Decode target parameters prior to saving a FlashMap

The target parameters for a FlashMap must be decoded to be able to
match them to the parameters of incoming requests given that the
HttpServletRequest returns decoded request parameters.

Issue: SPR-9657
Backport Issue: SPR-9701

org.springframework.web.servlet/src/main/java/org/springframework/web/servlet/support/AbstractFlashMapManager.java

@@ -177,12 +177,18 @@ public abstract class AbstractFlashMapManager implements FlashMapManager {
 		if (CollectionUtils.isEmpty(flashMap)) {
 			return;
 		}
+
 		String path = decodeAndNormalizePath(flashMap.getTargetRequestPath(), request);
 		flashMap.setTargetRequestPath(path);
-		flashMap.startExpirationPeriod(this.flashMapTimeout);
+
+		decodeParameters(flashMap.getTargetRequestParams(), request);
+
 		if (logger.isDebugEnabled()) {
 			logger.debug("Saving FlashMap=" + flashMap);
 		}
+
+		flashMap.startExpirationPeriod(this.flashMapTimeout);
+
 		synchronized (writeLock) {
 			List<FlashMap> allMaps = retrieveFlashMaps(request);
 			allMaps = (allMaps == null) ? new CopyOnWriteArrayList<FlashMap>() : allMaps;
@@ -203,6 +209,14 @@ public abstract class AbstractFlashMapManager implements FlashMapManager {
 		return path;
 	}
 
+	private void decodeParameters(MultiValueMap<String, String> params, HttpServletRequest request) {
+		for (String name : new ArrayList<String>(params.keySet())) {
+			for (String value : new ArrayList<String>(params.remove(name))) {
+				params.add(name, this.urlPathHelper.decodeRequestString(request, value));
+			}
+		}
+	}
+
 	/**
 	 * Update the FlashMap instances in some underlying storage.
 	 * @param flashMaps a non-empty list of FlashMap instances to save

org.springframework.web.servlet/src/test/java/org/springframework/web/servlet/support/AbstractFlashMapManagerTests.java

@@ -264,6 +264,22 @@ public class AbstractFlashMapManagerTests {
 		assertEquals("/once/only", flashMap.getTargetRequestPath());
 	}
 
+	@Test
+	public void saveOutputFlashMapDecodeParameters() throws Exception {
+		this.request.setCharacterEncoding("UTF-8");
+
+		FlashMap flashMap = new FlashMap();
+		flashMap.put("anyKey", "anyValue");
+
+		flashMap.addTargetRequestParam("key", "%D0%90%D0%90");
+		flashMap.addTargetRequestParam("key", "%D0%91%D0%91");
+		flashMap.addTargetRequestParam("key", "%D0%92%D0%92");
+		this.flashMapManager.saveOutputFlashMap(flashMap, this.request, this.response);
+
+		assertEquals(Arrays.asList("\u0410\u0410", "\u0411\u0411", "\u0412\u0412"),
+				flashMap.getTargetRequestParams().get("key"));
+	}
+
 
 	private static class TestFlashMapManager extends AbstractFlashMapManager {