前面 ，我們分析了Netty中的Channel元件，本篇我們來介紹一下與Channel關聯的另一個核心的元件 —— EventLoop。
Netty版本：4.1.30
概述
EventLoop定義了Netty的核心抽象，用於處理網路連線生命週期中所有發生的事件。
我們先來從一個比較高的視角來了解一下Channels、Thread、EventLoops、EventLoopGroups之間的關係。
上圖是表示了擁有4個EventLoop的EventLoopGroup處理IO的流程圖。它們之間的關係如下：
- 一個 EventLoopGroup包含一個或多個EventLoop
- 一個 EventLoop在它的生命週期內只和一個Thread繫結
- 所有由EventLoop處理的I/O事件都將在它專有的Thread上被處理
- 一個Channel在它的生命週期內只註冊於一個EventLoop
- 一個EventLoop可能會被分配給一個或多個Channel
EventLoop 原理
下圖是Netty EventLoop相關類的UML圖。從中我們可以看到EventLoop相關的類都是實現了 java.util.concurrent 包中的 ExecutorService 介面。我們可以直接將任務(Runable 或 Callable) 提交給EventLoop去立即執行或定時執行。
例如，使用EventLoop去執行定時任務，樣例程式碼：
public static void scheduleViaEventLoop() {
Channel ch = new NioSocketChannel();
ScheduledFuture<?> future = ch.eventLoop().schedule(
() -> System.out.println("60 seconds later"), 60, TimeUnit.SECONDS);
}
複製程式碼Thread 管理
Netty執行緒模型的高效能主要取決於當前所執行執行緒的身份的確定。一個執行緒提交到EventLoop執行的流程如下：
- 將Task任務提交給EventLoop執行
- 在Task傳遞到execute方法之後，檢查當前要執行的Task的執行緒是否是分配給EventLoop的那個執行緒
- 如果是，則該執行緒會立即執行
- 如果不是，則將執行緒放入任務佇列中，等待下一次執行
其中，Netty中的每一個EventLoop都有它自己的任務佇列，並且和其他的EventLoop的任務佇列獨立開來。
Thread 分配
服務於Channel的I/O和事件的EventLoop包含在EventLoopGroup中。根據不同的傳輸實現，EventLoop的建立和分配方式也不同。
NIO傳輸
在NIO傳輸方式中，使用盡可能少的EventLoop就可以服務多個Channel。如圖所示，EventLoopGroup採用順序迴圈的方式負責為每一個新建立的Channel分配EventLoop，每一個EventLoop會被分配給多個Channels。
一旦一個Channel被分配給了一個EventLoop，則這個Channel的生命週期內，只會繫結這個EventLoop。這就讓我們在ChannelHandler的實現省去了對執行緒安全和同步問題的擔心。
OIO傳輸
與NIO方式的不同在於，一個EventLoop只會服務於一個Channel。
NioEventLoop & NioEventLoopGroup 建立
初步瞭解了 EventLoop 以及 EventLoopGroup 的工作機制，接下來我們以 NioEventLoopGroup 為例，來深入分析 NioEventLoopGroup 是如何建立的，又是如何啟動的，它的內部執行邏輯又是怎樣的等等問題。
MultithreadEventExecutorGroup 構造器
我們從 NioEventLoopGroup 的建構函式開始分析：
EventLoopGroup acceptorEventLoopGroup = new NioEventLoopGroup(1);
複製程式碼NioEventLoopGroup建構函式會呼叫到父類 MultithreadEventLoopGroup 的建構函式，預設情況下，EventLoop的數量 = 處理器數量 x 2：
public abstract class MultithreadEventLoopGroup extends MultithreadEventExecutorGroup implements EventLoopGroup {
private static final InternalLogger logger = InternalLoggerFactory.getInstance(MultithreadEventLoopGroup.class);
private static final int DEFAULT_EVENT_LOOP_THREADS;
// 預設情況下，EventLoop的數量 = 處理器數量 x 2
static {
DEFAULT_EVENT_LOOP_THREADS = Math.max(1, SystemPropertyUtil.getInt(
"io.netty.eventLoopThreads", NettyRuntime.availableProcessors() * 2));
if (logger.isDebugEnabled()) {
logger.debug("-Dio.netty.eventLoopThreads: {}", DEFAULT_EVENT_LOOP_THREADS);
}
}
protected MultithreadEventLoopGroup(int nThreads, Executor executor, Object... args) {
super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, executor, args);
}
...
}
複製程式碼繼續呼叫父類，會呼叫到 MultithreadEventExecutorGroup 的構造器，主要做三件事情：
- 建立執行緒任務執行器 ThreadPerTaskExecutor
- 通過for迴圈建立數量為 nThreads 個的 EventLoop
- 建立 EventLoop 選擇器 EventExecutorChooser
protected MultithreadEventExecutorGroup(int nThreads, Executor executor,
EventExecutorChooserFactory chooserFactory, Object... args) {
if (nThreads <= 0) {
throw new IllegalArgumentException(String.format("nThreads: %d (expected: > 0)", nThreads));
}
// 建立任務執行器 ThreadPerTaskExecutor
if (executor == null) {
executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
}
// 建立 EventExecutor 陣列
children = new EventExecutor[nThreads];
// 通過for迴圈建立數量為 nThreads 個的 EventLoop
for (int i = 0; i < nThreads; i ++) {
boolean success = false;
try {
// 呼叫 newChild 介面
children[i] = newChild(executor, args);
success = true;
} catch (Exception e) {
// TODO: Think about if this is a good exception type
throw new IllegalStateException("failed to create a child event loop", e);
} finally {
if (!success) {
for (int j = 0; j < i; j ++) {
children[j].shutdownGracefully();
}
for (int j = 0; j < i; j ++) {
EventExecutor e = children[j];
try {
while (!e.isTerminated()) {
e.awaitTermination(Integer.MAX_VALUE, TimeUnit.SECONDS);
}
} catch (InterruptedException interrupted) {
// Let the caller handle the interruption.
Thread.currentThread().interrupt();
break;
}
}
}
}
}
// 建立選擇器
chooser = chooserFactory.newChooser(children);
final FutureListener<Object> terminationListener = new FutureListener<Object>() {
@Override
public void operationComplete(Future<Object> future) throws Exception {
if (terminatedChildren.incrementAndGet() == children.length) {
terminationFuture.setSuccess(null);
}
}
};
for (EventExecutor e: children) {
e.terminationFuture().addListener(terminationListener);
}
Set<EventExecutor> childrenSet = new LinkedHashSet<EventExecutor>(children.length);
Collections.addAll(childrenSet, children);
readonlyChildren = Collections.unmodifiableSet(childrenSet);
}
複製程式碼建立執行緒任務執行器 ThreadPerTaskExecutor
if (executor == null) {
executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
}
複製程式碼執行緒任務執行器 ThreadPerTaskExecutor 原始碼如下，具體的任務都由 ThreadFactory 去執行：
public final class ThreadPerTaskExecutor implements Executor {
private final ThreadFactory threadFactory;
public ThreadPerTaskExecutor(ThreadFactory threadFactory) {
if (threadFactory == null) {
throw new NullPointerException("threadFactory");
}
this.threadFactory = threadFactory;
}
// 使用 threadFactory 執行任務
@Override
public void execute(Runnable command) {
threadFactory.newThread(command).start();
}
}
複製程式碼來看看 newDefaultThreadFactory 方法：
protected ThreadFactory newDefaultThreadFactory() {
return new DefaultThreadFactory(getClass());
}
複製程式碼DefaultThreadFactory
接下來看看 DefaultThreadFactory 這個類，實現了 ThreadFactory 介面，我們可以瞭解到：
- EventLoopGroup的命名規則
- 具體的執行緒為 FastThreadLocalThread
public class DefaultThreadFactory implements ThreadFactory {
// 執行緒池ID編號自增器
private static final AtomicInteger poolId = new AtomicInteger();
// 執行緒ID自增器
private final AtomicInteger nextId = new AtomicInteger();
// 執行緒名稱字首
private final String prefix;
// 是否為守護程式
private final boolean daemon;
// 執行緒優先順序
private final int priority;
// 執行緒組
protected final ThreadGroup threadGroup;
public DefaultThreadFactory(Class<?> poolType) {
this(poolType, false, Thread.NORM_PRIORITY);
}
...
// 獲取執行緒名，返回結果：nioEventLoopGroup
public static String toPoolName(Class<?> poolType) {
if (poolType == null) {
throw new NullPointerException("poolType");
}
String poolName = StringUtil.simpleClassName(poolType);
switch (poolName.length()) {
case 0:
return "unknown";
case 1:
return poolName.toLowerCase(Locale.US);
default:
if (Character.isUpperCase(poolName.charAt(0)) && Character.isLowerCase(poolName.charAt(1))) {
return Character.toLowerCase(poolName.charAt(0)) + poolName.substring(1);
} else {
return poolName;
}
}
}
public DefaultThreadFactory(String poolName, boolean daemon, int priority, ThreadGroup threadGroup) {
if (poolName == null) {
throw new NullPointerException("poolName");
}
if (priority < Thread.MIN_PRIORITY || priority > Thread.MAX_PRIORITY) {
throw new IllegalArgumentException(
"priority: " + priority + " (expected: Thread.MIN_PRIORITY <= priority <= Thread.MAX_PRIORITY)");
}
// nioEventLoopGroup-2-
prefix = poolName + `-` + poolId.incrementAndGet() + `-`;
this.daemon = daemon;
this.priority = priority;
this.threadGroup = threadGroup;
}
public DefaultThreadFactory(String poolName, boolean daemon, int priority) {
this(poolName, daemon, priority, System.getSecurityManager() == null ?
Thread.currentThread().getThreadGroup() : System.getSecurityManager().getThreadGroup());
}
@Override
public Thread newThread(Runnable r) {
// 建立新執行緒 nioEventLoopGroup-2-1
Thread t = newThread(FastThreadLocalRunnable.wrap(r), prefix + nextId.incrementAndGet());
try {
if (t.isDaemon() != daemon) {
t.setDaemon(daemon);
}
if (t.getPriority() != priority) {
t.setPriority(priority);
}
} catch (Exception ignored) {
// Doesn`t matter even if failed to set.
}
return t;
}
// 建立新執行緒 FastThreadLocalThread
protected Thread newThread(Runnable r, String name) {
return new FastThreadLocalThread(threadGroup, r, name);
}
}
複製程式碼建立NioEventLoop
繼續從 MultithreadEventExecutorGroup 構造器開始，建立完任務執行器 ThreadPerTaskExecutor 之後，進入for迴圈，開始建立 NioEventLoop：
for (int i = 0; i < nThreads; i ++) {
boolean success = false;
try {
// 建立 nioEventLoop
children[i] = newChild(executor, args);
success = true;
} catch (Exception e) {
// TODO: Think about if this is a good exception type
throw new IllegalStateException("failed to create a child event loop", e);
}
...
}
複製程式碼NioEventLoopGroup類中的 newChild() 方法：
@Override
protected EventLoop newChild(Executor executor, Object... args) throws Exception {
return new NioEventLoop(this, executor, (SelectorProvider) args[0],
((SelectStrategyFactory) args[1]).newSelectStrategy(), (RejectedExecutionHandler) args[2]);
}
複製程式碼NioEventLoop 構造器：
public final class NioEventLoop extends SingleThreadEventLoop{
...
NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider, SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler) {
// 呼叫父類 SingleThreadEventLoop 構造器
super(parent, executor, false, DEFAULT_MAX_PENDING_TASKS, rejectedExecutionHandler);
if (selectorProvider == null) {
throw new NullPointerException("selectorProvider");
}
if (strategy == null) {
throw new NullPointerException("selectStrategy");
}
// 設定 selectorProvider
provider = selectorProvider;
// 獲取 SelectorTuple 物件，裡面封裝了原生的selector和優化過的selector
final SelectorTuple selectorTuple = openSelector();
// 設定優化過的selector
selector = selectorTuple.selector;
// 設定原生的selector
unwrappedSelector = selectorTuple.unwrappedSelector;
// 設定select策略
selectStrategy = strategy;
}
...
}
複製程式碼接下來我們看看 獲取多路複用選擇器 方法—— openSelector() ，
// selectKey 優化選項flag
private static final boolean DISABLE_KEYSET_OPTIMIZATION =
SystemPropertyUtil.getBoolean("io.netty.noKeySetOptimization", false);
private SelectorTuple openSelector() {
// JDK原生的selector
final Selector unwrappedSelector;
try {
// 通過 SelectorProvider 建立獲得selector
unwrappedSelector = provider.openSelector();
} catch (IOException e) {
throw new ChannelException("failed to open a new selector", e);
}
// 如果不優化，則直接返回
if (DISABLE_KEYSET_OPTIMIZATION) {
return new SelectorTuple(unwrappedSelector);
}
// 通過反射建立 sun.nio.ch.SelectorImpl 物件
Object maybeSelectorImplClass = AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
try {
return Class.forName(
"sun.nio.ch.SelectorImpl",
false,
PlatformDependent.getSystemClassLoader());
} catch (Throwable cause) {
return cause;
}
}
});
// 如果 maybeSelectorImplClass 不是 selector 的一個實現，則直接返回原生的Selector
if (!(maybeSelectorImplClass instanceof Class) ||
// ensure the current selector implementation is what we can instrument.
// 確保當前的選擇器實現是我們可以檢測的
!((Class<?>) maybeSelectorImplClass).isAssignableFrom(unwrappedSelector.getClass())) {
if (maybeSelectorImplClass instanceof Throwable) {
Throwable t = (Throwable) maybeSelectorImplClass;
logger.trace("failed to instrument a special java.util.Set into: {}", unwrappedSelector, t);
}
return new SelectorTuple(unwrappedSelector);
}
// maybeSelectorImplClass 是selector的實現，則轉化為 selector 實現類
final Class<?> selectorImplClass = (Class<?>) maybeSelectorImplClass;
// 建立新的 SelectionKey 集合 SelectedSelectionKeySet,內部採用的是 SelectionKey 陣列的形
// 式，而非 set 集合
final SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();
Object maybeException = AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
try {
// 通過反射的方式獲取 sun.nio.ch.SelectorImpl 的成員變數 selectedKeys
Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
// 通過反射的方式獲取 sun.nio.ch.SelectorImpl 的成員變數 publicSelectedKeys
Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");
if (PlatformDependent.javaVersion() >= 9 && PlatformDependent.hasUnsafe()) {
// Let us try to use sun.misc.Unsafe to replace the SelectionKeySet.
// This allows us to also do this in Java9+ without any extra flags.
long selectedKeysFieldOffset = PlatformDependent.objectFieldOffset(selectedKeysField);
long publicSelectedKeysFieldOffset =
PlatformDependent.objectFieldOffset(publicSelectedKeysField);
if (selectedKeysFieldOffset != -1 && publicSelectedKeysFieldOffset != -1) {
PlatformDependent.putObject( unwrappedSelector, selectedKeysFieldOffset, selectedKeySet);
PlatformDependent.putObject(unwrappedSelector, publicSelectedKeysFieldOffset, selectedKeySet);
return null;
}
// We could not retrieve the offset, lets try reflection as last-resort.
}
// 設定欄位 selectedKeys Accessible 為true
Throwable cause = ReflectionUtil.trySetAccessible(selectedKeysField, true);
if (cause != null) {
return cause;
}
// 設定欄位 publicSelectedKeys Accessible 為true
cause = ReflectionUtil.trySetAccessible(publicSelectedKeysField, true);
if (cause != null) {
return cause;
}
selectedKeysField.set(unwrappedSelector, selectedKeySet);
publicSelectedKeysField.set(unwrappedSelector, selectedKeySet);
return null;
} catch (NoSuchFieldException e) {
return e;
} catch (IllegalAccessException e) {
return e;
}
}
});
if (maybeException instanceof Exception) {
selectedKeys = null;
Exception e = (Exception) maybeException;
logger.trace("failed to instrument a special java.util.Set into: {}", unwrappedSelector, e);
return new SelectorTuple(unwrappedSelector);
}
// 設定 SelectedSelectionKeySet
selectedKeys = selectedKeySet;
logger.trace("instrumented a special java.util.Set into: {}", unwrappedSelector);
// 返回包含了原生selector和優化過的selector的SelectorTuple
return new SelectorTuple(unwrappedSelector,
new SelectedSelectionKeySetSelector(unwrappedSelector, selectedKeySet));
}
複製程式碼優化後的 SelectedSelectionKeySet 物件，內部採用 SelectionKey 陣列的形式：
final class SelectedSelectionKeySet extends AbstractSet<SelectionKey> {
SelectionKey[] keys;
int size;
SelectedSelectionKeySet() {
keys = new SelectionKey[1024];
}
// 使用陣列，來替代HashSet,可以降低時間複雜度為O(1)
@Override
public boolean add(SelectionKey o) {
if (o == null) {
return false;
}
keys[size++] = o;
if (size == keys.length) {
increaseCapacity();
}
return true;
}
@Override
public boolean remove(Object o) {
return false;
}
@Override
public boolean contains(Object o) {
return false;
}
@Override
public int size() {
return size;
}
@Override
public Iterator<SelectionKey> iterator() {
return new Iterator<SelectionKey>() {
private int idx;
@Override
public boolean hasNext() {
return idx < size;
}
@Override
public SelectionKey next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
return keys[idx++];
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
};
}
void reset() {
reset(0);
}
void reset(int start) {
Arrays.fill(keys, start, size, null);
size = 0;
}
// 擴容
private void increaseCapacity() {
SelectionKey[] newKeys = new SelectionKey[keys.length << 1];
System.arraycopy(keys, 0, newKeys, 0, size);
keys = newKeys;
}
}
複製程式碼SingleThreadEventLoop 構造器
public abstract class SingleThreadEventLoop extends SingleThreadEventExecutor implements EventLoop {
...
protected SingleThreadEventLoop(EventLoopGroup parent, Executor executor,
boolean addTaskWakesUp, int maxPendingTasks,
RejectedExecutionHandler rejectedExecutionHandler) {
// 呼叫 SingleThreadEventExecutor 構造器
super(parent, executor, addTaskWakesUp, maxPendingTasks, rejectedExecutionHandler);
tailTasks = newTaskQueue(maxPendingTasks);
}
...
}
複製程式碼SingleThreadEventExecutor 構造器，主要做兩件事情：
- 設定執行緒任務執行器。
- 設定任務佇列。前面講到EventLoop對於不能立即執行的Task會放入一個佇列中，就是這裡設定的。
public abstract class SingleThreadEventExecutor extends AbstractScheduledEventExecutor implements OrderedEventExecutor {
...
protected SingleThreadEventExecutor(EventExecutorGroup parent, Executor executor,
boolean addTaskWakesUp, int maxPendingTasks,
RejectedExecutionHandler rejectedHandler) {
super(parent);
this.addTaskWakesUp = addTaskWakesUp;
this.maxPendingTasks = Math.max(16, maxPendingTasks);
// 設定執行緒任務執行器
this.executor = ObjectUtil.checkNotNull(executor, "executor");
// 設定任務佇列
taskQueue = newTaskQueue(this.maxPendingTasks);
rejectedExecutionHandler = ObjectUtil.checkNotNull(rejectedHandler, "rejectedHandler");
}
...
}
複製程式碼NioEventLoop 中對 newTaskQueue 介面的實現，返回的是 JCTools 工具包 Mpsc 佇列。後面我們寫文章單獨介紹 JCTools 中的相關佇列。
Mpsc：Multi Producer Single Consumer (Lock less, bounded and unbounded)
多個生產者對單個消費者（無鎖、有界和無界都有實現）
public final class NioEventLoop extends SingleThreadEventLoop {
...
@Override
protected Queue<Runnable> newTaskQueue(int maxPendingTasks) {
// This event loop never calls takeTask()
return maxPendingTasks == Integer.MAX_VALUE ? PlatformDependent.<Runnable>newMpscQueue()
: PlatformDependent.<Runnable>newMpscQueue(maxPendingTasks);
}
...
}
複製程式碼建立執行緒執行選擇器chooser
接下來，我們看看 MultithreadEventExecutorGroup 構造器的最後一個部分內容，建立執行緒執行選擇器chooser，它的主要作用就是 EventLoopGroup 用於從 EventLoop 陣列中選擇一個 EventLoop 去執行任務。
// 建立選擇器
chooser = chooserFactory.newChooser(children);
複製程式碼EventLoopGroup 中定義的 next() 介面：
public interface EventLoopGroup extends EventExecutorGroup {
...
// 選擇下一個 EventLoop 用於執行任務
@Override
EventLoop next();
...
}
複製程式碼MultithreadEventExecutorGroup 中對 next() 的實現：
@Override
public EventExecutor next() {
// 呼叫 DefaultEventExecutorChooserFactory 中的next()
return chooser.next();
}
複製程式碼DefaultEventExecutorChooserFactory 對於如何從陣列中選擇任務執行器，也做了巧妙的優化。
public final class DefaultEventExecutorChooserFactory implements EventExecutorChooserFactory {
public static final DefaultEventExecutorChooserFactory INSTANCE = new DefaultEventExecutorChooserFactory();
private DefaultEventExecutorChooserFactory() { }
@SuppressWarnings("unchecked")
@Override
public EventExecutorChooser newChooser(EventExecutor[] executors) {
if (isPowerOfTwo(executors.length)) {
return new PowerOfTwoEventExecutorChooser(executors);
} else {
return new GenericEventExecutorChooser(executors);
}
}
// 判斷執行緒任務執行的個數是否為 2 的冪次方。e.g: 2、4、8、16
private static boolean isPowerOfTwo(int val) {
return (val & -val) == val;
}
// 冪次方選擇器
private static final class PowerOfTwoEventExecutorChooser implements EventExecutorChooser {
private final AtomicInteger idx = new AtomicInteger();
private final EventExecutor[] executors;
PowerOfTwoEventExecutorChooser(EventExecutor[] executors) {
this.executors = executors;
}
@Override
public EventExecutor next() {
// 通過二級制進行 & 運算，效率更高
return executors[idx.getAndIncrement() & executors.length - 1];
}
}
// 普通選擇器
private static final class GenericEventExecutorChooser implements EventExecutorChooser {
private final AtomicInteger idx = new AtomicInteger();
private final EventExecutor[] executors;
GenericEventExecutorChooser(EventExecutor[] executors) {
this.executors = executors;
}
@Override
public EventExecutor next() {
// 按照最普通的取模的方式從index=0開始向後開始選擇
return executors[Math.abs(idx.getAndIncrement() % executors.length)];
}
}
}
複製程式碼小結
通過本節內容，我們瞭解到了EventLoop與EventLoopGroup的基本原理，EventLoopGroup與EventLoop的建立過程：
- 建立執行緒任務執行器 ThreadPerTaskExecutor
- 建立EventLoop
- 建立任務選擇器 EventExecutorChooser