我正在研究的平台具有相当严格的内存限制，我正试图找到一种解析大JSON字符串的方法，而无需在内存中加载超过几百个字节。 JSON字符串存储在更大的芯片（闪存）上的文件中。
有两件事我无法找到一个好的解决方案：
foo["bar"][2]之类的“路径”来访问特定值
（如果值变成一个数组/对象，那么我们应该只返回它是一个数组/对象的事实，也许它是否为空。）所以基本上我需要的函数在调用时，一步一步地解析json，只保存我们实际需要继续解析的部分。
对于界面，我认为不可能有类似exampleJson["aa"].2.["gg]的内容，但我设法非常接近：exampleJson["aa"].2.["gg"]()。这将导致调用一个函数，然后可以轻松访问{'aa'，2，'gg'}并从文件中读取/解析json。
这是我的代码到目前为止，但我真的不知道如何继续：
https://repl.it/HfwS/2
-- Looks complicated, but is pretty simple. Using meta tables we create a json interface that can almost be accessed as if it was a lua table.
-- E.g. example["aa"][2]["gg"]() ; the only difference is that we have to use parentheses at the end
-- The problematic part starts where it says `THIS IS WHERE THE JSON PARSING WOULD HAPPEN`
json = {}
setmetatable(json, {
__call = function(path)
local jsonFile = _file.open(filePath)
local fileLen = jsonFile:stat().size
local patternTable = {} -- Will store `{'aa',2,'gg'}` for `example.['aa'].[2]['gg']()`
local fakeJson = {}
setmetatable(fakeJson, {
__index = function (t, k)
patternTable[#patternTable+1] = k
return fakeJson
end;
__call = function()
-- THIS IS WHERE THE JSON PARSING WOULD HAPPEN --
-- The patternTable contains {'aa',2,'gg'} at this point
-- Loop through the json file char by char
local valueToReturn = ''
local filePos = 0
for i=1, fileLen do
jsonFile:seek("set", filePos)
local currentChar = jsonFile:read(1) -- read character at current position
filePos = filePos + 1
-- print(currentChar)
-- Now the question is, how do we parse the json?
print('Magic to parse the json')
-- valueToReturn = ?
end
patternTable = {} -- Reset the patternTable
return valueToReturn
end;
})
return fakeJson
end;
})
local fakeParsedJson = json('example.json')
local value = fakeParsedJson["aa"][2]["gg"]() -- Notice the `()` in the end
print(value)
答案 0 :(得分：0)
我花了一些时间思考如何实现这一目标，并最终设法将其拉下来。检索值并迭代数组/对象就像一个魅力。如果您知道更好的方法，请告诉我。 （我对代码不太满意;看起来它可能会更清晰。）但是嘿它有效。
如果您想尝试一下，这是一个小提琴： https://repl.it/HfwS/31
json = {}
setmetatable(json, {
__call = function(filePath)
local jsonFile = _file.open(filePath)
local fileLen = jsonFile:stat().size
local jsonPath = {} -- Would store `{'aa',2,'gg'}` for `example['aa'][2]['gg']()`
local fakeJson = {}
setmetatable(fakeJson, {
__index = function (t, k)
jsonPath[#jsonPath+1] = k
return fakeJson
end;
__call = function()
-- THIS IS WHERE THE JSON PARSING WOULD HAPPEN --
-- The jsonPath contains {'aa',2,'gg'} at this point
local brcStack = {} -- will be used to push/pop braces/brackets
local jsonPathDim = 1 -- table dimension (['a'] == 1; ['a']['b'] == 2; ...)
-- Loop through the json file char by char
local valueToReturn
local filePos = 0
local nextChar = function()
jsonFile:seek("set", filePos)
filePos = filePos + 1
local char = jsonFile:read(1)
--print(char)
return char
end
local jsonValid = true
for o=1, fileLen do -- infinite
if jsonPathDim > #jsonPath then -- jsonPath followed. Now we can extract the value.
while true do
local currentChar = nextChar()
if currentChar == '"' then -- string
valueToReturn = ''
for i=1, fileLen do
currentChar = nextChar()
if currentChar == '"' then
break
elseif currentChar == nil then
jsonValid = false
break
else
valueToReturn = valueToReturn .. currentChar
end
end
break
elseif string.find(currentChar,'[%d.]') then -- numbers 0.3, .3, 99 etc
local rawValue = ''
if currentChar == '.' then
rawValue = '0'
end
for i=1, fileLen do
if string.find(currentChar, '[%s,\r\n%]%}]') then
break
elseif filePos > fileLen then
jsonValid = false
break
else
rawValue = rawValue .. currentChar
end
currentChar = nextChar()
end
valueToReturn = tonumber(rawValue)
break
elseif currentChar == 't' then -- true
valueToReturn = true
break
elseif currentChar == 'f' then -- false
valueToReturn = false
break
elseif currentChar == 'n' then -- null
valueToReturn = nil -- ?
break
elseif currentChar == '{' then -- null
valueToReturn = {}
brcStack[#brcStack+1] = '{'
local origBrcLvl = #brcStack
while true do
currentChar = nextChar()
if filePos > fileLen then
jsonValid = false
break
elseif currentChar == '\\' then
nextChar()
-- Continue
elseif origBrcLvl == #brcStack and currentChar == '"' then
local keyToPush = ''
while true do
currentChar = nextChar()
if currentChar == '"' then
while true do
currentChar = nextChar()
if currentChar == ':' then
valueToReturn[keyToPush] = 0
break
elseif filePos > fileLen then
break
end
end
break
elseif filePos > fileLen then
jsonValid = false
break
else
keyToPush = keyToPush .. currentChar
end
end
break
elseif currentChar == '[' or currentChar == '{' then
brcStack[#brcStack+1] = currentChar
elseif currentChar == ']' then
if brcStack[#brcStack] == ']' then
brcStack[#brcStack] = nil
else
jsonValid = false
break
end
elseif currentChar == '}' then
if brcStack[#brcStack] == '}' then
brcStack[#brcStack] = nil
else
jsonValid = false
break
end
end
end
break
elseif currentChar == '[' then
brcStack[#brcStack+1] = '['
valueToReturn = {}
local origBrcLvl = #brcStack
while true do
currentChar = nextChar()
if origBrcLvl == #brcStack and #valueToReturn == 0 and not string.find(currentChar, '[%s\r\n%]]') then
valueToReturn[#valueToReturn+1] = 0
end
if filePos > fileLen then
jsonValid = false
break
elseif currentChar == '\\' then
nextChar()
-- Continue
elseif origBrcLvl == #brcStack and currentChar == ',' then
valueToReturn[#valueToReturn+1] = 0
elseif currentChar == '[' or currentChar == '{' then
brcStack[#brcStack+1] = currentChar
elseif currentChar == ']' then
if brcStack[#brcStack] == ']' then
brcStack[#brcStack] = nil
else
jsonValid = false
break
end
elseif currentChar == '}' then
if brcStack[#brcStack] == '}' then
brcStack[#brcStack] = nil
else
jsonValid = false
break
end
end
end
break
end
end
break
end
local currentKey = jsonPath[jsonPathDim]
local currentKeyLen = string.len(currentKey)
if type(jsonPath[jsonPathDim]) == 'string' then -- Parsing { object
while true do
local currentChar = nextChar()
if currentChar == '{' then
brcStack[#brcStack+1] = '{'
local origBrcLvl = #brcStack
local keyFound = true
for z=1, fileLen do -- loop over keys until we find it
currentChar = nextChar()
if currentChar == '\\' then
nextChar()
-- Continue
elseif origBrcLvl == #brcStack and currentChar == '"' then
local keyMatched = false
for i=1, fileLen do
local expectedChar = string.sub(currentKey,i,i)
if nextChar() == expectedChar then
if i == currentKeyLen and nextChar() == '"' then
keyMatched = true
while true do
currentChar = nextChar()
if currentChar == ':' then
break
elseif currentChar == nil then
jsonValid = false
break
end
end
break
end
-- Continue
else
keyMatched = false
break
end
end
if keyMatched then
keyFound = true
break
end
elseif currentChar == '[' or currentChar == '{' then
brcStack[#brcStack+1] = currentChar
elseif currentChar == ']' then
if brcStack[#brcStack] == ']' then
brcStack[#brcStack] = nil
else
jsonValid = false
break
end
elseif currentChar == '}' then
if brcStack[#brcStack] == '}' then
brcStack[#brcStack] = nil
else
jsonValid = false
break
end
end
end
if keyFound then
jsonPathDim = jsonPathDim+1
end
break
elseif currentChar == nil then
jsonValid = false
break
end
end
elseif type(jsonPath[jsonPathDim]) == 'number' then -- Parsing [ array
while true do
local currentChar = nextChar()
if currentChar == '[' then
brcStack[#brcStack+1] = '['
local origBrcLvl = #brcStack
local currentIndex = 1
-- currentKey
local keyMatched = true
for i=1, fileLen do
currentChar = nextChar()
if currentChar == '\\' then
nextChar()
-- Continue
elseif origBrcLvl == #brcStack and currentChar == ',' then
currentIndex = currentIndex +1
if currentIndex == currentKey then
jsonPathDim = jsonPathDim+1
break
end
elseif currentChar == '[' or currentChar == '{' then
brcStack[#brcStack+1] = currentChar
elseif currentChar == ']' then
if brcStack[#brcStack] == ']' then
brcStack[#brcStack] = nil
else
jsonValid = false
break
end
elseif currentChar == '}' then
if brcStack[#brcStack] == '}' then
brcStack[#brcStack] = nil
else
jsonValid = false
break
end
else
-- Continue
end
end
break
elseif currentChar == nil then
jsonValid = false
break
end
end
else
jsonValid = false
break -- Invalid json
end
end
jsonPath = {} -- Reset the jsonPath
return valueToReturn
end;
})
return fakeJson
end;
})
local example = json('example.json')
-- Read a value
local value = example["aa"][2]['k1']()
print(value)
-- Loop over a key value table and print the keys and values
for key, value in pairs(example["aa"][2]()) do
print('key: ' .. key, 'value: ' .. example["aa"][2][key]())
end
JSON验证可能会更好，但如果您提供无效的json数据，那么您不应该期待任何事情。
答案 1 :(得分：0)
如果要解码单个JSON元素（对象，数组等）而不是解码整个JSON，则需要具有两个功能的JSON库：
例：
如何使用this module部分解码JSON：
-- This is content of data.txt file:
-- {"aa":["qq",{"k1":23,"gg":"YAY","Fermat_primes":[3, 5, 17, 257, 65537]}]}
-- We want to extract as Lua values only "Fermat_primes" array and "gg" string
local json = require('json')
-- Open file
local file = assert(io.open('data.txt', 'r'))
-- Define loader function which will read the file in 64-byte chunks
local function my_json_loader()
return file:read(64)
end
local FP, gg
-- Prepare callback function for traverse with partial decode
local function my_callback (path, json_type, value)
path = table.concat(path, '/')
if path == "aa/2/Fermat_primes" then
FP = value
return true -- we want to decode this array instead of traverse through it
elseif path == "aa/2/gg" then
gg = value
end
end
json.traverse(my_json_loader, my_callback)
-- Close file
file:close()
-- Display the results
print('aa.2.gg = '..gg)
print('aa.2.Fermat_primes:')
for k, v in ipairs(FP) do print(k, v) end
输出：
aa.2.gg = YAY
aa.2.Fermat_primes:
1 3
2 5
3 17
4 257
5 65537