CopyOnWriteArrayList Sample Code

A simple one from http://www.ibm.com/developerworks/java/library/j-tiger06164.html.

 

 

import java.util.*;
import java.util.concurrent.*;

public class CopyOnWrite {
  public static void main(String args[]) {
    List list1 = new CopyOnWriteArrayList(Arrays.asList(args));
    List list2 = new ArrayList(Arrays.asList(args));
    Iterator itor1 = list1.iterator();
    Iterator itor2 = list2.iterator();
    list1.add("New");
    list2.add("New");
    try {
      printAll(itor1);
    } catch (ConcurrentModificationException e) {
      System.err.println("Shouldn't get here");
    }
    try {
      printAll(itor2);
    } catch (ConcurrentModificationException e) {
      System.err.println("Will get here.");
    }
  }
  private static void printAll(Iterator itor) {
    while (itor.hasNext()) {
      System.out.println(itor.next());
    }
  }
}

A complex example using multiple threads. 

 

import java.util.Iterator;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.CountDownLatch;

public class MultiThreads {
  public static void main(String[] args) throws Exception {
    List<Val> ls = new CopyOnWriteArrayList<Val>();
    ls.add(new Val(1, 10));
    ls.add(new Val(2, 20));
    ls.add(new Val(3, 30));

    CountDownLatch latch = new CountDownLatch(1);
    Walker w = new Walker(ls.iterator(), latch);
    Modifer m = new Modifer(ls, latch);
    w.start();
    m.start();
    
    latch.await();
    System.out.println("===========");
    Iterator<Val> it = ls.iterator();
    while (it.hasNext())
      System.out.println("[main] " + it.next());
  }
}

class Walker extends Thread {
  Iterator<Val> it;
  CountDownLatch latch;
  Walker(Iterator<Val> it, CountDownLatch latch) {
    this.it = it;
    this.latch = latch;
  }

  public void run() {
    try {
      latch.await();
      System.out.println("===========");
      while (it.hasNext())
        System.out.println("[walker] " + it.next());
    } catch (Exception e) {
      throw new RuntimeException(e);
    }
  }
}

class Modifer extends Thread {
  List<Val> ls;
  CountDownLatch latch;

  public Modifer(List<Val> ls, CountDownLatch latch) {
    this.ls = ls;
    this.latch = latch;
  }

  public void run() {
    try {
      ls.set(0, new Val(1, 100));
      ls.remove(2);
      ls.add(new Val(3, 300));
      ls.add(new Val(4, 400));
      latch.countDown();
    } catch (Exception e) {
      throw new RuntimeException(e);
    }
  }
}

class Val {
  private int no;
  private int age;
  
  public Val(int no, int age) {
    this.no = no;
    this.age = age;
  }
  public String toString() {
    return "no: " + no + ", age: " + age;
  }
}