【记录】Java NIO实现网络模块遇到的BUG
1.背景
通过JavaNio实现一个简单的网络模块，有点像Netty的线程模型，一个线程（AcceptThread）建立新连接，把新连接绑定到某个SelectorThread，SelectorThread处理读/写。
- AcceptThread：拥有一个Selector，上面只注册了一个ServerSocketChannel，监听客户端新连接，当接收到新连接时，把新连接注册到SelectorThread上。
- SelectorThread：拥有一个Selector，用于注册客户端连接，并且处理读/写事件。
2.代码
public class NonblockingServer {
/** 处理客户端读写 */
private SelectorThread[] selectorThreads;
/** 建立新连接 */
private AcceptThread acceptThread;
/** 选择器 */
private SelectNextSelectorThread nextSelectorThread;
/** 默认selectorThread数量 */
private static int defaultSelectorNum = 3;
public NonblockingServer() {
this(defaultSelectorNum);
}
public NonblockingServer(int selectorThreadNum) {
if (selectorThreadNum < 1) {
throw new IllegalArgumentException("SelectorThread线程数量不能低于1");
}
selectorThreads = new SelectorThread[selectorThreadNum];
try {
for (int i = 0; i < selectorThreads.length; i++) {
selectorThreads[i] = new SelectorThread("selector-thread-" + i);
}
acceptThread = new AcceptThread("accept-thread");
nextSelectorThread = new SelectNextSelectorThread(Arrays.asList(selectorThreads));
} catch (Exception e) {
throw new RuntimeException(e);
}
}
public void listen(String host, int port) {
try {
acceptThread.listen(host, port);
} catch (IOException e) {
throw new RuntimeException(e);
}
}
/**
* 处理连接事件，把新连接注册到一个SelectorThread上
*/
class AcceptThread extends Thread {
private Selector selector;
private ServerSocketChannel serverChannel;
AcceptThread(String name) throws IOException {
super(name);
selector = Selector.open();
}
public void listen(String host, int port) throws IOException {
serverChannel = ServerSocketChannel.open();
serverChannel.configureBlocking(false);
serverChannel.register(selector, SelectionKey.OP_ACCEPT);
serverChannel.bind(new InetSocketAddress(host, port));
this.start();
}
@Override
public void run() {
while (!Thread.currentThread().isInterrupted()) {
try {
int select = selector.select();
if (select == 0) {
continue;
}
Set<SelectionKey> keys = selector.selectedKeys();
Iterator<SelectionKey> iterator = keys.iterator();
while (iterator.hasNext()) {
SelectionKey key = iterator.next();
iterator.remove();
if (key.isAcceptable()) {
handAccept();
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
private void handAccept() throws IOException {
SocketChannel clientChannel = serverChannel.accept();
SelectorThread selectorThread = nextSelectorThread.nextThread();
// 懒加载
if (!selectorThread.isStart()) {
selectorThread.start();
}
selectorThread.register(clientChannel);
}
}
/**
* 处理客户端连接的读/写事件
*/
class SelectorThread extends Thread {
private Selector selector;
private volatile boolean start = false;
SelectorThread(String name) throws IOException {
super(name);
selector = Selector.open();
}
@Override
public void run() {
while (start) {
try {
selector.select();
Set<SelectionKey> keys = selector.selectedKeys();
Iterator<SelectionKey> iterator = keys.iterator();
while (start && iterator.hasNext()) {
SelectionKey key = iterator.next();
iterator.remove();
SocketChannel channel = (SocketChannel) key.channel();
if (key.isReadable()) {
handRead(channel);
} else if (key.isWritable()) {
handWrite(channel);
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
/**
* 这里简单的把输出打印出来
*/
private void handRead(SocketChannel channel) throws IOException {
ByteBuffer buffer = ByteBuffer.allocate(1024);
int read = channel.read(buffer);
if (read == -1) {
System.out.println(channel.getRemoteAddress() + " 断开连接");
channel.close();
} else {
byte[] bytes = new byte[read];
buffer.flip();
buffer.get(bytes);
System.out.println(new String(bytes));
buffer.clear();
}
}
private void handWrite(SocketChannel channel) {
// nothing
}
/**
* 把新连接注册到本线程
*/
public void register(SocketChannel clientChannel) throws IOException {
clientChannel.configureBlocking(false);
clientChannel.register(selector, SelectionKey.OP_READ | SelectionKey.OP_WRITE);
}
@Override
public synchronized void start() {
start = true;
super.start();
}
public void shutdown() {
this.start = false;
this.interrupt();
}
public boolean isStart() {
return start;
}
}
static class SelectNextSelectorThread {
private final Collection<? extends SelectorThread> threads;
private Iterator<? extends SelectorThread> iterator;
public <T extends SelectorThread> SelectNextSelectorThread(Collection<T> threads) {
this.threads = threads;
iterator = this.threads.iterator();
}
/**
* 选择下一个SelectorThread，这里为轮询
*
* @return SelectorThread
*/
public SelectorThread nextThread() {
if (!iterator.hasNext()) {
iterator = threads.iterator();
}
return iterator.next();
}
}
}
NonblockingServer
3.问题
当用客户端连接几次后，发现AcceptThread被卡死了！新连接无法建立。
用jvisualVM可以看到AcceptThread状态为监视，说明此时的AcceptThread在等待某个锁。
通过IDEA的dump threads也可以看到：
观察SelectThread线程：
SelectorImpl.lockAndDoSelect代码：
private int lockAndDoSelect(long var1) throws IOException {
synchronized(this) {
if (!this.isOpen()) {
throw new ClosedSelectorException();
} else {
int var10000;
synchronized(this.publicKeys) {
synchronized(this.publicSelectedKeys) {
var10000 = this.doSelect(var1);
}
}
return var10000;
}
}
}
SelectorImpl.register代码：
protected final SelectionKey register(AbstractSelectableChannel var1, int var2, Object var3) {
if (!(var1 instanceof SelChImpl)) {
throw new IllegalSelectorException();
} else {
SelectionKeyImpl var4 = new SelectionKeyImpl((SelChImpl)var1, this);
var4.attach(var3);
synchronized(this.publicKeys) {
this.implRegister(var4);
}
var4.interestOps(var2);
return var4;
}
}
这两个方法都需要获取this.publicKeys的锁，经过调试发现，SeletorThread在获取Selector的产生的事件时selector.select()，一直在不断的循环执行SelectorImpl.lockAndDoSelect方法，一直在不停的获取this.publicKeys的锁，所以这里的register方法很难有机会拿到this.publicKeys的锁，AcceptThread就卡死在获取锁的过程中了。
4.解决方法
Netty在注册新连接时，是把这个注册过程封装成一个任务，交给SelectorThread执行的，就不会发生线程冲突。
Thrift是把新连接放到SelecttorThread中的BlockingQueue中，也是由SelectorThread执行的。
5.修正后的代码
public class NonblockingServer {
/** 处理客户端读写 */
private SelectorThread[] selectorThreads;
/** 建立新连接 */
private AcceptThread acceptThread;
/** 选择器 */
private SelectNextSelectorThread nextSelectorThread;
/** 默认selectorThread数量 */
private static int defaultSelectorNum = 3;
public NonblockingServer() {
this(defaultSelectorNum);
}
public NonblockingServer(int selectorThreadNum) {
if (selectorThreadNum < 1) {
throw new IllegalArgumentException("SelectorThread线程数量不能低于1");
}
selectorThreads = new SelectorThread[selectorThreadNum];
try {
for (int i = 0; i < selectorThreads.length; i++) {
selectorThreads[i] = new SelectorThread("selector-thread-" + i);
}
acceptThread = new AcceptThread("accept-thread");
nextSelectorThread = new SelectNextSelectorThread(Arrays.asList(selectorThreads));
} catch (Exception e) {
throw new RuntimeException(e);
}
}
public void listen(String host, int port) {
try {
acceptThread.listen(host, port);
} catch (IOException e) {
throw new RuntimeException(e);
}
}
/**
* 处理连接事件，把新连接注册到一个SelectorThread上
*/
class AcceptThread extends Thread {
private Selector selector;
private ServerSocketChannel serverChannel;
AcceptThread(String name) throws IOException {
super(name);
selector = Selector.open();
}
public void listen(String host, int port) throws IOException {
serverChannel = ServerSocketChannel.open();
serverChannel.configureBlocking(false);
serverChannel.register(selector, SelectionKey.OP_ACCEPT);
serverChannel.bind(new InetSocketAddress(host, port));
this.start();
}
@Override
public void run() {
while (!Thread.currentThread().isInterrupted()) {
try {
int select = selector.select();
if (select == 0) {
continue;
}
Set<SelectionKey> keys = selector.selectedKeys();
Iterator<SelectionKey> iterator = keys.iterator();
while (iterator.hasNext()) {
SelectionKey key = iterator.next();
iterator.remove();
if (key.isAcceptable()) {
handAccept();
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
private void handAccept() throws IOException {
SocketChannel clientChannel = serverChannel.accept();
SelectorThread selectorThread = nextSelectorThread.nextThread();
// 懒加载
if (!selectorThread.isStart()) {
selectorThread.start();
}
selectorThread.register(clientChannel);
}
}
/**
* 处理客户端连接的读/写事件
*/
class SelectorThread extends Thread {
private Selector selector;
private BlockingQueue<Runnable> tasks;
private volatile boolean start = false;
SelectorThread(String name) throws IOException {
super(name);
selector = Selector.open();
tasks = new LinkedBlockingQueue<>();
}
@Override
public void run() {
while (start) {
try {
selector.select();
handTask();
Set<SelectionKey> keys = selector.selectedKeys();
Iterator<SelectionKey> iterator = keys.iterator();
while (start && iterator.hasNext()) {
SelectionKey key = iterator.next();
iterator.remove();
SocketChannel channel = (SocketChannel) key.channel();
if (key.isReadable()) {
handRead(channel);
} else if (key.isWritable()) {
handWrite(channel);
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
private void handTask() {
Runnable task;
while (start && (task = tasks.poll()) != null) {
task.run();
}
}
/**
* 这里简单的把输出打印出来
*/
private void handRead(SocketChannel channel) throws IOException {
ByteBuffer buffer = ByteBuffer.allocate(1024);
int read = channel.read(buffer);
if (read == -1) {
System.out.println(channel.getRemoteAddress() + " 断开连接");
channel.close();
} else {
byte[] bytes = new byte[read];
buffer.flip();
buffer.get(bytes);
System.out.println(new String(bytes));
buffer.clear();
}
}
private void handWrite(SocketChannel channel) {
// nothing
}
/**
* 把新连接注册到本线程
*/
public void register(SocketChannel clientChannel) {
submit(() -> {
try {
clientChannel.configureBlocking(false);
clientChannel.register(selector, SelectionKey.OP_READ | SelectionKey.OP_WRITE);
} catch (Exception e) {
try {
clientChannel.close();
} catch (IOException ex) {
ex.printStackTrace();
}
}
});
selector.wakeup();
}
public void submit(Runnable task) {
tasks.offer(task);
}
@Override
public synchronized void start() {
start = true;
super.start();
}
public void shutdown() {
this.start = false;
this.interrupt();
}
public boolean isStart() {
return start;
}
}
static class SelectNextSelectorThread {
private final Collection<? extends SelectorThread> threads;
private Iterator<? extends SelectorThread> iterator;
public <T extends SelectorThread> SelectNextSelectorThread(Collection<T> threads) {
this.threads = threads;
iterator = this.threads.iterator();
}
/**
* 选择下一个SelectorThread，这里为轮询
*
* @return SelectorThread
*/
public SelectorThread nextThread() {
if (!iterator.hasNext()) {
iterator = threads.iterator();
}
return iterator.next();
}
}
}
NonblockingServer
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