An Introduction Into Java Threads

This article provides an introductory overview on Java threads; what is a thread, how it can be implemented, what is the lifecycle of a thread and some more important topics. Whether you are referring to this article as an interview guide, as a manual to get started with threads or as a quick note to brush up on threads, this article is able to provide an informative abstract on the key concepts of Java threads.

What Is A Thread?

Thread is simply a flow of execution within a program. Unlike many other programming languages, Java has in-built support for multithreading. Simply put, multithreading is a specialized form of multitasking where specific processes within a program are divided into individual threads. Each of these threads are run in parallel making optimal use of the available resources.

Lifecycle Of A Thread

A thread goes through various states in its life cycle. There are five different states a thread can be in at any given point of time; new, runnable, running, non runnable(waiting) and terminated states. The transition among these different states is controlled by the Java virtual machine.

An illustration of the thread lifecycle

New State

When a new thread has been created by instantiating an object from the Thread class, it is in the new state. It remains in this state till the start() method of the Thread class is invoked. At this state, calling methods other than the start() or stop() will cause an exception to be throwed.

Runnable State

When the Thread class’s start() method is invoked on the newly created thread, it transitions from new state to the runnable state but has not started running yet. The JVM thread scheduler is responsible for deciding which thread should start out of the thread pool.

Running State

When the scheduler selects a thread and starts running it, then the thread’s state is changed from runnable to running.

Non Runnable State

A thread will transition to non runnable state when one of the following four events occurs;

  • When sleep() method is invoked and the thread sleeps for a specific time period.
  • When suspend() method is invoked.
  • When the wait() method is invoked and the thread waits for a notification of a free resource, for a completion of another thread or waits to acquire a lock on an object.
  • When the thread is blocking on I/O and is waiting for the completion of it.

When a thread is in the non runnable state, it cannot continue its operations until it’s moved to the runnable state. Moreover threads in this state do not consume any CPU cycles.

Below mentions the scenarios that allows threads on non runnable state to be transitioned to the runnable state;

  • If a thread was on sleep, then once the specific time period expires or when the interrupt() method is called.
  • If a thread was suspended, then when the resume() method is called.
  • If a thread was waiting for a free resource or a completion of another thread, then when the other thread signals the waiting thread using notify() or notifyAll() methods.
  • If thread was blocked on I/O, then once the I/O completes.

Terminated State

A thread transitions to this state once the run method has finished its execution, when the stop() method is invoked or when a failure like an unhandled exception has occurred.

Thread Priority

Every thread in Java has a priority that helps the JVM thread scheduler to determine the order of execution. Java priority ranges from MIN_PRIORITY(minimum) of 1 to MAX_PRIORITY(maximum) of 10. The threads that have higher priority are given precedence over the lower priority threads by the thread scheduler but it does not guarantee that it will execute threads based on the priority because the scheduling mechanisms differ from JVM to JVM.

Does Threads Have A Default Priority?

The default priority of the main thread in an application is set to 5 by the JVM. All other threads that get created inherits the priority value of its parent thread. For example, after the priority of the main thread is explicitly changed to 8, and if a new thread is created in the main thread, the newly created thread will have a priority of 8.

How To Create Threads?

In Java, threads can be created by using two mechanisms;

  • Extending the Thread Class
  • Implementing the Runnable Interface

Creating Threads By Extending The Thread Class

Creation of a thread by extending the Thread class can be done in three steps. Initially create a class that extends the java.lang.Thread class. As the second step, override the run() method of the Thread class and write the code/logic that needs to be executed inside the run() method. As the final step, create an object of the new class and call the start() method using that object to start the thread execution. The start() method will invoke the run() method on the thread object.

Creating Threads By Implementing The Runnable Interface

Creation of a thread by implementing the runnable interface is also somewhat similar to the thread creation process using the Thread class. In this scenario, initially create a new class which implements the java.lang.Runnable interface. Once done, override the run() method and write the code/logic inside it. As the next step, create an object of the newly created class and use that object as a constructor argument to create an object from the Thread class. Finally, call the start() method using the created Thread class object.

Difference Between Thread Class And Runnable Interface Implementations?

  • Java does not support multiple inheritance, therefore cannot extend the Thread class in classes that already inherit another class. But if the runnable interface is implemented, then the class is still capable of extending another class.
  • Creating threads by extending the Thread class gives the advantage to use its inbuilt methods like yield(), sleep() etc. which are not available if the runnable interface is implemented.

Thread Class Methods

This section discusses about some of the frequently used static and non-static methods available in the thread class and how those methods work.

Set/Get Priority Methods

The getPriority() and setPriority() methods of the thread class are used to check and update the priorities of the threads which range from a minimum of one to a maximum of ten.

Join Method

When a thread invokes this method on a second thread, it notifies that the invoking thread will be waiting for a specified time(time as a method parameter is optional) till the execution of the second thread completes for reasons like getting the output etc. When this method is invoked, the invoking thread will be transitioned to the waiting state. Once transitioned, it cannot directly go back to the running state but rather to the runnable state under one of the following scenarios;

  • If second thread finishes before given time.
  • If second thread does not finish on given time.
  • If invoking thread gets interrupted.

Yield Method

Once the yield() method is invoked by a thread, it hints the thread scheduler that it’s willing to give a chance to other threads. It will also cause the thread to be transitioned from running state to runnable state thus relinquishing the CPU. The thread scheduler checks for threads in runnable state that have the same priority as the thread that got transitioned. If there are no any waiting threads at all or there are no any threads with the same priority then the yield() method will be ignored and the earlier thread will continue executing and if there are threads that match the priority, then the thread scheduler will execute one out of them.

Sleep Method

The sleep method causes the currently running thread to pause its execution for a given time period. The thread will be transitioned back to runnable state if the specified duration of sleep is over or when the thread is interrupted. The sleep method has two constructors; first one to specify the sleep time in milliseconds and the second one to specify the sleep time in milliseconds and nanoseconds.

Interrupt Method

This method is used to interrupt the thread executions of threads that are in the non runnable state(sleep, wait ,etc.) by throwing InterruptedException. If the interrupt() method is called on threads that are not in non-runnable state, those threads will not get interrupted but the interrupt flag will be set to true where if those threads go to non-runnable state at a later point, then those will be interrupted.

Conclusion

As discussed within the article, threading is a basic but one of the most important features in the Java programming language. This article was able to provide an informative overview about threads. You can use the knowledge gained from this to dive in and explore more advanced concepts and topics about threading.