| Uploader: | Ced |
| Date Added: | 03.01.2018 |
| File Size: | 65.76 Mb |
| Operating Systems: | Windows NT/2000/XP/2003/2003/7/8/10 MacOS 10/X |
| Downloads: | 31657 |
| Price: | Free* [*Free Regsitration Required] |
Multithreading in Java - GeeksforGeeks
Java concurrency (multi-threading). This article describes how to do concurrent programming with Java. It covers the concepts of parallel programming, immutability, threads, the executor framework (thread pools), futures, callables CompletableFuture and the fork-join framework. Jan 09, · Thread creation by extending the Thread class We create a class that extends the blogger.com class. This class overrides the run() method available in the Thread class. A thread begins its life inside run() method. We create an object of our new class and call start() method to start the execution of a thread/5. Unlike many other computer languages, Java provides built-in support for multithreading. Multithreading in Java contains two or more parts that can run concurrently. A Java thread is actually a.
Java multi thread download file and update array
By using our site, java multi thread download file and update array, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. Under what circumstances would an unsynchronized collection, say an ArrayList, cause a problem? I can't think of any, can someone please give me an example where an Java multi thread download file and update array causes a problem and a Vector solves it?
I wrote a program that have 2 threads both modifying an arraylist that has one element. One thread puts "bbb" into the arraylist while the other puts "aaa" into the arraylist. I don't really see an instance where the string is half modified, I am on the right track here? Also, I remember that I was told that multiple threads are not really running simultaneously, 1 thread is run for sometime and another java multi thread download file and update array runs after that on computers with a single CPU.
If that was correct, how could two threads ever access the same data at the same time? Maybe thread 1 will be stopped in the middle of modifying something and thread 2 will be started? There are three aspects of what might go wrong if you use an ArrayList for example without adequate synchronization.
The first scenario is that if two threads happen to update the ArrayList at the same time, then it may get corrupted. For instance, the logic of appending to a list goes something like this:. Suppose that the first thread gets to the point labeled HERE and is preempted. The second thread comes along, and overwrites the slot in elements that the first thread just updated with its own element, and then increments size. When the first thread finally gets control, it updates size.
The end result is that we've added the second thread's element and not the first thread's element, and most likely also added a null to the list. This is just illustrative. In reality, the native code compiler may have reordered the code, and so on.
But the point is that bad things can happen if updates happen simultaneously. The second scenario arises due to the caching of main memory contents in the CPU's cache memory.
Suppose that we have two threads, one adding elements to the list and the second one reading the list's size. When on thread adds an element, it will update the list's size attribute. However, since size is not volatilethe new value of size may not immediately be written out to main memory.
Instead, it could sit in the cache until a synchronization point where the Java memory model requires that cached writes get flushed. In the meantime, the second thread could call size on the list and get a stale value of size. In the worst case, the second thread calling get int for example might see inconsistent values of size and the elements array, resulting in unexpected exceptions.
Note that kind of problem can happen even when there is only one core and no memory caching. The third scenario arises when you synchronize operations on the ArrayList ; e.
If thread2's list is an ArrayList or LinkedList and the two threads run simultaneously, thread 2 will fail with a ConcurrentModificationException. If it is some other home brew list, then the results are unpredictable. If there is only one core available to run the application, obviously only one thread gets to run at a time. This makes some of the hazards impossible and others become much less likely likely to occur. However, it is possible for the OS to switch from one thread to another thread at any point in the code, and at any time.
That's possible. The probability of it happening is very small 1 but that just makes this kind of problem more insidious. A practical example. At the end list should contain 40 items, but for me it usually shows between 30 and Guess why? This is the famous problem of incrementing integer from two different threads. There's a nice explanation in Sun's Java tutorial on concurrency. That won't happen. However, what could happen is that only one of the strings gets added.
Or that an exception occurs during the call to add. If you want to access a java multi thread download file and update array from multiple threads, you need to synchronize this access. However, just using a Vector does not really solve the problem. You will not get the issues described above, but the following pattern will still not work:. The Vector itself will not get corrupted, but that does not mean that it cannot get into states that your business logic would not want to have.
You need to synchronize in your application code and then there is no need to use Vector. The concurrency utility packages also has a number of collections that provide atomic operations necessary for thread-safe queues and such.
Anytime that a thread is reading the ArrayList and the other one is writing, or when they are both writing. Here's a very known example. Yes, Single core cpus can execute only one instruction at a time not really, pipelining has been here for a while, but as a professor once said, thats "free" parallelism.
Even though, each process running in your computer is only executed for a period of time, then it goes to an idle state, java multi thread download file and update array. And then go into an idle state or finish. Processes execution are interleaved. With threads the same thing happens, only that they are contained inside a process. How they execute is dependant on the Operating System, but the concept remains the same.
They change from active to idle constantly through their lifetime. A great example is the producer-consumer problem. See here: link text. You cannot control java multi thread download file and update array one thread will be stopped and other will start. Thread 1 will not wait until it has completely finished adding data. There is always possible to corrupt data. Learn more.
Asked 9 years, 6 months ago. Active 4 years, 3 months ago. Viewed 29k times. Many Thanks in advance. Half and half on a string is straightly impossible, but if you have 2 threads adding strings to an unsynchronized list, you may not get strings in the end.
See my answer for an illustration of why this can happen. Linking few books is a good start, I seriously doubt lack of very basic knowledge can be mended by an answer.
Stephen C Stephen C k 80 80 gold badges silver badges bronze badges. Hi Stephen. I understand that adding the synchronized keyword to the function that threads enter simultaneously solved the problem? I mean, in your first example, simply defining the function as "public synchronized void add T element " solves the problem, isn't it? CAAY - Hypothetically yes.
But that code is a "sketch" of what is going on in the real ArrayList. Obviously you can't add synchronized to methods defined by standard classes. So this is not a solution to the problem this Question is describing, java multi thread download file and update array. Nikita Rybak Nikita Rybak A fair example, but your answer is incomplete. Thilo Thilo k 83 83 gold badges silver badges bronze badges. When will it cause trouble?
Tom Tom The first part of youe query has been already answered. I will try to answer the second part : Also, I remember that I was told that multiple threads are not really running simultaneously, 1 thread is run for sometime and another thread runs after that on computers with a single CPU. Amit Amit 6 6 silver badges 11 11 bronze badges. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password, java multi thread download file and update array.
Post as a guest Name. Email Required, but never shown. The Overflow Blog. Scaling your VPN overnight. How event-driven architecture solves modern web app problems. Featured on Meta. The Q1 Community Roadmap is on the Blog. Community and Moderator guidelines for escalating issues via new response….
How to speed up applications using threads in JAVA
, time: 8:37Java multi thread download file and update array
Jan 09, · Thread creation by extending the Thread class We create a class that extends the blogger.com class. This class overrides the run() method available in the Thread class. A thread begins its life inside run() method. We create an object of our new class and call start() method to start the execution of a thread/5. Initialize and sum an array in Java using threads. Ask Question Asked 3 years, 11 months ago. Adding values to the array, each index in the array gets a thread to initialize that value: Why is it not recommended to run blogger.com file directly from blogger.com file? Feb 27, · ArrayList and Multithreading in Java. Ask Question Asked 9 years, 2 months ago. When on thread adds an element, it will update the list's size attribute. However, since size is not volatile, (int) for example) might see inconsistent values of size and the elements array, resulting in unexpected exceptions. (Note that kind of problem can.
No comments:
Post a Comment