Add a Junit asserts to AssertJ guide

Fixes: #53

Very rudimentary guide -- just ran on the default group and saw what
changes were made. Could potentially use more in the future

SUMMARY.md

@@ -21,6 +21,7 @@
     * [Refactoring with declarative YAML recipes](running-recipes/popular-recipe-guides/authoring-declarative-yaml-recipes.md)
     * [Automating Maven dependency management](running-recipes/popular-recipe-guides/automating-maven-dependency-management.md)
     * [Migrate Hamcrest to AssertJ](running-recipes/popular-recipe-guides/hamcrest-to-assertj.md)
+    * [Migrate JUnit Asserts to AssertJ](/running-recipes/popular-recipe-guides/junit-asserts-to-assertj.md)
 * [Authoring Recipes](authoring-recipes/authoring-recipes.md)
   * [Recipe development environment](authoring-recipes/recipe-development-environment.md)
   * [The different types of recipes](authoring-recipes/types-of-recipes.md)

running-recipes/popular-recipe-guides/junit-asserts-to-assertj.md

@@ -0,0 +1,143 @@
+# Migrate to AssertJ from JUnit Asserts
+
+In this tutorial, we'll use OpenRewrite to perform an automated migration from [JUnit Asserts](https://junit.org/junit4/javadoc/4.13/org/junit/Assert.html) to [AssertJ](https://assertj.github.io/doc/). AssertJ is a Java library that provides a rich set of assertions and truly helpful error messages, improves test code readability, and is designed to be super easy to use within your favorite IDE.
+
+## Example Configuration
+
+The migrate to AssertJ recipe can be applied by adding OpenRewrite's plugin to your project and including a dependency on [rewrite-testing-frameworks](https://github.com/openrewrite/rewrite-testing-frameworks):
+
+{% tabs %}
+{% tab title="Maven" %}
+{% code title="pom.xml" %}
+```xml
+<build>
+  <plugins>
+    <plugin>
+      <groupId>org.openrewrite.maven</groupId>
+      <artifactId>rewrite-maven-plugin</artifactId>
+      <version>5.15.4</version>
+      <configuration>
+        <activeRecipes>
+          <recipe>org.openrewrite.java.testing.assertj.JUnitToAssertj</recipe>
+        </activeRecipes>
+      </configuration>
+      <dependencies>
+        <dependency>
+          <groupId>org.openrewrite.recipe</groupId>
+          <artifactId>rewrite-testing-frameworks</artifactId>
+          <version>2.1.4</version>
+        </dependency>
+      </dependencies>
+    </plugin>
+  </plugins>
+<build>
+```
+{% endcode %}
+{% endtab %}
+
+{% tab title="Gradle" %}
+{% code title="build.gradle" %}
+```groovy
+    plugins {
+        id("java")
+        id("org.openrewrite.rewrite") version("6.5.12")
+    }
+    
+    rewrite {
+        activeRecipe("org.openrewrite.java.testing.assertj.JUnitToAssertj")
+    }
+    
+    repositories {
+        mavenCentral() // rewrite-spring is published to Maven Central
+    }
+    
+    dependencies {
+        rewrite(platform("org.openrewrite.recipe:rewrite-recipe-bom:2.5.3"))
+        rewrite("org.openrewrite.recipe:rewrite-testing-frameworks")
+    
+        // Other project dependencies
+    }
+```
+{% endcode %}
+{% endtab %}
+{% endtabs %}
+
+At this point, you're ready to execute the migration by running `mvn rewrite:run` or `gradlew rewriteRun`. After running the migration you can inspect the results with `git diff` (or equivalent), manually fix anything that wasn't able to be migrated automatically, and commit the results.
+
+## Before and After
+
+For the full list of changes, this recipe will make, see its [reference page](/reference/recipes/java/testing/assertj/junittoassertj.md).
+
+{% tabs %}
+{% tab title="JUnit Assert Test Class (Before)" %}
+```java
+package org.openrewrite.example;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.fail;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class ExampleJunitTestClass {
+
+    @Test
+    void someTest() {
+        try {
+            QueryRegistry queryRegistry = new QueryRegistry.Builder().build();
+            fail();
+        } catch (IllegalArgumentException ignored) {
+        }
+
+        // ...
+
+        assertEquals(1, currentSubs.size());
+        assertEquals("myPrefix_subId", currentSubs.get(0).getSubscriptionId());
+        assertTrue(allSubscriptions.containsKey("myApp"));
+    }
+
+    @Test
+    void shouldFailToInitializeWithMissingTable() {
+        when(catalog.loadTable(any())).thenThrow(new RuntimeException());
+        IcebergWriterStage stage = new IcebergWriterStage();
+        assertThrows(RuntimeException.class, () -> stage.init(context));
+    }
+}
+```
+{% endtab %}
+
+{% tab title="AssertJ Test Class (After)" %}
+```java
+package org.openrewrite.example;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.fail;
+import static org.assertj.core.api.AssertionsForClassTypes.assertThatExceptionOfType;
+
+
+public class ExampleJunitTestClass {
+
+    @Test
+    void someTest() {
+        try {
+            QueryRegistry queryRegistry = new QueryRegistry.Builder().build();
+            fail("");
+        } catch (IllegalArgumentException ignored) {
+        }
+
+        // ...
+
+        assertThat(currentSubs.size()).isEqualTo(1);
+        assertThat(currentSubs.get(0).getSubscriptionId()).isEqualTo("myPrefix_subId");
+        assertThat(allSubscriptions.containsKey("myApp")).isTrue();
+    }
+
+    @Test
+    void shouldFailToInitializeWithMissingTable() {
+        when(catalog.loadTable(any())).thenThrow(new RuntimeException());
+        IcebergWriterStage stage = new IcebergWriterStage();
+        assertThatExceptionOfType(RuntimeException.class).isThrownBy(() -> stage.init(context));
+    }
+}
+```
+{% endtab %}
+{% endtabs %}

running-recipes/running-recipes.md

@@ -18,3 +18,4 @@
   * [Refactoring with declarative YAML recipes](/running-recipes/popular-recipe-guides/authoring-declarative-yaml-recipes.md)
   * [Automating Maven dependency management](/running-recipes/popular-recipe-guides/automating-maven-dependency-management.md)
   * [Migrate Hamcrest to AssertJ](/running-recipes/popular-recipe-guides/hamcrest-to-assertj.md)
+  * [Migrate JUnit Asserts to AssertJ](/running-recipes/popular-recipe-guides/junit-asserts-to-assertj.md)