Add SecurityAssertions

This commit introduces a simple, internal test API for
verifying aspects of an Authentication, like its name
and authorities.

Closes gh-17844

core/src/test/java/org/springframework/security/authentication/SecurityAssertions.java

@@ -0,0 +1,100 @@
+/*
+ * Copyright 2004-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.security.authentication;
+
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+import java.util.Set;
+import java.util.function.Predicate;
+
+import org.assertj.core.api.AbstractObjectAssert;
+import org.assertj.core.api.Assertions;
+import org.assertj.core.api.CollectionAssert;
+import org.assertj.core.api.Condition;
+import org.assertj.core.api.ObjectAssert;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
+
+import org.springframework.security.core.Authentication;
+import org.springframework.security.core.GrantedAuthority;
+import org.springframework.security.core.authority.AuthorityUtils;
+
+@NullMarked
+public final class SecurityAssertions {
+
+	private SecurityAssertions() {
+
+	}
+
+	public static AuthenticationAssert assertThat(@Nullable Authentication authentication) {
+		Assertions.assertThat(authentication).isNotNull();
+		return new AuthenticationAssert(authentication);
+	}
+
+	public static final class AuthenticationAssert extends AbstractObjectAssert<AuthenticationAssert, Authentication> {
+
+		private final Authentication authentication;
+
+		private AuthenticationAssert(Authentication authentication) {
+			super(authentication, AuthenticationAssert.class);
+			this.authentication = authentication;
+		}
+
+		public AuthenticationAssert name(String name) {
+			Assertions.assertThat(this.authentication.getName()).isEqualTo(name);
+			return this;
+		}
+
+		public ObjectAssert<GrantedAuthority> hasAuthority(String authority) {
+			Collection<? extends GrantedAuthority> actual = this.authentication.getAuthorities();
+			for (GrantedAuthority element : actual) {
+				if (element.getAuthority().equals(authority)) {
+					return new ObjectAssert<>(element);
+				}
+			}
+			throw new AssertionError(actual + " does not contain " + authority + " as expected");
+		}
+
+		public CollectionAssert<GrantedAuthority> hasAuthorities(String... authorities) {
+			HasAuthoritiesPredicate test = new HasAuthoritiesPredicate(authorities);
+			return authorities().has(new Condition<>(test, "contains %s", Arrays.toString(authorities)));
+		}
+
+		public CollectionAssert<GrantedAuthority> authorities() {
+			return new CollectionAssert<>(this.authentication.getAuthorities());
+		}
+
+	}
+
+	private static final class HasAuthoritiesPredicate implements Predicate<Collection<? extends GrantedAuthority>> {
+
+		private final Collection<String> expected;
+
+		private HasAuthoritiesPredicate(String... expected) {
+			this.expected = List.of(expected);
+		}
+
+		@Override
+		public boolean test(Collection<? extends GrantedAuthority> actual) {
+			Set<String> asString = AuthorityUtils.authorityListToSet(actual);
+			return asString.containsAll(this.expected);
+		}
+
+	}
+
+}