How can I locate (and instantiate) a class in Java when I only have its name but not its package? The code
Class<?> clazz = Class.forName( "className" );
throws a ClassNotFoundException althout "className" is on the classpath. It only works with the complete package name. The code
URL url = SomeClass.getClass().getClassLoader().findResource( "className.class" );
returns null.
This is not a duplicate since the solutions which are using package lists are not answering the question "without package". Instead they rely on a predefined or externally configured list of packages. So "Finding a class reflectively by its simple-name alone" is not a duplicate but provides a solution which invloves packages.
Are there any Java methods available to achieve this?
My own answer would be:
No. Neither Class nor ClassLoader or any other Java class provides this functionality. I have written a small class to solve this. The last line from System.out.println gives the location of the class. Please feel free to use and adapt it to your requirements.
package com.lemcke.util;
import java.io.File;
import java.io.IOException;
import java.util.Enumeration;
import java.util.jar.JarEntry;
import java.util.jar.JarFile;
public class ClassLocator {
/**
* Locates a class by its name. Searches the whole {@code classpath}.
*
* @param name
* Name of the class (without package)
* @return full package name of the class or {@code null} if not found
*/
public String locateClass( String name ) throws IOException {
if ( name == null || name.length() == 0 ) {
return null;
}
if ( name.endsWith( ".class" ) == false ) {
name += ".class";
}
String found = null;
String cp = System.getProperty( "java.class.path" );
String[] paths = cp.split( "[,;]" );
for ( String pathname : paths ) {
System.out.println( pathname );
if ( pathname.endsWith( ".jar" ) ) {
found = locateClassInJar( name, pathname );
if ( found != null ) {
break;
}
} else {
found = locateClassInFolder( name, new File( pathname ) );
if ( found != null ) {
found = found.substring( pathname.length() + 1 );
break;
}
}
}
if ( found != null ) {
found = found.substring( 0, found.length() - ".class".length() )
.replace( '/', '.' ).replace( '\\', '.' );
}
return found;
}
private String locateClassInFolder( String name, File path )
throws IOException {
File f = new File( path + "/" + name );
if ( f.exists() ) {
return f.getPath();
}
String found = null;
for ( File entry : path.listFiles() ) {
if ( entry.isDirectory() ) {
System.out.println( entry );
found = locateClassInFolder( name, entry );
if ( found != null ) {
break;
}
}
}
return found;
}
private String locateClassInJar( String name, String jarpath )
throws IOException {
name = "/" + name;
JarFile jarFile = new JarFile( jarpath );
Enumeration<JarEntry> entries = jarFile.entries();
String found = null, entry = null;
while ( entries.hasMoreElements() ) {
entry = entries.nextElement().getName();
if ( entry.endsWith( name ) ) {
found = entry;
break;
}
}
jarFile.close();
return found;
}
/**
* @param args
* @throws IOException
*/
public static void main( String[] args ) throws IOException {
ClassLocator locator = new ClassLocator();
String found = locator.locateClass( args[0] );
System.out.println( "found = " + found );
}
}
