FileSnap.java

找到100个snapshot文件，按zxid降序排序
遍历找到的snapshot文件，从zxid最大的开始
将快照文件反序列化化
校验快照完整性，通过提取内容的checksum与文件中的val字段比较
如果有一个文件是完整的，则退出循环。

 public long deserialize(DataTree dt, Map<Long, Integer> sessions) throws IOException {
        // we run through 100 snapshots (not all of them)
        // if we cannot get it running within 100 snapshots
        // we should  give up
        List<File> snapList = findNValidSnapshots(100);
        if (snapList.size() == 0) {
            return -1L;
        }
        File snap = null;
        long snapZxid = -1;
        boolean foundValid = false;
        for (int i = 0, snapListSize = snapList.size(); i < snapListSize; i++) {
            snap = snapList.get(i);
            LOG.info("Reading snapshot {}", snap);
            snapZxid = Util.getZxidFromName(snap.getName(), SNAPSHOT_FILE_PREFIX);
            try (CheckedInputStream snapIS = SnapStream.getInputStream(snap)) {
                InputArchive ia = BinaryInputArchive.getArchive(snapIS);
                deserialize(dt, sessions, ia);
                SnapStream.checkSealIntegrity(snapIS, ia);

                // Digest feature was added after the CRC to make it backward
                // compatible, the older code can still read snapshots which
                // includes digest.
                //
                // To check the intact, after adding digest we added another
                // CRC check.
                if (dt.deserializeZxidDigest(ia, snapZxid)) {
                    SnapStream.checkSealIntegrity(snapIS, ia);
                }

                foundValid = true;
                break;
            } catch (IOException e) {
                LOG.warn("problem reading snap file {}", snap, e);
            }
        }
        if (!foundValid) {
            throw new IOException("Not able to find valid snapshots in " + snapDir);
        }
        dt.lastProcessedZxid = snapZxid;
        lastSnapshotInfo = new SnapshotInfo(dt.lastProcessedZxid, snap.lastModified() / 1000);

        // compare the digest if this is not a fuzzy snapshot, we want to compare
        // and find inconsistent asap.
        if (dt.getDigestFromLoadedSnapshot() != null) {
            dt.compareSnapshotDigests(dt.lastProcessedZxid);
        }
        return dt.lastProcessedZxid;
    }