Java example source code file (Main.java)

This example Java source code file (Main.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

assertionerror, classformaterror, classnotfoundexception, exception, fileinputstream, hashtable, main, net, network, simpleclassloader, util, weakreference

The Main.java Java example source code

/*
 * Copyright (c) 2004, 2012, 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.
 */

/*
 * @test
 * @bug 5040740
 * @summary annotations cause memory leak
 * @author gafter
 *
 * @run shell LoaderLeak.sh
 */

import java.net.*;
import java.lang.ref.*;
import java.util.*;
import java.io.*;

public class Main {
    public static void main(String[] args) throws Exception {
        for (int i=0; i<100; i++)
            doTest(args.length != 0);
    }

    static void doTest(boolean readAnn) throws Exception {
        // URL classes = new URL("file://" + System.getProperty("user.dir") + "/classes");
        // URL[] path = { classes };
        // URLClassLoader loader = new URLClassLoader(path);
        ClassLoader loader = new SimpleClassLoader();
        WeakReference<Class c = new WeakReference(loader.loadClass("C"));
        if (c.get() == null) throw new AssertionError();
        if (c.get().getClassLoader() != loader) throw new AssertionError();
        if (readAnn) System.out.println(c.get().getAnnotations()[0]);
        if (c.get() == null) throw new AssertionError();
        System.gc();
        System.gc();
        if (c.get() == null) throw new AssertionError();
        System.gc();
        System.gc();
        loader = null;

        // Might require multiple calls to System.gc() for weak-references
        // processing to be complete. If the weak-reference is not cleared as
        // expected we will hang here until timed out by the test harness.
        while (true) {
            System.gc();
            Thread.sleep(20);
            if (c.get() == null) {
                break;
            }
        }
    }
}

class SimpleClassLoader extends ClassLoader {
    private Hashtable classes = new Hashtable();

    public SimpleClassLoader() {
    }
    private byte getClassImplFromDataBase(String className)[] {
        byte result[];
        try {
            FileInputStream fi = new FileInputStream("classes/"+className+".class");
            result = new byte[fi.available()];
            fi.read(result);
            return result;
        } catch (Exception e) {

            /*
             * If we caught an exception, either the class wasnt found or it
             * was unreadable by our process.
             */
            return null;
        }
    }
    public Class loadClass(String className) throws ClassNotFoundException {
        return (loadClass(className, true));
    }
    public synchronized Class loadClass(String className, boolean resolveIt)
        throws ClassNotFoundException {
        Class result;
        byte  classData[];

        /* Check our local cache of classes */
        result = (Class)classes.get(className);
        if (result != null) {
            return result;
        }

        /* Check with the primordial class loader */
        try {
            result = super.findSystemClass(className);
            return result;
        } catch (ClassNotFoundException e) {
        }

        /* Try to load it from our repository */
        classData = getClassImplFromDataBase(className);
        if (classData == null) {
            throw new ClassNotFoundException();
        }

        /* Define it (parse the class file) */
        result = defineClass(classData, 0, classData.length);
        if (result == null) {
            throw new ClassFormatError();
        }

        if (resolveIt) {
            resolveClass(result);
        }

        classes.put(className, result);
        return result;
    }
}