在Java并发编程实践中4.4中提到向客户端加锁的方法。此为验证示例，写的不好，但可以看出结果来。

package com.blackbread.test;

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

public class GoodListHelper<E> {
    public List<E> list = Collections.synchronizedList(new ArrayList<E>());

    public boolean putIfAbsent(E x) throws InterruptedException {
        synchronized (list) {
            boolean absent = !list.contains(x);
            if (absent) {
                if (list.contains(x))
                    System.out.println(list.contains(x));
                list.add(x);
            }
            return absent;
        }
    }

    public class PrintA extends Thread {
        private GoodListHelper<String> goodListHelper;
        private String value;

        public PrintA(GoodListHelper<String> goodListHelper, String value) {
            this.goodListHelper = goodListHelper;
            this.value = value;
        }

        @Override
        public void run() {
            try {
                goodListHelper.putIfAbsent(value);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public class PrintB extends Thread {
        private GoodListHelper<String> goodListHelper;
        private String value;

        public PrintB(GoodListHelper<String> goodListHelper, String value) {
            this.goodListHelper = goodListHelper;
            this.value = value;
        }

        @Override
        public void run() {
            goodListHelper.list.add(value);
        }
    }

    public static void main(String[] args) throws InterruptedException {
        final GoodListHelper<String> goodListHelper = new GoodListHelper<String>();
        new Thread(new Runnable() {
            @Override
            public void run() {
                ExecutorService executor = Executors.newFixedThreadPool(50);
                for (int i = 0; i < 1000; i++) {
                    Thread t = goodListHelper.new PrintA(goodListHelper,
                            String.valueOf(i));
                    executor.execute(t);
                }
                executor.shutdown();
            }
        }).start();
        new Thread(new Runnable() {
            @Override
            public void run() {
                ExecutorService executor = Executors.newFixedThreadPool(50);
                for (int i = 0; i < 1000; i++) {
                    Thread t = goodListHelper.new PrintB(goodListHelper,
                            String.valueOf(i));
                    executor.execute(t);
                }
                executor.shutdown();
            }
        }).start();
    }
}