Propagate backend error to worker (#4039)

java/api/src/main/java/org/ray/api/exception/RayActorException.java

@@ -0,0 +1,16 @@
+package org.ray.api.exception;
+
+/**
+ * Indicates that the actor died unexpectedly before finishing a task.
+ *
+ * This exception could happen either because the actor process dies while executing a task, or
+ * because a task is submitted to a dead actor.
+ */
+public class RayActorException extends RayException {
+
+  public static final RayActorException INSTANCE = new RayActorException();
+
+  private RayActorException() {
+    super("The actor died unexpectedly before finishing this task.");
+  }
+}

java/api/src/main/java/org/ray/api/exception/RayTaskException.java

@@ -0,0 +1,15 @@
+package org.ray.api.exception;
+
+/**
+ * Indicates that a task threw an exception during execution.
+ *
+ * If a task throws an exception during execution, a RayTaskException is stored in the object store
+ * as the task's output. Then when the object is retrieved from the object store, this exception
+ * will be thrown and propagate the error message.
+ */
+public class RayTaskException extends RayException {
+
+  public RayTaskException(String message, Throwable cause) {
+    super(message, cause);
+  }
+}

java/api/src/main/java/org/ray/api/exception/RayWorkerException.java

@@ -0,0 +1,13 @@
+package org.ray.api.exception;
+
+/**
+ * Indicates that the worker died unexpectedly while executing a task.
+ */
+public class RayWorkerException extends RayException {
+
+  public static final RayWorkerException INSTANCE = new RayWorkerException();
+
+  private RayWorkerException() {
+    super("The worker died unexpectedly while executing this task.");
+  }
+}

java/api/src/main/java/org/ray/api/exception/UnreconstructableException.java

@@ -0,0 +1,23 @@
+package org.ray.api.exception;
+
+import org.ray.api.id.UniqueId;
+
+/**
+ * Indicates that an object is lost (either evicted or explicitly deleted) and cannot be
+ * reconstructed.
+ *
+ * Note, this exception only happens for actor objects. If actor's current state is after object's
+ * creating task, the actor cannot re-run the task to reconstruct the object.
+ */
+public class UnreconstructableException extends RayException {
+
+  public final UniqueId objectId;
+
+  public UnreconstructableException(UniqueId objectId) {
+    super(String.format(
+        "Object %s is lost (either evicted or explicitly deleted) and cannot be reconstructed.",
+        objectId));
+    this.objectId = objectId;
+  }
+
+}