Change to lazy init values only

Instead of lazy init the whole array, we lazy init each values.

Author: Bao-Long Nguyen-Trong

api/all/src/main/java/io/opentelemetry/api/internal/OtelEncodingUtils.java

@@ -6,7 +6,6 @@
 package io.opentelemetry.api.internal;
 
 import java.util.Arrays;
-import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 
 /**
@@ -22,8 +21,16 @@ public final class OtelEncodingUtils {
   private static final int NUM_ASCII_CHARACTERS = 128;
   private static final char[] ENCODING = buildEncodingArray();
   private static final byte[] DECODING = buildDecodingArray();
-
-  @Nullable private static volatile boolean[] validHex = null;
+  /**
+   * Stores whether the character at that index is a valid HEX character. 
+   * We lazy init the array values to minimize impact during process startup esp. on mobile.
+   * A value of 0 means the character validity has not been determined yet.
+   * A value of 1 means the character is a valid HEX character.
+   * A value of -1 means the character is an invalid HEX character.
+   * 
+   * @see #isValidBase16Character(char) 
+   */
+  private static final int[] VALID_HEX = new int[Character.MAX_VALUE];
 
   private static char[] buildEncodingArray() {
     char[] encoding = new char[512];
@@ -44,14 +51,6 @@ private static byte[] buildDecodingArray() {
     return decoding;
   }
 
-  private static boolean[] buildValidHexArray() {
-    boolean[] validHex = new boolean[Character.MAX_VALUE];
-    for (int i = 0; i < Character.MAX_VALUE; i++) {
-      validHex[i] = (48 <= i && i <= 57) || (97 <= i && i <= 102);
-    }
-    return validHex;
-  }
-
   /**
    * Returns the {@code long} value whose base16 representation is stored in the first 16 chars of
    * {@code chars} starting from the {@code offset}.
@@ -154,15 +153,12 @@ public static boolean isValidBase16String(CharSequence value) {
 
   /** Returns whether the given {@code char} is a valid hex character. */
   public static boolean isValidBase16Character(char b) {
-    if (validHex == null) {
-      synchronized (OtelEncodingUtils.class) {
-        if (validHex == null) {
-          validHex = buildValidHexArray();
-        }
-      }
+    int isValid = VALID_HEX[b];
+    if (isValid == 0) {
+      isValid = (48 <= b && b <= 57) || (97 <= b && b <= 102) ? 1 : -1;
+      VALID_HEX[b] = isValid;
     }
-
-    return validHex[b];
+    return isValid == 1;
   }
 
   private OtelEncodingUtils() {}