Channel的Unsafe对Buffer的使用

• 在Channel的接口体系设计中，Channel只是与ChannelHandler，ChannelHandlerContext，ChannelPipeline完成对数据的处理，而不直接参与和底层socket进行数据交互，是通过一个内部Unsafe的接口设计来完成这项工作。
• 如对于数据读取，在Channel的实现中拿到的是Unsafe已经读取回来的数据，Unsafe将socket的字节数据读取后转为了对象Object msg的数据，或者将ServerSocket接收到的客户端socket连接，将socket对象交给Channel。具体为交给Channel所绑定的ChannelPipeline，从而开始Channel体系结构的数据流处理。
• 故Buffer包中相关缓存实现类主要是在Channel的Unsafe接口的实现类中引用，读取底层Java NIO的socket的数据，如SocketChannel的bytes：
  如下为AbstractNioByteChannel的NioByteUnsafe的read方法：
  AbstractNioByteChannel是NioSocketChannel的基类，主要是读取客户端发送过来的字节数据

@Override
public final void read() {
    final ChannelConfig config = config();
    if (shouldBreakReadReady(config)) {
        clearReadPending();
        return;
    }
    final ChannelPipeline pipeline = pipeline();
    // 获取allocator，allocator是创建ByteBuf对象
    final ByteBufAllocator allocator = config.getAllocator();
    final RecvByteBufAllocator.Handle allocHandle = recvBufAllocHandle();
    allocHandle.reset(config);

    ByteBuf byteBuf = null;
    boolean close = false;
    try {
        do {
            // 从allocator获取一个ByteBuf对象实例
            byteBuf = allocHandle.allocate(allocator);
            // doReadBytes从底层socket读取数据到byteBuf
            allocHandle.lastBytesRead(doReadBytes(byteBuf));
            if (allocHandle.lastBytesRead() <= 0) {
                // nothing was read. release the buffer.
                byteBuf.release();
                byteBuf = null;
                close = allocHandle.lastBytesRead() < 0;
                if (close) {
                    // There is nothing left to read as we received an EOF.
                    readPending = false;
                }
                break;
            }

            allocHandle.incMessagesRead(1);
            readPending = false;
            // 将byteBuf传给pipeline，从而开始数据处理
            pipeline.fireChannelRead(byteBuf);
            byteBuf = null;
        } while (allocHandle.continueReading());

        allocHandle.readComplete();
        pipeline.fireChannelReadComplete();

        if (close) {
            closeOnRead(pipeline);
        }
    } catch (Throwable t) {
        handleReadException(pipeline, byteBuf, t, close, allocHandle);
    } finally {
        // Check if there is a readPending which was not processed yet.
        // This could be for two reasons:
        // * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
        // * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
        //
        // See https://github.com/netty/netty/issues/2254
        if (!readPending && !config.isAutoRead()) {
            removeReadOp();
        }
    }
}

• 从以上代码可以看出是ByteBuf的allocator是从ChannelConfig中获取：

final ByteBufAllocator allocator = config.getAllocator();

Channel使用堆内存还是直接内存

1. allocator的实际类型代表了Channel具体是使用堆内存还是直接内存。这个可以通过用户代码通过childOption设置或者使用默认的：

2. 用户代码传入：主要是通过childOption传入，则ServerBootstrap在接收到客户端连接并创建SocketChannel时，会根据初始化ServerBootstrap时的childOption设置，对该SocketChannel的config进行配置。

// 处理客户端请求的配置
serverBootstrap.childOption(ChannelOption.TCP_NODELAY, true);
serverBootstrap.childOption(ChannelOption.SO_KEEPALIVE, true);
// 客户端SocketChannel所使用的allocator的实现类
serverBootstrap.childOption(ChannelOption.ALLOCATOR, UnpooledByteBufAllocator.DEFAULT);
serverBootstrap.childHandler(new NettyServerInitializer(webSocketService));

1. 默认实现，一般不需要通过用户代码传入，使用默认的即可，默认实现为：如果当前的运行平台是android，则使用unpooled，即不使用池化机制，因为android移动端，内存资源有限，不适合做缓存；其他平台则默认使用池化机制，提高性能，以空间换时间。其次是使用堆内存还是直接内存的问题，主要是根据：

（1）当前运行平台是否支持使用Java的unsafe来进行本地方法调用；

（2）程序的系统参数是否设置了io.netty.noPreferDirect=true。如果当前平台支持unsafe且io.netty.noPreferDirect=false或者没有设置，默认为false，则使用直接内存；否则使用堆内存。
这两个参数的默认值：

1. 是否运行通过底层api直接访问直接内存，默认：允许 
-Dio.netty.noPreferDirect
2. 是否允许使用sun.misc.Unsafe，默认：允许；注意：使用sun的私有类库存在平台移植问题，另外sun.misc.Unsafe类是不安全的，如果操作失败，不是抛出异常，而是虚拟机core dump，不建议使用Unsafe 
-Dio.netty.noUnsafe

源码分析如下：

1. 
public class DefaultChannelConfig implements ChannelConfig {
    ...
    
    private volatile ByteBufAllocator allocator = ByteBufAllocator.DEFAULT;
    
    ...
}

2. 
public interface ByteBufAllocator {
    
    ByteBufAllocator DEFAULT = ByteBufUtil.DEFAULT_ALLOCATOR;
}

3. DEFAULT_ALLOCATOR
public final class ByteBufUtil {
    ...
    
    static final ByteBufAllocator DEFAULT_ALLOCATOR;

    static {
        // android则是unpooled，其他为pooled
        String allocType = SystemPropertyUtil.get(
                "io.netty.allocator.type", PlatformDependent.isAndroid() ? "unpooled" : "pooled");
        allocType = allocType.toLowerCase(Locale.US).trim();

        ByteBufAllocator alloc;
        if ("unpooled".equals(allocType)) {
            alloc = UnpooledByteBufAllocator.DEFAULT;
            logger.debug("-Dio.netty.allocator.type: {}", allocType);
        } else if ("pooled".equals(allocType)) {
            alloc = PooledByteBufAllocator.DEFAULT;
            logger.debug("-Dio.netty.allocator.type: {}", allocType);
        } else {
            alloc = PooledByteBufAllocator.DEFAULT;
            logger.debug("-Dio.netty.allocator.type: pooled (unknown: {})", allocType);
        }

        DEFAULT_ALLOCATOR = alloc;
        
        ...
    }

    ...
}

4. UnpooledByteBufAllocator.DEFAULT：非池化机制默认alloctor
/** * Default instance which uses leak-detection for direct buffers. */
public static final UnpooledByteBufAllocator DEFAULT =
        new UnpooledByteBufAllocator(PlatformDependent.directBufferPreferred());
        
5. PooledByteBufAllocator.DEFAULT：池化机制默认allocator
public static final PooledByteBufAllocator DEFAULT =
        new PooledByteBufAllocator(PlatformDependent.directBufferPreferred());

6. 在4，5中，都调用了PlatformDependent.directBufferPreferred()，如果返回true，则使用直接内存，否则使用堆内存。PlatformDependent.directBufferPreferred()的底层实现如下：

private static final Throwable UNSAFE_UNAVAILABILITY_CAUSE = unsafeUnavailabilityCause0();
private static final boolean DIRECT_BUFFER_PREFERRED =
        UNSAFE_UNAVAILABILITY_CAUSE == null && !SystemPropertyUtil.getBoolean("io.netty.noPreferDirect", false);

unsafeUnavailabilityCause0的实现：判断当前平台是否支持使用Java的unsafe
private static Throwable unsafeUnavailabilityCause0() {
    if (isAndroid()) {
        logger.debug("sun.misc.Unsafe: unavailable (Android)");
        return new UnsupportedOperationException("sun.misc.Unsafe: unavailable (Android)");
    }
    Throwable cause = PlatformDependent0.getUnsafeUnavailabilityCause();
    if (cause != null) {
        return cause;
    }

    try {
        boolean hasUnsafe = PlatformDependent0.hasUnsafe();
        logger.debug("sun.misc.Unsafe: {}", hasUnsafe ? "available" : "unavailable");
        return hasUnsafe ? null : PlatformDependent0.getUnsafeUnavailabilityCause();
    } catch (Throwable t) {
        logger.trace("Could not determine if Unsafe is available", t);
        // Probably failed to initialize PlatformDependent0.
        return new UnsupportedOperationException("Could not determine if Unsafe is available", t);
    }
}