When a JSON-RPC request without params (i.e. them being null in Java) is emitted, that gets written as "params": null instead of just omitting the params field from the output. The latter should be done instead.

Specifications

JSON-RPC specification says:

params
A Structured value that holds the parameter values to be used during the invocation of the method. This member MAY be omitted.

where "two structured types (Objects and Arrays)".

LSP specification says (in TypeScript syntax):

/**
 * The method's params.
 */
params?: array | object;

which exactly matches the JSON-RPC specification with the ? meaning omission, but not null (which would have to be explicitly given as a | null variant).

Implementation

The writing of params here:
https://github.com/eclipse/lsp4j/blob/22f8edce55379bd9d4ae1b96bc614e74549bc0f9/org.eclipse.lsp4j.jsonrpc/src/main/java/org/eclipse/lsp4j/jsonrpc/json/adapters/MessageTypeAdapter.java#L404-L409
uses the writeNullValue function here:
https://github.com/eclipse/lsp4j/blob/22f8edce55379bd9d4ae1b96bc614e74549bc0f9/org.eclipse.lsp4j.jsonrpc/src/main/java/org/eclipse/lsp4j/jsonrpc/json/adapters/MessageTypeAdapter.java#L449-L457
which goes out of its way to output a null instead of omitting missing params.

This clearly violates the specifications and trips up other JSON-RPC/LSP implementations that require incoming requests to follow the specification to the letter.

Background

According to git blame, this behavior was introduced in #137, the goal of which was to fix the behavior for response result that indeed may not be omitted but may be null. Looks like the same logic was erroneously applied to request/notification params as well.