文件通道创建方式综述

程序员文章站 2022-04-24 13:27:05

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Reference定义（PhantomReference，Cleaner）:[url]http://donald-draper.iteye.com/blog/2371661[/url]
FileChanne定义：[url]http://donald-draper.iteye.com/blog/2374149[/url]
文件读写方式简单综述:[url]http://donald-draper.iteye.com/blog/2374237[/url]
文件读写方式简单综述后续（文件，流构造）:[url]http://donald-draper.iteye.com/blog/2374294[/url]
在看了FileChanne定义后，为了更好了理解FileChanne，我们简单的看了文件File，路径Path ，文件系统FileSystem，文件系统提供者FileSystemProvider。我们先回顾下相关概念：
File内部关联一个文件系统FileSystem，用于操作底层的系统，file的文件分隔符和路径分隔符都是从FileSystem获取，windows（\\,;）和unix(\,:)有所不同，FileSystem根据底层操作获取不同文件系统实现，windows默认为Win32FileSystem。file的创建，删除，list当前目录文件等待操作，实际是委托给Win32FileSystem。获取文件Path，首先获取文件的默认文件系统提供者，默认为WindowsFileSystemProvider，WindowsFileSystemProvider通过文件path（URI），创建文件Path（WindowsPath），这个主要用于创建文件通达需要。
今天我们来看一下通道的具体实现，从获取通道开始，获取通道有4中方法：
1.从FileOutputStream获取可写不可读文件通道
//FileOutputStream

 public FileChannel getChannel() {
        synchronized (this) {
            if (channel == null) {
	        //可写不可读
                channel = FileChannelImpl.open(fd, false, true, append, this);

                /*
                 * Increment fd's use count. Invoking the channel's close()
                 * method will result in decrementing the use count set for
                 * the channel.
                 */
                fd.incrementAndGetUseCount();
            }
            return channel;
        }
    }

2.从FileInputStream获取可读不可写文件通道
//FileInputStream

public FileChannel getChannel() {
        synchronized (this) {
            if (channel == null) {
	        //可读不可写
                channel = FileChannelImpl.open(fd, true, false, this);

                /*
                 * Increment fd's use count. Invoking the channel's close()
                 * method will result in decrementing the use count set for
                 * the channel.
                 */
                fd.incrementAndGetUseCount();
            }
            return channel;
        }
    }

3.从RandomAccessFile获取可读可写文件通道
//RandomAccessFile

public class RandomAccessFile implements DataOutput, DataInput, Closeable {

    private FileDescriptor fd;
    private FileChannel channel = null;
    private boolean rw;//是否读写

    private Object closeLock = new Object();
    private volatile boolean closed = false;

    private static final int O_RDONLY = 1;
    private static final int O_RDWR =   2;
    private static final int O_SYNC =   4;
    private static final int O_DSYNC =  8;
 /* @since 1.4
  * @spec JSR-51
  */
 public final FileChannel getChannel() {
     synchronized (this) {
         if (channel == null) {
	     //默认可读，在根据rw判断是否可写
             channel = FileChannelImpl.open(fd, true, rw, this);

             /*
              * FileDescriptor could be shared by FileInputStream or
              * FileOutputStream.
              * Ensure that FD is GC'ed only when all the streams/channels
              * are done using it.
              * Increment fd's use count. Invoking the channel's close()
              * method will result in decrementing the use count set for
              * the channel.
              */
             fd.incrementAndGetUseCount();
         }
         return channel;
     }
 }
 ...
 }

//FileChannelImpl

  public static FileChannel open(FileDescriptor filedescriptor, boolean flag, boolean flag1, Object obj)
    {
        return new FileChannelImpl(filedescriptor, flag, flag1, false, obj);
    }
    public static FileChannel open(FileDescriptor filedescriptor, boolean flag, boolean flag1, boolean flag2, Object obj)
    {
        return new FileChannelImpl(filedescriptor, flag, flag1, flag2, obj);
    }
    private FileChannelImpl(FileDescriptor filedescriptor, boolean flag, boolean flag1, boolean flag2, Object obj)
    {
        fd = filedescriptor;
        readable = flag;//可读标志
        writable = flag1;//可写标志
        append = flag2;//是否尾部追加文件，默认为fasle
	//创建文件通道的对象，为FileInput/OutputStream，
	//RandomAccessFile获取FileSystemProvider(WindowsFileSystemProvider)
        parent = obj;
        nd = new FileDispatcherImpl(flag2);
    }

4.从文件系统提供者获取文件通道(FileChannel#open)
//FileChannel

//根据文件Path和打开选项创建文件通道
public static FileChannel open(Path path, OpenOption... options)
        throws IOException
    {
        Set<OpenOption> set = new HashSet<OpenOption>(options.length);
        Collections.addAll(set, options);
	//委托给 FileChannel open(Path path, Set<? extends OpenOption> options,FileAttribute<?>... attrs)
        return open(path, set, NO_ATTRIBUTES);
    }
//根据文件Path，打开选项，及文件属性创建文件通道
public static FileChannel open(Path path,
                                   Set<? extends OpenOption> options,
                                   FileAttribute<?>... attrs)
        throws IOException
    {
        //这一步我们在前面一篇文章已讲，文件系统提供者为WindowsFileSystemProvider
        FileSystemProvider provider = path.getFileSystem().provider();
        return provider.newFileChannel(path, options, attrs);
    }

//WindowsFileSystemProvider

public transient FileChannel newFileChannel(Path path, Set set, FileAttribute afileattribute[])
        throws IOException
    {
        WindowsPath windowspath;//文件Path
        WindowsSecurityDescriptor windowssecuritydescriptor;//文件属性描述
        if(path == null)
            throw new NullPointerException();
        if(!(path instanceof WindowsPath))
            throw new ProviderMismatchException();
        windowspath = (WindowsPath)path;
        windowssecuritydescriptor = WindowsSecurityDescriptor.fromAttribute(afileattribute);
	我们需要关注的是这一点
        FileChannel filechannel = WindowsChannelFactory.newFileChannel(windowspath.getPathForWin32Calls(), windowspath.getPathForPermissionCheck(), set, windowssecuritydescriptor.address());
        if(windowssecuritydescriptor != null)
            windowssecuritydescriptor.release();
        return filechannel;
        ...
        windowsexception.rethrowAsIOException(windowspath);
        ...
        Exception exception;
        exception;
        if(windowssecuritydescriptor != null)
            windowssecuritydescriptor.release();
        throw exception;
    }

再来看WindowsChannelFactory创建文件通道：

class WindowsChannelFactory
{
    private static final JavaIOFileDescriptorAccess fdAccess = SharedSecrets.getJavaIOFileDescriptorAccess();
    static final OpenOption OPEN_REPARSE_POINT = new OpenOption() {};
    private WindowsChannelFactory()
    {
    }
    static FileChannel newFileChannel(String s, String s1, Set set, long l)
        throws WindowsException
    {
        //转换打开选项集为Flags
        Flags flags = Flags.toFlags(set);
        if(!flags.read && !flags.write)
            if(flags.append)
                flags.write = true;
            else
                flags.read = true;
        //可读可append，抛出选项配置错误
        if(flags.read && flags.append)
            throw new IllegalArgumentException("READ + APPEND not allowed");
	//可append，存在压缩抛出选项配置错误
        if(flags.append && flags.truncateExisting)
        {
            throw new IllegalArgumentException("APPEND + TRUNCATE_EXISTING not allowed");
        } else
        {
	    //创建文件描述符
            FileDescriptor filedescriptor = open(s, s1, flags, l);
	    //委托给FileChannelImpl
            return FileChannelImpl.open(filedescriptor, flags.read, flags.write, flags.append, null);
        }
    }
}

上述过程我们需要关注的有以下几点：
1.

//转换打开选项集为Flags
Flags flags = Flags.toFlags(set);

//创建文件描述符
FileDescriptor filedescriptor = open(s, s1, flags, l);

 //委托给FileChannelImpl
return FileChannelImpl.open(filedescriptor, flags.read, flags.write, flags.append, null);

下面分别来看：
1.

//转换打开选项集为Flags
Flags flags = Flags.toFlags(set);

//WindowsChannelFactory - Flags
 private static class Flags
    {
        boolean read;//是否可读
        boolean write;//是否可写
        boolean append;//追加文件
        boolean truncateExisting;//存在，则压缩
        boolean create;//文件不存在，则创建
        boolean createNew;//无论文件不在与否，创建文件
        boolean deleteOnClose;//关闭文件删除，可用于创建临时文件
        boolean sparse;//是否稀疏文件
        boolean overlapped;
        boolean sync;//同步文件更新（内容及元数据）到底层文件
        boolean dsync;同步文件更新（内容）到底层文件
        boolean shareRead;//共享读
        boolean shareWrite;//贡献写
        boolean shareDelete;//共享删除
        boolean noFollowLinks;
        boolean openReparsePoint;

        private Flags()
        {
            shareRead = true;
            shareWrite = true;
            shareDelete = true;
        }
	//将打开选项集转化为Flags
        static Flags toFlags(Set set)
        {

            Flags flags = new Flags();
	    //遍历打开选项集，转化选项配置为Flags对应的field
            for(Iterator iterator = set.iterator(); iterator.hasNext();)
            {
                OpenOption openoption = (OpenOption)iterator.next();
		//标准打开选项和拓展打开选项的配置转化类，将配置转化为Int，类型
                static class _cls2
                {

                    static final int $SwitchMap$java$nio$file$StandardOpenOption[];
                    static final int $SwitchMap$com$sun$nio$file$ExtendedOpenOption[];

                    static 
                    {
                        $SwitchMap$com$sun$nio$file$ExtendedOpenOption = new int[ExtendedOpenOption.values().length];
                        try
                        {
                            $SwitchMap$com$sun$nio$file$ExtendedOpenOption[ExtendedOpenOption.NOSHARE_READ.ordinal()] = 1;
                        }
                        catch(NoSuchFieldError nosuchfielderror) { }
                        try
                        {
                            $SwitchMap$com$sun$nio$file$ExtendedOpenOption[ExtendedOpenOption.NOSHARE_WRITE.ordinal()] = 2;
                        }
                        catch(NoSuchFieldError nosuchfielderror1) { }
                        try
                        {
                            $SwitchMap$com$sun$nio$file$ExtendedOpenOption[ExtendedOpenOption.NOSHARE_DELETE.ordinal()] = 3;
                        }
                        catch(NoSuchFieldError nosuchfielderror2) { }
                        $SwitchMap$java$nio$file$StandardOpenOption = new int[StandardOpenOption.values().length];
                        try
                        {
                            $SwitchMap$java$nio$file$StandardOpenOption[StandardOpenOption.READ.ordinal()] = 1;
                        }
                        catch(NoSuchFieldError nosuchfielderror3) { }
                        try
                        {
                            $SwitchMap$java$nio$file$StandardOpenOption[StandardOpenOption.WRITE.ordinal()] = 2;
                        }
                        catch(NoSuchFieldError nosuchfielderror4) { }
                        try
                        {
                            $SwitchMap$java$nio$file$StandardOpenOption[StandardOpenOption.APPEND.ordinal()] = 3;
                        }
                        catch(NoSuchFieldError nosuchfielderror5) { }
                        try
                        {
                            $SwitchMap$java$nio$file$StandardOpenOption[StandardOpenOption.TRUNCATE_EXISTING.ordinal()] = 4;
                        }
                        catch(NoSuchFieldError nosuchfielderror6) { }
                        try
                        {
                            $SwitchMap$java$nio$file$StandardOpenOption[StandardOpenOption.CREATE.ordinal()] = 5;
                        }
                        catch(NoSuchFieldError nosuchfielderror7) { }
                        try
                        {
                            $SwitchMap$java$nio$file$StandardOpenOption[StandardOpenOption.CREATE_NEW.ordinal()] = 6;
                        }
                        catch(NoSuchFieldError nosuchfielderror8) { }
                        try
                        {
                            $SwitchMap$java$nio$file$StandardOpenOption[StandardOpenOption.DELETE_ON_CLOSE.ordinal()] = 7;
                        }
                        catch(NoSuchFieldError nosuchfielderror9) { }
                        try
                        {
                            $SwitchMap$java$nio$file$StandardOpenOption[StandardOpenOption.SPARSE.ordinal()] = 8;
                        }
                        catch(NoSuchFieldError nosuchfielderror10) { }
                        try
                        {
                            $SwitchMap$java$nio$file$StandardOpenOption[StandardOpenOption.SYNC.ordinal()] = 9;
                        }
                        catch(NoSuchFieldError nosuchfielderror11) { }
                        try
                        {
                            $SwitchMap$java$nio$file$StandardOpenOption[StandardOpenOption.DSYNC.ordinal()] = 10;
                        }
                        catch(NoSuchFieldError nosuchfielderror12) { }
                    }
                }
               //标准打开选项
                if(openoption instanceof StandardOpenOption)
                    switch(_cls2..SwitchMap.java.nio.file.StandardOpenOption[((StandardOpenOption)openoption).ordinal()])
                    {
                    case 1: // '\001'
                        flags.read = true;
                        break;

                    case 2: // '\002'
                        flags.write = true;
                        break;

                    case 3: // '\003'
                        flags.append = true;
                        break;

                    case 4: // '\004'
                        flags.truncateExisting = true;
                        break;

                    case 5: // '\005'
                        flags.create = true;
                        break;

                    case 6: // '\006'
                        flags.createNew = true;
                        break;

                    case 7: // '\007'
                        flags.deleteOnClose = true;
                        break;

                    case 8: // '\b'
                        flags.sparse = true;
                        break;

                    case 9: // '\t'
                        flags.sync = true;
                        break;

                    case 10: // '\n'
                        flags.dsync = true;
                        break;

                    default:
                        throw new UnsupportedOperationException();
                    }
                else
		//拓展打开选项
                if(openoption instanceof ExtendedOpenOption)
                    switch(_cls2..SwitchMap.com.sun.nio.file.ExtendedOpenOption[((ExtendedOpenOption)openoption).ordinal()])
                    {
                    case 1: // '\001'
                        flags.shareRead = false;
                        break;

                    case 2: // '\002'
                        flags.shareWrite = false;
                        break;

                    case 3: // '\003'
                        flags.shareDelete = false;
                        break;

                    default:
                        throw new UnsupportedOperationException();
                    }
                else
                if(openoption == LinkOption.NOFOLLOW_LINKS)
                    flags.noFollowLinks = true;
                else
                if(openoption == WindowsChannelFactory.OPEN_REPARSE_POINT)
                    flags.openReparsePoint = true;
                else
                if(openoption == null)
                    throw new NullPointerException();
                else
                    throw new UnsupportedOperationException();
            }

            return flags;
        }
    }

//创建文件描述符
FileDescriptor filedescriptor = open(s, s1, flags, l);

private static FileDescriptor open(String s, String s1, Flags flags, long l)
        throws WindowsException
    {
        boolean flag = false;
        int i = 0;//标准配置项，读写配置
        if(flags.read)
            i |= -2147483648;
        if(flags.write)
            i |= 1073741824;
        int j = 0;//扩展配置项共享读写删除配置
        if(flags.shareRead)
            j |= 1;
        if(flags.shareWrite)
            j |= 2;
        if(flags.shareDelete)
            j |= 4;
        int k = 128;//记录createNew，create，truncateExisting，dsync，sync，overlapped，deleteOnClose
        byte byte0 = 3;
        if(flags.write)
            if(flags.createNew)
            {
                byte0 = 1;
                k |= 2097152;
            } else
            {
                if(flags.create)
                    byte0 = 4;
                if(flags.truncateExisting)
                    if(byte0 == 4)
                        flag = true;
                    else
                        byte0 = 5;
            }
        if(flags.dsync || flags.sync)
            k |= -2147483648;
        if(flags.overlapped)
            k |= 1073741824;
        if(flags.deleteOnClose)
            k |= 67108864;
        boolean flag1 = true;//记录noFollowLinks，openReparsePoint，deleteOnClose
        if(byte0 != 1 && (flags.noFollowLinks || flags.openReparsePoint || flags.deleteOnClose))
        {
            if(flags.noFollowLinks || flags.deleteOnClose)
                flag1 = false;
            k |= 2097152;
        }
        if(s1 != null)
        {
            SecurityManager securitymanager = System.getSecurityManager();
            if(securitymanager != null)
            {
	       //检查读写删除权限
                if(flags.read)
                    securitymanager.checkRead(s1);
                if(flags.write)
                    securitymanager.checkWrite(s1);
                if(flags.deleteOnClose)
                    securitymanager.checkDelete(s1);
            }
        }
	//创建文件
        long l1 = WindowsNativeDispatcher.CreateFile(s, i, j, l, byte0, k);
        ...
        FileDescriptor filedescriptor = new FileDescriptor();
	//设置文件描述的处理器
        fdAccess.setHandle(filedescriptor, l1);
        return filedescriptor;
    }

来看这一步的关键点：

//创建文件
long l1 = WindowsNativeDispatcher.CreateFile(s, i, j, l, byte0, k);

//WindowsNativeDispatcher

  static long CreateFile(String s, int i, int j, int k, int l)
        throws WindowsException
    {
        return CreateFile(s, i, j, 0L, k, l);
    }
static long CreateFile(String s, int i, int j, long l, int k, int i1)
        throws WindowsException
    {
        //将文件Path信息，放在本地buffer中
        NativeBuffer nativebuffer = asNativeBuffer(s);
        long l1 = CreateFile0(nativebuffer.address(), i, j, l, k, i1);
        //释放nativebuffer，放入线程本地缓存，以便重用
        nativebuffer.release();
        return l1;
        Exception exception;
        exception;
        nativebuffer.release();
        throw exception;
    }
 private static native long CreateFile0(long l, int i, int j, long l1, int k, int i1)
        throws WindowsException;

创建方法中我们还有两点要看
2.a

//将文件Path信息，放在本地buffer中
 NativeBuffer nativebuffer = asNativeBuffer(s);

2.b

 //释放nativebuffer，放入线程本地缓存，以便重用
 nativebuffer.release();

下面分别来看这两点：
再看这个之前先看一下NativeBuffer
//NativeBuffer

class NativeBuffer
{
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private final long address;//内存地址
    private final int size;//内存size
    private final Cleaner cleaner;//清理器
    private Object owner;//buffer拥有者
    private static class Deallocator
        implements Runnable
    {

        public void run()
        {
	    //释放本地buffer空间
            NativeBuffer.unsafe.freeMemory(address);
        }

        private final long address;

        Deallocator(long l)
        {
            address = l;
        }
    }
    NativeBuffer(int i)
    {
        address = unsafe.allocateMemory(i);
        size = i;
        cleaner = Cleaner.create(this, new Deallocator(address));
    }
    void release()
    {
        NativeBuffers.releaseNativeBuffer(this);
    }
    long address()
    {
        return address;
    }
    int size()
    {
        return size;
    }
    Cleaner cleaner()
    {
        return cleaner;
    }
    void setOwner(Object obj)
    {
        owner = obj;
    }
    Object owner()
    {
        return owner;
    }
}

再回到刚才创建文件中的两点
2.a

//将文件Path信息，放在本地buffer中
 NativeBuffer nativebuffer = asNativeBuffer(s);

//WindowsNativeDispatcher

static NativeBuffer asNativeBuffer(String s)
    {
        int i = s.length() << 1;
        int j = i + 2;
	//从线程本地缓冲获取本地buffer
        NativeBuffer nativebuffer = NativeBuffers.getNativeBufferFromCache(j);
	//如果获取本地buffer为空，则创建一个
        if(nativebuffer == null)
            nativebuffer = NativeBuffers.allocNativeBuffer(j);
        else//不为null，检查本地buffer的拥有者，是则直接返回
        if(nativebuffer.owner() == s)
            return nativebuffer;
        char ac[] = s.toCharArray();
	//否则,拷贝buffer内存
        unsafe.copyMemory(ac, Unsafe.ARRAY_CHAR_BASE_OFFSET, null, nativebuffer.address(), i);
        unsafe.putChar(nativebuffer.address() + (long)i, '\0');
	//设置buffer的拥有者
        nativebuffer.setOwner(s);
        return nativebuffer;
    }

2.b

//释放nativebuffer，放入线程本地缓存，以便重用
 nativebuffer.release();

//NativeBuffers

void release()
    {
        //委托给NativeBuffers
        NativeBuffers.releaseNativeBuffer(this);
    }

从以上a,b两点,我们需要关注的是getNativeBufferFromCache，allocNativeBuffer，releaseNativeBuffer方法，
下面我们单独来看一下NativeBuffers

class NativeBuffers
{
    private static native void initIDs();
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    static 
    {
        //在当前线程访问控制权限下，加载net和nio库
        AccessController.doPrivileged(new PrivilegedAction() {

            public Void run()
            {
                System.loadLibrary("net");
                System.loadLibrary("nio");
                return null;
            }

            public volatile Object run()
            {
                return run();
            }

        });
        initIDs();
    }
    private NativeBuffers()
    {
    }
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final int TEMP_BUF_POOL_SIZE = 3;//临时buffer
    private static ThreadLocal threadLocal = new ThreadLocal();//存放线程本地buffer
    static final boolean $assertionsDisabled = !sun/nio/fs/NativeBuffers.desiredAssertionStatus();
    //从线程本地缓存获取NativeBuffer
     static NativeBuffer getNativeBufferFromCache(int i)
    {
        //从线程本地缓存获取NativeBuffer数组
        NativeBuffer anativebuffer[] = (NativeBuffer[])threadLocal.get();
        if(anativebuffer != null)
        {
            for(int j = 0; j < 3; j++)
            {
                NativeBuffer nativebuffer = anativebuffer[j];
                if(nativebuffer != null && nativebuffer.size() >= i)
                {
		    //返回线程本地缓存NativeBuffer数组中，第一个可用的NativeBuffer(容量大于i)，
		    //将NativeBuffer数组index索引对应的置null
                    anativebuffer[j] = null;
                    return nativebuffer;
                }
            }

        }
        return null;
    }
    //创建本地buffer
     static NativeBuffer allocNativeBuffer(int i)
    {
        if(i < 2048)
            i = 2048;
	//size最小为2M
        return new NativeBuffer(i);
    }
    //获取容量大于等于i的NativeBuffer
     static NativeBuffer getNativeBuffer(int i)
    {
       //从线程本地缓存获取NativeBuffer
        NativeBuffer nativebuffer = getNativeBufferFromCache(i);
        if(nativebuffer != null)
        {
	   //拥有者不为null，则置null
            nativebuffer.setOwner(null);
            return nativebuffer;
        } else
        {
	    //否则创建一个NativeBuffer
            return allocNativeBuffer(i);
        }
    }
    //将字节序列存放到NativeBuffer
     static NativeBuffer asNativeBuffer(byte abyte0[])
    {
        //从线程本地缓存获取NativeBuffer
        NativeBuffer nativebuffer = getNativeBuffer(abyte0.length + 1);
         //拷贝字符串到nativebuffer
        copyCStringToNativeBuffer(abyte0, nativebuffer);
        return nativebuffer;
    }
    //拷贝字符串到nativebuffer
     static void copyCStringToNativeBuffer(byte abyte0[], NativeBuffer nativebuffer)
    {
        long l = Unsafe.ARRAY_BYTE_BASE_OFFSET;
        long l1 = abyte0.length;
	//断言开启，断言nativebuffer容量是否够用，不够用，则抛AssertionError
        if(!$assertionsDisabled && (long)nativebuffer.size() < l1 + 1L)
        {
            throw new AssertionError();
        } else
        {
            unsafe.copyMemory(abyte0, l, null, nativebuffer.address(), l1);
            unsafe.putByte(nativebuffer.address() + l1, (byte)0);
            return;
        }
    }
    //释放nativebuffer
    static void releaseNativeBuffer(NativeBuffer nativebuffer)
    {
        //从线程本地缓存获取NativeBuffer数组
        NativeBuffer anativebuffer[] = (NativeBuffer[])threadLocal.get();
        if(anativebuffer == null)
        {
	    //如果数组为空，则创长度为3的NativeBuffer数组，并将nativebuffer放入缓存中
            anativebuffer = new NativeBuffer[3];
            anativebuffer[0] = nativebuffer;
	    //将NativeBuffer数组添加到线程本地缓存
            threadLocal.set(anativebuffer);
            return;
        }
	//NativeBuffer数组不为null
        for(int i = 0; i < 3; i++)
            if(anativebuffer[i] == null)
            {
	        //将nativebuffer放到线程本地缓冲NativeBuffer数组，
		//索引对应的NativeBuffer为null的位置上
                anativebuffer[i] = nativebuffer;
                return;
            }
        //如果NativeBuffer的元素没有为null的，则将nativebuffer放在第一个容量小于它的index上，
	//并释放小于nativebuffer的内存空间
        for(int j = 0; j < 3; j++)
        {
            NativeBuffer nativebuffer1 = anativebuffer[j];
	    //将
            if(nativebuffer1.size() < nativebuffer.size())
            {
                nativebuffer1.cleaner().clean();
                anativebuffer[j] = nativebuffer;
                return;
            }
        }
        //释放nativebuffer内存空间
        nativebuffer.cleaner().clean();
    }
    //这里之所以将释放的NativeBuffer放在线程本地缓存中，主要为了重用NativeBuffer，因为NativeBuffer直接操作底层
    内存，创建一个要耗费一定的系统资源。

}

 //委托给FileChannelImpl
return FileChannelImpl.open(filedescriptor, flags.read, flags.write, flags.append, null);

在这一步我们又看到了FileChannelImpl#open方法，这个我们在后面再看。

[size=medium][b]总结：[/b][/size]
[color=blue]获取区文件的通道一共有四种，第一种从FileOutputStream获取写模式文件通道，第二种从FileInputStream获取读模式文件通道，第三种从RandomAccessFile获取读写模式文件通道，第四种调用FileChannelImpl#open方法，这个过程首先从参数文件Path（WindowsPath）获取文件系统的提供者，实际为WindowsFileSystemProvider，委托给WindowsFileSystemProvider创建文件通道，WindowsFileSystemProvider根据WindowsPath和，文件属性WindowsSecurityDescriptor（FileAttribute[]），和打开选项集，将实际创建通道任务交给WindowsChannelFactory，WindowsChannelFactory首先将打开选项装换为内部的通道配置标志Flags（读写模式（read，writer），同步方式（sync，dsync），append等），然后根据Flags,和Path信息等信息创建文件，创建文件实际由WindowsNativeDispatcher完成。WindowsNativeDispatcher首先从线程本地缓存获取NativeBuffer，将Path信息放在NativeBuffer中，然后创建文件，创建后，将NativeBuffer释放，即放入线程本地缓存，以便重用。具体选择哪种方式，根据需要选择。[/color]

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文件通道创建方式综述

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