Clarify requirements for ArgumentConverter/Aggregator implementations

Issue: #1397

Author: marcphilipp

junit-jupiter-params/src/main/java/org/junit/jupiter/params/aggregator/ArgumentsAccessor.java

@@ -32,6 +32,8 @@
  * In addition, {@link ArgumentsAggregator} implementations are given access to
  * an {@code ArgumentsAccessor}.
  *
+ * <p>This interface is not intended to be implemented by clients.
+ *
  * @since 5.2
  * @see ArgumentsAggregator
  * @see org.junit.jupiter.params.ParameterizedTest

junit-jupiter-params/src/main/java/org/junit/jupiter/params/aggregator/ArgumentsAggregator.java

@@ -33,7 +33,10 @@
  * in a CSV file into a domain object such as a {@code Person}, {@code Address},
  * {@code Order}, etc.
  *
- * <p>Implementations must provide a no-args constructor.
+ * <p>Implementations must provide a no-args constructor and should not make any
+ * assumptions on when they are instantiated or how often they are called. Since
+ * instances may potentially be cached and called from different threads they
+ * should be thread-safe and designed to be used as singletons.
  *
  * @since 5.2
  * @see AggregateWith

junit-jupiter-params/src/main/java/org/junit/jupiter/params/converter/ArgumentConverter.java

@@ -28,7 +28,10 @@
  * to know about the target type instead of the complete
  * {@link ParameterContext}.
  *
- * <p>Implementations must provide a no-args constructor.
+ * <p>Implementations must provide a no-args constructor and should not make any
+ * assumptions on when they are instantiated or how often they are called. Since
+ * instances may potentially be cached and called from different threads they
+ * should be thread-safe and designed to be used as singletons.
  *
  * @since 5.0
  * @see SimpleArgumentConverter