最近一直在看《Think In Java》里关于并发部分的章节，读到第二十一章有一个有趣的比喻：必须先挖房子的地基，但是接下来可以并行的铺设钢结构和构建水泥部件，而这两项任务必须在混凝土浇筑之前完成。管道必须在水泥板浇注之前到位，而水泥板必须在开始构筑房屋骨架之前到位。
在这些任务中，某些可以并行执行，但是某些步骤需要所有的任务都结束之后才能开动，这是线程之间协作的必要性。
在此之前，我们学习过使用notify()、notifyAll()和wait()来控制线程间的协作，让我们先来回顾一下。notify()、notifyAll()和wait()这三个方法同属于Object对象，wait()会使得当前线程等待并交出对象的锁，直到别的线程调用notify()或notifyAll()后可能会被唤醒。
对于一些简单的问题，这已经够用了，但是Java SE5中的concurrent包中提供了BlockingQueue、Condition等类来帮助我们完成更复杂的线程间协作的任务。
下面看一个例子，一台机器具有三个任务：一个制作吐司、一个给吐司抹黄油，另一个在抹过黄油的吐司上涂果酱。通过各个处理过程之间的BlockingQueue来运行这个程序。来自Think In Java (p.s. 我觉得这本书难懂的原因，在于你在理解它教导并发概念的同时，还得十分小心地注意其余的语法细节，一定要有耐心！)。
package concurrency;//: concurrency/ToastOMatic.java
// A toaster that uses queues.
import java.util.concurrent.*;
import java.util.*;
import static net.mindview.utill.Print.*; class Toast {
public enum Status { DRY, BUTTERED, JAMMED }
private Status status = Status.DRY;
private final int id;
public Toast(int idn) { id = idn; }
public void butter() { status = Status.BUTTERED; }
public void jam() { status = Status.JAMMED; }
public Status getStatus() { return status; }
public int getId() { return id; }
public String toString() {
return "Toast " + id + ": " + status;
}
} class ToastQueue extends LinkedBlockingQueue<Toast> {} class Toaster implements Runnable {
private ToastQueue toastQueue;
private int count = 0;
private Random rand = new Random(47);
public Toaster(ToastQueue tq) { toastQueue = tq; }
public void run() {
try {
while(!Thread.interrupted()) {
TimeUnit.MILLISECONDS.sleep(
100 + rand.nextInt(500));
// Make toast
Toast t = new Toast(count++);
print(t);
// Insert into queue
toastQueue.put(t);
}
} catch(InterruptedException e) {
print("Toaster interrupted");
}
print("Toaster off");
}
} // Apply butter to toast:
class Butterer implements Runnable {
private ToastQueue dryQueue, butteredQueue;
public Butterer(ToastQueue dry, ToastQueue buttered) {
dryQueue = dry;
butteredQueue = buttered;
}
public void run() {
try {
while(!Thread.interrupted()) {
// Blocks until next piece of toast is available:
Toast t = dryQueue.take();
t.butter();
print(t);
butteredQueue.put(t);
}
} catch(InterruptedException e) {
print("Butterer interrupted");
}
print("Butterer off");
}
} // Apply jam to buttered toast:
class Jammer implements Runnable {
private ToastQueue butteredQueue, finishedQueue;
public Jammer(ToastQueue buttered, ToastQueue finished) {
butteredQueue = buttered;
finishedQueue = finished;
}
public void run() {
try {
while(!Thread.interrupted()) {
// Blocks until next piece of toast is available:
Toast t = butteredQueue.take();
t.jam();
print(t);
finishedQueue.put(t);
}
} catch(InterruptedException e) {
print("Jammer interrupted");
}
print("Jammer off");
}
} // Consume the toast:
class Eater implements Runnable {
private ToastQueue finishedQueue;
private int counter = 0;
public Eater(ToastQueue finished) {
finishedQueue = finished;
}
public void run() {
try {
while(!Thread.interrupted()) {
// Blocks until next piece of toast is available:
Toast t = finishedQueue.take();
// Verify that the toast is coming in order,
// and that all pieces are getting jammed:
if(t.getId() != counter++ ||
t.getStatus() != Toast.Status.JAMMED) {
print(">>>> Error: " + t);
System.exit(1);
} else
print("Chomp! " + t);
}
} catch(InterruptedException e) {
print("Eater interrupted");
}
print("Eater off");
}
} public class ToastOMatic {
public static void main(String[] args) throws Exception {
ToastQueue dryQueue = new ToastQueue(),
butteredQueue = new ToastQueue(),
finishedQueue = new ToastQueue();
ExecutorService exec = Executors.newCachedThreadPool();
exec.execute(new Toaster(dryQueue));
exec.execute(new Butterer(dryQueue, butteredQueue));
exec.execute(new Jammer(butteredQueue, finishedQueue));
exec.execute(new Eater(finishedQueue));
TimeUnit.SECONDS.sleep(5);
exec.shutdownNow();
}
} /* (Execute to see output) *///:~
看完晕乎乎的？很正常，所以才需要我来给大家讲解啦 :)
首先可以注意到的，程序中并没有出现任何Lock对象或是synchronized关键字来同步，这是因为在实现BlockingQueue的队列类内部已经使用Condition在维护。这降低了程序的耦合度，使得每个类只需要和自己的BlockingQueue通信。
程序中定义了:
一个实体类：Toast。使用enum来管理状态是一个优秀的示例。
三个队列：dryQueue、butteredQueue、finishedQueue
四个Runnable任务：Toaster、Butterer、Jammer、Eater
根据字面意思理解，当线程不被中断的时候，Toaster负责制作吐司，所以只需要和dryQueue通信。Butterer在吐司上涂黄油，需要从dryQueue中取出原味土司，涂上黄油(t.butter())后放入butteredQueue。Jammer在抹过黄油的吐司上涂果酱，需要从butteredQueue中取出，涂上果酱后放入finishedQueue。Eater就只需要从finishedQueue中取出来吃啦。细心的读者还会发现Eater中做了检查，如果不是涂上果酱的吐司就不吃(傲娇的表情)。如果线程被中断，任务就打印信息并退出。
TimeUnit.SECONDS.sleep(3)的作用是当前线程等待3秒，等待后台制作吐司。exec.shutdownNow()停止当前线程池。
是不是觉得自己理解了？那么还有一道课后题留给大家：修改ToastOMatic.java，使用两个单独的组装线来创建涂有黄油和果酱的三明治（即不必先涂黄油再涂果酱，可以异步处理，明显提高工作效率）。
答案在这里:
//: concurrency/E29_ToastOMatic2.java
/********************** Exercise 29 ***********************
* Modify ToastOMatic.java to create peanut butter and jelly
* on toast sandwiches using two separate assembly lines
* (one for peanut butter, the second for jelly, then
* merging the two lines).
*********************************************************/
package concurrency;
import java.util.concurrent.*;
import java.util.*;
import static net.mindview.utill.Print.*; class Toast {
public enum Status {
DRY,
BUTTERED,
JAMMED,
READY {
public String toString() {
return
BUTTERED.toString() + " & " + JAMMED.toString();
}
}
}
private Status status = Status.DRY;
private final int id;
public Toast(int idn) { id = idn; }
public void butter() {
status =
(status == Status.DRY) ? Status.BUTTERED :
Status.READY;
}
public void jam() {
status =
(status == Status.DRY) ? Status.JAMMED :
Status.READY;
}
public Status getStatus() { return status; }
public int getId() { return id; }
public String toString() {
return "Toast " + id + ": " + status;
}
} class ToastQueue extends LinkedBlockingQueue<Toast> {} class Toaster implements Runnable {
private ToastQueue toastQueue;
private int count;
private Random rand = new Random(47);
public Toaster(ToastQueue tq) { toastQueue = tq; }
public void run() {
try {
while(!Thread.interrupted()) {
TimeUnit.MILLISECONDS.sleep(
100 + rand.nextInt(500));
// Make toast
Toast t = new Toast(count++);
print(t);
// Insert into queue
toastQueue.put(t);
}
} catch(InterruptedException e) {
print("Toaster interrupted");
}
print("Toaster off");
}
} // Apply butter to toast:
class Butterer implements Runnable {
private ToastQueue inQueue, butteredQueue;
public Butterer(ToastQueue in, ToastQueue buttered) {
inQueue = in;
butteredQueue = buttered;
}
public void run() {
try {
while(!Thread.interrupted()) {
// Blocks until next piece of toast is available:
Toast t = inQueue.take();
t.butter();
print(t);
butteredQueue.put(t);
}
} catch(InterruptedException e) {
print("Butterer interrupted");
}
print("Butterer off");
}
} // Apply jam to toast:
class Jammer implements Runnable {
private ToastQueue inQueue, jammedQueue;
public Jammer(ToastQueue in, ToastQueue jammed) {
inQueue = in;
jammedQueue = jammed;
}
public void run() {
try {
while(!Thread.interrupted()) {
// Blocks until next piece of toast is available:
Toast t = inQueue.take();
t.jam();
print(t);
jammedQueue.put(t);
}
} catch(InterruptedException e) {
print("Jammer interrupted");
}
print("Jammer off");
}
} // Consume the toast:
class Eater implements Runnable {
private ToastQueue finishedQueue;
public Eater(ToastQueue finished) {
finishedQueue = finished;
}
public void run() {
try {
while(!Thread.interrupted()) {
// Blocks until next piece of toast is available:
Toast t = finishedQueue.take();
// Verify that all pieces are ready for consumption:
if(t.getStatus() != Toast.Status.READY) {
print(">>>> Error: " + t);
System.exit(1);
} else
print("Chomp! " + t);
}
} catch(InterruptedException e) {
print("Eater interrupted");
}
print("Eater off");
}
} // Outputs alternate inputs on alternate channels:
class Alternator implements Runnable {
private ToastQueue inQueue, out1Queue, out2Queue;
private boolean outTo2; // control alternation
public Alternator(ToastQueue in, ToastQueue out1,
ToastQueue out2) {
inQueue = in;
out1Queue = out1;
out2Queue = out2;
}
public void run() {
try {
while(!Thread.interrupted()) {
// Blocks until next piece of toast is available:
Toast t = inQueue.take();
if(!outTo2)
out1Queue.put(t);
else
out2Queue.put(t);
outTo2 = !outTo2; // change state for next time
}
} catch(InterruptedException e) {
print("Alternator interrupted");
}
print("Alternator off");
}
} // Accepts toasts on either channel, and relays them on to
// a "single" successor
class Merger implements Runnable {
private ToastQueue in1Queue, in2Queue, toBeButteredQueue,
toBeJammedQueue, finishedQueue;
public Merger(ToastQueue in1, ToastQueue in2,
ToastQueue toBeButtered, ToastQueue toBeJammed,
ToastQueue finished) {
in1Queue = in1;
in2Queue = in2;
toBeButteredQueue = toBeButtered;
toBeJammedQueue = toBeJammed;
finishedQueue = finished;
}
public void run() {
try {
while(!Thread.interrupted()) {
// Blocks until next piece of toast is available:
Toast t = null;
while(t == null) {
t = in1Queue.poll(50, TimeUnit.MILLISECONDS);
if(t != null)
break;
t = in2Queue.poll(50, TimeUnit.MILLISECONDS);
}
// Relay toast onto the proper queue
switch(t.getStatus()) {
case BUTTERED:
toBeJammedQueue.put(t);
break;
case JAMMED:
toBeButteredQueue.put(t);
break;
default:
finishedQueue.put(t);
}
}
} catch(InterruptedException e) {
print("Merger interrupted");
}
print("Merger off");
}
} public class E29_ToastOMatic2 {
public static void main(String[] args) throws Exception {
ToastQueue
dryQueue = new ToastQueue(),
butteredQueue = new ToastQueue(),
toBeButteredQueue = new ToastQueue(),
jammedQueue = new ToastQueue(),
toBeJammedQueue = new ToastQueue(),
finishedQueue = new ToastQueue();
ExecutorService exec = Executors.newCachedThreadPool();
exec.execute(new Toaster(dryQueue));
exec.execute(new Alternator(dryQueue, toBeButteredQueue,
toBeJammedQueue));
exec.execute(
new Butterer(toBeButteredQueue, butteredQueue));
exec.execute(
new Jammer(toBeJammedQueue, jammedQueue));
exec.execute(new Merger(butteredQueue , jammedQueue,
toBeButteredQueue, toBeJammedQueue, finishedQueue));
exec.execute(new Eater(finishedQueue));
TimeUnit.SECONDS.sleep(5);
exec.shutdownNow();
}
} /* (Execute to see output) *///:~
没想清楚前不许偷看！
因为代码比较长，推荐把代码导入IDE查看。