java nio学习之Channel(三)

悠悠 2024-04-18 14:59 68阅读 0赞

# 实战 #

复制一张图片

public static void main(String[] args) throws IOException {
            String fileName = parentPath + "\\104.jpg";
            Pipe pipe = Pipe.open();
            ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
            try (WritableByteChannel writableByteChannel = Channels.newChannel(Channels.newOutputStream(pipe.sink()))){
                FileChannel channel = new FileInputStream(fileName).getChannel();
                channel.transferTo(0, new File(fileName).length(), writableByteChannel);
            }
    
            File file = new File(parentPath + "\\b.jpg");
            ReadableByteChannel readableByteChannel = pipe.source();
            FileChannel fileChannel = new FileOutputStream(file).getChannel();
    
            while (readableByteChannel.read(byteBuffer) > 0) {
                byteBuffer.flip();
                fileChannel.write(byteBuffer);
                byteBuffer.clear();
            }
    
        }
    
        private static String parentPath = "D:\\github\\zip";
        private static String path = parentPath + "\\104.jpg";

上面这样写是因为想熟悉一下NIO,Java NIO管道是2个线程之间的单向数据连接。`Pipe`有一个源通道和一个sink通道.

sink负责写通道，source负责读通道

# 为啥这么做 #

查看这一篇[https://mp.weixin.qq.com/s?\_\_biz=MzIwMDY0Nzk2Mw==&mid=2650320600&idx=1&sn=31b01b7887f1880510549ac28efe738a&chksm=8ef5faacb98273bad0de9b1ed1a94bc894cdd330e911d10828281c67f734b1548a39bd07fa70&mpshare=1&scene=2&srcid=&sharer\_sharetime=1567731655097&sharer\_shareid=2a6de52c25a6a3a0164993bffe122d8a&from=timeline&clicktime=1567731954\#rd][https_mp.weixin.qq.com_s_biz_MzIwMDY0Nzk2Mw_mid_2650320600_idx_1_sn_31b01b7887f1880510549ac28efe738a_chksm_8ef5faacb98273bad0de9b1ed1a94bc894cdd330e911d10828281c67f734b1548a39bd07fa70_mpshare_1_scene_2_srcid_sharer_sharetime_1567731655097_sharer_shareid_2a6de52c25a6a3a0164993bffe122d8a_from_timeline_clicktime_1567731954_rd]

## transferTo ##

首先是transferTo，

我们可以看到这里并没有使用`ByteBuffer`进行数据传输，而是使用了`transferTo`的方法。这个方法是将两个通道进行直连。

This method is potentially much more efficient than a simple loop* that reads from this channel and writes to the target channel.  Many* operating systems can transfer bytes directly from the filesystem cache* to the target channel without actually copying them.

这是源码上的描述文字，大概意思就是使用`transferTo`的效率比循环一个`Channel`读取出来然后再循环写入另一个`Channel`好。操作系统能够直接传输字节从文件系统缓存到目标的`Channel`中，而不需要实际的`copy`阶段。

> copy阶段就是从内核空间转到用户空间的一个过程

# 缓冲器介绍 #

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODMzNjY1OA_size_16_color_FFFFFF_t_70][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODMzNjY1OA_size_16_color_FFFFFF_t_70 1][]

我们可以看到一半的copy是从内核空间复制到用户空间，而transferTo就是开辟直接缓存。还可以开辟一个物理内存映射，MappedByteBuffer。

# sink和source相互阻塞 #

Java NIO 管道是2个线程之间的单向数据连接。Pipe有一个source通道和一个sink通道。其中source通道用于读取数据，sink通道用于写入数据。可以看到源码中的介绍，大概意思就是写入线程会阻塞至有读线程从通道中读取数据。如果没有数据可读，读线程也会阻塞至写线程写入数据。直至通道关闭。

Whether or not a thread writing bytes to a pipe will block until another thread reads those bytes

我们可以使用CompletableFuture将sink和source进行异步分开操作。

# 原理 #

[点这里][Link 1]

# 单纯的复制 #

WritableByteChannel writableByteChannel = Channels.newChannel(zipOut);
            try (FileChannel fileChannel = new FileInputStream(fileToZip).getChannel()) {
                fileChannel.transferTo(0, fileToZip.length(), writableByteChannel);
            }

很简单就实现了。

[https_mp.weixin.qq.com_s_biz_MzIwMDY0Nzk2Mw_mid_2650320600_idx_1_sn_31b01b7887f1880510549ac28efe738a_chksm_8ef5faacb98273bad0de9b1ed1a94bc894cdd330e911d10828281c67f734b1548a39bd07fa70_mpshare_1_scene_2_srcid_sharer_sharetime_1567731655097_sharer_shareid_2a6de52c25a6a3a0164993bffe122d8a_from_timeline_clicktime_1567731954_rd]: https://mp.weixin.qq.com/s?__biz=MzIwMDY0Nzk2Mw==&mid=2650320600&idx=1&sn=31b01b7887f1880510549ac28efe738a&chksm=8ef5faacb98273bad0de9b1ed1a94bc894cdd330e911d10828281c67f734b1548a39bd07fa70&mpshare=1&scene=2&srcid=&sharer_sharetime=1567731655097&sharer_shareid=2a6de52c25a6a3a0164993bffe122d8a&from=timeline&clicktime=1567731954#rd
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODMzNjY1OA_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/20190908164236197.JPG?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODMzNjY1OA==,size_16,color_FFFFFF,t_70
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODMzNjY1OA_size_16_color_FFFFFF_t_70 1]: https://img-blog.csdnimg.cn/20190908164249215.JPG?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODMzNjY1OA==,size_16,color_FFFFFF,t_70
[Link 1]: https://ifeve.com/pipe/