Every Class object contains a Class.getClassLoader()
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()
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(java.lang.String)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
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(java.lang.Class) If there is a security manager, its SecurityManager.checkCreateClassLoader()
parent
The parent class loaderSecurityException
If a security manager exists and its
checkCreateClassLoader method doesn't allow creation
of a new class loader. getSystemClassLoader() as the parent class loader.
If there is a security manager, its SecurityManager.checkCreateClassLoader()
SecurityException
If a security manager exists and its
checkCreateClassLoader method doesn't allow creation
of a new class loader.loadClass(java.lang.String,boolean)loadClass(name,
false).
name
The binary name of the classClassNotFoundException
If the class was not found Invoke findLoadedClass(java.lang.String)
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(java.lang.String)
If the class was found using the above steps, and the
resolve flag is true, this method will then invoke the resolveClass(java.lang.Class)
Subclasses of ClassLoader are encouraged to override findClass(java.lang.String)
name
The binary name of the classresolve
If true then resolve the classClassNotFoundException
If the class could not be foundloadClass method after checking the
parent class loader for the requested class. The default implementation
throws a ClassNotFoundException.
name
The binary name of the classClassNotFoundException
If the class could not be found 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 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.loadClass(java.lang.String,boolean)resolveClass(java.lang.Class) This method assigns a default ProtectionDomain to the newly defined class. The
ProtectionDomain is effectively granted the same set of
permissions returned when java.security.Policy.getPermissions(java.security.CodeSource)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 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 (which is unsigned), or if
name begins with "java.".loadClass(java.lang.String,boolean)resolveClass(java.lang.Class)java.security.CodeSourcejava.security.SecureClassLoaderdefineClass(java.lang.String,byte[],int,int) 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 classClassFormatError
If the data did not contain a valid classNoClassDefFoundError
If name is not equal to the binary
name of the class specified by bIndexOutOfBoundsException
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.".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(java.lang.String,byte[],int,int)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(java.lang.String,byte[],int,int,java.security.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 bSecurityException
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(java.lang.String,byte[],int,int,java.security.ProtectionDomain) "\"'s signer information does not match signer information of other classes in the same package");c
The class to linkNullPointerException
If c is null.defineClass(java.lang.String,byte[],int,int) This method loads the class through the system class loader (see
getSystemClassLoader()findClass(java.lang.String)
name
The binary name of the classClassNotFoundException
If the class could not be foundClassLoader(java.lang.ClassLoader) getParent()name
The binary name of the classThe 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(java.lang.String)
name
The resource nameThe name of a resource is a /-separated path name that identifies the resource.
The search order is described in the documentation for getResource(java.lang.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.java.io.IOException
If I/O errors occurfindResources(java.lang.String)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 nameURL objects for
the resourcesjava.io.IOException
If I/O errors occurgetSystemClassLoader()name
The resource nameURL object for reading the
resource, or null if the resource could not be foundEnumeration of java.net.URL The search order is described in the documentation for getSystemResource(java.lang.String)
name
The resource nameURL
objectsjava.io.IOException
If I/O errors occur The search order is described in the documentation for getResource(java.lang.String)
name
The resource namegetSystemClassLoader()name
The resource name 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 SecurityManager.checkPermission(java.security.Permission)RuntimePermission.
SecurityException
If a security manager exists and its checkPermission
method doesn't allow access to this class loader's parent class
loader.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 SecurityManager.checkPermission(java.security.Permission)RuntimePermission.
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()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 ancestorslibname
The library nameSystem.loadLibrary(java.lang.String)System.mapLibraryName(java.lang.String)Every native library requires a particular version of JNI. This is denoted by the private jniVersion field. This field is set by the VM when it loads the library, and used by the VM to pass the correct version of JNI to the native methods.
ClassLoadersetPackageAssertionStatus(java.lang.String,boolean)setClassAssertionStatus(java.lang.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.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(java.lang.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.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.className
The fully qualified class name of the class whose desired
assertion status is being queried.setClassAssertionStatus(java.lang.String,boolean)setPackageAssertionStatus(java.lang.String,boolean)setDefaultAssertionStatus(boolean)