Example showing why creating HashMaps in many threads is slower under JDK7 than under JDK6.

ParallelHashMapCreationTest.java

package com.github.punya.threadlocalrandomnumbers;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
import java.util.HashMap;

import org.junit.Test;

public final class ParallelHashMapCreationTest {
    private static final int THREADS = 16;
    private static final int MAPS_PER_THREAD = 1000000;

    @Test
    public void benchmark() throws InterruptedException {
        final CountDownLatch latch = new CountDownLatch(THREADS);

        class CreateMapsTask implements Runnable {
            @Override
            public void run() {
                for (int i = 0; i < MAPS_PER_THREAD; ++i) {
                    new HashMap<String, String>();
                }
                latch.countDown();
            }
        }

        ExecutorService exec = Executors.newFixedThreadPool(THREADS);
        for (int i = 0; i < THREADS; ++i) {
            exec.submit(new CreateMapsTask());
        }
        latch.await();
        exec.shutdownNow();
    }
}