Java class file: modules

From version 53.0 (Java SE 9), a module is declared in module-info.java, which compiles to module-info.class. That file uses the class file format but holds no class. Its class flags are just ACC_MODULE (0x8000), its this_class names module-info, super_class is 0, and it has no interfaces, fields or methods. Everything is in its attributes: a Module attribute, and usually ModulePackages and ModuleMainClass. Its constant pool may hold Module and Package entries, which no other class file may. The language side is on the Java packages and modules page.

Module

The module declaration: its name, flags and version, then five tables, one for each kind of directive.

Offset	Size	Type	Name	Meaning
6	2	u2	`module_name_index`	A Module entry: this module’s name
8	2	u2	`module_flags`	ACC_OPEN, ACC_SYNTHETIC, ACC_MANDATED
10	2	u2	`module_version_index`	A Utf8 entry: the version, or 0 for none
12	varies		requires	u2 `requires_count` and the requires entries
…	varies		exports	u2 `exports_count` and the exports entries
…	varies		opens	u2 `opens_count` and the opens entries
…	varies		uses	u2 `uses_count` and the uses indexes
…	varies		provides	u2 `provides_count` and the provides entries

requires

One 6-byte entry per requires directive. Every module except java.base must have exactly one entry for java.base; javac adds it, marked ACC_MANDATED, when the source does not say so.

Offset	Size	Type	Name	Meaning
0	2	u2	`requires_index`	A Module entry: the module depended on
2	2	u2	`requires_flags`	ACC_TRANSITIVE, ACC_STATIC_PHASE, ACC_SYNTHETIC, ACC_MANDATED
4	2	u2	`requires_version_index`	A Utf8 entry: the version of the module compiled against, or 0

exports

One entry per exports directive: a package whose public types other modules may use.

Offset	Size	Type	Name	Meaning
0	2	u2	`exports_index`	A Package entry
2	2	u2	`exports_flags`	ACC_SYNTHETIC, ACC_MANDATED
4	2	u2	`exports_to_count`	0 to export to every module; otherwise the number of modules that follow
6	2 × n	u2[]	`exports_to_index`	Module entries: the only modules it is exported to (`exports … to …`)

opens

One entry per opens directive: a package whose types, private members included, are open to reflection at run time. The layout is the same as exports, with opens_index, opens_flags, opens_to_count and opens_to_index. An open module (ACC_OPEN) has no opens entries.

uses

The services this module looks up with java.util.ServiceLoader.

Offset	Size	Type	Name	Meaning
0	2	u2	`uses_count`	The number of services
2	2 × n	u2[]	`uses_index`	Class entries: the service interfaces or classes

provides

One entry per provides … with … directive: implementations of a service that this module offers.

Offset	Size	Type	Name	Meaning
0	2	u2	`provides_index`	A Class entry: the service
2	2	u2	`provides_with_count`	The number of implementations, at least 1
4	2 × n	u2[]	`provides_with_index`	Class entries: the implementing classes

ModulePackages

Every package of the module, exported, opened or not. The compiler or the jar tool writes it, so the JVM need not scan the module’s files.

Offset	Size	Type	Name	Meaning
6	2	u2	`package_count`	The number of packages
8	2 × n	u2[]	`package_index`	Package entries

ModuleMainClass

The class java --module runs when no class is named. The length is always 2.

Offset	Size	Type	Name	Meaning
6	2	u2	`main_class_index`	A Class entry: the main class

The JDK’s own attributes

The JDK’s build tools write three more attributes into the module-info files of the JDK’s own modules. They are not in the JVM Specification, and other module-info files do not normally have them. The layouts below are the ones the explorer reads, from the OpenJDK sources; they were not checked against real files for this page. The explorer reads them but never turns a file down because of them.

ModuleHashes

Hashes of other modules that this one is tied to, so that they can be checked when the modules are linked.

Offset	Size	Type	Name	Meaning
6	2	u2	`algorithm_index`	A Utf8 entry: the hash algorithm, such as `SHA-256`
8	2	u2	`hashes_table_length`	The number of modules
…	2	u2	`module_name_index`	A Module entry
…	2	u2	`hash_length`	The length of the hash
…	hash_length	u1[]	`hash`	The hash

The last three fields repeat for each module.

ModuleResolution

How the module is treated when modules are resolved. The length is always 2.

Offset	Size	Type	Name	Meaning
6	2	u2	`resolution_flags`	0x0001 DO_NOT_RESOLVE_BY_DEFAULT, 0x0002 WARN_DEPRECATED, 0x0004 WARN_DEPRECATED_FOR_REMOVAL, 0x0008 WARN_INCUBATING

Incubator modules, such as those named jdk.incubator.…, carry DO_NOT_RESOLVE_BY_DEFAULT and WARN_INCUBATING.

ModuleTarget

The platform a module with native code was built for. The length is always 2.

Offset	Size	Type	Name	Meaning
6	2	u2	`target_platform_index`	A Utf8 entry: the platform, such as `macos-aarch64` or `linux-x64`

.CLASS File Format – Modules