# Class Configuration

java.lang.Object
java.lang.module.Configuration

public final class Configuration extends Object
A configuration that is the result of resolution or resolution with service binding.

A configuration encapsulates the readability graph that is the output of resolution. A readability graph is a directed graph whose vertices are of type `ResolvedModule` and the edges represent the readability amongst the modules. `Configuration` defines the `modules()` method to get the set of resolved modules in the graph. `ResolvedModule` defines the `reads()` method to get the set of modules that a resolved module reads. The modules that are read may be in the same configuration or may be in `parent` configurations.

Configuration defines the `resolve` method to resolve a collection of root modules, and the `resolveAndBind` method to do resolution with service binding. There are instance and static variants of both methods. The instance methods create a configuration with the receiver as the parent configuration. The static methods are for more advanced cases where there can be more than one parent configuration.

Each `layer` of modules in the Java virtual machine is created from a configuration. The configuration for the `boot` layer is obtained by invoking `ModuleLayer.boot().configuration()`. The configuration for the boot layer will often be the parent when creating new configurations.

## Optional Services

Resolution requires that if a module `M` '`uses`' a service or '`provides`' an implementation of a service, then the service must be available to `M` at run time, either because `M` itself contains the service's package or because `M` reads another module that exports the service's package. However, it is sometimes desirable for the service's package to come from a module that is optional at run time, as indicated by the use of 'requires static' in this example:

       module M {
           requires static Y;
           uses p.S;
       }

       module Y {
          exports p;
       }

Resolution is resilient when a service's package comes from a module that is optional at run time. That is, if a module `M` has an optional dependency on some module `Y`, but `Y` is not needed at run time (`Y` might be observable but no-one reads it), then resolution at run time assumes that `Y` exported the service's package at compile time. Resolution at run time does not attempt to check whether `Y` is observable or (if it is observable) whether `Y` exports the service's package.

The module that '`uses`' the service, or '`provides`' an implementation of it, may depend directly on the optional module, as `M` does above, or may depend indirectly on the optional module, as shown here:

        module M {
            requires X;
            uses p.S;
        }

        module X {
            requires static transitive Y;
        }

        module Y {
            exports p;
        }

In effect, the service that `M` '`uses`', or '`provides`' an implementation of, is optional if it comes from an optional dependency. In this case, code in `M` must be prepared to deal with the class or interface that denotes the service being unavailable at run time. This is distinct from the more regular case where the service is available but no implementations of the service are available.

## Example

The following example uses the `resolve` method to resolve a module named myapp with the configuration for the boot layer as the parent configuration. It prints the name of each resolved module and the names of the modules that each module reads.

       Path dir1 = ..., dir2 = ..., dir3 = ...;
       ModuleFinder finder = ModuleFinder.of(dir1, dir2, dir3);
       Configuration parent = ModuleLayer.boot().configuration();
       Configuration cf = parent.resolve(finder, ModuleFinder.of(), Set.of("myapp"));
       cf.modules().forEach(m -> {
           System.out.format("%s -> %s%n",
               m.name(),
               m.reads().stream()
                   .map(ResolvedModule::name)
                   .collect(Collectors.joining(", ")));
       });

Since:
  9
See Also:
  - `ModuleLayer`

- ## Method Summary

  Modifier and Type
  Method
  Description
  `static Configuration`
  `empty()`
  Returns the empty configuration.
  `Optional<ResolvedModule>`
  `findModule(String name)`
  Finds a resolved module in this configuration, or if not in this configuration, the parent configurations.
  `Set<ResolvedModule>`
  `modules()`
  Returns an unmodifiable set of the resolved modules in this configuration.
  `List<Configuration>`
  `parents()`
  Returns an unmodifiable list of this configuration's parents, in search order.
  `Configuration`
  `resolve(ModuleFinder before, ModuleFinder after, Collection<String> roots)`
  Resolves a collection of root modules, with this configuration as its parent, to create a new configuration.
  `static Configuration`
  `resolve(ModuleFinder before, List<Configuration> parents, ModuleFinder after, Collection<String> roots)`
  Resolves a collection of root modules to create a configuration.
  `Configuration`
  `resolveAndBind(ModuleFinder before, ModuleFinder after, Collection<String> roots)`
  Resolves a collection of root modules, with service binding, and with this configuration as its parent, to create a new configuration.
  `static Configuration`
  `resolveAndBind(ModuleFinder before, List<Configuration> parents, ModuleFinder after, Collection<String> roots)`
  Resolves a collection of root modules, with service binding, to create configuration.
  `String`
  `toString()`
  Returns a string describing this configuration.

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- ## Method Details

  - ### resolve

    public Configuration resolve(ModuleFinder before, ModuleFinder after, Collection<String> roots)
    Resolves a collection of root modules, with this configuration as its parent, to create a new configuration. This method works exactly as specified by the static `resolve` method when invoked with this configuration as the parent. In other words, if this configuration is `cf` then this method is equivalent to invoking:

            Configuration.resolve(before, List.of(cf), after, roots);

    Parameters:
      `before` - The before module finder to find modules
      `after` - The after module finder to locate modules when not located by the `before` module finder or in parent configurations
      `roots` - The possibly-empty collection of module names of the modules to resolve
    Returns:
      The configuration that is the result of resolving the given root modules
    Throws:
      `FindException` - If resolution fails for any of the observability-related reasons specified by the static `resolve` method
      `ResolutionException` - If resolution fails any of the consistency checks specified by the static `resolve` method

  - ### resolveAndBind

    public Configuration resolveAndBind(ModuleFinder before, ModuleFinder after, Collection<String> roots)
    Resolves a collection of root modules, with service binding, and with this configuration as its parent, to create a new configuration. This method works exactly as specified by the static `resolveAndBind` method when invoked with this configuration as the parent. In other words, if this configuration is `cf` then this method is equivalent to invoking:

            Configuration.resolveAndBind(before, List.of(cf), after, roots);

    Parameters:
      `before` - The before module finder to find modules
      `after` - The after module finder to locate modules when not located by the `before` module finder or in parent configurations
      `roots` - The possibly-empty collection of module names of the modules to resolve
    Returns:
      The configuration that is the result of resolving, with service binding, the given root modules
    Throws:
      `FindException` - If resolution fails for any of the observability-related reasons specified by the static `resolve` method
      `ResolutionException` - If resolution fails any of the consistency checks specified by the static `resolve` method

  - ### resolve

    public static Configuration resolve(ModuleFinder before, List<Configuration> parents, ModuleFinder after, Collection<String> roots)
    Resolves a collection of root modules to create a configuration.

    Each root module is located using the given `before` module finder. If a module is not found then it is located in the parent configuration as if by invoking the `findModule` method on each parent in iteration order. If not found then the module is located using the given `after` module finder. The same search order is used to locate transitive dependences. Root modules or dependences that are located in a parent configuration are resolved no further and are not included in the resulting configuration.

    When all modules have been enumerated then a readability graph is computed, and in conjunction with the module exports and service use, checked for consistency.

    Resolution may fail with `FindException` for the following observability-related reasons:

    - A root module, or a direct or transitive dependency, is not found.

    - An error occurs when attempting to find a module. Possible errors include I/O errors, errors detected parsing a module descriptor (`module-info.class`) or two versions of the same module are found in the same directory.

    Resolution may fail with `ResolutionException` if any of the following consistency checks fail:

    - A cycle is detected, say where module `m1` requires module `m2` and `m2` requires `m1`.

    - 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.

    - Two or more modules in the configuration export the same package 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`.

    - A module `M` declares that it '`uses p.S`' or '`provides p.S with ...`', but the package `p` is neither in module `M` nor exported to `M` by any module that `M` reads. Additionally, neither of the following is `true`:

      - `M` declares '`requires static`' for at least one module that is not in the readability graph.
      - `M` reads another module that declares '`requires transitive static`' for at least one module that is not in the readability graph.
    Implementation Note:
      In the implementation then observability of modules may depend on referential integrity or other checks that ensure different builds of tightly coupled modules or modules for specific operating systems or architectures are not combined in the same configuration.
    Parameters:
      `before` - The before module finder to find modules
      `parents` - The list parent configurations in search order
      `after` - The after module finder to locate modules when not located by the `before` module finder or in parent configurations
      `roots` - The possibly-empty collection of module names of the modules to resolve
    Returns:
      The configuration that is the result of resolving the given root modules
    Throws:
      `FindException` - If resolution fails for any of observability-related reasons specified above
      `ResolutionException` - If resolution fails for any of the consistency checks specified above
      `IllegalArgumentException` - If the list of parents is empty, or the list has two or more parents with modules for different target operating systems, architectures, or versions

  - ### resolveAndBind

    public static Configuration resolveAndBind(ModuleFinder before, List<Configuration> parents, ModuleFinder after, Collection<String> roots)
    Resolves a collection of root modules, with service binding, to create configuration.

    This method works exactly as specified by `resolve` except that the graph of resolved modules is augmented with modules induced by the service-use dependence relation.

    More specifically, the root modules are resolved as if by calling `resolve`. The resolved modules, and all modules in the parent configurations, with `service dependences` are then examined. All modules found by the given module finders that `provide` an implementation of one or more of the service types are added to the module graph and then resolved as if by calling the `resolve` method. Adding modules to the module graph may introduce new service-use dependences and so the process works iteratively until no more modules are added.

    As service binding involves resolution then it may fail with `FindException` or `ResolutionException` for exactly the same reasons specified in `resolve`.

    Parameters:
      `before` - The before module finder to find modules
      `parents` - The list parent configurations in search order
      `after` - The after module finder to locate modules when not located by the `before` module finder or in parent configurations
      `roots` - The possibly-empty collection of module names of the modules to resolve
    Returns:
      The configuration that is the result of resolving, with service binding, the given root modules
    Throws:
      `FindException` - If resolution fails for any of the observability-related reasons specified by the static `resolve` method
      `ResolutionException` - If resolution fails any of the consistency checks specified by the static `resolve` method
      `IllegalArgumentException` - If the list of parents is empty, or the list has two or more parents with modules for different target operating systems, architectures, or versions

  - ### empty

    public static Configuration empty()
    Returns the empty configuration. There are no modules in the empty configuration. It has no parents.
    Returns:
      The empty configuration

  - ### parents

    public List<Configuration> parents()
    Returns an unmodifiable list of this configuration's parents, in search order. If this is the empty configuration then an empty list is returned.
    Returns:
      A possibly-empty unmodifiable list of this parent configurations

  - ### modules

    public Set<ResolvedModule> modules()
    Returns an unmodifiable set of the resolved modules in this configuration.
    Returns:
      A possibly-empty unmodifiable set of the resolved modules in this configuration

  - ### findModule

    public Optional<ResolvedModule> findModule(String name)
    Finds a resolved module in this configuration, or if not in this configuration, the parent configurations. Finding a module in parent configurations is equivalent to invoking `findModule` on each parent, in search order, until the module is found or all parents have been searched. In a tree of configurations then this is equivalent to a depth-first search.
    Parameters:
      `name` - The module name of the resolved module to find
    Returns:
      The resolved module with the given name or an empty `Optional` if there isn't a module with this name in this configuration or any parent configurations

  - ### toString

    public String toString()
    Returns a string describing this configuration.
    Overrides:
      `toString` in class `Object`
    Returns:
      A possibly empty string describing this configuration