Liam Miller-Cushon created BEAM-13020:
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Summary: Fragile reference to private synthetic this$ field
Key: BEAM-13020
URL: https://issues.apache.org/jira/browse/BEAM-13020
Project: Beam
Issue Type: Bug
Components: sdk-java-core
Reporter: Liam Miller-Cushon
The following code is using reflection to access a field with a name starting
with `this$`:
[https://github.com/apache/beam/blob/4fbdcca377fb9d804433f3014a7e7dfcef2e02f9/runners/spark/src/main/java/org/apache/beam/runners/spark/SparkNativePipelineVisitor.java#L186]
The OpenJDK javac generates private synthetic fields with names starting with
`this$` 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:
{code:java}
class Outer {
int x;
class Inner1 {
int f() {
return x;
}
}
class Inner2 {
void g() {
System.err.println("hello");
}
}
}
{code}
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.
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