Refactor FilterWebHandler and DefaultWebFilterChain

The chain is initialized once and re-used vs creating the "next" chains
on every request.

Author: rstoyanchev

spring-web/src/main/java/org/springframework/web/server/handler/DefaultWebFilterChain.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -19,11 +19,12 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import java.util.ListIterator;
 
 import reactor.core.publisher.Mono;
 
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
-import org.springframework.util.ObjectUtils;
 import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.server.WebFilter;
 import org.springframework.web.server.WebFilterChain;
@@ -32,34 +33,80 @@
 /**
  * Default implementation of {@link WebFilterChain}.
  *
+ * <p>Each instance of this class represents one link in the chain. The public
+ * constructor {@link #DefaultWebFilterChain(WebHandler, List)}
+ * initializes the full chain and represents its first link.
+ *
+ * <p>This class is immutable and thread-safe. It can be created once and
+ * re-used to handle request concurrently.
+ *
  * @author Rossen Stoyanchev
  * @since 5.0
  */
 public class DefaultWebFilterChain implements WebFilterChain {
 
-	private final List<WebFilter> filters;
+	private final List<WebFilter> allFilters;
 
 	private final WebHandler handler;
 
-	private final int index;
+	@Nullable
+	private final WebFilter currentFilter;
 
+	@Nullable
+	private final DefaultWebFilterChain next;
 
-	public DefaultWebFilterChain(WebHandler handler, WebFilter... filters) {
+
+	/**
+	 * Public constructor with the list of filters and the target handler to use.
+	 * @param handler the target handler
+	 * @param filters the filters ahead of the handler
+	 * @since 5.1
+	 */
+	public DefaultWebFilterChain(WebHandler handler, List<WebFilter> filters) {
 		Assert.notNull(handler, "WebHandler is required");
-		this.filters = ObjectUtils.isEmpty(filters) ? Collections.emptyList() : Arrays.asList(filters);
+		this.allFilters = Collections.unmodifiableList(filters);
 		this.handler = handler;
-		this.index = 0;
+		DefaultWebFilterChain chain = initChain(filters, handler);
+		this.currentFilter = chain.currentFilter;
+		this.next = chain.next;
+	}
+
+	private static DefaultWebFilterChain initChain(List<WebFilter> filters, WebHandler handler) {
+		DefaultWebFilterChain chain = new DefaultWebFilterChain(filters, handler, null, null);
+		ListIterator<? extends WebFilter> iterator = filters.listIterator(filters.size());
+		while (iterator.hasPrevious()) {
+			chain = new DefaultWebFilterChain(filters, handler, iterator.previous(), chain);
+		}
+		return chain;
 	}
 
-	private DefaultWebFilterChain(DefaultWebFilterChain parent, int index) {
-		this.filters = parent.getFilters();
-		this.handler = parent.getHandler();
-		this.index = index;
+	/**
+	 * Private constructor to represent one link in the chain.
+	 */
+	private DefaultWebFilterChain(List<WebFilter> allFilters, WebHandler handler,
+			@Nullable WebFilter currentFilter, @Nullable DefaultWebFilterChain next) {
+
+		this.allFilters = allFilters;
+		this.currentFilter = currentFilter;
+		this.handler = handler;
+		this.next = next;
+	}
+
+	/**
+	 * Public constructor with the list of filters and the target handler to use.
+	 * @param handler the target handler
+	 * @param filters the filters ahead of the handler
+	 * @deprecated as of 5.1 this constructor is deprecated in favor of
+	 * {@link #DefaultWebFilterChain(WebHandler, List)}.
+	 */
+	@Deprecated
+	public DefaultWebFilterChain(WebHandler handler, WebFilter... filters) {
+		this(handler, Arrays.asList(filters));
 	}
 
 
 	public List<WebFilter> getFilters() {
-		return this.filters;
+		return this.allFilters;
 	}
 
 	public WebHandler getHandler() {
@@ -69,16 +116,10 @@ public WebHandler getHandler() {
 
 	@Override
 	public Mono<Void> filter(ServerWebExchange exchange) {
-		return Mono.defer(() -> {
-			if (this.index < this.filters.size()) {
-				WebFilter filter = this.filters.get(this.index);
-				WebFilterChain chain = new DefaultWebFilterChain(this, this.index + 1);
-				return filter.filter(exchange, chain);
-			}
-			else {
-				return this.handler.handle(exchange);
-			}
-		});
+		return Mono.defer(() ->
+				this.currentFilter != null && this.next != null ?
+						this.currentFilter.filter(exchange, this.next) :
+						this.handler.handle(exchange));
 	}
 
 }

spring-web/src/main/java/org/springframework/web/server/handler/FilteringWebHandler.java

@@ -16,7 +16,6 @@
 
 package org.springframework.web.server.handler;
 
-import java.util.Arrays;
 import java.util.List;
 
 import reactor.core.publisher.Mono;
@@ -34,32 +33,30 @@
  */
 public class FilteringWebHandler extends WebHandlerDecorator {
 
-	private final WebFilter[] filters;
+	private final DefaultWebFilterChain chain;
 
 
 	/**
 	 * Constructor.
 	 * @param filters the chain of filters
 	 */
-	public FilteringWebHandler(WebHandler webHandler, List<WebFilter> filters) {
-		super(webHandler);
-		this.filters = filters.toArray(new WebFilter[0]);
+	public FilteringWebHandler(WebHandler handler, List<WebFilter> filters) {
+		super(handler);
+		this.chain = new DefaultWebFilterChain(handler, filters);
 	}
 
 
 	/**
 	 * Return a read-only list of the configured filters.
 	 */
 	public List<WebFilter> getFilters() {
-		return Arrays.asList(this.filters);
+		return this.chain.getFilters();
 	}
 
 
 	@Override
 	public Mono<Void> handle(ServerWebExchange exchange) {
-		return this.filters.length != 0 ?
-				new DefaultWebFilterChain(getDelegate(), this.filters).filter(exchange) :
-				super.handle(exchange);
+		return this.chain.filter(exchange);
 	}
 
 }

spring-web/src/test/java/org/springframework/web/server/handler/FilteringWebHandlerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -42,6 +42,7 @@
 import static org.junit.Assert.assertTrue;
 
 /**
+ * Unit tests for {@link FilteringWebHandler}.
  * @author Rossen Stoyanchev
  */
 public class FilteringWebHandlerTests {