RFR: 8373452: DataFormat threading and API issues

[Andy Goryachev]

On Tue, 20 Jan 2026 22:01:58 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> ### Summary
>> 
>> This PR makes the `DataFormat` constructor private:
>> 
>> private DataFormat(@NamedArg("ids") String... ids)
>> 
>> 
>> and replaces it with
>> 
>> public static DataFormat of(String ... ids)
>> 
>> 
>> ### Problem
>> 
>> There seems to be several issues with DataFormat API and implementation discovered during a Clipboard-related code review:
>> 
>> 1. `static DataFormat::lookupMimeType(String)` is not thread safe: while iterating over previously registered entries in the `DATA_FORMAT_LIST` another thread might create a new instance (DataFormat L227)
>> 
>> 2. `public DataFormat(String...)` constructor might throw an `IllegalArgumentException` if one of the given mime types is already assigned to another `DataFormat`. The origin of this requirement is unclear, but one possible issue I can see is if the application has two libraries that both attempt to create a `DataFormat` for let's say `"text/css"`. Then, depending on the timing or the exact code path, an exception will be thrown for which the library(-ies) might not be prepared. The constructor is also not thread safe.
>> 
>> 3. To avoid a situation mentioned in bullet 2, a developer would is typically call `lookupMimeType()` to obtain an already registered instance, followed by a constructor call if such an instance has not been found. An example of such code can be seen in webkit/UIClientImpl:299 - but even then, despite that two-step process being synchronized, the code might still fail if *some other* library or the application attempts to create a new instance of DataFormat, since the constructor itself is not synchronized.
>> 
>> 4. `DataFormat(new String[] { null })` is allowed but makes no sense!
>> 
>> Why do we need to have the registry of previously created instances? Unclear. My theory is that the DataFormat allows to have multiple mime-types (ids) - example being `DataFormat.FILES = new DataFormat("application/x-java-file-list", "java.file-list");` - and the registry was added to prevent creation of a `DataFormat` with just one id for some reason.
>> 
>> What should be done?
>> - find out why we need this registry in the first place i.e. what could happen if we have multiple DataFormat instances with overlapping ids.
>> - if the registry is needed add a new factory method, something like `DataFormat::of(String ...)` which is properly synchronized. This method will be called by the constructor to retain the backward compatibility.
>> - deprecate (possibly for removal) `DataFormat::lookupMimeType(St...
>
> modules/javafx.graphics/src/main/java/javafx/scene/input/DataFormat.java line 133:
> 
>> 131:      */
>> 132:     @Deprecated(since = "27")
>> 133:     private DataFormat(@NamedArg("ids") String... ids) {
> 
> This is an incompatible change. You will need to go through the usual process for such changes where it is deprecated for removal in one release and removed in a subsequent release.

should we also print a warning to `stderr`?

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PR Review Comment: https://git.openjdk.org/jfx/pull/2006#discussion_r2710486058