Add hibernate-demo: get() vs load(), merge() vs refresh(), inserting objects (Hibernate 7.4.1.Final + Spring Boot 4.1.0)

Adds a JUnit test suite (GetVsGetReferenceTest, MergeRefreshTest, OptimisticLockTest,
IdentityBatchTest, SequenceBatchTest, AllocationSizeSweepTest, BatchSizeSweepTest) so every
surprising behavior described in the three companion posts has a reproducible test, alongside
the original CommandLineRunner scenarios. Rewrites all three doc chapters and the README around
the new experiments: the get()/getReference() same-session matrix, the merge()/refresh()
experiments (including exactly when OptimisticLockException surfaces and a corrected LAZY-plus-
cascade merge() result), and two new sweeps (allocationSize, batch_size) for batch inserts.

scripts/clean_output.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python3
+"""Turn a raw mvn spring-boot:run capture into a clean, reproducible transcript.
+
+Strips JVM/build noise (sun.misc.Unsafe warnings, JAVA_TOOL_OPTIONS proxy banners) and the
+duplicate un-prefixed echo of each SQL statement that org.hibernate.SQL's show_sql=true prints
+to stdout in addition to the "Hibernate: ..." line the logger emits -- same text twice, so only
+the logger-prefixed copy is kept.
+"""
+import re
+import sys
+
+DROP_PREFIXES = (
+    "WARNING:",
+    "Picked up JAVA_TOOL_OPTIONS",
+)
+
+
+def clean(lines):
+    out = []
+    for line in lines:
+        stripped = line.rstrip("\n")
+        if any(stripped.startswith(p) for p in DROP_PREFIXES):
+            continue
+        # Drop the bare SQL echo line that show_sql=true prints without the "Hibernate: " prefix
+        # -- it is always immediately followed by the same text WITH the prefix.
+        out.append(stripped)
+    deduped = []
+    i = 0
+    while i < len(out):
+        cur = out[i]
+        nxt = out[i + 1] if i + 1 < len(out) else None
+        if nxt is not None and nxt == "Hibernate: " + cur:
+            i += 1  # skip the bare echo, keep the prefixed one on the next iteration
+            continue
+        deduped.append(out[i])
+        i += 1
+    return deduped
+
+
+if __name__ == "__main__":
+    src, dst = sys.argv[1], sys.argv[2]
+    with open(src, encoding="utf-8") as f:
+        lines = f.readlines()
+    cleaned = clean(lines)
+    with open(dst, "w", encoding="utf-8") as f:
+        f.write("\n".join(cleaned) + "\n")
+    print(f"{src}: {len(lines)} -> {dst}: {len(cleaned)} lines")

scripts/run-all.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+# Regenerate every file in docs/output/ from a real run. This is the script referenced by
+# "regenerated by one command" in the post and the README -- if it stops producing the same
+# shape of output, the docs are wrong until it's fixed, not the other way around.
+set -eu
+
+cd "$(dirname "$0")/.."
+RAW_DIR="$(mktemp -d)"
+trap 'rm -rf "$RAW_DIR"' EXIT
+
+run_one() {
+  local profile="$1" outfile="$2"
+  echo "=== running profile: $profile ===" >&2
+  mvn -q -B org.springframework.boot:spring-boot-maven-plugin:run \
+    -Dspring-boot.run.profiles="$profile" 2>&1 \
+    | grep -v '^Picked up JAVA_TOOL_OPTIONS' \
+    | grep -v '^WARNING:' \
+    > "$RAW_DIR/$profile.raw.txt"
+  python3 scripts/clean_output.py "$RAW_DIR/$profile.raw.txt" "docs/output/$outfile"
+}
+
+run_one getvsload        get-vs-load.txt
+run_one mergerefresh      merge-vs-refresh.txt
+run_one insert-identity   insert-identity.txt
+run_one insert-sequence   insert-sequence.txt
+
+echo "docs/output/ regenerated." >&2

scripts/run.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# Run one scenario in the foreground and exit.
+#
+#   ./scripts/run.sh getvsload
+#   ./scripts/run.sh mergerefresh
+#   ./scripts/run.sh insert-identity
+#   ./scripts/run.sh insert-sequence
+#
+# Every scenario here is a CommandLineRunner against an in-memory H2 database with
+# spring.main.web-application-type=none, so there is no server to keep alive and nothing to
+# kill afterwards -- the process runs the scenario and exits on its own.
+set -eu
+
+PROFILE="${1:?usage: run.sh <getvsload|mergerefresh|insert-identity|insert-sequence>}"
+
+cd "$(dirname "$0")/.."
+mvn -q -B org.springframework.boot:spring-boot-maven-plugin:run \
+  -Dspring-boot.run.profiles="$PROFILE"