I'm using net beans for Java development. I'm working on a multi threading application and I want to easily identify code sections which are executed by more than one thread? Is there a easy way to do that?
For example, if some field of method of class ABC is executed by more than one thread?
This is something that can only be determined at runtime.
You can throw this method to the beginning you your method calls to determine the calling Thread.
public static void reportThread(String methodName) {
//Somehow LOG (println, logging framework)
LOG(methodName + " was ran on thread: " + Thread.currentThread().getName());
}
In general, it is not possible to do statically, i.e. by inspection of the code. (The problem is undecidable due to the halting problem.)
Your only option is to do the analysis runtime, that is, to log which actual thread executes with method. You have a few options. Here are two that I come to think of immediately.
Add System.out.println(Thread.currentThread()) on interesting methods
Use for example AspectJ to do something similar.
Related
In our Netty application. We are moving all blocking calls in our code to run in a special backgroundThreadGroup.
I'd like to be able to log in production the threadName and the lineNumber of the java code that is about to execute a blocking operation. (i.e. sync File and Network IO)
That way I can grep for the logs looking at places were we might have missed to move our blocking code to the backgroundThreadGroup.
Is there a way to instrument the JVM so that it can tell me that?
Depends on what you mean by a "blocking operation".
In a broad sense, any operation that causes a voluntary context switch is blocking. Trying to do something special about them is absolutely impractical.
For example, in Java, any method containing synchronized is potentially blocking. This includes
ConcurrentHashMap.put
SecureRandom.nextInt
System.getProperty
and many more. I don't think you really want to avoid calling all these methods that look normal at a first glance.
Even simple methods without any synchronization primitives can be blocking. E.g., ByteBuffer.get may result in a page fault and a blocking read on the OS level. Furthermore, as mentioned in comments, there are JVM level blocking operations that are not under your control.
In short, it's impractical if not impossible to find all places in the code where a blocking operation happens.
If, however, you are interested in finding particular method calls that you believe are bad (like Thread.sleep and Socket.read), you can definitely do so. There is a BlockHound project specifically for this purpose. It already has a predefined list of "bad" methods, but can be customized with your own list.
There is a library called BlockHound, that will throw an exception unless you have configured BlockHound to ignore that specific blocking call
This is how you configure BlockHound for Netty: https://github.com/violetagg/netty/blob/625f9d5781ed85bfaca6fa4e826d0d46d70fdbd8/common/src/main/java/io/netty/util/internal/Hidden.java
(You can improve the above code by replacing the last line with builder.nonBlockingThreadPredicate(
p -> p.or(thread -> thread instanceof FastThreadLocalThread)); )
see https://github.com/reactor/BlockHound
see https://blog.frankel.ch/blockhound-how-it-works/
I personally used it to find all blocking call within our Netty based service.
Good Luck
I am newbie learning selenium and wrote below java code. I am trying to run a for loop that is supposed to load the site 20 times. Right now it does loop in sequential order and I want that to be run in parallel.
public class lenders {
//ExtentReports logger = ExtentReports.get(lenders.class);
public static void main(String[] args) throws InterruptedException {
for (int i=0; i<20; i++) {
FirefoxDriver driver= new FirefoxDriver();
driver.manage().timeouts().pageLoadTimeout(1, TimeUnit.SECONDS);
try {
driver.get("https://www.google.com");
} catch (TimeoutException e) {
driver.quit();
}
}
Towards the end I want 20 browsers to be open and loading the site and all of them getting killed.
If you are on Java-8 you can run a parallel for loop using aparallelStream
IntStream.range(0,20).parallel().forEach(i->{
... do something here
});
In general and at a high level, trying to run code in parallel within java means that you are trying to run multi-threaded code (more than one thread executing at one time).
As individuals have been saying in comments, one must therefore give a warning with my answer. Multi-threading in itself is a complicated topic and one must enter this territory with caution as there can be many issues/topics regarding "thread safety" and even if this is the way you "should" approach the "business request".
In any case, if you really want to create something multi-threaded then I would direct you to a few technical items to get you STARTED on the topic training (and your own further research): You could create another class that implements the Callable interface. It will then have to have the "call" method by nature of implementing that interface. In this class and in the "call" method you would put the actual logic that you want to happen in parallel. In your case, all of the driver, etc.
Then in your parent class (the one you put code from above), you can use a FixedThreadPool and an associated ExecutorService that accepts this callable class. It will essentially run the "call" method in a separate thread so that your for loop can continue onwards at the same time that the logic in the "call" method is executed. It will go the second time around and create another thread, etc. You can manage how many threads are created using your thread pool. You can use different kind of thread pools and services, etc. Again, this is a really BIG topic and field. I am just trying to get your nose in a direction for you to start researching it further.
People might not like my answer because they think you should use completely different technologies other than using Selenium in this manner, etc. I totally understand that point of view and don't disagree with those alternate answers. However, your question was "how to get this running at the same time" and I have tried to give you the building block answer. I hope that helps! Let me know if you need some links to tutorials or anything, but google "ExecutorService" "Thread Pool" and "Callable" (or combinations of them) with the word java and tutorial should get you a variety of answers on the topic.
I hope that helps!
In the gevent tutorial there's an example that looks like this:
import gevent
def foo():
print('Running in foo')
gevent.sleep(0)
print('Explicit context switch to foo again')
def bar():
print('Explicit context to bar')
gevent.sleep(0)
print('Implicit context switch back to bar')
gevent.joinall([
gevent.spawn(foo),
gevent.spawn(bar),
])
and the output is
Running in foo
Explicit context to bar
Explicit context switch to foo again
Implicit context switch back to bar
I've tested it and seen for myself that it works. A friend of mine claims this is run entirely within a single thread. Except that I can't think of any implementation of gevent.sleep(0) which doesn't boil down to some form of "cheating" (ie: Swap the top two stack frames etc.)
Can someone explain how this works? If this were Java (or at least some language where that kind of stack manipulation is forbidden), would this be possible? (again, without using multiple threads).
It indeed runs on only 1 thread. gevent uses greenlets, which are coroutines, not threads. There is only 1 stack at each given time (unless you use multithreading, and then use greenlets in each threads).
In your example above, whenever you call sleep or joinall, the current coroutine (greenlet) actually yields to the hub. Think about the hub as a central dispatcher, responsible to decide which coroutine will run next.
To convince yourself of that, remove the gevent.sleep(0) call, and you'll see that it behaves differently.
Note that unlike threads, the execution is deterministic, so if you run the program twice, it will execute in the exact same order.
I have a scenario in which I am running unreliable code in java (the scenario is not unlike this). I am providing the framework classes, and the intent is for the third party to overwrite a base class method called doWork(). However, if the client doWork() enters a funked state (such as an infinite loop), I need to be able to terminate the worker.
Most of the solutions (I've found this example and this example) revolve around a loop check for a volatile boolean:
while (keepRunning) {
//some code
}
or checking the interrupted status:
while (isInterrupted()) {
//some code
}
However, neither of these solutions deal with the the following in the '//some code' section:
for (int i = 0; i < 10; i++) {
i = i - 1;
}
I understand the reasons thread.stop() was depreciated, and obviously, running faulty code isn't desirable, though my situation forces me to run code I can't verify myself. But I find it hard to believe Java doesn't have some mechanism for handling threads which get into an unacceptable state. So, I have two questions:
Is it possible to launch a Thread or Runnable in Java which can be reliably killed? Or does Java require cooperative multithreading to the point where a thread can effectively hose the system?
If not, what steps can be taken to pass live objects such that the code can be run in a Process instead (such as passing active network connections) where I can actually kill it.?
If you really don't want to (or probably cannot due to requirement of passing network connections) spawn new processes, you can try to instrument code of this 'plugin' when you load it's class. I mean change it's bytecode so it will include static calls to some utility method (eg ClientMentalHealthChecker.isInterrupted()). It's actually not that hard to do. Here you can find some tools that might help: https://java-source.net/open-source/bytecode-libraries. It won't be bullet proof because there are other ways of blocking execution. Also keep in mind that clients code can catch InterruptedExceptions.
In my application which runs user submitted code[1] in separate threads, there might be some cases where the code might take very long to run or it might even have an infinite loop! In that case how do I stop that particular thread?
I'm not in control of the user code, so I cannot check for Thread.interrupted() from the inside. Nor can I use Thread.stop() carelessly. I also cannot put those code in separate processes.
So, is there anyway to handle this situation?
[1] I'm using JRuby, and the user code is in ruby.
With the constraints you've provided:
User submitted code you have no control over.
Cannot force checks for Thread.interrupted().
Cannot use Thread.stop().
Cannot put the user code in a process jail.
The answer to your question is "no, there is no way of handling this situation". You've pretty much systematically designed things so that you have zero control over untrusted third-party code. This is ... a suboptimal design.
If you want to be able to handle anything, you're going to have to relax one (or preferably more!) of the above constraints.
Edited to add:
There might be a way around this for you without forcing your clients to change code if that is a(nother) constraint. Launch the Ruby code in another process and use some form of IPC mechanism to do interaction with your main code base. To avoid forcing the Ruby code to suddenly have to be coded to use explicit IPC, drop in a set of proxy objects for your API that do the IPC behind the scenes which themselves call proxy objects in your own server. That way your client code is given the illusion of working inside your server while you jail that code in its own process (which you can ultimately kill -9 as the ultimate sanction should it come to that).
Later you're going to want to wean your clients from the illusion since IPC and native calls are very different and hiding that behind a proxy can be evil, but it's a stopgap you can use while you deprecate APIs and move your clients over to the new APIs.
I'm not sure about the Ruby angle (or of the threading angle) of things here, but if you're running user-submitted code, you had best run it in a separate process rather than in a separate thread of the same process.
Rule number one: Never trust user input. Much less if the input is code!
Cheers
Usually you have a variable to indicate to stop a thread. Some other thread then would set this variable to true. Finally you periodically check, whether the variable is set or not.
But given that you can't change user code , I am afraid there isn't a safe way of doing it.
For Running Thread Thread.Interrupt wont actually stop as sfussenegger mentioned aforth (thanks sfussenegger recollected after reading spec).
using a shared variable to signal that it should stop what it is doing. The thread should check the variable periodically,(ex : use a while loop ) and exit in an orderly manner.
private boolean isExit= false;
public void beforeExit() {
isExit= true;
}
public void run() {
while (!isExit) {
}
}