In Java (and other programming languages), threads share the same address space, and, therefore, have access to all variables, objects, methods, and global scope of the application or program.
When more than one thread attempts to access the same piece of data at the same point of time, (ie, concurrently), there can be several problems, such as thread synchronization issues, thread interference issues, and so forth.
Fortunately, there are a few techniques that Java developers can use to mitigate these issues. In this programming tutorial, we will discuss thread interference in Java with relevant code examples to help illustrate the concept.
If you need a refresher on threading in Java, we suggest reading our tutorial: Introduction to Using Threads in Java.
What is Thread Interference?
A thread is a light-weight process that can be independently scheduled by the operating system. It is also defined as the independent path of execution within a program that allows for concurrent execution of code, meaning that multiple threads can execute at the same time. It should be noted that a processor can execute one and only one thread at any point in time.
When a programmer is building applications in Java that leverage threads, they might encounter thread interference problems. Thread interference occurs when threads are competing for the same resources, which can lead to unexpected behavior and unexpected results.
Two or more independent threads performing different actions on the same data may interleave if they share the same memory. This leads to inconsistent data in memory. Threads interleave when they perform operations in several steps, which can overlap. This phenomenon is also known as thread interference.
Java offers several synchronization mechanisms to avoid thread interference errors to ensure that only one thread can access an object at a time. This can be done through the use of locks, monitors, and other synchronization techniques. The most basic synchronization mechanism is the synchronized keyword, which can help developers block access to a method in its entirety.
Why Does Thread Interference Occur in Java?
One common cause of thread interference is when two threads simultaneously try to iterate over a collection of objects. This can happen even if both threads use the same iterator; each thread will still have its copy of the iterator, but they will be accessing the same underlying objects. If one thread modifies an object while another thread is iterating over it, the results will be unpredictable.
Another common cause of interference is when one thread calls a method on an object that another thread is using. For example, imagine two threads operating on a linked list: one thread is calling the add() method to add a new node to the list, while the other is calling the remove() method to remove a node from the list. If both threads operate on the same list, they could end up interfering with each other’s work and causing unexpected results.
Thread Interference vs Memory Consistency Errors
Java thread interference happens when multiple threads are trying to access the same object and they interfere with each other. This can happen when one thread is reading data while another thread is writing data, or when one thread is waiting for another thread to finish an operation.
Memory Consistency Errors happen when one thread is reading data that has been modified by another thread, but the first thread does not see the changes because it has not finished its operation yet.
Reading: Multithreading in Java
How to Avoid Thread Interference Errors in java
Developers can adhere to the following best practices to avoid thread interference errors in Java:
- Shared data should be synchronized. In doing so, only one thread will be able to access a piece of shared data at any given point of time.
- Declare variables as volatile if multiple threads will access them. It prevents the compiler from caching the value, which could cause issues if another thread changes it.
- If possible, use immutable objects. Since immutable objects cannot be modified, thread interference errors will not occur.
- Use multiple threads to process long-running tasks. Since each thread will only access a small portion of the data, the chances of thread interference errors will be reduced.
- When working with shared objects, use locks or other synchronization mechanisms. By doing so, you can prevent multiple threads from accessing an object concurrently.
How to Avoid Memory Consistency Errors in Java
Programmers can avoid memory consistency errors in Java in several ways:
- Use synchronization when accessing shared data. By doing this, you will be able to ensure that only one thread can access the data at a time, which will help prevent memory consistency errors from occurring.
- Shared data should be stored in atomic variables. With atomic variables, you are able to access and update variables in a thread-safe manner without the need for synchronization to be used.
- Use concurrent data structures. Many of these data structures are designed in such a way that they can be used concurrently by multiple threads and often feature built-in mechanisms for preventing memory consistency errors from occurring.
- Use a message passing mechanism. The threads will have the ability to communicate without directly accessing shared data and will be able to keep memory consistency errors at bay.
- Ensure all threads use the same memory ordering. Different threads can have different memory ordering conventions, which can lead to memory consistency errors if they are not handled properly.
Reading: The Best Tools for Remote Developers
Thread Interference in Java Code Example
Consider the following Java classes:
class MySharedClass { static int counter = 5; void increaseByOne() { for (int j = 0; j < 10; j++) { counter++; System.out.println("The value of the variable counter after increment is: " + counter); } } void decreaseByOne() { for (int j = 0; j < 10; j++) { counter--; System.out.println("The value of the variable counter after decrement is: " + counter); } } }
The class named MySharedClass contained a static or shared variable named counter that is set to an initial value. There are two methods in this class: increaseByOne and decreaseByOne. While the former increases the value of the shared variable by one, the latter decreases it by one.
The following Java code example shows how you can use two threads – one to invoke the increaseByOne method and the other to invoke the decreaseByOne method:
class ThreadInterferenceDemo { public static void main(String[] args) { final MySharedClass obj = new MySharedClass(); Thread t1 = new Thread() { @Override public void run() { obj.increaseByOne(); } }; Thread t2 = new Thread() { @Override public void run() { obj.decreaseByOne(); } }; t1.start(); t2.start(); } }
The image below shows the output upon execution of the program:
Note that the actual output would be different and unpredictable due to thread interference.
Final Thoughts on Java Thread Interference
Thread interference is one of the most challenging coding problems to solve in Java applications, but with the right approach and techniques, developers can handle it easily. With a proper understanding of Java thread interference, you can create efficient and reliable applications that leverage multithreading while avoiding such errors.
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