What is the ClassLoader in Java?

A ClassLoader is responsible for loading Java class files (.class bytecode) into the JVM at runtime. The JVM uses a delegation hierarchy: Bootstrap ClassLoader (loads core Java classes from rt.jar/modules), Extension/Platform ClassLoader (loads extension libraries), and Application ClassLoader (loads classes from the classpath). When a class is requested, the child loader asks the parent first (delegation model) — this prevents malicious classes from replacing core Java classes. You can create custom ClassLoaders for dynamic class loading, hot deployment (reloading classes without restarting), or loading from non-standard sources (databases, networks). This is how application servers like Tomcat isolate different web applications.