net.yagga.util
Class MetaJarClassLoader

java.lang.Object
  |
  +--java.lang.ClassLoader
        |
        +--net.yagga.util.MetaJarClassLoader

public class MetaJarClassLoader

extends java.lang.ClassLoader

A Meta class loader for loading jar files in many situations. It is interesting that the class, by using the MetajarResources class can load jar classes from Jars in the classpath

 ./Test.class
 ./net/yagga/util/MetaJarClassLoader.class
 ./sample.jar
   +- Difficult.class
 

In this simple case one can read the class Difficult from the file "sample.jar" via this class, in the main method of the Test class. This case is more or less the same as seen in JavaWorld's tip 70 (JarClassLoader). The class reads the specified Jar entry and returns the byte array and then instantiate an object of the Class using the bytes. The trick is a bit more complicated if we are in such a case, executing the jar file named "all.jar":

 ./all.jar
   +- Test.class
   +- net/yagga/util/MetaJarClassLoader.class
   +- sample.jar
     +- Difficult.class
 

In such a case JarClassLoader does not work. Instead we use the ability of MetJarResources to load bytes also from jars that are enclosedin the top-level (executing) jar.

Author:

Walter Gamba

See Also:

JavaWorld's tip 70, MetaJarResources

Inner classes inherited from class java.lang.ClassLoader

java.lang.ClassLoader.NativeLibrary

Field Summary

private java.lang.String	jarFile
private MetaJarResources	metaJarResources

Fields inherited from class java.lang.ClassLoader

bootstrapClassPath, classes, defaultDomain, defaultPermissions, domains, getClassLoaderPerm, initialized, loadedLibraryNames, nativeLibraries, nativeLibraryContext, nocerts, package2certs, packages, parent, scl, sclSet, sys_paths, systemNativeLibraries, usr_paths

Constructor Summary

MetaJarClassLoader(java.lang.String jFile)
Construct the class loader givena a Jar File.

Method Summary

protected java.lang.Class	findClass(java.lang.String className) Method that actually reads the class from raw bytes.
java.lang.String	getActualJarName() Return the actual jar name.
java.lang.String	getMainClassName() Returns the name of the jar file main class, or null if no "Main-Class" manifest attributes was defined.
void	invokeClass(java.lang.String name, java.lang.String[] args) Invokes the application in this jar file given the name of the main class and an array of arguments.
java.lang.Object	invokeMethod(java.lang.String className, java.lang.String method, java.lang.Object[] args) Invokes the application in this jar file given the name of the class, a static Method and an array of arguments.
java.lang.Object	simpleInvokeMethod(java.lang.String className, java.lang.String method, java.lang.Object[] args) Invokes a static method of a given class in this JAR.

Methods inherited from class java.lang.ClassLoader

, addClass, check, checkCerts, checkPackageAccess, compareCerts, defineClass, defineClass, defineClass, defineClass0, definePackage, findBootstrapClass, findBootstrapClass0, findLibrary, findLoadedClass, findNative, findResource, findResources, findSystemClass, getBootstrapClassPath, getBootstrapResource, getBootstrapResources, getCallerClassLoader, getDefaultDomain, getGetClassLoaderPerm, getPackage, getPackages, getParent, getResource, getResourceAsStream, getResources, getSystemClassLoader, getSystemResource, getSystemResourceAsStream, getSystemResources, initializePath, isAncestor, loadClass, loadClass, loadClassInternal, loadLibrary, loadLibrary0, resolveClass, resolveClass0, setSigners

Methods inherited from class java.lang.Object

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

Field Detail

metaJarResources

private MetaJarResources metaJarResources

jarFile

private java.lang.String jarFile

Constructor Detail

MetaJarClassLoader

public MetaJarClassLoader(java.lang.String jFile)

Construct the class loader givena a Jar File.

Parameters:

jFile - the name of the jar file.

Method Detail

getActualJarName

public java.lang.String getActualJarName()

Return the actual jar name. This can be the top level Jar (whose name we usually ignore) or the jar file name we have passed in the constructor.

Returns:

the file name of the top-level jar file (if searching a Jar from inside an executing jar) or the jar name as we have passed it to the Constructor.

getMainClassName

public java.lang.String getMainClassName()

Returns the name of the jar file main class, or null if no "Main-Class" manifest attributes was defined.

Returns:

the name of the main class of the Jar file

invokeMethod

public java.lang.Object invokeMethod(java.lang.String className,
                                     java.lang.String method,
                                     java.lang.Object[] args)
                              throws java.lang.ClassNotFoundException,
                                     java.lang.NoSuchMethodException,
                                     java.lang.reflect.InvocationTargetException

Invokes the application in this jar file given the name of the class, a static Method and an array of arguments. The class must define this static method which takes an array of Object arguments and can return anything.

Parameters:

name - the name of the class

method - the name of the method to invoke

args - the arguments to the method of this class of the jar file

invokeClass

public void invokeClass(java.lang.String name,
                        java.lang.String[] args)
                 throws java.lang.ClassNotFoundException,
                        java.lang.NoSuchMethodException,
                        java.lang.reflect.InvocationTargetException

Invokes the application in this jar file given the name of the main class and an array of arguments. The class must define a static method "main" which takes an array of String arguemtns and is of return type "void".

Parameters:

name - the name of the main class

args - the arguments to the main method of the main class of the jar file

simpleInvokeMethod

public java.lang.Object simpleInvokeMethod(java.lang.String className,
                                           java.lang.String method,
                                           java.lang.Object[] args)
                                    throws java.lang.ClassNotFoundException,
                                           java.lang.NoSuchMethodException,
                                           java.lang.reflect.InvocationTargetException

Invokes a static method of a given class in this JAR. The class must define this static method which takes any number of arguments and can return anything. One must pass Strings as arguemnts, and they are converted to the appropriate native types. Only native types can be used, not even arrays of native types.
Please note that no overloading/polymorohism of methods is used. The first method that has the same name as the one passed, and the same number of arguments, is used.
And of course, "first" has no predictable meaning, so please be sure that you don't have ambiguities.

Parameters:

name - the name of the class

method - the name of the method to invoke

args - the arguments to the method of this class of the jar file, as Strings

Returns:

an Object containing th return value or values.

findClass

protected java.lang.Class findClass(java.lang.String className)
                             throws java.lang.ClassNotFoundException

Method that actually reads the class from raw bytes. This method uses MetaJarResources to read raw bytes, then creates a Class with the given bytes

Overrides:

findClass in class java.lang.ClassLoader

Parameters:

className - the classname (fully qualified with "."). Teh name is then read friom the jar file substituting "." with "/" and appending ".class"

Returns:

a Class object of representing the given class