Java Exception Handling

Java Exception Handling – IllegalMonitorStateException

Moving along through our in-depth Java Exception Handling series, today we’ll get into the IllegalMonitorStateException. The IllegalMonitorStateException is thrown when a thread has been instructed to wait for an object’s monitor that the specified thread does not have ownership of.

Throughout this article we’ll explore the IllegalMonitorStateException in greater detail, starting with where it resides in the overallJava Exception Hierarchy. We’ll also look at a functional code sample that illustrates how multithreading and concurrent waiting might be implemented, and how IllegalMonitorStateExceptions can appear if such an implementation has some small flaws in it. Let’s get crackin’!

The Technical Rundown

All Java errors implement the java.lang.Throwable interface, or are extended from another inherited class therein. The full exception hierarchy of this error is:

Full Code Sample

Below is the full code sample we’ll be using in this article. It can be copied and pasted if you’d like to play with the code yourself and see how everything works.

This code sample also uses the class, the source of which can be found on GitHub.

When Should You Use It?

As described by the official documentation, an IllegalMonitorStateException can occur when a thread attempts to wait on an object’s monitor, or to notify other threads waiting for said object’s monitor, when that thread does not own the monitor in question. Put another way, if the Object.wait() method is called on an object which was not created by the current thread, an IllegalMonitorStateException will be thrown. A thread can become the owner of an object’s monitor by:

  • Executing a synchronized instance method of the object.
  • Executing a code block of a synchronized statement that synchronizes on the object.
  • Executing a synchronized static method of the object, if the object is a Class type.

This is easier to understand with some code examples, but before we get into that it’s worth briefly noting that the use of Object.wait() and notify() methods is generally not the go-to technique for performing multithreaded or concurrent programming in modern Java. If you’re interested in how most Java developments handle concurrency, take a look at the java.util.concurrent package.

The goal with our code sample is simple: To create two threads and assign each their own Runnable object, which is an interface that implements the run() method. A Runnable instance can be passed to a new Thread instance, and the method will be automatically executed by the thread once it’s started.

Thus, we begin with our Runner class, which implements the Runnable interface:

Aside from a few getters and setters, the core logic is found in the run() method. It starts by checking if the Thread thread property of the Runner exists via the getThread() method. If not, it explicitly calls the Main.main.wait() method, otherwise it calls the wait() method of the associated Thread object for this Runner instance.

The ThreadingManager class is just a helper class to make it easier to generate and keep track of Runners and Threads by storing them in private ArrayList<Runner> runners and ArrayList<Thread> threads properties. We’ll also use many of the helper methods, such as getRunner(String name), to retrieve specific instances of Runners and Threads for testing purposes later on:

Speaking of testing, let’s look at our Main class and the first test method, DualThreadingTest():

As you can see, our test begins with a new ThreadingManager instance, from which we create two new Runners named primary and secondary, respectively. We then create two new Threads with the same names, and retrieve the appropriate Runners from our manager instance to pass to those new Thread(...) constructor calls. As mentioned before, passing a Runnable instance (like a Runner object) to a Thread constructor ensures that the Runner's run() method is executed when the thread starts execution. Consequently, our last step is to start() both threads.

Executing the DualThreadingTest() code produces the following output, which includes throwing some IllegalMonitorStateExceptions:

Since we didn’t explicitly tell our Runner instances which specific Thread instance each was assigned, they both attempted to call the wait() method of the Main.main object, as seen in this snippet from the method:

At the most basic level, every object in Java has an intrinsic lock associated with it. Anytime a thread needs exclusive access to that object it must acquire ownership of the object’s intrinsic lock. Once exclusive access requirements are finished, it can release the lock. In some scenarios, like the one we created above, a thread may attempt an action on an object for which it doesn’t have an exclusive lock, which can cause an IllegalMonitorStateException in some situations.

One solution is to use the built-in synchronized statement, which specifies an object that will be providing the intrinsic lock to the code block within the statement. You can see an example of this in the else { ... } block of our method above. By using the synchronized (getThread()) statement, we give the code within the synchronized block access to the lock of the associated thread of this Runner instance (as acquired by getThread()). It’s worth noting that it’s usually best programming practice to use a local variable and retrieve the Thread instance via getThread() one time, then reuse it throughout the code. However, in this case, using a local variable within a synchronized statement can be tricky to control, since synchronization may occur out of order (slower or faster) than code outside of it that actually produced the local variable. Thus, it’s safer to make the explicit call every time in this case.

To test this behavior we have the DualThreadingWithOwnershipTest() method, which is similar to DualThreadingTest(), except we explicitly assign Thread instances to the Runner instances before start() is called for both Threads:

As a result, this invokes the synchronized(...) statement for each appropriate thread within the method, ensuring our concurrency works as expected and doesn’t throw any inels in the output:

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