JavaDubbo协议下的服务端线程使用详解
Provider端线程模型
在了解服务端线程模型之前，先了解一下dubbo对Channel上的操作抽象，Dubbo将Channel上的操作成了5中行为，分别是：建立连接、断开连接、发送消息、接收消息、异常捕获，Channel上的操作的接口为org.apache.dubbo.remoting.ChannelHandler，该接口是SPI的，用户可以自己扩展，接口代码如下：
该接口抽象的五种Channel上的行为解释如下：
- 建立连接：connected，主要是的职责是在channel记录read、write的时间，以及处理建立连接后的回调逻辑，比如dubbo支持在断开后自定义回调的hook（onconnect），即在该操作中执行。
- 断开连接：disconnected，主要是的职责是在channel移除read、write的时间，以及处理端开连接后的回调逻辑，比如dubbo支持在断开后自定义回调的hook（ondisconnect），即在该操作中执行。
- 发送消息：sent，包括发送请求和发送响应。记录write的时间。
- 接收消息：received，包括接收请求和接收响应。记录read的时间。
- 异常捕获：caught，用于处理在channel上发生的各类异常。
Dubbo框架的线程模型与以上这五种行为息息相关，Dubbo协议Provider端线程模型提供了五种实现，虽说都是五种但是别把二者混淆，线程模型的顶级接口是org.apache.dubbo.remoting.Dispatcher，该接口也是SPI的，提供的五种实现分别是AllDispatcher、DirectDispatcher、MessageOnlyDispatcher、ExecutionDispatcher、ConnectionOrderedDispatcher，默认的使用的是AllDispatcher。
org.apache.dubbo.remoting.ChannelHandler作为Channel上的行为的顶级接口对应Dubbo协议Provider端的5种线程模型同样也提供了对应的5种实现，分别是AllChannelHandler、DirectChannelHandler、MessageOnlyChannelHandler、ExecutionChannelHandler、ConnectionOrderedChannelHandler，这里Channel上行为的具体实现不展开讨论。
Channel上行为和线程模型之间使用策略可以参考org.apache.dubbo.remoting.transport.dispatcher.ChannelHandlers的源代码，这里不做详细的介绍，下面的各个章节只针对5种线程模型做简单的介绍。
AllDispatcher
IO线程上的操作：
- 接口响应序列化
- sent操作
Dubbo线程池上的操作：
- received、connected、disconnected、caught都是在Dubbo线程池上执行
- 服务端反序列化操作的Dubbo线程池上执行
AllDispatcher代码如下，AllDispatcher的dispatch方法实例化了AllChannelHandler，AllChannelHandler实现了received、connected、disconnected、caught操作在dubbo线程池中，代码如下：
public class AllDispatcher implements Dispatcher {
public static final String NAME = "all";
@Override
public ChannelHandler dispatch(ChannelHandler handler, URL url) {
return new AllChannelHandler(handler, url);
}
}
public class AllChannelHandler extends WrappedChannelHandler {
public AllChannelHandler(ChannelHandler handler, URL url) {
super(handler, url);
}
@Override
public void connected(Channel channel) throws RemotingException {
ExecutorService executor = getSharedExecutorService();
try {
executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.CONNECTED));
} catch (Throwable t) {
throw new ExecutionException("connect event", channel, getClass() + " error when process connected event .", t);
}
}
@Override
public void disconnected(Channel channel) throws RemotingException {
ExecutorService executor = getSharedExecutorService();
try {
executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.DISCONNECTED));
} catch (Throwable t) {
throw new ExecutionException("disconnect event", channel, getClass() + " error when process disconnected event .", t);
}
}
@Override
public void received(Channel channel, Object message) throws RemotingException {
ExecutorService executor = getPreferredExecutorService(message);
try {
executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
} catch (Throwable t) {
if(message instanceof Request && t instanceof RejectedExecutionException){
sendFeedback(channel, (Request) message, t);
return;
}
throw new ExecutionException(message, channel, getClass() + " error when process received event .", t);
}
}
@Override
public void caught(Channel channel, Throwable exception) throws RemotingException {
ExecutorService executor = getSharedExecutorService();
try {
executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.CAUGHT, exception));
} catch (Throwable t) {
throw new ExecutionException("caught event", channel, getClass() + " error when process caught event .", t);
}
}
}DirectDispatcher
该线程模型Channel上的所有行为均在IO线程中执行，并没有在Dubbo线程池中执行
DirectDispatcher与AllDispatcher相似，实例化了DirectChannelHandler，DirectChannelHandler只实现了received行为，但是received中获取的线程池如果是ThreadlessExecutor才会提交task，否则也是在ChannelHandler中执行received行为，ThreadlessExecutor和普通线程池最大的区别是不会管理任何线程，这里不展开讨论。
public class DirectDispatcher implements Dispatcher {
public static final String NAME = "direct";
@Override
public ChannelHandler dispatch(ChannelHandler handler, URL url) {
return new DirectChannelHandler(handler, url);
}
}
public class DirectChannelHandler extends WrappedChannelHandler {
public DirectChannelHandler(ChannelHandler handler, URL url) {
super(handler, url);
}
@Override
public void received(Channel channel, Object message) throws RemotingException {
ExecutorService executor = getPreferredExecutorService(message);
if (executor instanceof ThreadlessExecutor) {
try {
executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
} catch (Throwable t) {
throw new ExecutionException(message, channel, getClass() + " error when process received event .", t);
}
} else {
handler.received(channel, message);
}
}
}
ExecutionDispatcher
在IO线程中执行的操作有：
- sent、connected、disconnected、caught操作在IO线程上执行。
- 序列化响应在IO线程上执行。
在Dubbo线程中执行的操作有：
- received都是在Dubbo线程上执行的。
- 反序列化请求的行为在Dubbo中做的。
同样的，我们可以直接看ExecutionChannelHandler源码，逻辑是当message的类型是Request时received行为在Dubbo线程池执行。感兴趣的可以自己看源码，这里不做介绍。
MessageOnlyDispatcher
Message Only Dispatcher所有的received行为和反序列化都是在dubbo线程池中执行的
public class MessageOnlyChannelHandler extends WrappedChannelHandler {
public MessageOnlyChannelHandler(ChannelHandler handler, URL url) {
super(handler, url);
}
@Override
public void received(Channel channel, Object message) throws RemotingException {
ExecutorService executor = getPreferredExecutorService(message);
try {
executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
} catch (Throwable t) {
if(message instanceof Request && t instanceof RejectedExecutionException){
sendFeedback(channel, (Request) message, t);
return;
}
throw new ExecutionException(message, channel, getClass() + " error when process received event .", t);
}
}
}
ConnectionOrderedDispatcher
该线程模型与AllDispatcher类似，sent操作和相应的序列化是在IO线程上执行；connected、disconnected、received、caught操作在dubbo线程池上执行，他们的区别是在connected、disconnected行为上ConnectionOrderedDispatcher做了线程池隔离，并且在Dubbo connected thread pool中提供了链接限制、告警灯能力，我们直接看ConnectionOrderedChannelHandler源码，代码如下：
public class ConnectionOrderedChannelHandler extends WrappedChannelHandler {
protected final ThreadPoolExecutor connectionExecutor;
private final int queueWarningLimit;
public ConnectionOrderedChannelHandler(ChannelHandler handler, URL url) {
super(handler, url);
String threadName = url.getParameter(THREAD_NAME_KEY, DEFAULT_THREAD_NAME);
connectionExecutor = new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>(url.getPositiveParameter(CONNECT_QUEUE_CAPACITY, Integer.MAX_VALUE)),
new NamedThreadFactory(threadName, true),
new AbortPolicyWithReport(threadName, url)
); // FIXME There's no place to release connectionExecutor!
queueWarningLimit = url.getParameter(CONNECT_QUEUE_WARNING_SIZE, DEFAULT_CONNECT_QUEUE_WARNING_SIZE);
}
@Override
public void connected(Channel channel) throws RemotingException {
try {
checkQueueLength();
connectionExecutor.execute(new ChannelEventRunnable(channel, handler, ChannelState.CONNECTED));
} catch (Throwable t) {
throw new ExecutionException("connect event", channel, getClass() + " error when process connected event .", t);
}
}
@Override
public void disconnected(Channel channel) throws RemotingException {
try {
checkQueueLength();
connectionExecutor.execute(new ChannelEventRunnable(channel, handler, ChannelState.DISCONNECTED));
} catch (Throwable t) {
throw new ExecutionException("disconnected event", channel, getClass() + " error when process disconnected event .", t);
}
}
@Override
public void received(Channel channel, Object message) throws RemotingException {
ExecutorService executor = getPreferredExecutorService(message);
try {
executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
} catch (Throwable t) {
if (message instanceof Request && t instanceof RejectedExecutionException) {
sendFeedback(channel, (Request) message, t);
return;
}
throw new ExecutionException(message, channel, getClass() + " error when process received event .", t);
}
}
@Override
public void caught(Channel channel, Throwable exception) throws RemotingException {
ExecutorService executor = getSharedExecutorService();
try {
executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.CAUGHT, exception));
} catch (Throwable t) {
throw new ExecutionException("caught event", channel, getClass() + " error when process caught event .", t);
}
}
private void checkQueueLength() {
if (connectionExecutor.getQueue().size() > queueWarningLimit) {
logger.warn(new IllegalThreadStateException("connectionordered channel handler queue size: " + connectionExecutor.getQueue().size() + " exceed the warning limit number :" + queueWarningLimit));
}
}
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