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Strategy for Offline/Online data synchronization

My requirement is I have server J2EE web application and client J2EE web application. Sometimes client can go offline. When client comes online he should be able to synchronize changes to and fro. Also I should be able to control which rows/tables need to be synchronized based on some filters/rules. Is there any existing Java frameworks for doing it? If I need to implement on my own, what are the different strategies that you can suggest?

One solution in my mind is maintaining sql logs and executing same statements at other side during synchronization. Do you see any problems with this strategy?

Answer

There are a number of Java libraries for data synchronizing/replication. Two that I’m aware of are daffodil and SymmetricDS. In a previous life I foolishly implemented (in Java) my own data replication process. It seems like the sort of thing that should be fairly straightforward, but if the data can be updated in multiple places simultaneously, it’s hellishly complicated. I strongly recommend you use one of the aforementioned projects to try and bypass dealing with this complexity yourself.

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Why can’t System.setProperty() change the classpath at runtime?

I am refering to the question on changing the classpath programmatically.

I read and found out that there is some function under System class as getproperties where we can retrieve the properties and then also can set it using setProperties().

The answers however I got was that It Wont work. I have not tried this myself, however, i am taking the call.

Just to clarify, then why these setProperty() and getProperty() methods are there if they cannot alter it at run time. Or is this specific to the classpath property only ?

I will appreciate if someone can present a scenario where they are really helpful?

Answer

You can certainly set any system properties you want at any point of time. The question is, will it have any effect? In the case of classpath, the answer is NO. The system class loader is initialized at a very early point in the startup sequence. It copies the classpath into its own data structures, and the classpath property is not read again. Changing it affect nothing in the system.

The reason for this may be two-fold. The lesser reason is performance. You may need to have some sort of data structure built for quick lookup of resources, and re-parsing classpath every time may be inefficient. The more important reason is security. You don’t want a rogue class change the classpath under you and load compromised version of another class.

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What is the most useful type of java.awt.image.BufferedImage for off-screen rendering? [closed]

I am creating a buffered image that is going to be a snapshot of a JComponent (via paint()) and rendered inside an ImageIcon. There are a large amount of types in the BufferedImage(int width, int height, int imageType) constructor, but which one should I use?

I am sure that any of them would work, but which ones are better than the others? How should I pick one? And Why?

Answer

See GraphicsConfiguration.createCompatibleImage(int, int) for a helper to create a BufferedImage of a “good” type among the many available types.

How to get your hands on a GraphicsConfiguration instance to make this call? It depends on where your code is executing. See the many methods for getting your hands on a GraphicsConfiguration via methods like getGraphicsConfiguration() or getDeviceConfiguration().

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Scroll to bottom of div?

I am creating a chat using Ajax requests and I’m trying to get messages div to scroll to the bottom without much luck.

I am wrapping everything in this div:

#scroll {
    height:400px;
    overflow:scroll;
}

Is there a way to keep it scrolled to the bottom by default using JS?

Is there a way to keep it scrolled to the bottom after an ajax request?

Answer

Here’s what I use on my site:

var objDiv = document.getElementById("your_div");
objDiv.scrollTop = objDiv.scrollHeight;
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Rounding negative numbers in Java

According to Wikipedia when rounding a negative number, you round the absolute number. So by that reasoning, -3.5 would be rounded to -4. But when I use java.lang.Math.round(-3.5) returns -3. Can someone please explain this?

Answer

According to the javadoc

Returns the closest long to the
argument. The result is rounded to an
integer by adding 1/2, taking the
floor of the result, and casting the
result to type long. In other words,
the result is equal to the value of
the expression:

(long)Math.floor(a + 0.5d)

Conceptually, you round up. In other words, to the next integer greater than the value and -3 is greater than -3.5, while -4 is less.

Source: stackoverflow
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