Java NIO ServerSocketChannel backlog原理、源码解读与默认值

backlog默认值为50

backlog原理

backlog影响的accept队列大小

客户端把数据放到accept队列，JVM再从accept队列取。

如果accept队列太小，JVM取数据不够快，那么accept队列就会溢出

源码找到ServerSocketChannel

无backlog参数，默认为0

public final ServerSocketChannel bind(SocketAddress local)
    throws IOException
{
    return bind(local, 0);
}

源码找到实现类ServerSocketChannelImpl，查看bind方法

Net.listen(this.fd, var2 < 1 ? 50 : var2); 此处代码，可以看出backlog默认值为< 1时，返回是50

public ServerSocketChannel bind(SocketAddress var1, int var2) throws IOException {
    synchronized(this.lock) {
        if (!this.isOpen()) {
            throw new ClosedChannelException();
        } else if (this.isBound()) {
            throw new AlreadyBoundException();
        } else {
            InetSocketAddress var4 = var1 == null ? new InetSocketAddress(0) : Net.checkAddress(var1);
            SecurityManager var5 = System.getSecurityManager();
            if (var5 != null) {
                var5.checkListen(var4.getPort());
            }

            NetHooks.beforeTcpBind(this.fd, var4.getAddress(), var4.getPort());
            Net.bind(this.fd, var4.getAddress(), var4.getPort());
            Net.listen(this.fd, var2 < 1 ? 50 : var2);
            synchronized(this.stateLock) {
                this.localAddress = Net.localAddress(this.fd);
            }

            return this;
        }
    }
}