Java Properties variabile: set property while iterating keys

Question

I have a code which collects all the system-properties keys which starts with a given prefix

private static List<String> getKeysByPrefix(String prefix) {
    Set<?> keySet = System.getProperties().keySet();
    Iterator<?> iterator = keySet.iterator();

    List<String> list = new ArrayList<>();
    while (iterator.hasNext()) {
        String key = (String) iterator.next();

        if (key.startsWith(prefix)) {
            list.add(key);
        }
    }

    return list;
}

This code is running inside a Jetty web-server, and while a request is executing this code, it can happen that another request write a new key inside the java System properties like a simple System.setProperty("test", "foo");

With this scenario I'm getting a ConcurrentModificationException while I'm iterating the properties and another request changes it.

java.util.ConcurrentModificationException
    at java.util.Hashtable$Enumerator.next(Hashtable.java:1378)

The code which set the new property is totally another flow compared to the one is iterating the keys, but because the SystemProperties are shared among all the JVM instance, I'm getting this error.

I already found other topics which treats this topic, and the most suggested solution is using ConcurrentHashMap. This option is unavailable for me because I'm not the one which creates the SystemProperties map but i'm only reading/writing it.

In order to reproduce the issue you can simply add a setProperty call right before the closing bracket of the iterator loop

Which is a suggested practice here in order to solve the problem?

You probably want only a single thread to be able to modify system properties at any given time. — Mena, Apr 10 '17 at 14:29
How are you updating properties? Using `System.setProperty`? — M A, Apr 10 '17 at 15:56

cactuschibre · Answer 1 · 2017-04-10T15:20:50.000

0

You cannot change a list while iterating on it.

Two options :

Use the java.util.concurrent package to avoid this exception (http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/package-summary.html)
READ then WRITE (not a the same time). Iterate first to make the list. Then make your modifications once the loop is finished.

Try this with CopyOnWriteArraySet :

    Set<?> keySet = new CopyOnWriteArraySet<Object>(System.getProperties().keySet());
    Iterator<?> iterator = keySet.iterator();

    List<String> list = new ArrayList<String>();
    while (iterator.hasNext()) {
        String key = (String) iterator.next();

        if (key.startsWith(prefix)) {
            list.add(key);
        }
    }

    return list;

edited Apr 10 '17 at 15:20

answered Apr 10 '17 at 14:36

cactuschibre

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About the second point, the problem here is that the modification is not applied after the iteration, they are 2 different e separated flows. – Deviling Master Apr 10 '17 at 14:38
So you need to clone the list before beginning loop OR encapsulate the original Set in a type of java.util.concurrent package (like CopyOnWriteArraySet http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CopyOnWriteArraySet.html) – cactuschibre Apr 10 '17 at 14:44
The problem is that when you go to clone the collection, then you can have the same problem as the OP because it iterates through the property `HashTable`. – Gray Apr 10 '17 at 16:28

score 0 · Answer 2 · edited Apr 10 '17 at 18:13

0

I believe that you should create a lock object and use a synchronized block whenever you try to read/write to properties object.

public final Object systemPropLockObject = new Object();

synchronized ( systemPropLockObject ) {
    // write the code to read/write to the system properties
}

edited Apr 10 '17 at 18:13

Renats Stozkovs

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answered Apr 10 '17 at 14:47

Shereef Sakr

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Gray · Answer 3 · 2017-04-10T15:45:06.293

With this scenario I'm getting a ConcurrentModificationException while I'm iterating the properties and another request changes it.

I think that this error is trying to tell you that you are using the System properties inappropriately. Even though they are backed by a HashTable, they are not designed to be iterated and updated concurrently.

In looking at the HashTable code I see a lot of comments like:

If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation, or through the setValue operation on a map entry returned by the iterator) the results of the iteration are undefined.

Needless to sat, this isn't good.

I already found other topics which treats this topic, and the most suggested solution is using ConcurrentHashMap. This option is unavailable for me because I'm not the one which creates the SystemProperties map but i'm only reading/writing it.

Can't you have a shared class that initializes a CHM with the contents of the initial System properties map but then does the read/write into the CHM?

If you edit your question and talk about your needs without talking about properties then we might be able to come up with a better answer.

Lastly, if you are desperate, a lock around your iteration may work:

Properties props = System.getProperties();
List<String> list = new ArrayList<>();
synchronized (props) {
    // get the properties which match with an enumeration inside the lock
    ...
}

This depends a ton on how the system properties lock itself but this may work for you. Implementing a different solution with a CHM seems better to me.

Java Properties variabile: set property while iterating keys

3 Answers3