Deadlock in Java

Executing multiple threads at once is known as "multithreading," and it's one of the most important concepts that Java supports. However, everything has its downside—especially if used incorrectly. For example, avoiding thread deadlock in Java requires a thorough understanding of multithreading and its best practices.

Even seasoned developers can run into deadlock if they don't keep a keen eye out for common mistakes. So, here's a quick tutorial to help you understand issues with dependency and concurrency in a Java program, along with some crucial advice for troubleshooting and avoiding them.

What is Multithreading?

To understand what a deadlock in Java is, you must first understand the concept of multithreading. Multithreading is a process that allows for multiple threads to be executed simultaneously. This is important for efficiency and speed in a Java application, but when handled incorrectly, issues can arise.

When multiple threads are being executed simultaneously, some threads may get stuck in a holding pattern.

In this case, thread 1 and thread 2 will not be executed because they are blocked, or rather, they are blocking each other. This blocking causes a condition known as deadlock, or object lock, in Java.

What is a Deadlock in Java?

There are many use cases for Java multithreading, but it can cause problems if you don't take an informed approach.

The most common issue that results from bad multithreading practices is that two Java threads (e.g., thread t1 and thread t2) try to use the same resources simultaneously, putting them into a Java deadlock situation.

When threads compete against each other for the same resource, they will continuously view the resource as "busy" and wait indefinitely, which leads to issues with your application's performance and functionality.

Unfortunately, deadlock can potentially occur in any situation where there are at least two threads and at least two resources.

Anytime that more than one thread tries to access a given resource at the same time, developers will have to find a workaround. Luckily, you can take steps to avoid the deadlock condition as much as possible.

Detecting Deadlock in Java

Since deadlock can occur if there are at least two threads and at least two resources, it's something developers need to familiarize themselves with.

When testing code, a deadlock in Java will read as "Locked resource" in the command-line interface.

For starters, you can detect a deadlock in Java by examining the application's thread dump. You can even use a command to generate the thread dump with some operating systems. You can view it in the command-line interface or download it in a text file for review and reference.

How to avoid Deadlock in Java?

Anytime you deal with multithreading, which is extremely valuable and often necessary, there is some chance that a deadlock can occur.

In other words, you can't avoid the risk of deadlock in Java altogether, but you can significantly minimize the risk through smart programming.

Here are some preventative tactics you should take the time to implement if you're using multithreading in Java.

Use only necessary locks

By far, the simplest way to avoid deadlock in Java is to use locks only when they are necessary. If you are accustomed to using locks in Java, it's important to understand the best alternatives.

For instance, you might want to switch to a data structure that does not use locks at all, which is considered ideal. A good example is using ConcurrentLinkedQueue over the synchronized ArrayList.

Utilize thread joins

Deadlock means two threads are in an indefinite waiting state, but you can avoid this if you use the Thread.join() method to set a maximum wait time.

This helps you avoid deadlock by simply limiting how long a thread will wait for a resource. It's more time-consuming to go through and set the wait time for each join, but clean coding will pay off in the long run.

Avoid nested locks at all costs

In general, avoiding unnecessary locks is important, but you should absolutely avoid nested locks at all costs. A nest lock allows you to give multiple threads access to a given resource, but it's a primary cause of deadlocks.

While an easy workaround for developers, nested locks can cause problems because they set threads up to compete with each other.

So, instead of using a nested lock, be mindful of whether a resource is already in use. If a lock is assigned to a thread, you should not assign it to another thread.

Timeout and prioritize locks

Another key way to avoid deadlock in Java is to limit how long a thread has to acquire a lock. If a lock cannot be acquired in the given timeframe, it will have a set wait time (i.e., timeout) before it tries again.

You should also prioritize locks by assigning a numeric value to each, with the lowest value taking the highest priority. This alone won't allow you to avoid deadlock, but it can help you continue running efficiently even when you set things like a timeout for your locks.

Tips and considerations

We know that avoiding struggles like deadlock is challenging but not impossible. With the right coding practices, you can be well on your way to a fully functional application that performs great even under intense demand.

Our biggest tip for avoiding deadlock is to make sure you fully understand the problem. If you've never dealt with it first-hand, consider yourself lucky, and also consider running some sample code that will show you what a deadlock example looks like.

From there, you can better understand how to avoid it using the methods shared above.

It's also important to stress why a deadlock is so bad. When it comes to a live application, deadlock can bring an application to its knees.

Deadlock is, indeed, as serious as it sounds, and it will lock up your application until you restart it or put in a workaround. It can quite literally break down business logic, which is why it needs to be avoided.

While it will take extra time to ensure your application is safe from deadlock, it's worth the effort. If you're not sure where to start, we can offer guidance.

Need help with your Java application?

Putting speed above code quality is never a good idea, but deadlock is tough to avoid.

Are you confused about synchronized blocks, printstacktrace, thread.sleep, public static void, or ways to override a Java deadlock?

Even seasoned developers can get hung up, and generalized Java tutorials often aren't enough. If you've tried a new thread, different order, the synchronized method, and so on, but you're still having problems, it might be time to call in a professional.

Adservio's code quality experts are standing by to help you improve the performance of your Java application by evaluating it from the ground up.

Interested in learning more about how we can help? Schedule a consultation today to speak with our team and explore our solutions for application performance and advanced design.