock:
public void addName(String name) {
synchronized(this) {
lastName = name;
nameCount++;
}
nameList.add(name);
}
In this example, the addName method needs to synchronize changes to lastName and nameCount, but also needs to avoid synchronizing invocations of other objects' methods. (Invoking other objects' methods from synchronized code can create problems that are described in the section on Liveness.) Without synchronized statements, there would have to be a separate, unsynchronized method for the sole purpose of invoking nameList.add.”
I understand their point about the flexibility Synchronized gives. But why did Oracle decide that nameList.add did not need to be synchronized? More generally, how can I determine which objects methods need to be synchronized and which don't?
Synchronizing has its price, performance wise. The general rule of thumb (which the JDK itself also follows, in this case too) is not to synchronize unless it's absolutely required.
Since there are many, many cases where you'd want to add an element to a list without requiring synchronization (e.g., if said list is a local variable), it was not defined as synchronized. When you need to synchronize such an operation, you can always use a CopyOnWWriteArrayList, or synchronize your access explicitly.
Synchronize based on what you know the threads will access.
If they are trying to access a list then synchronize the getter. In this case, Oracle was synchronizing the call to the method ONLY.
If they are trying to modify the list, then synchronize the reference to the list inside your block!
So:
synchronized(this.nameList){
//now I'm safe to modify
nameList.add(name);
}
You can create a thread safe ArrayList of Strings like this:
ArrayList<String> list = Collections.synchronizedList(new ArrayList<String>());
the methods add() and remove() are thread safe. If you don't need to iterate over the elements you're just fine.
Note: It is imperative that you manually synchronize on the returned collection when iterating over it
synchronized (list) {
Iterator i = list.iterator(); // Must be in the synchronized block
while (list.hasNext())
yourmethod(list.next()); // Your logic on elements...
}
Before using read the docs here: Synchronized Collection
I'm wondering what the difference is between these ways of synchronization
List<Integer> intList = Collections.synchronizedList(new ArrayList<Integer>());
synchronized (intList) {
//Stuff
}
and using an object lock
Object objectLock = new Object();
List<Integer> intList = new ArrayList<Integer>();
synchronized (objectLock) {
//Stuff
}
The first approach makes sure that individual method calls are synchronized, and it avoids needing to manage a separate lock object. One thread can call
intList.add(3);
and another can call
intList.clear();
without a synchronized block, and it'll be properly synchronized. (Unfortunately, this doesn't help when you need to hold the lock for a group of function calls; then, you need a synchronized block around those calls.) Also, if you need to pass the list around, you can use
otherObject.doStuffWith(intList);
and
return intList;
instead of
otherObject.doStuffWith(intList, objectLock);
and
return ListAndLock(intList, objectLock);
The code you show is not necessarily thread safe yet!!
The only difference between one excerpt and the other is the object you use as a monitor for synchronization. This difference will determine which object should be used for synchronization by other threads that need access to the mutable data you're trying to protect
great read for this: java concurrency in practice
I am new to multi-threading in Java and don't quite understand what's going on.
From online tutorials and lecture notes, I know that the synchronized block, which must be applied to a non-null object, ensures that only one thread can execute that block of code. Since an array is an object in Java, synchronize can be applied to it. Further, if the array stores objects, I should be able to synchronize each element of the array too.
My program has several threads updated an array of numbers, hence I created an array of Long objects:
synchronized (grid[arrayIndex]){
grid[arrayIndex] += a.getNumber();
}
This code sits inside the run() method of the thread class which I have extended. The array, grid, is shared by all of my threads. However, this does not return the correct results while running the same program on one thread does.
This will not work. It is important to realize that grid[arrayIndex] += ... is actually replacing the element in the grid with a new object. This means that you are synchronizing on an object in the array and then immediately replacing the object with another in the array. This will cause other threads to lock on a different object so they won't block. You must lock on a constant object.
You can instead lock on the entire array object, if it is never replaced with another array object:
synchronized (grid) {
// this changes the object to another Long so can't be used to lock
grid[arrayIndex] += a.getNumber();
}
This is one of the reasons why it is a good pattern to lock on a final object. See this answer with more details:
Why is it not a good practice to synchronize on Boolean?
Another option would be to use an array of AtomicLong objects, and use their addAndGet() or getAndAdd() method. You wouldn't need synchronization to increment your objects, and multiple objects could be incremented concurrently.
The java class Long is immutable, you cannot change its value. So when you perform an action:
grid[arrayIndex] += a.getNumber();
it is not changing the value of grid[arrayIndex], which you are locking on, but is actually creating a new Long object and setting its value to the old value plus a.getNumber. So you will end up with different threads synchronizing on different objects, which leads to the results you are seeing
The synchronized block you have here is no good. When you synchronize on the array element, which is presumably a number, you're synchronizing only on that object. When you reassign the element of the array to a different object than the one you started with, the synchronization is no longer on the correct object and other threads will be able to access that index.
One of these two options would be more correct:
private final int[] grid = new int[10];
synchronized (grid) {
grid[arrayIndex] += a.getNumber();
}
If grid can't be final:
private final Object MUTEX = new Object();
synchronized (MUTEX) {
grid[arrayIndex] += a.getNumber();
}
If you use the second option and grid is not final, any assignment to grid should also be synchronized.
synchronized (MUTEX) {
grid = new int[20];
}
Always synchronize on something final, always synchronize on both access and modification, and once you have that down, you can start looking into other locking mechanisms, such as Lock, ReadWriteLock, and Semaphore. These can provide more complex locking mechanisms than synchronization that is better for scenarios where Java's default synchronization alone isn't enough, such as locking data in a high-throughput system (read/write locking) or locking in resource pools (counting semaphores).
If I do something to a list inside a synchronized block, does it prevent other threads from accessing that list elsewhere?
List<String> myList = new ArrayList<String>();
synchronized {
mylist.add("Hello");
}
Does this prevent other threads from iterating over myList and removing/adding values?
I'm looking to add/remove values from a list, but at the same time protect it from other threads/methods from iterating over it (as the values in the list might be invalidated)
No, it does not.
The synchronized block only prevents other threads from entering the block (more accurately, it prevents other threads from entering all blocks synchronized on the same object instance - in this case blocks synchronized on this).
You need to use the instance you want to protect in the synchronized block:
synchronized(myList) {
mylist.add("Hello");
}
The whole area is quite well explained in the Java tutorial:
http://download.oracle.com/javase/tutorial/essential/concurrency/syncmeth.html
Yes, but only if all other accesses to myList are protected by synchronized blocks on the same object. The code sample you posted is missing an object on which you synchronize (i.e., the object whose mutex lock you acquire). If you synchronize on different objects or fail to synchronize at all in one instance, then other threads may very well access the list concurrently. Therefore, you must ensure that all threads have to enter a synchronized block on the same object (e.g., using synchronized (myList) { ... } consistently) before accessing the list. In fact, there is already a factory method that will wrap each method of your list with synchronized methods for you: Collections.synchronizedList.
However, you can certainly use Collections.synchronizedList to wrap your list so that all of its methods are individually synchronized, but that doesn't necessarily mean that your application's invariants are maintained. Individually marking each method of the list as synchronized will ensure that the list's internal state remains consistent, but your application may wish for more, in which case you will need to write some more complex synchronization logic or see if you can take advantage of the Concurrency API (highly recommended).
here the sychronized makes sure that only one thread is adding Hello to the myList at a time...
to be more specific about synchronizing wrt objects yu can use
synchronized( myList ) //object name
{
//other code
}
vinod
From my limited understanding of concurrency control in Java I would say that it is unlikely that the code above would present the behaviour you are looking for.
The synchronised block would use the lock of whatever object you are calling said code in, which would in no way stop any other code from accessing that list unless said other code was also synchronised using the same lock object.
I have no idea if this would work, or if its in any way advised, but I think that:
List myList = new ArrayList();
synchronized(myList) {
mylist.add("Hello");
}
would give the behaviour you describe, by synchronizing on the lock object of the list itself.
However, the Java documentation recommends this way to get a synchronized list:
List list = Collections.synchronizedList(new ArrayList(...));
See: http://download.oracle.com/javase/1.4.2/docs/api/java/util/ArrayList.html
Is the following code set up to correctly synchronize the calls on synchronizedMap?
public class MyClass {
private static Map<String, List<String>> synchronizedMap = Collections.synchronizedMap(new HashMap<String, List<String>>());
public void doWork(String key) {
List<String> values = null;
while ((values = synchronizedMap.remove(key)) != null) {
//do something with values
}
}
public static void addToMap(String key, String value) {
synchronized (synchronizedMap) {
if (synchronizedMap.containsKey(key)) {
synchronizedMap.get(key).add(value);
}
else {
List<String> valuesList = new ArrayList<String>();
valuesList.add(value);
synchronizedMap.put(key, valuesList);
}
}
}
}
From my understanding, I need the synchronized block in addToMap() to prevent another thread from calling remove() or containsKey() before I get through the call to put() but I do not need a synchronized block in doWork() because another thread cannot enter the synchronized block in addToMap() before remove() returns because I created the Map originally with Collections.synchronizedMap(). Is that correct? Is there a better way to do this?
Collections.synchronizedMap() guarantees that each atomic operation you want to run on the map will be synchronized.
Running two (or more) operations on the map however, must be synchronized in a block.
So yes - you are synchronizing correctly.
If you are using JDK 6 then you might want to check out ConcurrentHashMap
Note the putIfAbsent method in that class.
There is the potential for a subtle bug in your code.
[UPDATE: Since he's using map.remove() this description isn't totally valid. I missed that fact the first time thru. :( Thanks to the question's author for pointing that out. I'm leaving the rest as is, but changed the lead statement to say there is potentially a bug.]
In doWork() you get the List value from the Map in a thread-safe way. Afterward, however, you are accessing that list in an unsafe matter. For instance, one thread may be using the list in doWork() while another thread invokes synchronizedMap.get(key).add(value) in addToMap(). Those two access are not synchronized. The rule of thumb is that a collection's thread-safe guarantees don't extend to the keys or values they store.
You could fix this by inserting a synchronized list into the map like
List<String> valuesList = new ArrayList<String>();
valuesList.add(value);
synchronizedMap.put(key, Collections.synchronizedList(valuesList)); // sync'd list
Alternatively you could synchronize on the map while you access the list in doWork():
public void doWork(String key) {
List<String> values = null;
while ((values = synchronizedMap.remove(key)) != null) {
synchronized (synchronizedMap) {
//do something with values
}
}
}
The last option will limit concurrency a bit, but is somewhat clearer IMO.
Also, a quick note about ConcurrentHashMap. This is a really useful class, but is not always an appropriate replacement for synchronized HashMaps. Quoting from its Javadocs,
This class is fully interoperable with Hashtable in programs that rely on its thread safety but not on its synchronization details.
In other words, putIfAbsent() is great for atomic inserts but does not guarantee other parts of the map won't change during that call; it guarantees only atomicity. In your sample program, you are relying on the synchronization details of (a synchronized) HashMap for things other than put()s.
Last thing. :) This great quote from Java Concurrency in Practice always helps me in designing an debugging multi-threaded programs.
For each mutable state variable that may be accessed by more than one thread, all accesses to that variable must be performed with the same lock held.
Yes, you are synchronizing correctly. I will explain this in more detail.
You must synchronize two or more method calls on the synchronizedMap object only in a case you have to rely on results of previous method call(s) in the subsequent method call in the sequence of method calls on the synchronizedMap object.
Let’s take a look at this code:
synchronized (synchronizedMap) {
if (synchronizedMap.containsKey(key)) {
synchronizedMap.get(key).add(value);
}
else {
List<String> valuesList = new ArrayList<String>();
valuesList.add(value);
synchronizedMap.put(key, valuesList);
}
}
In this code
synchronizedMap.get(key).add(value);
and
synchronizedMap.put(key, valuesList);
method calls are relied on the result of the previous
synchronizedMap.containsKey(key)
method call.
If the sequence of method calls were not synchronized the result might be wrong.
For example thread 1 is executing the method addToMap() and thread 2 is executing the method doWork()
The sequence of method calls on the synchronizedMap object might be as follows:
Thread 1 has executed the method
synchronizedMap.containsKey(key)
and the result is "true".
After that operating system has switched execution control to thread 2 and it has executed
synchronizedMap.remove(key)
After that execution control has been switched back to the thread 1 and it has executed for example
synchronizedMap.get(key).add(value);
believing the synchronizedMap object contains the key and NullPointerException will be thrown because synchronizedMap.get(key)
will return null.
If the sequence of method calls on the synchronizedMap object is not dependent on the results of each other then you don't need to synchronize the sequence.
For example you don't need to synchronize this sequence:
synchronizedMap.put(key1, valuesList1);
synchronizedMap.put(key2, valuesList2);
Here
synchronizedMap.put(key2, valuesList2);
method call does not rely on the results of the previous
synchronizedMap.put(key1, valuesList1);
method call (it does not care if some thread has interfered in between the two method calls and for example has removed the key1).
That looks correct to me. If I were to change anything, I would stop using the Collections.synchronizedMap() and synchronize everything the same way, just to make it clearer.
Also, I'd replace
if (synchronizedMap.containsKey(key)) {
synchronizedMap.get(key).add(value);
}
else {
List<String> valuesList = new ArrayList<String>();
valuesList.add(value);
synchronizedMap.put(key, valuesList);
}
with
List<String> valuesList = synchronziedMap.get(key);
if (valuesList == null)
{
valuesList = new ArrayList<String>();
synchronziedMap.put(key, valuesList);
}
valuesList.add(value);
The way you have synchronized is correct. But there is a catch
Synchronized wrapper provided by Collection framework ensures that the method calls I.e add/get/contains will run mutually exclusive.
However in real world you would generally query the map before putting in the value. Hence you would need to do two operations and hence a synchronized block is needed. So the way you have used it is correct. However.
You could have used a concurrent implementation of Map available in Collection framework. 'ConcurrentHashMap' benefit is
a. It has a API 'putIfAbsent' which would do the same stuff but in a more efficient manner.
b. Its Efficient: dThe CocurrentMap just locks keys hence its not blocking the whole map's world. Where as you have blocked keys as well as values.
c. You could have passed the reference of your map object somewhere else in your codebase where you/other dev in your tean may end up using it incorrectly. I.e he may just all add() or get() without locking on the map's object. Hence his call won't run mutually exclusive to your sync block. But using a concurrent implementation gives you a peace of mind that it
can never be used/implemented incorrectly.
Check out Google Collections' Multimap, e.g. page 28 of this presentation.
If you can't use that library for some reason, consider using ConcurrentHashMap instead of SynchronizedHashMap; it has a nifty putIfAbsent(K,V) method with which you can atomically add the element list if it's not already there. Also, consider using CopyOnWriteArrayList for the map values if your usage patterns warrant doing so.