public abstract class ClassLoader extends Object
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.
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
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).
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 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
. . .
}
}
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)| Modifier and Type | Method and Description |
|---|---|
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.
|
ClassLoader |
getParent()
Returns the parent class loader for delegation.
|
java.net.URL |
getResource(String name)
Finds the resource with the given name.
|
InputStream |
getResourceAsStream(String name)
Returns an input stream for reading the specified resource.
|
java.util.Enumeration<java.net.URL> |
getResources(String name)
Finds all the resources with the given name.
|
static ClassLoader |
getSystemClassLoader()
Returns the system class loader for delegation.
|
static java.net.URL |
getSystemResource(String name)
Find a resource of the specified name from the search path used to load
classes.
|
static InputStream |
getSystemResourceAsStream(String name)
Open for reading, a resource of the specified name from the search path
used to load classes.
|
static java.util.Enumeration<java.net.URL> |
getSystemResources(String name)
Finds all resources of the specified name from the search path used to
load classes.
|
Class<?> |
loadClass(String name)
Loads the class with the specified binary 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.
|
public Class<?> loadClass(String name) throws ClassNotFoundException
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 classClassNotFoundException - If the class was not foundpublic java.net.URL getResource(String name)
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.
name - The resource namepublic java.util.Enumeration<java.net.URL> getResources(String name) throws IOException
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).
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)public static java.net.URL getSystemResource(String name)
getSystemClassLoader()). name - The resource nameURL object for reading the
resource, or null if the resource could not be foundpublic static java.util.Enumeration<java.net.URL> getSystemResources(String name) throws IOException
Enumeration of URL objects.
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)
The search order is described in the documentation for getResource(String).
name - The resource namepublic static InputStream getSystemResourceAsStream(String name)
getSystemClassLoader()). name - The resource namepublic final ClassLoader getParent()
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 checkPermission method with a RuntimePermission("getClassLoader") permission to verify
access to the parent class loader is permitted. If not, a
SecurityException will be thrown.
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()
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 checkPermission method with a RuntimePermission("getClassLoader") permission to verify
access to the system class loader. If not, a
SecurityException will be thrown.
SecurityException - 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.public void setDefaultAssertionStatus(boolean enabled)
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)
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)
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()