Add PostAuthenticationEntryPoint

This is a handy implementation that allows an entry point to
operate differently when there is already a known user in
context. In some cases, it is not desireable to show the
end user another form and ask them for their username when
we already know it, for example.

Author: jzheaux

web/src/main/java/org/springframework/security/web/authentication/PostAuthenticationEntryPoint.java

@@ -0,0 +1,84 @@
+/*
+ * 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.web.authentication;
+
+import java.io.IOException;
+import java.util.Map;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
+import jakarta.servlet.ServletException;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
+
+import org.springframework.security.core.Authentication;
+import org.springframework.security.core.AuthenticationException;
+import org.springframework.security.core.context.SecurityContextHolder;
+import org.springframework.security.core.context.SecurityContextHolderStrategy;
+import org.springframework.security.web.AuthenticationEntryPoint;
+import org.springframework.security.web.FormPostRedirectStrategy;
+import org.springframework.security.web.RedirectStrategy;
+import org.springframework.security.web.csrf.CsrfToken;
+import org.springframework.util.Assert;
+import org.springframework.web.util.UriComponentsBuilder;
+
+public final class PostAuthenticationEntryPoint implements AuthenticationEntryPoint {
+
+	private final String entryPointUri;
+
+	private final Map<String, Function<Authentication, String>> params;
+
+	private SecurityContextHolderStrategy securityContextHolderStrategy = SecurityContextHolder
+		.getContextHolderStrategy();
+
+	private RedirectStrategy redirectStrategy = new FormPostRedirectStrategy();
+
+	public PostAuthenticationEntryPoint(String entryPointUri, Map<String, Function<Authentication, String>> params) {
+		this.entryPointUri = entryPointUri;
+		this.params = params;
+	}
+
+	@Override
+	public void commence(HttpServletRequest request, HttpServletResponse response,
+			AuthenticationException authException) throws IOException, ServletException {
+		Authentication authentication = getAuthentication(authException);
+		Assert.notNull(authentication, "could not find authentication in order to perform post");
+		Map<String, String> params = this.params.entrySet()
+			.stream()
+			.collect(Collectors.toMap(Map.Entry::getKey, (entry) -> entry.getValue().apply(authentication)));
+		UriComponentsBuilder builder = UriComponentsBuilder.fromUriString(this.entryPointUri);
+		CsrfToken csrf = (CsrfToken) request.getAttribute(CsrfToken.class.getName());
+		if (csrf != null) {
+			builder.queryParam(csrf.getParameterName(), csrf.getToken());
+		}
+		String entryPointUrl = builder.build(false).expand(params).toUriString();
+		this.redirectStrategy.sendRedirect(request, response, entryPointUrl);
+	}
+
+	private Authentication getAuthentication(AuthenticationException authException) {
+		Authentication authentication = authException.getAuthenticationRequest();
+		if (authentication != null && authentication.isAuthenticated()) {
+			return authentication;
+		}
+		authentication = this.securityContextHolderStrategy.getContext().getAuthentication();
+		if (authentication != null && authentication.isAuthenticated()) {
+			return authentication;
+		}
+		return null;
+	}
+
+}