Real‑world examples of examples of class not found exception examples

Real examples of ClassNotFoundException in everyday Java

Before definitions and theory, let’s walk through some real examples of examples of class not found exception examples that show up in ordinary Java development. These are the failures you see in stack traces, CI logs, and late‑night Slack messages.

One classic example of ClassNotFoundException looks like this:

Exception in thread "main" java.lang.ClassNotFoundException: com.mysql.cj.jdbc.Driver
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520)
    at java.base/java.lang.Class.forName0(Native Method)
    at java.base/java.lang.Class.forName(Class.java:375)
    at com.example.App.main(App.java:18)

You wrote code that calls Class.forName("com.mysql.cj.jdbc.Driver"), it works on your machine, but fails on a coworker’s laptop or in production. That’s a textbook example of a missing JDBC driver JAR on the runtime classpath.

Another example of this runtime error appears when you move from a monolith to microservices and forget to include a library in a smaller service. A call to a class that used to be on the giant monolith classpath now explodes with ClassNotFoundException. These examples include logging frameworks, JSON mappers, and internal SDKs that were quietly bundled before.

Examples of examples of class not found exception examples in build tools

Modern build tools create some of the best examples of ClassNotFoundException, because they hide a lot of classpath complexity behind plugins and configuration.

In Maven, a common example of this exception appears when you mark a dependency as provided, assuming the application server will supply it, but you later run the code as a standalone JAR. For instance, you depend on javax.servlet-api as provided and then run:

java -jar target/myapp.jar

At runtime, you suddenly get:

java.lang.ClassNotFoundException: javax.servlet.http.HttpServlet

The class exists in your IDE during compilation, but not in the runtime environment. This is one of the clearest examples of examples of class not found exception examples caused by mismatched compile‑time and runtime dependencies.

Gradle provides another example of this pattern. Suppose you declare a library only under testImplementation but later move some test utility code into your main source set. Your tests pass locally, but when you package and run the app, the dependency is missing and you hit ClassNotFoundException for that helper class. Here, the build configuration silently created an environment where the class was visible in tests but absent in production.

Shadow/fat JAR plugins add their own twist. A typical example of ClassNotFoundException shows up when you use a shading plugin that accidentally excludes a package. The build completes, the JAR runs, and then some reflection-based framework tries to load a class that never made it into the shaded artifact. This is one of the more subtle real examples, because the code compiles, unit tests might pass, but the final packaged artifact is missing pieces.

Android-focused examples of class not found exception examples

Android developers see some of the most confusing examples of examples of class not found exception examples, thanks to the way the Android runtime and build tools work.

A very common Android example of this error involves ProGuard or R8. Imagine you have a JSON serialization library that uses reflection to load model classes. During minification, R8 decides a class looks unused and strips it out. On a debug build (without shrinking), everything works. On a release build, your app suddenly crashes with something like:

java.lang.ClassNotFoundException: Didn't find class "com.example.app.models.UserProfile" on path: DexPathList(...)

The class was present in source, present in debug builds, but removed from the optimized release APK. These examples include networking libraries, dependency injection frameworks, and any plugin that relies on reflection.

Another Android example: dynamic feature modules. Suppose you move some screens into a dynamic feature module, but your base module still tries to reference a class directly instead of going through the dynamic loading API. On devices where the module hasn’t been downloaded yet, the system throws ClassNotFoundException when trying to start that Activity or Fragment. This is a modern 2024‑2025 pattern as apps get more modular and on‑demand.

You also see examples of this exception when using third‑party SDKs (analytics, ads, payments). If your Gradle configuration excludes transitive dependencies to keep the APK smaller, you might accidentally drop a required support library. The app compiles, but when the SDK initializes and tries to load its helper classes, you get a runtime ClassNotFoundException referencing a class from the excluded dependency.

Framework and library integration: best examples from Spring, Hibernate, and friends

Enterprise frameworks produce some of the best examples of ClassNotFoundException because they rely heavily on reflection and classpath scanning.

In Spring Boot, a typical example is a missing JDBC driver or template engine. Your application.properties references a database URL, Spring Boot auto‑configures a DataSource, and behind the scenes it attempts to load the driver class. If the driver JAR is not on the classpath of your Docker image, you see an exception like:

Caused by: java.lang.ClassNotFoundException: org.postgresql.Driver

Locally, your IDE’s classpath includes the driver. In the container, your trimmed‑down image omits it. These real examples highlight the gap between development and deployment environments.

Hibernate gives another example of examples of class not found exception examples. You might have an entity with a custom user type defined by class name in XML or annotations. If that class was moved to a different package or removed from the module, Hibernate’s startup process fails when it tries to load the type by name.

Logging frameworks contribute their own examples. If you use SLF4J with Logback in development but deploy to an environment that only has Log4j 2 bindings, you can end up with missing implementation classes. Sometimes this manifests as a NoClassDefFoundError, sometimes as ClassNotFoundException, depending on when the failure occurs. Either way, the pattern is similar: configuration or packaging differences between environments lead to missing classes.

Modern JVM trends (2024–2025): modules, containers, and cloud

Recent JVM trends have created new examples of examples of class not found exception examples that didn’t exist a decade ago.

With the Java Platform Module System (JPMS), you can have a class physically present on the module path but not readable by a given module. In some situations, reflective access fails with errors that look very similar to classic class loading problems. Misconfigured module-info.java files can produce examples where a library works perfectly in a classpath‑based app but fails with ClassNotFoundException or related errors when you move to a fully modular setup.

Containers and serverless platforms add more real examples. A common scenario in 2024 is trimming Docker images for faster cold starts. Teams aggressively remove “unused” JARs to shrink image size. Then, at runtime, an optional feature is triggered—say, a particular authentication provider—and the app tries to load a class from a dependency that was removed. The service blows up with ClassNotFoundException only when that code path is executed in production traffic.

Multi‑stage Docker builds also create subtle examples. The build stage may include dev‑only dependencies, while the final runtime stage copies only part of the build output. If you forget to include all needed JARs, you end up with a container that starts, but fails as soon as it reaches code relying on the missing library.

In the cloud, managed platforms sometimes provide their own class loaders (application servers, function runtimes, or plugin systems). If you assume the platform will provide certain classes and it doesn’t—or if versions differ—you get yet another example of ClassNotFoundException. These examples include outdated JDBC drivers on managed database platforms, older servlet APIs on legacy app servers, or missing observability agents.

Diagnosing these examples: patterns to watch for

Looking across all these real examples of examples of class not found exception examples, the same patterns keep showing up.

You can usually categorize each example of ClassNotFoundException into one of a few buckets:

• The class never made it into the runtime environment (missing JAR, wrong Gradle/Maven scope, excluded transitive dependency).
• The class exists, but the class loader cannot see it (modular boundaries, container class loaders, OSGi or plugin isolation).
• The class was removed or renamed (refactoring, ProGuard/R8 shrinking, aggressive image trimming).
• The class name is wrong (typo, outdated configuration, mismatched package name after refactor).

When you hit one of these examples in real life, the debugging playbook is fairly consistent:

• Read the full stack trace, not just the top line. The context (reflection calls, framework startup, JDBC initialization) usually hints at which dependency is missing.
• Confirm the class is present in the built artifact. Open the JAR or APK as a zip file and look for the .class file in the expected package.
• Compare environments. Many of the best examples of this exception only appear in production or CI because the classpath differs from local development.
• Check build configuration for provided, runtimeOnly, or testImplementation scopes that might hide a missing dependency.

For a deeper background on Java class loading and error types, the official Java documentation on exceptions and the JVM specification (via Oracle’s Java docs) remain the most authoritative references.

Preventing future examples of class not found exception examples

The most valuable part of studying these examples of examples of class not found exception examples is learning how to avoid creating new ones.

Teams that rarely see this error in 2024 tend to follow a few habits:

• They use reproducible builds and containerized dev environments so the classpath in development matches production more closely.
• They add integration tests that run the actual packaged artifact (JAR, WAR, or APK) rather than only testing in the IDE.
• They monitor logs in staging and production for early signs of ClassNotFoundException and related errors, treating them as deployment problems, not just coding mistakes.
• They document reflection‑based configuration (like fully qualified class names in YAML, XML, or annotations) so refactors don’t silently break runtime loading.

If you want to go deeper into software reliability patterns and error handling, universities such as MIT and Stanford publish open courseware on software engineering and systems design that often touches on these runtime failure modes, even if they don’t mention every specific exception by name.

Studying real examples of these failures—and keeping a team “post‑mortem log” of your own best examples—pays off. The next time your logs show a ClassNotFoundException, you’ll have a mental catalog of examples to compare against, and you’ll spend minutes, not hours, tracking down the missing class.

FAQ: common questions and examples

Q: Can you give a simple example of ClassNotFoundException for beginners?
A straightforward example of this error is a small Java app that uses MySQL. The code calls Class.forName("com.mysql.cj.jdbc.Driver"), but the MySQL Connector/J JAR is not on the runtime classpath. The program compiles, but when you run it, you get ClassNotFoundException for the driver class.

Q: Are there examples of this exception that only appear in production?
Yes. Many examples include missing dependencies in Docker images, misconfigured provided scopes in Maven, or ProGuard/R8 stripping classes in release builds. These scenarios often work fine in a developer’s IDE while failing only in CI or production.

Q: How do I distinguish an example of ClassNotFoundException from NoClassDefFoundError?
Both relate to missing classes, but ClassNotFoundException is a checked exception that usually appears when code explicitly tries to load a class by name (for example, Class.forName). NoClassDefFoundError is an error thrown by the JVM when a class that was present at compile time can’t be found at runtime. The debugging techniques overlap, and real‑world examples often involve the same underlying classpath issues.

Q: What are the best examples of tools that help catch these errors early?
Good examples include integration test setups that run the final JAR or APK, dependency analysis plugins in Maven or Gradle, and CI pipelines that spin up containers matching production. While not Java‑specific, general software quality practices promoted by universities like Carnegie Mellon’s SEI help teams design processes that reduce these runtime surprises.

Q: Do modern trends like microservices and serverless create more examples of ClassNotFoundException?
They often do. Splitting a monolith into many smaller services multiplies the number of classpaths you manage. Each service can have its own missing‑dependency example, and serverless platforms sometimes provide only a subset of libraries you’re used to having. That’s why keeping track of these examples—and learning from them—is so important for JVM teams in 2024 and beyond.