1.通过hash从mapBlockIndex获得CBlockIndex

2.通过CBlockIndex得到该区块在文件中的存储位置，读取文件，获得CBlock对象

3.从CBlockIndex和CBlock获取所需要的值，封装成json对象返回

src/rpc/blockchain.cpp

static UniValue getblock(const JSONRPCRequest& request)
{
  LOCK(cs_main);

    std::string strHash = request.params[0].get_str(); //获得参数 hash
    uint256 hash(uint256S(strHash));

    int verbosity = 1;
    if (!request.params[1].isNull()) {
        if(request.params[1].isNum())
            verbosity = request.params[1].get_int();
        else
            verbosity = request.params[1].get_bool() ? 1 : 0;
    }

    const CBlockIndex* pblockindex = LookupBlockIndex(hash); //第一步
    if (!pblockindex) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
    }

    const CBlock block = GetBlockChecked(pblockindex); //第二步

    if (verbosity <= 0)
    {
        CDataStream ssBlock(SER_NETWORK, PROTOCOL_VERSION | RPCSerializationFlags());
        ssBlock << block;
        std::string strHex = HexStr(ssBlock.begin(), ssBlock.end());
        return strHex;
    }

    return blockToJSON(block, pblockindex, verbosity >= 2); //第三步
}

第一步 通过hash从mapBlockIndex获得CBlockIndex

src/validation.h

inline CBlockIndex* LookupBlockIndex(const uint256& hash)
{
    AssertLockHeld(cs_main);
    BlockMap::const_iterator it = mapBlockIndex.find(hash);
    return it == mapBlockIndex.end() ? nullptr : it->second;
}

第二步 通过CBlockIndex读取文件，获得CBlock对象

src/rpc/blockchain.cpp

static CBlock GetBlockChecked(const CBlockIndex* pblockindex)
{
    CBlock block;
    if (IsBlockPruned(pblockindex)) { //Prune 修剪
        throw JSONRPCError(RPC_MISC_ERROR, "Block not available (pruned data)");
    }

    if (!ReadBlockFromDisk(block, pblockindex, Params().GetConsensus())) {
        // Block not found on disk. This could be because we have the block
        // header in our index but don't have the block (for example if a
        // non-whitelisted node sends us an unrequested long chain of valid
        // blocks, we add the headers to our index, but don't accept the
        // block).
        throw JSONRPCError(RPC_MISC_ERROR, "Block not found on disk");
    }

    return block;
}

从CBlockIndex构建CDiskBlockPos对象

src/validation.cpp

bool ReadBlockFromDisk(CBlock& block, const CBlockIndex* pindex, const Consensus::Params& consensusParams)
{
    CDiskBlockPos blockPos;
    {
        LOCK(cs_main);
        blockPos = pindex->GetBlockPos();
    }

    if (!ReadBlockFromDisk(block, blockPos, consensusParams))
        return false;
    if (block.GetHash() != pindex->GetBlockHash())
        return error("ReadBlockFromDisk(CBlock&, CBlockIndex*): GetHash() doesn't match index for %s at %s",
                pindex->ToString(), pindex->GetBlockPos().ToString());
    return true;
}

从文件读取构建CBlock

bool ReadBlockFromDisk(CBlock& block, const CDiskBlockPos& pos, const Consensus::Params& consensusParams)
{
    block.SetNull();

    // Open history file to read
    CAutoFile filein(OpenBlockFile(pos, true), SER_DISK, CLIENT_VERSION);
    if (filein.IsNull())
        return error("ReadBlockFromDisk: OpenBlockFile failed for %s", pos.ToString());

    // Read block
    try {
        filein >> block; 
    }
    catch (const std::exception& e) {
        return error("%s: Deserialize or I/O error - %s at %s", __func__, e.what(), pos.ToString());
    }

    // Check the header
    if (!CheckProofOfWork(block.GetHash(), block.nBits, consensusParams)) //工作量证明检测
        return error("ReadBlockFromDisk: Errors in block header at %s", pos.ToString());

    return true;
}