深入浅出NIO之Channel、Buffer

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前言

Java NIO 由以下几个核心部分组成:
1 、Buffer
2、Channel
3、Selector

传统的IO操作面向数据流,意味着每次从流中读一个或多个字节,直至完成,数据没有被缓存在任何地方。

NIO操作面向缓冲区,数据从Channel读取到Buffer缓冲区,随后在Buffer中处理数据。

本文着重介绍Channel和Buffer的概念以及在文件读写方面的应用和内部实现原理。

Buffer

A buffer is a linear, finite sequence of elements of a specific primitive type.

一块缓存区,内部使用字节数组存储数据,并维护几个特殊变量,实现数据的反复利用。
1、mark:初始值为-1,用于备份当前的position;
2、position:初始值为0,position表示当前可以写入或读取数据的位置,当写入或读取一个数据后,position向前移动到下一个位置;
3、limit:写模式下,limit表示最多能往Buffer里写多少数据,等于capacity值;读模式下,limit表示最多可以读取多少数据。
4、capacity:缓存数组大小

mark():把当前的position赋值给mark

public final Buffer mark() {
    mark = position;
    return this;
}

reset():把mark值还原给position

public final Buffer reset() {
    int m = mark;
    if (m < 0)
        throw new InvalidMarkException();
    position = m;
    return this;
}

clear():一旦读完Buffer中的数据,需要让Buffer准备好再次被写入,clear会恢复状态值,但不会擦除数据。

public final Buffer clear() {
    position = 0;
    limit = capacity;
    mark = -1;
    return this;
}

flip():Buffer有两种模式,写模式和读模式,flip后Buffer从写模式变成读模式。

public final Buffer flip() {
    limit = position;
    position = 0;
    mark = -1;
    return this;
}

rewind():重置position为0,从头读写数据。

public final Buffer rewind() {
    position = 0;
    mark = -1;
    return this;
}

目前Buffer的实现类有以下几种:

  • ByteBuffer
  • CharBuffer
  • DoubleBuffer
  • FloatBuffer
  • IntBuffer
  • LongBuffer
  • ShortBuffer
  • MappedByteBuffer

ByteBuffer

A byte buffer,extend from Buffer

ByteBuffer的实现类包括"HeapByteBuffer"和"DirectByteBuffer"两种。

HeapByteBuffer

public static ByteBuffer allocate(int capacity) {
    if (capacity < 0)
        throw new IllegalArgumentException();
    return new HeapByteBuffer(capacity, capacity);
}
HeapByteBuffer(int cap, int lim) {  
    super(-1, 0, lim, cap, new byte[cap], 0);
}

HeapByteBuffer通过初始化字节数组hd,在虚拟机堆上申请内存空间。

DirectByteBuffer

public static ByteBuffer allocateDirect(int capacity) {
    return new DirectByteBuffer(capacity);
}
DirectByteBuffer(int cap) {
    super(-1, 0, cap, cap);
    boolean pa = VM.isDirectMemoryPageAligned();
    int ps = Bits.pageSize();
    long size = Math.max(1L, (long)cap + (pa ? ps : 0));
    Bits.reserveMemory(size, cap);

    long base = 0;
    try {
        base = unsafe.allocateMemory(size);
    } catch (OutOfMemoryError x) {
        Bits.unreserveMemory(size, cap);
        throw x;
    }
    unsafe.setMemory(base, size, (byte) 0);
    if (pa && (base % ps != 0)) {
        // Round up to page boundary
        address = base + ps - (base & (ps - 1));
    } else {
        address = base;
    }
    cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
    att = null;
}

DirectByteBuffer通过unsafe.allocateMemory申请堆外内存,并在ByteBuffer的address变量中维护指向该内存的地址。
unsafe.setMemory(base, size, (byte) 0)方法把新申请的内存数据清零。

Channel

A channel represents an open connection to an entity such as a hardware device, a file, a network socket, or a program component that is capable of performing one or more distinct I/O operations, for example reading or writing.

NIO把它支持的I/O对象抽象为Channel,Channel又称“通道”,类似于原I/O中的流(Stream),但有所区别:
1、流是单向的,通道是双向的,可读可写。
2、流读写是阻塞的,通道可以异步读写。
3、流中的数据可以选择性的先读到缓存中,通道的数据总是要先读到一个缓存中,或从缓存中写入,如下所示:

目前已知Channel的实现类有:

  • FileChannel
  • DatagramChannel
  • SocketChannel
  • ServerSocketChannel

FileChannel

A channel for reading, writing, mapping, and manipulating a file.
一个用来写、读、映射和操作文件的通道。

FileChannel的read、write和map通过其实现类FileChannelImpl实现。

read实现
public int read(ByteBuffer dst) throws IOException {
    ensureOpen();
    if (!readable)
        throw new NonReadableChannelException();
    synchronized (positionLock) {
        int n = 0;
        int ti = -1;
        try {
            begin();
            ti = threads.add();
            if (!isOpen())
                return 0;
            do {
                n = IOUtil.read(fd, dst, -1, nd);
            } while ((n == IOStatus.INTERRUPTED) && isOpen());
            return IOStatus.normalize(n);
        } finally {
            threads.remove(ti);
            end(n > 0);
            assert IOStatus.check(n);
        }
    }
}

FileChannelImpl的read方法通过IOUtil的read实现:

static int read(FileDescriptor fd, ByteBuffer dst, long position,
                NativeDispatcher nd) IOException {
    if (dst.isReadOnly())
        throw new IllegalArgumentException("Read-only buffer");
    if (dst instanceof DirectBuffer)
        return readIntoNativeBuffer(fd, dst, position, nd);

    // Substitute a native buffer
    ByteBuffer bb = Util.getTemporaryDirectBuffer(dst.remaining());
    try {
        int n = readIntoNativeBuffer(fd, bb, position, nd);
        bb.flip();
        if (n > 0)
            dst.put(bb);
        return n;
    } finally {
        Util.offerFirstTemporaryDirectBuffer(bb);
    }
}

通过上述实现可以看出,基于channel的文件数据读取步骤如下:
1、申请一块和缓存同大小的DirectByteBuffer bb。
2、读取数据到缓存bb,底层由NativeDispatcher的read实现。
3、把bb的数据读取到dst(用户定义的缓存,在jvm中分配内存)。
read方法导致数据复制了两次

write实现
public int write(ByteBuffer src) throws IOException {
    ensureOpen();
    if (!writable)
        throw new NonWritableChannelException();
    synchronized (positionLock) {
        int n = 0;
        int ti = -1;
        try {
            begin();
            ti = threads.add();
            if (!isOpen())
                return 0;
            do {
                n = IOUtil.write(fd, src, -1, nd);
            } while ((n == IOStatus.INTERRUPTED) && isOpen());
            return IOStatus.normalize(n);
        } finally {
            threads.remove(ti);
            end(n > 0);
            assert IOStatus.check(n);
        }
    }
}

和read实现一样,FileChannelImpl的write方法通过IOUtil的write实现:

static int write(FileDescriptor fd, ByteBuffer src, long position,
                 NativeDispatcher nd) throws IOException {
    if (src instanceof DirectBuffer)
        return writeFromNativeBuffer(fd, src, position, nd);
    // Substitute a native buffer
    int pos = src.position();
    int lim = src.limit();
    assert (pos <= lim);
    int rem = (pos <= lim ? lim - pos : 0);
    ByteBuffer bb = Util.getTemporaryDirectBuffer(rem);
    try {
        bb.put(src);
        bb.flip();
        // Do not update src until we see how many bytes were written
        src.position(pos);
        int n = writeFromNativeBuffer(fd, bb, position, nd);
        if (n > 0) {
            // now update src
            src.position(pos + n);
        }
        return n;
    } finally {
        Util.offerFirstTemporaryDirectBuffer(bb);
    }
}

通过上述实现可以看出,基于channel的文件数据写入步骤如下:
1、申请一块DirectByteBuffer,bb大小为byteBuffer中的limit - position。
2、复制byteBuffer中的数据到bb中。
3、把数据从bb中写入到文件,底层由NativeDispatcher的write实现,具体如下:

private static int writeFromNativeBuffer(FileDescriptor fd, 
        ByteBuffer bb, long position, NativeDispatcher nd)
    throws IOException {
    int pos = bb.position();
    int lim = bb.limit();
    assert (pos <= lim);
    int rem = (pos <= lim ? lim - pos : 0);

    int written = 0;
    if (rem == 0)
        return 0;
    if (position != -1) {
        written = nd.pwrite(fd,
                            ((DirectBuffer)bb).address() + pos,
                            rem, position);
    } else {
        written = nd.write(fd, ((DirectBuffer)bb).address() + pos, rem);
    }
    if (written > 0)
        bb.position(pos + written);
    return written;
}

write方法也导致了数据复制了两次

Channel和Buffer示例

File file = new RandomAccessFile("data.txt", "rw");
FileChannel channel = file.getChannel();
ByteBuffer buffer = ByteBuffer.allocate(48);

int bytesRead = channel.read(buffer);
while (bytesRead != -1) {
    System.out.println("Read " + bytesRead);
    buffer.flip();
    while(buffer.hasRemaining()){
        System.out.print((char) buffer.get());
    }
    buffer.clear();
    bytesRead = channel.read(buffer);
}
file.close();

注意buffer.flip() 的调用,首先将数据写入到buffer,然后变成读模式,再从buffer中读取数据。

总结

通过本文的介绍,希望大家对Channel和Buffer在文件读写方面的应用和内部实现有了一定了解,努力做到不被一叶障目。

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