NIO之Buffer源码分析

https://dongzl.github.io/netty-handbook/#/_content/chapter03

测试代码

public class BasicBuffer {
    public static void main(String[] args) {
        LongBuffer longBuffer = LongBuffer.allocate(4);
        System.out.println(longBuffer.mark() + "," + longBuffer.position()
        + "," + longBuffer.limit() + "," + longBuffer.capacity());
        longBuffer.put(1234);
        longBuffer.put(12345);
        longBuffer.put(12346);
        longBuffer.put(12347);

        System.out.println("before flip: " + longBuffer.mark() + "," + longBuffer.position()
                + "," + longBuffer.limit() + "," + longBuffer.capacity());

        longBuffer.flip();

        System.out.println("after flip: " + longBuffer.mark() + "," + longBuffer.position()
                + "," + longBuffer.limit() + "," + longBuffer.capacity());

        while(longBuffer.hasRemaining()) {
            System.out.println(longBuffer.get());
            System.out.println("after get: " + longBuffer.mark() + "," + longBuffer.position()
                    + "," + longBuffer.limit() + "," + longBuffer.capacity());
        }
    }
}

解读

最开始 LongBuffer.allocate(4)，可以看到初始
mark=-1，position=0，limit=4，capacity=4。

public abstract class LongBuffer
    extends Buffer
    implements Comparable<LongBuffer>
{
    public static LongBuffer allocate(int capacity) {
        if (capacity < 0)
            throw new IllegalArgumentException();
        return new HeapLongBuffer(capacity, capacity);
    }
}

class HeapLongBuffer
    extends LongBuffer
{
    HeapLongBuffer(int cap, int lim) {            // package-private

        super(-1, 0, lim, cap, new long[cap], 0);
        /*
        hb = new long[cap];
        offset = 0;
        */
    }
}

public abstract class LongBuffer
    extends Buffer
    implements Comparable<LongBuffer>
{
    final long[] hb;                  // Non-null only for heap buffers
    final int offset;
    
    LongBuffer(int mark, int pos, int lim, int cap,   // package-private
                 long[] hb, int offset)
    {
        super(mark, pos, lim, cap);
        this.hb = hb;
        this.offset = offset;
    }
}

而put数据过程，就是position 索引不断增加过程；
调用的是HeapLongBuffer#put()

class HeapLongBuffer
    extends LongBuffer
{
    public LongBuffer put(long x) {

        hb[ix(nextPutIndex())] = x;
        return this;
    }
    protected int ix(int i) {
        return i + offset;
    }
}

flip()则是将position赋值给limit，position置为0

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

get（）则调用HeapLongBuffer#get（），取hb数组中position值

class HeapLongBuffer
    extends LongBuffer
{
    public long get() {
        return hb[ix(nextGetIndex())];
    }

    final int nextGetIndex() {                          // package-private
        if (position >= limit)
            throw new BufferUnderflowException();
        return position++;
    }
}