【原创】java NIO FileChannel 学习笔记 FileChannel实现分析 即FileChannelImpl分析

上文已经说了FileChannel是一个抽象类，FileChannelImpl是其实现，接下来介绍FileChannelImpl，参考代码来自OpenJDK7

首先 

public class FileChannelImpl extends FileChannel

该类的成员有：

// Memory allocation size for mapping buffers
private static final long allocationGranularity;

// Used to make native read and write calls
private final FileDispatcher nd;

// File descriptor
private final FileDescriptor fd;

// File access mode (immutable)
private final boolean writable;
private final boolean readable;
private final boolean append;

// Required to prevent finalization of creating stream (immutable)
private final Object parent;

// Thread-safe set of IDs of native threads, for signalling
private final NativeThreadSet threads = new NativeThreadSet(2);

// Lock for operations involving position and size
private final Object positionLock = new Object();

//发现NIO这些类里面好多直接拿一个Object当做锁用的，没有用ReentrantLock等显式锁，毕竟1.6之后内置锁的效率已经得到了提升，并且更方便

关于FileDescriptor的描述：

Instances of the file descriptor class serve as an opaque handle to the underlying machine-specific structure representing an open file, an open socket, or another source or sink of bytes. The main practical use for a file descriptor is to create a FileInputStream or FileOutputStream to contain it.

Applications should not create their own file descriptors.

接下来介绍私有构造函数

private FileChannelImpl(FileDescriptor fd, boolean readable,
boolean writable, boolean append, Object parent)
{
this.fd = fd;
this.readable = readable;
this.writable = writable;
this.append = append;
this.parent = parent;
this.nd = new FileDispatcherImpl(append);
}

既然是私有的那么我们只能使用Open来获取一个实例，关于参数总的parent 我认为是这个FileChannel对应的FileInputStream之类。

// Used by FileInputStream.getChannel() and RandomAccessFile.getChannel()
public static FileChannel open(FileDescriptor fd,
boolean readable, boolean writable,
Object parent)
{
return new FileChannelImpl(fd, readable, writable, false, parent);
}

// Used by FileOutputStream.getChannel
public static FileChannel open(FileDescriptor fd,
boolean readable, boolean writable,
boolean append, Object parent)
{
return new FileChannelImpl(fd, readable, writable, append, parent);
}

接下来将介绍FileChannel的close 、read 和write position方法

先说close方法是AbstractInterruptibleChannel中定义的方法。一下是关于该类的简单信息

public abstract class AbstractInterruptibleChannel
implements Channel, InterruptibleChannel

该类中close的定义：

public final void close() throws IOException {
synchronized (closeLock) {
　　if (!open)
　　return;
　　open = false;
　　implCloseChannel();
　　}
}

先说closeLock,  声明为一个final 类成员 private final Object closeLock = new Object();

再说implCloseChannel()       protected abstract void implCloseChannel() throws IOException;显而易见继承该抽象类的类来实现该抽象方法

FileChannelImpl的implCloseChannel() 方法

protected void implCloseChannel() throws IOException {
// Release and invalidate any locks that we still hold
if (fileLockTable != null) {
　　for (FileLock fl: fileLockTable.removeAll()) {
　　synchronized (fl) {
　　　　if (fl.isValid()) {
　　　　　　　　　　　　nd.release(fd, fl.position(), fl.size());
　　　　　　　　　　　　((FileLockImpl)fl).invalidate();
　　　　　　}
　　　　}
　　　}
}

nd.preClose(fd);
threads.signalAndWait();

if (parent != null) {

// Close the fd via the parent stream's close method. The parent
// will reinvoke our close method, which is defined in the
// superclass AbstractInterruptibleChannel, but the isOpen logic in
// that method will prevent this method from being reinvoked.
//
((java.io.Closeable)parent).close();
} else {
nd.close(fd);
}

}