We Keep Coding

How to make all class methods run only on single thread? (synchronized class?)

I know synchronized keyword makes method run only on single class at a time. But here is the problem.
I have a database class with methods e.g. insertAccount, updateSetting, etc. If I make insertAccount, updateSetting synchronized, each of them will be able to run only on one thread at a time.
If there was one method for whole database, it would be great, but there are not one. If one thread calls insertAccount and another thread calls updateSetting at the same time, it will go bad, right?
Because only one of these methods can be run at any time. So what do I do?
Is there a way to apply something like synchronized to the whole class? So that if 1st thread calls insertAccount and 2nd thread calls updateSetting at the same time, 2nd thread has to wait until 1st thread finishes accessing database.

The real answer here: step back and do some studying. You should not be using synchronized here, but rather look into a lock object that a reader/writer needs to acquire prior turning to that "DB class". See here for more information.
On the other hand, you should understand what transactions are, and how your database supports those. Meaning: there are different kinds of problems; and the different layers (application code, database) have different responsibilities.
You see, using "trial and error" isn't an approach that will work out here. You should spend some serious time studying the underlying concepts. Otherwise you are risking to damage your data set; and worse: you risk writing code that works fine most of the time; but fails in obscure ways "randomly". Because that is what happens when multiple threads manipulate shared data in an uncontrolled manner.

You misunderstood how synchronized work.
If you mark two method of class by synchronized only one of them could be executed at any moment of time (except if you invoke wait).
Also note that if you have several instances of this class you can execute methods of different instances simultaneously.

#Test(singleThreaded = true) Use above annotation above class and its tests will be run using a single thread even though you have used parallel="methods" in your testng.xml file

java - sharing data between threads - atomicreference or synchronize

I am making a 2 player videogame, and the oponent's position gets updated on a thread, because it has a socket that is continuously listening. What I want to share is position and rotation.
As it is a videogame I don't want the main thread to be blocked (or be just the minimum time possible) and I don't want the performance to be affected. So from what I've seen to share this info the normal thing to do would be something like
class sharedinfo
{
public synchronized read();
public synchronized write();
}
but this would block the read in the main thread (the same that draws the videogame) until the three values (or even more info in the future are written) are written, and also I've read that synchronized is very expensive (also it is important to say this game is for android also, so performance is very important).
But I was thinking that maybe having sharedInfo inside an AtomicReference and eliminating synchronized would make it more efficient, because it would only stop when the reference itself is being updated (the write would not exist, I would create a new object and put it on the atomicreference), also they say that atomic* use hardware operations and are more efficient than synchronized.
What do you think?

Consider using a queue for this, Java has some nice concurrent queue implementations. Look up the BlockingQueue interface in java.util.concurrent, and who implements it. Chances are you fill find strategies implemented that you hadn't even considered.
Before you know it, you will want to communicate more than just positions between your threads, and with a queue you can stick different type of objects in there, maybe at different priorities, etc.
If in your code you use Interfaces (like Queue or BlockingQueue) as much as possible (i.e. anywhere but the place where the specific instance is constructed), it is really easy to swap out what exact type of Queue you are using, if you need different functionality, or just want to play around.

Manually Increasing the Amount of CPU a Java Application Uses

I've just made a program with Eclipse that takes a really long time to execute. It's taking even longer because it's loading my CPU to 25% only (I'm assuming that is because I'm using a quad-core and the program is only using one core). Is there any way to make the program use all 4 cores to max it out? Java is supposed to be natively multi-threaded, so I don't understand why it would only use 25%.

You still have to create and manage threads manually in your application. Java can't determine that two tasks can run asynchronously and automatically split the work into several threads.

This is a pretty vague question because we don't know much about what your program does. If your program is single-threaded, then no number of cores on your machine is going to make it run any faster. Java does have threading support, but it won't automatically parallelize your code for you. To speed it up, you'll need to identify parts of the computation that can be run in parallel with one another and add code as appropriate to split up and reconstitute the work. Without more info on what your program does, I can't help you out.
Another important detail to note is that Java threads are not the same as system threads. The JVM often has its own thread scheduler that tries to put Java threads onto actual system threads in a way that's fair, but there's no actual guarantee that it will do so.

Yes, Java is multi-threaded, but the multi-threading doesn't happen "by magic".
Have a look at either at the Thread class or at the Executor framework. Essentially you need to split your job into "subtasks" each of which can run on a single processor, then do something like this:
Executor ex = Executors.newFixedThreadPool(4);
while (thereAreMoreSubtasksToDo) {
ex.execute(new Runnable() {
public void run() {
... do subtask ...
}
});
}
Turning a serial routine/algorithm into a parallel one isn't necessarily trivial: you need to know in particular about a range of issues broadly termed "thread-safety". You may be interested in some material I've written about thread-safety in Java, and threading in general if you follow the links: the key thing to bear in mind is that if any data/objects are being shared among the different threads running, then you need to take special precautions. That said, for independent things that you just want to "run at the same time", then the above pattern will get you started.

Java is multi-threaded but if your application runs in only one thread, only one thread will be used. (Apart from the internal threads Java uses for finalization, garbage collection and so on.)
If you want your code to use multiple threads, you have to split it up manually, either by starting threads by yourself or using a third party thread pool. I'd suggest the latter option as it's safer but both can work equally well.

You've got a bit of learning ahead of you (actually, quite a bit of learning) - but it's learning you should do if you are going to be doing any serious programming.
Here's a starting point: http://download.oracle.com/javase/tutorial/essential/concurrency/
But you might want to look into a good book on Java multi-threading (I did this so long ago that any book I could recommend would be out of print). This sort of hard topic is well suited for learning from a text instead of online tutorials.

Best way to reuse a Runnable

I have a class that implements Runnable and am currently using an Executor as my thread pool to run tasks (indexing documents into Lucene).
executor.execute(new LuceneDocIndexer(doc, writer));
My issue is that my Runnable class creates many Lucene Field objects and I would rather reuse them then create new ones every call. What's the best way to reuse these objects (Field objects are not thread safe so I cannot simple make them static) - should I create my own ThreadFactory? I notice that after a while the program starts to degrade drastically and the only thing I can think of is it's GC overhead. I am currently trying to profile the project to be sure this is even an issue - but for now lets just assume it is.

Your question asks how to reuse a Runnable, so I am going to ignore the other details adn simply answer that question.
If you are using a ThreadPoolExecutor, you can use the [ThreadPoolExecutor#afterExecute][1] method to return the Runnable object to a pool/queue of 'cached' Runnables.
[1]: http://java.sun.com/javase/6/docs/api/java/util/concurrent/ThreadPoolExecutor.html#afterExecute(java.lang.Runnable, java.lang.Throwable)

A Runnable object is reusable. It is thread object which is not.
Best way ? it is your way :-)
I think it is more a lucene question than a runnable question.

You might want to do some more benchmarking to nail down what's causing your slowdowns.
I'm willing to bet that your problem is not related to the creation of Field instances. Field doesn't have any finalizers, and they're not designed to be pooled.

For now I have decided to just use a simple Producer->Consumer model. I pass a BlockingQueue to each indexer, rather then a document to index, and then have the main driver of the program add new documents to that queue. The Indexers then feed off that [bounded] queue and reuse the Field objects and share the thread-safe IndexWriter.
I did find a place where I was possibly not calling HttpMethod.releaseConnection() so that could have caused my memory issues (uncertain).

How good is the JVM at parallel processing? When should I create my own Threads and Runnables? Why might threads interfere?

I have a Java program that runs many small simulations. It runs a genetic algorithm, where each fitness function is a simulation using parameters on each chromosome. Each one takes maybe 10 or so seconds if run by itself, and I want to run a pretty big population size (say 100?). I can't start the next round of simulations until the previous one has finished. I have access to a machine with a whack of processors in it and I'm wondering if I need to do anything to make the simulations run in parallel. I've never written anything explicitly for multicore processors before and I understand it's a daunting task.
So this is what I would like to know: To what extent and how well does the JVM parallel-ize? I have read that it creates low level threads, but how smart is it? How efficient is it? Would my program run faster if I made each simulation a thread? I know this is a huge topic, but could you point me towards some introductory literature concerning parallel processing and Java?
Thanks very much!
Update:
Ok, I've implemented an ExecutorService and made my small simulations implement Runnable and have run() methods. Instead of writing this:
Simulator sim = new Simulator(args);
sim.play();
return sim.getResults();
I write this in my constructor:
ExecutorService executor = Executors.newFixedThreadPool(32);
And then each time I want to add a new simulation to the pool, I run this:
RunnableSimulator rsim = new RunnableSimulator(args);
exectuor.exectue(rsim);
return rsim.getResults();
The RunnableSimulator::run() method calls the Simulator::play() method, neither have arguments.
I think I am getting thread interference, because now the simulations error out. By error out I mean that variables hold values that they really shouldn't. No code from within the simulation was changed, and before the simulation ran perfectly over many many different arguments. The sim works like this: each turn it's given a game-piece and loops through all the location on the game board. It checks to see if the location given is valid, and if so, commits the piece, and measures that board's goodness. Now, obviously invalid locations are being passed to the commit method, resulting in index out of bounds errors all over the place.
Each simulation is its own object right? Based on the code above? I can pass the exact same set of arguments to the RunnableSimulator and Simulator classes and the runnable version will throw exceptions. What do you think might cause this and what can I do to prevent it? Can I provide some code samples in a new question to help?

Java Concurrency Tutorial
If you're just spawning a bunch of stuff off to different threads, and it isn't going to be talking back and forth between different threads, it isn't too hard; just write each in a Runnable and pass them off to an ExecutorService.
You should skim the whole tutorial, but for this particular task, start here.
Basically, you do something like this:
ExecutorService executorService = Executors.newFixedThreadPool(n);
where n is the number of things you want running at once (usually the number of CPUs). Each of your tasks should be an object that implements Runnable, and you then execute it on your ExecutorService:
executorService.execute(new SimulationTask(parameters...));
Executors.newFixedThreadPool(n) will start up n threads, and execute will insert the tasks into a queue that feeds to those threads. When a task finishes, the thread it was running on is no longer busy, and the next task in the queue will start running on it. Execute won't block; it will just put the task into the queue and move on to the next one.
The thing to be careful of is that you really AREN'T sharing any mutable state between tasks. Your task classes shouldn't depend on anything mutable that will be shared among them (i.e. static data). There are ways to deal with shared mutable state (locking), but if you can avoid the problem entirely it will be a lot easier.
EDIT: Reading your edits to your question, it looks like you really want something a little different. Instead of implementing Runnable, implement Callable. Your call() method should be pretty much the same as your current run(), except it should return getResults();. Then, submit() it to your ExecutorService. You will get a Future in return, which you can use to test if the simulation is done, and, when it is, get your results.

You can also see the new fork join framework by Doug Lea. One of the best book on the subject is certainly Java Concurrency in Practice. I would strong recommend you to take a look at the fork join model.

Java threads are just too heavyweight. We have implement parallel branches in Ateji PX as very lightweight scheduled objects. As in Erlang, you can create tens of millions of parallel branches before you start noticing an overhead. But it's still Java, so you don't need to switch to a different language.

If you are doing full-out processing all the time in your threads, you won't benefit from having more threads than processors. If your threads occasionally wait on each other or on the system, then Java scales well up to thousands of threads.
I wrote an app that discovered a class B network (65,000) in a few minutes by pinging each node, and each ping had retries with an increasing delay. When I put each ping on a separate thread (this was before NIO, I could probably improve it now), I could run to about 4000 threads in windows before things started getting flaky. Linux the number was nearer 1000 (Never figured out why).
No matter what language or toolkit you use, if your data interacts, you will have to pay some attention to those areas where it does. Java uses a Synchronized keyword to prevent two threads from accessing a section at the same time. If you write your Java in a more functional manner (making all your members final) you can run without synchronization, but it can be--well let's just say solving problems takes a different approach that way.
Java has other tools to manage units of independent work, look in the "Concurrent" package for more information.

Java is pretty good at parallel processing, but there are two caveats:
Java threads are relatively heavyweight (compared with e.g. Erlang), so don't start creating them in the hundreds or thousands. Each thread gets its own stack memory (default: 256KB) and you could run out of memory, among other things.
If you run on a very powerful machine (especially with a lot of CPUs and a large amount of RAM), then the VM's default settings (especially concerning GC) may result in suboptimal performance and you may have to spend some times tuning them via command line options. Unfortunately, this is not a simple task and requires a lot of knowledge.