Class URLClassLoader

All Implemented Interfaces:
Closeable, AutoCloseable
Direct Known Subclasses:
MLet

public class URLClassLoader
extends SecureClassLoader
implements Closeable

This class loader is used to load classes and resources from a search path of URLs referring to both JAR files and directories. Any URL that ends with a '/' is assumed to refer to a directory. Otherwise, the URL is assumed to refer to a JAR file which will be opened as needed.

The AccessControlContext of the thread that created the instance of URLClassLoader will be used when subsequently loading classes and resources.

The classes that are loaded are by default granted permission only to access the URLs specified when the URLClassLoader was created.

Since:
1.2

Constructors

Constructor and Description
URLClassLoader(URL[] urls)

Constructs a new URLClassLoader for the specified URLs using the default delegation parent ClassLoader.
URLClassLoader(URL[] urls, ClassLoader parent)

Constructs a new URLClassLoader for the given URLs.
URLClassLoader(URL[] urls, ClassLoader parent, URLStreamHandlerFactory factory)

Constructs a new URLClassLoader for the specified URLs, parent class loader, and URLStreamHandlerFactory.

Methods

Modifier and Type Method and Description
protected void addURL(URL url)

Appends the specified URL to the list of URLs to search for classes and resources.
void close()

Closes this URLClassLoader, so that it can no longer be used to load new classes or resources that are defined by this loader.
protected Package definePackage(String name, Manifest man, URL url)

Defines a new package by name in this ClassLoader.
protected Class<?> findClass(String name)

Finds and loads the class with the specified name from the URL search path.
URL findResource(String name)

Finds the resource with the specified name on the URL search path.
Enumeration<URL> findResources(String name)

Returns an Enumeration of URLs representing all of the resources on the URL search path having the specified name.
protected PermissionCollection getPermissions(CodeSource codesource)

Returns the permissions for the given codesource object.
InputStream getResourceAsStream(String name)

Returns an input stream for reading the specified resource.
URL[] getURLs()

Returns the search path of URLs for loading classes and resources.
static URLClassLoader newInstance(URL[] urls)

Creates a new instance of URLClassLoader for the specified URLs and default parent class loader.
static URLClassLoader newInstance(URL[] urls, ClassLoader parent)

Creates a new instance of URLClassLoader for the specified URLs and parent class loader.

Methods inherited from class java.security.SecureClassLoader

defineClass, defineClass

Methods inherited from class java.lang.ClassLoader

clearAssertionStatus, defineClass, defineClass, defineClass, defineClass, definePackage, findLibrary, findLoadedClass, findSystemClass, getClassLoadingLock, getPackage, getPackages, getParent, getResource, getResources, getSystemClassLoader, getSystemResource, getSystemResourceAsStream, getSystemResources, loadClass, loadClass, registerAsParallelCapable, resolveClass, setClassAssertionStatus, setDefaultAssertionStatus, setPackageAssertionStatus, setSigners

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructors

URLClassLoader

public URLClassLoader(URL[] urls,
                      ClassLoader parent)

Constructs a new URLClassLoader for the given URLs. The URLs will be searched in the order specified for classes and resources after first searching in the specified parent class loader. Any URL that ends with a '/' is assumed to refer to a directory. Otherwise, the URL is assumed to refer to a JAR file which will be downloaded and opened as needed.

If there is a security manager, this method first calls the security manager's checkCreateClassLoader method to ensure creation of a class loader is allowed.

Parameters:
urls - the URLs from which to load classes and resources
parent - the parent class loader for delegation
Throws:
SecurityException - if a security manager exists and its checkCreateClassLoader method doesn't allow creation of a class loader.
NullPointerException - if urls is null.
See Also:
SecurityManager.checkCreateClassLoader()

URLClassLoader

public URLClassLoader(URL[] urls)

Constructs a new URLClassLoader for the specified URLs using the default delegation parent ClassLoader. The URLs will be searched in the order specified for classes and resources after first searching in the parent class loader. Any URL that ends with a '/' is assumed to refer to a directory. Otherwise, the URL is assumed to refer to a JAR file which will be downloaded and opened as needed.

If there is a security manager, this method first calls the security manager's checkCreateClassLoader method to ensure creation of a class loader is allowed.

Parameters:
urls - the URLs from which to load classes and resources
Throws:
SecurityException - if a security manager exists and its checkCreateClassLoader method doesn't allow creation of a class loader.
NullPointerException - if urls is null.
See Also:
SecurityManager.checkCreateClassLoader()

URLClassLoader

public URLClassLoader(URL[] urls,
                      ClassLoader parent,
                      URLStreamHandlerFactory factory)

Constructs a new URLClassLoader for the specified URLs, parent class loader, and URLStreamHandlerFactory. The parent argument will be used as the parent class loader for delegation. The factory argument will be used as the stream handler factory to obtain protocol handlers when creating new jar URLs.

If there is a security manager, this method first calls the security manager's checkCreateClassLoader method to ensure creation of a class loader is allowed.

Parameters:
urls - the URLs from which to load classes and resources
parent - the parent class loader for delegation
factory - the URLStreamHandlerFactory to use when creating URLs
Throws:
SecurityException - if a security manager exists and its checkCreateClassLoader method doesn't allow creation of a class loader.
NullPointerException - if urls is null.
See Also:
SecurityManager.checkCreateClassLoader()

Methods

getResourceAsStream

public InputStream getResourceAsStream(String name)

Returns an input stream for reading the specified resource. If this loader is closed, then any resources opened by this method will be closed.

The search order is described in the documentation for ClassLoader.getResource(String).

Overrides:
getResourceAsStream in class ClassLoader
Parameters:
name - The resource name
Returns:
An input stream for reading the resource, or null if the resource could not be found
Since:
1.7

close

public void close()
           throws IOException

Closes this URLClassLoader, so that it can no longer be used to load new classes or resources that are defined by this loader. Classes and resources defined by any of this loader's parents in the delegation hierarchy are still accessible. Also, any classes or resources that are already loaded, are still accessible.

In the case of jar: and file: URLs, it also closes any files that were opened by it. If another thread is loading a class when the close method is invoked, then the result of that load is undefined.

The method makes a best effort attempt to close all opened files, by catching IOExceptions internally. Unchecked exceptions and errors are not caught. Calling close on an already closed loader has no effect.

Specified by:
close in interface Closeable
Specified by:
close in interface AutoCloseable
Throws:
IOException - if closing any file opened by this class loader resulted in an IOException. Any such exceptions are caught internally. If only one is caught, then it is re-thrown. If more than one exception is caught, then the second and following exceptions are added as suppressed exceptions of the first one caught, which is then re-thrown.
SecurityException - if a security manager is set, and it denies RuntimePermission("closeClassLoader")
Since:
1.7

addURL

protected void addURL(URL url)

Appends the specified URL to the list of URLs to search for classes and resources.

If the URL specified is null or is already in the list of URLs, or if this loader is closed, then invoking this method has no effect.

Parameters:
url - the URL to be added to the search path of URLs

getURLs

public URL[] getURLs()

Returns the search path of URLs for loading classes and resources. This includes the original list of URLs specified to the constructor, along with any URLs subsequently appended by the addURL() method.

Returns:
the search path of URLs for loading classes and resources.

findClass

protected Class<?> findClass(String name)
                      throws ClassNotFoundException

Finds and loads the class with the specified name from the URL search path. Any URLs referring to JAR files are loaded and opened as needed until the class is found.

Overrides:
findClass in class ClassLoader
Parameters:
name - the name of the class
Returns:
the resulting class
Throws:
ClassNotFoundException - if the class could not be found, or if the loader is closed.
NullPointerException - if name is null.

definePackage

protected Package definePackage(String name,
                                Manifest man,
                                URL url)
                         throws IllegalArgumentException

Defines a new package by name in this ClassLoader. The attributes contained in the specified Manifest will be used to obtain package version and sealing information. For sealed packages, the additional URL specifies the code source URL from which the package was loaded.

Parameters:
name - the package name
man - the Manifest containing package version and sealing information
url - the code source url for the package, or null if none
Returns:
the newly defined Package object
Throws:
IllegalArgumentException - if the package name duplicates an existing package either in this class loader or one of its ancestors

findResource

public URL findResource(String name)

Finds the resource with the specified name on the URL search path.

Overrides:
findResource in class ClassLoader
Parameters:
name - the name of the resource
Returns:
a URL for the resource, or null if the resource could not be found, or if the loader is closed.

findResources

public Enumeration<URL> findResources(String name)
                               throws IOException

Returns an Enumeration of URLs representing all of the resources on the URL search path having the specified name.

Overrides:
findResources in class ClassLoader
Parameters:
name - the resource name
Returns:
an Enumeration of URLs If the loader is closed, the Enumeration will be empty.
Throws:
IOException - if an I/O exception occurs

getPermissions

protected PermissionCollection getPermissions(CodeSource codesource)

Returns the permissions for the given codesource object. The implementation of this method first calls super.getPermissions and then adds permissions based on the URL of the codesource.

If the protocol of this URL is "jar", then the permission granted is based on the permission that is required by the URL of the Jar file.

If the protocol is "file" and there is an authority component, then permission to connect to and accept connections from that authority may be granted. If the protocol is "file" and the path specifies a file, then permission to read that file is granted. If protocol is "file" and the path is a directory, permission is granted to read all files and (recursively) all files and subdirectories contained in that directory.

If the protocol is not "file", then permission to connect to and accept connections from the URL's host is granted.

Overrides:
getPermissions in class SecureClassLoader
Parameters:
codesource - the codesource
Returns:
the permissions granted to the codesource
Throws:
NullPointerException - if codesource is null.

newInstance

public static URLClassLoader newInstance(URL[] urls,
                                         ClassLoader parent)

Creates a new instance of URLClassLoader for the specified URLs and parent class loader. If a security manager is installed, the loadClass method of the URLClassLoader returned by this method will invoke the SecurityManager.checkPackageAccess method before loading the class.

Parameters:
urls - the URLs to search for classes and resources
parent - the parent class loader for delegation
Returns:
the resulting class loader
Throws:
NullPointerException - if urls is null.

newInstance

public static URLClassLoader newInstance(URL[] urls)

Creates a new instance of URLClassLoader for the specified URLs and default parent class loader. If a security manager is installed, the loadClass method of the URLClassLoader returned by this method will invoke the SecurityManager.checkPackageAccess before loading the class.

Parameters:
urls - the URLs to search for classes and resources
Returns:
the resulting class loader
Throws:
NullPointerException - if urls is null.

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Documentation extracted from Debian's OpenJDK Development Kit package.
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https://docs.oracle.com/javase/8/docs/api/java/net/URLClassLoader.html