Add compact-strings module: Compact Strings (JEP 254) measured with JOL on JDK 25, with and without -XX:-CompactStrings, and compact object headers on 25 vs 27

Co-Authored-By: Claude Sonnet 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_01KqJyCidz3ZgRyHABv2GVJh
2026-10-01 04:23:27 +00:00
parent fadfd05670
commit fc05eed4b5
26 changed files with 943 additions and 0 deletions
@@ -0,0 +1,57 @@
+import java.lang.reflect.Field;
+import org.openjdk.jol.info.ClassLayout;
+import org.openjdk.jol.info.GraphLayout;
+import org.openjdk.jol.vm.VM;
+
+/**
+ * What one String really costs: the String object, the byte[] it points to, and the coder byte that says how to read it.
+ * Measured with JOL (org.openjdk.jol:jol-core). Run by run.sh under several JVM flag sets.
+ */
+public class StringLayout {
+
+    static byte coder(String s) throws Exception {
+        Field f = String.class.getDeclaredField("coder");
+        f.setAccessible(true);                       // needs --add-opens java.base/java.lang=ALL-UNNAMED
+        return f.getByte(s);
+    }
+
+    static byte[] value(String s) throws Exception {
+        Field f = String.class.getDeclaredField("value");
+        f.setAccessible(true);
+        return (byte[]) f.get(s);
+    }
+
+    static String repeat(char c, int n) { return String.valueOf(c).repeat(n); }
+
+    public static void main(String[] args) throws Exception {
+        System.out.println("java.version = " + System.getProperty("java.version"));
+        System.out.println("vm.flags     = " + String.join(" ", java.lang.management.ManagementFactory.getRuntimeMXBean().getInputArguments()
+                .stream().filter(a -> a.startsWith("-XX")).toList()));
+        System.out.println(VM.current().details().lines().filter(l -> l.startsWith("# Objects are 8 bytes") || l.startsWith("# Field sizes")
+                || l.contains("Array element") || l.contains("compressed")).reduce("", (a, b) -> a + b + "\n"));
+
+        System.out.println("=== java.lang.String, the class");
+        System.out.println(ClassLayout.parseClass(String.class).toPrintable());
+
+        System.out.println("=== one Latin-1 string and one UTF-16 string, field by field");
+        System.out.println(ClassLayout.parseInstance("hello").toPrintable());
+        System.out.println(ClassLayout.parseInstance("h\u20ACllo").toPrintable());
+
+        System.out.println("=== the byte[] behind each");
+        System.out.println(ClassLayout.parseInstance(value("hello")).toPrintable());
+        System.out.println(ClassLayout.parseInstance(value("h\u20ACllo")).toPrintable());
+
+        System.out.println("=== footprint by length (String object + its byte[], via GraphLayout)");
+        System.out.println("chars  content   coder  byte[].length  String  byte[]  total");
+        for (int n : new int[] {0, 1, 5, 10, 20, 40, 100, 1000}) {
+            for (char c : new char[] {'a', '\u00E9', '\u20AC'}) {          // a = ASCII, e-acute = Latin-1 supplement, euro = needs UTF-16
+                String s = new String(repeat(c, n).toCharArray());           // a fresh object, never the interned literal
+                long total = GraphLayout.parseInstance(s).totalSize();
+                long str = ClassLayout.parseInstance(s).instanceSize();
+                long arr = ClassLayout.parseInstance(value(s)).instanceSize();
+                System.out.printf("%5d  %-8s  %5d  %13d  %6d  %6d  %5d%n", n,
+                        c == 'a' ? "ASCII" : c == '\u00E9' ? "U+00E9" : "U+20AC", coder(s), value(s).length, str, arr, total);
+            }
+        }
+    }
+}