public abstract class ClassLoader extends Object
A class loader is an object that is responsible for loading classes. The class 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
object contains a Class
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.
Class loaders may typically be used by security managers to indicate security domains.
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 delegate the search for the class or resource to its parent class loader before attempting to find the class or resource itself. The virtual machine's built-in class loader, called the "bootstrap class loader", does not itself have a parent but may serve as the parent of a ClassLoader
instance.
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
method. Note that the ClassLoader.registerAsParallelCapable
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
methods). loadClass
Normally, the Java virtual machine loads classes from the local file system in a platform-dependent manner. For example, on UNIX systems, the virtual machine loads classes from the directory defined by the CLASSPATH
environment variable.
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
converts an array of bytes into an instance of class defineClass
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
method of the class loader that originally created the class. loadClass
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
and findClass
loadClassData
to load a class from the network. Once it has downloaded the bytes that make up the class, it should use the method
to create a class instance. A sample implementation is: defineClass
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 . . . } }
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"
resolveClass(Class)
protected ClassLoader(ClassLoader parent)
Creates a new class loader using the specified parent class loader for delegation.
If there is a security manager, its
method is invoked. This may result in a security exception. checkCreateClassLoader
parent
- The parent class loaderSecurityException
- If a security manager exists and its checkCreateClassLoader
method doesn't allow creation of a new class loader.protected ClassLoader()
Creates a new class loader using the ClassLoader
returned by the method
as the parent class loader. getSystemClassLoader()
If there is a security manager, its
method is invoked. This may result in a security exception. checkCreateClassLoader
SecurityException
- If a security manager exists and its checkCreateClassLoader
method doesn't allow creation of a new class loader.public Class<?> loadClass(String name) throws ClassNotFoundException
Loads the class with the specified binary name. This method searches for classes in the same manner as the loadClass(String, boolean)
method. It is invoked by the Java virtual machine to resolve class references. Invoking this method is equivalent to invoking
.loadClass(name,
false)
name
- The binary name of the classClass
objectClassNotFoundException
- If the class was not foundprotected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException
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
method on the parent class loader. If the parent is loadClass
null
the class loader built-in to 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 the resolveClass(Class)
method on the resulting Class
object.
Subclasses of ClassLoader
are encouraged to override findClass(String)
, rather than this method.
Unless overridden, this method synchronizes on the result of
method during the entire class loading process.getClassLoadingLock
name
- The binary name of the classresolve
- If true
then resolve the classClass
objectClassNotFoundException
- If the class could not be foundprotected Object getClassLoadingLock(String className)
Returns the lock object for class loading operations. For backward compatibility, the default implementation of this method behaves as follows. If this ClassLoader object is registered as parallel capable, the method returns a dedicated object associated with the specified class name. Otherwise, the method returns this ClassLoader object.
className
- The name of the to-be-loaded classNullPointerException
- If registered as parallel capable and className
is nullloadClass(String, boolean)
protected Class<?> findClass(String name) throws ClassNotFoundException
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 the
method after checking the parent class loader for the requested class. The default implementation throws a loadClass
ClassNotFoundException
.
name
- The binary name of the classClass
objectClassNotFoundException
- If the class could not be found@Deprecated protected final Class<?> defineClass(byte[] b, int off, int len) throws ClassFormatError
Deprecated. Replaced by defineClass(String, byte[], int, int)
Converts an array of bytes into an instance of class Class
. Before the Class
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.
b
- The bytes that make up the class data. The bytes in positions off
through off+len-1
should have the format of a valid class file as defined by The Java™ Virtual Machine Specification.off
- The start offset in b
of the class datalen
- The length of the class dataClass
object that was created from the specified class dataClassFormatError
- If the data did not contain a valid classIndexOutOfBoundsException
- If either off
or len
is negative, or if off+len
is greater than b.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.
".loadClass(String, boolean)
, resolveClass(Class)
protected final Class<?> defineClass(String name, byte[] b, int off, int len) throws ClassFormatError
Converts an array of bytes into an instance of class Class
. Before the Class
can be used it must be resolved.
This method assigns a default
to the newly defined class. The ProtectionDomain
ProtectionDomain
is effectively granted the same set of permissions returned when
is invoked. The default domain is created on the first invocation of Policy.getPolicy().getPermissions(new CodeSource(null, null))
, and re-used on subsequent invocations. defineClass
To assign a specific ProtectionDomain
to the class, use the
method that takes a defineClass
ProtectionDomain
as one of its arguments.
name
- The expected binary name of the class, or null
if not knownb
- The bytes that make up the class data. The bytes in positions off
through off+len-1
should have the format of a valid class file as defined by The Java™ Virtual Machine Specification.off
- The start offset in b
of the class datalen
- The length of the class dataClass
object that was created from the specified class data.ClassFormatError
- If the data did not contain a valid classIndexOutOfBoundsException
- If either off
or len
is negative, or if off+len
is greater than b.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 if name
begins with "java.
".loadClass(String, boolean)
, resolveClass(Class)
, CodeSource
, SecureClassLoader
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 class Class
, with an optional ProtectionDomain
. If the domain is null
, then a default domain will be assigned to the class as specified in the documentation for defineClass(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
within the CodeSource
ProtectionDomain
of the class. Any classes added to that package must contain the same set of certificates or a SecurityException
will be thrown. Note that if name
is null
, 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.
The specified name
cannot begin with "java.
", since all classes in the "java.*
packages can only be defined by the bootstrap class loader. If name
is not null
, it must be equal to the binary name of the class specified by the byte array "b
", otherwise a
will be thrown. NoClassDefFoundError
name
- The expected binary name of the class, or null
if not knownb
- The bytes that make up the class data. The bytes in positions off
through off+len-1
should have the format of a valid class file as defined by The Java™ Virtual Machine Specification.off
- The start offset in b
of the class datalen
- The length of the class dataprotectionDomain
- The ProtectionDomain of the classClass
object created from the data, and optional ProtectionDomain
.ClassFormatError
- If the data did not contain a valid classNoClassDefFoundError
- If name
is not equal to the binary name of the class specified by b
IndexOutOfBoundsException
- If either off
or len
is negative, or if off+len
is greater than b.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 name
begins with "java.
".protected final Class<?> defineClass(String name, ByteBuffer b, ProtectionDomain protectionDomain) throws ClassFormatError
Converts a
into an instance of class ByteBuffer
Class
, with an optional ProtectionDomain
. If the domain is null
, then a default domain will be assigned to the class as specified in the documentation for defineClass(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, and the restrictions on class names 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);
return cl.defineClass
(name, temp, 0, temp.length, pd);
name
- The expected binary name. of the class, or null
if not knownb
- The bytes that make up the class data. The bytes from positions b.position()
through b.position() + b.limit() -1
should have the format of a valid class file as defined by The Java™ Virtual Machine Specification.protectionDomain
- The ProtectionDomain of the class, or null
.Class
object created from the data, and optional ProtectionDomain
.ClassFormatError
- If the data did not contain a valid class.NoClassDefFoundError
- If name
is not equal to the binary name of the class specified by b
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 name
begins with "java.
".defineClass(String, byte[], int, int, ProtectionDomain)
protected final void resolveClass(Class<?> c)
Links the specified class. This (misleadingly named) method may be used by a class loader to link a class. If the class c
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.
c
- The class to linkNullPointerException
- If c
is null
.defineClass(String, byte[], int, int)
protected final Class<?> findSystemClass(String name) throws ClassNotFoundException
Finds a class with the specified binary name, loading it if necessary.
This method loads the class through the system class loader (see getSystemClassLoader()
). The Class
object returned might have more than one ClassLoader
associated with it. Subclasses of ClassLoader
need not usually invoke this method, because most class loaders need to override just findClass(String)
.
name
- The binary name of the classClass
object for the specified name
ClassNotFoundException
- If the class could not be foundClassLoader(ClassLoader)
, getParent()
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. Otherwise null
is returned.
name
- The binary name of the classClass
object, or null
if the class has not been loadedprotected final void setSigners(Class<?> c, Object[] signers)
Sets the signers of a class. This should be invoked after defining a class.
c
- The Class
objectsigners
- The signers for the classpublic URL getResource(String name)
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.
This method will first search the parent class loader for the resource; if the parent is null
the path of the class loader built-in to the virtual machine is searched. That failing, this method will invoke findResource(String)
to find the resource.
getResources(String)
method.name
- The resource nameURL
object for reading the resource, or null
if the resource could not be found or the invoker doesn't have adequate privileges to get the resource.public Enumeration<URL> getResources(String name) throws IOException
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.
The search order is described in the documentation for getResource(String)
.
getResource(String)
method. This should ensure that the first element returned by the Enumeration's nextElement
method is the same resource that the getResource(String)
method would return.name
- The resource nameURL
objects for the resource. If no resources could be found, the enumeration will be empty. Resources that the class loader doesn't have access to will not be in the enumeration.IOException
- If I/O errors occurfindResources(String)
protected URL findResource(String name)
Finds the resource with the given name. Class loader implementations should override this method to specify where to find resources.
name
- The resource nameURL
object for reading the resource, or null
if the resource could not be foundprotected Enumeration<URL> findResources(String name) throws IOException
Returns an enumeration of
objects representing all the resources with the given name. Class loader implementations should override this method to specify where to load resources from.URL
name
- The resource nameURL
objects for the resourcesIOException
- If I/O errors occurprotected static boolean registerAsParallelCapable()
Registers the caller as parallel capable. The registration succeeds if and only if all of the following conditions are met:
Note that once a class loader is registered as parallel capable, there is no way to change it back.
public static URL getSystemResource(String name)
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 (see getSystemClassLoader()
).
name
- The resource nameURL
object for reading the resource, or null
if the resource could not be foundpublic static Enumeration<URL> getSystemResources(String name) throws IOException
Finds all resources of the specified name from the search path used to load classes. The resources thus found are returned as an
of Enumeration
objects. URL
The search order is described in the documentation for getSystemResource(String)
.
name
- The resource nameURL
objectsIOException
- If I/O errors occurpublic InputStream getResourceAsStream(String name)
Returns an input stream for reading the specified resource.
The search order is described in the documentation for getResource(String)
.
name
- The resource namenull
if the resource could not be foundpublic static InputStream getSystemResourceAsStream(String name)
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 (see getSystemClassLoader()
).
name
- The resource namenull
if the resource could not be foundpublic final ClassLoader getParent()
Returns the parent class loader for delegation. Some implementations may use null
to represent the bootstrap class loader. This method will return null
in such implementations if this class loader's parent is the bootstrap class loader.
If a security manager is present, and the invoker's class loader is not null
and is not an ancestor of this class loader, then this method invokes the security manager's
method with a checkPermission
permission to verify access to the parent class loader is permitted. If not, a RuntimePermission("getClassLoader")
SecurityException
will be thrown.
ClassLoader
SecurityException
- If a security manager exists and its checkPermission
method doesn't allow access to this class loader's parent class loader.public static ClassLoader getSystemClassLoader()
Returns the system class loader for delegation. This is the default delegation parent for new ClassLoader
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 and sets it as the context class loader of the invoking Thread
.
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 type ClassLoader
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.
If a security manager is present, and the invoker's class loader is not null
and the invoker's class loader is not the same as or an ancestor of the system class loader, then this method invokes the security manager's
method with a checkPermission
permission to verify access to the system class loader. If not, a RuntimePermission("getClassLoader")
SecurityException
will be thrown.
ClassLoader
for delegation, or null
if noneSecurityException
- If a security manager exists and its checkPermission
method doesn't allow access to the system class loader.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 the Throwable.getCause()
method.protected Package definePackage(String name, String specTitle, String specVersion, String specVendor, String implTitle, String implVersion, String implVendor, URL sealBase) throws IllegalArgumentException
Defines a package by name in this ClassLoader
. This allows class loaders to define the packages for their classes. Packages must be created before the class is defined, and package names must be unique within a class loader and cannot be redefined or changed once created.
name
- The package namespecTitle
- The specification titlespecVersion
- The specification versionspecVendor
- The specification vendorimplTitle
- The implementation titleimplVersion
- The implementation versionimplVendor
- The implementation vendorsealBase
- If not null
, then this package is sealed with respect to the given code source URL
object. Otherwise, the package is not sealed.Package
objectIllegalArgumentException
- If package name duplicates an existing package either in this class loader or one of its ancestorsprotected Package getPackage(String name)
Returns a Package
that has been defined by this class loader or any of its ancestors.
name
- The package namePackage
corresponding to the given name, or null
if not foundprotected Package[] getPackages()
Returns all of the Packages
defined by this class loader and its ancestors.
Package
objects defined by this ClassLoader
protected String findLibrary(String libname)
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 returns null
, the VM searches the library along the path specified as the "java.library.path
" property.
libname
- The library nameSystem.loadLibrary(String)
, System.mapLibraryName(String)
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 invoking setPackageAssertionStatus(String, boolean)
or setClassAssertionStatus(String, boolean)
.
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.public void setPackageAssertionStatus(String packageName, boolean enabled)
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 of javax.swing
, and both java.util
and java.lang.reflect
are subpackages of java
.
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
and javax.lang.reflect
both have package defaults associated with them, the latter package default applies to classes in javax.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)
.
packageName
- The name of the package whose package default assertion status is to be set. A null
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.public 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. 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.
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.public void clearAssertionStatus()
Sets the default assertion status for this class loader to false
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."
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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
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