[Java] Add Plasma Free to Java code path (#2802)

java/api/src/main/java/org/ray/api/Ray.java

@@ -108,7 +108,7 @@ public final class Ray extends RayCall {
   /**
    * Get the underlying runtime instance.
    */
-  static RayRuntime internal() {
+  public static RayRuntime internal() {
     return runtime;
   }
 }

java/api/src/main/java/org/ray/api/runtime/RayRuntime.java

@@ -52,6 +52,14 @@ public interface RayRuntime {
    */
   <T> WaitResult<T> wait(List<RayObject<T>> waitList, int numReturns, int timeoutMs);
 
+  /**
+   * Free a list of objects from Plasma Store.
+   *
+   * @param objectIds The object ids to free.
+   * @param localOnly Whether only free objects for local object store or not.
+   */
+  void free(List<UniqueId> objectIds, boolean localOnly);
+
   /**
    * Invoke a remote function.
    *