versionhelper12.java

java1.6众多例子参考

/*
 * @(#)VersionHelper12.java	1.11 06/07/19
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package com.sun.naming.internal;

import java.io.InputStream;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URLClassLoader;
import java.net.URL;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.util.Enumeration;
import java.util.Hashtable;
import java.util.NoSuchElementException;
import java.util.Properties;

import javax.naming.*;

/**
 * VersionHelper was used by JNDI to accommodate differences between
 * JDK 1.1.x and the Java 2 platform. As this is no longer necessary
 * since JNDI's inclusion in the platform, this class currently
 * serves as a set of utilities for performing system-level things,
 * such as class-loading and reading system properties.
 * 
 * @author Rosanna Lee
 * @author Scott Seligman
 * @version 1.11 06/07/19
 */

final class VersionHelper12 extends VersionHelper {

    private boolean getSystemPropsFailed = false;

    VersionHelper12() {} // Disallow external from creating one of these.

    public Class loadClass(String className) throws ClassNotFoundException {
	ClassLoader cl = getContextClassLoader();
	return Class.forName(className, true, cl);
    }

    /**
      * Package private.
      */
    Class loadClass(String className, ClassLoader cl) 
	throws ClassNotFoundException {
	return Class.forName(className, true, cl);
    }

    /**
     * @param className A non-null fully qualified class name.
     * @param codebase A non-null, space-separated list of URL strings.
     */
    public Class loadClass(String className, String codebase) 
	throws ClassNotFoundException, MalformedURLException {
	ClassLoader cl;

	ClassLoader parent = getContextClassLoader();
	cl = URLClassLoader.newInstance(getUrlArray(codebase), parent);

	return Class.forName(className, true, cl);
    }

    String getJndiProperty(final int i) {
	return (String) AccessController.doPrivileged(
	    new PrivilegedAction() {
		public Object run() {
		    try {
			return System.getProperty(PROPS[i]);
		    } catch (SecurityException e) {
			return null;
		    }
	        }
	    }
	);
    }

    String[] getJndiProperties() {
	if (getSystemPropsFailed) {
	    return null;	// after one failure, don't bother trying again
	}
	Properties sysProps = (Properties) AccessController.doPrivileged(
	    new PrivilegedAction() {
		public Object run() {
		    try {
			return System.getProperties();
		    } catch (SecurityException e) {
			getSystemPropsFailed = true;
			return null;
		    }
		}
	    }
	);
	if (sysProps == null) {
	    return null;
	}
	String[] jProps = new String[PROPS.length];
	for (int i = 0; i < PROPS.length; i++) {
	    jProps[i] = sysProps.getProperty(PROPS[i]);
	}
	return jProps;
    }

    InputStream getResourceAsStream(final Class c, final String name) {
	return (InputStream) AccessController.doPrivileged(
	    new PrivilegedAction() {
		public Object run() {
	            return c.getResourceAsStream(name);
	        }
	    }
	);
    }

    InputStream getJavaHomeLibStream(final String filename) {
	return (InputStream) AccessController.doPrivileged(
	    new PrivilegedAction() {
		public Object run() {
		    try {
			String javahome = System.getProperty("java.home");
			if (javahome == null) {
			    return null;
			}
			String pathname = javahome + java.io.File.separator +
			    "lib" + java.io.File.separator + filename;
			return new java.io.FileInputStream(pathname);
		    } catch (Exception e) {
			return null;
		    }
		}
	    }
	);
    }

    NamingEnumeration getResources(final ClassLoader cl, final String name)
	    throws IOException
    {
	Enumeration urls;
	try {
	    urls = (Enumeration) AccessController.doPrivileged(
		new PrivilegedExceptionAction() {
		    public Object run() throws IOException {
			return (cl == null)
			    ? ClassLoader.getSystemResources(name)
			    : cl.getResources(name);
		    }
		}
	    );
	} catch (PrivilegedActionException e) {
	    throw (IOException)e.getException();
	}
	return new InputStreamEnumeration(urls);
    }

    ClassLoader getContextClassLoader() {
	return (ClassLoader) AccessController.doPrivileged(
	    new PrivilegedAction() {
		public Object run() {
		    return Thread.currentThread().getContextClassLoader();
		}
	    }
	);
    }


    /**
     * Given an enumeration of URLs, an instance of this class represents
     * an enumeration of their InputStreams.  Each operation on the URL
     * enumeration is performed within a doPrivileged block.
     * This is used to enumerate the resources under a foreign codebase.
     * This class is not MT-safe.
     */
    class InputStreamEnumeration implements NamingEnumeration {

	private final Enumeration urls;

	private Object nextElement = null;

	InputStreamEnumeration(Enumeration urls) {
	    this.urls = urls;
	}

	/*
	 * Returns the next InputStream, or null if there are no more.
	 * An InputStream that cannot be opened is skipped.
	 */
	private Object getNextElement() {
	    return AccessController.doPrivileged(
		new PrivilegedAction() {
		    public Object run() {
			while (urls.hasMoreElements()) {
			    try {
				return ((URL)urls.nextElement()).openStream();
			    } catch (IOException e) {
				// skip this URL
			    }
			}
			return null;
		    }
		}
	    );
	}

	public boolean hasMore() {
	    if (nextElement != null) {
		return true;
	    }
	    nextElement = getNextElement();
	    return (nextElement != null);
	}

	public boolean hasMoreElements() {
	    return hasMore();
	}

	public Object next() {
	    if (hasMore()) {
		Object res = nextElement;
		nextElement = null;
		return res;
	    } else {
		throw new NoSuchElementException();
	    }
	}

	public Object nextElement() {
	    return next();
	}

	public void close() {
	}
    }
}