java.lang.ClassLoader
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Java™ Platform Standard Ed. 6 |
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java.lang.Object
public abstract class ClassLoader
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 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.
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)| Constructor Summary | |
|---|---|
protected |
ClassLoader()
Creates a new class loader using the ClassLoader returned by the method getSystemClassLoader() as the parent class loader. |
protected |
ClassLoader(ClassLoader parent)
Creates a new class loader using the specified parent class loader for delegation. |
| Method Summary | |
|---|---|
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. |
protected Class<?> |
defineClass(byte[] b,
int off,
int len)
Deprecated. Replaced by defineClass(String, byte[], int, int) |
protected Class<?> |
defineClass(String name,
byte[] b,
int off,
int len)
Converts an array of bytes into an instance of class Class. |
protected Class<?> |
defineClass(String name,
byte[] b,
int off,
int len,
ProtectionDomain protectionDomain)
Converts an array of bytes into an instance of class Class, with an optional ProtectionDomain. |
protected Class<?> |
defineClass(String name,
ByteBuffer b,
ProtectionDomain protectionDomain)
Converts a ByteBuffer
into an instance of class Class,
with an optional 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 this ClassLoader. |
protected Class<?> |
findClass(String name)
Finds the class with the specified binary name. |
protected String |
findLibrary(String libname)
Returns the absolute path name of a native library. |
protected 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 Enumeration<URL> |
findResources(String name)
Returns an enumeration of URL objects
representing all the resources with the given name. |
protected Class<?> |
findSystemClass(String name)
Finds a class with the specified binary name, loading it if necessary. |
protected Package |
getPackage(String name)
Returns a Package that has been defined by this class loader or any of its ancestors. |
protected Package[] |
getPackages()
Returns all of the Packages defined by this class loader and its ancestors. |
ClassLoader |
getParent()
Returns the parent class loader for delegation. |
URL |
getResource(String name)
Finds the resource with the given name. |
InputStream |
getResourceAsStream(String name)
Returns an input stream for reading the specified resource. |
Enumeration<URL> |
getResources(String name)
Finds all the resources with the given name. |
static ClassLoader |
getSystemClassLoader()
Returns the system class loader for delegation. |
static 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 Enumeration<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. |
protected Class<?> |
loadClass(String name,
boolean resolve)
Loads the class with the specified binary name. |
protected void |
resolveClass(Class<?> c)
Links the specified class. |
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 void |
setSigners(Class<?> c,
Object[] signers)
Sets the signers of a class. |
| Methods inherited from class java.lang.Object |
|---|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
| Constructor Detail |
|---|
protected ClassLoader(ClassLoader parent)
If there is a security manager, its checkCreateClassLoader method is invoked. This may result in
a security exception.
parent - The parent class loader
SecurityException - If a security manager exists and its
checkCreateClassLoader method doesn't allow creation
of a new class loader.protected ClassLoader()
getSystemClassLoader() as the parent class loader.
If there is a security manager, its checkCreateClassLoader method is invoked. This may result in
a security exception.
SecurityException - If a security manager exists and its
checkCreateClassLoader method doesn't allow creation
of a new class loader.| Method Detail |
|---|
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 class
ClassNotFoundException - If the class was not found
protected Class<?> loadClass(String name,
boolean resolve)
throws ClassNotFoundException
Invoke findLoadedClass(String) to check if the class
has already been loaded.
Invoke the loadClass method
on the parent class loader. If the parent is 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.
name - The binary name of the classresolve - If true then resolve the class
ClassNotFoundException - If the class could not be found
protected Class<?> findClass(String name)
throws ClassNotFoundException
loadClass method after checking the
parent class loader for the requested class. The default implementation
throws a ClassNotFoundException.
name - The binary name of the class
ClassNotFoundException - If the class could not be found
@Deprecated
protected final Class<?> defineClass(byte[] b,
int off,
int len)
throws ClassFormatError
defineClass(String, byte[], int, int)
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 data
ClassFormatError - If the data did not contain a valid class
IndexOutOfBoundsException - If either off or len is negative, or if
off+len is greater than b.length.loadClass(String, boolean),
resolveClass(Class)
protected final Class<?> defineClass(String name,
byte[] b,
int off,
int len)
throws ClassFormatError
This method assigns a default ProtectionDomain to the newly defined class. The
ProtectionDomain is effectively granted the same set of
permissions returned when Policy.getPolicy().getPermissions(new CodeSource(null, null))
is invoked. The default domain is created on the first invocation of
defineClass,
and re-used on subsequent invocations.
To assign a specific ProtectionDomain to the class, use
the defineClass method that takes a
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 data
ClassFormatError - If the data did not contain a valid class
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 (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
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
CodeSource within the
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 NoClassDefFoundError will be thrown.
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 class
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
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
ByteBuffer
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 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);
returncl.defineClass(name, temp, 0, temp.length, pd);
name - The expected binary namenull 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.
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)
c - The class to link
NullPointerException - If c is null.defineClass(String, byte[], int, int)
protected final Class<?> findSystemClass(String name)
throws ClassNotFoundException
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 class
ClassNotFoundException - If the class could not be foundClassLoader(ClassLoader),
getParent()protected final Class<?> findLoadedClass(String name)
name - The binary name of the class
protected final void setSigners(Class<?> c,
Object[] signers)
c - The Class objectsigners - The signers for the classpublic 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 name
public Enumeration<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 name
URL 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)
name - The resource name
protected Enumeration<URL> findResources(String name)
throws IOException
URL objects
representing all the resources with the given name. Class loader
implementations should override this method to specify where to load
resources from.
name - The resource name
URL objects for
the resources
IOException - If I/O errors occurpublic static URL getSystemResource(String name)
getSystemClassLoader()).
name - The resource name
URL object for reading the
resource, or null if the resource could not be found
public static Enumeration<URL> getSystemResources(String name)
throws IOException
Enumeration of URL objects.
The search order is described in the documentation for getSystemResource(String).
name - The resource name
URL
objects
IOException - If I/O errors occurpublic InputStream getResourceAsStream(String name)
The search order is described in the documentation for getResource(String).
name - The resource name
public static InputStream getSystemResourceAsStream(String name)
getSystemClassLoader()).
name - The resource name
public 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.
protected Package definePackage(String name,
String specTitle,
String specVersion,
String specVendor,
String implTitle,
String implVersion,
String implVendor,
URL sealBase)
throws IllegalArgumentException
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.
IllegalArgumentException - If package name duplicates an existing package either in this
class loader or one of its ancestorsprotected Package getPackage(String name)
name - The package name
protected Package[] getPackages()
protected String findLibrary(String libname)
libname - The library name
System.loadLibrary(String),
System.mapLibraryName(String)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"
(Java Language
Specification, section 7.4.2).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()
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Java™ Platform Standard Ed. 6 |
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