Class ClassLoader
- Direct Known Subclasses:
SecureClassLoader
ClassLoader
is an abstract class. Given the binary name of a class, a class loader should attempt to
locate or generate data that constitutes a definition for the class. A
typical strategy is to transform the name into a file name and then read a
"class file" of that name from a file system.
Every Class
object contains a reference
to the ClassLoader
that defined
it.
Class
objects for array classes are not created by class
loaders, but are created automatically as required by the Java runtime.
The class loader for an array class, as returned by Class.getClassLoader()
is the same as the class loader for its element
type; if the element type is a primitive type, then the array class has no
class loader.
Applications implement subclasses of ClassLoader
in order to
extend the manner in which the Java virtual machine dynamically loads
classes.
In addition to loading classes, a class loader is also responsible for
locating resources. A resource is some data (a ".class
" file,
configuration data, or an image for example) that is identified with an
abstract '/'-separated path name. Resources are typically packaged with an
application or library so that they can be located by code in the
application or library. In some cases, the resources are included so that
they can be located by other libraries.
The ClassLoader
class uses a delegation model to search for
classes and resources. Each instance of ClassLoader
has an
associated parent class loader. When requested to find a class or
resource, a ClassLoader
instance will usually delegate the search
for the class or resource to its parent class loader before attempting to
find the class or resource itself.
Class loaders that support concurrent loading of classes are known as
parallel capable class
loaders and are required to register themselves at their class initialization
time by invoking the ClassLoader.registerAsParallelCapable
method. Note that the ClassLoader
class is registered as parallel
capable by default. However, its subclasses still need to register themselves
if they are parallel capable.
In environments in which the delegation model is not strictly
hierarchical, class loaders need to be parallel capable, otherwise class
loading can lead to deadlocks because the loader lock is held for the
duration of the class loading process (see loadClass
methods).
Run-time Built-in Class Loaders
The Java run-time has the following built-in class loaders:Bootstrap class loader. It is the virtual machine's built-in class loader, typically represented as
null
, and does not have a parent.Platform class loader. The platform class loader is responsible for loading the platform classes. Platform classes include Java SE platform APIs, their implementation classes and JDK-specific run-time classes that are defined by the platform class loader or its ancestors. The platform class loader can be used as the parent of a
ClassLoader
instance.To allow for upgrading/overriding of modules defined to the platform class loader, and where upgraded modules read modules defined to class loaders other than the platform class loader and its ancestors, then the platform class loader may have to delegate to other class loaders, the application class loader for example. In other words, classes in named modules defined to class loaders other than the platform class loader and its ancestors may be visible to the platform class loader.
System class loader. It is also known as application class loader and is distinct from the platform class loader. The system class loader is typically used to define classes on the application class path, module path, and JDK-specific tools. The platform class loader is the parent or an ancestor of the system class loader, so the system class loader can load platform classes by delegating to its parent.
Normally, the Java virtual machine loads classes from the local file
system in a platform-dependent manner.
However, some classes may not originate from a file; they may originate
from other sources, such as the network, or they could be constructed by an
application. The method defineClass
converts an array of bytes into an instance of class
Class
. Instances of this newly defined class can be created using
Class.newInstance
.
The methods and constructors of objects created by a class loader may
reference other classes. To determine the class(es) referred to, the Java
virtual machine invokes the loadClass
method of
the class loader that originally created the class.
For example, an application could create a network class loader to download class files from a server. Sample code might look like:
ClassLoader loader = new NetworkClassLoader(host, port); Object main = loader.loadClass("Main", true).newInstance(); . . .
The network class loader subclass must define the methods findClass
and loadClassData
to load a class
from the network. Once it has downloaded the bytes that make up the class,
it should use the method defineClass
to
create a class instance. A sample implementation is:
class NetworkClassLoader extends ClassLoader { String host; int port; public Class findClass(String name) { byte[] b = loadClassData(name); return defineClass(name, b, 0, b.length); } private byte[] loadClassData(String name) { // load the class data from the connection . . . } }
Binary names
Any class name provided as a String
parameter to methods in
ClassLoader
must be a binary name as defined by
The Java Language Specification.
Examples of valid class names include:
"java.lang.String" "javax.swing.JSpinner$DefaultEditor" "java.security.KeyStore$Builder$FileBuilder$1" "java.net.URLClassLoader$3$1"
Any package name provided as a String
parameter to methods in
ClassLoader
must be either the empty string (denoting an unnamed package)
or a fully qualified name as defined by
The Java Language Specification.
- See Java Language Specification:
-
6.7 Fully Qualified Names and Canonical Names
13.1 The Form of a Binary - Since:
- 1.0
- See Also:
-
Constructor Summary
ModifierConstructorDescriptionprotected
Creates a new class loader using theClassLoader
returned by the methodgetSystemClassLoader()
as the parent class loader.protected
ClassLoader
(ClassLoader parent) Creates a new class loader using the specified parent class loader for delegation.protected
ClassLoader
(String name, ClassLoader parent) Creates a new class loader of the specified name and using the specified parent class loader for delegation. -
Method Summary
Modifier and TypeMethodDescriptionvoid
Sets the default assertion status for this class loader tofalse
and discards any package defaults or class assertion status settings associated with the class loader.protected final Class
<?> defineClass
(byte[] b, int off, int len) Deprecated.protected final Class
<?> defineClass
(String name, byte[] b, int off, int len) Converts an array of bytes into an instance of classClass
.protected final Class
<?> defineClass
(String name, byte[] b, int off, int len, ProtectionDomain protectionDomain) Converts an array of bytes into an instance of classClass
, with a givenProtectionDomain
.protected final Class
<?> defineClass
(String name, ByteBuffer b, ProtectionDomain protectionDomain) protected Package
definePackage
(String name, String specTitle, String specVersion, String specVendor, String implTitle, String implVersion, String implVendor, URL sealBase) Defines a package by name in thisClassLoader
.protected Class
<?> Finds the class with the specified binary name.protected Class
<?> Finds the class with the given binary name in a module defined to this class loader.protected String
findLibrary
(String libname) Returns the absolute path name of a native library.protected final Class
<?> findLoadedClass
(String name) Returns the class with the given binary name if this loader has been recorded by the Java virtual machine as an initiating loader of a class with that binary name.protected URL
findResource
(String name) Finds the resource with the given name.protected URL
findResource
(String moduleName, String name) Returns a URL to a resource in a module defined to this class loader.protected Enumeration
<URL> findResources
(String name) Returns an enumeration ofURL
objects representing all the resources with the given name.protected final Class
<?> findSystemClass
(String name) Finds a class with the specified binary name, loading it if necessary.protected Object
getClassLoadingLock
(String className) Returns the lock object for class loading operations.final Package
getDefinedPackage
(String name) Returns aPackage
of the given name that has been defined by this class loader.final Package[]
Returns all of thePackage
s that have been defined by this class loader.getName()
Returns the name of this class loader ornull
if this class loader is not named.protected Package
getPackage
(String name) Deprecated.If multiple class loaders delegate to each other and define classes with the same package name, and one such loader relies on the lookup behavior ofgetPackage
to return aPackage
from a parent loader, then the properties exposed by thePackage
may not be as expected in the rest of the program.protected Package[]
Returns all of thePackage
s that have been defined by this class loader and its ancestors.final ClassLoader
Returns the parent class loader for delegation.static ClassLoader
Returns the platform class loader.getResource
(String name) Finds the resource with the given name.getResourceAsStream
(String name) Returns an input stream for reading the specified resource.getResources
(String name) Finds all the resources with the given name.static ClassLoader
Returns the system class loader.static URL
getSystemResource
(String name) Find a resource of the specified name from the search path used to load classes.static InputStream
Open for reading, a resource of the specified name from the search path used to load classes.static Enumeration
<URL> getSystemResources
(String name) Finds all resources of the specified name from the search path used to load classes.final Module
Returns the unnamedModule
for this class loader.final boolean
Class
<?> Loads the class with the specified binary name.protected Class
<?> Loads the class with the specified binary name.protected static boolean
Registers the caller as parallel capable.protected final void
resolveClass
(Class<?> c) Links the specified class.Returns a stream whose elements are the URLs of all the resources with the given name.void
setClassAssertionStatus
(String className, boolean enabled) Sets the desired assertion status for the named top-level class in this class loader and any nested classes contained therein.void
setDefaultAssertionStatus
(boolean enabled) Sets the default assertion status for this class loader.void
setPackageAssertionStatus
(String packageName, boolean enabled) Sets the package default assertion status for the named package.protected final void
setSigners
(Class<?> c, Object[] signers) Sets the signers of a class.
-
Constructor Details
-
ClassLoader
Creates a new class loader of the specified name and using the specified parent class loader for delegation.- API Note:
- If the parent is specified as
null
(for the bootstrap class loader) then there is no guarantee that all platform classes are visible. - Parameters:
name
- class loader name; ornull
if not namedparent
- the parent class loader- Throws:
IllegalArgumentException
- if the given name is empty.- Since:
- 9
-
ClassLoader
Creates a new class loader using the specified parent class loader for delegation.- API Note:
- If the parent is specified as
null
(for the bootstrap class loader) then there is no guarantee that all platform classes are visible. - Parameters:
parent
- The parent class loader- Since:
- 1.2
-
ClassLoader
protected ClassLoader()Creates a new class loader using theClassLoader
returned by the methodgetSystemClassLoader()
as the parent class loader.
-
-
Method Details
-
getName
Returns the name of this class loader ornull
if this class loader is not named.- API Note:
- This method is non-final for compatibility. If this method is overridden, this method must return the same name as specified when this class loader was instantiated.
- Returns:
- name of this class loader; or
null
if this class loader is not named. - Since:
- 9
-
loadClass
Loads the class with the specified binary name. This method searches for classes in the same manner as theloadClass(String, boolean)
method. It is invoked by the Java virtual machine to resolve class references. Invoking this method is equivalent to invokingloadClass(name, false)
.- Parameters:
name
- The binary name of the class- Returns:
- The resulting
Class
object - Throws:
ClassNotFoundException
- If the class was not found
-
loadClass
Loads the class with the specified binary name. The default implementation of this method searches for classes in the following order:Invoke
findLoadedClass(String)
to check if the class has already been loaded.Invoke the
loadClass
method on the parent class loader. If the parent isnull
the class loader built into the virtual machine is used, instead.Invoke the
findClass(String)
method to find the class.
If the class was found using the above steps, and the
resolve
flag is true, this method will then invoke theresolveClass(Class)
method on the resultingClass
object.Subclasses of
ClassLoader
are encouraged to overridefindClass(String)
, rather than this method.Unless overridden, this method synchronizes on the result of
getClassLoadingLock
method during the entire class loading process.- Parameters:
name
- The binary name of the classresolve
- Iftrue
then resolve the class- Returns:
- The resulting
Class
object - Throws:
ClassNotFoundException
- If the class could not be found
-
getClassLoadingLock
Returns the lock object for class loading operations.- API Note:
- This method allows parallel capable class loaders to implement
finer-grained locking schemes such that multiple threads may load classes
concurrently without deadlocks. For non-parallel-capable class loaders,
the
ClassLoader
object is synchronized on during the class loading operations. Class loaders with non-hierarchical delegation should be registered as parallel capable to prevent deadlocks. - Implementation Requirements:
- If this
ClassLoader
object is registered as parallel capable, this method returns a dedicated object associated with the specified class name. Otherwise, this method returns thisClassLoader
object. - Parameters:
className
- The name of the to-be-loaded class- Returns:
- the lock for class loading operations
- Throws:
NullPointerException
- If registered as parallel capable andclassName
isnull
- Since:
- 1.7
- See Also:
-
findClass
Finds the class with the specified binary name. This method should be overridden by class loader implementations that follow the delegation model for loading classes, and will be invoked by theloadClass
method after checking the parent class loader for the requested class.- Implementation Requirements:
- The default implementation throws
ClassNotFoundException
. - Parameters:
name
- The binary name of the class- Returns:
- The resulting
Class
object - Throws:
ClassNotFoundException
- If the class could not be found- Since:
- 1.2
-
findClass
Finds the class with the given binary name in a module defined to this class loader. Class loader implementations that support loading from modules should override this method.- API Note:
- This method returns
null
rather than throwingClassNotFoundException
if the class could not be found. - Implementation Requirements:
- The default implementation attempts to find the class by
invoking
findClass(String)
when themoduleName
isnull
. It otherwise returnsnull
. - Parameters:
moduleName
- The module name; ornull
to find the class in the unnamed module for this class loadername
- The binary name of the class- Returns:
- The resulting
Class
object, ornull
if the class could not be found. - Since:
- 9
-
defineClass
@Deprecated(since="1.1") protected final Class<?> defineClass(byte[] b, int off, int len) throws ClassFormatError Deprecated.Replaced bydefineClass(String, byte[], int, int)
Converts an array of bytes into an instance of classClass
. Before theClass
can be used it must be resolved. This method is deprecated in favor of the version that takes a binary name as its first argument, and is more secure.- Parameters:
b
- The bytes that make up the class data. The bytes in positionsoff
throughoff+len-1
should have the format of a valid class file as defined by The Java Virtual Machine Specification.off
- The start offset inb
of the class datalen
- The length of the class data- Returns:
- The
Class
object that was created from the specified class data - Throws:
ClassFormatError
- If the data did not contain a valid classIndexOutOfBoundsException
- If eitheroff
orlen
is negative, or ifoff+len
is greater thanb.length
.SecurityException
- If an attempt is made to add this class to a package that contains classes that were signed by a different set of certificates than this class, or if an attempt is made to define a class in a package with a fully-qualified name that starts with "java.
".- See Also:
-
defineClass
protected final Class<?> defineClass(String name, byte[] b, int off, int len) throws ClassFormatError Converts an array of bytes into an instance of classClass
. Before theClass
can be used it must be resolved.This method assigns a default
ProtectionDomain
to the newly defined class. ThegetPermissions
method of theProtectionDomain
always returnsnull
. The default protection domain is created on the first invocation ofdefineClass
, and re-used on subsequent invocations.To assign a specific
ProtectionDomain
to the class, use thedefineClass
method that takes aProtectionDomain
as one of its arguments.This method defines a package in this class loader corresponding to the package of the
Class
(if such a package has not already been defined in this class loader). The name of the defined package is derived from the binary name of the class specified by the byte arrayb
. Other properties of the defined package are as specified byPackage
.- Parameters:
name
- The expected binary name of the class, ornull
if not knownb
- The bytes that make up the class data. The bytes in positionsoff
throughoff+len-1
should have the format of a valid class file as defined by The Java Virtual Machine Specification.off
- The start offset inb
of the class datalen
- The length of the class data- Returns:
- The
Class
object that was created from the specified class data. - Throws:
ClassFormatError
- If the data did not contain a valid classIndexOutOfBoundsException
- If eitheroff
orlen
is negative, or ifoff+len
is greater thanb.length
.SecurityException
- If an attempt is made to add this class to a package that contains classes that were signed by a different set of certificates than this class (which is unsigned), or ifname
begins with "java.
".- Since:
- 1.1
- See Also:
-
defineClass
protected final Class<?> defineClass(String name, byte[] b, int off, int len, ProtectionDomain protectionDomain) throws ClassFormatError Converts an array of bytes into an instance of classClass
, with a givenProtectionDomain
.If the given
ProtectionDomain
isnull
, then a default protection domain will be assigned to the class as specified in the documentation fordefineClass(String, byte[], int, int)
. Before the class can be used it must be resolved.The first class defined in a package determines the exact set of certificates that all subsequent classes defined in that package must contain. The set of certificates for a class is obtained from the
CodeSource
within theProtectionDomain
of the class. Any classes added to that package must contain the same set of certificates or aSecurityException
will be thrown. Note that ifname
isnull
, this check is not performed. You should always pass in the binary name of the class you are defining as well as the bytes. This ensures that the class you are defining is indeed the class you think it is.If the specified
name
begins with "java.
", it can only be defined by the platform class loader or its ancestors; otherwiseSecurityException
will be thrown. Ifname
is notnull
, it must be equal to the binary name of the class specified by the byte arrayb
, otherwise aNoClassDefFoundError
will be thrown.This method defines a package in this class loader corresponding to the package of the
Class
(if such a package has not already been defined in this class loader). The name of the defined package is derived from the binary name of the class specified by the byte arrayb
. Other properties of the defined package are as specified byPackage
.- Parameters:
name
- The expected binary name of the class, ornull
if not knownb
- The bytes that make up the class data. The bytes in positionsoff
throughoff+len-1
should have the format of a valid class file as defined by The Java Virtual Machine Specification.off
- The start offset inb
of the class datalen
- The length of the class dataprotectionDomain
- TheProtectionDomain
of the class- Returns:
- The
Class
object created from the data, andProtectionDomain
. - Throws:
ClassFormatError
- If the data did not contain a valid classNoClassDefFoundError
- Ifname
is notnull
and not equal to the binary name of the class specified byb
IndexOutOfBoundsException
- If eitheroff
orlen
is negative, or ifoff+len
is greater thanb.length
.SecurityException
- If an attempt is made to add this class to a package that contains classes that were signed by a different set of certificates than this class, or ifname
begins with "java.
" and this class loader is not the platform class loader or its ancestor.
-
defineClass
protected final Class<?> defineClass(String name, ByteBuffer b, ProtectionDomain protectionDomain) throws ClassFormatError Converts aByteBuffer
into an instance of classClass
, with the givenProtectionDomain
. If the givenProtectionDomain
isnull
, then a default protection domain will be assigned to the class as specified in the documentation fordefineClass(String, byte[], int, int)
. Before the class can be used it must be resolved.The rules about the first class defined in a package determining the set of certificates for the package, the restrictions on class names, and the defined package of the class are identical to those specified in the documentation for
defineClass(String, byte[], int, int, ProtectionDomain)
.An invocation of this method of the form cl
.defineClass(
name,
bBuffer,
pd)
yields exactly the same result as the statements...
byte[] temp = new byte[bBuffer.remaining
()];
bBuffer.get
(temp);
returncl.defineClass
(name, temp, 0, temp.length, pd);
- Parameters:
name
- The expected binary name. of the class, ornull
if not knownb
- The bytes that make up the class data. The bytes from positionsb.position()
throughb.position() + b.limit() -1
should have the format of a valid class file as defined by The Java Virtual Machine Specification.protectionDomain
- TheProtectionDomain
of the class, ornull
.- Returns:
- The
Class
object created from the data, andProtectionDomain
. - Throws:
ClassFormatError
- If the data did not contain a valid class.NoClassDefFoundError
- Ifname
is notnull
and not equal to the binary name of the class specified byb
SecurityException
- If an attempt is made to add this class to a package that contains classes that were signed by a different set of certificates than this class, or ifname
begins with "java.
".- Since:
- 1.5
- See Also:
-
resolveClass
Links the specified class. This (misleadingly named) method may be used by a class loader to link a class. If the classc
has already been linked, then this method simply returns. Otherwise, the class is linked as described in the "Execution" chapter of The Java Language Specification.- Parameters:
c
- The class to link- Throws:
NullPointerException
- Ifc
isnull
.- See Also:
-
findSystemClass
Finds a class with the specified binary name, loading it if necessary.This method loads the class through the system class loader (see
getSystemClassLoader()
). TheClass
object returned might have more than oneClassLoader
associated with it. Subclasses ofClassLoader
need not usually invoke this method, because most class loaders need to override justfindClass(String)
.- Parameters:
name
- The binary name of the class- Returns:
- The
Class
object for the specifiedname
- Throws:
ClassNotFoundException
- If the class could not be found- See Also:
-
findLoadedClass
Returns the class with the given binary name if this loader has been recorded by the Java virtual machine as an initiating loader of a class with that binary name. Otherwisenull
is returned.- Parameters:
name
- The binary name of the class- Returns:
- The
Class
object, ornull
if the class has not been loaded - Since:
- 1.1
-
setSigners
-
findResource
Returns a URL to a resource in a module defined to this class loader. Class loader implementations that support loading from modules should override this method.- API Note:
- This method is the basis for the
Class.getResource
,Class.getResourceAsStream
, andModule.getResourceAsStream
methods. It is not subject to the rules for encapsulation specified byModule.getResourceAsStream
. - Implementation Requirements:
- The default implementation attempts to find the resource by
invoking
findResource(String)
when themoduleName
isnull
. It otherwise returnsnull
. - Parameters:
moduleName
- The module name; ornull
to find a resource in the unnamed module for this class loadername
- The resource name- Returns:
- A URL to the resource;
null
if the resource could not be found, a URL could not be constructed to locate the resource, or there isn't a module of the given name defined to the class loader. - Throws:
IOException
- If I/O errors occur- Since:
- 9
- See Also:
-
getResource
Finds the resource with the given name. A resource is some data (images, audio, text, etc) that can be accessed by class code in a way that is independent of the location of the code.The name of a resource is a '
/
'-separated path name that identifies the resource.Resources in named modules are subject to the encapsulation rules specified by
Module.getResourceAsStream
. Additionally, and except for the special case where the resource has a name ending with ".class
", this method will only find resources in packages of named modules when the package isopened
unconditionally (even if the caller of this method is in the same module as the resource).- API Note:
- Where several modules are defined to the same class loader,
and where more than one module contains a resource with the given name,
then the ordering that modules are searched is not specified and may be
very unpredictable.
When overriding this method it is recommended that an implementation
ensures that any delegation is consistent with the
getResources(String)
method. - Implementation Requirements:
- The default implementation will first search the parent class
loader for the resource; if the parent is
null
the path of the class loader built into the virtual machine is searched. If not found, this method will invokefindResource(String)
to find the resource. - Parameters:
name
- The resource name- Returns:
URL
object for reading the resource;null
if the resource could not be found, aURL
could not be constructed to locate the resource, or the resource is in a package that is not opened unconditionally.- Throws:
NullPointerException
- Ifname
isnull
- Since:
- 1.1
-
getResources
Finds all the resources with the given name. A resource is some data (images, audio, text, etc) that can be accessed by class code in a way that is independent of the location of the code.The name of a resource is a
/
-separated path name that identifies the resource.Resources in named modules are subject to the encapsulation rules specified by
Module.getResourceAsStream
. Additionally, and except for the special case where the resource has a name ending with ".class
", this method will only find resources in packages of named modules when the package isopened
unconditionally (even if the caller of this method is in the same module as the resource).- API Note:
- Where several modules are defined to the same class loader,
and where more than one module contains a resource with the given name,
then the ordering is not specified and may be very unpredictable.
When overriding this method it is recommended that an
implementation ensures that any delegation is consistent with the
getResource(String)
method. This should ensure that the first element returned by the Enumeration'snextElement
method is the same resource that thegetResource(String)
method would return. - Implementation Requirements:
- The default implementation will first search the parent class
loader for the resource; if the parent is
null
the path of the class loader built into the virtual machine is searched. It then invokesfindResources(String)
to find the resources with the name in this class loader. It returns an enumeration whose elements are the URLs found by searching the parent class loader followed by the elements found withfindResources
. - Parameters:
name
- The resource name- Returns:
- An enumeration of
URL
objects for the resource. If no resources could be found, the enumeration will be empty. Resources for which aURL
cannot be constructed, or are in a package that is not opened unconditionally, are not returned in the enumeration. - Throws:
IOException
- If I/O errors occurNullPointerException
- Ifname
isnull
- Since:
- 1.2
-
resources
Returns a stream whose elements are the URLs of all the resources with the given name. A resource is some data (images, audio, text, etc) that can be accessed by class code in a way that is independent of the location of the code.The name of a resource is a
/
-separated path name that identifies the resource.The resources will be located when the returned stream is evaluated. If the evaluation results in an
IOException
then the I/O exception is wrapped in anUncheckedIOException
that is then thrown.Resources in named modules are subject to the encapsulation rules specified by
Module.getResourceAsStream
. Additionally, and except for the special case where the resource has a name ending with ".class
", this method will only find resources in packages of named modules when the package isopened
unconditionally (even if the caller of this method is in the same module as the resource).- API Note:
- When overriding this method it is recommended that an
implementation ensures that any delegation is consistent with the
getResource(String)
method. This should ensure that the first element returned by the stream is the same resource that thegetResource(String)
method would return. - Implementation Requirements:
- The default implementation invokes
getResources
to find all the resources with the given name and returns a stream with the elements in the enumeration as the source. - Parameters:
name
- The resource name- Returns:
- A stream of resource
URL
objects. If no resources could be found, the stream will be empty. Resources for which aURL
cannot be constructed, or are in a package that is not opened unconditionally, will not be in the stream. - Throws:
NullPointerException
- Ifname
isnull
- Since:
- 9
-
findResource
Finds the resource with the given name. Class loader implementations should override this method.For resources in named modules then the method must implement the rules for encapsulation specified in the
Module
getResourceAsStream
method. Additionally, it must not find non-".class
" resources in packages of named modules unless the package isopened
unconditionally.- Implementation Requirements:
- The default implementation returns
null
. - Parameters:
name
- The resource name- Returns:
URL
object for reading the resource;null
if the resource could not be found, aURL
could not be constructed to locate the resource, or the resource is in a package that is not opened unconditionally.- Since:
- 1.2
-
findResources
Returns an enumeration ofURL
objects representing all the resources with the given name. Class loader implementations should override this method.For resources in named modules then the method must implement the rules for encapsulation specified in the
Module
getResourceAsStream
method. Additionally, it must not find non-".class
" resources in packages of named modules unless the package isopened
unconditionally.- Implementation Requirements:
- The default implementation returns an enumeration that contains no elements.
- Parameters:
name
- The resource name- Returns:
- An enumeration of
URL
objects for the resource. If no resources could be found, the enumeration will be empty. Resources for which aURL
cannot be constructed, or are in a package that is not opened unconditionally, are not returned in the enumeration. - Throws:
IOException
- If I/O errors occur- Since:
- 1.2
-
registerAsParallelCapable
protected static boolean registerAsParallelCapable()Registers the caller as parallel capable. The registration succeeds if and only if all of the following conditions are met:- no instance of the caller has been created
- all of the super classes (except class Object) of the caller are registered as parallel capable
Note that once a class loader is registered as parallel capable, there is no way to change it back.
In cases where this method is called from a context where the caller is not a subclass of
ClassLoader
or there is no caller frame on the stack (e.g. when called directly from a JNI attached thread),IllegalCallerException
is thrown.- Returns:
true
if the caller is successfully registered as parallel capable andfalse
if otherwise.- Throws:
IllegalCallerException
- if the caller is not a subclass ofClassLoader
- Since:
- 1.7
- See Also:
-
isRegisteredAsParallelCapable
public final boolean isRegisteredAsParallelCapable()- Returns:
true
if this class loader is parallel capable, otherwisefalse
.- Since:
- 9
- See Also:
-
getSystemResource
Find a resource of the specified name from the search path used to load classes. This method locates the resource through the system class loader (seegetSystemClassLoader()
).Resources in named modules are subject to the encapsulation rules specified by
Module.getResourceAsStream
. Additionally, and except for the special case where the resource has a name ending with ".class
", this method will only find resources in packages of named modules when the package isopened
unconditionally.- Parameters:
name
- The resource name- Returns:
- A
URL
to the resource;null
if the resource could not be found, a URL could not be constructed to locate the resource, or the resource is in a package that is not opened unconditionally. - Since:
- 1.1
-
getSystemResources
Finds all resources of the specified name from the search path used to load classes. The resources thus found are returned as anEnumeration
ofURL
objects.The search order is described in the documentation for
getSystemResource(String)
.Resources in named modules are subject to the encapsulation rules specified by
Module.getResourceAsStream
. Additionally, and except for the special case where the resource has a name ending with ".class
", this method will only find resources in packages of named modules when the package isopened
unconditionally.- Parameters:
name
- The resource name- Returns:
- An enumeration of
URL
objects for the resource. If no resources could be found, the enumeration will be empty. Resources for which aURL
cannot be constructed, or are in a package that is not opened unconditionally, are not returned in the enumeration. - Throws:
IOException
- If I/O errors occur- Since:
- 1.2
-
getResourceAsStream
Returns an input stream for reading the specified resource.The search order is described in the documentation for
getResource(String)
.Resources in named modules are subject to the encapsulation rules specified by
Module.getResourceAsStream
. Additionally, and except for the special case where the resource has a name ending with ".class
", this method will only find resources in packages of named modules when the package isopened
unconditionally.- Parameters:
name
- The resource name- Returns:
- An input stream for reading the resource;
null
if the resource could not be found, or the resource is in a package that is not opened unconditionally. - Throws:
NullPointerException
- Ifname
isnull
- Since:
- 1.1
-
getSystemResourceAsStream
Open for reading, a resource of the specified name from the search path used to load classes. This method locates the resource through the system class loader (seegetSystemClassLoader()
).Resources in named modules are subject to the encapsulation rules specified by
Module.getResourceAsStream
. Additionally, and except for the special case where the resource has a name ending with ".class
", this method will only find resources in packages of named modules when the package isopened
unconditionally.- Parameters:
name
- The resource name- Returns:
- An input stream for reading the resource;
null
if the resource could not be found, or the resource is in a package that is not opened unconditionally. - Since:
- 1.1
-
getParent
Returns the parent class loader for delegation. Some implementations may usenull
to represent the bootstrap class loader. This method will returnnull
in such implementations if this class loader's parent is the bootstrap class loader.- Returns:
- The parent
ClassLoader
- Since:
- 1.2
-
getUnnamedModule
Returns the unnamedModule
for this class loader.- Returns:
- The unnamed Module for this class loader
- Since:
- 9
- See Also:
-
getPlatformClassLoader
Returns the platform class loader. All platform classes are visible to the platform class loader.- Implementation Note:
- The name of the builtin platform class loader is
"platform"
. - Returns:
- The platform
ClassLoader
. - Since:
- 9
-
getSystemClassLoader
Returns the system class loader. This is the default delegation parent for newClassLoader
instances, and is typically the class loader used to start the application.This method is first invoked early in the runtime's startup sequence, at which point it creates the system class loader. This class loader will be the context class loader for the main application thread (for example, the thread that invokes the
main
method of the main class).The default system class loader is an implementation-dependent instance of this class.
If the system property "
java.system.class.loader
" is defined when this method is first invoked then the value of that property is taken to be the name of a class that will be returned as the system class loader. The class is loaded using the default system class loader and must define a public constructor that takes a single parameter of typeClassLoader
which is used as the delegation parent. An instance is then created using this constructor with the default system class loader as the parameter. The resulting class loader is defined to be the system class loader. During construction, the class loader should take great care to avoid callinggetSystemClassLoader()
. If circular initialization of the system class loader is detected then anIllegalStateException
is thrown.- Implementation Note:
- The system property to override the system class loader is not
examined until the VM is almost fully initialized. Code that executes
this method during startup should take care not to cache the return
value until the system is fully initialized.
The name of the built-in system class loader is
"app"
. The system property "java.class.path
" is read during early initialization of the VM to determine the class path. An empty value of "java.class.path
" property is interpreted differently depending on whether the initial module (the module containing the main class) is named or unnamed: If named, the built-in system class loader will have no class path and will search for classes and resources using the application module path; otherwise, if unnamed, it will set the class path to the current working directory.JAR files on the class path may contain a
Class-Path
manifest attribute to specify dependent JAR files to be included in the class path.Class-Path
entries must meet certain conditions for validity (see the JAR File Specification for details). InvalidClass-Path
entries are ignored. For debugging purposes, ignored entries can be printed to the console if thejdk.net.URLClassPath.showIgnoredClassPathEntries
system property is set totrue
. - Returns:
- The system
ClassLoader
- Throws:
IllegalStateException
- If invoked recursively during the construction of the class loader specified by the "java.system.class.loader
" property.Error
- If the system property "java.system.class.loader
" is defined but the named class could not be loaded, the provider class does not define the required constructor, or an exception is thrown by that constructor when it is invoked. The underlying cause of the error can be retrieved via theThrowable.getCause()
method.
-
definePackage
protected Package definePackage(String name, String specTitle, String specVersion, String specVendor, String implTitle, String implVersion, String implVendor, URL sealBase) Defines a package by name in thisClassLoader
.Package names must be unique within a class loader and cannot be redefined or changed once created.
If a class loader wishes to define a package with specific properties, such as version information, then the class loader should call this
definePackage
method before callingdefineClass
. Otherwise, thedefineClass
method will define a package in this class loader corresponding to the package of the newly defined class; the properties of this defined package are specified byPackage
.- API Note:
- A class loader that wishes to define a package for classes in a JAR
typically uses the specification and implementation titles, versions, and
vendors from the JAR's manifest. If the package is specified as
sealed in the JAR's manifest,
the
URL
of the JAR file is typically used as thesealBase
. If classes of package'p'
defined by this class loader are loaded from multiple JARs, thePackage
object may contain different information depending on the first class of package'p'
defined and which JAR's manifest is read first to explicitly define package'p'
.It is strongly recommended that a class loader does not call this method to explicitly define packages in named modules; instead, the package will be automatically defined when a class is being defined. If it is desirable to define
Package
explicitly, it should ensure that all packages in a named module are defined with the properties specified byPackage
. Otherwise, somePackage
objects in a named module may be for example sealed with different seal base. - Parameters:
name
- The package namespecTitle
- The specification titlespecVersion
- The specification versionspecVendor
- The specification vendorimplTitle
- The implementation titleimplVersion
- The implementation versionimplVendor
- The implementation vendorsealBase
- If notnull
, then this package is sealed with respect to the given code sourceURL
object. Otherwise, the package is not sealed.- Returns:
- The newly defined
Package
object - Throws:
NullPointerException
- ifname
isnull
.IllegalArgumentException
- if a package of the givenname
is already defined by this class loader- See Java Virtual Machine Specification:
-
5.3 Creation and Loading
- Since:
- 1.2
- See Also:
-
getDefinedPackage
Returns aPackage
of the given name that has been defined by this class loader.- Parameters:
name
- The package name- Returns:
- The
Package
of the given name that has been defined by this class loader, ornull
if not found - Throws:
NullPointerException
- ifname
isnull
.- See Java Virtual Machine Specification:
-
5.3 Creation and Loading
- Since:
- 9
-
getDefinedPackages
Returns all of thePackage
s that have been defined by this class loader. The returned array has no duplicatedPackage
s of the same name.- API Note:
- This method returns an array rather than a
Set
orStream
for consistency with the existinggetPackages()
method. - Returns:
- The array of
Package
objects that have been defined by this class loader; or a zero length array if no package has been defined by this class loader. - See Java Virtual Machine Specification:
-
5.3 Creation and Loading
- Since:
- 9
-
getPackage
Deprecated.If multiple class loaders delegate to each other and define classes with the same package name, and one such loader relies on the lookup behavior ofgetPackage
to return aPackage
from a parent loader, then the properties exposed by thePackage
may not be as expected in the rest of the program. For example, thePackage
will only expose annotations from thepackage-info.class
file defined by the parent loader, even if annotations exist in apackage-info.class
file defined by a child loader. A more robust approach is to use thegetDefinedPackage(java.lang.String)
method which returns aPackage
for the specified class loader.Finds a package by name in this class loader and its ancestors.If this class loader defines a
Package
of the given name, thePackage
is returned. Otherwise, the ancestors of this class loader are searched recursively (parent by parent) for aPackage
of the given name.- API Note:
- The
platform class loader
may delegate to the application class loader but the application class loader is not its ancestor. When invoked on the platform class loader, this method will not find packages defined to the application class loader. - Parameters:
name
- The package name- Returns:
- The
Package
of the given name that has been defined by this class loader or its ancestors, ornull
if not found. - Throws:
NullPointerException
- ifname
isnull
.- Since:
- 1.2
- See Also:
-
getPackages
Returns all of thePackage
s that have been defined by this class loader and its ancestors. The returned array may contain more than onePackage
object of the same package name, each defined by a different class loader in the class loader hierarchy.- API Note:
- The
platform class loader
may delegate to the application class loader. In other words, packages in modules defined to the application class loader may be visible to the platform class loader. On the other hand, the application class loader is not its ancestor and hence when invoked on the platform class loader, this method will not return any packages defined to the application class loader. - Returns:
- The array of
Package
objects that have been defined by this class loader and its ancestors - Since:
- 1.2
- See Also:
-
findLibrary
Returns the absolute path name of a native library. The VM invokes this method to locate the native libraries that belong to classes loaded with this class loader. If this method returnsnull
, the VM searches the library along the path specified as the "java.library.path
" property.- Parameters:
libname
- The library name- Returns:
- The absolute path of the native library
- Since:
- 1.2
- See Also:
-
setDefaultAssertionStatus
public void setDefaultAssertionStatus(boolean enabled) Sets the default assertion status for this class loader. This setting determines whether classes loaded by this class loader and initialized in the future will have assertions enabled or disabled by default. This setting may be overridden on a per-package or per-class basis by invokingsetPackageAssertionStatus(String, boolean)
orsetClassAssertionStatus(String, boolean)
.- Parameters:
enabled
-true
if classes loaded by this class loader will henceforth have assertions enabled by default,false
if they will have assertions disabled by default.- Since:
- 1.4
-
setPackageAssertionStatus
Sets the package default assertion status for the named package. The package default assertion status determines the assertion status for classes initialized in the future that belong to the named package or any of its "subpackages".A subpackage of a package named p is any package whose name begins with "
p.
". For example,javax.swing.text
is a subpackage ofjavax.swing
, and bothjava.util
andjava.lang.reflect
are subpackages ofjava
.In the event that multiple package defaults apply to a given class, the package default pertaining to the most specific package takes precedence over the others. For example, if
javax.lang
andjavax.lang.reflect
both have package defaults associated with them, the latter package default applies to classes injavax.lang.reflect
.Package defaults take precedence over the class loader's default assertion status, and may be overridden on a per-class basis by invoking
setClassAssertionStatus(String, boolean)
.- Parameters:
packageName
- The name of the package whose package default assertion status is to be set. Anull
value indicates the unnamed package that is "current" (see section 7.4.2 of The Java Language Specification.)enabled
-true
if classes loaded by this classloader and belonging to the named package or any of its subpackages will have assertions enabled by default,false
if they will have assertions disabled by default.- Since:
- 1.4
-
setClassAssertionStatus
Sets the desired assertion status for the named top-level class in this class loader and any nested classes contained therein. This setting takes precedence over the class loader's default assertion status, and over any applicable per-package default. This method has no effect if the named class has already been initialized. (Once a class is initialized, its assertion status cannot change.)If the named class is not a top-level class, this invocation will have no effect on the actual assertion status of any class.
- Parameters:
className
- The fully qualified class name of the top-level class whose assertion status is to be set.enabled
-true
if the named class is to have assertions enabled when (and if) it is initialized,false
if the class is to have assertions disabled.- Since:
- 1.4
-
clearAssertionStatus
public void clearAssertionStatus()Sets the default assertion status for this class loader tofalse
and discards any package defaults or class assertion status settings associated with the class loader. This method is provided so that class loaders can be made to ignore any command line or persistent assertion status settings and "start with a clean slate."- Since:
- 1.4
-
defineClass(String, byte[], int, int)