NIO的JVM内存和机器内存的选择

NIO到底是使用JVM的内存还是使用机器内存呢？其实，在ByteBuffer中提供了两个申请内存的方法供我们选择。

申请JVM堆内存：

/**
 * Allocates a new byte buffer.
 *
 * <p> The new buffer's position will be zero, its limit will be its
 * capacity, its mark will be undefined, and each of its elements will be
 * initialized to zero.  It will have a {@link #array backing array},
 * and its {@link #arrayOffset array offset} will be zero.
 *
 * @param  capacity
 *         The new buffer's capacity, in bytes
 *
 * @return  The new byte buffer
 *
 * @throws  IllegalArgumentException
 *          If the <tt>capacity</tt> is a negative integer
 */
public static ByteBuffer allocate(int capacity) {
    if (capacity < 0)
        throw new IllegalArgumentException();
    return new HeapByteBuffer(capacity, capacity);
}

申请机器内存：

/**
 * Allocates a new direct byte buffer.
 *
 * <p> The new buffer's position will be zero, its limit will be its
 * capacity, its mark will be undefined, and each of its elements will be
 * initialized to zero.  Whether or not it has a
 * {@link #hasArray backing array} is unspecified.
 *
 * @param  capacity
 *         The new buffer's capacity, in bytes
 *
 * @return  The new byte buffer
 *
 * @throws  IllegalArgumentException
 *          If the <tt>capacity</tt> is a negative integer
 */
public static ByteBuffer allocateDirect(int capacity) {
    return new DirectByteBuffer(capacity);
}