java.util.concurrent包中有许多涉及多线程并发的工具，使用这些现成工具可以大大简化编码，并且安全高效。

Exchanger提供了一个同步点，即调用exchange方法，当两个线程都到达这个同步点时，两个线程交换数据。

比如，有两个人，一个人需要买一本书，另一个人当然卖一本书，这两个人约定在某地交易，但这两个人距离约定地不同，到达目的地的时间也不同，先到的必须等待直到另一个人到达完成交易。

使用Exchanger类可以很容易实现！

import java.util.*;
import java.util.concurrent.*;
interface Exchangable {
}
class Money implements Exchangable {
    private int value;
    public Money(int value) {
        this.value = value;
    }
    public String toString() {
        return "$" + value;
    }
}
class Book implements Exchangable {
    private int value;
    private String name;
    public Book(String name, int value) {
        this.name = name;
        this.value = value;
    }
    public String toString() {
        String format = "Book: %s($%d)";
        return String.format(format, name, value);
    }
}
class Seller implements Runnable {
    Exchanger<Exchangable> exchanger;
    Exchangable book;
    public Seller(Exchanger<Exchangable> exchanger) {
        this.exchanger = exchanger;
        this.book = new Book("Love Java", 88);
    }
    public void run() {
        try {
            Random random = new Random(System.currentTimeMillis());
            int seconds = random.nextInt(10) + 1;
            System.out.println("Hi, I am Seller, I am going to sell a book,waiting " + seconds + "s");
            TimeUnit.SECONDS.sleep(seconds);
            System.out.println("Hi, I am Seller, I get " + exchanger.exchange(book));
        } catch (Exception e) {

        }
    }
}
class Buyer implements Runnable {
    Exchanger<Exchangable> exchanger;
    Exchangable money;
    public Buyer(Exchanger<Exchangable> exchanger) {
        this.exchanger = exchanger;
        this.money = new Money(88);
    }
    public void run() {
        try {
            Random random = new Random(System.currentTimeMillis() + 99);
            int seconds = random.nextInt(10) + 1;
            System.out.println("Hi, I am Buyer, I am going to buy a book,waiting " + seconds + "s");
            TimeUnit.SECONDS.sleep(seconds);
            System.out.println("Hi, I am Buyer, I get " + exchanger.exchange(money));
        } catch (Exception e) {

        }
    }
}
public class ExchangerDemo {
    public static void main(String[] args) {
        Exchanger<Exchangable> exchanger = new Exchanger<>();
        Seller seller = new Seller(exchanger);
        Buyer buyer = new Buyer(exchanger);
        new Thread(seller).start();
        new Thread(buyer).start();
    }
}

Exchanger类使用很简单，只有两个public方法，一个是exchange(V x)方法，死等待直到完成交易，另一个是exchange(V x, long time, TimeUnit timeunit)，其中x是要交换的数据，time是时间，timeunit指定时间单位，合起来表示最长等待时间，比如exchange(x, 5, TimeUnit.SECONDS)表示最长等待5秒！

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