文件通道解析二（文件锁，关闭通道）

文件通道解析一（读写操作，通道数据传输等）:http://donald-draper.iteye.com/blog/2374603
引言：
上一篇文章看了文件通道，读写操作，通道数据传输操作，先来回顾一下：
    文件通道的构造，主要是初始化通道读写模式，追加模式append及文件分发器，FileDispatcherImpl。
    文件通道的读写操作的实际操作都是由IOUtil协助FileDispatcherImpl完成，这一点和SocketChannel通道读写思路基本相同。
    文件通道传输方法transferTo，首先确保当前文件通道是否打开，是否可读，然后检查目的通道是否关闭，是否可写；然后先调用文件通道本地方法传输通道的数据到目的通道，如果失败，则将文件通道数据，映射到内存MappedByteBuffer，然后调用目的通道的写操作（MappedByteBuffer），如果再失败，则将通道数据，写到DirectByteBuffer中，然后在调用目的通道的写操作（DirectByteBuffer）。
     文件通道传输方法transferFrom，确保当前通道可写，打开，源通道可读打开，如果源通道为文件通道，将源通道数据映射的内存MappedByteBuffer，然后由IOUtil协助FileDispatcherImpl，将MappedByteBuffer
写入当前通道，如果源通道非文件通道，则先调用源通道的读操作，从源通道读取数据，写到临时DirectByteBuffer，委托write，写DirectByteBuffer到当前通道，即由IOUtil协助FileDispatcherImpl，将DirectByteBuffer写入当前通道。
在往下看之前我们先把文件通道的相关filed贴出，以便我们理解:

public class FileChannelImpl extends FileChannel
{
 private static final long allocationGranularity = initIDs();
    private final FileDispatcher nd;//文件分发器
    private final FileDescriptor fd;//文件描述
    private final boolean writable;//通道是否可写
    private final boolean readable;//通道是否可读
    private final boolean append;//通道写是否为追加模式
    private final Object parent;//创建通道的对象
    //下面这些属性，暂时不能确定是具体什么意思，只能先从字面上去理解，
    //这里我们先放在这里，后面用到在讲
    private final NativeThreadSet threads = new NativeThreadSet(2);
    private final Object positionLock = new Object();//文件读写是位置锁
    private static volatile boolean transferSupported = true;//是否支持通道传输
    private static volatile boolean pipeSupported = true;//是否支持管道
    private static volatile boolean fileSupported = true;//是否支持文件
    private static final long MAPPED_TRANSFER_SIZE = 8388608L;/
    private static final int TRANSFER_SIZE = 8192;
    private static final int MAP_RO = 0;
    private static final int MAP_RW = 1;
    private static final int MAP_PV = 2;
    private volatile FileLockTable fileLockTable;//存放文件锁的Table
    private static boolean isSharedFileLockTable;//文件锁table是否为共享
    private static volatile boolean propertyChecked;
    static final boolean $assertionsDisabled = !sun/nio/ch/FileChannelImpl.desiredAssertionStatus();

    static 
    {
        //加载nio，net资源库
        Util.load();
    }
     private static native long initIDs();
 }

现在我们来文件的lock和trylock方法
再来看锁文件region方法：

public FileLock lock(long l, long l1, boolean flag)
        throws IOException
    {
        FileLockImpl filelockimpl;
        FileLockTable filelocktable;
        boolean flag1;
        int i;
        ensureOpen();
        if(flag && !readable)//如果写模式，锁不能为共享模式
            throw new NonReadableChannelException();
        if(!flag && !writable)//如果为读模，则锁必须共享模式
            throw new NonWritableChannelException();
	//创建文件锁
        filelockimpl = new FileLockImpl(this, l, l1, flag);
	//获取文件通道的文件锁table
        filelocktable = fileLockTable();
	//将文件锁，加入通道文件锁table中
        filelocktable.add(filelockimpl);
        flag1 = false;
        i = -1;
        FileLock filelock;
        begin();
        ...
        int j;
       //不断的尝试加锁，直到成功
        do
	    //尝试锁文件region
            j = nd.lock(fd, true, l, l1, flag);
        while(j == 2 && isOpen());
        if(isOpen())
        {
            if(j == 1)
            {
                if(!$assertionsDisabled && !flag)
                    throw new AssertionError();
		//创建新的文件锁
                FileLockImpl filelockimpl1 = new FileLockImpl(this, l, l1, false);
		//替换filelocktable旧的文件锁
                filelocktable.replace(filelockimpl, filelockimpl1);
                filelockimpl = filelockimpl1;
            }
            flag1 = true;
        }
	如果锁文件region失败，则从filelocktable移除
        if(!flag1)
            filelocktable.remove(filelockimpl);
        threads.remove(i);
        try
        {
            end(flag1);
        }
        catch(ClosedByInterruptException closedbyinterruptexception)
        {
            throw new FileLockInterruptionException();
        }
        ...
        return filelockimpl;
    }
 

//尝试加锁

public FileLock tryLock(long l, long l1, boolean flag)
        throws IOException
    {
        FileLockImpl filelockimpl;
        FileLockTable filelocktable;
        int j;
        ensureOpen();
        if(flag && !readable)//如果写模式，锁不能为共享模式
            throw new NonReadableChannelException();
        if(!flag && !writable)//如果为读模，则锁必须共享模式
            throw new NonWritableChannelException();
	 //创建文件锁
        filelockimpl = new FileLockImpl(this, l, l1, flag);
        filelocktable = fileLockTable();//获取文件通道的文件锁table
        filelocktable.add(filelockimpl);//将文件锁，加入通道文件锁table中
        j = threads.add();
        int i;
        Object obj;
        try
        {
            ensureOpen();
	    //尝试锁文件region
            i = nd.lock(fd, false, l, l1, flag);
        }
        catch(IOException ioexception)
        {
            filelocktable.remove(filelockimpl);
            throw ioexception;
        }
        ...
	//创建新的文件锁
        obj = new FileLockImpl(this, l, l1, false);
	//替换filelocktable旧的文件锁
        filelocktable.replace(filelockimpl, ((FileLock) (obj)));
        filelockimpl1 = ((FileLockImpl) (obj));
        threads.remove(j);
        return filelockimpl1;
        ...
    }

从lock方法和try方法来看，首先检查共享模式参数shared与当前通道的读写模式是否匹配，然后根具postion，size和shared信息构造文件锁FileLockImpl，添加到通道文件锁表filelocktable中，再通过文件分发器FileDispatcherImpl锁文件region，如果成功，则创建新的文件锁替换旧的文件锁。如果lock失败，则从通道文件锁表filelocktable移除先前添加的文件锁。这里为什么要先创建文件锁添加到通道文件锁表filelocktable中，可能是为了先抢占通道文件锁表的位置，再去通FileDispatcherImpl锁文件region，成功则创建新的文件锁替换旧的文件锁。lock方法和trylock方法不同的是在FileDispatcherImpl锁文件region这个过程，lock为循环文件region，直到成功，而trylock方法，只锁一次，成功则则创建新的文件锁替换旧的文件锁，失败则从通道文件锁表filelocktable移除先前添加的文件锁，返回。

lock方法和trylock方法我们几点要关注：
1.

 //创建文件锁
filelockimpl = new FileLockImpl(this, l, l1, flag);

2.

filelocktable = fileLockTable();//获取文件通道的文件锁table

3.

j = nd.lock(fd, true, l, l1, flag);

下面分别来看这几点：
1.

 //创建文件锁
filelockimpl = new FileLockImpl(this, l, l1, flag);

//FileLockImpl

public class FileLockImpl extends FileLock
{
    private volatile boolean valid;//有效性标志
    static final boolean $assertionsDisabled = !sun/nio/ch/FileLockImpl.desiredAssertionStatus();
    //构造
    FileLockImpl(FileChannel filechannel, long l, long l1, boolean flag)
    {
        super(filechannel, l, l1, flag);
        valid = true;//初始化有效
    }

    FileLockImpl(AsynchronousFileChannel asynchronousfilechannel, long l, long l1, boolean flag)
    {
        super(asynchronousfilechannel, l, l1, flag);
        valid = true;
    }
    //判断文件锁是否有效
    public boolean isValid()
    {
        return valid;
    }
    //使文件锁无效
    void invalidate()
    {
        //如果断言开启，断言线程是否持有锁
        if(!$assertionsDisabled && !Thread.holdsLock(this))
        {
            throw new AssertionError();
        } else
        {
            valid = false;
            return;
        }
    }
    //释放文件锁
    public synchronized void release()
        throws IOException
    {
       //获取锁关联的通道
        Channel channel = acquiredBy();
        if(!channel.isOpen())//通道关闭
            throw new ClosedChannelException();
        if(valid)//在通道有效的情况下
        {
            if(channel instanceof FileChannelImpl)
	        //将实际工作委托相应的通道
                ((FileChannelImpl)channel).release(this);
            else
            if(channel instanceof AsynchronousFileChannelImpl)
                ((AsynchronousFileChannelImpl)channel).release(this);
            else
                throw new AssertionError();
            valid = false;
        }
    }
}

//FileLock

public abstract class FileLock implements AutoCloseable {
    private final Channel channel;//锁关联通道
    private final long position;//文件锁region的起始位置
    private final long size;//region大小
    private final boolean shared;//是否为共享模式
    //构造文件锁
    protected FileLock(FileChannel channel,
                       long position, long size, boolean shared)
    {
        if (position < 0)
            throw new IllegalArgumentException("Negative position");
        if (size < 0)
            throw new IllegalArgumentException("Negative size");
        if (position + size < 0)
            throw new IllegalArgumentException("Negative position + size");
        this.channel = channel;
        this.position = position;
        this.size = size;
        this.shared = shared;
    }
   /**
     * Returns the file channel upon whose file this lock was acquired.
     *返回用有本文件锁的通道
     *  This method has been superseded by the {@link #acquiredBy acquiredBy}
     * method.
     *
     * @return  The file channel, or {@code null} if the file lock was not
     *          acquired by a file channel.
     */
    public final FileChannel channel() {
        return (channel instanceof FileChannel) ? (FileChannel)channel : null;
    }
     /**
     * Returns the channel upon whose file this lock was acquired.
     *//获取锁关联通道
     * @return  The channel upon whose file this lock was acquired.
     *
     * @since 1.7
     */
    public Channel acquiredBy() {
        return channel;
    }
    /**
     * Returns the position within the file of the first byte of the locked
     * region.
     *获取锁文件region的起始position
     *  A locked region need not be contained within, or even overlap, the
     * actual underlying file, so the value returned by this method may exceed
     * the file's current size.  

     *
     * @return  The position
     */
    public final long position() {
        return position;
    }

    /**
     * Returns the size of the locked region in bytes.
     *获取锁文件region的size
     *  A locked region need not be contained within, or even overlap, the
     * actual underlying file, so the value returned by this method may exceed
     * the file's current size.  

     *
     * @return  The size of the locked region
     */
    public final long size() {
        return size;
    }

    /**
     * Tells whether this lock is shared.  
     *判断锁是否为共享模式
     * @return <tt>true</tt> if lock is shared,
     *         <tt>false</tt> if it is exclusive
     */
    public final boolean isShared() {
        return shared;
    }

    /**
     * Tells whether or not this lock overlaps the given lock range.  </p>
     *//判断加锁的文件region是否重叠
     * @return  <tt>true</tt> if, and only if, this lock and the given lock
     *          range overlap by at least one byte
     */
    public final boolean overlaps(long position, long size) {
        if (position + size <= this.position)
            return false;               // That is below this
        if (this.position + this.size <= position)
            return false;               // This is below that
        return true;
    }

    /**
     * Tells whether or not this lock is valid.
     *判断锁是否有效
     *  A lock object remains valid until it is released or the associated
     * file channel is closed, whichever comes first.  

     *
     * @return  <tt>true</tt> if, and only if, this lock is valid
     */
    public abstract boolean isValid();

    /**
     * Releases this lock.
     *
     *  If this lock object is valid then invoking this method releases the
     * lock and renders the object invalid.  If this lock object is invalid
     * then invoking this method has no effect.  

     *释放锁
     * @throws  ClosedChannelException
     *          If the channel that was used to acquire this lock
     *          is no longer open
     *
     * @throws  IOException
     *          If an I/O error occurs
     */
    public abstract void release() throws IOException;

    /**
     * This method invokes the {@link #release} method. It was added
     * to the class so that it could be used in conjunction with the
     * automatic resource management block construct.
     *关闭文件锁
     * @since 1.7
     */
    public final void close() throws IOException {
        release();
    }
}

2.

filelocktable = fileLockTable();//获取文件通道的文件锁table

//获取文件通道的文件锁table
private FileLockTable fileLockTable()
        throws IOException
    {
        if(fileLockTable != null)
            break MISSING_BLOCK_LABEL_96;
        FileChannelImpl filechannelimpl = this;
        JVM INSTR monitorenter ;
        int i;
        if(fileLockTable != null)
            break MISSING_BLOCK_LABEL_84;
        if(!isSharedFileLockTable())
            break MISSING_BLOCK_LABEL_73;
        i = threads.add();
        ensureOpen();
	//如果为共享模式
        fileLockTable = FileLockTable.newSharedFileLockTable(this, fd);
        threads.remove(i);
        break MISSING_BLOCK_LABEL_84;
        Exception exception;
        exception;
        threads.remove(i);
        throw exception;
	//否则
        fileLockTable = new SimpleFileLockTable();
        break MISSING_BLOCK_LABEL_96;
        Exception exception1;
        exception1;
        throw exception1;
        return fileLockTable;
    }

从上面可以看出如果为通道文件锁表为共享模式，则创建SharedFileLockTable，否则为
SimpleFileLockTable；
上述方法中break MISSING_BLOCK_LABEL_73类似的语句，这就由于反编译错误导致的，break语句有两种，一种为平时我们用的无label的break，即在for或while，switch语句中直接break;,break还有一种方式为break Label；即跳出循环到指定的Label：
实例：

testLable:
for (obj : list){
     if (flag) 
        break testLable;
}

由于反编译插件无法识别这样的语句，所以出现break MISSING_BLOCK_LABEL_73类似的语句。

Branching Statements-The break Statement：http://docs.oracle.com/javase/tutorial/java/nutsandbolts/branch.html
Dealing with labels in decompiled code：http://*.com/questions/6347930/dealing-with-labels-in-decompiled-code
我们先看一下FileLockTable的定义

abstract class FileLockTable
{
    protected FileLockTable()
    {
    }
    //创建共享文件锁表
    public static FileLockTable newSharedFileLockTable(Channel channel, FileDescriptor filedescriptor)
        throws IOException
    {
        return new SharedFileLockTable(channel, filedescriptor);
    }
    public abstract void add(FileLock filelock)//添加
        throws OverlappingFileLockException;
    public abstract void remove(FileLock filelock);//移除
    public abstract List removeAll();//移除所有
    public abstract void replace(FileLock filelock, FileLock filelock1);//替换
}

再来看SimpleFileLockTable
//FileChannelImpl-SimpleFileLockTable

private static class SimpleFileLockTable extends FileLockTable
    {
        private final List lockList = new ArrayList(2);//文件锁集合
        static final boolean $assertionsDisabled = !sun/nio/ch/FileChannelImpl.desiredAssertionStatus();
        public SimpleFileLockTable()
        {
        }
	//添加文件锁
	 public void add(FileLock filelock)
            throws OverlappingFileLockException
        {
            synchronized(lockList)
            {
                checkList(filelock.position(), filelock.size());
                lockList.add(filelock);
            }
        }
	//查看添加的文件锁，是否与文件锁集合中，已经加锁的文件region重叠
	private void checkList(long l, long l1)
            throws OverlappingFileLockException
        {
	   //如果断言开启，当前线程不只有文件锁集合，则抛出AssertionError
            if(!$assertionsDisabled && !Thread.holdsLock(lockList))
                throw new AssertionError();
            //遍历当前文件锁集合，查看添加的文件锁，是否与已经加锁的文件region重叠，
	    //重叠则抛出OverlappingFileLockException
            for(Iterator iterator = lockList.iterator(); iterator.hasNext();)
            {
                FileLock filelock = (FileLock)iterator.next();
                if(filelock.overlaps(l, l1))
                    throw new OverlappingFileLockException();
            }

        }
        //移除文件锁
        public void remove(FileLock filelock)
        {
            synchronized(lockList)
            {
                lockList.remove(filelock);
            }
        }
       //移除所有文件锁
        public List removeAll()
        {
            List list = lockList;
            JVM INSTR monitorenter ;
            ArrayList arraylist;
            arraylist = new ArrayList(lockList);
            lockList.clear();//直接clear文件锁集合
            return arraylist;
            Exception exception;
            exception;
            throw exception;
        }
       //替换文件锁，先移除filelock，后添加filelock1
        public void replace(FileLock filelock, FileLock filelock1)
        {
            synchronized(lockList)
            {
                lockList.remove(filelock);
                lockList.add(filelock1);
            }
        }
    }

再来看SharedFileLockTable

class SharedFileLockTable extends FileLockTable
{
    private static ConcurrentHashMap lockMap = new ConcurrentHashMap();//文件锁Map
    private static ReferenceQueue queue = new ReferenceQueue();
    private final Channel channel;//关联通道
    private final FileKey fileKey;//文件锁key
    static final boolean $assertionsDisabled = !sun/nio/ch/SharedFileLockTable.desiredAssertionStatus();
    //文件锁引用
    private static class FileLockReference extends WeakReference
    {

        FileKey fileKey()
        {
            return fileKey;
        }

        private FileKey fileKey;

        FileLockReference(FileLock filelock, ReferenceQueue referencequeue, FileKey filekey)
        {
            super(filelock, referencequeue);
            fileKey = filekey;
        }
    }

    //构造共享文件锁table
    SharedFileLockTable(Channel channel1, FileDescriptor filedescriptor)
        throws IOException
    {
        channel = channel1;
	//创建文件key
        fileKey = FileKey.create(filedescriptor);
    }
    //添加文件锁
    public void add(FileLock filelock)
        throws OverlappingFileLockException
    {
       //从文件锁Map获取，文件key对应的锁集合
        Object obj = (List)lockMap.get(fileKey);
_L3:
        List list;
label0:
        {
            if(obj != null)
                break MISSING_BLOCK_LABEL_95;
	    //文件锁集合为null，则创建共享锁集合
            obj = new ArrayList(2);
            synchronized(obj)
            {
	        //将文件key与文件锁集合放入Map中
                list = (List)lockMap.putIfAbsent(fileKey, obj);
                if(list != null)
                    break label0;
		//将文件锁索引添加到锁集合中
                ((List) (obj)).add(new FileLockReference(filelock, queue, fileKey));
            }
            break; /* Loop/switch isn't completed */
        }
        obj2;
        JVM INSTR monitorexit ;
          goto _L1
        exception;
        throw exception;
_L1:
        obj = list;
        List list1;
label1:
        {
            synchronized(obj)
            {
                list1 = (List)lockMap.get(fileKey);
                if(obj != list1)
                    break label1;
                checkList(((List) (obj)), filelock.position(), filelock.size());
                ((List) (obj)).add(new FileLockReference(filelock, queue, fileKey));
            }
            break; /* Loop/switch isn't completed */
        }
        obj = list1;
        obj1;
        JVM INSTR monitorexit ;
        if(true) goto _L3; else goto _L2
        exception1;
        throw exception1;
_L2:
        removeStaleEntries();
        return;
    }
    //如果文件key对应的，文件锁集合为空，则从map中移除文件key Entry
    private void removeKeyIfEmpty(FileKey filekey, List list)
    {
        if(!$assertionsDisabled && !Thread.holdsLock(list))
            throw new AssertionError();
        if(!$assertionsDisabled && lockMap.get(filekey) != list)
            throw new AssertionError();
        if(list.isEmpty())
            lockMap.remove(filekey);
    }
    //移除文件锁
    public void remove(FileLock filelock)
    {
        if(!$assertionsDisabled && filelock == null)
            throw new AssertionError();
	//从锁Map中获取文件key对应的集合 
        List list = (List)lockMap.get(fileKey);
        if(list == null)
            return;
        synchronized(list)
        {
            int i = 0;
            do
            {
                if(i >= list.size())
                    break;
		//获取文件锁引用
                FileLockReference filelockreference = (FileLockReference)list.get(i);
		//参文件锁引用获取文件锁
                FileLock filelock1 = (FileLock)filelockreference.get();
                if(filelock1 == filelock)
                {
                    if(!$assertionsDisabled && (filelock1 == null || filelock1.acquiredBy() != channel))
                        throw new AssertionError();
		    //找到，则清除引用，help GC
                    filelockreference.clear();
                    list.remove(i);//从文件锁集合中移除
                    break;
                }
                i++;
            } while(true);
        }
    }
    //移除所有文件锁
    public List removeAll()
    {
        ArrayList arraylist = new ArrayList();
	//获取文件key的文件锁集合
        List list = (List)lockMap.get(fileKey);
        if(list != null)
            synchronized(list)
            {
	        //遍历锁集合，将通读文件的文件锁添加到arraylist
                for(int i = 0; i < list.size();)
                {
                    FileLockReference filelockreference = (FileLockReference)list.get(i);
                    FileLock filelock = (FileLock)filelockreference.get();
                    if(filelock != null && filelock.acquiredBy() == channel)
                    {
                        filelockreference.clear();
                        list.remove(i);
                        arraylist.add(filelock);
                    } else
                    {
                        i++;
                    }
                }
                //移除文件key的锁集合
                removeKeyIfEmpty(fileKey, list);
            }
        return arraylist;
    }
    //替换文件锁
    public void replace(FileLock filelock, FileLock filelock1)
    {
        //获取文件锁集合
        List list = (List)lockMap.get(fileKey);
        if(!$assertionsDisabled && list == null)
            throw new AssertionError();
        synchronized(list)
        {
            int i = 0;
            do
            {
                if(i >= list.size())
                    break;
                FileLockReference filelockreference = (FileLockReference)list.get(i);
                FileLock filelock2 = (FileLock)filelockreference.get();
                if(filelock2 == filelock)
                {
		    //找到对应的文件锁，则清除引用，help gc
                    filelockreference.clear();
                    list.set(i, new FileLockReference(filelock1, queue, fileKey));//替换文件锁引用
                    break;
                }
                i++;
            } while(true);
        }
    }
   //检查文件key的锁集中的文件锁，锁住文件的region是否重叠
    private void checkList(List list, long l, long l1)
        throws OverlappingFileLockException
    {
        if(!$assertionsDisabled && !Thread.holdsLock(list))
            throw new AssertionError();
        for(Iterator iterator = list.iterator(); iterator.hasNext();)
        {
            FileLockReference filelockreference = (FileLockReference)iterator.next();
            FileLock filelock = (FileLock)filelockreference.get();
            if(filelock != null && filelock.overlaps(l, l1))
                throw new OverlappingFileLockException();
        }

    }
    //清空文件锁Map
    private void removeStaleEntries()
    {
        do
        {
            FileLockReference filelockreference;
	    //从引用队列中取出文件锁引用
            if((filelockreference = (FileLockReference)queue.poll()) == null)
                break;
            //从文件锁引用获取文件key
            FileKey filekey = filelockreference.fileKey();
	    //获取文件key对应的文件锁
            List list = (List)lockMap.get(filekey);
            if(list != null)
                synchronized(list)
                {
                    list.remove(filelockreference);
		    //移除文件key对应的文件锁集合
                    removeKeyIfEmpty(filekey, list);
                }
        } while(true);
    }
}

SharedFileLockTable用于存储文件共享锁，SimpleFileLockTable用于存储文件互质锁；
两者最大的不同是SimpleFileLockTable用list去管理文件锁，而SharedFileLockTable
用于ConcurrentHashMap管理文件锁。
3.

j = nd.lock(fd, true, l, l1, flag);

//FileDispatcherImpl

int lock(FileDescriptor filedescriptor, boolean flag, long l, long l1, boolean flag1)
        throws IOException
    {
        return lock0(filedescriptor, flag, l, l1, flag1);
    }
 static native int lock0(FileDescriptor filedescriptor, boolean flag, long l, long l1, boolean flag1)
        throws IOException;

来看释放文件锁

 void release(FileLockImpl filelockimpl)
        throws IOException
    {
        int i = threads.add();
        ensureOpen();
	//文件分发器先释放锁住的文件region
        nd.release(fd, filelockimpl.position(), filelockimpl.size());
        ...
        if(!$assertionsDisabled && fileLockTable == null)
        {
            throw new AssertionError();
        } else
        {
	    //从文件锁table中移除文件锁
            fileLockTable.remove(filelockimpl);
            return;
        }
    }

来看这一句：

//文件分发器先释放锁住的文件region
nd.release(fd, filelockimpl.position(), filelockimpl.size());

//FileDispatcherImpl

void release(FileDescriptor filedescriptor, long l, long l1)
        throws IOException
    {
        release0(filedescriptor, l, l1);
    }
  static native void release0(FileDescriptor filedescriptor, long l, long l1)
        throws IOException

//获取文件锁table共享模式

private static boolean isSharedFileLockTable()
    {
        if(!propertyChecked)
            synchronized(sun/nio/ch/FileChannelImpl)
            {
                if(!propertyChecked)
                {
                    String s = (String)AccessController.doPrivileged(new GetPropertyAction("sun.nio.ch.disableSystemWideOverlappingFileLockCheck"));
                    isSharedFileLockTable = s == null || s.equals("false");
                    propertyChecked = true;
                }
            }
        return isSharedFileLockTable;
    }

再来看关闭通道

protected void implCloseChannel()
        throws IOException
    {
        if(fileLockTable != null)
        {
	    //清空文件锁table
            for(Iterator iterator = fileLockTable.removeAll().iterator(); iterator.hasNext();)
            {
                FileLock filelock = (FileLock)iterator.next();
                synchronized(filelock)
                {
                    if(filelock.isValid())
                    {
		        //释放文件锁，锁住的文件region
                        nd.release(fd, filelock.position(), filelock.size());
                        ((FileLockImpl)filelock).invalidate();//使文件锁无效
                    }
                }
            }

        }
	//预先关闭文件描述
        nd.preClose(fd);
        threads.signalAndWait();
	//如果父对象不为null，则关闭父对象，否则关闭文件分发器
        if(parent != null)
            ((Closeable)parent).close();
        else
            nd.close(fd);
    }

//FileDispatcherImpl

void close(FileDescriptor filedescriptor)
        throws IOException
    {
        close0(filedescriptor);
    }
 static native void close0(FileDescriptor filedescriptor)
        throws IOException;

总结：
lock方法和try方法，首先检查共享模式参数shared与当前通道的读写模式是否匹配，然后根据具postion，size和shared信息构造文件锁FileLockImpl，添加到通道文件锁表filelocktable中，再通过文件分发器FileDispatcherImpl锁文件region，如果成功，则创建新的文件锁替换旧的文件锁。如果lock失败，则从通道文件锁表filelocktable移除先前添加的文件锁。这里为什么要先创建文件锁添加到通道文件锁表filelocktable中，可能是为了先抢占通道文件锁表的位置，再去通FileDispatcherImpl锁文件region，成功则创建新的文件锁替换旧的文件锁。lock方法和trylock方法不同的是在FileDispatcherImpl锁文件region这个过程，lock为循环文件region，直到成功，而trylock方法，只锁一次，成功则则创建新的文件锁替换旧的文件锁，失败则从通道文件锁表filelocktable移除先前添加的文件锁，返回。


附：

public class FileKey
{
    private static native void initIDs();
    private long dwVolumeSerialNumber;
    private long nFileIndexHigh;
    private long nFileIndexLow;

    static 
    {
        initIDs();
    }
    private FileKey()
    {
    }

    public static FileKey create(FileDescriptor filedescriptor)
    {
        FileKey filekey = new FileKey();
        try
        {
            filekey.init(filedescriptor);
        }
        catch(IOException ioexception)
        {
            throw new Error(ioexception);
        }
        return filekey;
    }
    private native void init(FileDescriptor filedescriptor)
        throws IOException;
    public int hashCode()
    {
        return (int)(dwVolumeSerialNumber ^ dwVolumeSerialNumber >>> 32) + (int)(nFileIndexHigh ^ nFileIndexHigh >>> 32) + (int)(nFileIndexLow ^ nFileIndexHigh >>> 32);
    }

    public boolean equals(Object obj)
    {
        if(obj == this)
            return true;
        if(!(obj instanceof FileKey))
            return false;
        FileKey filekey = (FileKey)obj;
        return dwVolumeSerialNumber == filekey.dwVolumeSerialNumber && nFileIndexHigh == filekey.nFileIndexHigh && nFileIndexLow == filekey.nFileIndexLow;
    }
}