[ID Refactor] Shorten the length of JobID to 4 bytes (#5110)

* WIP

* Fix

* Add jobid test

* Fix

* Add python part

* Fix

* Fix tes

* Remove TODOs

* Fix C++ tests

* Lint

* Fix

* Fix exporting functions in multiple ray.init

* Fix java test

* Fix lint

* Fix linting

* Address comments.

* FIx

* Address and fix linting

* Refine and fix

* Fix

* address

* Address comments.

* Fix linting

* Fix

* Address

* Address comments.

* Address

* Address

* Fix

* Fix

* Fix

* Fix lint

* Fix

* Fix linting

* Address comments.

* Fix linting

* Address comments.

* Fix linting

* address comments.

* Fix

java/api/src/main/java/org/ray/api/id/JobId.java

@@ -0,0 +1,62 @@
+package org.ray.api.id;
+
+import java.io.Serializable;
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+import java.util.Arrays;
+
+/**
+ * Represents the id of a Ray job.
+ */
+public class JobId extends BaseId implements Serializable {
+
+  // Note that the max value of a job id is NIL which value is (2^32 - 1).
+  public static final Long MAX_VALUE = (long) Math.pow(2, 32) - 1;
+
+  public static final int LENGTH = 4;
+
+  public static final JobId NIL = genNil();
+
+  /**
+   * Create a JobID instance according to the given bytes.
+   */
+  private JobId(byte[] id) {
+    super(id);
+  }
+
+  /**
+   * Create a JobId from a given hex string.
+   */
+  public static JobId fromHexString(String hex) {
+    return new JobId(hexString2Bytes(hex));
+  }
+
+  /**
+   * Creates a JobId from the given ByteBuffer.
+   */
+  public static JobId fromByteBuffer(ByteBuffer bb) {
+    return new JobId(byteBuffer2Bytes(bb));
+  }
+
+  public static JobId fromInt(int value) {
+    byte[] bytes = new byte[JobId.LENGTH];
+    ByteBuffer wbb = ByteBuffer.wrap(bytes);
+    wbb.order(ByteOrder.LITTLE_ENDIAN);
+    wbb.putInt(value);
+    return new JobId(bytes);
+  }
+
+  /**
+   * Generate a nil JobId.
+   */
+  private static JobId genNil() {
+    byte[] b = new byte[LENGTH];
+    Arrays.fill(b, (byte) 0xFF);
+    return new JobId(b);
+  }
+
+  @Override
+  public int size() {
+    return LENGTH;
+  }
+}

java/api/src/main/java/org/ray/api/runtimecontext/RuntimeContext.java

@@ -1,6 +1,7 @@
 package org.ray.api.runtimecontext;
 
 import java.util.List;
+import org.ray.api.id.JobId;
 import org.ray.api.id.UniqueId;
 
 /**
@@ -11,7 +12,7 @@ public interface RuntimeContext {
   /**
    * Get the current Job ID.
    */
-  UniqueId getCurrentJobId();
+  JobId getCurrentJobId();
 
   /**
    * Get the current actor ID.