Java 寫時拷貝容器CopyOnWriteArrayList的測試

測試代碼：

package copyOnWriteArrayListTest;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

class AddThread implements Runnable {
    private List<Double> list;
    public AddThread(List<Double> list) {
        this.list = list;
   }
  
  @Override
   public void run(){
      for ( int i = 0; i < 10000; i++){
        list.add(Math.random());
      }
   }
}

public class CopyOnWriteArrayListTest {

	public static final int THREAD_POOL_SIZE = 2;
	
	public static void main(String[] args) throws InterruptedException {
		// List<Double> list = new ArrayList<>();
		List<Double> list = new CopyOnWriteArrayList<>();
		ExecutorService es = Executors.newFixedThreadPool(THREAD_POOL_SIZE);
		es.execute(new AddThread(list));
		es.execute(new AddThread(list));
		
		es.shutdown();
		
		Thread.sleep(4000);
		System.out.println("OK, list length: " + list.size());

	}

}

輸出結果：

OK, list length: 20000

如果把支持寫時拷貝的list替換成普通的ArrayList：

List list = new ArrayList<>();

因爲兩個線程同時對這個普通的ArrayList進行寫操作，結果如下：

OK, list length: 17578

看下CopyOnWriteArrayList實現源代碼裏Add方法的實現：

/**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        synchronized (lock) {
            Object[] es = getArray();
            int len = es.length;
            es = Arrays.copyOf(es, len + 1);
            es[len] = e;
            setArray(es);
            return true;
        }
    }

lock初始化的地方：

final transient Object lock = new Object();