Module java.base
Package java.lang

## Class ModuleLayer

- java.lang.Object
- - java.lang.ModuleLayer

-     public final class ModuleLayer
      extends Object

  A layer of modules in the Java virtual machine.

  A layer is created from a graph of modules in a `Configuration` and a function that maps each module to a `ClassLoader`. Creating a layer informs the Java virtual machine about the classes that may be loaded from the modules so that the Java virtual machine knows which module that each class is a member of.

  Creating a layer creates a `Module` object for each `ResolvedModule` in the configuration. For each resolved module that is `read`, the `Module` `reads` the corresponding run-time `Module`, which may be in the same layer or a `parent` layer.

  The `defineModulesWithOneLoader` and `defineModulesWithManyLoaders` methods provide convenient ways to create a module layer where all modules are mapped to a single class loader or where each module is mapped to its own class loader. The `defineModules` method is for more advanced cases where modules are mapped to custom class loaders by means of a function specified to the method. Each of these methods has an instance and static variant. The instance methods create a layer with the receiver as the parent layer. The static methods are for more advanced cases where there can be more than one parent layer or where a `Controller` is needed to control modules in the layer

  A Java virtual machine has at least one non-empty layer, the `boot` layer, that is created when the Java virtual machine is started. The boot layer contains module `java.base` and is the only layer in the Java virtual machine with a module named "`java.base`". The modules in the boot layer are mapped to the bootstrap class loader and other class loaders that are built-in into the Java virtual machine. The boot layer will often be the `parent` when creating additional layers.

  Each `Module` in a layer is created so that it `exports` and `opens` the packages described by its `ModuleDescriptor`. Qualified exports (where a package is exported to a set of target modules rather than all modules) are reified when creating the layer as follows:

  - If module `X` exports a package to `Y`, and if the runtime `Module` `X` reads `Module` `Y`, then the package is exported to `Module` `Y` (which may be in the same layer as `X` or a parent layer).
  - If module `X` exports a package to `Y`, and if the runtime `Module` `X` does not read `Y` then target `Y` is located as if by invoking `findModule` to find the module in the layer or its parent layers. If `Y` is found then the package is exported to the instance of `Y` that was found. If `Y` is not found then the qualified export is ignored.

  Qualified opens are handled in same way as qualified exports.

  As when creating a `Configuration`, `automatic` modules receive special treatment when creating a layer. An automatic module is created in the Java virtual machine as a `Module` that reads every unnamed `Module` in the Java virtual machine.

  Unless otherwise specified, passing a `null` argument to a method in this class causes a `NullPointerException` to be thrown.

  ### Example usage:

  This example creates a configuration by resolving a module named "`myapp`" with the configuration for the boot layer as the parent. It then creates a new layer with the modules in this configuration. All modules are defined to the same class loader.

           ModuleFinder finder = ModuleFinder.of(dir1, dir2, dir3);

           ModuleLayer parent = ModuleLayer.boot();

           Configuration cf = parent.configuration().resolve(finder, ModuleFinder.of(), Set.of("myapp"));

           ClassLoader scl = ClassLoader.getSystemClassLoader();

           ModuleLayer layer = parent.defineModulesWithOneLoader(cf, scl);

           Class<?> c = layer.findLoader("myapp").loadClass("app.Main");

  Since:
    9
  See Also:
    `Module.getLayer()`

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class | Description
    --- | --- | ---
    `static class` | `ModuleLayer.Controller` | Controls a module layer.

  - ### Method Summary

    All Methods Static Methods Instance Methods Concrete Methods
    Modifier and Type | Method | Description
    --- | --- | ---
    `static ModuleLayer` | `boot()` | Returns the boot layer.
    `Configuration` | `configuration()` | Returns the configuration for this layer.
    `ModuleLayer` | `defineModules​(Configuration cf, Function<String,​ClassLoader> clf)` | Creates a new module layer, with this layer as its parent, by defining the modules in the given `Configuration` to the Java virtual machine.
    `static ModuleLayer.Controller` | `defineModules​(Configuration cf, List<ModuleLayer> parentLayers, Function<String,​ClassLoader> clf)` | Creates a new module layer by defining the modules in the given `Configuration` to the Java virtual machine.
    `ModuleLayer` | `defineModulesWithManyLoaders​(Configuration cf, ClassLoader parentLoader)` | Creates a new module layer, with this layer as its parent, by defining the modules in the given `Configuration` to the Java virtual machine.
    `static ModuleLayer.Controller` | `defineModulesWithManyLoaders​(Configuration cf, List<ModuleLayer> parentLayers, ClassLoader parentLoader)` | Creates a new module layer by defining the modules in the given `Configuration` to the Java virtual machine.
    `ModuleLayer` | `defineModulesWithOneLoader​(Configuration cf, ClassLoader parentLoader)` | Creates a new module layer, with this layer as its parent, by defining the modules in the given `Configuration` to the Java virtual machine.
    `static ModuleLayer.Controller` | `defineModulesWithOneLoader​(Configuration cf, List<ModuleLayer> parentLayers, ClassLoader parentLoader)` | Creates a new module layer by defining the modules in the given `Configuration` to the Java virtual machine.
    `static ModuleLayer` | `empty()` | Returns the empty layer.
    `ClassLoader` | `findLoader​(String name)` | Returns the `ClassLoader` for the module with the given name.
    `Optional<Module>` | `findModule​(String name)` | Returns the module with the given name in this layer, or if not in this layer, the parent layers.
    `Set<Module>` | `modules()` | Returns the set of the modules in this layer.
    `List<ModuleLayer>` | `parents()` | Returns the list of this layer's parents unless this is the empty layer, which has no parents and so an empty list is returned.
    `String` | `toString()` | Returns a string describing this module layer.

    - ### Methods declared in class java.lang.Object

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

- - ### Method Detail

    - #### defineModulesWithOneLoader

          public ModuleLayer defineModulesWithOneLoader​(Configuration cf,
                                                        ClassLoader parentLoader)

      Creates a new module layer, with this layer as its parent, by defining the modules in the given `Configuration` to the Java virtual machine. This method creates one class loader and defines all modules to that class loader. The `parent` of each class loader is the given parent class loader. This method works exactly as specified by the static `defineModulesWithOneLoader` method when invoked with this layer as the parent. In other words, if this layer is `thisLayer` then this method is equivalent to invoking:

               ModuleLayer.defineModulesWithOneLoader(cf, List.of(thisLayer), parentLoader).layer();

      Parameters:
        `cf` - The configuration for the layer
        `parentLoader` - The parent class loader for the class loader created by this method; may be `null` for the bootstrap class loader
      Returns:
        The newly created layer
      Throws:
        `IllegalArgumentException` - If the given configuration has more than one parent or the parent of the configuration is not the configuration for this layer
        `LayerInstantiationException` - If the layer cannot be created for any of the reasons specified by the static `defineModulesWithOneLoader` method
        `SecurityException` - If `RuntimePermission("createClassLoader")` or `RuntimePermission("getClassLoader")` is denied by the security manager
      See Also:
        `findLoader(java.lang.String)`

    - #### defineModulesWithManyLoaders

          public ModuleLayer defineModulesWithManyLoaders​(Configuration cf,
                                                          ClassLoader parentLoader)

      Creates a new module layer, with this layer as its parent, by defining the modules in the given `Configuration` to the Java virtual machine. Each module is defined to its own `ClassLoader` created by this method. The `parent` of each class loader is the given parent class loader. This method works exactly as specified by the static `defineModulesWithManyLoaders` method when invoked with this layer as the parent. In other words, if this layer is `thisLayer` then this method is equivalent to invoking:

               ModuleLayer.defineModulesWithManyLoaders(cf, List.of(thisLayer), parentLoader).layer();

      Parameters:
        `cf` - The configuration for the layer
        `parentLoader` - The parent class loader for each of the class loaders created by this method; may be `null` for the bootstrap class loader
      Returns:
        The newly created layer
      Throws:
        `IllegalArgumentException` - If the given configuration has more than one parent or the parent of the configuration is not the configuration for this layer
        `LayerInstantiationException` - If the layer cannot be created for any of the reasons specified by the static `defineModulesWithManyLoaders` method
        `SecurityException` - If `RuntimePermission("createClassLoader")` or `RuntimePermission("getClassLoader")` is denied by the security manager
      See Also:
        `findLoader(java.lang.String)`

    - #### defineModules

          public ModuleLayer defineModules​(Configuration cf,
                                           Function<String,​ClassLoader> clf)

      Creates a new module layer, with this layer as its parent, by defining the modules in the given `Configuration` to the Java virtual machine. Each module is mapped, by name, to its class loader by means of the given function. This method works exactly as specified by the static `defineModules` method when invoked with this layer as the parent. In other words, if this layer is `thisLayer` then this method is equivalent to invoking:

               ModuleLayer.defineModules(cf, List.of(thisLayer), clf).layer();

      Parameters:
        `cf` - The configuration for the layer
        `clf` - The function to map a module name to a class loader
      Returns:
        The newly created layer
      Throws:
        `IllegalArgumentException` - If the given configuration has more than one parent or the parent of the configuration is not the configuration for this layer
        `LayerInstantiationException` - If the layer cannot be created for any of the reasons specified by the static `defineModules` method
        `SecurityException` - If `RuntimePermission("getClassLoader")` is denied by the security manager

    - #### defineModulesWithOneLoader

          public static ModuleLayer.Controller defineModulesWithOneLoader​(Configuration cf,
                                                                          List<ModuleLayer> parentLayers,
                                                                          ClassLoader parentLoader)

      Creates a new module layer by defining the modules in the given `Configuration` to the Java virtual machine. This method creates one class loader and defines all modules to that class loader.

      The class loader created by this method implements direct delegation when loading classes from modules. If the `loadClass` method is invoked to load a class then it uses the package name of the class to map it to a module. This may be a module in this layer and hence defined to the same class loader. It may be a package in a module in a parent layer that is exported to one or more of the modules in this layer. The class loader delegates to the class loader of the module, throwing `ClassNotFoundException` if not found by that class loader. When `loadClass` is invoked to load classes that do not map to a module then it delegates to the parent class loader.

      The class loader created by this method locates resources (`getResource`, `getResources`, and other resource methods) in all modules in the layer before searching the parent class loader.

      Attempting to create a layer with all modules defined to the same class loader can fail for the following reasons:

      - Overlapping packages: Two or more modules in the configuration have the same package.

      - Split delegation: The resulting class loader would need to delegate to more than one class loader in order to load classes in a specific package.

      In addition, a layer cannot be created if the configuration contains a module named "`java.base`", or a module contains a package named "`java`" or a package with a name starting with "`java.`".

      If there is a security manager then the class loader created by this method will load classes and resources with privileges that are restricted by the calling context of this method.

      Parameters:
        `cf` - The configuration for the layer
        `parentLayers` - The list of parent layers in search order
        `parentLoader` - The parent class loader for the class loader created by this method; may be `null` for the bootstrap class loader
      Returns:
        A controller that controls the newly created layer
      Throws:
        `IllegalArgumentException` - If the parent(s) of the given configuration do not match the configuration of the parent layers, including order
        `LayerInstantiationException` - If all modules cannot be defined to the same class loader for any of the reasons listed above
        `SecurityException` - If `RuntimePermission("createClassLoader")` or `RuntimePermission("getClassLoader")` is denied by the security manager
      See Also:
        `findLoader(java.lang.String)`

    - #### defineModulesWithManyLoaders

          public static ModuleLayer.Controller defineModulesWithManyLoaders​(Configuration cf,
                                                                            List<ModuleLayer> parentLayers,
                                                                            ClassLoader parentLoader)

      Creates a new module layer by defining the modules in the given `Configuration` to the Java virtual machine. Each module is defined to its own `ClassLoader` created by this method. The `parent` of each class loader is the given parent class loader.

      The class loaders created by this method implement direct delegation when loading classes from modules. If the `loadClass` method is invoked to load a class then it uses the package name of the class to map it to a module. The package may be in the module defined to the class loader. The package may be exported by another module in this layer to the module defined to the class loader. It may be in a package exported by a module in a parent layer. The class loader delegates to the class loader of the module, throwing `ClassNotFoundException` if not found by that class loader. When `loadClass` is invoked to load a class that does not map to a module then it delegates to the parent class loader.

      The class loaders created by this method locate resources (`getResource`, `getResources`, and other resource methods) in the module defined to the class loader before searching the parent class loader.

      If there is a security manager then the class loaders created by this method will load classes and resources with privileges that are restricted by the calling context of this method.

      Parameters:
        `cf` - The configuration for the layer
        `parentLayers` - The list of parent layers in search order
        `parentLoader` - The parent class loader for each of the class loaders created by this method; may be `null` for the bootstrap class loader
      Returns:
        A controller that controls the newly created layer
      Throws:
        `IllegalArgumentException` - If the parent(s) of the given configuration do not match the configuration of the parent layers, including order
        `LayerInstantiationException` - If the layer cannot be created because the configuration contains a module named "`java.base`" or a module contains a package named "`java`" or a package with a name starting with "`java.`"
        `SecurityException` - If `RuntimePermission("createClassLoader")` or `RuntimePermission("getClassLoader")` is denied by the security manager
      See Also:
        `findLoader(java.lang.String)`

    - #### defineModules

          public static ModuleLayer.Controller defineModules​(Configuration cf,
                                                             List<ModuleLayer> parentLayers,
                                                             Function<String,​ClassLoader> clf)

      Creates a new module layer by defining the modules in the given `Configuration` to the Java virtual machine. The given function maps each module in the configuration, by name, to a class loader. Creating the layer informs the Java virtual machine about the classes that may be loaded so that the Java virtual machine knows which module that each class is a member of.

      The class loader delegation implemented by the class loaders must respect module readability. The class loaders should be `parallel-capable` so as to avoid deadlocks during class loading. In addition, the entity creating a new layer with this method should arrange that the class loaders be ready to load from these modules before there are any attempts to load classes or resources.

      Creating a layer can fail for the following reasons:

      - Two or more modules with the same package are mapped to the same class loader.

      - A module is mapped to a class loader that already has a module of the same name defined to it.

      - A module is mapped to a class loader that has already defined types in any of the packages in the module.

      In addition, a layer cannot be created if the configuration contains a module named "`java.base`", a configuration contains a module with a package named "`java`" or a package name starting with "`java.`", or the function to map a module name to a class loader returns `null` or the platform class loader.

      If the function to map a module name to class loader throws an error or runtime exception then it is propagated to the caller of this method.

      API Note:
        It is implementation specific as to whether creating a layer with this method is an atomic operation or not. Consequentially it is possible for this method to fail with some modules, but not all, defined to the Java virtual machine.
      Parameters:
        `cf` - The configuration for the layer
        `parentLayers` - The list of parent layers in search order
        `clf` - The function to map a module name to a class loader
      Returns:
        A controller that controls the newly created layer
      Throws:
        `IllegalArgumentException` - If the parent(s) of the given configuration do not match the configuration of the parent layers, including order
        `LayerInstantiationException` - If creating the layer fails for any of the reasons listed above
        `SecurityException` - If `RuntimePermission("getClassLoader")` is denied by the security manager

    - #### configuration

          public Configuration configuration()

      Returns the configuration for this layer.
      Returns:
        The configuration for this layer

    - #### parents

          public List<ModuleLayer> parents()

      Returns the list of this layer's parents unless this is the empty layer, which has no parents and so an empty list is returned.
      Returns:
        The list of this layer's parents

    - #### modules

          public Set<Module> modules()

      Returns the set of the modules in this layer.
      Returns:
        A possibly-empty unmodifiable set of the modules in this layer

    - #### findModule

          public Optional<Module> findModule​(String name)

      Returns the module with the given name in this layer, or if not in this layer, the parent layers. Finding a module in parent layers 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 layers then this is equivalent to a depth-first search.
      Parameters:
        `name` - The name of the module to find
      Returns:
        The module with the given name or an empty `Optional` if there isn't a module with this name in this layer or any parent layer

    - #### findLoader

          public ClassLoader findLoader​(String name)

      Returns the `ClassLoader` for the module with the given name. If a module of the given name is not in this layer then the `parent` layers are searched in the manner specified by `findModule`.

      If there is a security manager then its `checkPermission` method is called with a `RuntimePermission("getClassLoader")` permission to check that the caller is allowed to get access to the class loader.

      API Note:
        This method does not return an `Optional<ClassLoader>` because `null` must be used to represent the bootstrap class loader.
      Parameters:
        `name` - The name of the module to find
      Returns:
        The ClassLoader that the module is defined to
      Throws:
        `IllegalArgumentException` - if a module of the given name is not defined in this layer or any parent of this layer
        `SecurityException` - if denied by the security manager

    - #### toString

          public String toString()

      Returns a string describing this module layer.
      Overrides:
        `toString` in class `Object`
      Returns:
        A possibly empty string describing this module layer

    - #### empty

          public static ModuleLayer empty()

      Returns the empty layer. There are no modules in the empty layer. It has no parents.
      Returns:
        The empty layer

    - #### boot

          public static ModuleLayer boot()

      Returns the boot layer. The boot layer contains at least one module, `java.base`. Its parent is the `empty` layer.
      API Note:
        This method returns `null` during startup and before the boot layer is fully initialized.
      Returns:
        The boot layer