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

appcontext, concurrentlinkedqueue, countdownlatch, failed, file, hashmap, imageinputstream, interruptedexception, myclassloader, net, network, reflection, runnable, runtimeexception, string, test, threading, threads, throwable, util

The Main.java Java example source code

/*
 * Copyright (c) 2009, 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.
 */

package test;

import java.io.File;
import java.io.IOException;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.HashMap;
import java.util.Map.Entry;
import java.util.Set;
import java.util.WeakHashMap;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.CountDownLatch;
import javax.imageio.stream.ImageInputStream;
import sun.awt.AppContext;
import sun.awt.SunToolkit;

public class Main {

    private static ThreadGroup appsThreadGroup;

    private static WeakHashMap<MyClassLoader, String> refs =
            new WeakHashMap<MyClassLoader, String>();

    /** Collection to simulate forgrotten streams **/
    private static HashMap<String, ImageInputStream> strongRefs =
            new HashMap<String, ImageInputStream>();

    private static ConcurrentLinkedQueue<Throwable> problems =
            new ConcurrentLinkedQueue<Throwable>();

    private static AppContext mainAppContext = null;

    private static CountDownLatch doneSignal;

    private static final int gcTimeout =
        Integer.getInteger("gcTimeout", 10).intValue();

    private static boolean forgetSomeStreams =
            Boolean.getBoolean("forgetSomeStreams");

    public static void main(String[] args) throws IOException {
        mainAppContext = SunToolkit.createNewAppContext();
        System.out.println("Current context class loader: " +
                Thread.currentThread().getContextClassLoader());

        appsThreadGroup = new ThreadGroup("MyAppsThreadGroup");

        File jar = new File("TestApp.jar");
        if (!jar.exists()) {
            System.out.println(jar.getAbsolutePath() + " was not found!\n" +
                    "Please install the jar with test application correctly!");
            throw new RuntimeException("Test failed: no TestApp.jar");
        }

        URL[] urls = new URL[]{jar.toURL()};

        int numApps = Integer.getInteger("numApps", 20).intValue();

        doneSignal = new CountDownLatch(numApps);
        int cnt = 0;
        while (cnt++ < numApps) {
            launch(urls, "testapp.Main", "launch");

            checkErrors();
        }

        System.out.println("Wait for apps completion....");

        try {
            doneSignal.await();
        } catch (InterruptedException e) {
        }

        System.out.println("All apps finished.");

        System.gc();

        System.out.flush();

        System.out.println("Enumerate strong refs:");
        for (String is : strongRefs.keySet()) {
            System.out.println("-> " + is);
        }

        System.out.println("=======================");

        // wait few seconds
        waitAndGC(gcTimeout);

        doneSignal = new CountDownLatch(1);

        Runnable workaround = new Runnable() {

            public void run() {
                AppContext ctx = null;
                try {
                    ctx = SunToolkit.createNewAppContext();
                } catch (Throwable e) {
                    // ignore...
                } finally {
                    doneSignal.countDown();
                }
            }
        };

        Thread wt = new Thread(appsThreadGroup, workaround, "Workaround");
        wt.setContextClassLoader(new MyClassLoader(urls, "workaround"));
        wt.start();
        wt = null;
        workaround = null;

        System.out.println("Wait for workaround completion...");

        try {
            doneSignal.await();
        } catch (InterruptedException e) {
        }

        // give a chance to GC
        waitAndGC(gcTimeout);

        if (!refs.isEmpty()) {
            System.out.println("Classloaders still alive:");

            for (MyClassLoader l : refs.keySet()) {
                String val = refs.get(l);

                if (val == null) {
                    throw new RuntimeException("Test FAILED: Invalid classloader name");
                }
                System.out.println("->" + val + (strongRefs.get(val) != null ?
                                    " (has strong ref)" : ""));
                if (strongRefs.get(val) == null) {
                    throw new RuntimeException("Test FAILED: exta class loader is detected! ");
                }
            }
        } else {
            System.out.println("No alive class loaders!!");
        }
        System.out.println("Test PASSED.");
    }

    private static void waitAndGC(int sec) {
        int cnt = sec;
        System.out.print("Wait ");
        while (cnt-- > 0) {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
            }
            // do GC every 3 seconds
            if (cnt % 3 == 2) {
                System.gc();
                System.out.print("+");
            } else {
                System.out.print(".");
            }
            checkErrors();
        }
        System.out.println("");
    }

    private static void checkErrors() {
        while (!problems.isEmpty()) {
            Throwable theProblem = problems.poll();
            System.out.println("Test FAILED!");
            do {
                theProblem.printStackTrace(System.out);
                theProblem = theProblem.getCause();
            } while (theProblem != null);
            throw new RuntimeException("Test FAILED");
        }
    }
    static int counter = 0;

    private static void launch(URL[] urls, final String className,
                               final String methodName)
    {
        final String uniqClassName = "testapp/Uniq" + counter;
        final boolean saveStrongRef = forgetSomeStreams ? (counter % 5 == 4) : false;

        System.out.printf("%s: launch the app\n", uniqClassName);
        Runnable launchIt = new Runnable() {
            public void run() {
                AppContext ctx = SunToolkit.createNewAppContext();

                try {
                    Class appMain =
                        ctx.getContextClassLoader().loadClass(className);
                    Method launch = appMain.getDeclaredMethod(methodName,
                                strongRefs.getClass());

                    Constructor c = appMain.getConstructor(String.class,
                                                           problems.getClass());

                    Object o = c.newInstance(uniqClassName, problems);

                    if (saveStrongRef) {
                        System.out.printf("%s: force strong ref\n",
                                          uniqClassName);
                        launch.invoke(o, strongRefs);
                    } else {
                        HashMap<String, ImageInputStream> empty = null;
                        launch.invoke(o, empty);
                    }

                    ctx = null;
                } catch (Throwable e) {
                    problems.add(e);
                } finally {
                    doneSignal.countDown();
                }
            }
        };

        MyClassLoader appClassLoader = new MyClassLoader(urls, uniqClassName);

        refs.put(appClassLoader, uniqClassName);

        Thread appThread = new Thread(appsThreadGroup, launchIt,
                                      "AppThread" + counter++);
        appThread.setContextClassLoader(appClassLoader);

        appThread.start();
        launchIt = null;
        appThread = null;
        appClassLoader = null;
    }

    private static class MyClassLoader extends URLClassLoader {

        private static boolean verbose =
            Boolean.getBoolean("verboseClassLoading");
        private String uniqClassName;

        public MyClassLoader(URL[] urls, String uniq) {
            super(urls);

            uniqClassName = uniq;
        }

        public Class loadClass(String name) throws ClassNotFoundException {
            if (verbose) {
                System.out.printf("%s: load class %s\n", uniqClassName, name);
            }
            if (uniqClassName.equals(name)) {
                return Object.class;
            }
            return super.loadClass(name);
        }

        public String toString() {
            return "MyClassLoader(" + uniqClassName + ")";
        }
    }
}