MAXSTRING trying to read string from JSON object.

Question

Question

I receive JSON from a webservice and parse it to an object:

Set wout=0
set stream=##class(%Stream.TmpCharacter).%New()
For {
	Quit:(httprequest.HttpResponse.Data.AtEnd || wout)
	d stream.Write(httprequest.HttpResponse.Data.Read(32000,.tSC))
}
Set jsonob={}.%FromJSON(stream)

jsonob contains a sub entity "pdf" which is a base64 string and %GetTypeOf() tells me it is a string.
However I am unable to read the nested jsonob.pdf into a stream or anything else as any attempt to use it result in a MAXLEN error.

Product version: IRIS 2021.1

$ZV: IRIS for UNIX (Red Hat Enterprise Linux for x86-64) 2021.1.1 (Build 324U) Thu Feb 24 2022 13:20:39 EST

Discussion (9)5

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Julius Kavay · Jan 31

You have right, I overlooked the [ character, sorry (usually one posts a piece of code and not a piece of picture!). So the above line would be

if jsonobj, jsonobj.statusCode = 200 {
   for i=0:1:jsonobj.value.labReports.%Size()-1 {
      set pdf(i)=jsonobj.value.labReports.%Get(i).%Get("pdf",,"stream<base64")
   }
   ... // do something with pfd(i) streams
}

0 0

score 1 · Answer 1 · 2024-01-24T09:47:48-05:00

@Nicki Vallentgoed
You can convert the HttpResponse.Data into a %DynamicObject without using an intermediate stream:

Set jsonob = ##class(%DynamicObject).%FromJSON(httprequest.HttpResponse.Data)

score 1 · Answer 2 · 2024-01-24T14:10:47-05:00

There are two places where a <MAXSTRING> signal could give you problems with JSON stored in a %DynamciAbstractObject. It can occur if the input is too long and it can occur when evaluating a JSON element that is too large.

The %FromJSON(stream) method can read JSON syntax directly from a stream of any length providing the operating system will give you enough virtual memory to hold the %DynamicAbstractObject. You seem to already understand this although you don't have to make a copy of an existing %Stream value (although it might need a stream.Rewind() call before you pass the %Stream to %FromJSON.)

Evaluating dynobj.%Get("pdfkey") can give you a <MAXSTRING> signal if the "pdfkey" element of the %DynamicObject referenced by the oref in the local variable 'dynobj' is very long. But look at the class reference description of the %Get method in the %DynamicObject class and you will see that the %Get method can have optional 2nd and 3rd parameters. The value of dynobj.%Get("pdfkey",,"stream") will return a %Stream containing the characters of the "pdfkey" element without generating a <MAXSTRING> signal. Even better, the value of dynobj.%Get("pdfkey",,"stream<base64") will be a %Stream containing the "pdfkey" element decoded out of the base64 encoding.

If you want to iterate through all the elements in a %DynamicObject then you can use the dynobj%GetIterator() method to create a %Iterator.Object class object. The %GetNext(key,.value,.type) method of the %Iterator.Object class can assign a %Stream to the .value parameter when the selected "key" element contains a long string value *providing* the optional .type parameter is also present. The .type parameter receives the %GetTypeOf value of the original object element so you can tell the difference between a long "string" type element returning a %Stream versus an "object" type element that contains a %Stream oref value.

score 0 · Answer 3 · 2024-01-30T06:10:34-05:00

My code:

try{
  Set jsonob = ##class(%DynamicObject).%FromJSON(httprequest.HttpResponse.Data)
  w !,"statusCodes: "_jsonob.%Get("statusCode"),!
  Set iter = jsonob.value.labReports.%GetIterator()
  While iter.%GetNext(.key, .labReport) {
  w !,"reportName: "_labReport.%Get("reportName")
  Set pdf=labReport.%Get("pdf",,"stream<base64")
  w !,"pdf: "_pdf.Read(20),!
  }
} catch(tException) {		
  w !,tException.DisplayString(),!
}

JSON looks like this( pdf the offending big string):

Output:

statusCodes: 200

reportName: Long-Oeloff-Badenhorst.pdf
<MAXSTRING> 5 ztestpdf+36^Test.Testy.1

ztestpdf+36 = Set pdf=labReport.%Get("pdf",,"stream<base64")

score 0 · Answer 4 · 2024-01-30T18:56:45-05:00

Two notes to your (above) code

- first, if you use the iterator method on an object which can contain long strings (longer what IRIS can handle) then you must specify the thrid argument to the %GetNext() method too, i.e.

while iterator.%GetNext(.key, .value, "stream") { ... }

- second, if you know the name and location of a stream property in an JSON object, like in your case, then just grab the data without using an iterator:

try { set jsonobj = {}.%FromJSON(httprequest.HttpResponse.Data) } catch { jsonobj=0 }
if jsonobj, jsonobj.statusCode = 200 {
   set pdf=jsonobj.value.labReport.%Get("pdf",,"stream<base64")  // according to the picture you provided
   ... // do something with the pdf-stream
}

For another example, how to work with long strings see this thread.

score 0 · Answer 5 · 2024-01-31T02:59:14-05:00

Please note that jsonob.value.labReports is an array, I assume that it possibly can contains multiple pdfs, so an iterator is required.

score 0 · Answer 6 · 2024-01-31T12:55:28-05:00

The third argument to %GetNext is not a string value. It is a variable passed by reference: iterator.%GetNext(.key,.value,.type). When this returns then the variable type will contain a string value like “string”, “number”,”object”, etc. If type contains “string” and $isobject(value) is true then value is a %Stream object containing your long string.

score 0 · Answer 7 · 2024-02-01T04:37:30-05:00

Thanks, the .%Get("pdf",,"stream<base64") works.

It is super frustrating though.
This produces an error: <PARAMETER> 127 %GetNext^%Iterator.Array.1

Set iter = jsonob.value.labReports.%GetIterator()
While iter.%GetNext(.key,.labReport,.type) {
    w !,"type: "_type
}

I do not know if it a bug on my side or the documentation:

score 0 · Answer 8 · 2024-01-30T12:31:45-05:00

Take a look to this article, maybe it's helpful:

https://community.intersystems.com/post/retrieving-base64-files-post-cal...