Netty分布式NioEventLoop優(yōu)化selector源碼解析

更新時(shí)間：2022年03月25日 13:12:40 作者：向南是個(gè)萬(wàn)人迷

這篇文章主要介紹了Netty分布式NioEventLoop優(yōu)化selector源碼解析，有需要的朋友可以借鑒參考下，希望能夠有所幫助，祝大家多多進(jìn)步，早日升職加薪

優(yōu)化selector

selector的創(chuàng)建過(guò)程

在剖析selector輪詢之前, 我們先講解一下selector的創(chuàng)建過(guò)程

回顧之前的小節(jié), 在創(chuàng)建NioEventLoop中初始化了唯一綁定的selector:

NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider, 
             SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler) {
    super(parent, executor, false, DEFAULT_MAX_PENDING_TASKS, rejectedExecutionHandler);
    //代碼省略
    provider = selectorProvider;
    selector = openSelector();
    selectStrategy = strategy;
}

這里 selector = openSelector() 初始化了selector

我們跟到openSelector()中:

private Selector openSelector() {
    final Selector selector;
    try {
        //調(diào)用jdk底層的api
        selector = provider.openSelector();
    } catch (IOException e) {
        throw new ChannelException("failed to open a new selector", e);
    }
    //判斷是否需要關(guān)閉優(yōu)化(默認(rèn)false, 也就是默認(rèn)需要優(yōu)化)
    if (DISABLE_KEYSET_OPTIMIZATION) {
        return selector;
    }
    //用這個(gè)數(shù)據(jù)結(jié)構(gòu)替換原生的SelectionKeySet
    final SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();
    Object maybeSelectorImplClass = AccessController.doPrivileged(new PrivilegedAction<Object>() {
        @Override
        public Object run() {
            try {
                //通過(guò)反射拿到sun.nio.ch.SelectorImpl這個(gè)類的class對(duì)象
                return Class.forName("sun.nio.ch.SelectorImpl", false, PlatformDependent.getSystemClassLoader());
            } catch (ClassNotFoundException e) {
                return e;
            } catch (SecurityException e) {
                return e;
            }
        }
    });
    //判斷拿到的是不是class對(duì)象并且是不是Selector的實(shí)現(xiàn)類
    if (!(maybeSelectorImplClass instanceof Class) ||!((Class<?>) maybeSelectorImplClass).isAssignableFrom(selector.getClass())) {
        if (maybeSelectorImplClass instanceof Exception) {
            Exception e = (Exception) maybeSelectorImplClass;
            logger.trace("failed to instrument a special java.util.Set into: {}", selector, e);
        }
        //如果不是他的實(shí)現(xiàn), 就直接返回原生select
        return selector;
    }
    //如果是它的實(shí)現(xiàn), 就拿到其class對(duì)象
    final Class<?> selectorImplClass = (Class<?>) maybeSelectorImplClass;
    Object maybeException = AccessController.doPrivileged(new PrivilegedAction<Object>() {
        @Override
        public Object run() {
            try {
                //通過(guò)反射拿到selectedKeys和publicSelectedKeys兩個(gè)屬性, 默認(rèn)這兩個(gè)屬性底層都是hashSet方式實(shí)現(xiàn)的
                Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
                Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");
                //設(shè)置成可修改的
                selectedKeysField.setAccessible(true);
                publicSelectedKeysField.setAccessible(true);
                //將selector的這兩個(gè)屬性替換成Netty的selectedKeySet
                selectedKeysField.set(selector, selectedKeySet);
                publicSelectedKeysField.set(selector, selectedKeySet);
                return null;
            } catch (NoSuchFieldException e) {
                return e;
            } catch (IllegalAccessException e) {
                return e;
            } catch (RuntimeException e) {
                if ("java.lang.reflect.InaccessibleObjectException".equals(e.getClass().getName())) {
                    return e;
                } else {
                    throw e;
                }
            }
        }
    });
    if (maybeException instanceof Exception) {
        selectedKeys = null;
        Exception e = (Exception) maybeException;
        logger.trace("failed to instrument a special java.util.Set into: {}", selector, e);
    } else {
        //將優(yōu)化后的keySet保存成NioEventLoop的成員變量
        selectedKeys = selectedKeySet;
        logger.trace("instrumented a special java.util.Set into: {}", selector);
    }
    return selector;
}

代碼剖析

這里代碼比較長(zhǎng), 我們一點(diǎn)一點(diǎn)的剖析:

首先 selector = provider.openSelector() 這里創(chuàng)建了jdk底層的selector

if (DISABLE_KEYSET_OPTIMIZATION) {
    return selector;
}

這里判斷了是否關(guān)閉優(yōu)化功能, 默認(rèn)是false, 也就是需要優(yōu)化, 這里的意思就是netty需要對(duì)jdk原生的selector進(jìn)行了優(yōu)化, 我們知道selector在select()操作時(shí)候, 會(huì)通過(guò)selector.selectedKeys()操作返回一個(gè)Set<SelectionKey>, 這個(gè)是Set類型, netty對(duì)這個(gè)set進(jìn)行了處理, 使用SelectedSelectionKeySet的數(shù)據(jù)結(jié)構(gòu)進(jìn)行替換, 當(dāng)在select()操作時(shí)將key存入一個(gè)SelectedSelectionKeySet的數(shù)據(jù)結(jié)構(gòu)中

final SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();

這里一步創(chuàng)建了這個(gè)優(yōu)化后的數(shù)據(jù)結(jié)構(gòu)

簡(jiǎn)單跟一下SelectedSelectctionKeySet這個(gè)類的構(gòu)造方法:

SelectedSelectionKeySet() {
    keysA = new SelectionKey[1024];
    keysB = keysA.clone();
}

初始化了兩個(gè)屬性keysA和keysB, 說(shuō)明這類其實(shí)底層是通過(guò)數(shù)組實(shí)現(xiàn)的, 通過(guò)操作數(shù)組下標(biāo)會(huì)有更高的效率

這個(gè)類的的flip()方法, 則返SelectionKey[]數(shù)組

SelectionKey[] flip() {
    if (isA) {
        isA = false;
        keysA[keysASize] = null;
        keysBSize = 0;
        return keysA;
    } else {
        isA = true;
        keysB[keysBSize] = null;
        keysASize = 0;
        return keysB;
    }
}

再看下其他方法:

@Override
public boolean remove(Object o) {
    return false;
}
@Override
public boolean contains(Object o) {
    return false;
}
@Override
public Iterator<SelectionKey> iterator() {
    throw new UnsupportedOperationException();
}

我們看到remove()方法, contains()方法都返回了false, 說(shuō)明其不支持刪除方法和包含方法, iterator()方法則直接拋出異常, 說(shuō)明其不支持迭代器操作

回到openSelector()中:

再往下看, 這里通過(guò) Class.forName("sun.nio.ch.SelectorImpl", false, PlatformDependent.getSystemClassLoader()) 創(chuàng)建了一個(gè)SelectorImpl的class對(duì)象

if(!(maybeSelectorImplClass instanceof Class) ||!((Class<?>) maybeSelectorImplClass).isAssignableFrom(selector.getClass()))

這里判斷拿到的對(duì)象是否為class對(duì)象并且是否為Selector的實(shí)現(xiàn)類, 如果不是, 則直接返回jdk的selector

如果是, 就繼續(xù)轉(zhuǎn)化成class對(duì)象

然后就做了真正的替換操作:

//通過(guò)反射拿到selectedKeys和publicSelectedKeys兩個(gè)屬性, 默認(rèn)這兩個(gè)屬性底層都是hashSet方式實(shí)現(xiàn)的
Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");
//設(shè)置成可修改的
selectedKeysField.setAccessible(true);
publicSelectedKeysField.setAccessible(true);
//將selector的這兩個(gè)屬性替換成Netty的selectedKeySet
selectedKeysField.set(selector, selectedKeySet);
publicSelectedKeysField.set(selector, selectedKeySet);

通過(guò)注釋我們不難看出, 這里將新創(chuàng)建selectedKeySet替換了selector對(duì)象中的selectedKeysField, 和selectedKeysField兩個(gè)屬性

最后通過(guò) selectedKeys = selectedKeySet 將優(yōu)化的數(shù)據(jù)結(jié)構(gòu)selectedKeySet保存在NioEventLoop的成員變量中