WIP AssertJ 3.x specific module in order to address #203

Right now this is a repackaged copy of xmlunit-assertj with the
workarounds for #181 and #161 removed.

This is not the final solution, I'll try to extract common APIs (and
tests) for xmlunit-assertj and xmlunit-assertj3 in order to reduce
code duplication later.

pom.xml

@@ -488,6 +488,9 @@
         <activeByDefault>false</activeByDefault>
         <jdk>[1.8,)</jdk>
       </activation>
+      <modules>
+        <module>xmlunit-assertj3</module>
+      </modules>
       <properties>
         <javadoc.additionalparam>-Xdoclint:html,syntax,accessibility,reference</javadoc.additionalparam>
       </properties>

xmlunit-assertj3/pom.xml

@@ -0,0 +1,68 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  This file is licensed to You 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
+
+  http://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.
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <parent>
+    <artifactId>xmlunit-parent</artifactId>
+    <groupId>org.xmlunit</groupId>
+    <version>2.8.1-SNAPSHOT</version>
+  </parent>
+  <modelVersion>4.0.0</modelVersion>
+
+  <properties>
+    <automatic.module.name>${project.groupId}.assertj3</automatic.module.name>
+    <maven.compile.source>8</maven.compile.source>
+    <maven.compile.target>8</maven.compile.target>
+  </properties>
+
+  <artifactId>xmlunit-assertj3</artifactId>
+  <name>org.xmlunit:xmlunit-assertj3</name>
+  <description>XMLUnit with AssertJ 3.x fluent API</description>
+  <url>https://www.xmlunit.org/</url>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.xmlunit</groupId>
+      <artifactId>xmlunit-core</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.xmlunit</groupId>
+      <artifactId>xmlunit-core</artifactId>
+      <classifier>tests</classifier>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.assertj</groupId>
+      <artifactId>assertj-core</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.assertj</groupId>
+      <artifactId>assertj-core</artifactId>
+      <version>3.18.0</version>
+    </dependency>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.mockito</groupId>
+      <artifactId>mockito-core</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+
+</project>

xmlunit-assertj3/src/main/java/org/xmlunit/assertj3/AssertionsAdapter.java

@@ -0,0 +1,67 @@
+/*
+  This file is licensed to You 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
+
+  http://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.xmlunit.assertj3;
+
+import org.assertj.core.api.AbstractBooleanAssert;
+import org.assertj.core.api.AbstractCharSequenceAssert;
+import org.assertj.core.api.AbstractDoubleAssert;
+import org.assertj.core.api.AbstractIntegerAssert;
+import org.assertj.core.api.AbstractObjectArrayAssert;
+import org.assertj.core.api.AbstractObjectAssert;
+import org.assertj.core.api.BooleanAssert;
+import org.assertj.core.api.DoubleAssert;
+import org.assertj.core.api.IntegerAssert;
+import org.assertj.core.api.ObjectArrayAssert;
+import org.assertj.core.api.ObjectAssert;
+import org.assertj.core.api.StringAssert;
+
+/**
+ * Class that is proxy for AssertJ assertions used by org.xmlunit.assertj.*Assert classes.
+ *
+ * <p>XMLUnit AssertJ tends to break binary compatibility of the
+ * assertThat methods from time to time so we shield ourselves from
+ * these changes a bit.</p>
+ *
+ * @see <a href="https://github.com/xmlunit/xmlunit/issues/135">GitHub issue #135</a>
+ * @since XMLUnit 2.8.1
+ */
+final class AssertionsAdapter {
+
+    private AssertionsAdapter() {
+    }
+
+    static <T> AbstractObjectAssert<?, T> assertThat(T actual) {
+        return new ObjectAssert<>(actual);
+    }
+
+    static <T> AbstractObjectArrayAssert<?, T> assertThat(T[] actual) {
+        return new ObjectArrayAssert<>(actual);
+    }
+
+    static AbstractCharSequenceAssert<?, String> assertThat(String actual) {
+        return new StringAssert(actual);
+    }
+
+    static AbstractIntegerAssert<?> assertThat(int actual) {
+        return new IntegerAssert(actual);
+    }
+
+    static AbstractDoubleAssert<?> assertThat(double actual) {
+        return new DoubleAssert(actual);
+    }
+
+    static AbstractBooleanAssert<?> assertThat(boolean actual) {
+        return new BooleanAssert(actual);
+    }
+}