concurrency vs. synchronization

which way to go? synchronized? ConcurrentHashMap?

After getting up this morning I thought it would be good to contribute something back to my favorite ORM tool: iBATIS. I picked up IBATIS-508 and tried to create a unit test before applying the patch. Unfortunately I was not able to create one so I wanted to do at least some manual testing in order to find out in which way to fix the issue most efficiently. So I created a test case that compares different options for synchronizing access to a resource (namely a map) used by multiple threads at the same time.

The original issue was that the implementation of com.ibatis.common.beans.ClassInfo could cause performance problems under heavy load because of double synchronization. The implementation as of iBATIS 2.3.4 looks like this:

Obviously this is kind of redundant. I was not sure if I should apply the attached patch or just remove the synchronized clause. Therefore I created the test case that can be seen below. The results were:

No synchronization was fastest, with ConcurrentHashMap being also quite fast and the implementation as of 2.3.4 beeing the slowest. Therefore I decided to go for the patch originally attached to the issue. Please see bellow for the complete implementation of the test case. What are the results on your machine? Mine was a Dual Core T7500@2,2 GHZ with 3 GB Ram, Windows XP and Java 1.6.0_13

package threads;

import java.text.NumberFormat;
import java.util.Collections;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import junit.framework.TestCase;

/**
 * integration test to decide for the best fix of
 * https://issues.apache.org/jira/browse/IBATIS-508
 * 
 * @see com.ibatis.common.beans.ClassInfo
 * 
 * @author Kai Grabfelder (nospam@kaigrabfelder.de)
 */
@SuppressWarnings("unchecked")
public class ClassInfoTest extends TestCase {

public void testNoSynch() throws Exception {
        doTest(new ClassInfo1());
    }

public void testSynchronized() throws Exception {
        doTest(new ClassInfo2());
    }

public void testDoubleSynchro() throws Exception {
        doTest(new ClassInfo3());
    }

public void testCollectionsSynchro() throws Exception {
        doTest(new ClassInfo4());
    }

public void testJavaConcurrent() throws Exception {
        doTest(new ClassInfo5());
    }

/**
     * performs the test, using the given classInfoInstance, representing a
     * certain synchronization strategy
     * 
     * @param classInfoInstance
     */
    public void doTest(ClassInfo classInfoInstance) throws Exception {

List<MyThread> threads = new LinkedList<MyThread>();

//create 10 threads that try to read classinformation for 5 different classes
        int count = 6;
        for (int i = 1; i < count; i++) {
            MyThread thread = new MyThread(Class
                    .forName("threads.ClassInfoTest$A" + i), classInfoInstance);
            thread.start();
            threads.add(thread);
        }
        for (int i = 1; i < count; i++) {
            MyThread thread = new MyThread(Class
                    .forName("threads.ClassInfoTest$A" + i), classInfoInstance);
            thread.start();
            threads.add(thread);
        }

//let the threads work for 10 seconds
        Thread.sleep(10000);

long cTotal = 0;

// finish the threads & count the results
        for (MyThread myThread : threads) {
            //            System.out.println(myThread.toString());
            cTotal = cTotal + myThread.counter;
            myThread.interrupt();
        }

System.out.println("impl. strategy: " + classInfoInstance.toString()
                + " avg calls per thread: "
                + NumberFormat.getInstance().format(cTotal / threads.size()));

}

/**
     * no synchronization
     */
    public static class ClassInfo {

private String className;

public ClassInfo() {
        }

public ClassInfo(Class clazz) {
            this.className = clazz.getName();
        }

public ClassInfo getInstance(Class clazz) {
            return null;
        }
    }

/**
     * no synchronization
     */
    public static class ClassInfo1 extends ClassInfo {

@Override
        public String toString() {
            return "no synchronization";
        }

private static final Map CLASS_INFO_MAP = (new HashMap());