Jonathan Lalou's Blog

Archive for the ‘en-US’ Category

Today in interview I have also been asked the following question: in Java, what is the difference between the methods wait() and sleep()?

First of all, wait() is a method of Object, meanwhile sleep() is a static method of Thread.

More important: Thread.sleep() freezes the execution of the complete thread for a given time. wait(), on its side, gives a maximum time on which the application is suspended: the waiting period may be interrupted by a call to the method notify() on the same object.

Today, in recruting interview, I have been asked the following question: what is the difference between the Java reserved word synchronized used on a method and this very word in a block? Happily I have known the answer:

Indeed, synchronized uses an Object to lock on. When a methid is synchronized, this means the current object is the locker.
Eg: this piece of code:
[java] public synchronized void foo(){
System.out.println("hello world!!!");
}
[/java]
is equivalent to that:
[java] public void foo(){
synchronized (this) {
System.out.println("hello world!!!");
}
}
[/java]
Besides, when synchronized is used on a static method, the class itself is the locker.
Eg: this piece of code:
[java] public static synchronized void goo() {
System.out.println("Chuck Norris");
}
[/java]is equivalent to that:
[java] public static void goo() {
synchronized (MyClass.class) {
System.out.println("Chuck Norris");
}
}
[/java]

I wrote the following notice in Ronan PIERRE‘s profile on LinkedIn:

I have reported to Ronan for more than two years as I was software architect at BNP Paribas. Ronannot only holds title: he *is* an actual team leader and project director. Efficient, skilled, open-minded, you can trust him: as a provider, he is used to following requirements and keep delays ; as a manager, he will do all that is necessary and sufficient to keep his commitments and find solutions if, ever, an issue appears. I strongly recommend Ronan as a very valuable professional.

I wrote the following notice on Ludovic MAURILLON‘s profile on LinkedIN:

I had reported to Ludovic for more than two years. Ludovic gathers eclectic skills: functionnal, technical and managerial. Ludovic is as fluent in Java as in prime brokerage. As a good leader, Ludovic always listens to his teammates. He is able to create trust and confidence between him and the people he works with. This trust creates strong synergy within and between teams: this is one of the factors that explain the successes Ludovic has encountered in all the projects he was involved in and/or led since I have met him. Shall I recommend Ludovic? Of course! Hire Ludovic, he is among the most valuable professionals you may work with!

I wrote the following notice on Denis PIHAN‘s profile on LinkedIn:

I worked under Denis’ indirect management. Denis is one of the most valuable professionals I have worked with. His knowledge is wide and impressive. Developer, architect, team leader, COO… He has gone up all the stairs. Nothing can stop him. His skills are numerous. Denis inspires trust. His pieces of advice are searched and followed by the tens of peoples who have worked with him. And do not let me forget to mention the enjoyment of listening to Denis’ Aussie accent!

Case

Since a couple of days I have met the following issue on my WebLogic server: one application is deployed on a cluster, which references two nodes. Load-balancing (in Round-Robin) is activated for JMS dispatching.

• Yet, all JMS messages are received only by one node (let’s say “the first”), none is received by the other (let’s say “the second”).
• When the 1st node falls, the 2nd receives the messages.
• When the 1st node is started up again, the 2nd keeps on receving the messages.
• When the 2nd node falls, the 1st receives the messages
• and so on

Fix

In WebLogic console go to JMS Modules. In the table of resources, select the connection factory. Then go to the tab Configuration and Load Balance. Uncheck “Server Affinity Enabled“.

Now it should work.

Many thanks to Jeffrey A. West for his help via Twitter.

The following error is among of the most hated ones in Mule ESB:
[java]java.util.MissingResourceException: Can’t find bundle for base name META-INF.services.org.mule.i18n.core-messages, locale en_US[/java].

I am not going to describe how to fix it, since several and various reasons may induce it. Anyway, here is an action plan to track and isolate the bug when it happens.

Usually, this MissingResourceException hides the actual error, instead of encapsulating and throwing it. For instance, consider the following stacktrace:
[java]Can’t find bundle for base name META-INF.services.org.mule.i18n.core-messages, locale en_USjava.util.MissingResourceException: Can’t find bundle for base name META-INF.services.org.mule.i18n.core-messages, locale en_US
at java.util.ResourceBundle.throwMissingResourceException(ResourceBundle.java:1427)
at java.util.ResourceBundle.getBundleImpl(ResourceBundle.java:1250)
at java.util.ResourceBundle.getBundle(ResourceBundle.java:952)
at org.mule.config.i18n.MessageFactory.getBundle(MessageFactory.java:211)
at org.mule.config.i18n.MessageFactory.getString(MessageFactory.java:181)
at org.mule.config.i18n.MessageFactory.createMessage(MessageFactory.java:108)
at org.mule.config.i18n.MessageFactory.createMessage(MessageFactory.java:59)
at org.mule.config.i18n.CoreMessages.initialisationFailure(CoreMessages.java:400)
at org.mule.module.scripting.expression.AbstractScriptExpressionEvaluator.getScript(AbstractScriptExpressionEvaluator.java:93)
at org.mule.module.scripting.expression.AbstractScriptExpressionEvaluator.evaluate(AbstractScriptExpressionEvaluator.java:54)
at org.mule.expression.DefaultExpressionManager.evaluate(DefaultExpressionManager.java:274)
at org.mule.expression.DefaultExpressionManager.evaluate(DefaultExpressionManager.java:210)
at org.mule.expression.DefaultExpressionManager.evaluate(DefaultExpressionManager.java:170)
at org.mule.transformer.simple.MessagePropertiesTransformer.addProperties(MessagePropertiesTransformer.java:165)
at org.mule.transformer.simple.MessagePropertiesTransformer.transformMessage(MessagePropertiesTransformer.java:93)
at org.mule.transformer.AbstractMessageTransformer.transform(AbstractMessageTransformer.java:145)
at org.mule.transformer.AbstractMessageTransformer.transform(AbstractMessageTransformer.java:93)
at org.mule.DefaultMuleMessage.applyAllTransformers(DefaultMuleMessage.java:1305)
at org.mule.DefaultMuleMessage.applyTransformers(DefaultMuleMessage.java:1265)
at org.mule.DefaultMuleMessage.applyTransformers(DefaultMuleMessage.java:1258)
at org.mule.transformer.AbstractTransformer.process(AbstractTransformer.java:118)
at org.mule.processor.ExceptionHandlingMessageProcessor.process(ExceptionHandlingMessageProcessor.java:25)
at org.mule.transport.AbstractMessageReceiver.routeMessage(AbstractMessageReceiver.java:185)
at org.mule.transport.AbstractReceiverWorker$1.doInTransaction(AbstractReceiverWorker.java:126)
at org.mule.transaction.TransactionTemplate.execute(TransactionTemplate.java:116)
at org.mule.transport.AbstractReceiverWorker.doRun(AbstractReceiverWorker.java:149)
at org.mule.transport.AbstractReceiverWorker.run(AbstractReceiverWorker.java:64)
at org.mule.work.WorkerContext.run(WorkerContext.java:309)
at org.mule.work.SyncWorkExecutor.doExecute(SyncWorkExecutor.java:41)
at org.mule.work.MuleWorkManager.executeWork(MuleWorkManager.java:251)
at org.mule.work.MuleWorkManager.doWork(MuleWorkManager.java:175)
at org.mule.transport.jms.MultiConsumerJmsMessageReceiver$SubReceiver.onMessage(MultiConsumerJmsMessageReceiver.java:326)
at com.ibm.mq.jms.MQMessageConsumer$FacadeMessageListener.onMessage(MQMessageConsumer.java:399)
at com.ibm.msg.client.jms.internal.JmsMessageConsumerImpl$JmsProviderMessageListener.onMessage(JmsMessageConsumerImpl.java:1023)
at com.ibm.msg.client.wmq.internal.WMQAsyncConsumerShadow.honourNoLocal(WMQAsyncConsumerShadow.java:566)
at com.ibm.msg.client.wmq.internal.WMQAsyncConsumerShadow.consumer(WMQAsyncConsumerShadow.java:400)
at com.ibm.mq.jmqi.remote.internal.RemoteAsyncConsume.driveConsumer(RemoteAsyncConsume.java:1527)
at com.ibm.mq.jmqi.remote.internal.RemoteDispatchThread.run(RemoteDispatchThread.java:395)
at com.ibm.msg.client.commonservices.workqueue.WorkQueueItem.runTask(WorkQueueItem.java:209)
at com.ibm.msg.client.commonservices.workqueue.SimpleWorkQueueItem.runItem(SimpleWorkQueueItem.java:100)
at com.ibm.msg.client.commonservices.workqueue.WorkQueueItem.run(WorkQueueItem.java:224)
at com.ibm.msg.client.commonservices.workqueue.WorkQueueManager.runWorkQueueItem(WorkQueueManager.java:298)
at com.ibm.msg.client.commonservices.j2se.workqueue.WorkQueueManagerImplementation$ThreadPoolWorker.run(WorkQueueManagerImplementation.java:1220)V.
[/java]

In IntelliJ IDEA, do Analyse > Analyse Stacktrace... > paste the trace (I have been shown the same tool for Eclipse, somewhere in the Console view). Now you can navigate easily in the code. Then you can notice, line #11, that the Exception you receive is raised by AbstractScriptExpressionEvaluator.
The corresponding piece of code is:
[java] try
{
script.initialise();
}
catch (InitialisationException e)
{
throw new MuleRuntimeException(
CoreMessages.initialisationFailure("An error occurred initialising script."), e);
}[/java]

Add a breakpoint at the level of the throw. Start the debugger, reproduce the bug. In the debugger, you can see the actual error, that is hidden in the stacktrace. In my case, it was a:
[java]org.mule.api.lifecycle.InitialisationException: Scripting engine ‘groovy’ not found. Available engines are: [com.sun.script.javascript.RhinoScriptEngineFactory@3d2227][/java]. Without the debugger, it would have been hard to guess that the original MissingResourceException was linked to a missing Groovy resource ;-).

How to fix the Sonar’s following report?
[java]Multithreaded correctness – Synchronization on interned String could lead to deadlock [/java]

Replace lock on String with a lock on an immutable and/or non-serializable object, ie a simple Object.
Eg:
Replace:
[java] synchronized ("myString") {…}[/java]
With:
[java] synchronized (myObject) {…}[/java]

How to fix the Sonar’s following report?
[java]Performance – Inefficient use of keySet iterator instead of entrySet iterator[/java]

The report is explicit: replace loop on keySet with a loop on entrySet, which is faster
Eg:
Replace:
[java]for (Integer key : stressList.keySet()){
Stress stress = stressList.get(key);
}[/java]
With:
[java]for (Map.Entry<Integer, Stress>integerStressEntry : stressList.entrySet()){
Stress stress = integerStressEntry.getValue();
}[/java]

Yet, if you need only keys or only values of your Map, then use rather keySet() or values().

Let’s say immediatly there is no miracle solution and it required a bit of work…

If the title is not explicit enough, the goal of this post is to give a way to include Twitter flow within your GoogleReader. IMHO, the main features of GoogleReader over regular Twitter are the following:

• posts are stored on long term
• posts can be searched or filtered by keywords
• easy integration with Google Plus share button
• ability to give a priority to feeds (as in Google Plus’s circles)
• a simple interface to monitor all feeds

Unlike, following Twitter accounts on Google Reader (or even any other RSS aggregator) makes you lose the real time feature of Twitter.

This said, how to include your Twitter follow list within Google Reader syndication?

• For each of the accounts you follow on Twitter, get the userId.
• The easiest is to select a tweet > right click > source code > search the attribute “data-user-id”, get the associated number, eg: 813286 for @BarackObama, 50055701 for @MittRomney and 248309482 for my prefered one: @John_the_Cowboy ;-).
• Alternatively, you can request the following service: http://www.idfromuser.com
• In Google Reader, add a feed: https://twitter.com/statuses/user_timeline/XYZ.rss. Replace “XYZ” with the actual number retrieved before

Of course, this is not easy at all, as said in the disclaimer above. It is long, takes much time, and can become fastidious if you follow tens of people. Anyway, Google Reader users are geeks rather than newbies. You may create a Groovy script of setup a Mule instance to automatize de process ;-).

I suggest you to create one or many folders to gather the Twitter feeds.