My Android app has an IntentService where it requests a list of MessageThreads objects from Facebook, parses the JSON response to build an ArrayList of the objects:
ArrayList<MessageThread> mMessageThreads = new ArrayList<MessageThread>();
Then it calls FB again in the same service, get the names for the MessageThread ids and matches them with the MessageThread objects. At this point I have an ArrayList with complete MessageThread objects and I insert them into an SQLite db.
// save to db and broadcast
for (MessageThread message : mMessageThreads) {
((FBClientApplication)getApplication()).getMessagesData().insertOrIgnore(message.toContentValues();
}
where:
public void insertOrIgnore(ContentValues values) {
SQLiteDatabase db = this.dbHelper.getWritableDatabase();
db.insertWithOnConflict(TABLE, null, values, SQLiteDatabase.CONFLICT_IGNORE);
}
Via ACRA reports I see that intermittently the line
for (MessageThread message : mMessageThreads)
throws an ConcurrentModificationException and the app forcloses. I haven't been able to isolate under what conditions. I read about this Exception and as I understand it it happens when we remove items from an ArrayList while iterating over it, but I'm not removing items from the list. Any pointers to help with this problem are greatly appreciated.
It also happens when you add items to an ArrayList while iterating over it, which it looks like you might do in this code.
In general, it's any "structural modification" that occurs to the ArrayList that can cause a CME while iterating.
What you can try to do is when you iterates your Collection instead of using the original you can make a copy right there, so you will have something like:
for (MessageThread message : new List<MessageThread>(mMessageThreads))
That will help you to avoid CuncurrentModificationException.
Now if you really want to get fancy you can protect your code using synchronized blocks such as:
synchronized(mMessageThreads){
for (MessageThread message : new List<MessageThread>(mMessageThreads)){
...
}
With this last pice of code you will restrict the access to mMessageThreads, if somebody it's using it it will get locked, so if somebody else wants to use it needs to wait until the first one is done.
Related
I want to update a document with a User object that I have, but I do not want the document to be created if it does not exist, and therefore I cannot use "DocumentReference.set" with "SetOptions.Merge()" (to my understanding).
However, according to this post (Difference between set with {merge: true} and update), "update" is actually the command I need. My problem is, it doesn't seem like update accepts a Java object.
I do not want to check whether or not the document exists myself, as this will result in an unnecessary read.
Is there any way around this?
Here is my code (I have removed success and failure listeners for simplicity):
public void saveUser(User user)
{
CollectionReference collection = db.collection("users");
String id = user.getId();
if (id.equals(""))
{
collection.add(user);
}
else
{
// I need to ensure that the ID variable for my user corresponds
// with an existing ID, as I do not want a new ID to be generated by
// my Java code (all IDs should be generated by Firestore auto-ID)
collection.document(ID).set(user);
}
}
It sounds like you:
Want to update an existing document
Are unsure if it already exists
Are unwilling to read the document to see if it exists
If this is the case, simply call update() and let it fail if the document doesn't exist. It won't crash your app. Simply attach an error listener to the task it returns, and decide what you want to do if it fails.
However you will need to construct a Map of fields and values to update using the source object you have. There are no workarounds for that.
I'm getting a java.util.ConcurrentModificationException on the line where the for-loop starts (see comment in code).
Why am i getting ConcurrentModificationException on this unmodifiableSet?
final Set<Port> portSet = Collections.unmodifiableSet(node.getOpenPorts());
if (!portSet.isEmpty()) {
StringBuilder tmpSb = new StringBuilder();
for (Port pp : portSet) { // <------- exception happening here
tmpSb.append(pp.getNum()).append(" ");
}
}
I've never witnessed this, but I'm getting crash reports from Google.
Something must be modifying the underlying set; i.e. the set returned by node.getOpenPorts().
Instead of wrapping the set with an "unmodifiable" wrapper, you could copy it.
final Set<Port> portSet = new HashSet<>(node.getOpenPorts());
But as a commenter (#Slaw) pointed out, that just moves the iteration inside the constructor and you would still get CCMEs.
The only real solutions are:
Change the implementation of the node class to use a concurrent set class for the port list that won't throw CCMEs if the collection is mutated while you are iterating it.
Change the implementation of the node class to return a copy of the port list. Deal with the updates-while-copying race condition with some internal locking.
Put a try / catch around the code and repeat the operation if you get a CCME
I've never witnessed this, but I'm getting crash reports from Google.
Yes. The problem only occurs if this code is executed while the open port list is changing.
The question may sound weird. I have the following custom Object that I named ItemUser:
private UserInfo user_info;
private List<UserAchievement> user_achievements;
Both fields have getters and setters. My Firestore's database looks like this:
I would like to get the List size instead of re-calling the database and getting the size of the collection from a separated call that would consume much resources and take a lot of time (3-4s).
Firstly I'm getting the data using this:
mDB.collection("COLLECTION_NAME").document("USER_ID").get()
Inside the onCompletedListener I'm getting the custom object as the following:
ItemUser mUser = task.getResult().toObject(ItemUser.class);
Now, when I'm trying to get the size of the user_achievements, a NullPointerException popups saying I can't get the size of a null reference.
Therefore the user_achievements is null. I think the way I'm defining user_achievements in my custom Object is the reason for this exception.
The question is: How could this be possible done without recalling the database to count only the size?
I have the main custom Object ItemUser and its children are 'healthy' except user_achievements because of the way it's defined - List<UserAchievement>.
So, any suggestions to overpass this issue?
How could this be possible done without recalling the database to count only the size?
No, because Cloud Firestore is a real-time database and items can be added or deleted, so to get the size of a list you need to query the database and use a get() call.
If you want to count the number of documents beneath a collection (which can be added to a list), please see my answer from this post in which I have explained that task.getResult().size() can help you solve the problem.
Edit:
mDB.collection("COLLECTION_NAME").document("USER_ID").get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
#Override
public void onComplete(#NonNull Task<QuerySnapshot> task) {
if (task.isSuccessful()) {
int size = task.getResult().size();
Log.d(TAG, String.valueOf(size));
}
}
});
I have a stateful EJB which calls an EJB stateless method of Web parsing pages.
Here is my stateful code :
#Override
public void parse() {
while(true) {
if(false == _activeMode) {
break;
}
for(String url : _urls){
if(false == _activeMode) {
break;
}
for(String prioritaryUrl : _prioritaryUrls) {
if(false == _activeMode)
break;
boursoramaStateless.parseUrl(prioritaryUrl);
}
boursoramaStateless.parseUrl(url);
}
}
}
No problem here.
I have some asynchronously call (with JMS) that add to my _urls variable (a List) some value. Goal is to parse new url inside my infinity loop.
I receive ConcurrentModificationException when I try to add new url in my List via JMS onMessage method but it seems to be working because this new url is parsed.
When I try to wrap a synchronized block :
while(true){
synchronized(_url){
// code...
}
}
My new url is never parsed, I expected to be parsed after a for() loop finished...
So my question is : how can I modify List when it's accessed inside a loop without having ConcurrentModificationException please ?
I just want 2 threads to modify some shared resource at same time without synchronized block...
You may want a CopyOnWriteArrayList.
For (String s : urls) uses an Iterator internally. The iterator checks for concurrent modification so that its behavior is well defined.
You can use a for(int i= ... loop. This way, no exception is thrown, and if elements are only added to the end of the List, you still get a consistent snapshot (the list as it exists at some time during the iteration). If the elements in the list are moved around, you may get missing entries.
If you want to use synchronised, you need to synchronise on both ends, but that way you lose concurrent reads.
If you want concurrent access AND consistent snapshots, you can use any of the collections in the java.util.concurrent package.
CopyOnWriteArrayList has already been mentioned. The other interesting are LinkedBlockingQueue and ArrayBlockingQueue (Collections but not Lists) but that's about all.
ok thank you guys.
So I made some modifications.
1) added iterator and leaving synchronized block (inside parse() function and around addUrl() function which add new url to my List)
--> it's work like a charm, no ConcurrentModificationException launched
2) added iterator and removed synchronized blocks
--> ConcurrentModificationException is still launched...
For now, I will read more about your answers and test your solutions.
Thank you again guys
First, forget about synchronized when running into Java EE container. It bothers the container to optimize threads utilization and will not work in clustered environment.
Second, it seems that your design is wrong. You should not update private field of the bean using JMS. This thing causes ConcurrentModificationException. You probably should modify your bean to retrieve the collection from database and your MDB to store the URL into the Database.
Other, easier for you solution is the following.
Retrieve the currently existing URLs and copy them to other collection. Then iterate over this collection. When the global collection is updated via JMS the update is not visible in the copied collection, so no exceptions will be thrown:
while(true) {
for (String url : copyUrls(_prioritaryUrls)) {
// deal with url
}
}
private List<String> copyUrls(List<Stirng> urls) {
return new ArrayList<String>(urls); // this create copy of the source list
}
//........
public void onMessage(Message message) {
_prioritaryUrls.add(((TextMessage)message).getText());
}
I am updating multiple records in database. Now whenever UI sends the list of records to be updated, I have to just update those records in database. I am using JDBC template for that.
Earlier Case
Earlier what I was whenever I got records from UI, I just do
jdbcTemplate.batchUpdate(Query, List<object[]> params)
Whenever there was an exception, I used to rollback whole transaction.
(Updated : Is batchUpdate multi-threaded or faster than batch update in some way?)
Later Case
But later as requirement changed whenever there was exception. So, whenever there is some exception, I should know which records failed to update. So I had to sent the records back to UI in case of exception with a reason, why did they failed.
so I had to do something similar to this:
for(Record record : RecordList)
{
try{
jdbcTemplate.update(sql, Object[] param)
}catch(Exception ex){
record.setReason("Exception : "+ex.getMessage());
continue;
}
}
So am I doing this in right fashion, by using the loop?
If yes, can someone suggest me how to make it multi-threaded.
Or is there anything wrong in this case.
To be true, I was hesitating to use try catch block inside the loop :( .
Please correct me, really need to learn a better way because I myself feel, there must be a better way , thanks.
make all update-operation to a Collection Callable<>,
send it to java.util.concurrent.ThreadPoolExecutor. the pool is multithreaded.
make Callable:
class UpdateTask implements Callable<Exception> {
//constructor with jdbctemplate,sql,param goes here.
#Override
public Exception call() throws Exception {
try{
jdbcTemplate.update(sql, Object[] param)
}catch(Exception ex){
return ex;
}
return null;
}
invoke call:
<T> List<Future<T>> java.util.concurrent.ExecutorService.invokeAll(Collection<? extends Callable<T>> tasks) throws InterruptedException
Your case looks like you need to use validation in java and filter out the valid data alone and send to the data base for updating.
BO layer
-> filter out the Valid Record.
-> Invalid Record should be send back with some validation text.
In DAO layer
-> batch update your RecordList
This will give you the best performance.
Never use database insert exception as a validation mechanism.
Exceptions are costly as the stack trace has to be created
Connection to database is another costly process and will take time to get a connection
Java If-Else will run much faster for same data-base validation