前面 ，我們分析了Netty Pipeline的初始化及節點新增與刪除邏輯。接下來，我們將來分析Pipeline的事件傳播機制。
Netty版本：4.1.30
inBound事件傳播
示例
我們通過下面這個例子來演示Netty Pipeline的事件傳播機制。
public class NettyPipelineInboundExample {
public static void main(String[] args) {
EventLoopGroup group = new NioEventLoopGroup(1);
ServerBootstrap strap = new ServerBootstrap();
strap.group(group)
.channel(NioServerSocketChannel.class)
.localAddress(new InetSocketAddress(8888))
.childOption(ChannelOption.TCP_NODELAY, true)
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new InboundHandlerA());
ch.pipeline().addLast(new InboundHandlerB());
ch.pipeline().addLast(new InboundHandlerC());
}
});
try {
ChannelFuture future = strap.bind().sync();
future.channel().closeFuture().sync();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
group.shutdownGracefully();
}
}
}
class InboundHandlerA extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
System.out.println("InboundHandler A : " + msg);
// 傳播read事件到下一個channelhandler
ctx.fireChannelRead(msg);
}
}
class InboundHandlerB extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
System.out.println("InboundHandler B : " + msg);
// 傳播read事件到下一個channelhandler
ctx.fireChannelRead(msg);
}
@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
// channel啟用，觸發channelRead事件，從pipeline的heandContext節點開始往下傳播
ctx.channel().pipeline().fireChannelRead("Hello world");
}
}
class InboundHandlerC extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
System.out.println("InboundHandler C : " + msg);
// 傳播read事件到下一個channelhandler
ctx.fireChannelRead(msg);
}
}
複製程式碼通過 telnet 來連線上面啟動好的netty服務，觸發channel active事件：
$ telnet 127.0.0.1 8888
複製程式碼按照InboundHandlerA、InboundHandlerB、InboundHandlerC的新增順序，控制檯輸出如下資訊：
InboundHandler A : Hello world
InboundHandler B : Hello world
InboundHandler C : Hello world
複製程式碼若是呼叫它們的新增順序，則會輸出對應順序的資訊，e.g:
...
ch.pipeline().addLast(new InboundHandlerB());
ch.pipeline().addLast(new InboundHandlerA());
ch.pipeline().addLast(new InboundHandlerC());
...
複製程式碼輸出如下資訊：
InboundHandler B : Hello world
InboundHandler A : Hello world
InboundHandler C : Hello world
複製程式碼原始碼分析
強烈建議 下面的流程，自己通過IDE的Debug模式來分析
待netty啟動成功，通過telnet連線到netty，然後通過telnet終端輸入任意字元（這一步才開啟Debug模式），進入Debug模式。
觸發channel read事件，從下面的入口開始呼叫
public class DefaultChannelPipeline implements ChannelPipeline {
...
// 出發channel read事件
@Override
public final ChannelPipeline fireChannelRead(Object msg) {
// 從head節點開始往下傳播read事件
AbstractChannelHandlerContext.invokeChannelRead(head, msg);
return this;
}
...
}
複製程式碼呼叫 AbstractChannelHandlerContext 中的 invokeChannelRead(head, msg) 介面：
abstract class AbstractChannelHandlerContext extends DefaultAttributeMap
implements ChannelHandlerContext, ResourceLeakHint {
...
// 呼叫channel read
static void invokeChannelRead(final AbstractChannelHandlerContext next, Object msg) {
// 獲取訊息
final Object m = next.pipeline.touch(ObjectUtil.checkNotNull(msg, "msg"), next);
// 獲取 EventExecutor
EventExecutor executor = next.executor();
// true
if (executor.inEventLoop()) {
// 呼叫下面的invokeChannelRead介面：invokeChannelRead(Object msg)
next.invokeChannelRead(m);
} else {
executor.execute(new Runnable() {
@Override
public void run() {
next.invokeChannelRead(m);
}
});
}
}
private void invokeChannelRead(Object msg) {
if (invokeHandler()) {
try {
// handler():獲取當前遍歷到的channelHandler，第一個為HeandContext，最後為TailContext
// 呼叫channel handler的channelRead介面
((ChannelInboundHandler) handler()).channelRead(this, msg);
} catch (Throwable t) {
notifyHandlerException(t);
}
} else {
fireChannelRead(msg);
}
}
...
@Override
public ChannelHandlerContext fireChannelRead(final Object msg) {
// 調回到上面的 invokeChannelRead(final AbstractChannelHandlerContext next, Object msg)
invokeChannelRead(findContextInbound(), msg);
return this;
}
...
// 遍歷出下一個ChannelHandler
private AbstractChannelHandlerContext findContextInbound() {
AbstractChannelHandlerContext ctx = this;
do {
//獲取下一個inbound型別的節點
ctx = ctx.next;
// 必須為inbound型別
} while (!ctx.inbound);
return ctx;
}
...
}
複製程式碼Pipeline中的第一個節點為HeadContext，它對於channelRead事件的處理，是直接往下傳播，程式碼如下：
final class HeadContext extends AbstractChannelHandlerContext
...
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
// HeadContext往下傳播channelRead事件，
// 呼叫HeandlerContext中的介面：fireChannelRead(final Object msg)
ctx.fireChannelRead(msg);
}
...
}
複製程式碼就這樣一直迴圈下去，依次會呼叫到 InboundHandlerA、InboundHandlerB、InboundHandlerC 中的 channelRead(ChannelHandlerContext ctx, Object msg) 介面。
到最後一個TailContext節點，它對channelRead事件的處理如下：
public class DefaultChannelPipeline implements ChannelPipeline {
final class TailContext extends AbstractChannelHandlerContext implements ChannelInboundHandler {
...
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
// 呼叫onUnhandledInboundMessage介面
onUnhandledInboundMessage(msg);
}
...
}
...
// 對未處理inbound訊息做最後的處理
protected void onUnhandledInboundMessage(Object msg) {
try {
logger.debug("Discarded inbound message {} that reached at the tail of the pipeline. Please check your pipeline configuration.", msg);
} finally {
// 對msg物件的引用數減1，當msg物件的引用數為0時，釋放該物件的記憶體
ReferenceCountUtil.release(msg);
}
}
...
}
複製程式碼以上就是pipeline對inBound訊息的處理流程。
SimpleChannelInboundHandler
在前面的例子中，假如中間有一個ChannelHandler未對channelRead事件進行傳播，就會導致訊息物件無法得到釋放，最終導致記憶體洩露。
我們還可以繼承 SimpleChannelInboundHandler 來自定義ChannelHandler，它的channelRead方法，對訊息物件做了msg處理，防止記憶體洩露。
public abstract class SimpleChannelInboundHandler<I> extends ChannelInboundHandlerAdapter {
...
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
boolean release = true;
try {
if (acceptInboundMessage(msg)) {
@SuppressWarnings("unchecked")
I imsg = (I) msg;
channelRead0(ctx, imsg);
} else {
release = false;
ctx.fireChannelRead(msg);
}
} finally {
if (autoRelease && release) {
// 對msg物件的引用數減1，當msg物件的引用數為0時，釋放該物件的記憶體
ReferenceCountUtil.release(msg);
}
}
}
...
}
複製程式碼outBound事件傳播
接下來，我們來分析Pipeline的outBound事件傳播機制。程式碼示例如下：
示例
public class NettyPipelineOutboundExample {
public static void main(String[] args) {
EventLoopGroup group = new NioEventLoopGroup(1);
ServerBootstrap strap = new ServerBootstrap();
strap.group(group)
.channel(NioServerSocketChannel.class)
.localAddress(new InetSocketAddress(8888))
.childOption(ChannelOption.TCP_NODELAY, true)
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new OutboundHandlerA());
ch.pipeline().addLast(new OutboundHandlerB());
ch.pipeline().addLast(new OutboundHandlerC());
}
});
try {
ChannelFuture future = strap.bind().sync();
future.channel().closeFuture().sync();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
group.shutdownGracefully();
}
}
}
class OutboundHandlerA extends ChannelOutboundHandlerAdapter {
@Override
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
// 輸出訊息
System.out.println("OutboundHandlerA: " + msg);
// 傳播write事件到下一個節點
ctx.write(msg, promise);
}
}
class OutboundHandlerB extends ChannelOutboundHandlerAdapter {
@Override
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
// 輸出訊息
System.out.println("OutboundHandlerB: " + msg);
// 傳播write事件到下一個節點
ctx.write(msg, promise);
}
@Override
public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
// 待handlerAdded事件觸發3s後，模擬觸發一個
ctx.executor().schedule(() -> {
// ctx.write("Hello world ! ");
ctx.channel().write("Hello world ! ");
}, 3, TimeUnit.SECONDS);
}
}
class OutboundHandlerC extends ChannelOutboundHandlerAdapter {
@Override
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
// 輸出訊息
System.out.println("OutboundHandlerC: " + msg);
// 傳播write事件到下一個節點
ctx.write(msg, promise);
}
}
複製程式碼通過 telnet 來連線上面啟動好的netty服務，觸發channel added事件：
$ telnet 127.0.0.1 8888
複製程式碼按照OutboundHandlerA、OutboundHandlerB、OutboundHandlerC的新增順序，控制檯輸出如下資訊：
OutboundHandlerC: Hello world !
OutboundHandlerB: Hello world !
OutboundHandlerA: Hello world !
複製程式碼輸出的順序正好與ChannelHandler的新增順序相反。
若是呼叫它們的新增順序，則會輸出對應順序的資訊，e.g:
...
ch.pipeline().addLast(new InboundHandlerB());
ch.pipeline().addLast(new InboundHandlerA());
ch.pipeline().addLast(new InboundHandlerC());
...
複製程式碼輸出如下資訊：
OutboundHandlerC: Hello world !
OutboundHandlerA: Hello world !
OutboundHandlerB: Hello world !
複製程式碼原始碼分析
強烈建議 下面的流程，自己通過IDE的Debug模式來分析
從channel的write方法開始，往下傳播write事件：
public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {
...
@Override
public ChannelFuture write(Object msg) {
// 呼叫pipeline往下傳播wirte事件
return pipeline.write(msg);
}
...
}
複製程式碼接著來看看Pipeline中的write介面：
public class DefaultChannelPipeline implements ChannelPipeline {
...
@Override
public final ChannelFuture write(Object msg) {
// 從tail節點開始傳播
return tail.write(msg);
}
...
}
複製程式碼呼叫ChannelHandlerContext中的write介面：
abstract class AbstractChannelHandlerContext extends DefaultAttributeMap
implements ChannelHandlerContext, ResourceLeakHint {
...
@Override
public ChannelFuture write(Object msg) {
// 往下呼叫write介面
return write(msg, newPromise());
}
@Override
public ChannelFuture write(final Object msg, final ChannelPromise promise) {
if (msg == null) {
throw new NullPointerException("msg");
}
try {
if (isNotValidPromise(promise, true)) {
ReferenceCountUtil.release(msg);
// cancelled
return promise;
}
} catch (RuntimeException e) {
ReferenceCountUtil.release(msg);
throw e;
}
// 往下呼叫write介面
write(msg, false, promise);
return promise;
}
...
private void write(Object msg, boolean flush, ChannelPromise promise) {
// 尋找下一個outbound型別的channelHandlerContext
AbstractChannelHandlerContext next = findContextOutbound();
final Object m = pipeline.touch(msg, next);
EventExecutor executor = next.executor();
if (executor.inEventLoop()) {
if (flush) {
next.invokeWriteAndFlush(m, promise);
} else {
// 呼叫介面 invokeWrite(Object msg, ChannelPromise promise)
next.invokeWrite(m, promise);
}
} else {
AbstractWriteTask task;
if (flush) {
task = WriteAndFlushTask.newInstance(next, m, promise);
} else {
task = WriteTask.newInstance(next, m, promise);
}
safeExecute(executor, task, promise, m);
}
}
// 尋找下一個outbound型別的channelHandlerContext
private AbstractChannelHandlerContext findContextOutbound() {
AbstractChannelHandlerContext ctx = this;
do {
ctx = ctx.prev;
} while (!ctx.outbound);
return ctx;
}
private void invokeWrite(Object msg, ChannelPromise promise) {
if (invokeHandler()) {
// 繼續往下呼叫
invokeWrite0(msg, promise);
} else {
write(msg, promise);
}
}
private void invokeWrite0(Object msg, ChannelPromise promise) {
try {
// 獲取當前的channelHandler，呼叫其write介面
// handler()依次會返回 OutboundHandlerC OutboundHandlerB OutboundHandlerA
((ChannelOutboundHandler) handler()).write(this, msg, promise);
} catch (Throwable t) {
notifyOutboundHandlerException(t, promise);
}
}
...
}
複製程式碼最終會呼叫到HeadContext的write介面：
@Override
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
// 呼叫unsafe進行寫資料操作
unsafe.write(msg, promise);
}
複製程式碼異常傳播
瞭解了Pipeline的入站與出站事件的機制之後，我們再來看看Pipeline的異常處理機制。
示例
public class NettyPipelineExceptionCaughtExample {
public static void main(String[] args) {
EventLoopGroup group = new NioEventLoopGroup(1);
ServerBootstrap strap = new ServerBootstrap();
strap.group(group)
.channel(NioServerSocketChannel.class)
.localAddress(new InetSocketAddress(8888))
.childOption(ChannelOption.TCP_NODELAY, true)
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new InboundHandlerA());
ch.pipeline().addLast(new InboundHandlerB());
ch.pipeline().addLast(new InboundHandlerC());
ch.pipeline().addLast(new OutboundHandlerA());
ch.pipeline().addLast(new OutboundHandlerB());
ch.pipeline().addLast(new OutboundHandlerC());
}
});
try {
ChannelFuture future = strap.bind().sync();
future.channel().closeFuture().sync();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
group.shutdownGracefully();
}
}
static class InboundHandlerA extends ChannelInboundHandlerAdapter {
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
System.out.println("InboundHandlerA.exceptionCaught:" + cause.getMessage());
ctx.fireExceptionCaught(cause);
}
}
static class InboundHandlerB extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
throw new Exception("ERROR !!!");
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
System.out.println("InboundHandlerB.exceptionCaught:" + cause.getMessage());
ctx.fireExceptionCaught(cause);
}
}
static class InboundHandlerC extends ChannelInboundHandlerAdapter {
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
System.out.println("InboundHandlerC.exceptionCaught:" + cause.getMessage());
ctx.fireExceptionCaught(cause);
}
}
static class OutboundHandlerA extends ChannelOutboundHandlerAdapter {
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
System.out.println("OutboundHandlerA.exceptionCaught:" + cause.getMessage());
ctx.fireExceptionCaught(cause);
}
}
static class OutboundHandlerB extends ChannelOutboundHandlerAdapter {
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
System.out.println("OutboundHandlerB.exceptionCaught:" + cause.getMessage());
ctx.fireExceptionCaught(cause);
}
}
static class OutboundHandlerC extends ChannelOutboundHandlerAdapter {
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
System.out.println("OutboundHandlerC.exceptionCaught:" + cause.getMessage());
ctx.fireExceptionCaught(cause);
}
}
}
複製程式碼通過 telnet 來連線上面啟動好的netty服務，並在控制檯傳送任意字元：
$ telnet 127.0.0.1 8888
複製程式碼觸發channel read事件並丟擲異常，控制檯輸出如下資訊：
InboundHandlerB.exceptionCaught:ERROR !!!
InboundHandlerC.exceptionCaught:ERROR !!!
OutboundHandlerA.exceptionCaught:ERROR !!!
OutboundHandlerB.exceptionCaught:ERROR !!!
OutboundHandlerC.exceptionCaught:ERROR !!!
複製程式碼可以看到異常的捕獲與我們新增的ChannelHandler順序相同。
原始碼分析
在我們的示例中，InboundHandlerB的ChannelRead介面丟擲異常，導致從InboundHandlerA將ChannelRead事件傳播到InboundHandlerB的過程中出現異常，異常被捕獲。
abstract class AbstractChannelHandlerContext extends DefaultAttributeMap
implements ChannelHandlerContext, ResourceLeakHint {
...
@Override
public ChannelHandlerContext fireExceptionCaught(final Throwable cause) {
//呼叫invokeExceptionCaught介面
invokeExceptionCaught(next, cause);
return this;
}
static void invokeExceptionCaught(final AbstractChannelHandlerContext next, final Throwable cause) {
ObjectUtil.checkNotNull(cause, "cause");
EventExecutor executor = next.executor();
if (executor.inEventLoop()) {
// 呼叫下一個節點的invokeExceptionCaught介面
next.invokeExceptionCaught(cause);
} else {
try {
executor.execute(new Runnable() {
@Override
public void run() {
next.invokeExceptionCaught(cause);
}
});
} catch (Throwable t) {
if (logger.isWarnEnabled()) {
logger.warn("Failed to submit an exceptionCaught() event.", t);
logger.warn("The exceptionCaught() event that was failed to submit was:", cause);
}
}
}
}
...
private void invokeChannelRead(Object msg) {
if (invokeHandler()) {
try {
// 丟擲異常
((ChannelInboundHandler) handler()).channelRead(this, msg);
} catch (Throwable t) {
// 異常捕獲，往下傳播
notifyHandlerException(t);
}
} else {
fireChannelRead(msg);
}
}
// 通知Handler發生異常事件
private void notifyHandlerException(Throwable cause) {
if (inExceptionCaught(cause)) {
if (logger.isWarnEnabled()) {
logger.warn(
"An exception was thrown by a user handler " +
"while handling an exceptionCaught event", cause);
}
return;
}
// 往下呼叫invokeExceptionCaught介面
invokeExceptionCaught(cause);
}
private void invokeExceptionCaught(final Throwable cause) {
if (invokeHandler()) {
try {
// 呼叫當前ChannelHandler的exceptionCaught介面
// 在我們的案例中，依次會呼叫InboundHandlerB、InboundHandlerC、
// OutboundHandlerA、OutboundHandlerB、OutboundHandlC
handler().exceptionCaught(this, cause);
} catch (Throwable error) {
if (logger.isDebugEnabled()) {
logger.debug(
"An exception {}" +
"was thrown by a user handler's exceptionCaught() " +
"method while handling the following exception:",
ThrowableUtil.stackTraceToString(error), cause);
} else if (logger.isWarnEnabled()) {
logger.warn(
"An exception '{}' [enable DEBUG level for full stacktrace] " +
"was thrown by a user handler's exceptionCaught() " +
"method while handling the following exception:", error, cause);
}
}
} else {
fireExceptionCaught(cause);
}
}
...
}
複製程式碼最終會呼叫到TailContext節點的exceptionCaught介面，如果我們中途沒有對異常進行攔截處理，做會列印出一段警告資訊！
public class DefaultChannelPipeline implements ChannelPipeline {
...
final class TailContext extends AbstractChannelHandlerContext implements ChannelInboundHandler {
...
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
onUnhandledInboundException(cause);
}
...
protected void onUnhandledInboundException(Throwable cause) {
try {
logger.warn(
"An exceptionCaught() event was fired, and it reached at the tail of the pipeline. " +
"It usually means the last handler in the pipeline did not handle the exception.",
cause);
} finally {
ReferenceCountUtil.release(cause);
}
}
}
...
}
複製程式碼在實際的應用中，一般會定一個ChannelHandler，放置Pipeline末尾，專門用來處理中途出現的各種異常。
最佳異常處理實踐
單獨定義ExceptionCaughtHandler來處理異常：
...
class ExceptionCaughtHandler extends ChannelInboundHandlerAdapter {
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
if (cause instanceof Exception) {
// TODO
System.out.println("Successfully caught exception ! ");
} else {
// TODO
}
}
}
...
ch.pipeline().addLast(new ExceptionCaughtHandler());
...
複製程式碼輸出：
InboundHandlerB.exceptionCaught:ERROR !!!
InboundHandlerC.exceptionCaught:ERROR !!!
OutboundHandlerA.exceptionCaught:ERROR !!!
OutboundHandlerB.exceptionCaught:ERROR !!!
OutboundHandlerC.exceptionCaught:ERROR !!!
Successfully caught exception ! // 成功捕獲日誌
複製程式碼Pipeline回顧與總結
至此，我們對Pipeline的原理的解析就完成了。
- Pipeline是在什麼時候建立的？
- Pipeline新增與刪除節點的邏輯是怎麼樣的？
- netty是如何判斷ChannelHandler型別的？
- 如何處理ChannelHandler中丟擲的異常？
- 對於ChannelHandler的新增應遵循什麼樣的順序？