Bael 4805 Decode a JWT Token in Java (#10535)

* BAEL-4805 Added the code for Decoding a JWT Token in Java tutorial

* BAEL-4805 Added newline at end of test file to conform to standards

* BAEL-4805 renamed test class to conform to naming standard for unit tests

* BAEL-4805 removed @DisplayName annotations as they are redundant

* BAEL-4805 changed assertion to be the user name in payload section to be more meaningful to reader

Co-authored-by: Liam Garvie <liamgarvie@Liams-MacBook-Pro.local>

jjwt/src/test/java/io/jsonwebtoken/jjwtfun/util/JWTDecoderUtilUnitTest.java

@@ -0,0 +1,32 @@
+package io.jsonwebtoken.jjwtfun.util;
+
+import io.jsonwebtoken.SignatureAlgorithm;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+class JWTDecoderUtilUnitTest {
+
+    private final static String SIMPLE_TOKEN = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkJhZWxkdW5nIFVzZXIiLCJpYXQiOjE1MTYyMzkwMjJ9";
+    private final static String SIGNED_TOKEN = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkJhZWxkdW5nIFVzZXIiLCJpYXQiOjE1MTYyMzkwMjJ9.qH7Zj_m3kY69kxhaQXTa-ivIpytKXXjZc1ZSmapZnGE";
+
+    @Test
+    void givenSimpleToken_whenDecoding_thenStringOfHeaderPayloadAreReturned() {
+        assertThat(JWTDecoderUtil.decodeJWTToken(SIMPLE_TOKEN))
+          .contains(SignatureAlgorithm.HS256.getValue());
+    }
+
+    @Test
+    void givenSignedToken_whenDecodingWithInvalidSecret_thenIntegrityIsNotValidated() {
+        assertThatThrownBy(() -> JWTDecoderUtil.decodeJWTToken(SIGNED_TOKEN, "BAD_SECRET"))
+          .hasMessage("Could not verify JWT token integrity!");
+    }
+
+    @Test
+    void givenSignedToken_whenDecodingWithValidSecret_thenIntegrityIsValidated() throws Exception {
+        assertThat(JWTDecoderUtil.decodeJWTToken(SIGNED_TOKEN, "MySecretKey"))
+          .contains("Baeldung User");
+    }
+}