Package net.jini.loader.pref

Class PreferredClassLoader

java.lang.Object
java.lang.ClassLoader
java.security.SecureClassLoader
java.net.URLClassLoader
net.jini.loader.pref.PreferredClassLoader
All Implemented Interfaces:
Closeable, AutoCloseable, ClassAnnotation
public class PreferredClassLoader extends URLClassLoader implements ClassAnnotation
A class loader that supports preferred classes.

A preferred class is a class that is to be loaded by a class loader without the loader delegating to its parent class loader first. Resources may also be preferred.

Like URLClassLoader, PreferredClassLoader loads classes and resources from a search path of URLs. If a URL in the path ends with a '/', it is assumed to refer to a directory; otherwise, the URL is assumed to refer to a JAR file.

The location of the first URL in the path can contain a preferred list for the entire path. A preferred list declares names of certain classes and other resources throughout the path as being preferred or not. When a PreferredClassLoader is asked to load a class or resource that is preferred (according to the preferred list) and the class or resource exists in the loader's path of URLs, the loader will not delegate first to its parent class loader as it otherwise would do; instead, it will attempt to load the class or resource from its own path of URLs only.

The preferred list for a path of URLs, if one exists, is located relative to the first URL in the path. If the first URL refers to a JAR file, then the preferred list is the contents of the file named "META-INF/PREFERRED.LIST" within that JAR file. If the first URL refers to a directory, then the preferred list is the contents of the file at the location "META-INF/PREFERRED.LIST" relative to that directory URL. If there is no preferred list at the required location, then no classes or resources are preferred for the path of URLs. A preferred list at any other location (such as relative to one of the other URLs in the path) is ignored.

Note that a class or resource is only considered to be preferred if the preferred list declares the name of the class or resource as being preferred and the class or resource actually exists in the path of URLs.

Preferred List Syntax

A preferred list is a UTF-8 encoded text file, with lines separated by CR LF, LF, or CR (not followed by an LF). Multiple whitespace characters in a line are equivalent to a single whitespace character, and whitespace characters at the beginning or end of a line are ignored. If the first non-whitespace character of a line is '#', the line is a comment and is equivalent to a blank line.

The first line of a preferred list must contain a version number in the following format: 

     PreferredResources-Version: 1.x
This specification defines only version 1.0, but PreferredClassLoader will parse any version 1.x, x>=0 with the format and semantics specified here.

After the version number line, a preferred list comprises an optional default preferred entry followed by zero or more named preferred entries. A preferred list must contain either a default preferred entry or at least one named preferred entry. Blank lines are allowed before and after preferred entries, as well as between the lines of a named preferred entry.

A default preferred entry is a single line in the following format: 

     Preferred: preferred-setting
where preferred-setting is a non-empty sequence of characters. If preferred-setting equals "true" (case insensitive), then resource names not matched by any of the named preferred entries are by default preferred; otherwise, resource names not matched by any of the named preferred entries are by default not preferred. If there is no default preferred entry, then resource names are by default not preferred.

A named preferred entry is two lines in the following format: 

     Name: name-expression
     Preferred: preferred-setting
where name-expression and preferred-setting are non-empty sequences of characters. If preferred-setting equals "true" (case insensitive), then resource names that are matched by name-expression (and not any more specific named preferred entries) are preferred; otherwise, resource names that are matched by name-expression (and not any more specific named preferred entries) are not preferred.

If name-expression ends with ".class", it matches a class whose binary name is name-expression without the ".class" suffix and with each '/' character replaced with a '.'. It also matches any class whose binary name starts with that same value followed by a '$'; this rule is intended to match nested classes that have an enclosing class of that name, so that the preferred settings of a class and all of its nested classes are the same by default. It is possible, but strongly discouraged, to override the preferred setting of a nested class with a named preferred entry that explicitly matches the nested class's binary name.

name-expression may match arbitrary resource names as well as class names, with path elements separated by '/' characters.

If name-expression ends with "/" or "/*", then the entry is a directory wildcard entry that matches all resources (including classes) in the named directory. If name-expression ends with "/-", then the entry is a namespace wildcard entry that matches all resources (including classes) in the named directory and all of its subdirectories.

When more than one named preferred entry matches a class or resource name, then the most specific entry takes precedence. A non-wildcard entry is more specific than a wildcard entry. A directory wildcard entry is more specific than a namespace wildcard entry. A namespace wildcard entry with more path elements is more specific than a namespace wildcard entry with fewer path elements. Given two non-wildcard entries, the entry with the longer name-expression is more specific (this rule is only significant when matching a class). The order of named preferred entries is insignificant.

Example Preferred List

Following is an example preferred list: 

     PreferredResources-Version: 1.0
     Preferred: false

     Name: com/foo/FooBar.class
     Preferred: true

     Name: com/foo/*
     Preferred: false

     Name: com/foo/-
     Preferred: true

     Name: image-files/*
     Preferred: mumble

The class com.foo.FooBar is preferred, as well as any nested classes that have it as an enclosing class. All other classes in the com.foo package are not preferred because of the directory wildcard entry. Classes in subpackages of com.foo are preferred because of the namespace wildcard entry. Resources in the directory "com/foo/" are not preferred, and resources in subdirectories of "com/foo/" are preferred. Resources in the directory "image-files/" are not preferred because preferred settings other than "true" are interpreted as false. Classes that are in a package named com.bar are not preferred because of the default preferred entry.

Since:
2.0
Author:
Sun Microsystems, Inc.

  • Constructor Details

    • PreferredClassLoader

      public PreferredClassLoader(URL[] urls, ClassLoader parent, String exportAnnotation, boolean requireDlPerm)
      Creates a new PreferredClassLoader that loads classes and resources from the specified path of URLs and delegates to the specified parent class loader.

      If exportAnnotation is not null, then it will be used as the return value of the loader's getClassAnnotation method. If exportAnnotation is null, the loader's getClassAnnotation method will return a space-separated list of the URLs in the specified path. The exportAnnotation parameter can be used to specify so-called "export" URLs, from which other parties should load classes defined by the loader and which are different from the "import" URLs that the classes are actually loaded from.

      If requireDlPerm is true, the loader's getPermissions method will require that the CodeSource of any class defined by the loader is granted DownloadPermission.

      Parameters:
      urls - the path of URLs to load classes and resources from
      parent - the parent class loader for delegation
      exportAnnotation - the export class annotation string to use for classes defined by this loader, or null
      requireDlPerm - if true, the loader will only define classes with a CodeSource that is granted DownloadPermission
      Throws:
      SecurityException - if there is a security manager and an invocation of its checkCreateClassLoader method fails

    • PreferredClassLoader

      public PreferredClassLoader(URL[] urls, ClassLoader parent, String exportAnnotation, boolean requireDlPerm, URLStreamHandlerFactory factory)
      Creates a new PreferredClassLoader that loads classes and resources from the specified path of URLs, delegates to the specified parent class loader, and uses the specified URLStreamHandlerFactory when creating new URL objects. This constructor passes factory to the superclass constructor that has a URLStreamHandlerFactory parameter.

      If exportAnnotation is not null, then it will be used as the return value of the loader's getClassAnnotation method. If exportAnnotation is null, the loader's getClassAnnotation method will return a space-separated list of the URLs in the specified path. The exportAnnotation parameter can be used to specify so-called "export" URLs, from which other parties should load classes defined by the loader and which are different from the "import" URLs that the classes are actually loaded from.

      If requireDlPerm is true, the loader's getPermissions method will require that the CodeSource of any class defined by the loader is granted DownloadPermission.

      Parameters:
      urls - the path of URLs to load classes and resources from
      parent - the parent class loader for delegation
      exportAnnotation - the export class annotation string to use for classes defined by this loader, or null
      requireDlPerm - if true, the loader will only define classes with a CodeSource that is granted DownloadPermission
      factory - the URLStreamHandlerFactory to use when creating new URL objects, or null
      Throws:
      SecurityException - if there is a security manager and an invocation of its checkCreateClassLoader method fails
      Since:
      2.1

  • Method Details

    • newInstance

      public static PreferredClassLoader newInstance(URL[] urls, ClassLoader parent, String exportAnnotation, boolean requireDlPerm)
      Creates a new instance of PreferredClassLoader that loads classes and resources from the specified path of URLs and delegates to the specified parent class loader.

      The exportAnnotation and requireDlPerm parameters have the same semantics as they do for the constructors.

      The loadClass method of the returned PreferredClassLoader will, if there is a security manager, invoke its checkPackageAccess method with the package name of the class to load before attempting to load the class; this could result in a SecurityException being thrown from loadClass.

      Parameters:
      urls - the path of URLs to load classes and resources from
      parent - the parent class loader for delegation
      exportAnnotation - the export class annotation string to use for classes defined by this loader, or null
      requireDlPerm - if true, the loader will only define classes with a CodeSource that is granted DownloadPermission
      Returns:
      the new PreferredClassLoader instance
      Throws:
      SecurityException - if the current security context does not have the permissions necessary to connect to all of the URLs in urls

    • isPreferredResource

      protected boolean isPreferredResource(String name, boolean isClass) throws IOException
      Returns true if a class or resource with the specified name is preferred for this class loader, and false if a class or resource with the specified name is not preferred for this loader.

      If isClass is true, then name is interpreted as the binary name of a class; otherwise, name is interpreted as the full path of a resource.

      This method only returns true if a class or resource with the specified name exists in the this loader's path of URLs and the name is preferred in the preferred list. This method returns false if the name is not preferred in the preferred list or if the name is preferred with the default preferred entry or a wildcard preferred entry and the class or resource does not exist in the path of URLs.

      Parameters:
      name - the name of the class or resource
      isClass - true if name is a binary class name, and false if name is the full path of a resource
      Returns:
      true if a class or resource named name is preferred for this loader, and false if a class or resource named name is not preferred for this loader
      Throws:
      IOException - if the preferred list cannot definitely be determined to exist or not exist, or if the preferred list contains a syntax error, or if the name is preferred with the default preferred entry or a wildcard preferred entry and the class or resource cannot definitely be determined to exist or not exist in the path of URLs, or if the name is preferred with a non-wildcard entry and the class or resource does not exist or cannot definitely be determined to exist in the path of URLs

    • loadClass

      protected Class loadClass(String name, boolean resolve) throws ClassNotFoundException
      Loads a class with the specified name.

      PreferredClassLoader implements this method as follows:

      This method first invokes findLoadedClass with name; if findLoadedClass returns a non-null Class, then this method returns that Class.

      Otherwise, this method invokes isPreferredResource with name as the first argument and true as the second argument:

      • If isPreferredResource throws an IOException, then this method throws a ClassNotFoundException containing the IOException as its cause.
      • If isPreferredResource returns true, then this method invokes findClass with name. If findClass throws an exception, then this method throws that exception. Otherwise, this method returns the Class returned by findClass, and if resolve is true, resolveClass is invoked with the Class before returning.
      • If isPreferredResource returns false, then this method invokes the superclass implementation of loadClass with name and resolve and returns the result. If the superclass's loadClass throws an exception, then this method throws that exception.
      Overrides:
      loadClass in class ClassLoader
      Parameters:
      name - the binary name of the class to load
      resolve - if true, then resolveClass will be invoked with the loaded class before returning
      Returns:
      the loaded class
      Throws:
      ClassNotFoundException - if the class could not be found

    • getResource

      public URL getResource(String name)
      Gets a resource with the specified name.

      PreferredClassLoader implements this method as follows:

      This method invokes isPreferredResource with name as the first argument and false as the second argument:

      • If isPreferredResource throws an IOException, then this method returns null.
      • If isPreferredResource returns true, then this method invokes findResource with name and returns the result.
      • If isPreferredResource returns false, then this method invokes the superclass implementation of getResource with name and returns the result.
      Overrides:
      getResource in class ClassLoader
      Parameters:
      name - the name of the resource to get
      Returns:
      a URL for the resource, or null if the resource could not be found

    • definePackage

      protected Package definePackage(String name, String specTitle, String specVersion, String specVendor, String implTitle, String implVersion, String implVendor, URL sealBase)
      Overrides:
      definePackage in class ClassLoader

    • getClassAnnotation

      public String getClassAnnotation()
      Returns the class annotation string for classes defined by this class loader. The returned value may be null.

      PreferredClassLoader implements this method as follows:

      If this PreferredClassLoader was constructed with a non-null export class annotation string, then this method returns that string. Otherwise, this method returns a space-separated list of this loader's path of URLs.

      Specified by:
      getClassAnnotation in interface ClassAnnotation
      Returns:
      the class annotation string for classes defined by this loader, or null

    • getPermissions

      protected PermissionCollection getPermissions(CodeSource codeSource)
      Returns the static permissions to be automatically granted to classes loaded from the specified CodeSource and defined by this class loader.

      PreferredClassLoader implements this method as follows:

      If there is a security manager and this PreferredClassLoader was constructed to enforce DownloadPermission, then this method checks that the current security policy grants the specified CodeSource the permission DownloadPermission("permit"); if that check fails, then this method throws a SecurityException.

      Then this method invokes the superclass implementation of getPermissions and returns the result.

      Overrides:
      getPermissions in class URLClassLoader
      Parameters:
      codeSource - the CodeSource to return the permissions to be granted to
      Returns:
      the permissions to be granted to the CodeSource
      Throws:
      SecurityException - if there is a security manager, this PreferredClassLoader was constructed to enforce DownloadPermission, and the current security policy does not grant the specified CodeSource the permission DownloadPermission("permit")

    • toString

      public String toString()
      Returns a string representation of this class loader.
      Overrides:
      toString in class Object