diff --git a/CHANGES.md b/CHANGES.md
index 55eba05..451d713 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -47,6 +47,8 @@ Swift-Pfade relativ zu `filius-ipad/ios/FiliusPad/TopologyEditor/` (Stand
 | `test/topology.test.ts` | — | neu: alle 23 Korpus-Dateien Feld für Feld wie Java (Geräte, Schnittstellen, Kabel, Doku-Elemente), dazu die Regeln, die der Korpus nicht berührt |
 | `test/javaIp.test.ts`, `test/ip-check.*` | — | neu: `IP.ipCheck`-Fälle, Ergebnis aus Java |
 | `src/javaEmail.ts` | Java `EmailServer` (Konten-Bean, `kontenLaden`/`stringZuListeBenutzerkonten`), `EmailAnwendung` (Ordner, Adressbuch, `laden`), `EmailKonto`, `Email`, `AddressEntry`, `EmailUtils` | neu nach Java: E-Mail-Beans über ihre Setter, `/mailserver/konten.txt` und `/konten.txt` im Format von Javas Lesern, ohne ihre Fehler (leere Felder zählen, eine kurze Zeile fällt allein weg, Nachrichten mit leerem Text oder Betreff bleiben, `;` im Betreff bleibt, CRLF, beim Client die letzte vollständige Zeile); Empfänger älterer Dateien als Zeichenketten; gegen Java auf dem Korpus und an 43 Fällen geprüft (13 benannte Java-Fehler) |
+| `src/javaBeansWriter.ts` | — | neu: Schreiber für das `XMLDecoder`-Format, Gegenstück zu `javabeans.ts`; ausdrücklich typisierte Eingabe, geteilte Objekte über `id`/`idref`, was XML nicht fassen kann (und CR) als `<char code>`; von Java selbst geprüft (`BeansDump`) |
+| `test/javaBeansWriter.test.ts`, `test/beans/` | — | neu: 9 Dateien des Schreibers, von Javas `XMLDecoder` gelesen und beschrieben; Erwartungen von Hand |
 | `src/importEmail.ts` | `Persistence/TopologyProjectStore.swift` (`registerJavaEmailConfigurations`, `assignedJavaEmailMessages`) | übertragen, Eingabe ist Javas Sicht: Server-Konten aus `/mailserver/konten.txt`, ohne Datei aus der Bean-Liste (Java verlöre sie); Client-Konto aus `/konten.txt`, ohne Datei aus der älteren `kontoListe` (Filius 2.14 liest sie nicht mehr); Konten und Nachrichten bleiben, wo Swift verwirft, was seine Prüfung nicht besteht; Vor- und Nachname getrennt; Empfangszeit nur aus der Datei (Swift nummeriert); gegen Swifts Modell geprüft |
 | `test/importEmail.test.ts`, `test/email-*.swift.json` | `State/TopologyEmail.swift` (`decodeJavaServer`, `decodeJavaClient`) als Vergleich | neu: dieselben 43 Kontodateien mit Swifts Leser; wie Swift bis auf benannte Fälle, in denen Swift prüft und die ganze Datei verwirft, wo Java liest |
 | `src/javaFileSystem.ts`, `src/javaString.ts` | Java `Dateisystem.holeDatei`, `String.trim`/`split`/`equalsIgnoreCase` | neu nach Java; die Groß-/Kleinschreibung gegenüber ASCII am JDK über die ganze BMP gemessen |
diff --git a/packages/fls/src/javaBeansWriter.ts b/packages/fls/src/javaBeansWriter.ts
new file mode 100644
index 0000000..3d0bc25
--- /dev/null
+++ b/packages/fls/src/javaBeansWriter.ts
@@ -0,0 +1,292 @@
+// Writes the format java.beans.XMLDecoder reads — the counterpart of
+// javabeans.ts. The input says what Java should do, explicitly typed: which
+// class to construct with which arguments, which setters, getters and methods
+// to call. An object reached twice (the same JS object) is written once with
+// an id and referenced after (idref), as XMLEncoder does.
+//
+// Unlike XMLEncoder this writer does not leave out what equals a
+// constructor's default: it writes what it is given. Checked by Java itself
+// (tools/java-oracle BeansDump reads what this writes; javaBeansWriter.test.ts).
+
+export type BeanPrimitive =
+  | { readonly kind: 'int' | 'short' | 'byte'; readonly value: number }
+  | { readonly kind: 'long'; readonly value: number | bigint }
+  | { readonly kind: 'float' | 'double'; readonly value: number }
+  | { readonly kind: 'char'; readonly value: string };
+
+export type BeanValue =
+  | null
+  | string
+  | boolean
+  | BeanPrimitive
+  | BeanObject
+  | BeanArray
+  | { readonly kind: 'class'; readonly name: string };
+
+export interface BeanObject {
+  readonly kind: 'object';
+  readonly className: string;
+  /** Constructor arguments. */
+  readonly args?: readonly BeanValue[];
+  /** `C.F`: a static field instead of a new object. */
+  readonly staticField?: string;
+  readonly statements?: readonly BeanStatement[];
+}
+
+export interface BeanArray {
+  readonly kind: 'array';
+  readonly componentClass: string;
+  readonly items: readonly BeanValue[];
+}
+
+export type BeanStatement =
+  /** target.setP(value) */
+  | { readonly set: string; readonly value: BeanValue }
+  /** target.getP(), then the statements on its result */
+  | { readonly get: string; readonly statements: readonly BeanStatement[] }
+  /** target.m(args) */
+  | { readonly call: string; readonly args: readonly BeanValue[] }
+  /** target.set(index, value) on a list */
+  | { readonly setIndex: number; readonly value: BeanValue }
+  /** target.get(index), then the statements on its result */
+  | { readonly getIndex: number; readonly statements: readonly BeanStatement[] };
+
+export const int = (value: number): BeanPrimitive => ({ kind: 'int', value });
+export const long = (value: number | bigint): BeanPrimitive => ({ kind: 'long', value });
+export const double = (value: number): BeanPrimitive => ({ kind: 'double', value });
+export const float = (value: number): BeanPrimitive => ({ kind: 'float', value });
+
+export function object(className: string, statements: readonly BeanStatement[] = [], args: readonly BeanValue[] = []): BeanObject {
+  return { kind: 'object', className, args, statements };
+}
+
+/** An XMLDecoder document: the values in order, as `new XMLDecoder(in).readObject()` returns them. */
+export function encodeJavaBeans(values: readonly BeanValue[]): string {
+  const writer = new Writer(sharedObjects(values));
+  const lines = ['<?xml version="1.0" encoding="UTF-8"?>', '<java version="17" class="java.beans.XMLDecoder">'];
+  for (const value of values) writer.value(value, 1, lines);
+  lines.push('</java>', '');
+  return lines.join('\n');
+}
+
+class Writer {
+  private readonly ids = new Map<BeanObject | BeanArray, string>();
+  private readonly counters = new Map<string, number>();
+  /** Objects written more than once need an id; found before writing. */
+  constructor(private readonly shared: ReadonlySet<BeanObject | BeanArray>) {}
+
+  value(value: BeanValue, depth: number, lines: string[]): void {
+    const indent = ' '.repeat(depth);
+    if (value === null) {
+      lines.push(`${indent}<null/>`);
+    } else if (typeof value === 'string') {
+      lines.push(`${indent}<string>${stringContent(value)}</string>`);
+    } else if (typeof value === 'boolean') {
+      lines.push(`${indent}<boolean>${value}</boolean>`);
+    } else {
+      switch (value.kind) {
+        case 'int':
+        case 'short':
+        case 'byte':
+          lines.push(`${indent}<${value.kind}>${integerText(value.value)}</${value.kind}>`);
+          break;
+        case 'long':
+          lines.push(`${indent}<long>${typeof value.value === 'bigint' ? value.value : integerText(value.value)}</long>`);
+          break;
+        case 'float':
+        case 'double':
+          lines.push(`${indent}<${value.kind}>${floatingText(value.value)}</${value.kind}>`);
+          break;
+        case 'char':
+          lines.push(`${indent}${charElement(value.value)}`);
+          break;
+        case 'class':
+          lines.push(`${indent}<class>${escapeText(value.name)}</class>`);
+          break;
+        case 'array':
+          this.array(value, depth, lines);
+          break;
+        case 'object':
+          this.object(value, depth, lines);
+          break;
+      }
+    }
+  }
+
+  private object(value: BeanObject, depth: number, lines: string[]): void {
+    const indent = ' '.repeat(depth);
+    const existing = this.ids.get(value);
+    if (existing !== undefined) {
+      lines.push(`${indent}<object idref="${existing}"/>`);
+      return;
+    }
+    const id = this.idFor(value, simpleName(value.className));
+    const idAttribute = id === undefined ? '' : ` id="${id}"`;
+    if (value.staticField !== undefined) {
+      lines.push(`${indent}<object class="${escapeAttribute(value.className)}"${idAttribute} field="${escapeAttribute(value.staticField)}"/>`);
+      return;
+    }
+    const args = value.args ?? [];
+    const statements = value.statements ?? [];
+    if (args.length === 0 && statements.length === 0) {
+      lines.push(`${indent}<object class="${escapeAttribute(value.className)}"${idAttribute}/>`);
+      return;
+    }
+    lines.push(`${indent}<object class="${escapeAttribute(value.className)}"${idAttribute}>`);
+    for (const arg of args) this.value(arg, depth + 1, lines);
+    for (const statement of statements) this.statement(statement, depth + 1, lines);
+    lines.push(`${indent}</object>`);
+  }
+
+  private array(value: BeanArray, depth: number, lines: string[]): void {
+    const indent = ' '.repeat(depth);
+    const existing = this.ids.get(value);
+    if (existing !== undefined) {
+      lines.push(`${indent}<object idref="${existing}"/>`);
+      return;
+    }
+    const id = this.idFor(value, `${simpleName(value.componentClass)}Array`);
+    const idAttribute = id === undefined ? '' : ` id="${id}"`;
+    const head = `${indent}<array class="${escapeAttribute(value.componentClass)}"${idAttribute} length="${value.items.length}"`;
+    if (value.items.length === 0) {
+      lines.push(`${head}/>`);
+      return;
+    }
+    lines.push(`${head}>`);
+    for (const [index, item] of value.items.entries()) {
+      lines.push(`${indent} <void index="${index}">`);
+      this.value(item, depth + 2, lines);
+      lines.push(`${indent} </void>`);
+    }
+    lines.push(`${indent}</array>`);
+  }
+
+  private statement(statement: BeanStatement, depth: number, lines: string[]): void {
+    const indent = ' '.repeat(depth);
+    const nested = (open: string, body: () => void) => {
+      lines.push(`${indent}${open}>`);
+      body();
+      lines.push(`${indent}</void>`);
+    };
+    if ('set' in statement) {
+      nested(`<void property="${escapeAttribute(statement.set)}"`, () => this.value(statement.value, depth + 1, lines));
+    } else if ('get' in statement) {
+      if (statement.statements.length === 0) return;
+      nested(`<void property="${escapeAttribute(statement.get)}"`, () => {
+        for (const inner of statement.statements) this.statement(inner, depth + 1, lines);
+      });
+    } else if ('call' in statement) {
+      if (statement.args.length === 0) {
+        lines.push(`${indent}<void method="${escapeAttribute(statement.call)}"/>`);
+        return;
+      }
+      nested(`<void method="${escapeAttribute(statement.call)}"`, () => {
+        for (const arg of statement.args) this.value(arg, depth + 1, lines);
+      });
+    } else if ('setIndex' in statement) {
+      nested(`<void index="${statement.setIndex}"`, () => this.value(statement.value, depth + 1, lines));
+    } else {
+      if (statement.statements.length === 0) return;
+      nested(`<void index="${statement.getIndex}"`, () => {
+        for (const inner of statement.statements) this.statement(inner, depth + 1, lines);
+      });
+    }
+  }
+
+  private idFor(value: BeanObject | BeanArray, name: string): string | undefined {
+    if (!this.shared.has(value)) return undefined;
+    const count = this.counters.get(name) ?? 0;
+    this.counters.set(name, count + 1);
+    const id = `${name}${count}`;
+    this.ids.set(value, id);
+    return id;
+  }
+}
+
+/** The objects and arrays the values reach more than once. */
+function sharedObjects(roots: readonly BeanValue[]): Set<BeanObject | BeanArray> {
+  const seen = new Set<BeanObject | BeanArray>();
+  const shared = new Set<BeanObject | BeanArray>();
+  const visitStatement = (statement: BeanStatement): void => {
+    if ('set' in statement || 'setIndex' in statement) visit(statement.value);
+    else if ('call' in statement) statement.args.forEach(visit);
+    else statement.statements.forEach(visitStatement);
+  };
+  const visit = (value: BeanValue): void => {
+    if (value === null || typeof value !== 'object' || (value.kind !== 'object' && value.kind !== 'array')) return;
+    if (seen.has(value)) {
+      shared.add(value);
+      return;
+    }
+    seen.add(value);
+    if (value.kind === 'array') {
+      value.items.forEach(visit);
+    } else {
+      (value.args ?? []).forEach(visit);
+      (value.statements ?? []).forEach(visitStatement);
+    }
+  };
+  roots.forEach(visit);
+  return shared;
+}
+
+function simpleName(className: string): string {
+  return className.slice(className.lastIndexOf('.') + 1).replace(/\$/g, '_');
+}
+
+function integerText(value: number): string {
+  if (!Number.isInteger(value)) throw new RangeError(`not an integer: ${value}`);
+  return String(value);
+}
+
+/** Text Double.valueOf reads back to the same value. */
+function floatingText(value: number): string {
+  if (Object.is(value, -0)) return '-0.0';
+  if (Number.isNaN(value)) return 'NaN';
+  if (value === Infinity) return 'Infinity';
+  if (value === -Infinity) return '-Infinity';
+  return String(value);
+}
+
+/**
+ * A string's content: what XML 1.0 cannot hold, and a CR (which XML would
+ * read as a line break), as `<char code="#…"/>`, the rest escaped.
+ */
+function stringContent(value: string): string {
+  let out = '';
+  for (let index = 0; index < value.length; index++) {
+    const unit = value.charCodeAt(index);
+    if (unit >= 0xd800 && unit <= 0xdbff && index + 1 < value.length) {
+      const next = value.charCodeAt(index + 1);
+      if (next >= 0xdc00 && next <= 0xdfff) {
+        out += value[index]! + value[index + 1]!;
+        index += 1;
+        continue;
+      }
+    }
+    out += needsCharElement(unit) ? charCode(unit) : escapeText(value[index]!);
+  }
+  return out;
+}
+
+function charElement(value: string): string {
+  const unit = value.charCodeAt(0);
+  if (value.length !== 1) throw new RangeError('a char is one UTF-16 unit');
+  return needsCharElement(unit) ? `<char code="#${unit.toString(16).toUpperCase()}"/>` : `<char>${escapeText(value)}</char>`;
+}
+
+function needsCharElement(unit: number): boolean {
+  return (unit < 0x20 && unit !== 0x09 && unit !== 0x0a) || (unit >= 0xd800 && unit <= 0xdfff) || unit === 0xfffe || unit === 0xffff;
+}
+
+function charCode(unit: number): string {
+  return `<char code="#${unit.toString(16).toUpperCase()}"/>`;
+}
+
+function escapeText(text: string): string {
+  return text.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;');
+}
+
+function escapeAttribute(text: string): string {
+  return escapeText(text).replace(/"/g, '&quot;');
+}
diff --git a/packages/fls/test/beans/arrays.xml b/packages/fls/test/beans/arrays.xml
new file mode 100644
index 0000000..cac8fb3
--- /dev/null
+++ b/packages/fls/test/beans/arrays.xml
@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<java version="17" class="java.beans.XMLDecoder">
+ <array class="java.lang.String" length="3">
+  <void index="0">
+   <string>a</string>
+  </void>
+  <void index="1">
+   <null/>
+  </void>
+  <void index="2">
+   <string>c</string>
+  </void>
+ </array>
+ <array class="int" length="2">
+  <void index="0">
+   <int>0</int>
+  </void>
+  <void index="1">
+   <int>5</int>
+  </void>
+ </array>
+</java>
diff --git a/packages/fls/test/beans/awt.xml b/packages/fls/test/beans/awt.xml
new file mode 100644
index 0000000..3313758
--- /dev/null
+++ b/packages/fls/test/beans/awt.xml
@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<java version="17" class="java.beans.XMLDecoder">
+ <object class="java.awt.Rectangle">
+  <int>1</int>
+  <int>2</int>
+  <int>3</int>
+  <int>4</int>
+ </object>
+ <object class="java.awt.Color">
+  <int>10</int>
+  <int>20</int>
+  <int>30</int>
+  <int>40</int>
+ </object>
+</java>
diff --git a/packages/fls/test/beans/beans.java.json b/packages/fls/test/beans/beans.java.json
new file mode 100644
index 0000000..cb763bc
--- /dev/null
+++ b/packages/fls/test/beans/beans.java.json
@@ -0,0 +1,411 @@
+{
+  "arrays.xml": {
+    "values": [
+      {
+        "id": 0,
+        "class": "[Ljava.lang.String;",
+        "items": [
+          {
+            "string": "a"
+          },
+          null,
+          {
+            "string": "c"
+          }
+        ]
+      },
+      {
+        "id": 1,
+        "class": "[I",
+        "items": [
+          {
+            "int": 0
+          },
+          {
+            "int": 5
+          }
+        ]
+      }
+    ],
+    "errors": []
+  },
+  "awt.xml": {
+    "values": [
+      {
+        "rectangle": [
+          1,
+          2,
+          3,
+          4
+        ]
+      },
+      {
+        "color": [
+          10,
+          20,
+          30,
+          40
+        ]
+      }
+    ],
+    "errors": []
+  },
+  "beans.xml": {
+    "values": [
+      {
+        "id": 0,
+        "class": "filius.software.system.Datei",
+        "properties": {
+          "dateiInhalt": {
+            "string": "a.de. A 3600 1.2.3.4\\u000A"
+          },
+          "dateiTyp": {
+            "string": "text"
+          },
+          "name": {
+            "string": "hosts"
+          }
+        }
+      },
+      {
+        "id": 1,
+        "class": "filius.software.email.EmailKonto",
+        "properties": {
+          "benutzername": {
+            "string": "bob"
+          },
+          "emailAdresse": null,
+          "nachname": {
+            "string": ""
+          },
+          "nachrichten": {
+            "id": 2,
+            "class": "java.util.LinkedList",
+            "items": [
+              {
+                "id": 3,
+                "class": "filius.software.email.Email",
+                "properties": {
+                  "absender": {
+                    "id": 4,
+                    "class": "filius.software.email.AddressEntry",
+                    "properties": {
+                      "mailAddress": {
+                        "string": "a@x"
+                      },
+                      "name": {
+                        "string": "Alice"
+                      }
+                    }
+                  },
+                  "bcc": {
+                    "id": 5,
+                    "class": "java.util.ArrayList",
+                    "items": []
+                  },
+                  "betreff": {
+                    "string": "Hi"
+                  },
+                  "cc": {
+                    "id": 6,
+                    "class": "java.util.ArrayList",
+                    "items": []
+                  },
+                  "dateReceived": {
+                    "string": ""
+                  },
+                  "delete": false,
+                  "empfaenger": {
+                    "id": 7,
+                    "class": "java.util.ArrayList",
+                    "items": [
+                      {
+                        "id": 8,
+                        "class": "filius.software.email.AddressEntry",
+                        "properties": {
+                          "mailAddress": {
+                            "string": "b@y"
+                          },
+                          "name": null
+                        }
+                      }
+                    ]
+                  },
+                  "neu": false,
+                  "text": {
+                    "string": ""
+                  },
+                  "versendet": false
+                }
+              }
+            ]
+          },
+          "passwort": null,
+          "pop3port": null,
+          "pop3server": null,
+          "smtpport": null,
+          "smtpserver": null,
+          "vorname": {
+            "string": ""
+          }
+        }
+      },
+      {
+        "id": 9,
+        "class": "filius.software.email.Kontakt",
+        "properties": {
+          "email": null,
+          "hausnr": {
+            "int": 12
+          },
+          "name": {
+            "string": "Lang"
+          },
+          "plz": {
+            "int": 0
+          },
+          "strasse": null,
+          "telefon": null,
+          "vorname": null,
+          "wohnort": null
+        }
+      }
+    ],
+    "errors": []
+  },
+  "collections.xml": {
+    "values": [
+      {
+        "id": 0,
+        "class": "java.util.Vector",
+        "items": [
+          {
+            "int": 1
+          },
+          {
+            "id": 1,
+            "class": "java.util.ArrayList",
+            "items": [
+              {
+                "string": "nested"
+              }
+            ]
+          }
+        ]
+      },
+      {
+        "id": 2,
+        "class": "java.util.HashMap",
+        "entries": [
+          [
+            {
+              "string": "key"
+            },
+            {
+              "int": 8
+            }
+          ]
+        ]
+      }
+    ],
+    "errors": []
+  },
+  "constants.xml": {
+    "values": [
+      {
+        "int": 0
+      }
+    ],
+    "errors": []
+  },
+  "indexed.xml": {
+    "values": [
+      {
+        "id": 0,
+        "class": "java.util.LinkedList",
+        "items": [
+          {
+            "id": 1,
+            "class": "filius.software.system.Datei",
+            "properties": {
+              "dateiInhalt": null,
+              "dateiTyp": null,
+              "name": {
+                "string": "renamed"
+              }
+            }
+          },
+          {
+            "string": "replaced"
+          }
+        ]
+      }
+    ],
+    "errors": []
+  },
+  "numbers.xml": {
+    "values": [
+      {
+        "int": -1
+      },
+      {
+        "int": 2147483647
+      },
+      {
+        "long": "-9007199254740991"
+      },
+      {
+        "long": "9223372036854775807"
+      },
+      {
+        "double": "0.1"
+      },
+      {
+        "double": "-0.0"
+      },
+      {
+        "double": "1.0E300"
+      },
+      {
+        "double": "NaN"
+      },
+      {
+        "double": "-Infinity"
+      },
+      {
+        "float": "1.5"
+      },
+      {
+        "short": -3
+      },
+      {
+        "byte": 127
+      },
+      {
+        "char": 120
+      },
+      {
+        "char": 1
+      },
+      true,
+      false,
+      null,
+      {
+        "class": "java.lang.String"
+      }
+    ],
+    "errors": []
+  },
+  "shared.xml": {
+    "values": [
+      {
+        "id": 0,
+        "class": "java.util.LinkedList",
+        "items": [
+          {
+            "id": 1,
+            "class": "java.util.ArrayList",
+            "items": [
+              {
+                "string": "shared"
+              }
+            ]
+          },
+          {
+            "id": 2,
+            "class": "java.util.ArrayList",
+            "items": [
+              {
+                "string": "other"
+              }
+            ]
+          },
+          {
+            "ref": 1
+          },
+          {
+            "ref": 2
+          }
+        ]
+      },
+      {
+        "id": 3,
+        "class": "filius.software.system.Datei",
+        "properties": {
+          "dateiInhalt": {
+            "string": "a.de. A 3600 1.2.3.4\\u000A"
+          },
+          "dateiTyp": {
+            "string": "text"
+          },
+          "name": {
+            "string": "hosts"
+          }
+        }
+      },
+      {
+        "ref": 3
+      }
+    ],
+    "errors": []
+  },
+  "strings.xml": {
+    "values": [
+      {
+        "id": 0,
+        "class": "java.util.LinkedList",
+        "items": [
+          {
+            "string": "plain"
+          },
+          {
+            "string": ""
+          },
+          {
+            "string": "  spaced  "
+          },
+          {
+            "string": "<&>\"'"
+          },
+          {
+            "string": "]]>"
+          },
+          {
+            "string": "tab\\u0009here"
+          },
+          {
+            "string": "line\\u000Abreak"
+          },
+          {
+            "string": "cr\\u000Dhere"
+          },
+          {
+            "string": "crlf\\u000D\\u000Ahere"
+          },
+          {
+            "string": "\\u0000\\u0001\\u001F\\u007F"
+          },
+          {
+            "string": "\\u0085\\u2028\\u2029"
+          },
+          {
+            "string": "\\uFEFFbom"
+          },
+          {
+            "string": "Gr\\u00FC\\u00DFe \\uD83D\\uDE00"
+          },
+          {
+            "string": "lone \\uD800 high"
+          },
+          {
+            "string": "lone \\uDC00 low"
+          },
+          {
+            "string": "\\uFFFE\\uFFFF"
+          }
+        ]
+      }
+    ],
+    "errors": []
+  }
+}
diff --git a/packages/fls/test/beans/beans.xml b/packages/fls/test/beans/beans.xml
new file mode 100644
index 0000000..9a5043d
--- /dev/null
+++ b/packages/fls/test/beans/beans.xml
@@ -0,0 +1,65 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<java version="17" class="java.beans.XMLDecoder">
+ <object class="filius.software.system.Datei">
+  <void property="name">
+   <string>hosts</string>
+  </void>
+  <void property="dateiTyp">
+   <string>text</string>
+  </void>
+  <void property="dateiInhalt">
+   <string>a.de. A 3600 1.2.3.4
+</string>
+  </void>
+ </object>
+ <object class="filius.software.email.EmailKonto">
+  <void property="benutzername">
+   <string>bob</string>
+  </void>
+  <void property="passwort">
+   <null/>
+  </void>
+  <void property="nachname">
+   <null/>
+  </void>
+  <void property="nachrichten">
+   <void method="add">
+    <object class="filius.software.email.Email">
+     <void property="absender">
+      <object class="filius.software.email.AddressEntry">
+       <void property="name">
+        <string>Alice</string>
+       </void>
+       <void property="mailAddress">
+        <string>a@x</string>
+       </void>
+      </object>
+     </void>
+     <void property="empfaenger">
+      <void method="add">
+       <object class="filius.software.email.AddressEntry">
+        <void property="mailAddress">
+         <string>b@y</string>
+        </void>
+       </object>
+      </void>
+     </void>
+     <void property="betreff">
+      <string>Hi</string>
+     </void>
+     <void property="neu">
+      <boolean>false</boolean>
+     </void>
+    </object>
+   </void>
+  </void>
+ </object>
+ <object class="filius.software.email.Kontakt">
+  <void property="hausnr">
+   <int>12</int>
+  </void>
+  <void property="name">
+   <string>Lang</string>
+  </void>
+ </object>
+</java>
diff --git a/packages/fls/test/beans/collections.xml b/packages/fls/test/beans/collections.xml
new file mode 100644
index 0000000..c2f586e
--- /dev/null
+++ b/packages/fls/test/beans/collections.xml
@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<java version="17" class="java.beans.XMLDecoder">
+ <object class="java.util.Vector">
+  <void method="add">
+   <int>1</int>
+  </void>
+  <void method="add">
+   <object class="java.util.ArrayList">
+    <void method="add">
+     <string>nested</string>
+    </void>
+   </object>
+  </void>
+ </object>
+ <object class="java.util.HashMap">
+  <void method="put">
+   <string>key</string>
+   <int>7</int>
+  </void>
+  <void method="put">
+   <string>key</string>
+   <int>8</int>
+  </void>
+ </object>
+</java>
diff --git a/packages/fls/test/beans/constants.xml b/packages/fls/test/beans/constants.xml
new file mode 100644
index 0000000..3846c84
--- /dev/null
+++ b/packages/fls/test/beans/constants.xml
@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<java version="17" class="java.beans.XMLDecoder">
+ <object class="javax.swing.SwingConstants" field="CENTER"/>
+</java>
diff --git a/packages/fls/test/beans/indexed.xml b/packages/fls/test/beans/indexed.xml
new file mode 100644
index 0000000..c3105fb
--- /dev/null
+++ b/packages/fls/test/beans/indexed.xml
@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<java version="17" class="java.beans.XMLDecoder">
+ <object class="java.util.LinkedList">
+  <void method="add">
+   <object class="filius.software.system.Datei">
+    <void property="name">
+     <string>first</string>
+    </void>
+   </object>
+  </void>
+  <void index="0">
+   <void property="name">
+    <string>renamed</string>
+   </void>
+  </void>
+  <void method="add">
+   <string>second</string>
+  </void>
+  <void index="1">
+   <string>replaced</string>
+  </void>
+ </object>
+</java>
diff --git a/packages/fls/test/beans/numbers.xml b/packages/fls/test/beans/numbers.xml
new file mode 100644
index 0000000..2828a68
--- /dev/null
+++ b/packages/fls/test/beans/numbers.xml
@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<java version="17" class="java.beans.XMLDecoder">
+ <int>-1</int>
+ <int>2147483647</int>
+ <long>-9007199254740991</long>
+ <long>9223372036854775807</long>
+ <double>0.1</double>
+ <double>-0.0</double>
+ <double>1e+300</double>
+ <double>NaN</double>
+ <double>-Infinity</double>
+ <float>1.5</float>
+ <short>-3</short>
+ <byte>127</byte>
+ <char>x</char>
+ <char code="#1"/>
+ <boolean>true</boolean>
+ <boolean>false</boolean>
+ <null/>
+ <class>java.lang.String</class>
+</java>
diff --git a/packages/fls/test/beans/shared.xml b/packages/fls/test/beans/shared.xml
new file mode 100644
index 0000000..deedcb5
--- /dev/null
+++ b/packages/fls/test/beans/shared.xml
@@ -0,0 +1,38 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<java version="17" class="java.beans.XMLDecoder">
+ <object class="java.util.LinkedList">
+  <void method="add">
+   <object class="java.util.ArrayList" id="ArrayList0">
+    <void method="add">
+     <string>shared</string>
+    </void>
+   </object>
+  </void>
+  <void method="add">
+   <object class="java.util.ArrayList" id="ArrayList1">
+    <void method="add">
+     <string>other</string>
+    </void>
+   </object>
+  </void>
+  <void method="add">
+   <object idref="ArrayList0"/>
+  </void>
+  <void method="add">
+   <object idref="ArrayList1"/>
+  </void>
+ </object>
+ <object class="filius.software.system.Datei" id="Datei0">
+  <void property="name">
+   <string>hosts</string>
+  </void>
+  <void property="dateiTyp">
+   <string>text</string>
+  </void>
+  <void property="dateiInhalt">
+   <string>a.de. A 3600 1.2.3.4
+</string>
+  </void>
+ </object>
+ <object idref="Datei0"/>
+</java>
diff --git a/packages/fls/test/beans/strings.xml b/packages/fls/test/beans/strings.xml
new file mode 100644
index 0000000..25b978d
--- /dev/null
+++ b/packages/fls/test/beans/strings.xml
@@ -0,0 +1,55 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<java version="17" class="java.beans.XMLDecoder">
+ <object class="java.util.LinkedList">
+  <void method="add">
+   <string>plain</string>
+  </void>
+  <void method="add">
+   <string></string>
+  </void>
+  <void method="add">
+   <string>  spaced  </string>
+  </void>
+  <void method="add">
+   <string>&lt;&amp;&gt;"'</string>
+  </void>
+  <void method="add">
+   <string>]]&gt;</string>
+  </void>
+  <void method="add">
+   <string>tab	here</string>
+  </void>
+  <void method="add">
+   <string>line
+break</string>
+  </void>
+  <void method="add">
+   <string>cr<char code="#D"/>here</string>
+  </void>
+  <void method="add">
+   <string>crlf<char code="#D"/>
+here</string>
+  </void>
+  <void method="add">
+   <string><char code="#0"/><char code="#1"/><char code="#1F"/></string>
+  </void>
+  <void method="add">
+   <string>…

</string>
+  </void>
+  <void method="add">
+   <string>bom</string>
+  </void>
+  <void method="add">
+   <string>Grüße 😀</string>
+  </void>
+  <void method="add">
+   <string>lone <char code="#D800"/> high</string>
+  </void>
+  <void method="add">
+   <string>lone <char code="#DC00"/> low</string>
+  </void>
+  <void method="add">
+   <string><char code="#FFFE"/><char code="#FFFF"/></string>
+  </void>
+ </object>
+</java>
diff --git a/packages/fls/test/javaBeansWriter.test.ts b/packages/fls/test/javaBeansWriter.test.ts
new file mode 100644
index 0000000..56590a8
--- /dev/null
+++ b/packages/fls/test/javaBeansWriter.test.ts
@@ -0,0 +1,193 @@
+// The XMLDecoder format as javaBeansWriter.ts writes it, read by Java
+// itself: every case is written to test/beans/<name>.xml (UPDATE_BEAN_CASES=1
+// rewrites them), tools/java-oracle BeansDump reads each with XMLDecoder and
+// describes what it got (beans.java.json). The descriptions must match the
+// expectations written here by hand, and Java must report no error.
+
+import { existsSync, readFileSync, writeFileSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { describe, expect, it } from 'vitest';
+import { decodeJavaBeans } from '../src/javabeans.js';
+import {
+  double,
+  encodeJavaBeans,
+  float,
+  int,
+  long,
+  object,
+  type BeanObject,
+  type BeanValue,
+} from '../src/javaBeansWriter.js';
+
+const directory = join(dirname(fileURLToPath(import.meta.url)), 'beans');
+
+const list = (className: string, items: BeanValue[]): BeanObject =>
+  object(className, items.map((item) => ({ call: 'add', args: [item] })));
+/** BeansDump's escaping: every unit outside printable ASCII, and the backslash, as backslash-u and four hex digits. */
+const text = (value: string) => ({
+  string: value.replace(/[^\x20-\x7E]|\\/g, (unit) => `\\u${unit.charCodeAt(0).toString(16).toUpperCase().padStart(4, '0')}`),
+});
+const bean = (className: string, properties: Record<string, unknown>) => ({ class: className, properties });
+const items = (className: string, values: unknown[]) => ({ class: className, items: values });
+
+const STRINGS = ['plain', '', '  spaced  ', '<&>"\'', ']]>', 'tab\there', 'line\nbreak', 'cr\rhere', 'crlf\r\nhere',
+  '\u0000\u0001\u001F\u007F', '\u0085\u2028\u2029', '\uFEFFbom', 'Grüße 😀', 'lone \uD800 high', 'lone \uDC00 low', '\uFFFE\uFFFF'];
+
+const inner = list('java.util.ArrayList', ['shared']);
+const other = list('java.util.ArrayList', ['other']);
+const file = object('filius.software.system.Datei', [
+  { set: 'name', value: 'hosts' },
+  { set: 'dateiTyp', value: 'text' },
+  { set: 'dateiInhalt', value: 'a.de. A 3600 1.2.3.4\n' },
+]);
+
+const CASES: Record<string, { values: BeanValue[]; expected: unknown[] }> = {
+  strings: {
+    values: [list('java.util.LinkedList', STRINGS)],
+    expected: [items('java.util.LinkedList', STRINGS.map(text))],
+  },
+  numbers: {
+    values: [int(-1), int(2_147_483_647), long(-9_007_199_254_740_991), long(9_223_372_036_854_775_807n), double(0.1), double(-0),
+      double(1e300), double(Number.NaN), double(-Infinity), float(1.5), { kind: 'short', value: -3 }, { kind: 'byte', value: 127 },
+      { kind: 'char', value: 'x' }, { kind: 'char', value: '\u0001' }, true, false, null, { kind: 'class', name: 'java.lang.String' }],
+    expected: [{ int: -1 }, { int: 2_147_483_647 }, { long: '-9007199254740991' }, { long: '9223372036854775807' }, { double: 0.1 },
+      { double: -0 }, { double: 1e300 }, { double: Number.NaN }, { double: -Infinity }, { float: 1.5 }, { short: -3 }, { byte: 127 },
+      { char: 120 }, { char: 1 }, true, false, null, { class: 'java.lang.String' }],
+  },
+  collections: {
+    values: [
+      list('java.util.Vector', [int(1), list('java.util.ArrayList', ['nested'])]),
+      object('java.util.HashMap', [{ call: 'put', args: ['key', int(7)] }, { call: 'put', args: ['key', int(8)] }]),
+    ],
+    expected: [
+      items('java.util.Vector', [{ int: 1 }, items('java.util.ArrayList', [text('nested')])]),
+      { class: 'java.util.HashMap', entries: [[text('key'), { int: 8 }]] },
+    ],
+  },
+  shared: {
+    values: [list('java.util.LinkedList', [inner, other, inner, other]), file, file],
+    expected: [
+      {
+        id: 0,
+        class: 'java.util.LinkedList',
+        items: [items('java.util.ArrayList', [text('shared')]), items('java.util.ArrayList', [text('other')]), { ref: 1 }, { ref: 2 }],
+      },
+      { id: 3, class: 'filius.software.system.Datei' },
+      { ref: 3 },
+    ],
+  },
+  awt: {
+    values: [object('java.awt.Rectangle', [], [int(1), int(2), int(3), int(4)]), object('java.awt.Color', [], [int(10), int(20), int(30), int(40)])],
+    expected: [{ rectangle: [1, 2, 3, 4] }, { color: [10, 20, 30, 40] }],
+  },
+  beans: {
+    values: [
+      file,
+      object('filius.software.email.EmailKonto', [
+        { set: 'benutzername', value: 'bob' },
+        { set: 'passwort', value: null },
+        { set: 'nachname', value: null },
+        {
+          get: 'nachrichten',
+          statements: [
+            {
+              call: 'add',
+              args: [
+                object('filius.software.email.Email', [
+                  { set: 'absender', value: object('filius.software.email.AddressEntry', [{ set: 'name', value: 'Alice' }, { set: 'mailAddress', value: 'a@x' }]) },
+                  { get: 'empfaenger', statements: [{ call: 'add', args: [object('filius.software.email.AddressEntry', [{ set: 'mailAddress', value: 'b@y' }])] }] },
+                  { set: 'betreff', value: 'Hi' },
+                  { set: 'neu', value: false },
+                ]),
+              ],
+            },
+          ],
+        },
+      ]),
+      object('filius.software.email.Kontakt', [{ set: 'hausnr', value: int(12) }, { set: 'name', value: 'Lang' }]),
+    ],
+    expected: [
+      bean('filius.software.system.Datei', { name: text('hosts'), dateiTyp: text('text'), dateiInhalt: text('a.de. A 3600 1.2.3.4\n') }),
+      bean('filius.software.email.EmailKonto', {
+        benutzername: text('bob'),
+        passwort: null,
+        // setNachname keeps "" for null.
+        nachname: text(''),
+        nachrichten: items('java.util.LinkedList', [
+          bean('filius.software.email.Email', {
+            betreff: text('Hi'),
+            neu: false,
+            empfaenger: items('java.util.ArrayList', [bean('filius.software.email.AddressEntry', { name: null, mailAddress: text('b@y') })]),
+          }),
+        ]),
+      }),
+      bean('filius.software.email.Kontakt', { hausnr: { int: 12 }, name: text('Lang') }),
+    ],
+  },
+  arrays: {
+    values: [{ kind: 'array', componentClass: 'java.lang.String', items: ['a', null, 'c'] }, { kind: 'array', componentClass: 'int', items: [int(0), int(5)] }],
+    expected: [items('[Ljava.lang.String;', [text('a'), null, text('c')]), items('[I', [{ int: 0 }, { int: 5 }])],
+  },
+  indexed: {
+    values: [object('java.util.LinkedList', [
+      { call: 'add', args: [object('filius.software.system.Datei', [{ set: 'name', value: 'first' }])] },
+      { getIndex: 0, statements: [{ set: 'name', value: 'renamed' }] },
+      { call: 'add', args: ['second'] },
+      { setIndex: 1, value: 'replaced' },
+    ])],
+    expected: [items('java.util.LinkedList', [bean('filius.software.system.Datei', { name: text('renamed') }), text('replaced')])],
+  },
+  constants: {
+    values: [{ kind: 'object', className: 'javax.swing.SwingConstants', staticField: 'CENTER' }],
+    expected: [{ int: 0 }],
+  },
+};
+
+/** Whether Java's description has everything expected: listed properties only, numbers by value, ids only where given. */
+function matches(actual: unknown, expected: unknown): boolean {
+  if (expected === null || typeof expected !== 'object') return Object.is(actual, expected);
+  if (actual === null || typeof actual !== 'object') return false;
+  const want = expected as Record<string, unknown>;
+  const have = actual as Record<string, unknown>;
+  for (const [key, value] of Object.entries(want)) {
+    if ((key === 'double' || key === 'float') && typeof have[key] === 'string') {
+      if (!Object.is(Number(have[key]), value)) return false;
+      continue;
+    }
+    if (Array.isArray(value)) {
+      const list = have[key];
+      if (!Array.isArray(list) || list.length !== value.length || !value.every((item, index) => matches(list[index], item))) return false;
+      continue;
+    }
+    if (!(key in have) || !matches(have[key], value)) return false;
+  }
+  return true;
+}
+
+const javaDump = existsSync(join(directory, 'beans.java.json'))
+  ? (JSON.parse(readFileSync(join(directory, 'beans.java.json'), 'utf8')) as Record<string, { values: unknown[]; errors: string[] }>)
+  : {};
+
+describe('javaBeansWriter, read by Java', () => {
+  for (const [name, { values, expected }] of Object.entries(CASES)) {
+    it(name, () => {
+      const path = join(directory, `${name}.xml`);
+      const xml = encodeJavaBeans(values);
+      if (process.env.UPDATE_BEAN_CASES === '1') writeFileSync(path, xml);
+      expect(readFileSync(path, 'utf8')).toBe(xml);
+      const java = javaDump[`${name}.xml`];
+      expect(java?.errors).toEqual([]);
+      expect(java!.values).toHaveLength(expected.length);
+      java!.values.forEach((value, index) => {
+        expect(matches(value, expected[index]), `value ${index}: ${JSON.stringify(value)}`).toBe(true);
+      });
+    });
+  }
+
+  it('reads back with the reader of this package', () => {
+    expect(decodeJavaBeans(encodeJavaBeans(CASES.strings!.values))).toEqual([
+      expect.objectContaining({ calls: STRINGS.map((value) => ({ method: 'add', args: [value] })) }),
+    ]);
+  });
+});
diff --git a/scripts/code_language_allow.txt b/scripts/code_language_allow.txt
index 97c759f..bc91a02 100644
--- a/scripts/code_language_allow.txt
+++ b/scripts/code_language_allow.txt
@@ -56,3 +56,9 @@ packages/fls/::gesendete
 packages/fls/::erstellte
 packages/fls/::konten
 packages/fls/::laden
+# Property names of Java FILIUS's Datei (dateiTyp, dateiInhalt) and Kontakt
+# (hausnr), as Java's XMLDecoder and the Java oracle name them.
+packages/fls/::datei
+packages/fls/::hausnr
+packages/fls/::typ
+packages/fls/::inhalt
diff --git a/tools/java-oracle/README.md b/tools/java-oracle/README.md
index 770295a..c27b55a 100644
--- a/tools/java-oracle/README.md
+++ b/tools/java-oracle/README.md
@@ -85,6 +85,16 @@ wird leer, der Eintrag bleibt trotzdem; Typen außer A/MX/NS (auch `a`,
 `CNAME`, eine Kommentarzeile) gelten als gültig; eine TTL, die kein `int`
 ist, wird still 3600.
 
+## Was Java aus unserem XML macht
+
+`BeansDump` liest jede `*.xml` unter `packages/fls/test/beans/` mit
+`XMLDecoder` und beschreibt, was dabei herauskommt (Beans über ihre
+Eigenschaften mit Lese- und Schreibmethode, Listen, Maps, Arrays,
+`Rectangle`, `Color`; ein zweites Mal gesehenes Objekt als Verweis) —
+`beans.java.json`. Fehler, die der Decoder sonst nur ins Log schreibt,
+sammelt ein `ExceptionListener`. Zeichenketten stehen als ASCII mit
+`\uXXXX`, weil ein einsames Surrogat in UTF-8 sonst zu „?“ würde.
+
 ## E-Mail-Kontodateien
 
 `EmailFileCases server|client` legt für jede Zeile von
diff --git a/tools/java-oracle/run.sh b/tools/java-oracle/run.sh
index 892673f..089ebc7 100755
--- a/tools/java-oracle/run.sh
+++ b/tools/java-oracle/run.sh
@@ -21,7 +21,7 @@ if [ ! -f "$upstream/target/filius.jar" ]; then
 fi
 classpath="$upstream/target/filius.jar:$upstream/target/lib/*"
 mkdir -p "$here/.build"
-javac -encoding UTF-8 -d "$here/.build" -cp "$classpath" "$here/src/FlsOracle.java" "$here/src/GetterDefaults.java" "$here/src/IpCheckCases.java" "$here/src/DnsHostsCases.java" "$here/src/EmailFileCases.java"
+javac -encoding UTF-8 -d "$here/.build" -cp "$classpath" "$here/src/FlsOracle.java" "$here/src/GetterDefaults.java" "$here/src/IpCheckCases.java" "$here/src/DnsHostsCases.java" "$here/src/EmailFileCases.java" "$here/src/BeansDump.java"
 # FILIUS writes its settings below $HOME; keep them out of the real one.
 mkdir -p "$here/.build/home"
 # Removed first: a crashed run must not leave the previous result standing.
@@ -82,3 +82,9 @@ for kind in server client; do
     HOME="$here/.build/home" xvfb-run -a java -cp "$classpath:$here/.build" EmailFileCases "$kind" "$cases" "$output" 2>&1 | grep -v 'logback\|^$' || true
     test -s "$output"
 done
+
+# What XMLDecoder makes of the files packages/fls/src/javaBeansWriter.ts writes.
+beans="$root/packages/fls/test/beans"
+rm -f "$beans/beans.java.json"
+HOME="$here/.build/home" xvfb-run -a java -cp "$classpath:$here/.build" BeansDump "$beans" "$beans/beans.java.json" 2>&1 | grep -v 'logback\|^$' || true
+test -s "$beans/beans.java.json"
diff --git a/tools/java-oracle/src/BeansDump.java b/tools/java-oracle/src/BeansDump.java
new file mode 100644
index 0000000..c319087
--- /dev/null
+++ b/tools/java-oracle/src/BeansDump.java
@@ -0,0 +1,141 @@
+// Reads every *.xml of a directory with java.beans.XMLDecoder and describes
+// the objects it gets — the check that Java reads what
+// packages/fls/src/javaBeansWriter.ts writes. Errors the decoder would only
+// log are collected; strings are written escaped (see escaped). Beans are described by their read-write properties
+// (java.beans.Introspector), lists, maps, arrays, Rectangle and Color by
+// their content; a value met again is a reference to its first description.
+//
+// Usage: java -cp filius.jar:lib/*:classes BeansDump <directory> <output json>
+
+import java.awt.Color;
+import java.awt.Rectangle;
+import java.beans.Introspector;
+import java.beans.PropertyDescriptor;
+import java.beans.XMLDecoder;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.lang.reflect.Array;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.IdentityHashMap;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.TreeMap;
+import java.util.stream.Stream;
+
+public class BeansDump {
+
+    public static void main(String[] args) throws Exception {
+        Map<String, Object> result = new TreeMap<>();
+        List<Path> files;
+        try (Stream<Path> list = Files.list(Path.of(args[0]))) {
+            files = list.filter(file -> file.toString().endsWith(".xml")).sorted().toList();
+        }
+        for (Path file : files) {
+            List<Object> errors = new ArrayList<>();
+            List<Object> values = new ArrayList<>();
+            try (XMLDecoder decoder = new XMLDecoder(new FileInputStream(file.toFile()), null,
+                    error -> errors.add(error.getClass().getSimpleName() + ": " + error.getMessage()))) {
+                BeansDump dump = new BeansDump();
+                while (true) {
+                    Object value;
+                    try {
+                        value = decoder.readObject();
+                    } catch (ArrayIndexOutOfBoundsException end) {
+                        break;
+                    }
+                    values.add(dump.describe(value, 0));
+                }
+            }
+            Map<String, Object> entry = new LinkedHashMap<>();
+            entry.put("values", values);
+            entry.put("errors", errors);
+            result.put(file.getFileName().toString(), entry);
+        }
+        try (FileOutputStream out = new FileOutputStream(args[1])) {
+            out.write((FlsOracle.Json.write(result, "") + "\n").getBytes(StandardCharsets.UTF_8));
+        }
+        System.out.println("beans dump: " + files.size() + " files");
+        System.exit(0);
+    }
+
+    private final IdentityHashMap<Object, Integer> seen = new IdentityHashMap<>();
+
+    Object describe(Object value, int depth) throws Exception {
+        if (value == null) return null;
+        if (value instanceof Boolean flag) return flag;
+        if (value instanceof String text) return tagged("string", escaped(text));
+        if (value instanceof Integer number) return tagged("int", number);
+        if (value instanceof Short number) return tagged("short", (int) number);
+        if (value instanceof Byte number) return tagged("byte", (int) number);
+        if (value instanceof Long number) return tagged("long", number.toString());
+        if (value instanceof Double || value instanceof Float) return tagged(value.getClass().getSimpleName().toLowerCase(), value.toString());
+        if (value instanceof Character character) return tagged("char", (int) character.charValue());
+        if (value instanceof Class<?> type) return tagged("class", type.getName());
+        if (value instanceof Rectangle rectangle) {
+            return tagged("rectangle", List.of(rectangle.x, rectangle.y, rectangle.width, rectangle.height));
+        }
+        if (value instanceof Color color) {
+            return tagged("color", List.of(color.getRed(), color.getGreen(), color.getBlue(), color.getAlpha()));
+        }
+        Integer reference = seen.get(value);
+        if (reference != null) return tagged("ref", reference);
+        int id = seen.size();
+        seen.put(value, id);
+        Map<String, Object> json = new LinkedHashMap<>();
+        json.put("id", id);
+        json.put("class", value.getClass().getName());
+        if (value.getClass().isArray()) {
+            List<Object> items = new ArrayList<>();
+            for (int index = 0; index < Array.getLength(value); index++) items.add(describe(Array.get(value, index), depth + 1));
+            json.put("items", items);
+        } else if (value instanceof Collection<?> collection) {
+            List<Object> items = new ArrayList<>();
+            for (Object item : collection) items.add(describe(item, depth + 1));
+            json.put("items", items);
+        } else if (value instanceof Map<?, ?> map) {
+            List<Object> entries = new ArrayList<>();
+            for (Map.Entry<?, ?> entry : map.entrySet()) {
+                entries.add(Arrays.asList(describe(entry.getKey(), depth + 1), describe(entry.getValue(), depth + 1)));
+            }
+            json.put("entries", entries);
+        } else if (depth < 8) {
+            Map<String, Object> properties = new TreeMap<>();
+            for (PropertyDescriptor property : Introspector.getBeanInfo(value.getClass()).getPropertyDescriptors()) {
+                if (property.getReadMethod() == null || property.getWriteMethod() == null) continue;
+                try {
+                    properties.put(property.getName(), describe(property.getReadMethod().invoke(value), depth + 1));
+                } catch (Exception error) {
+                    properties.put(property.getName(), tagged("error", error.getClass().getSimpleName()));
+                }
+            }
+            json.put("properties", properties);
+        }
+        return json;
+    }
+
+    /**
+     * Every UTF-16 unit outside printable ASCII, and the backslash, as
+     * backslash-u and four hex digits, so that no encoding step can change
+     * a unit (a lone surrogate would become "?" in UTF-8).
+     */
+    static String escaped(String text) {
+        StringBuilder out = new StringBuilder();
+        for (char unit : text.toCharArray()) {
+            if (unit < 0x20 || unit > 0x7E || unit == '\\') out.append(String.format("\\u%04X", (int) unit));
+            else out.append(unit);
+        }
+        return out.toString();
+    }
+
+    private static Map<String, Object> tagged(String tag, Object value) {
+        Map<String, Object> json = new LinkedHashMap<>();
+        json.put(tag, value);
+        return json;
+    }
+}