[Java] Support multiple workers in Java worker process (#5505)

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

@@ -1,6 +1,7 @@
 package org.ray.api;
 
 import java.util.List;
+import java.util.concurrent.Callable;
 import org.ray.api.id.ObjectId;
 import org.ray.api.id.UniqueId;
 import org.ray.api.runtime.RayRuntime;
@@ -117,6 +118,28 @@ public final class Ray extends RayCall {
     return runtime.wait(waitList, waitList.size(), Integer.MAX_VALUE);
   }
 
+  /**
+   * If users want to use Ray API in there own threads, they should wrap their {@link Runnable}
+   * objects with this method.
+   *
+   * @param runnable The runnable to wrap.
+   * @return The wrapped runnable.
+   */
+  public static Runnable wrapRunnable(Runnable runnable) {
+    return runtime.wrapRunnable(runnable);
+  }
+
+  /**
+   * If users want to use Ray API in there own threads, they should wrap their {@link Callable}
+   * objects with this method.
+   *
+   * @param callable The callable to wrap.
+   * @return The wrapped callable.
+   */
+  public static Callable wrapCallable(Callable callable) {
+    return runtime.wrapCallable(callable);
+  }
+
   /**
    * Get the underlying runtime instance.
    */

java/api/src/main/java/org/ray/api/options/ActorCreationOptions.java

@@ -30,7 +30,7 @@ public class ActorCreationOptions extends BaseTaskOptions {
 
     private Map<String, Double> resources = new HashMap<>();
     private int maxReconstructions = NO_RECONSTRUCTION;
-    private String jvmOptions = "";
+    private String jvmOptions = null;
 
     public Builder setResources(Map<String, Double> resources) {
       this.resources = resources;

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

@@ -1,6 +1,7 @@
 package org.ray.api.runtime;
 
 import java.util.List;
+import java.util.concurrent.Callable;
 import org.ray.api.RayActor;
 import org.ray.api.RayObject;
 import org.ray.api.RayPyActor;
@@ -141,4 +142,18 @@ public interface RayRuntime {
    */
   RayPyActor createPyActor(String moduleName, String className, Object[] args,
       ActorCreationOptions options);
+
+  /**
+   * Wrap a {@link Runnable} with necessary context capture.
+   * @param runnable The runnable to wrap.
+   * @return The wrapped runnable.
+   */
+  Runnable wrapRunnable(Runnable runnable);
+
+  /**
+   * Wrap a {@link Callable} with necessary context capture.
+   * @param callable The callable to wrap.
+   * @return The wrapped callable.
+   */
+  Callable wrapCallable(Callable callable);
 }