Question

trying to use a concurrent skip list map. i had problems with how to use a synchronized linked hash map correctly, so i decided to give concurrent skip list map a try.

i have the same sort of problem. the unit test below fails because when i get the entry set, it has null values when size() indicates that the map is not empty. naict, i have all access to the map synchronized.

i would think that one would not need to do this (synchronized), since this a concurrent map.

the server just puts the numbers 0,1,2,3, … into the map, keeping it’s size below a threshold. it tries to put one number in for each millisecond that has passed since the server was started.

any pointers will be appreciated.

thanks

import static org.junit.Assert.*;
import java.util.*;
import java.util.Map.Entry;
import java.util.concurrent.ConcurrentSkipListMap;
import org.junit.*;
class DummyServer implements Runnable {
    DummyServer(int pieces) {
        t0=System.currentTimeMillis();
        this.pieces=pieces;
        max=pieces;
        lruMap=new ConcurrentSkipListMap<Long,Long>();
    }
    Set<Map.Entry<Long,Long>> entrySet() {
        Set<Entry<Long,Long>> entries=null;
        synchronized(lruMap) {
            entries=Collections.unmodifiableSet(lruMap.entrySet());
        }
        return entries;
    }
    Set<Long> keySet() {
        Set<Long> entries=null;
        synchronized(lruMap) {
            entries=Collections.unmodifiableSet(lruMap.keySet());
        }
        return entries;
    }
    @Override public void run() {
        int n=0;
        while(piece<stopAtPiece) {
            long target=piece(System.currentTimeMillis()-t0);
            long n0=piece;
            for(;piece<target;piece++,n++)
                put(piece);
            if(n>max+max/10) {
                Long[] keys=keySet().toArray(new Long[0]);
                synchronized(lruMap) {
                    for(int i=0;n>max;i++,n--)
                        lruMap.remove(keys[i]);
                }
            }
            try {
                Thread.sleep(10);
            } catch(InterruptedException e) {
                e.printStackTrace();
                break;
            }
        }
    }
    private void put(long piece) {
        synchronized(lruMap) {
            lruMap.put(piece,piece);
        }
    }
    public long piece() {
        return piece;
    }
    public Long get(long piece) {
        synchronized(lruMap) {
            return lruMap.get(piece);
        }
    }
    public int size() {
        synchronized(lruMap) {
            return lruMap.size();
        }
    }
    public long piece(long dt) {
        return dt/period*pieces+dt%period*pieces/period;
    }
    private long piece;
    int period=2000;
    private volatile Map<Long,Long> lruMap;
    public final long t0;
    protected final int pieces;
    public final int max;
    public long stopAtPiece=Long.MAX_VALUE;
}
public class DummyServerTestCase {
    void checkMap(Long n) {
        if(server.size()>0) {
            final Set<Map.Entry<Long,Long>> mapValues=server.entrySet();
            @SuppressWarnings("unchecked") final Map.Entry<Long,Long>[] entries=new Map.Entry[mapValues.size()];
            mapValues.toArray(entries);
            try {
                if(entries[0]==null)
                    System.out.println(server.piece());
                assertNotNull(entries[0]);
            } catch(Exception e) {
                fail(e.toString());
            }
        }
    }
    @Test public void testRunForFirstIsNotZero() {
        server.stopAtPiece=1*server.pieces;
        Thread thread=new Thread(server);
        thread.start();
        while(thread.isAlive()) {
            for(long i=0;i<server.piece();i++) {
                server.get(i);
                Thread.yield();
                checkMap(server.piece());
                Thread.yield();
            }
        }
    }
    DummyServer server=new DummyServer(1000);
}

Answer 1

The problem is that you are performing

final Map.Entry<Long,Long>[] entries=new Map.Entry[mapValues.size()]; // size>0
mapValues.toArray(entries); // size is 0.

Between creating the array and calling toArray you are clearing the map.

If you take a copy using the Iterator you will not get this race condition.

void checkMap(Long n) {
    final Set<Map.Entry<Long, Long>> mapValues = server.entrySet();
    Set<Map.Entry<Long, Long>> entries = new LinkedHashSet<>(mapValues);

    for (Entry<Long, Long> entry : entries) {
        assertNotNull(entry);
    }
}

or

void checkMap(Long n) {
    for (Entry<Long, Long> entry : server.entrySet())
        assertNotNull(entry);
}