Method readPage

src/java/simpledb/index/BTreeFile.java:89–136

Read a page from the file on disk. This should not be called directly but should be called from the BufferPool via getPage() @param pid - the id of the page to read from disk @return the page constructed from the contents on disk

(PageId pid)

	 * @return the page constructed from the contents on disk
	 */
	public Page readPage(PageId pid) {
		BTreePageId id = (BTreePageId) pid;

        try (BufferedInputStream bis = new BufferedInputStream(new FileInputStream(f))) {
            if (id.pgcateg() == BTreePageId.ROOT_PTR) {
				//指向根节点指针
                byte[] pageBuf = new byte[BTreeRootPtrPage.getPageSize()];
                int retval = bis.read(pageBuf, 0, BTreeRootPtrPage.getPageSize());
                if (retval == -1) {
                    throw new IllegalArgumentException("Read past end of table");
                }
                if (retval < BTreeRootPtrPage.getPageSize()) {
                    throw new IllegalArgumentException("Unable to read "
                            + BTreeRootPtrPage.getPageSize() + " bytes from BTreeFile");
                }
                Debug.log(1, "BTreeFile.readPage: read page %d", id.getPageNumber());
                return new BTreeRootPtrPage(id, pageBuf);
            } else {
                byte[] pageBuf = new byte[BufferPool.getPageSize()];
                if (bis.skip(BTreeRootPtrPage.getPageSize() + (long) (id.getPageNumber() - 1) * BufferPool.getPageSize()) !=
                        BTreeRootPtrPage.getPageSize() + (long) (id.getPageNumber() - 1) * BufferPool.getPageSize()) {
                    throw new IllegalArgumentException(
                            "Unable to seek to correct place in BTreeFile");
                }
                int retval = bis.read(pageBuf, 0, BufferPool.getPageSize());
                if (retval == -1) {
                    throw new IllegalArgumentException("Read past end of table");
                }
                if (retval < BufferPool.getPageSize()) {
                    throw new IllegalArgumentException("Unable to read "
                            + BufferPool.getPageSize() + " bytes from BTreeFile");
                }
                Debug.log(1, "BTreeFile.readPage: read page %d", id.getPageNumber());
				//分别是三种节点
                if (id.pgcateg() == BTreePageId.INTERNAL) {
                    return new BTreeInternalPage(id, pageBuf, keyField);
                } else if (id.pgcateg() == BTreePageId.LEAF) {
                    return new BTreeLeafPage(id, pageBuf, keyField);
                } else { // id.pgcateg() == BTreePageId.HEADER
                    return new BTreeHeaderPage(id, pageBuf);
                }
            }
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        // Close the file on success or error
        // Ignore failures closing the file
    }

	/**
	 * Write a page to disk.  This should not be called directly but should