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I tried to serialize a map with null value (api 45):

Map<String, String> bodyMap = new Map<String, String> 'key'=>'value', 'key2'=>null;
System.debug(JSON.serialize(bodyMap,true));

but it does not suppress null value (as it does in a custom object):

"key2":null,"key":"value"

Does it work as expected? Is there any other way than writing custom serialiser?

Yep, that's the expected behavior.

For simple maps, you can get rid of null values by iterating over the map keys, keeping track of which ones have null values, and then a little map manipulation.

Set<String> mapKeysToRemoveSet = new Set<String>();

for(String key :myMap.keySet())
 if(myMap.get(key) == null)
 mapKeysToRemoveSet.add(key);
 


// The Map class only comes with a remove() method, which only removes one item from the
// map at a time.
// It's not a great idea to modify a collection while you're iterating over it.
// The solution is to get the map's keyset, then use the set class's removeAll() method.
// This has the effect of removing multiple items from the map in one shot.
myMap.keySet().removeAll(mapKeysToRemoveSet);

System.debug(JSON.serialize(myMap));

The documentation for JSON.serialize(objectToSerialize, suppressApexObjectNulls)
mentions the type of objectToSerialize parameter as:

Type: Object

The Apex object to serialize.

And if you refer to other documentations (mentioned below), a Collection is always categorized differently than an Apex Object. Apex Object always refers to instances of custom/system classes within the platform.

So the behavior what you are experiencing is expected. Your option is to either go with a custom class or serialize only after removing the null values from the collection.

References for data types in Apex which mentions Collection differently than Apex Objects:

• Data Types


• Expressions

This is a problem which I had faced, and I came across a string manipulation method that strips out nulls from null json keys.

 public static string stripJsonNulls(string JsonString)


 if(JsonString != null)
 
 JsonString = JsonString.replaceAll('"[^"]*":null',''); //basic removeal of null values
 JsonString = JsonString.replaceAll(',2,', ','); //remove duplicate/multiple commas
 JsonString = JsonString.replace(',', ''); //prevent opening brace from having a comma after it
 JsonString = JsonString.replace(',', ''); //prevent closing brace from having a comma before it
 JsonString = JsonString.replace('[,', '['); //prevent opening bracket from having a comma after it
 JsonString = JsonString.replace(',]', ']'); //prevent closing bracket from having a comma before it
 

 return JsonString;



Map<String, String> bodyMap = new Map<String, String> 'key'=>'value', 'key2'=>null;
System.debug(stripJsonNulls(JSON.serialize(bodyMap,false)));

OP : "key":"value"

Src: https://iwritecrappycode.wordpress.com/2014/07/16/stripping-nulls-from-a-json-object-in-apex/