[Java] Generate head redis port randomly (#6879)

* Random head port

* address comments.

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

@@ -7,6 +7,8 @@ import org.testng.annotations.Test;
 
 public class RayConfigTest {
 
+  public final static int NUM_RETRIES = 5;
+
   @Test
   public void testCreateRayConfig() {
     try {
@@ -18,6 +20,27 @@ public class RayConfigTest {
       // Unset system properties.
       System.clearProperty("ray.job.resource-path");
     }
+  }
 
+  @Test
+  public void testGenerateHeadPortRandomly() {
+    boolean isSame = true;
+    final int port1 = RayConfig.create().headRedisPort;
+    // If we the 2 ports are the same, let's retry.
+    // This is used to avoid any flaky chance.
+    for (int i = 0; i < NUM_RETRIES; ++i) {
+      final int port2 = RayConfig.create().headRedisPort;
+      if (port1 != port2) {
+        isSame = false;
+        break;
+      }
+    }
+    Assert.assertFalse(isSame);
+  }
+
+  @Test
+  public void testSpecifyHeadPort() {
+    System.setProperty("ray.redis.head-port", "11111");
+    Assert.assertEquals(RayConfig.create().headRedisPort, 11111);
   }
 }