Icsme2016. Java - Alternative to spinlock. Chapter 17. Threads and Locks. While most of the discussion in the preceding chapters is concerned only with the behavior of code as executed a single statement or expression at a time, that is, by a single thread, the Java Virtual Machine can support many threads of execution at once. These threads independently execute code that operates on values and objects residing in a shared main memory. Threads may be supported by having many hardware processors, by time-slicing a single hardware processor, or by time-slicing many hardware processors. Threads are represented by the Thread class. The only way for a user to create a thread is to create an object of this class; each thread is associated with such an object.
A thread will start when the start() method is invoked on the corresponding Thread object. The behavior of threads, particularly when not correctly synchronized, can be confusing and counterintuitive. These semantics do not prescribe how a multithreaded program should be executed. 17.2. 17.2.4. Table 17.1. Java Concurrency / Multithreading Tutorial. Back in the old days a computer had a single CPU, and was only capable of executing a single program at a time. Later came multitasking which meant that computers could execute multiple programs (AKA tasks or processes) at the same time.
It wasn't really "at the same time" though. The single CPU was shared between the programs. The operating system would switch between the programs running, executing each of them for a little while before switching. Along with multitasking came new challenges for software developers. Programs can no longer assume to have all the CPU time available, nor all memory or any other computer resources. Later yet came multithreading which mean that you could have multiple threads of execution inside the same program.
Multithreading can be a great way to increase the performance of some types of programs. If a thread reads a memory location while another thread writes to it, what value will the first thread end up reading? Multithreading and Concurrency in Java. Java.util.concurrent - Java Concurrency Utilities. Last updated: 2014-06-23 Java 5 added a new Java package to the Java platform, the java.util.concurrent package. This package contains a set of classes that makes it easier to develop concurrent (multithreaded) applications in Java. Before this package was added, you would have to program your utility classes yourself. In this tutorial I will take you through the new java.util.concurrent classes, one by one, so you can learn how to use them. I will use the versions in Java 6.
I will not explain the core issues of concurrency in Java - the theory behind it, that is. Work in Progress This tutorial is very much "work in progress", so if you spot a missing class or interface, please be patient. Table of Contents Here is a list of the topics covered in this java.util.concurrent trail. Feel Free to Contact Me If you disagree with anything I write here about the java.util.concurrent utilities, or just have comments, questions, etc, feel free to send me an email. Java Thread Local – How to use and code sample. Thread Local is an interesting and useful concept, yet most of the Java developers are not aware of how to use that. In this post, I'll explain what is Thread Local and when to use it, with an example code.
Since it'll be little tough to understand this concept at first, I'll keep the explanation as simple as possible (corollary: you shouldn't use this code as it is in a production environment. Grasp the concept and modify upon it!) Let's begin. What is Thread Local? Thread Local can be considered as a scope of access, like a request scope or session scope. Values stored in Thread Local are global to the thread, meaning that they can be accessed from anywhere inside that thread.
Well, that's the concept of Thread Local. When to use Thread Local? We saw what is thread local in the above section. I can point out one use case where I used thread local. To solve that, you can use Thread Local. This servlet might be servicing more that one request at a time. Got it!? How to use Thread Local? Problems with RejectedExecutionHandler and Futures | A Concurrent Affair. I’ve been working with ThreadPoolExecutor and RejectedExecutionHandler. The executor framework provides a convenient way to distribute concurrent tasks to multiple threads, and the different RejectedExecutionHandler policies, ThreadPoolExecutor.DiscardOldestPolicy, for instance, enable your program to shed load if there is more work than it can reliably handle.
Unfortunately, when I started using Futures, things became complicated. Consider the following example: It creates a thread pool with a maximum of 10 threads, and a work queue that can hold another 10 items. The policy to shed load is ThreadPoolExecutor.DiscardOldestPolicy, which means it will favor newer tasks over older tasks, if the queue has reached its capacity.
Then the program creates 30 tasks and submits them to the executor service. The tasks each sleep for a second to make sure we actually hit the capacity of the thread pool executor. I haven’t found a good solution with the built-in classes of the JDK yet. Bug ID: JDK-6625725 (coll) modCount should not be volatile. Java - HashMap and visibility.