22. Concurrency and Threads in Java

22. Concurrency and Threads in Java

Concurrency is a fundamental concept in modern programming, especially in Java. With the increase in the number of processors and cores in computer systems, it becomes increasingly important to know how to perform tasks simultaneously to make the most of available hardware resources. In this chapter, we will explore the concepts of concurrency and threads in Java, diving from the basics to advanced aspects.

Thread Fundamentals

In Java, a thread is the smallest unit of execution that can be scheduled by the operating system. A Java application can have several threads running simultaneously, each one handling a different part of the work. This allows programs to multitask, such as responding to user events while performing calculations in the background.

To create a thread in Java, there are two main ways:

• Extending the Thread class: Create a class that extends java.lang.Thread and overrides the run() method. After creating an instance of your class, call the start() method to run the thread.
• Implementing the Runnable interface: Create a class that implements the Runnable interface and implements the run() method. You can pass an instance of this class to the Thread constructor and then call start().

class MyThread extends Thread {
  public void run() {
    // Code to be executed in a new thread
  }
}

class MyRunnable implements Runnable {
  public void run() {
    // Code to be executed in a new thread
  }
}

public class ExampleThread {
  public static void main(String[] args) {
    MyThread mt = new MyThread();
    mt.start();
    
    Thread t = new Thread(new MyRunnable());
    t.start();
  }
}

Thread Management

Managing threads manually can be complex and error-prone. Java provides a set of tools to help manage thread execution, including methods for:

• Synchronization: Keyword synchronized and classes in the java.util.concurrent package, such as Locks, Semaphores and CountDownLatch help manage access to shared resources.
• Inter-Thread Communication: Methods like wait(), notify() and notifyAll() are used for communication between threads.
• State management: Methods like interrupt(), join() and isAlive() help to manage the state and lifecycle of threads.

Competition Issues

Developing competing applications brings unique challenges, such as:

• Deadlock: Occurs when two or more threads are waiting indefinitely for each other to release resources, resulting in a deadlock.
• Starvation: Occurs when one or more threads are prevented from advancing due to resource monopolization by other threads.
• Race conditions: Arise when two or more threads access a shared resource simultaneously and the final result depends on the order of execution of the threads.

To avoid these problems, it is essential to understand and apply proper synchronization and blocking techniques.

Executors and Executor Services

Starting with Java 5, the Concurrency API introduced the executor framework, which simplifies thread execution and management. Executors abstract the creation and management of threads, providing an executor service that can be used to execute tasks asynchronously.

Examples of executors include:

• ThreadPoolExecutor: Manages a pool of reusable threads.
• ScheduledThreadPoolExecutor: Allows the execution of tasks with a delay or periodically.
• Executors.newCachedThreadPool: Creates a thread pool that creates new threads as needed, but will reuse previously constructed threads when they become available.

ExecutorService executor = Executors.newFixedThreadPool(4);

executor.execute(new Runnable() {
  public void run() {
    // Task to be executed
  }
});

executor.shutdown();

High Level Competition

In addition to executors, the Java Concurrency API offers high-level abstractions such as:

• Future and Callable to work with results from asynchronous tasks.
• CompletionService to manage a set of asynchronous tasks.
• Concurrent Collections such as ConcurrentHashMap and BlockingQueue to work with collections in concurrent environments.

Good Competition Practices

To write robust and efficient concurrent programs in Java, it is important to follow some good practices:

• Minimize the scope of critical sections by using synchronization only where it is strictly necessary.
• Avoid long-running deadlocks to reduce the risk of deadlock and improve application responsiveness.
• Prefer the high-level abstractions of the Java Concurrency API over managing threads and synchronization manually.
• Use concurrent collections to manage access to data shared between threads.
• Be aware of common concurrency issues such as race conditions, deadlocks, and starvation and know how to avoid them.

In summary, concurrency in Java is a powerful feature that, when used correctly, can lead to more responsive and efficient applications. However, it also introduces complexity and potential pitfalls. With a solid understanding of the concepts and tools provided by the Java Concurrency API, developers can create safe and effective concurrent programs.