Add support for JSON assertions using JSON compare

This commit moves JSON content AssertJ support from Spring Boot.

See gh-21178

Co-authored-by: Brian Clozel <brian.clozel@broadcom.com>

spring-test/spring-test.gradle

@@ -32,6 +32,7 @@ dependencies {
 	optional("org.apache.groovy:groovy")
 	optional("org.apache.tomcat.embed:tomcat-embed-core")
 	optional("org.aspectj:aspectjweaver")
+	optional("org.assertj:assertj-core")
 	optional("org.hamcrest:hamcrest")
 	optional("org.htmlunit:htmlunit") {
 		exclude group: "commons-logging", module: "commons-logging"

spring-test/src/main/java/org/springframework/test/json/JsonContent.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2002-2024 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.test.json;
+
+import org.assertj.core.api.AssertProvider;
+
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
+
+/**
+ * JSON content usually created from a JSON tester. Generally used only to
+ * {@link AssertProvider provide} {@link JsonContentAssert} to AssertJ
+ * {@code assertThat} calls.
+ *
+ * @author Phillip Webb
+ * @author Diego Berrueta
+ * @since 6.2
+ */
+public final class JsonContent implements AssertProvider<JsonContentAssert> {
+
+	private final String json;
+
+	@Nullable
+	private final Class<?> resourceLoadClass;
+
+	/**
+	 * Create a new {@link JsonContent} instance.
+	 * @param json the actual JSON content
+	 * @param resourceLoadClass the source class used to load resources
+	 */
+	JsonContent(String json, @Nullable Class<?> resourceLoadClass) {
+		Assert.notNull(json, "JSON must not be null");
+		this.json = json;
+		this.resourceLoadClass = resourceLoadClass;
+	}
+
+	/**
+	 * Use AssertJ's {@link org.assertj.core.api.Assertions#assertThat assertThat}
+	 * instead.
+	 */
+	@Override
+	public JsonContentAssert assertThat() {
+		return new JsonContentAssert(this.json, this.resourceLoadClass, null);
+	}
+
+	/**
+	 * Return the actual JSON content string.
+	 * @return the JSON content
+	 */
+	public String getJson() {
+		return this.json;
+	}
+
+	@Override
+	public String toString() {
+		return "JsonContent " + this.json;
+	}
+
+}