Buffer是一个抽象类，位于java.nio包中，主要用作缓冲区。注意：Buffer是非线程安全类。capacity一旦初始化后就不会改变，其值一直为常量。在使用中我们一般使用Buffer的抽象子类ByteBuffer.allocate()方法，实际上是生成ByteArrayBuffer类。

(1)Buffer中定义的变量含义

/** * UNSET_MARK means the mark has not been set. */

static final int UNSET_MARK = -1;

/** * The capacity of this buffer, which never changes. */

final int capacity;

/** * limit - 1 is the last element that can be read or written. * Limit must be no less than zero and no greater than capacity. */

int limit;

/** * Mark is where position will be set when reset() is called. * Mark is not set by default. Mark is always no less than zero and no * greater than position. */

int mark = UNSET_MARK;

/** * The current position of this buffer. Position is always no less than zero * and no greater than limit. */

int position = 0;

/** * The log base 2 of the element size of this buffer. Each typed subclass * (ByteBuffer, CharBuffer, etc.) is responsible for initializing this * value. The value is used by JNI code in frameworks/base/ to avoid the * need for costly 'instanceof' tests. */

final int _elementSizeShift;

/** * For direct buffers, the effective address of the data; zero otherwise. * This is set in the constructor. */

final long effectiveDirectAddress;

(2)clear()方法用于写模式，其作用为清空Buffer中的内容，所谓清空是指写上限与Buffer的真实容量相同，即limit==capacity,同时将当前写位置置为最前端下标为0处。代码如下：

/** * UNSET_MARK means the mark has not been set. */

static final int UNSET_MARK = -1;

/** * Clears this buffer. *

* While the content of this buffer is not changed, the following internal * changes take place: the current position is reset back to the start of * the buffer, the value of the buffer limit is made equal to the capacity * and mark is cleared. * *@return this buffer. */

public final Buffer clear() {

position = 0; //设置当前下标为0

mark = UNSET_MARK; //取消标记

limit = capacity; //设置写越界位置与和Buffer容量相同

return this;

}

(3)reset()方法和clear()方法一样用于写模式，区别是reset()的作用是丢弃mark位置以后的数据，重新从mark位置开始写入，且mark不能未设置；而clear是从0位置开始重新写入。

/** * Resets the position of this buffer to the mark. * *@return this buffer. *@throws InvalidMarkException * if the mark is not set. */

public final Buffer reset() {

if (mark == UNSET_MARK) {

throw new InvalidMarkException("Mark not set");

}

position = mark;

return this;

}

(4)rewind()在读写模式下都可用，它单纯的将当前位置置0，同时取消mark标记，仅此而已；也就是说写模式下limit仍保持与Buffer容量相同，只是重头写而已；读模式下limit仍然与rewind()调用之前相同，也就是为flip()调用之前写模式下的position的最后位置，flip()调用后此位置变为了读模式的limit位置，即越界位置，代码如下：

/** * Rewinds this buffer. *

* The position is set to zero, and the mark is cleared. The content of this * buffer is not changed. * *@return this buffer. */

public final Buffer rewind() {

position = 0;

mark = UNSET_MARK;

return this;

}

(5)flip()函数的作用是将写模式转变为读模式，即将写模式下的Buffer中内容的最后位置变为读模式下的limit位置，作为读越界位置，同时将当前读位置置为0，表示转换后重头开始读，同时再消除写模式下的mark标记，代码如下

/** * Flips this buffer. *

* The limit is set to the current position, then the position is set to * zero, and the mark is cleared. *

* The content of this buffer is not changed. * *@return this buffer. */

public final Buffer flip() {

limit = position;

position = 0;

mark = UNSET_MARK;

return this;

}

(6)remaining()仅在读模式下使用，用来获取还未读出的字节数。

/** * Returns the number of remaining elements in this buffer, that is * {@code limit - position}. * *@return the number of remaining elements in this buffer. */

public final int remaining() {

return limit - position;

}

(7)Buffer的抽象子类ByteBuffer的compact()方法也蛮重要的。compact()的作用是压缩数据。比如当前EOF是6，当前指针指向2(即0,1的数据已经写出了，没用了)，那么compact方法将把2,3,4,5的数据挪到0,1,2,3的位置，然后指针指向4的位置。这样的意思是，从4的位置接着再写入数据。

/**

* Compacts this byte buffer.

*

* The remaining bytes will be moved to the head of the

* buffer, starting from position zero. Then the position is set to

* {@code remaining()}; the limit is set to capacity; the mark is

* cleared.

*

* @return {@code this}

* @throws ReadOnlyBufferException

* if no changes may be made to the contents of this buffer.

*/

public abstract ByteBuffer compact();