Add Java 15 to JavaVersion enum

See gh-21691

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/system/JavaVersion.java

@@ -66,7 +66,12 @@ public enum JavaVersion {
 	/**
 	 * Java 14.
 	 */
-	FOURTEEN("14", MethodHandles.Lookup.class, "hasFullPrivilegeAccess");
+	FOURTEEN("14", MethodHandles.Lookup.class, "hasFullPrivilegeAccess"),
+
+	/**
+	 * Java 15.
+	 */
+	FIFTEEN("15", CharSequence.class, "isEmpty");
 
 	private final String name;
 

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/system/JavaVersionTests.java

@@ -121,4 +121,10 @@ class JavaVersionTests {
 		assertThat(JavaVersion.getJavaVersion()).isEqualTo(JavaVersion.FOURTEEN);
 	}
 
+	@Test
+	@EnabledOnJre(JRE.JAVA_15)
+	void currentJavaVersionFifteen() {
+		assertThat(JavaVersion.getJavaVersion()).isEqualTo(JavaVersion.FIFTEEN);
+	}
+
 }