Fragile reference to private synthetic this$ field

[damccorm]

The following code is using reflection to access a field with a name starting with this$:

beam/runners/spark/src/main/java/org/apache/beam/runners/spark/SparkNativePipelineVisitor.java

Line 186 in 4fbdcca

Field parent = fnClass.getDeclaredField("this$0");

The OpenJDK javac generates private synthetic fields with names starting with this$ as an implementation detail of inner classes. In the future that implementation detail may be changing, and the this$ field will no longer be generated for all inner classes. For more information about the proposed change, see: https://bugs.openjdk.java.net/browse/JDK-8271717

Please consider alternatives to accessing the private synthetic this$ field to ensure this code continues to work after the change. For example, consider passing an explicit copy of the enclosing instance to code that needs access to it, or adding an explicit getter to the inner class.

For example, given:

class Outer {
  int x;
  class Inner1 {
    int f() {
      return x;
    }
  }
  class Inner2
{
    void g() {
      System.err.println("hello");
    }
  }
}

Currently Inner1 and Inner2 both have a synthetic field named this$0 that stores a reference to Outer.

In the future the implementation detail might be changing to omit the field from classes that don't reference the enclosing instance. So in the example, Inner1 would still have the synthetic field because it accesses the field x in the enclosing instance Outer. However Inner2 would no longer have a synthetic field, because it doesn't access any state from its enclosing instance.

Imported from Jira BEAM-13020. Original Jira may contain additional context.
Reported by: cushon.

[blazingbhavneek]

Hey there! 👋 I'm new to this repository and eager to contribute! 🌟 Could you kindly suggest some entry point or files to look into?

[damccorm]

Hey, saw you added this comment several places. I'd recommend focusing on a single issue at first (I answered the underlying question here - #20298 (comment))

[lukaszspyra]

.take-issue

[lukaszspyra]

Hi @damccorm ,

Please have a look at the proposed changes in PR #27868.

Replaced the synthetic field this$0 by explicit one, to get rid of the not recommended way of accessing instances.
However, the drawback of this solution is there will be always two fields generated with outer class instance (explicit and this$0).
If we kept using this$0 in the code and just add getter instead, like MapElement getOuter(){ return MapElements.this}; we would have only one field.