PoolSubPage内部数据结构分析

final class PoolSubpage<T> {
    final PoolChunk<T> chunk;                 // 所属的PoolChunk
    private final int memoryMapIdx;       // 在memoryMap的索引 id memoryMap[id]
    private final int runOffset;                  // 在PoolChunk的运行时内存偏移量
    private final int pageSize;                  // pageSize
    private final long[] bitmap;                
    PoolSubpage<T> prev;
    PoolSubpage<T> next;
    boolean doNotDestroy;
    int elemSize;
    private int maxNumElems;
    private int bitmapLength;                   // bitmap中实际使用的长度
    private int nextAvail;                           //下一个可分配的elemSize块
    private int numAvail;                          //目前可用的elemSize块数量。
    // TODO: Test if adding padding helps under contention
    //private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;
    /** Special constructor that creates a linked list head */
    PoolSubpage(int pageSize) {
        chunk = null;
        memoryMapIdx = -1;
        runOffset = -1;
        elemSize = -1;
        this.pageSize = pageSize;
        bitmap = null;
    }
    PoolSubpage(PoolChunk<T> chunk, int memoryMapIdx, int runOffset, int pageSize, int elemSize) {
        this.chunk = chunk;
        this.memoryMapIdx = memoryMapIdx;
        this.runOffset = runOffset;
        this.pageSize = pageSize;
        bitmap = new long[pageSize >>> 10]; // pageSize / 16 / 64
        init(elemSize);
    }

PoolSubpage由大小相等的elemSize组成，elemSize表示第一次在该PoolSubpage申请内存的大小，用bitmap来表示每一个elemSize是否被占用。

void init(int elemSize) {
        doNotDestroy = true;
        this.elemSize = elemSize;
        if (elemSize != 0) {
            maxNumElems = numAvail = pageSize / elemSize;   
            nextAvail = 0;
            bitmapLength = maxNumElems >>> 6; // @1
            if ((maxNumElems & 63) != 0) {  //@2
                bitmapLength ++;
            }
            for (int i = 0; i < bitmapLength; i ++) {
                bitmap[i] = 0;
            }
        }
        addToPool();
    }

这里主要是来计算maxNumElems,numAvail。bitmapLenth为bitmap实际需要的长度。其实等于 maxNumElems /64 = maxNumElems >>>6,代码@2，如果maxNumElems如果小于64，那么maxNumElems>>>6则等为0，所以需要加一。

private void addToPool() {
        PoolSubpage<T> head = chunk.arena.findSubpagePoolHead(elemSize);
        assert prev == null && next == null;
        prev = head;
        next = head.next;
        next.prev = this;
        head.next = this;
    }