Class ModuleLayer
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 toY
, and if the runtimeModule
X
readsModule
Y
, then the package is exported toModule
Y
(which may be in the same layer asX
or a parent layer). - If module
X
exports a package toY
, and if the runtimeModule
X
does not readY
then targetY
is located as if by invokingfindModule
to find the module in the layer or its parent layers. IfY
is found then the package is exported to the instance ofY
that was found. IfY
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
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:
-
Nested Class Summary
Modifier and TypeClassDescriptionstatic final class
Controls a module layer. -
Method Summary
Modifier and TypeMethodDescriptionstatic ModuleLayer
boot()
Returns the boot layer.Returns the configuration for this layer.defineModules
(Configuration cf, Function<String, ClassLoader> clf) Creates a new module layer, with this layer as its parent, by defining the modules in the givenConfiguration
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 givenConfiguration
to the Java virtual machine.defineModulesWithManyLoaders
(Configuration cf, ClassLoader parentLoader) Creates a new module layer, with this layer as its parent, by defining the modules in the givenConfiguration
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 givenConfiguration
to the Java virtual machine.defineModulesWithOneLoader
(Configuration cf, ClassLoader parentLoader) Creates a new module layer, with this layer as its parent, by defining the modules in the givenConfiguration
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 givenConfiguration
to the Java virtual machine.static ModuleLayer
empty()
Returns the empty layer.findLoader
(String name) Returns theClassLoader
for the module with the given name.findModule
(String name) Returns the module with the given name in this layer, or if not in this layer, the parent layers.modules()
Returns an unmodifiable set of the modules in this layer.parents()
Returns an unmodifiable list of this layer's parents, in search order.toString()
Returns a string describing this module layer.
-
Method Details
-
defineModulesWithOneLoader
Creates a new module layer, with this layer as its parent, by defining the modules in the givenConfiguration
to the Java virtual machine. This method creates one class loader and defines all modules to that class loader. Theparent
of each class loader is the given parent class loader. This method works exactly as specified by the staticdefineModulesWithOneLoader
method when invoked with this layer as the parent. In other words, if this layer isthisLayer
then this method is equivalent to invoking:ModuleLayer.defineModulesWithOneLoader(cf, List.of(thisLayer), parentLoader).layer();
- Parameters:
cf
- The configuration for the layerparentLoader
- The parent class loader for the class loader created by this method; may benull
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 layerLayerInstantiationException
- If the layer cannot be created for any of the reasons specified by the staticdefineModulesWithOneLoader
methodSecurityException
- IfRuntimePermission("createClassLoader")
orRuntimePermission("getClassLoader")
is denied by the security manager- See Also:
-
defineModulesWithManyLoaders
Creates a new module layer, with this layer as its parent, by defining the modules in the givenConfiguration
to the Java virtual machine. Each module is defined to its ownClassLoader
created by this method. Theparent
of each class loader is the given parent class loader. This method works exactly as specified by the staticdefineModulesWithManyLoaders
method when invoked with this layer as the parent. In other words, if this layer isthisLayer
then this method is equivalent to invoking:ModuleLayer.defineModulesWithManyLoaders(cf, List.of(thisLayer), parentLoader).layer();
- Parameters:
cf
- The configuration for the layerparentLoader
- The parent class loader for each of the class loaders created by this method; may benull
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 layerLayerInstantiationException
- If the layer cannot be created for any of the reasons specified by the staticdefineModulesWithManyLoaders
methodSecurityException
- IfRuntimePermission("createClassLoader")
orRuntimePermission("getClassLoader")
is denied by the security manager- See Also:
-
defineModules
Creates a new module layer, with this layer as its parent, by defining the modules in the givenConfiguration
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 staticdefineModules
method when invoked with this layer as the parent. In other words, if this layer isthisLayer
then this method is equivalent to invoking:ModuleLayer.defineModules(cf, List.of(thisLayer), clf).layer();
- Parameters:
cf
- The configuration for the layerclf
- 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 layerLayerInstantiationException
- If the layer cannot be created for any of the reasons specified by the staticdefineModules
methodSecurityException
- IfRuntimePermission("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 givenConfiguration
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, throwingClassNotFoundException
if not found by that class loader. WhenloadClass
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 layerparentLayers
- The list of parent layers in search orderparentLoader
- The parent class loader for the class loader created by this method; may benull
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 orderLayerInstantiationException
- If all modules cannot be defined to the same class loader for any of the reasons listed aboveSecurityException
- IfRuntimePermission("createClassLoader")
orRuntimePermission("getClassLoader")
is denied by the security manager- See Also:
-
defineModulesWithManyLoaders
public static ModuleLayer.Controller defineModulesWithManyLoaders(Configuration cf, List<ModuleLayer> parentLayers, ClassLoader parentLoader) Creates a new module layer by defining the modules in the givenConfiguration
to the Java virtual machine. Each module is defined to its ownClassLoader
created by this method. Theparent
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, throwingClassNotFoundException
if not found by that class loader. WhenloadClass
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 layerparentLayers
- The list of parent layers in search orderparentLoader
- The parent class loader for each of the class loaders created by this method; may benull
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 orderLayerInstantiationException
- 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
- IfRuntimePermission("createClassLoader")
orRuntimePermission("getClassLoader")
is denied by the security manager- See Also:
-
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 givenConfiguration
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 returnsnull
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. Consequently 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 layerparentLayers
- The list of parent layers in search orderclf
- 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 orderLayerInstantiationException
- If creating the layer fails for any of the reasons listed aboveSecurityException
- IfRuntimePermission("getClassLoader")
is denied by the security manager
-
configuration
Returns the configuration for this layer.- Returns:
- The configuration for this layer
-
parents
Returns an unmodifiable list of this layer's parents, in search order. If this is the empty layer then an empty list is returned.- Returns:
- A possibly-empty unmodifiable list of this layer's parents
-
modules
-
findModule
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 invokingfindModule
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
Returns theClassLoader
for the module with the given name. If a module of the given name is not in this layer then theparent
layers are searched in the manner specified byfindModule
.If there is a security manager then its
checkPermission
method is called with aRuntimePermission("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 layerSecurityException
- if denied by the security manager
-
toString
-
empty
Returns the empty layer. There are no modules in the empty layer. It has no parents.- Returns:
- The empty layer
-
boot
Returns the boot layer. The boot layer contains at least one module,java.base
. Its parent is theempty
layer.- API Note:
- This method returns
null
during startup and before the boot layer is fully initialized. - Returns:
- The boot layer
-