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Java example source code file (ByteClassLoader.java)

This example Java source code file (ByteClassLoader.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

bufferedoutputstream, byteclassloader, class, classnotfoundexception, filenotfoundexception, fileoutputstream, ioexception, jar, jarentry, jaroutputstream, net, network, runtimeexception, string, url, urlclassloader

The ByteClassLoader.java Java example source code

import java.io.BufferedOutputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.jar.JarEntry;
import java.util.jar.JarOutputStream;

/*
 * Copyright (c) 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

/**
 * A ByteClassLoader is used to define classes from collections of bytes, as
 * well as loading classes in the usual way. It includes options to write the
 * classes to files in a jar, or to read the classes from jars in a later or
 * debugging run.
 *
 * If Boolean property byteclassloader.verbose is true, be chatty about jar
 * file operations.
 *
 */
public class ByteClassLoader extends URLClassLoader {

    final static boolean verbose
            = Boolean.getBoolean("byteclassloader.verbose");

    final boolean read;
    final JarOutputStream jos;
    final String jar_name;

    /**
     * Make a new ByteClassLoader.
     *
     * @param jar_name  Basename of jar file to be read/written by this classloader.
     * @param read      If true, read classes from jar file instead of from parameter.
     * @param write     If true, write classes to jar files for offline study/use.
     *
     * @throws FileNotFoundException
     * @throws IOException
     */
    public ByteClassLoader(String jar_name, boolean read, boolean write)
            throws FileNotFoundException, IOException {
        super(read
                ? new URL[]{new URL("file:" + jar_name + ".jar")}
                : new URL[0]);
        this.read = read;
        this.jar_name = jar_name;
        this.jos = write
                ? new JarOutputStream(
                new BufferedOutputStream(
                new FileOutputStream(jar_name + ".jar"))) : null;
        if (read && write) {
            throw new Error("At most one of read and write may be true.");
        }
    }

    private static void writeJarredFile(JarOutputStream jos, String file, String suffix, byte[] bytes) {
        String fileName = file.replace(".", "/") + "." + suffix;
        JarEntry ze = new JarEntry(fileName);
        try {
            ze.setSize(bytes.length);
            jos.putNextEntry(ze);
            jos.write(bytes);
            jos.closeEntry();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    /**
     * (pre)load class name using classData for the definition.
     *
     * @param name
     * @param classData
     * @return
     */
    public Class<?> loadBytes(String name, byte[] classData) throws ClassNotFoundException {
        if (jos != null) {
            if (verbose) {
                System.out.println("ByteClassLoader: writing " + name);
            }
            writeJarredFile(jos, name, "class", classData);
        }

        Class<?> clazz = null;
        if (read) {
            if (verbose) {
                System.out.println("ByteClassLoader: reading " + name + " from " + jar_name);
            }
            clazz = loadClass(name);
        } else {
            clazz = defineClass(name, classData, 0, classData.length);
            resolveClass(clazz);
        }
        return clazz;
    }

    public void close() {
        if (jos != null) {
            try {
                if (verbose) {
                    System.out.println("ByteClassLoader: closing " + jar_name);
                }
                jos.close();
            } catch (IOException ex) {
            }
        }
    }
}

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