W3cubDocs

Class ClassLoader

java.lang.Object

java.lang.ClassLoader

Direct Known Subclasses:: SecureClassLoader

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 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.

In addition to loading classes, a class loader is also responsible for locating resources. A resource is some data (a ".class" file, configuration data, or an image for example) that is identified with an abstract '/'-separated path name. Resources are typically packaged with an application or library so that they can be located by code in the application or library. In some cases, the resources are included so that they can be located by other libraries.

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 usually delegate the search for the class or resource to its parent class loader before attempting to find the class or resource itself.

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).

Run-time Built-in Class Loaders

The Java run-time has the following built-in class loaders:

Bootstrap class loader. It is the virtual machine's built-in class loader, typically represented as null, and does not have a parent.
Platform class loader. The platform class loader is responsible for loading the platform classes. Platform classes include Java SE platform APIs, their implementation classes and JDK-specific run-time classes that are defined by the platform class loader or its ancestors. The platform class loader can be used as the parent of a ClassLoader instance.

To allow for upgrading/overriding of modules defined to the platform class loader, and where upgraded modules read modules defined to class loaders other than the platform class loader and its ancestors, then the platform class loader may have to delegate to other class loaders, the application class loader for example. In other words, classes in named modules defined to class loaders other than the platform class loader and its ancestors may be visible to the platform class loader.
System class loader. It is also known as application class loader and is distinct from the platform class loader. The system class loader is typically used to define classes on the application class path, module path, and JDK-specific tools. The platform class loader is the parent or an ancestor of the system class loader, so the system class loader can load platform classes by delegating to its parent.

Normally, the Java virtual machine loads classes from the local file system in a platform-dependent manner. 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
              . . .
         }
     }

Binary names

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"

Any package name provided as a String parameter to methods in ClassLoader must be either the empty string (denoting an unnamed package) or a fully qualified name as defined by The Java Language Specification.

See Java Language Specification:

6.7 Fully Qualified Names
13.1 The Form of a Binary

Since:

1.0

See Also:

Constructor Summary

ClassLoader()

ClassLoader(ClassLoader parent)

ClassLoader(String name,
 ClassLoader parent)

Modifier	Constructor	Description
`protected`	Creates a new class loader using the `ClassLoader` returned by the method `getSystemClassLoader()` as the parent class loader.
`protected`	Creates a new class loader using the specified parent class loader for delegation.
`protected`	Creates a new class loader of the specified name and using the specified parent class loader for delegation.

Method Summary

Modifier and Type	Method	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.
`protected final Class<?>`	`defineClass(byte[] b, int off, int len)`	Deprecated. Replaced by `defineClass(String, byte[], int, int)`
`protected final Class<?>`	`defineClass(String name, byte[] b, int off, int len)`	Converts an array of bytes into an instance of class `Class`.
`protected final Class<?>`	`defineClass(String name, byte[] b, int off, int len, ProtectionDomain protectionDomain)`	Converts an array of bytes into an instance of class `Class`, with a given `ProtectionDomain`.
`protected final Class<?>`	`defineClass(String name, ByteBuffer b, ProtectionDomain protectionDomain)`	Converts a `ByteBuffer` into an instance of class `Class`, with the given `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 Class<?>`	`findClass(String moduleName, String name)`	Finds the class with the given binary name in a module defined to this class loader.
`protected String`	`findLibrary(String libname)`	Returns the absolute path name of a native library.
`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.
`protected URL`	`findResource(String name)`	Finds the resource with the given name.
`protected URL`	`findResource(String moduleName, String name)`	Returns a URL to a resource in a module defined to this class loader.
`protected Enumeration<URL>`	`findResources(String name)`	Returns an enumeration of `URL` objects representing all the resources with the given name.
`protected final Class<?>`	`findSystemClass(String name)`	Finds a class with the specified binary name, loading it if necessary.
`protected Object`	`getClassLoadingLock(String className)`	Returns the lock object for class loading operations.
`final Package`	`getDefinedPackage(String name)`	Returns a `Package` of the given name that has been defined by this class loader.
`final Package[]`	`getDefinedPackages()`	Returns all of the `Package`s that have been defined by this class loader.
`String`	`getName()`	Returns the name of this class loader or `null` if this class loader is not named.
`protected Package`	`getPackage(String name)`	Deprecated. If multiple class loaders delegate to each other and define classes with the same package name, and one such loader relies on the lookup behavior of `getPackage` to return a `Package` from a parent loader, then the properties exposed by the `Package` may not be as expected in the rest of the program.
`protected Package[]`	`getPackages()`	Returns all of the `Package`s that have been defined by this class loader and its ancestors.
`final ClassLoader`	`getParent()`	Returns the parent class loader for delegation.
`static ClassLoader`	`getPlatformClassLoader()`	Returns the platform class loader.
`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.
`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.
`final Module`	`getUnnamedModule()`	Returns the unnamed `Module` for this class loader.
`final boolean`	`isRegisteredAsParallelCapable()`	Returns `true` if this class loader is registered as parallel capable, otherwise `false`.
`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 static boolean`	`registerAsParallelCapable()`	Registers the caller as parallel capable.
`protected final void`	`resolveClass(Class<?> c)`	Links the specified class.
`Stream<URL>`	`resources(String name)`	Returns a stream whose elements are the URLs of all the resources with the given 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.
`protected final void`	`setSigners(Class<?> c, Object[] signers)`	Sets the signers of a class.

Methods declared in class java.lang.Object

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

Constructor Details

ClassLoader

protected ClassLoader(String name, ClassLoader parent)

Creates a new class loader of the specified name and using the specified parent class loader for delegation.

API Note:: If the parent is specified as null (for the bootstrap class loader) then there is no guarantee that all platform classes are visible.
Parameters:: name - class loader name; or null if not named; parent - the parent class loader
Throws:: IllegalArgumentException - if the given name is empty.; SecurityException - If a security manager exists and its SecurityManager.checkCreateClassLoader() method doesn't allow creation of a new class loader.
Since:: 9

ClassLoader

protected ClassLoader(ClassLoader parent)

Creates a new class loader using the specified parent class loader for delegation.

If there is a security manager, its checkCreateClassLoader method is invoked. This may result in a security exception.

API Note:: If the parent is specified as null (for the bootstrap class loader) then there is no guarantee that all platform classes are visible.
Parameters:: parent - The parent class loader
Throws:: SecurityException - If a security manager exists and its checkCreateClassLoader method doesn't allow creation of a new class loader.
Since:: 1.2

ClassLoader

protected ClassLoader()

Creates a new class loader using the ClassLoader returned by the method getSystemClassLoader() as the parent class loader.

If there is a security manager, its checkCreateClassLoader method is invoked. This may result in a security exception.

Throws:: SecurityException - If a security manager exists and its checkCreateClassLoader method doesn't allow creation of a new class loader.

Method Details

getName

public String getName()

Returns the name of this class loader or null if this class loader is not named.

API Note:: This method is non-final for compatibility. If this method is overridden, this method must return the same name as specified when this class loader was instantiated.
Returns:: name of this class loader; or null if this class loader is not named.
Since:: 9

loadClass

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)

Parameters:: name - The binary name of the class
Returns:: The resulting Class object
Throws:: ClassNotFoundException - If the class was not found

loadClass

protected 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 loadClass method on the parent class loader. If the parent is null the class loader built into 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 getClassLoadingLock method during the entire class loading process.

Parameters:: name - The binary name of the class; resolve - If true then resolve the class
Returns:: The resulting Class object
Throws:: ClassNotFoundException - If the class could not be found

getClassLoadingLock

protected Object getClassLoadingLock(String className)

Returns the lock object for class loading operations.

API Note:

This method allows parallel capable class loaders to implement finer-grained locking schemes such that multiple threads may load classes concurrently without deadlocks. For non-parallel-capable class loaders, the ClassLoader object is synchronized on during the class loading operations. Class loaders with non-hierarchical delegation should be registered as parallel capable to prevent deadlocks.

Implementation Requirements:

If this ClassLoader object is registered as parallel capable, this method returns a dedicated object associated with the specified class name. Otherwise, this method returns this ClassLoader object.

Parameters:

className - The name of the to-be-loaded class

Returns:

the lock for class loading operations

Throws:

NullPointerException - If registered as parallel capable and className is null

Since:

1.7

See Also:

findClass

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 loadClass method after checking the parent class loader for the requested class.

Implementation Requirements:: The default implementation throws ClassNotFoundException.
Parameters:: name - The binary name of the class
Returns:: The resulting Class object
Throws:: ClassNotFoundException - If the class could not be found
Since:: 1.2

findClass

protected Class<?> findClass(String moduleName, String name)

Finds the class with the given binary name in a module defined to this class loader. Class loader implementations that support loading from modules should override this method.

API Note:: This method returns null rather than throwing ClassNotFoundException if the class could not be found.
Implementation Requirements:: The default implementation attempts to find the class by invoking findClass(String) when the moduleName is null. It otherwise returns null.
Parameters:: moduleName - The module name; or null to find the class in the unnamed module for this class loader; name - The binary name of the class
Returns:: The resulting Class object, or null if the class could not be found.
Since:: 9

defineClass

@Deprecated(since="1.1") 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.

Parameters:

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 data

len - The length of the class data

Returns:

The Class object that was created from the specified class data

Throws:

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, or if an attempt is made to define a class in a package with a fully-qualified name that starts with "java.".

See Also:

defineClass

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 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 protection 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.

This method defines a package in this class loader corresponding to the package of the Class (if such a package has not already been defined in this class loader). The name of the defined package is derived from the binary name of the class specified by the byte array b. Other properties of the defined package are as specified by Package.

Parameters:

name - The expected binary name of the class, or null if not known

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 data

len - The length of the class data

Returns:

The Class object that was created from the specified class data.

Throws:

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.".

Since:

1.1

See Also:

defineClass

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 a given ProtectionDomain.

If the given ProtectionDomain is null, then a default protection 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 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.

If the specified name begins with "java.", it can only be defined by the platform class loader or its ancestors; otherwise SecurityException will be thrown. 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.

Parameters:: name - The expected binary name of the class, or null if not known; 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 data; len - The length of the class data; protectionDomain - The ProtectionDomain of the class
Returns:: The Class object created from the data, and ProtectionDomain.
Throws:: ClassFormatError - If the data did not contain a valid class; NoClassDefFoundError - If name is not null and 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." and this class loader is not the platform class loader or its ancestor.

defineClass

protected final Class<?> defineClass(String name, ByteBuffer b, ProtectionDomain protectionDomain) throws ClassFormatError

Converts a ByteBuffer into an instance of class Class, with the given ProtectionDomain. If the given ProtectionDomain is null, then a default protection 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, the restrictions on class names, and the defined package of the class 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);

Parameters:

name - The expected binary name. of the class, or null if not known

b - 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.

Returns:

The Class object created from the data, and ProtectionDomain.

Throws:

ClassFormatError - If the data did not contain a valid class.

NoClassDefFoundError - If name is not null and 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.".

Since:

1.5

See Also:

resolveClass

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.

Parameters:

c - The class to link

Throws:

NullPointerException - If c is null.

See Also:

findSystemClass

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).

Parameters:

name - The binary name of the class

Returns:

The Class object for the specified name

Throws:

ClassNotFoundException - If the class could not be found

See Also:

findLoadedClass

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.

Parameters:: name - The binary name of the class
Returns:: The Class object, or null if the class has not been loaded
Since:: 1.1

setSigners

protected final void setSigners(Class<?> c, Object[] signers)

Sets the signers of a class. This should be invoked after defining a class.

Parameters:: c - The Class object; signers - The signers for the class
Since:: 1.1

findResource

protected URL findResource(String moduleName, String name) throws IOException

Returns a URL to a resource in a module defined to this class loader. Class loader implementations that support loading from modules should override this method.

API Note:

This method is the basis for the Class.getResource, Class.getResourceAsStream, and Module.getResourceAsStream methods. It is not subject to the rules for encapsulation specified by Module.getResourceAsStream.

Implementation Requirements:

The default implementation attempts to find the resource by invoking findResource(String) when the moduleName is null. It otherwise returns null.

Parameters:

moduleName - The module name; or null to find a resource in the unnamed module for this class loader

name - The resource name

Returns:

A URL to the resource; null if the resource could not be found, a URL could not be constructed to locate the resource, access to the resource is denied by the security manager, or there isn't a module of the given name defined to the class loader.

Throws:

IOException - If I/O errors occur

Since:

See Also:

getResource

public 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.

Resources in named modules are subject to the encapsulation rules specified by Module.getResourceAsStream. Additionally, and except for the special case where the resource has a name ending with ".class", this method will only find resources in packages of named modules when the package is opened unconditionally (even if the caller of this method is in the same module as the resource).

API Note:: Where several modules are defined to the same class loader, and where more than one module contains a resource with the given name, then the ordering that modules are searched is not specified and may be very unpredictable. When overriding this method it is recommended that an implementation ensures that any delegation is consistent with the getResources(String) method.
Implementation Requirements:: The default implementation will first search the parent class loader for the resource; if the parent is null the path of the class loader built into the virtual machine is searched. If not found, this method will invoke findResource(String) to find the resource.
Parameters:: name - The resource name
Returns:: URL object for reading the resource; null if the resource could not be found, a URL could not be constructed to locate the resource, the resource is in a package that is not opened unconditionally, or access to the resource is denied by the security manager.
Throws:: NullPointerException - If name is null
Since:: 1.1

getResources

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.

API Note:: Where several modules are defined to the same class loader, and where more than one module contains a resource with the given name, then the ordering is not specified and may be very unpredictable. When overriding this method it is recommended that an implementation ensures that any delegation is consistent with the 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.
Implementation Requirements:: The default implementation will first search the parent class loader for the resource; if the parent is null the path of the class loader built into the virtual machine is searched. It then invokes findResources(String) to find the resources with the name in this class loader. It returns an enumeration whose elements are the URLs found by searching the parent class loader followed by the elements found with findResources.
Parameters:: name - The resource name
Returns:: An enumeration of URL objects for the resource. If no resources could be found, the enumeration will be empty. Resources for which a URL cannot be constructed, are in a package that is not opened unconditionally, or access to the resource is denied by the security manager, are not returned in the enumeration.
Throws:: IOException - If I/O errors occur; NullPointerException - If name is null
Since:: 1.2

resources

public Stream<URL> resources(String name)

Returns a stream whose elements are the URLs of 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 resources will be located when the returned stream is evaluated. If the evaluation results in an IOException then the I/O exception is wrapped in an UncheckedIOException that is then thrown.

API Note:: When overriding this method it is recommended that an implementation ensures that any delegation is consistent with the getResource(String) method. This should ensure that the first element returned by the stream is the same resource that the getResource(String) method would return.
Implementation Requirements:: The default implementation invokes getResources to find all the resources with the given name and returns a stream with the elements in the enumeration as the source.
Parameters:: name - The resource name
Returns:: A stream of resource URL objects. If no resources could be found, the stream will be empty. Resources for which a URL cannot be constructed, are in a package that is not opened unconditionally, or access to the resource is denied by the security manager, will not be in the stream.
Throws:: NullPointerException - If name is null
Since:: 9

findResource

protected URL findResource(String name)

Finds the resource with the given name. Class loader implementations should override this method.

For resources in named modules then the method must implement the rules for encapsulation specified in the Module getResourceAsStream method. Additionally, it must not find non-".class" resources in packages of named modules unless the package is opened unconditionally.

Implementation Requirements:: The default implementation returns null.
Parameters:: name - The resource name
Returns:: URL object for reading the resource; null if the resource could not be found, a URL could not be constructed to locate the resource, the resource is in a package that is not opened unconditionally, or access to the resource is denied by the security manager.
Since:: 1.2

findResources

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

Returns an enumeration of URL objects representing all the resources with the given name. Class loader implementations should override this method.

Implementation Requirements:: The default implementation returns an enumeration that contains no elements.
Parameters:: name - The resource name
Returns:: An enumeration of URL objects for the resource. If no resources could be found, the enumeration will be empty. Resources for which a URL cannot be constructed, are in a package that is not opened unconditionally, or access to the resource is denied by the security manager, are not returned in the enumeration.
Throws:: IOException - If I/O errors occur
Since:: 1.2

registerAsParallelCapable

protected static boolean registerAsParallelCapable()

Registers the caller as parallel capable. The registration succeeds if and only if all of the following conditions are met:

no instance of the caller has been created
all of the super classes (except class Object) of the caller are registered as parallel capable

Note that once a class loader is registered as parallel capable, there is no way to change it back.

In cases where this method is called from a context where the caller is not a subclass of ClassLoader or there is no caller frame on the stack (e.g. when called directly from a JNI attached thread), IllegalCallerException is thrown.

Returns:

true if the caller is successfully registered as parallel capable and false if otherwise.

Throws:

IllegalCallerException - if the caller is not a subclass of ClassLoader

Since:

1.7

See Also:

isRegisteredAsParallelCapable

public final boolean isRegisteredAsParallelCapable()

Returns true if this class loader is registered as parallel capable, otherwise false.

Returns:

true if this class loader is parallel capable, otherwise false.

Since:

See Also:

getSystemResource

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()).

Parameters:: name - The resource name
Returns:: A URL to the resource; null if the resource could not be found, a URL could not be constructed to locate the resource, the resource is in a package that is not opened unconditionally or access to the resource is denied by the security manager.
Since:: 1.1

getSystemResources

public 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 Enumeration of URL objects.

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

Parameters:: name - The resource name
Returns:: An enumeration of URL objects for the resource. If no resources could be found, the enumeration will be empty. Resources for which a URL cannot be constructed, are in a package that is not opened unconditionally, or access to the resource is denied by the security manager, are not returned in the enumeration.
Throws:: IOException - If I/O errors occur
Since:: 1.2

getResourceAsStream

public InputStream getResourceAsStream(String name)

Returns an input stream for reading the specified resource.

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

Parameters:: name - The resource name
Returns:: An input stream for reading the resource; null if the resource could not be found, the resource is in a package that is not opened unconditionally, or access to the resource is denied by the security manager.
Throws:: NullPointerException - If name is null
Since:: 1.1

getSystemResourceAsStream

public 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()).

Parameters:: name - The resource name
Returns:: An input stream for reading the resource; null if the resource could not be found, the resource is in a package that is not opened unconditionally, or access to the resource is denied by the security manager.
Since:: 1.1

getParent

public 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.

Returns:: The parent ClassLoader
Throws:: SecurityException - If a security manager is present, and the caller's class loader is not null and is not an ancestor of this class loader, and the caller does not have the RuntimePermission("getClassLoader")
Since:: 1.2

getUnnamedModule

public final Module getUnnamedModule()

Returns the unnamed Module for this class loader.

Returns:

The unnamed Module for this class loader

Since:

See Also:

getPlatformClassLoader

public static ClassLoader getPlatformClassLoader()

Returns the platform class loader. All platform classes are visible to the platform class loader.

Implementation Note:: The name of the builtin platform class loader is "platform".
Returns:: The platform ClassLoader.
Throws:: SecurityException - If a security manager is present, and the caller's class loader is not null, and the caller's class loader is not the same as or an ancestor of the platform class loader, and the caller does not have the RuntimePermission("getClassLoader")
Since:: 9

getSystemClassLoader

public static ClassLoader getSystemClassLoader()

Returns the system class loader. 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. This class loader will be the context class loader for the main application thread (for example, the thread that invokes the main method of the main class).

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. During construction, the class loader should take great care to avoid calling getSystemClassLoader(). If circular initialization of the system class loader is detected then an IllegalStateException is thrown.

Implementation Note:

The system property to override the system class loader is not examined until the VM is almost fully initialized. Code that executes this method during startup should take care not to cache the return value until the system is fully initialized.

The name of the built-in system class loader is "app". The system property "java.class.path" is read during early initialization of the VM to determine the class path. An empty value of "java.class.path" property is interpreted differently depending on whether the initial module (the module containing the main class) is named or unnamed: If named, the built-in system class loader will have no class path and will search for classes and resources using the application module path; otherwise, if unnamed, it will set the class path to the current working directory.

JAR files on the class path may contain a Class-Path manifest attribute to specify dependent JAR files to be included in the class path. Class-Path entries must meet certain conditions for validity (see the JAR File Specification for details). Invalid Class-Path entries are ignored. For debugging purposes, ignored entries can be printed to the console if the jdk.net.URLClassPath.showIgnoredClassPathEntries system property is set to true.

Returns:

The system ClassLoader

Throws:

SecurityException - If a security manager is present, and the caller's class loader is not null and is not the same as or an ancestor of the system class loader, and the caller does not have the RuntimePermission("getClassLoader")

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.

definePackage

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.

Package names must be unique within a class loader and cannot be redefined or changed once created.

If a class loader wishes to define a package with specific properties, such as version information, then the class loader should call this definePackage method before calling defineClass. Otherwise, the defineClass method will define a package in this class loader corresponding to the package of the newly defined class; the properties of this defined package are specified by Package.

API Note:

A class loader that wishes to define a package for classes in a JAR typically uses the specification and implementation titles, versions, and vendors from the JAR's manifest. If the package is specified as sealed in the JAR's manifest, the URL of the JAR file is typically used as the sealBase. If classes of package 'p' defined by this class loader are loaded from multiple JARs, the Package object may contain different information depending on the first class of package 'p' defined and which JAR's manifest is read first to explicitly define package 'p'.

It is strongly recommended that a class loader does not call this method to explicitly define packages in named modules; instead, the package will be automatically defined when a class is being defined. If it is desirable to define Package explicitly, it should ensure that all packages in a named module are defined with the properties specified by Package. Otherwise, some Package objects in a named module may be for example sealed with different seal base.

Parameters:

name - The package name

specTitle - The specification title

specVersion - The specification version

specVendor - The specification vendor

implTitle - The implementation title

implVersion - The implementation version

implVendor - The implementation vendor

sealBase - If not null, then this package is sealed with respect to the given code source URL object. Otherwise, the package is not sealed.

Returns:

The newly defined Package object

Throws:

NullPointerException - if name is null.

IllegalArgumentException - if a package of the given name is already defined by this class loader

See Java Virtual Machine Specification:

5.3 Creation and Loading

Since:

1.2

See Also:

getDefinedPackage

public final Package getDefinedPackage(String name)

Returns a Package of the given name that has been defined by this class loader.

Parameters:: name - The package name
Returns:: The Package of the given name that has been defined by this class loader, or null if not found
Throws:: NullPointerException - if name is null.
See Java Virtual Machine Specification:: 5.3 Creation and Loading
Since:: 9

getDefinedPackages

public final Package[] getDefinedPackages()

Returns all of the Packages that have been defined by this class loader. The returned array has no duplicated Packages of the same name.

API Note:: This method returns an array rather than a Set or Stream for consistency with the existing getPackages() method.
Returns:: The array of Package objects that have been defined by this class loader; or a zero length array if no package has been defined by this class loader.
See Java Virtual Machine Specification:: 5.3 Creation and Loading
Since:: 9

getPackage

@Deprecated(since="9") protected Package getPackage(String name)

Deprecated.

If multiple class loaders delegate to each other and define classes with the same package name, and one such loader relies on the lookup behavior of getPackage to return a Package from a parent loader, then the properties exposed by the Package may not be as expected in the rest of the program. For example, the Package will only expose annotations from the package-info.class file defined by the parent loader, even if annotations exist in a package-info.class file defined by a child loader. A more robust approach is to use the getDefinedPackage(java.lang.String) method which returns a Package for the specified class loader.

Finds a package by name in this class loader and its ancestors.

If this class loader defines a Package of the given name, the Package is returned. Otherwise, the ancestors of this class loader are searched recursively (parent by parent) for a Package of the given name.

API Note:

The platform class loader may delegate to the application class loader but the application class loader is not its ancestor. When invoked on the platform class loader, this method will not find packages defined to the application class loader.

Parameters:

name - The package name

Returns:

The Package of the given name that has been defined by this class loader or its ancestors, or null if not found.

Throws:

NullPointerException - if name is null.

Since:

1.2

See Also:

getPackages

protected Package[] getPackages()

Returns all of the Packages that have been defined by this class loader and its ancestors. The returned array may contain more than one Package object of the same package name, each defined by a different class loader in the class loader hierarchy.

API Note:

The platform class loader may delegate to the application class loader. In other words, packages in modules defined to the application class loader may be visible to the platform class loader. On the other hand, the application class loader is not its ancestor and hence when invoked on the platform class loader, this method will not return any packages defined to the application class loader.

Returns:

The array of Package objects that have been defined by this class loader and its ancestors

Since:

1.2

See Also:

findLibrary

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.

Parameters:

libname - The library name

Returns:

The absolute path of the native library

Since:

1.2

See Also:

setDefaultAssertionStatus

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).

Parameters:: 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.
Since:: 1.4

setPackageAssertionStatus

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).

Parameters:: 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.
Since:: 1.4

setClassAssertionStatus

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.

Parameters:: 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.
Since:: 1.4

clearAssertionStatus

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."

Since:: 1.4

© 1993, 2023, Oracle and/or its affiliates. All rights reserved.
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).
Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/ClassLoader.html