[Java] Fix serializing issues of RaySerializer (#4887)

* Fix

* Address comment.

java/dependencies.bzl

@@ -11,7 +11,7 @@ def gen_java_deps():
             "com.sun.xml.bind:jaxb-impl:2.3.0",
             "com.typesafe:config:1.3.2",
             "commons-io:commons-io:2.5",
-            "de.ruedigermoeller:fst:2.47",
+            "de.ruedigermoeller:fst:2.57",
             "javax.xml.bind:jaxb-api:2.3.0",
             "org.apache.commons:commons-lang3:3.4",
             "org.ow2.asm:asm:6.0",

java/runtime/pom.xml

@@ -54,7 +54,7 @@
 <dependency>
   <groupId>de.ruedigermoeller</groupId>
   <artifactId>fst</artifactId>
-  <version>2.47</version>
+  <version>2.57</version>
 </dependency>
 <dependency>
   <groupId>org.apache.commons</groupId>

java/runtime/src/main/java/org/ray/runtime/RayPyActorImpl.java

@@ -20,7 +20,9 @@ public class RayPyActorImpl extends RayActorImpl implements RayPyActor {
    */
   private String className;
 
-  private RayPyActorImpl() {}
+  // Note that this empty constructor must be public
+  // since it'll be needed when deserializing.
+  public RayPyActorImpl() {}
 
   public RayPyActorImpl(UniqueId id, String moduleName, String className) {
     super(id);

java/test/src/main/java/org/ray/api/test/RaySerializerTest.java

@@ -0,0 +1,23 @@
+package org.ray.api.test;
+
+import org.ray.api.RayPyActor;
+import org.ray.api.id.UniqueId;
+import org.ray.runtime.RayPyActorImpl;
+import org.ray.runtime.util.Serializer;
+import org.testng.Assert;
+import org.testng.annotations.Test;
+
+public class RaySerializerTest {
+
+  @Test
+  public void testSerializePyActor() {
+    final UniqueId pyActorId = UniqueId.randomId();
+    RayPyActor pyActor = new RayPyActorImpl(pyActorId, "test", "RaySerializerTest");
+    byte[] bytes = Serializer.encode(pyActor);
+    RayPyActor result = Serializer.decode(bytes);
+    Assert.assertEquals(result.getId(), pyActorId);
+    Assert.assertEquals(result.getModuleName(), "test");
+    Assert.assertEquals(result.getClassName(), "RaySerializerTest");
+  }
+
+}