Standard Chartered

Java 7 used 16 lock-striped Segments (each a ReentrantLock over a mini hash table). Java 8+ removed segments: an empty bucket is filled with a lock-free CAS; otherwise it uses synchronized on the bucket's first node. Resizing is cooperative (multiple threads migrate bins), and bins treeify like HashMap.

• Reads are lock-free (volatile fields); size() uses striped CounterCells (like LongAdder), so it is an estimate under concurrency.
• No null keys or values.

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.

With REQUIRED, inner and outer share one physical transaction. When the inner proxy sees a runtime exception, it marks the shared transaction rollback-only. Catching the exception outside does not clear the mark, so the outer commit attempt becomes a rollback and Spring reports UnexpectedRollbackException: Transaction silently rolled back because it has been marked as rollback-only.

Solutions: use REQUIRES_NEW (or NESTED with savepoints) for the part that may fail independently, or avoid calling a transactional bean that throws when you intend to recover.

Behaviour when a transaction already exists / does not:

• REQUIRED (default): join / create new.
• REQUIRES_NEW: suspend current and create new / create new.
• NESTED: savepoint within current / create new (like REQUIRED).
• SUPPORTS: join / run non-transactionally.
• MANDATORY: join / throw IllegalTransactionStateException.
• NOT_SUPPORTED: suspend and run non-transactionally / non-transactionally.
• NEVER: throw / non-transactionally.

Composite indexes are sorted by a, then b within a, then c. The leftmost-prefix rule says the index is usable efficiently only for a prefix of the columns.

• a=1: yes, full use.
• b=2: not as an efficient seek (MySQL cannot; PostgreSQL may do a full index scan, MySQL 8.0 has skip scan in limited cases).
• a=1 AND c=3: seeks on a, then c is only filtered inside the range of a (an index condition, not a seek).
• a=1 AND b>5 AND c=3: seeks on a and range on b; c cannot narrow the range because after a range predicate the following columns are not sorted contiguously.

Rule of thumb: equality columns first, then the range or sort column, in order of selectivity and query frequency.

CREATE INDEX idx_orders_cust_status_created ON orders (customer_id, status, created_at);