hardConceptScenarioMidSeniorStaff
Where does a class come from at runtime, and what is the difference between ClassNotFoundException and NoClassDefFoundError?
Loaders delegate to their parent before searching themselves, so the JDK always wins a name clash. ClassNotFoundException means a loader was asked for bytes it could not find; NoClassDefFoundError means the JVM needed a class that was there at compile time, or one whose static initialiser had already failed.
6 min readUpdated 2026-08-15
Answer
Short answer
ClassLoader delegation means a loader asks its parent first, so class identity depends on both the fully qualified class name and the defining loader. ClassNotFoundException means loading failed when requested, while NoClassDefFoundError usually means a class that was present at compile time cannot be resolved or initialized at runtime.
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