https://airbrake.io/blog/java/exceptionininitializererror



Moving along through our in-depth Java Exception Handling series, today we’ll dive into the ExceptionInInitializerError, which is thrown when an error occurs within the static initializer of a class or object. Since an ExceptionInInitializerError isn’t ever the cause of a thrown error, catching such an exception provides an underlying causal exception that indicates what the actual source of the issue was.

Throughout this article we’ll examine the ExceptionInInitializerError by looking at where it resides in the overall Java Exception Hierarchy. We’ll then explore a simple and fully-functional code sample that illustrates how a static initializer works, and what might lead to a thrown ExceptionInInitializerError in such an initializer, so let’s get to it!

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.

When Should You Use It?

Since the appearance of an ExceptionInInitializerError indicates a problem within a static initializer, we should briefly examine what a static initializer is and how they’re typically used. Put simply, a static initializer block is a normal code block that is executed when the class is loaded.

A static initializer is created by enclosing code anywhere inside a class definition surrounded by braces ({ & }) and also preceded by the static keyword. For example:

As you can see in the code above, there can be as many static initializer blocks as desired within a class definition. Each will be executed in a top-down manner when the class is first loaded.

Static initializers are not to be confused with instance initializers (or instance members), which are executed when the class is instantiated (i.e. a new object of that class type is created). Instance initializers are also executed just before the constructor method.

An instance initializer is written inside a class definition using two braces ({ & }), but without a preceding static keyword:

Just as with static versions, multiple instance initializers can be defined and will be executed from top to bottom.

The purpose of a static initializer is typically to execute any initialization code that must occur before a class can be used. Critically, static initializers are only executed once ever, the first time that class is loaded. Thus, they are great for classes that are frequently used throughout the application, but only need some basic configuration code to be executed once before multiple instances are used elsewhere, such as a database connection class.

To test a static initializer in “real” code we’ve added a simple snippet and field to our trusty Book class:

A static initializer block has been added near to the top of the class definition, and merely attempts to ensure the publicationType static field is an upper case value:

With this static initializer in place let’s try creating a new Book instance:

Executing this main(String[] args) method produces the following output:

Sure enough, this threw an ExceptionInInitializerError at us. Just as importantly, since ExceptionInInitializerErrors aren’t ever going to cause a problem themselves, catching such an exception always contains an actual causal exception, which is the error that was thrown within a static initializer that led to the caught ExceptionInInitializerError. In this case, we see the cause was a NullPointerException. With a bit of digging and consideration, we can see that the problem is within the static initializer code inside the Book class. It attempts to get the static publicationType value, but we don’t explicitly set an initial value for this field, so calling this references a null value, hence the NullPointerException. The solution is to either set an initial value for publicationType, or to include a try-catchblock within the static initializer. In this case, a try-catch block handles the error, but doesn’t resolve the root cause, so it’s probably better to specify a default value instead:

Now, rerunning the same main(String[] args) method instantiates our Book just fine and outputs the results:

A few things worth noting. First, since our code to force the publicationType value to be upper case occurs within a static initializer, this occurs before our Book instance is initialized, as well as before the Book(String title, String author, Integer pageCount, Date publishedAt, String publicationType) constructor method is executed. Thus, even though we successfully change the publicationType from the default of Book to novel, it won’t be upper case’d unless we do so inside an instance or member initializer.

Additionally, since publicationType is a static field, this means it is unique to the entire Book class. If we were to create a second Book instance with a publicationTypespecified in the constructor of poem, the publicationType of our Game of Thrones Bookinstance would also have its publicationType changed to poem:

This code produces the following output:

As we can see, the publicationType of both instances is now the new value of poem, so just be ware that static members and initializers are global to all instances of a particular class.

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