張黃矚 , 軟件工程師
2006 年 1 月 18 日
Java5
增加了新的類庫并發(fā)集java.util.concurrent，該類庫為并發(fā)程序提供了豐富的API多線程編程在Java
5中更加容易，靈活。本文通過一個(gè)網(wǎng)絡(luò)服務(wù)器模型，來實(shí)踐Java5的多線程編程，該模型中使用了Java5中的線程池，阻塞隊(duì)列，可重入鎖等，還實(shí)踐了
Callable， Future等接口，并使用了Java 5的另外一個(gè)新特性泛型。
簡介
本文將實(shí)現(xiàn)一個(gè)網(wǎng)絡(luò)服務(wù)器模型，一旦有客戶端連接到該服務(wù)器，則啟動(dòng)一個(gè)新線程為該連接服務(wù)，服務(wù)內(nèi)容為往客戶端輸送一些字符信息。一個(gè)典型的網(wǎng)絡(luò)服務(wù)器模型如下：
1. 建立監(jiān)聽端口。
2. 發(fā)現(xiàn)有新連接，接受連接，啟動(dòng)線程，執(zhí)行服務(wù)線程。
3. 服務(wù)完畢，關(guān)閉線程。
這
個(gè)模型在大部分情況下運(yùn)行良好，但是需要頻繁的處理用戶請(qǐng)求而每次請(qǐng)求需要的服務(wù)又是簡短的時(shí)候，系統(tǒng)會(huì)將大量的時(shí)間花費(fèi)在線程的創(chuàng)建銷毀。Java
5的線程池克服了這些缺點(diǎn)。通過對(duì)重用線程來執(zhí)行多個(gè)任務(wù)，避免了頻繁線程的創(chuàng)建與銷毀開銷，使得服務(wù)器的性能方面得到很大提高。因此，本文的網(wǎng)絡(luò)服務(wù)器
模型將如下：
1. 建立監(jiān)聽端口，創(chuàng)建線程池。
2. 發(fā)現(xiàn)有新連接，使用線程池來執(zhí)行服務(wù)任務(wù)。
3. 服務(wù)完畢，釋放線程到線程池。
下面詳細(xì)介紹如何使用Java 5的concurrent包提供的API來實(shí)現(xiàn)該服務(wù)器。
初始化
初
始化包括創(chuàng)建線程池以及初始化監(jiān)聽端口。
創(chuàng)建線程池可以通過調(diào)用java.util.concurrent.Executors類里的靜態(tài)方法newChahedThreadPool或是
newFixedThreadPool來創(chuàng)建，也可以通過新建一個(gè)java.util.concurrent.ThreadPoolExecutor實(shí)例
來執(zhí)行任務(wù)。這里我們采用newFixedThreadPool方法來建立線程池。
ExecutorService pool = Executors.newFixedThreadPool(10);
表示新建了一個(gè)線程池，線程池里面有10個(gè)線程為任務(wù)隊(duì)列服務(wù)。
使用ServerSocket對(duì)象來初始化監(jiān)聽端口。
private static final int PORT = 19527;
serverListenSocket = new ServerSocket(PORT);
serverListenSocket.setReuseAddress(true);
serverListenSocket.setReuseAddress(true);
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服務(wù)新連接
當(dāng)有新連接建立時(shí)，accept返回時(shí)，將服務(wù)任務(wù)提交給線程池執(zhí)行。
while(true){
Socket socket = serverListenSocket.accept();
pool.execute(new ServiceThread(socket));
}
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這里使用線程池對(duì)象來執(zhí)行線程，減少了每次線程創(chuàng)建和銷毀的開銷。任務(wù)執(zhí)行完畢，線程釋放到線程池。
服務(wù)任務(wù)
服
務(wù)線程ServiceThread維護(hù)一個(gè)count來記錄服務(wù)線程被調(diào)用的次數(shù)。每當(dāng)服務(wù)任務(wù)被調(diào)用一次時(shí)，count的值自增1，因此
ServiceThread提供一個(gè)increaseCount和getCount的方法，分別將count值自增1和取得該count值。由于可能多個(gè)
線程存在競爭，同時(shí)訪問count，因此需要加鎖機(jī)制，在Java 5之前，我們只能使用synchronized來鎖定。Java
5中引入了性能更加粒度更細(xì)的重入鎖ReentrantLock。我們使用ReentrantLock保證代碼線程安全。下面是具體代碼：
private static ReentrantLock lock = new ReentrantLock ();
private static int count = 0;
private int getCount(){
int ret = 0;
try{
lock.lock();
ret = count;
}finally{
lock.unlock();
}
return ret;
}
private void increaseCount(){
try{
lock.lock();
++count;
}finally{
lock.unlock();
}
}
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服務(wù)線程在開始給客戶端打印一個(gè)歡迎信息，
increaseCount();
int curCount = getCount();
helloString = "hello, id = " + curCount+"\r\n";
dos = new DataOutputStream(connectedSocket.getOutputStream());
dos.write(helloString.getBytes());
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然后使用ExecutorService的submit
方法提交一個(gè)Callable的任務(wù)，返回一個(gè)Future接口的引用。這種做法對(duì)費(fèi)時(shí)的任務(wù)非常有效，submit任務(wù)之后可以繼續(xù)執(zhí)行下面的代碼，然
后在適當(dāng)?shù)奈恢每梢允褂肍uture的get方法來獲取結(jié)果，如果這時(shí)候該方法已經(jīng)執(zhí)行完畢，則無需等待即可獲得結(jié)果，如果還在執(zhí)行，則等待到運(yùn)行完畢。
ExecutorService executor = Executors.newSingleThreadExecutor();
Future <String> future = executor.submit(new TimeConsumingTask());
dos.write("let's do soemthing other".getBytes());
String result = future.get();
dos.write(result.getBytes());
其中TimeConsumingTask實(shí)現(xiàn)了Callable接口
class TimeConsumingTask implements Callable <String>{
public String call() throws Exception {
System.out.println
("It's a time-consuming task,
you'd better retrieve your result in the furture");
return "ok, here's the result: It takes me lots of time to produce this result";
}
}
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這里使用了Java
5的另外一個(gè)新特性泛型，聲明TimeConsumingTask的時(shí)候使用了String做為類型參數(shù)。必須實(shí)現(xiàn)Callable接口的call函數(shù)，
其作用類似與Runnable中的run函數(shù)，在call函數(shù)里寫入要執(zhí)行的代碼，其返回值類型等同于在類聲明中傳入的類型值。在這段程序中，我們提交了
一個(gè)Callable的任務(wù)，然后程序不會(huì)堵塞，而是繼續(xù)執(zhí)行dos.write("let's do soemthing
other".getBytes());當(dāng)程序執(zhí)行到String result =
future.get()時(shí)如果call函數(shù)已經(jīng)執(zhí)行完畢，則取得返回值，如果還在執(zhí)行，則等待其執(zhí)行完畢。
服務(wù)器端的完整實(shí)現(xiàn)
服務(wù)器端的完整實(shí)現(xiàn)代碼如下：
package com.andrew;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.Serializable;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantLock;
public class Server {
private static int produceTaskSleepTime = 100;
private static int consumeTaskSleepTime = 1200;
private static int produceTaskMaxNumber = 100;
private static final int CORE_POOL_SIZE = 2;
private static final int MAX_POOL_SIZE = 100;
private static final int KEEPALIVE_TIME = 3;
private static final int QUEUE_CAPACITY = (CORE_POOL_SIZE + MAX_POOL_SIZE) / 2;
private static final TimeUnit TIME_UNIT = TimeUnit.SECONDS;
private static final String HOST = "127.0.0.1";
private static final int PORT = 19527;
private BlockingQueue<Runnable> workQueue = new ArrayBlockingQueue<Runnable>(
QUEUE_CAPACITY);
//private ThreadPoolExecutor serverThreadPool = null;
private ExecutorService pool = null;
private RejectedExecutionHandler rejectedExecutionHandler = new
ThreadPoolExecutor.DiscardOldestPolicy();
private ServerSocket serverListenSocket = null;
private int times = 5;
public void start() {
// You can also init thread pool in this way.
/*serverThreadPool = new ThreadPoolExecutor(CORE_POOL_SIZE,
MAX_POOL_SIZE, KEEPALIVE_TIME, TIME_UNIT, workQueue,
rejectedExecutionHandler);*/
pool = Executors.newFixedThreadPool(10);
try {
serverListenSocket = new ServerSocket(PORT);
serverListenSocket.setReuseAddress(true);
System.out.println("I'm listening");
while (times-- > 0) {
Socket socket = serverListenSocket.accept();
String welcomeString = "hello";
//serverThreadPool.execute(new ServiceThread(socket, welcomeString));
pool.execute(new ServiceThread(socket));
}
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
cleanup();
}
public void cleanup() {
if (null != serverListenSocket) {
try {
serverListenSocket.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
//serverThreadPool.shutdown();
pool.shutdown();
}
public static void main(String args[]) {
Server server = new Server();
server.start();
}
}
class ServiceThread implements Runnable, Serializable {
private static final long serialVersionUID = 0;
private Socket connectedSocket = null;
private String helloString = null;
private static int count = 0;
private static ReentrantLock lock = new ReentrantLock();
ServiceThread(Socket socket) {
connectedSocket = socket;
}
public void run() {
increaseCount();
int curCount = getCount();
helloString = "hello, id = " + curCount + "\r\n";
ExecutorService executor = Executors.newSingleThreadExecutor();
Future<String> future = executor.submit(new TimeConsumingTask());
DataOutputStream dos = null;
try {
dos = new DataOutputStream(connectedSocket.getOutputStream());
dos.write(helloString.getBytes());
try {
dos.write("let's do soemthing other.\r\n".getBytes());
String result = future.get();
dos.write(result.getBytes());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
if (null != connectedSocket) {
try {
connectedSocket.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
if (null != dos) {
try {
dos.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
executor.shutdown();
}
}
private int getCount() {
int ret = 0;
try {
lock.lock();
ret = count;
} finally {
lock.unlock();
}
return ret;
}
private void increaseCount() {
try {
lock.lock();
++count;
} finally {
lock.unlock();
}
}
}
class TimeConsumingTask implements Callable<String> {
public String call() throws Exception {
System.out
.println("It's a time-consuming task,
you'd better retrieve your result in the furture");
return "ok, here's the result: It takes me lots of time to produce this result";
}
}
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運(yùn)行程序
運(yùn)行服務(wù)端，客戶端只需使用telnet 127.0.0.1 19527 即可看到信息如下：
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