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-->1.3.0<!-- -->).</div></div><div class=docItemContainer_Djhp><article><nav class="theme-doc-breadcrumbs breadcrumbsContainer_Z_bl" aria-label=Breadcrumbs><ul class=breadcrumbs><li class=breadcrumbs__item><a aria-label="Home page" class=breadcrumbs__link href=/><svg viewBox="0 0 24 24" class=breadcrumbHomeIcon_YNFT><path d="M10 19v-5h4v5c0 .55.45 1 1 1h3c.55 0 1-.45 1-1v-7h1.7c.46 0 .68-.57.33-.87L12.67 3.6c-.38-.34-.96-.34-1.34 0l-8.36 7.53c-.34.3-.13.87.33.87H5v7c0 .55.45 1 1 1h3c.55 0 1-.45 1-1z" fill=currentColor /></svg></a><li class="breadcrumbs__item breadcrumbs__item--active"><span class=breadcrumbs__link>Java Serialization Format</span></ul></nav><span class="theme-doc-version-badge badge badge--secondary">Version: 0.17</span><div class="tocCollapsible_ETCw theme-doc-toc-mobile tocMobile_ITEo"><button type=button class="clean-btn tocCollapsibleButton_TO0P">On this page</button></div><div class="theme-doc-markdown markdown"><header><h1>Java Serialization Format</h1></header><h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=spec-overview>Spec overview<a href=#spec-overview class=hash-link aria-label="Direct link to Spec overview" title="Direct link to Spec overview" translate=no>​</a></h2>
<p>Apache Fory Java serialization is a dynamic binary format for Java object graphs. It supports
shared references, circular references, polymorphism, and optional schema evolution. The format is
stream friendly: shared type metadata is written inline when needed and there is no meta start
offset.</p>
<p>The Java native format is an extension of the xlang wire format and reuses the same core framing
and encodings; see <code>docs/specification/xlang_serialization_spec.md</code> for the shared baseline.</p>
<p>Overall layout:</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| fory header | object ref meta | object type meta | object value data |</span><br/></div></code></pre></div></div>
<p>All data is encoded in little endian byte order. When running on a big endian platform, array
serializers swap byte order on write/read so the on-wire layout remains little endian.</p>
<h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=fory-header>Fory header<a href=#fory-header class=hash-link aria-label="Direct link to Fory header" title="Direct link to Fory header" translate=no>​</a></h2>
<p>Java native serialization writes a one byte bitmap header. The header layout mirrors the xlang
bitmap and uses the same flag bits.</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| 5 bits | 1 bit | 1 bit | 1 bit |</span><br/></div><div class=token-line style=color:#393A34><span class="token plain">+--------------+-------+-------+-------+</span><br/></div><div class=token-line style=color:#393A34><span class="token plain">| reserved | oob | xlang | null |</span><br/></div></code></pre></div></div>
<ul>
<li class="">null flag: 1 when object is null, 0 otherwise. If object is null, other bits are not set.</li>
<li class="">xlang flag: 1 when serialization uses xlang format, 0 when serialization uses Java native format.</li>
<li class="">oob flag: 1 when <code>BufferCallback</code> is not null, 0 otherwise.</li>
</ul>
<p>The header is always a single byte; no language ID is written.</p>
<h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=reference-meta>Reference meta<a href=#reference-meta class=hash-link aria-label="Direct link to Reference meta" title="Direct link to Reference meta" translate=no>​</a></h2>
<p>Reference tracking uses the same flags as the xlang specification.</p>
<table><thead><tr><th>Flag<th>Byte Value<th>Description<tbody><tr><td>NULL FLAG<td><code>-3</code><td>Object is null. No further bytes are written for this object.<tr><td>REF FLAG<td><code>-2</code><td>Object was already serialized. Followed by unsigned varint32 reference ID.<tr><td>NOT_NULL VALUE FLAG<td><code>-1</code><td>Object is non-null but reference tracking is disabled for this type. Object data follows immediately.<tr><td>REF VALUE FLAG<td><code>0</code><td>Object is referencable and this is its first occurrence. Object data follows. Assigns next reference ID.</table>
<p>When reference tracking is disabled globally or for a specific field/type, only <code>NULL FLAG</code> and
<code>NOT_NULL VALUE FLAG</code> are used.</p>
<h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=type-system-and-type-ids>Type system and type IDs<a href=#type-system-and-type-ids class=hash-link aria-label="Direct link to Type system and type IDs" title="Direct link to Type system and type IDs" translate=no>​</a></h2>
<p>Java native serialization uses the unified type ID layout shared with xlang:</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">full_type_id = (user_type_id &lt;&lt; 8) | internal_type_id</span><br/></div></code></pre></div></div>
<ul>
<li class=""><code>internal_type_id</code> is the low 8 bits describing the kind (enum/struct/ext, named variants, or a
built-in type).</li>
<li class=""><code>user_type_id</code> is the numeric registration ID (0-based) for user-defined enum/struct/ext types.</li>
<li class="">Named types use <code>NAMED_*</code> internal IDs and carry names in metadata rather than embedding a user
ID.</li>
</ul>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=shared-internal-type-ids-0-63>Shared internal type IDs (0-63)<a href=#shared-internal-type-ids-0-63 class=hash-link aria-label="Direct link to Shared internal type IDs (0-63)" title="Direct link to Shared internal type IDs (0-63)" translate=no>​</a></h3>
<p>Java native mode shares the xlang internal IDs for all values below 64. IDs <code>0~56</code> are defined by
the xlang spec, while <code>57~63</code> are reserved for future internal use. These IDs are stable across
languages.</p>
<p>See the internal type ID table in
<a class="" href=/docs/0.17/specification/xlang_serialization_spec#internal-type-id-table>Xlang Serialization Format</a>.
Java shares all IDs <code>&lt; 64</code>, with <code>57~63</code> reserved for future internal use.</p>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=java-native-built-in-type-ids>Java native built-in type IDs<a href=#java-native-built-in-type-ids class=hash-link aria-label="Direct link to Java native built-in type IDs" title="Direct link to Java native built-in type IDs" translate=no>​</a></h3>
<p>Java native serialization assigns Java-specific built-ins starting at
<code>Types.BOUND + 5</code> (<code>Types.BOUND</code> is 64; 5 IDs are reserved for future use).
Type IDs in <code>0~56</code> are shared with xlang; <code>57~63</code> are reserved; <code>64+</code> are only
valid in Java native mode.</p>
<table><thead><tr><th>Type ID<th>Name<th>Description<tbody><tr><td>69<td>VOID_ID<td>java.lang.Void<tr><td>70<td>CHAR_ID<td>java.lang.Character<tr><td>71<td>PRIMITIVE_VOID_ID<td>void<tr><td>72<td>PRIMITIVE_BOOL_ID<td>boolean<tr><td>73<td>PRIMITIVE_INT8_ID<td>byte<tr><td>74<td>PRIMITIVE_CHAR_ID<td>char<tr><td>75<td>PRIMITIVE_INT16_ID<td>short<tr><td>76<td>PRIMITIVE_INT32_ID<td>int<tr><td>77<td>PRIMITIVE_FLOAT32_ID<td>float<tr><td>78<td>PRIMITIVE_INT64_ID<td>long<tr><td>79<td>PRIMITIVE_FLOAT64_ID<td>double<tr><td>80<td>PRIMITIVE_BOOLEAN_ARRAY_ID<td>boolean[]<tr><td>81<td>PRIMITIVE_BYTE_ARRAY_ID<td>byte[]<tr><td>82<td>PRIMITIVE_CHAR_ARRAY_ID<td>char[]<tr><td>83<td>PRIMITIVE_SHORT_ARRAY_ID<td>short[]<tr><td>84<td>PRIMITIVE_INT_ARRAY_ID<td>int[]<tr><td>85<td>PRIMITIVE_FLOAT_ARRAY_ID<td>float[]<tr><td>86<td>PRIMITIVE_LONG_ARRAY_ID<td>long[]<tr><td>87<td>PRIMITIVE_DOUBLE_ARRAY_ID<td>double[]<tr><td>88<td>STRING_ARRAY_ID<td>String[]<tr><td>89<td>OBJECT_ARRAY_ID<td>Object[]<tr><td>90<td>ARRAYLIST_ID<td>java.util.ArrayList<tr><td>91<td>HASHMAP_ID<td>java.util.HashMap<tr><td>92<td>HASHSET_ID<td>java.util.HashSet<tr><td>93<td>CLASS_ID<td>java.lang.Class<tr><td>94<td>EMPTY_OBJECT_ID<td>empty object stub<tr><td>95<td>LAMBDA_STUB_ID<td>lambda stub<tr><td>96<td>JDK_PROXY_STUB_ID<td>JDK proxy stub<tr><td>97<td>REPLACE_STUB_ID<td>writeReplace/readResolve stub<tr><td>98<td>NONEXISTENT_META_SHARED_ID<td>meta-shared unknown class stub</table>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=registration-and-named-types>Registration and named types<a href=#registration-and-named-types class=hash-link aria-label="Direct link to Registration and named types" title="Direct link to Registration and named types" translate=no>​</a></h3>
<p>User-defined enum/struct/ext types can be registered by numeric ID or by name.</p>
<ul>
<li class="">Numeric registration: <code>full_type_id = (user_id &lt;&lt; 8) | internal_type_id</code>.</li>
<li class="">Name registration: type meta uses namespace and type name (see below).</li>
<li class="">Unregistered types are encoded as named types using namespace = package name and type name =
simple class name.</li>
</ul>
<p>Named type selection rules for unregistered types:</p>
<ul>
<li class="">enum -> NAMED_ENUM</li>
<li class="">struct-like serializers -> NAMED_STRUCT (or NAMED_COMPATIBLE_STRUCT in compatible mode)</li>
<li class="">all other custom serializers -> NAMED_EXT</li>
</ul>
<h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=type-meta-encoding>Type meta encoding<a href=#type-meta-encoding class=hash-link aria-label="Direct link to Type meta encoding" title="Direct link to Type meta encoding" translate=no>​</a></h2>
<p>Every value is written with a type ID followed by optional type metadata:</p>
<ol>
<li class="">Write <code>type_id</code> using varuint32 small7 encoding.</li>
<li class="">For <code>NAMED_ENUM</code>, <code>NAMED_STRUCT</code>, <code>NAMED_EXT</code>, <code>NAMED_COMPATIBLE_STRUCT</code>:<!-- -->
<ul>
<li class="">If meta share is enabled: write shared class meta (streaming format).</li>
<li class="">Otherwise: write namespace and type name as meta strings.</li>
</ul>
</li>
<li class="">For <code>COMPATIBLE_STRUCT</code>:<!-- -->
<ul>
<li class="">If meta share is enabled: write shared class meta (streaming format).</li>
<li class="">Otherwise: no extra meta (type ID is sufficient).</li>
</ul>
</li>
<li class="">All other types: no extra meta.</li>
</ol>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=shared-class-meta-streaming>Shared class meta (streaming)<a href=#shared-class-meta-streaming class=hash-link aria-label="Direct link to Shared class meta (streaming)" title="Direct link to Shared class meta (streaming)" translate=no>​</a></h3>
<p>When meta share is enabled, Java uses the streaming shared meta protocol and writes TypeDef
bytes inline on first use.</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| varuint32: index_marker | [class def bytes if new] |</span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain">index_marker = (index &lt;&lt; 1) | flag</span><br/></div><div class=token-line style=color:#393A34><span class="token plain">flag = 1 -> reference</span><br/></div><div class=token-line style=color:#393A34><span class="token plain">flag = 0 -> new type</span><br/></div></code></pre></div></div>
<ul>
<li class="">If <code>flag == 1</code>, this is a reference to a previously written type. No class def bytes follow.</li>
<li class="">If <code>flag == 0</code>, this is a new type definition and class def bytes are written inline.</li>
</ul>
<p>The index is assigned sequentially in the order types are first encountered.</p>
<h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=schema-modes>Schema modes<a href=#schema-modes class=hash-link aria-label="Direct link to Schema modes" title="Direct link to Schema modes" translate=no>​</a></h2>
<p>Java native serialization supports two schema modes:</p>
<ul>
<li class="">Schema consistent (compatible mode disabled): fields are serialized in a fixed order and no
ClassDef is required. Type meta uses <code>STRUCT</code> or <code>NAMED_STRUCT</code> for user-defined classes.</li>
<li class="">Schema evolution (compatible mode enabled): fields are serialized with schema evolution metadata
(ClassDef). Type meta uses <code>COMPATIBLE_STRUCT</code> or <code>NAMED_COMPATIBLE_STRUCT</code>.</li>
</ul>
<h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=classdef-format-compatible-mode>ClassDef format (compatible mode)<a href=#classdef-format-compatible-mode class=hash-link aria-label="Direct link to ClassDef format (compatible mode)" title="Direct link to ClassDef format (compatible mode)" translate=no>​</a></h2>
<p>ClassDef is the schema evolution metadata encoded for compatible structs. It is written inline
when shared meta is enabled, or referenced by index when already seen.</p>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=binary-layout>Binary layout<a href=#binary-layout class=hash-link aria-label="Direct link to Binary layout" title="Direct link to Binary layout" translate=no>​</a></h3>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| 8 bytes header | [varuint32 extra size] | class meta bytes |</span><br/></div></code></pre></div></div>
<p>Header layout (lower bits on the right):</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| 50-bit hash | 4 bits reserved | 1 bit compress | 1 bit has_fields_meta | 8-bit size |</span><br/></div></code></pre></div></div>
<ul>
<li class="">size: lower 8 bits. If size equals the mask (0xFF), write extra size as varuint32 and add it.</li>
<li class="">compress: set when payload is compressed.</li>
<li class="">has_fields_meta: set when field metadata is present.</li>
<li class="">reserved: bits 10-13 are reserved for future use and must be zero.</li>
<li class="">hash: 50-bit hash of the payload and flags.</li>
</ul>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=class-meta-bytes>Class meta bytes<a href=#class-meta-bytes class=hash-link aria-label="Direct link to Class meta bytes" title="Direct link to Class meta bytes" translate=no>​</a></h3>
<p>Class meta encodes a linearized class hierarchy (from parent to leaf) and field metadata:</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| num_classes | class_layer_0 | class_layer_1 | ... |</span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain">class_layer:</span><br/></div><div class=token-line style=color:#393A34><span class="token plain">| num_fields &lt;&lt; 1 | registered_flag | [type_id if registered] |</span><br/></div><div class=token-line style=color:#393A34><span class="token plain">| namespace | type_name | field_infos |</span><br/></div></code></pre></div></div>
<ul>
<li class=""><code>num_classes</code> stores <code>(num_layers - 1)</code> in a single byte.<!-- -->
<ul>
<li class="">If it equals <code>0b1111</code>, read an extra varuint32 small7 and add it.</li>
<li class="">The actual number of layers is <code>num_classes + 1</code>.</li>
</ul>
</li>
<li class=""><code>registered_flag</code> is 1 if the class is registered by numeric ID.</li>
<li class="">If registered by ID, the class type ID follows (varuint32 small7).</li>
<li class="">If registered by name or unregistered, namespace and type name are written as meta strings.</li>
</ul>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=field-info>Field info<a href=#field-info class=hash-link aria-label="Direct link to Field info" title="Direct link to Field info" translate=no>​</a></h3>
<p>Each field uses a compact header followed by its name bytes (omitted when TAG_ID is used) and its
type info:</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| field_header | [field_name_bytes] | field_type |</span><br/></div></code></pre></div></div>
<p><code>field_header</code> bits:</p>
<ul>
<li class="">bit 0: trackingRef</li>
<li class="">bit 1: nullable</li>
<li class="">bits 2-3: field name encoding</li>
<li class="">bits 4-6: name length (len-1), or tag ID when TAG_ID is used; value 7 indicates extended length</li>
<li class="">bit 7: reserved (0)</li>
</ul>
<p>Field name encoding:</p>
<ul>
<li class="">0: UTF8</li>
<li class="">1: ALL_TO_LOWER_SPECIAL</li>
<li class="">2: LOWER_UPPER_DIGIT_SPECIAL</li>
<li class="">3: TAG_ID (field name omitted, tag ID stored in size field)</li>
</ul>
<p>If length is extended (size==7), an extra varuint32 small7 storing <code>(len-1) - 7</code> follows.</p>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=field-type-encoding>Field type encoding<a href=#field-type-encoding class=hash-link aria-label="Direct link to Field type encoding" title="Direct link to Field type encoding" translate=no>​</a></h3>
<p>Field types are encoded with a type tag and optional nested type info. For nested types, the header
includes nullable/trackingRef flags in the low bits.
Top-level field types use the tag only (no flags).</p>
<p>Type tags:</p>
<table><thead><tr><th>Tag<th>Field type<tbody><tr><td>0<td>Object (ObjectFieldType)<tr><td>1<td>Map (MapFieldType)<tr><td>2<td>Collection/List/Set (CollectionFieldType)<tr><td>3<td>Array (ArrayFieldType)<tr><td>4<td>Enum (EnumFieldType)<tr><td>5+<td>Registered type (RegisteredFieldType)</table>
<p>Encoding rules:</p>
<ul>
<li class="">ObjectFieldType: write tag 0.</li>
<li class="">MapFieldType: write tag 1, then key type, then value type.</li>
<li class="">CollectionFieldType: write tag 2, then element type.</li>
<li class="">ArrayFieldType: write tag 3, then dimensions, then component type.</li>
<li class="">EnumFieldType: write tag 4.</li>
<li class="">RegisteredFieldType: write tag <code>5 + type_id</code>.</li>
</ul>
<p>For nested types, nullable/trackingRef flags are stored in the low bits of the header as
<code>(type_tag &lt;&lt; 2) | (nullable &lt;&lt; 1) | tracking_ref</code>.</p>
<h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=meta-string-encoding>Meta string encoding<a href=#meta-string-encoding class=hash-link aria-label="Direct link to Meta string encoding" title="Direct link to Meta string encoding" translate=no>​</a></h2>
<p>Namespace, type names, and field names use the same meta string encodings as the xlang spec.</p>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=package-and-type-names>Package and type names<a href=#package-and-type-names class=hash-link aria-label="Direct link to Package and type names" title="Direct link to Package and type names" translate=no>​</a></h3>
<p>Header format:</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| 6 bits size | 2 bits encoding |</span><br/></div></code></pre></div></div>
<ul>
<li class="">size is the byte length of the encoded name.</li>
<li class="">if size == 63, write extra length <code>(size - 63)</code> as varuint32 small7.</li>
</ul>
<p>Encodings:</p>
<ul>
<li class="">Package name: UTF8, ALL_TO_LOWER_SPECIAL, LOWER_UPPER_DIGIT_SPECIAL</li>
<li class="">Type name: UTF8, LOWER_UPPER_DIGIT_SPECIAL, FIRST_TO_LOWER_SPECIAL, ALL_TO_LOWER_SPECIAL</li>
</ul>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=field-names>Field names<a href=#field-names class=hash-link aria-label="Direct link to Field names" title="Direct link to Field names" translate=no>​</a></h3>
<p>Field name encoding is described in the ClassDef field header section. When using TAG_ID, the
field name bytes are omitted and the tag ID is stored in the size field.</p>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=encoding-algorithms>Encoding algorithms<a href=#encoding-algorithms class=hash-link aria-label="Direct link to Encoding algorithms" title="Direct link to Encoding algorithms" translate=no>​</a></h3>
<p>See the xlang specification for encoding algorithms and tables:
<code>docs/specification/xlang_serialization_spec.md#meta-string</code>.</p>
<h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=value-encodings>Value encodings<a href=#value-encodings class=hash-link aria-label="Direct link to Value encodings" title="Direct link to Value encodings" translate=no>​</a></h2>
<p>This section describes the byte layouts for common built-in serializers used in Java native
serialization. Custom serializers (EXT) may define additional formats but must still follow the
reference and type meta rules described above.</p>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=primitives>Primitives<a href=#primitives class=hash-link aria-label="Direct link to Primitives" title="Direct link to Primitives" translate=no>​</a></h3>
<ul>
<li class="">boolean: 1 byte (0x00 or 0x01).</li>
<li class="">byte: 1 byte.</li>
<li class="">short: 2 bytes little endian.</li>
<li class="">char: 2 bytes little endian (UTF-16 code unit).</li>
<li class="">int:<!-- -->
<ul>
<li class="">fixed: 4 bytes little endian.</li>
<li class="">varint: signed varint32 (ZigZag) when <code>compressInt</code> is enabled.</li>
</ul>
</li>
<li class="">long:<!-- -->
<ul>
<li class="">fixed: 8 bytes little endian.</li>
<li class="">varint: signed varint64 (ZigZag) when <code>longEncoding=VARINT</code>.</li>
<li class="">tagged: tagged int64 when <code>longEncoding=TAGGED</code>.</li>
</ul>
</li>
<li class="">float: IEEE 754 float32, little endian.</li>
<li class="">double: IEEE 754 float64, little endian.</li>
</ul>
<p>Varint encodings follow the xlang spec:
<code>docs/specification/xlang_serialization_spec.md#unsigned-varint32</code>.</p>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=string>String<a href=#string class=hash-link aria-label="Direct link to String" title="Direct link to String" translate=no>​</a></h3>
<p>Strings are encoded as:</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| varuint36_small: (num_bytes &lt;&lt; 2) | coder | string bytes |</span><br/></div></code></pre></div></div>
<ul>
<li class="">coder: 2-bit value<!-- -->
<ul>
<li class="">0: LATIN1</li>
<li class="">1: UTF16</li>
<li class="">2: UTF8</li>
</ul>
</li>
<li class="">num_bytes: byte length of the encoded string payload.</li>
</ul>
<p>UTF16 is encoded as little endian 2-byte code units.</p>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=enum>Enum<a href=#enum class=hash-link aria-label="Direct link to Enum" title="Direct link to Enum" translate=no>​</a></h3>
<ul>
<li class="">If <code>serializeEnumByName</code> is enabled: write enum name as a meta string.</li>
<li class="">Otherwise: write an enum tag as varuint32 small7.<!-- -->
<ul>
<li class="">By default the tag is the declaration ordinal.</li>
<li class="">If the enum configures <code>@ForyEnumId</code>, write the configured stable id instead. Java supports
annotating exactly one id field, exactly one zero-argument id getter, or every enum constant
with explicit tag values.</li>
</ul>
</li>
</ul>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=binary-byte>Binary (byte[])<a href=#binary-byte class=hash-link aria-label="Direct link to Binary (byte[])" title="Direct link to Binary (byte[])" translate=no>​</a></h3>
<p>Primitive byte arrays are encoded as:</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| varuint32: num_bytes | raw bytes |</span><br/></div></code></pre></div></div>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=primitive-arrays>Primitive arrays<a href=#primitive-arrays class=hash-link aria-label="Direct link to Primitive arrays" title="Direct link to Primitive arrays" translate=no>​</a></h3>
<p>Primitive arrays use <code>writePrimitiveArrayWithSize</code> unless compression is enabled:</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| varuint32: byte_length | raw bytes |</span><br/></div></code></pre></div></div>
<ul>
<li class=""><code>compressIntArray</code>: int[] encoded as <code>| varuint32: length | varint32... |</code>.</li>
<li class=""><code>compressLongArray</code>: long[] encoded as <code>| varuint32: length | varint64/tagged... |</code>.</li>
</ul>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=object-arrays>Object arrays<a href=#object-arrays class=hash-link aria-label="Direct link to Object arrays" title="Direct link to Object arrays" translate=no>​</a></h3>
<p>Object arrays encode length and a monomorphic flag:</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| varuint32_small7: (length &lt;&lt; 1) | mono_flag |</span><br/></div></code></pre></div></div>
<ul>
<li class="">If <code>mono_flag == 1</code>, all elements share a known component serializer. Each element uses ref
flags and the component serializer writes the value.</li>
<li class="">If <code>mono_flag == 0</code>, each element uses ref flags and writes its own class info and data.</li>
</ul>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=collections-listset>Collections (List/Set)<a href=#collections-listset class=hash-link aria-label="Direct link to Collections (List/Set)" title="Direct link to Collections (List/Set)" translate=no>​</a></h3>
<p>Collections encode length and a one-byte elements header:</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| varuint32_small7: length | elements_header | [elem_class_info] | elements... |</span><br/></div></code></pre></div></div>
<p><code>elements_header</code> bits (see <code>CollectionFlags</code>):</p>
<ul>
<li class="">bit 0: TRACKING_REF</li>
<li class="">bit 1: HAS_NULL</li>
<li class="">bit 2: IS_DECL_ELEMENT_TYPE</li>
<li class="">bit 3: IS_SAME_TYPE</li>
</ul>
<p>If <code>IS_SAME_TYPE</code> is set and <code>IS_DECL_ELEMENT_TYPE</code> is not set, the element class info is written
once before the elements. Element values then follow with either ref flags (if TRACKING_REF) or
per-element null flags (if HAS_NULL).</p>
<p>If <code>IS_SAME_TYPE</code> is not set, each element is written with its own class info and data (and
optionally ref flags).</p>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=maps>Maps<a href=#maps class=hash-link aria-label="Direct link to Maps" title="Direct link to Maps" translate=no>​</a></h3>
<p>Maps encode entry count and then a sequence of chunks. Each chunk groups entries that share key
and value types.</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| varuint32_small7: size | chunk_1 | chunk_2 | ... |</span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain">chunk (non-null entries):</span><br/></div><div class=token-line style=color:#393A34><span class="token plain">| header | chunk_size | [key_class_info] | [value_class_info] | entries... |</span><br/></div></code></pre></div></div>
<p><code>header</code> bits (see <code>MapFlags</code>):</p>
<ul>
<li class="">bit 0: TRACKING_KEY_REF</li>
<li class="">bit 1: KEY_HAS_NULL</li>
<li class="">bit 2: KEY_DECL_TYPE</li>
<li class="">bit 3: TRACKING_VALUE_REF</li>
<li class="">bit 4: VALUE_HAS_NULL</li>
<li class="">bit 5: VALUE_DECL_TYPE</li>
</ul>
<p>If <code>KEY_DECL_TYPE</code> or <code>VALUE_DECL_TYPE</code> is unset, the corresponding class info is written once at
the start of the chunk. <code>chunk_size</code> is a single byte (1..255) and <code>MAX_CHUNK_SIZE</code> is 255.</p>
<h4 class="anchor anchorTargetStickyNavbar_Vzrq" id=null-keyvalue-entries>Null key/value entries<a href=#null-keyvalue-entries class=hash-link aria-label="Direct link to Null key/value entries" title="Direct link to Null key/value entries" translate=no>​</a></h4>
<p>Entries with null key or null value are encoded as special single-entry chunks without a
<code>chunk_size</code> byte:</p>
<ul>
<li class="">null key, non-null value: <code>NULL_KEY_VALUE_DECL_TYPE*</code> flags, then value payload</li>
<li class="">null value, non-null key: <code>NULL_VALUE_KEY_DECL_TYPE*</code> flags, then key payload</li>
<li class="">null key and null value: <code>KV_NULL</code> header only</li>
</ul>
<p>These chunks always represent exactly one entry.</p>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=objects-and-structs>Objects and structs<a href=#objects-and-structs class=hash-link aria-label="Direct link to Objects and structs" title="Direct link to Objects and structs" translate=no>​</a></h3>
<p>Object values are encoded as:</p>
<div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">| ref meta | type meta | field data |</span><br/></div></code></pre></div></div>
<p>Field data is written by the serializer selected by the class info. For standard object
serialization:</p>
<ul>
<li class="">Fields are sorted deterministically using <code>DescriptorGrouper</code> order:
primitives, boxed primitives, built-ins, collections, maps, then other fields, with names sorted
within each category.</li>
<li class="">For compatible mode, <code>MetaSharedSerializer</code> uses ClassDef field metadata to read and skip
unknown fields.</li>
<li class="">For each field, the serializer uses field metadata (nullable, trackingRef, polymorphic) to decide
whether to write ref flags and/or type meta before the field value.</li>
</ul>
<h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=extensions-ext>Extensions (EXT)<a href=#extensions-ext class=hash-link aria-label="Direct link to Extensions (EXT)" title="Direct link to Extensions (EXT)" translate=no>​</a></h3>
<p>Extension types are encoded by their registered serializer. Type meta is still written before the
value as described above. The serializer is responsible for the value layout.</p>
<h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=out-of-band-buffers>Out-of-band buffers<a href=#out-of-band-buffers class=hash-link aria-label="Direct link to Out-of-band buffers" title="Direct link to Out-of-band buffers" translate=no>​</a></h2>
<p>When a <code>BufferCallback</code> is provided, the oob flag is set in the header and serializers may emit
buffer references instead of inline bytes (for example, large primitive arrays). The out-of-band
buffer protocol is specific to the callback implementation; the main stream only contains
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