本文主要利用nio包中的SocketChannel和Selector模拟实现ping命令(linux)，下面的示例是Sun提供的，有助于理解SocketChannel与Selector的使用。对于初学者而言，有明白以下几件事情：
<1> 调用Selector.selector()方法将会导致当前线程阻塞；
<2> Selector阻塞后，以下事件将导致Selector活跃：
(1)其他线程或者当前线程为通道向Selector中注册了感兴趣的事件(Connect、Accept、Read、Write);
(2)其他线程调用了selector的wakeup方法；
明白了这些以下代码对你来说，就是小菜一叠，以上是我看代码的一点体会，希望指正！
public class Ping 
{
// The default daytime port
static int DAYTIME_PORT = 13;
// The port we'll actually use
static int port = DAYTIME_PORT;
// Representation of a ping target
//
static class Target {
InetSocketAddress address;
SocketChannel channel;
Exception failure;
long connectStart;
long connectFinish = 0;
boolean shown = false;
Target(String host) {
try {
address = new InetSocketAddress(InetAddress.getByName(host),
port);
} catch (IOException x) {
failure = x;
}
}
/**//*打印信息*/
void show() {
String result;
if (connectFinish != 0)
result = Long.toString(connectFinish - connectStart) + "ms";
else if (failure != null)
result = failure.toString();
else
result = "Timed out";
System.out.println(address + " : " + result);
shown = true;
}
}
// Thread for printing targets as they're heard from
//
static class Printer extends Thread
{
LinkedList pending = new LinkedList();
Printer() {
setName("Printer");
setDaemon(true);
}
void add(Target t) {
synchronized (pending) {
pending.add(t);
pending.notify();
}
}
public void run() {
try {
for (;;) {
Target t = null;
synchronized (pending) {
while (pending.size() == 0)
pending.wait();
//唤醒后删除target
t = (Target)pending.removeFirst();
}
t.show();
}
} catch (InterruptedException x) {
return;
}
}
}
// Thread for connecting to all targets in parallel via a single selector
//
static class Connector extends Thread{
Selector sel;
Printer printer;
// List of pending targets. We use this list because if we try to
// register a channel with the selector while the connector thread is
// blocked in the selector then we will block.
//
LinkedList pending = new LinkedList();
Connector(Printer pr) throws IOException {
printer = pr;
sel = Selector.open();
setName("Connector");
}
// Initiate a connection sequence to the given target and add the
// target to the pending-target list
void add(Target t) {
SocketChannel sc = null;
try {
// Open the channel, set it to non-blocking, initiate connect
//在收到添加一个target的请求时，打开通道
sc = SocketChannel.open();
//把通道注册为非阻塞的
sc.configureBlocking(false);
boolean connected = sc.connect(t.address);
System.out.println("if connected:"+connected);
// Record the time we started
t.channel = sc;
t.connectStart = System.currentTimeMillis();
if (connected) {
t.connectFinish = t.connectStart;
sc.close();
printer.add(t);
} else {
// Add the new channel to the pending list
// synchronized (pending) {
// pending.add(t);
// }
// Nudge the selector so that it will process the pending list
sel.wakeup();
}
} catch (IOException x) {
if (sc != null) {
try {
sc.close();
} catch (IOException xx) { }
}
t.failure = x;
printer.add(t);
}
}
// Process any targets in the pending list
//
void processPendingTargets() throws IOException {
synchronized (pending) {
while (pending.size() > 0) {
Target t = (Target)pending.removeFirst();
try {
// Register the channel with the selector, indicating
// interest in connection completion and attaching the
// target object so that we can get the target back
// after the key is added to the selector's
// selected-key set
t.channel.register(sel, SelectionKey.OP_CONNECT, t);
} catch (IOException x) {
// Something went wrong, so close the channel and
// record the failure
t.channel.close();
t.failure = x;
printer.add(t);
}
}
}
}
// Process keys that have become selected
//
void processSelectedKeys() throws IOException {
for (Iterator i = sel.selectedKeys().iterator(); i.hasNext();) {
// Retrieve the next key and remove it from the set
SelectionKey sk = (SelectionKey)i.next();
i.remove();
// Retrieve the target and the channel
Target t = (Target)sk.attachment();
SocketChannel sc = (SocketChannel)sk.channel();
// Attempt to complete the connection sequence
try {
if (sc.finishConnect()) {
sk.cancel();
t.connectFinish = System.currentTimeMillis();
sc.close();
printer.add(t);
}
} catch (IOException x) {
sc.close();
t.failure = x;
printer.add(t);
}
}
}
volatile boolean shutdown = false;
// Invoked by the main thread when it's time to shut down
void shutdown() {
shutdown = true;
sel.wakeup();
}
// connector线程执行的过程描述
// 启动-->调用select()-->导致线程阻塞
// -->add()方法调用selecor.wakeup激活阻塞-->n=0-->执行注册感兴趣的事件-->select()-->n=1-->处理target连接-->select()-->线程阻塞
// -->main函数中的sleep过后，调用shutdown()方法，shutdown()中调用selector.wakeup()，唤醒线程，线程执行完成。
public void run() {
for (;;) {
try {
int n = sel.select();//这里需要注意：线程启动后，select方法的执行将会导致线程阻塞,直到有感兴趣的事件或者selector.wakeup()调用激活。
if (n > 0)processSelectedKeys();//由于通道没有注册感兴趣事件，即使是selector.wakeup()激活阻塞，这里的n也等于0
processPendingTargets();
if (shutdown) {
sel.close();
return;
}
} catch (IOException x) {
x.printStackTrace();
}
}
}
}
public static void main(String[] args)throws InterruptedException, IOException{
// if (args.length < 1) {
// System.err.println("Usage: java Ping [port] host");
// return;
// }
//
int firstArg = 1;
//
// // If the first argument is a string of digits then we take that
// // to be the port number to use
// if (Pattern.matches("[0-9]+", args[0])) {
// port = Integer.parseInt(args[0]);
// firstArg = 1;
// }
port = new Integer(80);
// Create the threads and start them up
Printer printer = new Printer();
printer.start();
Connector connector = new Connector(printer);
connector.start();
// Create the targets and add them to the connector
LinkedList targets = new LinkedList();
for (int i = firstArg; i < 2; i++) {
Target t = new Target("localhost");
targets.add(t);
connector.add(t);
}
//等待指定的时间关闭connector线程
Thread.sleep(50000);
connector.shutdown();//设置connector线程关闭标识位，同时唤醒阻塞的selector
//阻塞当前调用线程，等待connector线程执行完毕（join的经典用法）
connector.join();
// Print status of targets that have not yet been shown
for (Iterator i = targets.iterator(); i.hasNext();) {
Target t = (Target)i.next();
if (!t.shown)
t.show();
}
}
打印结果:
/10.10.10.153:80 : 50024ms
posted on 2012-02-07 16:53
zhangxl 阅读(912)
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java concurrency