HDFS-10725. Caller context should always be constructed by a builder. (Contributed by Mingliang Liu)

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/CallerContext.java

@@ -35,7 +35,7 @@ import java.util.Arrays;
 @InterfaceAudience.LimitedPrivate({"HBase", "HDFS", "Hive", "MapReduce",
     "Pig", "YARN"})
 @InterfaceStability.Evolving
-public class CallerContext {
+public final class CallerContext {
   public static final Charset SIGNATURE_ENCODING = StandardCharsets.UTF_8;
   /** The caller context.
    *
@@ -54,7 +54,7 @@ public class CallerContext {
    */
   private final byte[] signature;
 
-  public CallerContext(Builder builder) {
+  private CallerContext(Builder builder) {
     this.context = builder.context;
     this.signature = builder.signature;
   }