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();
}
}