java-NIO-ByteBuffer
ByteBuf
同样是例子
RandomAccessFile randFile = new RandomAccessFile(filename, "rw");
FileChannel inChannel = randFile.getChannel();
ByteBuffer buf = ByteBuffer.allocate(48);
int bytesRead = inChannel.read(buf); //read into buffer.
while (bytesRead != -1) {
buf.flip(); //make buffer ready for read
while(buf.hasRemaining()){
System.out.print((char) buf.get()); // read 1 byte at a time
}
buf.clear(); //make buffer ready for writing
bytesRead = inChannel.read(buf);
}
randFile.close();
用来存储byte类型的缓冲区，可以在这种缓冲区中存储任意其他基本类型的二进制值
此外还具有其他类型的buf
CharBuffer
DoubleBuffer
FloatBuffer
IntBuffer
LongBuffer
ShortBuffer
关于buf的特性，这个博客总结的不错http://www.cnblogs.com/youngKen/p/4923635.html
回顾例子代码
ByteBuffer buf = ByteBuffer.allocate(48);
int bytesRead = inChannel.read(buf);
ByteBuffer有两种，一种是堆内存中的，另一种是利用unsafe操作的
申请了
public static ByteBuffer allocate(int capacity) {
if (capacity < 0)
throw new IllegalArgumentException();
return new HeapByteBuffer(capacity, capacity);
}
继承关系
HeapByteBuffer->ByteBuffer->Buffer
HeapByteBuffer
HeapByteBuffer(int cap, int lim) {
super(-1, 0, lim, cap, new byte[cap], 0);
}
进而调用ByteBuffer的构造方法
ByteBuffer(int mark, int pos, int lim, int cap, // package-private
byte[] hb, int offset) {
super(mark, pos, lim, cap);
this.hb = hb;
this.offset = offset;
}
继续构造父类Buffer
Buffer(int mark, int pos, int lim, int cap) { // package-private
if (cap < 0)
throw new IllegalArgumentException("Negative capacity: " + cap);
this.capacity = cap;
limit(lim);
position(pos);
if (mark >= 0) {
if (mark > pos)
throw new IllegalArgumentException("mark > position: ("
+ mark + " > " + pos + ")");
this.mark = mark;
}
}
可以看出ByteBuffer是在Buffer的基础上增加了hb和offset，hb是创建的byte数组
FileChannel的read方法
FileChannelImpl中有个positionLock，首先锁住位置
回到例子中的inChannel.read(buf)把read传入
public int read(ByteBuffer dst) throws IOException {
//验证channel是否打开
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);
}
}
}
begin是父类的方法,为channel设置一个interruptor
protected final void begin() {
if (interruptor == null) {
interruptor = new Interruptible() {
public void interrupt(Thread target) {
synchronized (closeLock) {
if (!open)
return;
open = false;
interrupted = target;
try {
AbstractInterruptibleChannel.this.implCloseChannel();
} catch (IOException x) { }
}
}};
}
blockedOn(interruptor);
Thread me = Thread.currentThread();
if (me.isInterrupted())
interruptor.interrupt(me);
}
implCloseChannel调子类的方法，执行关闭操作
protected void implCloseChannel() throws IOException {
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) {
((java.io.Closeable)parent).close();
} else {
nd.close(fd);
}
}
NativeThreadSet的add方法
在集合中添加native线程
int add() {
long th = NativeThread.current();
// 0 and -1 are treated as placeholders, not real thread handles
if (th == 0)
th = -1;
synchronized (this) {
int start = 0;
if (used >= elts.length) {
int on = elts.length;
int nn = on * 2;
long[] nelts = new long[nn];
System.arraycopy(elts, 0, nelts, 0, on);
elts = nelts;
start = on;
}
for (int i = start; i < elts.length; i++) {
if (elts[i] == 0) {
elts[i] = th;
used++;
return i;
}
}
assert false;
return -1;
}
}
二. 读取方法
读取方法是，把文件描述符关联的文件内容拷贝到临时buf中，然后把临时buf内容拷贝到目标buf中
do {
n = IOUtil.read(fd, dst, -1, nd);
} while ((n == IOStatus.INTERRUPTED) && isOpen());
IOUtil.read方法中，先生成一个临时的DirectByteBuffer，把文件中的内容读取到这个临时buf中，最后把临时buf设置为只读状态，然后写入到目标buf中
static int read(FileDescriptor fd, ByteBuffer dst, long position,
NativeDispatcher nd)
throws IOException {
if (dst.isReadOnly())
throw new IllegalArgumentException("Read-only buffer");
if (dst instanceof DirectBuffer)
return readIntoNativeBuffer(fd, dst, position, nd);
//申请一个临时的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);
}
}
dst.put(bb);的过程如下，就是把bb的内容拷贝到接收buf中，然后位置指针都向前移动
HeapByteBuffer sb = (HeapByteBuffer)src;
//limit-position还能够读取的量
int n = sb.remaining();
//如果写入的内容比可以使用的多，那么溢出
if (n > remaining()) throw new BufferOverflowException();
System.arraycopy(sb.hb, sb.ix(sb.position()), hb, ix(position()), n);
sb.position(sb.position() + n);
position(position() + n);
三. 写入方法
do {
n = IOUtil.write(fd, src, -1, nd);
} while ((n == IOStatus.INTERRUPTED) && isOpen());
不断调用IOUtil的写方法，获取ByteBuffer的pos，lim等
static int write(FileDescriptor fd, ByteBuffer src, long position,
NativeDispatcher nd)
throws IOException {
if (src instanceof DirectBuffer)
return writeFromNativeBuffer(fd, src, position, nd);
int pos = src.position();
int lim = src.limit();
assert (pos <= lim);
int rem = (pos <= lim ? lim - pos : 0);
ByteBuffer bb = Util.getTemporaryDirectBuffer(rem);
try {
//把原始内容写入到临时buf中
bb.put(src);
//临时buf变为读状态
bb.flip();
// 更新positiojn
src.position(pos);
//临时buf内容写到文件中
int n = writeFromNativeBuffer(fd, bb, position, nd);
if (n > 0) {
// now update src
src.position(pos + n);
}
return n;
} finally {
Util.offerFirstTemporaryDirectBuffer(bb);
}
}
getTemporaryDirectBuffer返回一个临时的buffer,这里的bufferCache是一个ThreadLocal，内部存储的BufferCache对象，BufferCache是Util的内部类
这个BufferCache是ByteBuffer的缓存，具体实现可以参见sun.nio.ch.Util的这个内部类
public static ByteBuffer getTemporaryDirectBuffer(int size) {
BufferCache cache = bufferCache.get();
ByteBuffer buf = cache.get(size); //获取一个临时的ByteBuffer，获取至少这么大的
if (buf != null) {
return buf;
} else {
if (!cache.isEmpty()) {
buf = cache.removeFirst();
free(buf);
}
return ByteBuffer.allocateDirect(size);
}
}
四. 其他
public final boolean hasRemaining() {
return position < limit;
}
buf.get()方法只读取一个内容，然后递增index
public byte get() {
return hb[ix(nextGetIndex())];
}
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