Cisco

Use two stacks: one for pending repeat counts and one for the string built before each [. On ], pop both, append the current segment count times to the saved prefix, and continue. Time is proportional to the output length.

static String decode(String s) {
    Deque<Integer> counts = new ArrayDeque<>();
    Deque<StringBuilder> prefixes = new ArrayDeque<>();
    StringBuilder cur = new StringBuilder();
    int k = 0;
    for (char ch : s.toCharArray()) {
        if (Character.isDigit(ch)) k = k * 10 + (ch - '0');
        else if (ch == '[') { counts.push(k); prefixes.push(cur); cur = new StringBuilder(); k = 0; }
        else if (ch == ']') {
            StringBuilder prev = prefixes.pop();
            for (int i = counts.pop(); i > 0; i--) prev.append(cur);
            cur = prev;
        } else cur.append(ch);
    }
    return cur.toString();
}

decode("3[a2[bc]]") returns abcbcabcbcabcbc; decode("10[x]") handles multi-digit counts through k * 10.

Each redeploy creates a new web-app class loader. If anything outside it keeps a reference to a class or object from the old loader (a ThreadLocal, a JDBC driver registered in DriverManager, a thread, a shutdown hook), the old loader and all its classes stay in Metaspace.

• Heap dump: find multiple instances of WebappClassLoader and trace the GC root path.
• Use jcmd <pid> VM.classloader_stats and GC.class_stats.
• Fix: deregister drivers, stop threads, clear ThreadLocals in contextDestroyed; do not hot-redeploy in production.

Map the busiest OS thread to a Java thread: find the thread ID with top -H -p <pid>, convert to hex, and look up nid=0x... in a jstack dump. Then confirm with JFR or async-profiler.

top -H -p 1234            # note TID 1260
printf '%x\n' 1260        # 4ec
jstack 1234 | grep -A 15 'nid=0x4ec'

• If the hot thread is "GC Thread" or "C2 CompilerThread", it is GC or JIT activity, not business code.
• Take several dumps; one sample can mislead.

No. Declarative transactions are applied by a proxy around a Spring bean. The proxy only intercepts calls arriving through it to methods it can override, so new-created objects, private, and final methods (CGLIB cannot override them) get no transaction and fail silently.

• Annotate public methods on beans; since Spring 6, protected and package-private methods are also supported with class-based (CGLIB) proxies.
• Check with TransactionSynchronizationManager.isActualTransactionActive() or by logging org.springframework.transaction at DEBUG.
• Put @Transactional on the service layer, not controllers or repository interfaces' callers' internals.

Self-invocation bypasses the proxy, so the REQUIRES_NEW annotation is never evaluated; saveAudit() runs inside the caller's transaction and rolls back with it.

Fix by moving it to a separate bean:

@Service
class AuditService {
    @Transactional(propagation = Propagation.REQUIRES_NEW)
    public void save(AuditEvent e) { repo.save(e); }
}

Alternatives: self-injection, TransactionTemplate inside the method, or AspectJ mode.

By default Spring rolls back only on unchecked exceptions (RuntimeException and Error). A checked exception commits whatever was flushed. Declare rollbackFor = Exception.class (or specific types) to roll back on checked ones.

@Transactional(rollbackFor = IOException.class)
public void importFile(Path p) throws IOException { ... }

This rule mirrors EJB convention. noRollbackFor does the inverse.