Java reflection performance issues3/23/2023 ![]() Application servers and OSGi containers use this feature of classloaders to load different applications or parts of applications in isolation. It is the very purpose of a classloader to load classes in isolation to each other. Unused members still increase the size of the class! If you have a class with 20 members, but depending on the use case you use only a subset, it makes sense to split the class. The solution is quite simply to split large classes into several smaller ones, especially if you know that not all of the constants are needed at the same time. Due to a coding error the application was loading not just one, but every language class during startup. Each class was quite large and memory-hungry. ![]() In one case a customer had one class per language to contain all language-specific literals. Although it is a good approach to keep all literals in static class constants, we should not keep them all in a single class. Quite often the root cause of large classes is too many static class constants. Too many large classes, and the heap is maxed out. Depending on the number of fields and class constants, the needed memory varies. Here we'll cover the most common classloader-related memory issues (plus one rare but educational one), and how to identify and solve them.Ī class is an object and consumes memory. Instead of a PermGen out-of-memory error, which at least tells us that the problem is class-related, we get a generic out-of-memory error. The other JVMs do not have a permanent generation, but that does not solve the problem, as classes can still fill up the heap. If this area is full, no more classes can be loaded and an out-of-memory error occurs in the PermGen. It represents a separate memory area, and its size must be configured separately. In the HotSpot JVM, classes are located in the permanent generation or PermGen (See the section Not All JVMs Are Created Equal). However, in Java classes, objects are managed on the heap, as well. When there are memory problems, one thinks primarily of normal objects. While Java classloader issues lead to the same runtime issues as the aforementioned dll-hell (multiple versions of the same class/method), they also lead to memory leaks and shortages that need to be addressed in any book about Java performance. However, modern enterprise Java applications often load thousands of classes, use different isolated classloaders, and generate classes on the fly. The second line just means that this annotation can be applied to methods only.Īnd the more important part is how the subscriber registry finds all its subscribing methods.Sometimes I think the Java classloader is to Java what dll-hell was to Microsoft Windows (Read here, and here if you want to know more about this sort of hell). The retention policy is there because some annotation do not mean anything after compilation (such as Override and SuppressWarnings), so it does not make sense to keep the annotation after compiling. The first line tells Java that this annotation must not be discarded during compile time so it will be available during runtime. ![]() ![]() ![]() Take for example the following class: public class StudentĪnd that is it! Important parts of the code to note are the first two lines before the declaration. The class object itself holds information such as its name and the package it resides in while an instance of a class holds the instanced values and methods as defined in the class. This immutable Class object is fundamentally different from instances of a class. Every unique Object is assigned an immutable Class object by the JVM. From the Java Class API, we see that Class is a subclass of Object. Create new instance of existing objects, invoke methods and change values of fields of existing objects.īefore getting started with reflections in Java, it is important to realize that a class is also an object.This is very useful if you are interested to implement test cases for private methods. Access private fields and methods of a class without having to modify the visibility modifier of the class itself.In particular, reflections in Java allows the inspection of classes, methods and fields during runtime, without having any knowledge of it during compile time. Reflection is the ability of a computer program to examine, inspect and modify its own behaviour at runtime. ![]()
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