A Closer Look at the Java VirtualMachineError

Java.lang.VirtualMachineError is thrown when the Java virtual machine encounters an internal error or resource limitation, which prevents it from functioning. It’s a self-defensive mechanism employed by JVM to prevent the entire application from crashing. In this article, let's discuss different types of VirtualMachineError, their characteristics, reasons why they get triggered, and potential solutions to fix them.

Types of VirtualMachineError

There are four different types of VirtualMachineError:

a. OutOfMemoryError

b. StackOverflowError

c. InternalError

d. UnknownError

Let’s review these types in detail in this section:

Fig: Java Throwable class hierarchy

OutOfMemoryError

Just like the OMG (Oh My God) acronym, OOM ( OutOfMemoryError) is quite popular among the DevOps community. Most DevOps engineers think that there is one OutOfMemoryError. But there are eight different kinds of OutOfMemoryError:

1. java.lang.OutOfMemoryError: Java heap space
2. java.lang.OutOfMemoryError: GC Overhead limit exceeded
3. java.lang.OutOfMemoryError: Requested array size exceeds VM limit
4. java.lang.OutOfMemoryError: Permgen space
5. java.lang.OutOfMemoryError: Metaspace
6. java.lang.OutOfMemoryError: Unable to create new native thread
7. java.lang.OutOfMemoryError: Kill process or sacrifice child
8. java.lang.OutOfMemoryError: reason stack_trace_with_native_method

Each flavor is triggered for different reasons. Similarly, solutions are also different for each type of OutOfMemoryError. Here is a beautiful, one-page document that summarizes all the different types of OutOfMemoryError, their causes, and solutions.

In general, the OutOfMemoryError can be diagnosed and fixed by analyzing Garbage Collection logs and Heap Dumps. Since analyzing Garbage Collection logs manually can be tedious, you may consider using free tools like GCeasy, HP Jmeter, and IBM GC analyzer. Similarly to analyze heap dumps, you may consider using free tools like HeapHero, Eclipse MAT.

StackOverflowError

Thread stack is storing information about the methods it’s executing, primitive datatype values, local variables, object pointers, and return values. All of them consume memory. If thread stack sizes grow beyond the allocated memory limit, then the java.lang.StackOverflowError is thrown. This problem typically happens when a thread recursively invokes the same function again and again as a result of a bug in the executing program. More details on how to debug StackOverflowError and all possible solutions to fix it can be found in this article.

InternalError

java.lang.InternalError is thrown by the JVM when there is a:

1. Fault in the software implementing the virtual machine
2. Fault in the underlying host system software
3. Fault in the hardware.

But rarely you will encounter the InternalError. To understand what specific scenarios may cause the InternalError, you may search for ‘InternalError’ string in Oracle’s Java Bug database. At the time of writing this article (Dec’ 20, 2018), there are only 200 defects reported for this error in Oracle Java bug database. Most of them are fixed.

UnknownError

java.lang.UnknownError is thrown when an exception or error has occurred, but the Java virtual machine is unable to report the actual exception or error. Seldom you will see the UnknownError. In fact, when searching for ‘UnknownError’ in the Oracle Java Bug database, at the time of writing this article (Dec’ 20, 2018), there are only two defects found and reported.

Characteristics

 VirtualMachineError has a couple of primary characteristics:

1. Unchecked Exception
2. Synchronous and asynchronous delivery

Let’s discuss these two characteristics in this section.

Unchecked Exception

There are two types of Exceptions:

• Checked exceptions
• Unchecked exceptions

Exceptions that are checked at compile time are called Checked Exceptions. If some methods in your code throw a checked exception, then the method must either handle the exception or it must specify the exception using throws keyword. Examples of checked exceptions are IOException, SQLException , DataAccessException, ClassNotFoundException , etc.

Unchecked exceptions do not have this requirement. They don’t have to be caught or declared thrown. All types of VirtualMachineError are unchecked exceptions.

Synchronous and Asynchronous Delivery

Exceptions can be thrown in two modes:

• Synchronous
• Asynchronous

Synchronous exceptions happen at a specific program statement, no matter how many numbers of times the program is executed in a similar environment. Examples of synchronous exceptions are  NullPointerException, ArrayIndexOutOfBoundException, etc.

Asynchronous exceptions can happen at any point in time and it can happen in any part of the program statement. There will be no consistency where it can be thrown. All  VirtualMachineErrors are thrown asynchronously, but sometimes, they can also be thrown synchronously. StackOverflowError may be thrown synchronously by method invocation as well as asynchronously due to native method execution or Java Virtual Machine resource limitations. Similarly, OutOfMemoryError may be thrown synchronously during object creation, array creation, class initialization, and boxing conversion, as well as asynchronously.