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/**

 * Classes to support module descriptors and creating configurations of modules

 * by means of resolution and service binding.

 *

 * <p> Unless otherwise noted, passing a {@code null} argument to a constructor

 * or method of any class or interface in this package will cause a {@link

 * java.lang.NullPointerException NullPointerException} to be thrown. Additionally,

 * invoking a method with an array or collection containing a {@code null} element

 * will cause a {@code NullPointerException}, unless otherwise specified. </p>

 *

 *

 * <h2><a id="resolution"></a>{@index "Module Resolution"}</h2>

 *

 * <p> Resolution is the process of computing how modules depend on each other.

 * The process occurs at compile time and run time. </p>

 *

 * <p> Resolution is a two-step process. The first step recursively enumerates

 * the 'requires' directives of a set of root modules. If all the enumerated

 * modules are observable, then the second step computes their readability graph.

 * The readability graph embodies how modules depend on each other, which in

 * turn controls access across module boundaries. </p>

 *

 * <h3> Step 1: Recursive enumeration </h3>

 *

 * <p> Recursive enumeration takes a set of module names, looks up each of their

 * module declarations, and for each module declaration, recursively enumerates:

 *

 * <ul>

 *   <li> <p> the module names given by the 'requires' directives with the

 *   'transitive' modifier, and </p></li>

 *   <li> <p> at the discretion of the host system, the module names given by

 *   the 'requires' directives without the 'transitive' modifier. </p></li>

 * </ul>

 *

 * <p> Module declarations are looked up in a set of observable modules. The set

 * of observable modules is determined in an implementation specific manner. The

 * set of observable modules may include modules with explicit declarations

 * (that is, with a {@code module-info.java} source file or {@code module-info.class}

 * file) and modules with implicit declarations (that is,

 * <a href="ModuleFinder.html#automatic-modules">automatic modules</a>).

 * Because an automatic module has no explicit module declaration, it has no

 * 'requires' directives of its own, although its name may be given by a

 * 'requires' directive of an explicit module declaration. </p>

 *

 * <p> The set of root modules, whose names are the initial input to this

 * algorithm, is determined in an implementation specific manner. The set of

 * root modules may include automatic modules. </p>

 *

 * <p> If at least one automatic module is enumerated by this algorithm, then

 * every observable automatic module must be enumerated, regardless of whether

 * any of their names are given by 'requires' directives of explicit module

 * declarations. </p>

 *

 * <p> If any of the following conditions occur, then resolution fails:

 * <ul>

 *   <li><p> Any root module is not observable. </p></li>

 *   <li><p> Any module whose name is given by a 'requires' directive with the

 *   'transitive' modifier is not observable. </p></li>

 *   <li><p> At the discretion of the host system, any module whose name is given

 *   by a 'requires' directive without the 'transitive' modifier is not

 *   observable. </p></li>

 *   <li><p> The algorithm in this step enumerates the same module name twice. This

 *   indicates a cycle in the 'requires' directives, disregarding any 'transitive'

 *   modifiers. </p></li>

 * </ul>

 *

 * <p> Otherwise, resolution proceeds to step 2. </p>

 *

 * <h3> Step 2: Computing the readability graph </h3>

 *

 * <p> A 'requires' directive (irrespective of 'transitive') expresses that

 * one module depends on some other module. The effect of the 'transitive'

 * modifier is to cause additional modules to also depend on the other module.

 * If module M 'requires transitive N', then not only does M depend on N, but

 * any module that depends on M also depends on N. This allows M to be

 * refactored so that some or all of its content can be moved to a new module N

 * without breaking modules that have a 'requires M' directive. </p>

 *

 * <p> Module dependencies are represented by the readability graph. The

 * readability graph is a directed graph whose vertices are the modules

 * enumerated in step 1 and whose edges represent readability between pairs of

 * modules. The edges are specified as follows:

 *

 * <p> First, readability is determined by the 'requires' directives of the

 * enumerated modules, disregarding any 'transitive' modifiers:

 *

 * <ul>

 *   <li><p> For each enumerated module A that 'requires' B: A "reads" B. </p></li>

 *   <li><p> For each enumerated module X that is automatic: X "reads" every

 *   other enumerated module (it is "as if" an automatic module has 'requires'

 *   directives for every other enumerated module). </p></li>

 * </ul>

 *

 * <p> Second, readability is augmented to account for 'transitive' modifiers:

 * <ul>

 *   <li> <p> For each enumerated module A that "reads" B: </p>

 *     <ul>

 *     <li><p> If B 'requires transitive' C, then A "reads" C as well as B. This

 *     augmentation is recursive: since A "reads" C, if C 'requires transitive'

 *     D, then A "reads" D as well as C and B. </p></li>

 *     <li><p> If B is an automatic module, then A "reads" every other enumerated

 *     automatic module. (It is "as if" an automatic module has 'requires transitive'

 *     directives for every other enumerated automatic module).</p> </li>

 *     </ul>

 *   </li>

 * </ul>

 *

 * <p> Finally, every module "reads" itself. </p>

 *

 * <p> If any of the following conditions occur in the readability graph, then

 * resolution fails:

 * <ul>

 *   <li><p> A module "reads" two or more modules with the same name. This includes

 *   the case where a module "reads" another with the same name as itself. </p></li>

 *   <li><p> Two or more modules export a package with the same name to a module

 *   that "reads" both. This includes the case where a module M containing package

 *   p "reads" another module that exports p to M. </p></li>

 *   <li><p> A module M declares that it 'uses p.S' or 'provides p.S with ...' but

 *   package p is neither in module M nor exported to M by any module that M

 *   "reads". </p></li>

 * </ul>

 * <p> Otherwise, resolution succeeds, and the result of resolution is the

 * readability graph.

 *

 * <h3><a id="root-modules"></a> Root modules </h3>

 *

 * <p> The set of root modules at compile-time is usually the set of modules

 * being compiled. At run-time, the set of root modules is usually the

 * application module specified to the 'java' launcher. When compiling code in

 * the unnamed module, or at run-time when the main application class is loaded

 * from the class path, then the default set of root modules is implementation

 * specific. In the JDK the default set of root modules contains every module

 * that is observable on the upgrade module path or among the system modules,

 * and that exports at least one package without qualification. </p>

 *

 * <h3> Observable modules </h3>

 *

 * <p> The set of observable modules at both compile-time and run-time is

 * determined by searching several different paths, and also by searching

 * the compiled modules built in to the environment. The search order is as

 * follows: </p>

 *

 * <ol>

 *   <li><p> At compile time only, the compilation module path. This path

 *   contains module definitions in source form.  </p></li>

 *

 *   <li><p> The upgrade module path. This path contains compiled definitions of

 *   modules that will be observed in preference to the compiled definitions of

 *   any <i>upgradeable modules</i> that are present in (3) and (4). See the Java

 *   SE Platform for the designation of which standard modules are upgradeable.

 *   </p></li>

 *

 *   <li><p> The system modules, which are the compiled definitions built in to

 *   the environment. </p></li>

 *

 *   <li><p> The application module path. This path contains compiled definitions

 *   of library and application modules. </p></li>

 *

 * </ol>

 *

 * <h3> 'requires' directives with 'static' modifier </h3>

 *

 * <p> 'requires' directives that have the 'static' modifier express an optional

 * dependence at run time. If a module declares that it 'requires static M' then

 * resolution does not search the observable modules for M to satisfy the dependency.

 * However, if M is recursively enumerated at step 1 then all modules that are

 * enumerated and `requires static M` will read M. </p>

 *

 * <h3> Completeness </h3>

 *

 * <p> Resolution may be partial at compile-time in that the complete transitive

 * closure may not be required to compile a set of modules. Minimally, the

 * readability graph that is constructed and validated at compile-time includes

 * the modules being compiled, their direct dependences, and all implicitly

 * declared dependences (requires transitive). </p>

 *

 * <p> At run-time, resolution is an additive process. The recursive enumeration

 * at step 1 may be relative to previous resolutions so that a root module,

 * or a module named in a 'requires' directive, is not enumerated when it was

 * enumerated by a previous (or parent) resolution. The readability graph that

 * is the result of resolution may therefore have a vertex for a module enumerated

 * in step 1 but with an edge to represent that the module reads a module that

 * was enumerated by previous (or parent) resolution. </p>

 *

 * @since 9

 */

package java.lang.module;