What major APIs were removed between Java 17 and Java 21?

Java 21 continues the cleanup of APIs that were deprecated in earlier releases. Understanding what was removed helps interviewers assess whether a candidate's Java knowledge is current.

// finalize() — use Cleaner instead (Java 9+)
// Old pattern:
class OldResource {
    @Override protected void finalize() { /* cleanup */ } // unreliable, deprecated
}

// Modern pattern:
class ManagedResource implements AutoCloseable {
    private static final Cleaner CLEANER = Cleaner.create();
    private final Cleaner.Cleanable cleanable;

    ManagedResource() {
        cleanable = CLEANER.register(this, new CleanupAction());
    }

    @Override public void close() { cleanable.clean(); }

    static class CleanupAction implements Runnable {
        @Override public void run() { /* release native resource */ }
    }
}

// SecurityManager — Java 17 deprecated, Java 21 warns, will be removed
// If your code calls System.setSecurityManager() → update now
// Migration: use OS-level security, module system, or third-party frameworks

The finalize() deprecation is particularly important: it was unreliable (not guaranteed to run, can be called on a different thread, delays GC), and caused memory leaks. The Cleaner API provides a reliable, predictable cleanup mechanism without the pitfalls of finalisation.

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