Tackling the Java Beast: Unraveling the LayerInstantiationException
As Java developers, we might have come across various exceptions while designing and coding our applications. One such exception is the LayerInstantiationException, which can be perplexing for many developers. In this in-depth guide, we will break down the LayerInstantiationException in Java, understand why it happens, and see how we can prevent it in our applications.
Trip Down Memory Lane: Java 9 and Modules
To fully understand LayerInstantiationException, we have to dabble into Java 9’s new feature - the Java Platform Module System(JPMS). This major upgrade has reshaped how we structure and encapsulate our applications in Java.
A Module is a named, self-describing collection of code and data. With the introduction of modules, your code can now be part of large, complex applications without exposing your code to all parts of those applications.
You can find a detailed overview of Java 9 modules here.
Layering in Java Modules
Modules are grouped into layers for managing dependencies and class loaders. An interesting capability in the JPMS is defining ModuleLayers. These are hierarchical arrangements of modules, as visualized in a tree of layers. By creating layers, we can create configurations where multiple versions of a module can co-exist within a single JVM instance, each in their separate layer.
Into the Depths: LayerInstantiationException
LayerInstantiationException is part of java.lang.module package and is a checked exception. The JavaDoc states that it’s “Thrown when creating a ModuleLayer and a ClassLoader cannot be created”.
This exception usually arises when the class loader we are referring to does not exist or is improperly defined. Let’s understand this with a simplistic code example.
1
2
3
4
5
6
7
8
9
10
try {
var cf = ModuleLayer.boot()
.configuration()
.resolveRequires(ModuleFinder.of(), List.of(ModuleLayer.boot()));
var parentClassLoader = ClassLoader.getPlatformClassLoader();
var layer = ModuleLayer.defineModulesWithOneLoader(cf, List.of(ModuleLayer.boot()), parentClassLoader);
} catch (LayerInstantiationException e) {
e.printStackTrace();
}
In the code snippet, if ClassLoader.getPlatformClassLoader(); returns invalid or undefined class loader, then this would result in an LayerInstantiationException when defineModulesWithOneLoader is invoked.
Causes of LayerInstantiationException
LayerInstantiationException can manifest because of several reasons. Some of the most frequent rationales for this exception include:
One or more of the modules in the configuration don’t have a corresponding module defined in the parent layer or layers.
An attempt to create a configuration specifies a module that cannot be located.
Here is an example:
1
2
3
4
ModuleLayer parent = ModuleLayer.boot();
Configuration cf = new Configuration(Optional.of(parent.configuration()), ModuleFinder.of(), Set.of("nonexistent.module"));
ModuleLayer layer = parent.defineModulesWithOneLoader(cf, ClassLoader.getSystemClassLoader());
Here, since nonexistent.module can’t be located, it leads to a LayerInstantiationException.
Dodge it: Tips to Avoid LayerInstantiationException
The best solution to avoid LayerInstantiationException is to ensure the class loader referred to while defining the module layer is correctly defined. Here, the integrity of the class loader is vital.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
ClassLoader parentClassLoader;
try {
parentClassLoader = new URLClassLoader(new URL[]{}, null);
} catch (MalformedURLException e) {
throw new RuntimeException("This should not have happened", e);
}
try {
var cf = ModuleLayer.boot()
.configuration()
.resolveRequires(ModuleFinder.of(), List.of(ModuleLayer.boot()));
var layer = ModuleLayer.defineModulesWithOneLoader(cf, List.of(ModuleLayer.boot()), parentClassLoader);
} catch (LayerInstantiationException e) {
e.printStackTrace();
}
In this snippet, we have defined a class loader using URLClassLoader, ensuring it’s correctly defined before it’s used.
Conclusion
Understanding and resolving exceptions throws us into the complexities and intricacies of a language. LayerInstantiationException might seem overwhelming at first, but once you grasp the concepts of ModuleLayers in Java 9, it will become easier to tackle.
Remember, coding is all about learning and evolving. Let your code break, make mistakes and learn from them, only then you will truly master a language!